Real-Time Laser Ultrasonic Monitoring of Laser-Induced Thermal Processes

Real-Time Laser Ultrasonic Monitoring of Laser-Induced Thermal Processes | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Real-Time Laser Ultrasonic Monitoring of Laser-Induced Thermal Processes Rosa E. Morales, Kathryn J. Harke, Joseph W. Tringe, David M. Stobbe, and 1 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-1347814/v1 This work is licensed under a CC BY 4.0 License Status: Under Review Version 1 posted 9 You are reading this latest preprint version Abstract Intra- and inter-layer integrity of components fabricated with advanced manufacturing techniques, such as laser powder bed fusion, is dependent upon rapid heating, melting, and solidification processes. There is a need for new techniques to provide in situ feedback of these processes. Here a laser-based ultrasonic technique to probe thermal effects induced by a high-power continuous wave laser in titanium samples is described. Numerical simulations were performed to show that, for a spatially uniform heating beam, laser-induced surface acoustic waves are strongly influenced by surface heating conditions, are dispersive in the case of rapid heating, and that an abrupt velocity reduction happens upon the onset of surface melting. Furthermore, laser-based ultrasound experimental results which monitor the transient change of surface wave travel time associated with high power laser surface heating are provided. A pulsed laser is used to generate high frequency surface acoustic waves that propagate through the laser-heated region and are detected using a photorefractive crystal-based interferometer. Qualitative agreement is observed between theory and experiment with both showing a rapid delay in the surface wave velocity at the onset of illumination and further decrease in surface wave velocity associated with melting. It is demonstrated that changes in the surface wave velocity can be used to track local heating and detect the onset of surface melting in real time. Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Introduction Lasers have seen widespread use in materials processing and manufacturing applications including laser cutting, welding, ablation, drilling, surface texturing, and advanced manufacturing 1 . In these applications, laser energy is absorbed by a material leading to local heating, melting, and vaporization, and in situ control of these laser-induced processes is critical to ensure the integrity of the final product 2 . For example, in laser powder bed fusion an object is built in consecutive layers by laser melting of powder mechanically distributed over the build surface 3 , 4 . Intra- and inter-layer integrity is dependent upon the rapid heating, melting, and solidification processes during which defects and material discontinuities are likely to form 3 – 6 . If the heating laser power is too high for a given scan speed, it can lead to evaporation of the material from a molten pool, a subsequent recoil force, and collapse of the melt pool, resulting in porosity in the component. Alternatively, a low laser heating power can lead to an incomplete melting and solidification process and lack of fusion defects 7 , 8 . The optimal laser parameters, defined as those required to produce a part with the lowest number of processing defects, are difficult to determine a priori due to the complexity of the process and additional variables including powder quality and machine to machine variations in laser characteristics 9 . The success of laser-based manufacturing techniques, such as powder bed fusion, hinges on the ability of the operator to set and control process variables such as laser power and speed 10 , 11 . Nondestructive evaluation can aid in this process and provide in situ feedback on the build quality such that process variables can be adjusted in real time 12 . Several nondestructive evaluation methods such as thermal imaging, optical imaging, and conventional ultrasonics have been employed to monitor laser powder bed fusion builds 9 , 12 . These techniques offer remote access to the high temperature laser-material interaction site. Thermal imaging can provide information regarding the surface temperature distribution, and optical imaging can be used to ascertain surface morphology changes 9 . Conventional acoustic emission and ultrasound can potentially provide additional data regarding the build process 13 . In the case of acoustic emission, the sound generated during the laser heating process is detected using a microphone or transducer and analyzed to infer information about the process 14 . Using signal processing techniques, including those employing machine learning, it is possible to categorize the laser-material interaction into different regimes 15 . In addition, the physical, mechanical, optical, and thermal properties of materials are functions of temperature. It follows that the velocity of ultrasonic waves is also temperature-dependent with an increase in temperature generally leading to a softening of the material and lower longitudinal, shear, and surface acoustic wave velocities. Ultrasound thermography is a well-known technique used to determine the temperature of a uniformly heated sample or to map out temperature distributions below the surface 16 . Ultrasound thermography has attracted much attention in the medical ultrasonics community to continuously monitor and provide feedback to thermal treatment processes in biological tissue such as high intensity focused ultrasound therapy 16 , 17 . Some of the advantages of conventional ultrasonic techniques for process monitoring are that they are not limited to surface temperature determination and that the transducers can be placed remote from the process zone. For additive manufacturing, they do require physical access to the build surface, but the contact location can be somewhat removed from the high temperature environment. The dependence of bulk wave propagation on temperature, mechanical properties, and phase state of the material has, for example, been used to infer the temperature in the processing zone, and to predict the melt pool size based on monitoring bulk waves reflected and scattered from the melt pool 18 , 19 . Laser-based ultrasonic techniques are well suited for the real-time monitoring and nondestructive evaluation of laser-induced thermal processes. The laser detection probe beam and laser source exciting the ultrasonic waves can be separated from the high temperature manufacturing process environment using optically transparent windows or other means. Laser ultrasonic techniques have been used to evaluate advanced manufacturing builds by using surface acoustic waves and bulk waves to detect surface and near subsurface defects ex situ 20 – 23 . Surface acoustic waves and bulk waves have also been used to evaluate material microstructure and grain size 24 , to infer the surface temperature in laser-induced thermal processes 22 , 25 , to predict internal temperature distributions based on waves propagating over multiple paths 26 , and to observe melting and solidification during crystal growth 27 . Finally, laser ultrasonic methods have been used for high temperature measurements of materials properties 28 and for phase transformation studies in metals 29 – 31 . In this work, laser-based ultrasound is used to monitor laser-induced heating and melting processes. Numerical simulations of a spatially-uniform heating beam show that laser-induced surface acoustic waves are strongly influenced by surface heating conditions. They are dispersive in the case of rapid heating where the thermally-induced mechanical property change is on the same spatial scale as the wavelength of the surface acoustic waves, and show an abrupt velocity reduction upon the onset of surface melting. Furthermore, using a laser line source for surface acoustic wave excitation and a point interferometric detector, experimental results demonstrate the transient change of surface wave travel time associated with high power laser surface heating with a Gaussian beam placed between the source and receiver positions. In agreement with the numerical simulations, a deviation in the response is observed when the heating laser power is sufficient to cause local surface melting. The proposed technique may find effective application in the mapping of transient laser-induced thermal fields and melt zones, providing critical information for real-time process control. Background And Theory The thermal and elastic properties of materials are dependent on temperature, with an increase in temperature generally resulting in a decrease in both material density and stiffness. A uniformly heated sample of a titanium alloy (Ti-6Al-4V) that is assumed to be homogeneous and isotropic is considered first. The temperature-dependent density, elastic modulus, and Poisson’s ratio are obtained from the software JMatPro 32 and are used to determine the longitudinal, \({c}_{L}\) , and shear, \({c}_{T}\) , wave speeds as a function of temperature. These values are, in turn, used to calculate the Rayleigh wave speed using the characteristic equation 33 . The result is shown in Fig. 1 where the Rayleigh wave speed is seen to decrease from 3000 m/s at room temperature (293 K) to 1880 m/s at the melting temperature of 1943 K. The decrease is relatively monotonic outside of a small region between 1100 and 1275 K, the temperature range at which Ti-6Al-4V undergoes an hcp (α) → bcc (β) phase transformation 34 . In the case of uniform heating, the thermal field and elastic properties are not depth-dependent and the Rayleigh waves propagate without dispersion. Rayleigh wave propagation becomes more complex in the case of transient heating, such as that produced by a high power laser source, as the thermal field and temperature-dependent elastic properties are functions of both time and space. Furthermore, the thermal properties are also temperature-dependent, and, at sufficiently high heating powers, the material will undergo a phase transformation resulting in surface melt. Here, the elastic displacement response generated by a nanosecond pulsed laser source incident upon a surface that is being heated by a spatially uniform continuous wave (CW) laser with a step-function time dependence is calculated. First, the temperature field produced by a CW laser is calculated using the implicit finite difference method presented by Singh and Narayan 35 . Temperature-dependent thermal properties and density are included for both the solid and liquid Ti-6Al-4V phases 36 – 39 . These properties are summarized in Appendix A. The model is used to determine the temperature as a function of time ( t ) at each depth ( d ) below the surface. Figure 2 (a) shows the surface temperature (at d = 0 ) as a function of time where the heating laser is turned on at t = 0 and a heating laser power density of 6 kW/cm 2 . The surface temperature rises until it reaches the melting temperature of 1943 K at a time of 465 ms, where it briefly remains until the net heat absorbed exceeds the latent heat of the phase change 35 , 40 . The melt front then begins to propagate into the material and, as shown on the right axis of Fig. 2 (a), proceeds rapidly to a depth of over 35 µm. Figure 2 (b) shows the full extent of the calculated thermal data, with the color bar indicating the temperature rise at each depth and heating time. Surface acoustic waves are confined to propagate in the near surface region with a penetration depth on the order of the wavelength. For example, for a frequency of 30 MHz, surface waves in Ti-6Al-4V will be sensitive to mechanical property changes that occur over a depth of approximately 100 µm. In Fig. 2 (b), the temperature rise is somewhat uniform over the near surface region, and thus it is expected that the mechanical property changes will also be relatively constant over the penetration depth, resulting in a surface acoustic wave delay that is frequency independent. The surface temperature for a significantly higher heating power density of 250 kW/cm 2 is shown in Fig. 2 (c). Here, surface melting occurs at about 300 µs and there are marked thermal gradients in the near surface region within the 500 µs time window as shown in Fig. 2 (d). These thermal gradients can cause dispersion of surface waves since the higher frequency waves, with a shorter wavelength, will be more influenced by the near surface region while lower frequency waves will penetrate further into the cooler substrate. This model allows for the calculation of temperature as a function of depth and melt front position at any time after the heating laser is turned on. Next, the pulsed laser excitation and interferometric detection of the CW laser-heated surface at a given time is modeled. The material near the surface is discretized into 400 layers, with a layer thickness of 0.6 µm. The elastic properties of each layer are calculated from the mean temperature of the layer, and the elastic wave propagation problem is then reduced to an analogous problem of wave propagation in a homogeneous, isotropic layered media 41 . When surface melting occurs, the thickness of the surface layer is set as the thickness of the melt pool and the density 38 (3920 kg/m 3 ) and longitudinal wave velocity 39 (4407 m/s) of liquid Ti-6Al-4V are used. Laser generation of ultrasound in plates 42 – 44 and in multi-layer plates on a semi-infinite substrate 45 have been previously addressed by others. The approach presented by Cheng et al . is followed in which the excitation laser source is represented as an equivalent elastic boundary source (Gaussian in space with a 10 ns pulse width), and the transfer matrix technique is used to enforce the continuity of stress and displacement across all homogeneous and isotropic layer boundaries 41 . The problem is solved in cylindrical coordinates using the integral transform technique where a Hankel transform of the elastic wave equation is taken with respect to the radial coordinate ( r ) and a Laplace transform is taken with respect to time. The normal surface displacement as a function of time at a given r is found through numerical inversion of the Hankel-Laplace transforms. The excitation laser spot size was set to 100 µm full width at half maximum (FWHM) with a detection location at r = 1.0 mm. The normal surface displacement as a function of time is given in Fig. 3 (a) for a heating power of 6 kW/cm 2 . The top curve shows the room temperature response in the absence of CW laser surface heating. A small wave amplitude arrival corresponding to the surface skimming longitudinal wave is seen at 0.16 µs followed by the larger surface acoustic wave (SAW) amplitude arrival at about 0.30 µs. The other curves show the displacement response at various times after the CW heating laser is turned on. For the signals between t = 0 and t = 450 ms, the shape of the surface acoustic wave remains relatively uniform, but the arrival is delayed as heating proceeds. During the last three time steps: t = 508.5, 517.0, and 525.6 ms, melting has occurred with melt depths of 6.0, 16.8 and 27.0 µm, respectively. More prominent dispersion is seen when the surface waves traverse the molten layer; the higher frequency, short wavelength, components are delayed due to the strong interaction with the melt layer. Note that in this case, the longitudinal wave velocity in the molten liquid is higher than the shear wave velocity in the substrate so the surface waves are not leaky 46 . In general, the velocity of waves propagating on a liquid-covered half space transition from the Rayleigh wave velocity at zero thickness to the Scholte wave velocity when the thickness of the liquid is large with respect to the wavelength 47 − 48 . Figure 3 (b) shows the evolution of the displacement field calculated throughout the heating time. Here the abscissa gives the time after the heating laser is turned on while the ordinate gives the time after the excitation laser pulse. The color bar represents the normal displacement of the surface. In this image, the SAW arrival has the largest negative amplitude and is shown in red. The initial pronounced change in the SAW arrival time is associated with the rapid rise in the near surface temperature as seen in Figs. 2 (a) and 2(b). The SAW arrival time is then relatively constant between 75 and 100 ms during which the α → β phase transformation in Ti-6Al-4V occurs. After this transition region, the arrival of the SAW continues to be delayed with heating time in a monotonic fashion until approximately 508 ms when a sharp break in the curve associated with surface melting is observed. The SAW signal sensitivity to the presence of melt makes it an attractive option for sensing melt depth. Figure 3 (c) shows the normal displacement of the surface for a higher heating power density, 250 kW/cm 2 . The excitation source characteristics are the same as those given above. In this case, however, the heating takes place much more rapidly and surface melting starts at about 305 µs. Such rapid heating leads to strong near-surface thermal gradients (see Figs. 2 (c) and 2(d)) which, in turn, lead to sharp changes in the mechanical properties within the wavelength range of the broadband surface acoustic wave. At heating times between t = 0 and t = 300 µs, a significant degree of surface acoustic wave dispersion is evident, with the higher frequency components that probe the near-surface temperature delayed with respect to the lower frequencies that penetrate further into the cooler bulk of the material. This effect is more pronounced at later times ( t > 300 µs) where the higher frequency SAW components are also delayed by the presence of surface melt. Figure 3 (d) shows the temporal evolution of the displacement field with surface heating. While the dispersion is certainly more pronounced than in Fig. 3 (b), the onset of surface melting is not as evident. Note that the dispersion of the SAWs in a multilayer system can be used to back out the depth-dependent mechanical properties using a model-based inversion approach 49 . For the heating case, depth-dependent mechanical properties could ultimately be related to the subsurface temperature profile. Experimental Setup A laser ultrasonic system was used to study surface acoustic wave propagation through a laser-heated region of Ti-6Al-4V samples. A schematic of this experimental configuration is shown in Fig. 4 . A pulsed Nd:YAG laser operating at the fundamental frequency (λ = 1064 nm) and a repetition rate of 15 Hz is used to generate broad band surface acoustic waves. The generation laser is collimated and focused to a line on the sample surface using a cylindrical lens. At the sample surface, the line source was approximately 15 mm in length and had a Gaussian FWHM of 60 µm. The laser energy at the sample was 1.6 mJ, sufficiently low that generation remained in the thermoelastic regime. The displacement normal to the surface associated with the laser-generated acoustic wave was detected using a photorefractive crystal (PRC) based interferometer using a Bismuth Silicon Oxide (BSO) PRC 50 – 52 . The detection laser was a 300 mW frequency doubled Nd:YAG laser operating at 532 nm. The laser output was directed to a beamsplitter where it was divided into reference and signal beams. The reference beam was sent directly to the PRC, while the signal beam was focused onto the specimen surface and, upon reflection, was sent to the PRC where it interferes with the reference beam and creates an index grating in the crystal. A portion of the reference beam diffracts from the grating in the two-wave mixing process and interferes with the signal beam at the photodetector. Polarization optics after the PRC were used to ensure that the diffracted reference beam and transmitted signal beam were in quadrature, optimizing the detection sensitivity. The output of the photodetector was sent to a digital oscilloscope with a 200 MHz bandwidth limit and subsequently transferred to a computer and filtered using a 40 MHz second order low pass Butterworth filter. The distance between the SAW excitation and detection was set to 4.0 mm. A fiber-coupled 60 W continuous wave Nd:YAG laser operating at a wavelength of 1064 nm was used to heat the sample surface. The laser output was collimated and sent through a spherical lens to the surface. The Gaussian spot size at the surface was 644 µm and the heating laser was positioned directly in between the SAW excitation laser line and the detection point using translation stages. The samples were polished Ti-6Al-4V disks with a diameter of 25 mm and a height of 13 mm. A LabVIEW code was used to control the heating laser power and to acquire laser ultrasonic signals during the heating process at a data acquisition rate of 15 Hz (corresponding to the excitation laser repetition rate). At a given heating laser power, data acquisition commenced at a heating time t h = -3 s, and single shot laser ultrasonic signals were acquired continuously throughout the experiment. At t h = 0 s the heating laser was switched on and the sample was illuminated with a constant power for 10 s, after which the heating laser was turned off. To monitor sample cooling, data acquisition continued for 5 s after the heating laser was turned off. The sample was then allowed to cool to room temperature and translated to a new position. The experimental procedure was repeated a total of 10 times at each heating power and the laser ultrasonic signals collected at each time, with respect to heating laser turn on at t h = 0, were averaged to improve the signal-to-noise ratio. After each experiment, the sample surface was inspected using an optical microscope for signs of surface melting and discoloration. Results And Discussion Figure 5 (a) shows the ultrasonic signals detected at several heating times, with the prominent feature (negative dip) corresponding to the arrival of the surface acoustic wave. The top waveform shows the response in the absence of surface heating ( t h < 0) and the negative peak in the surface wave arrival is seen at about 1.33 µs. The next four curves show the displacement responses at various times after the heating laser is turned on, and the last two curves show the response at 1 and 3 s after the heating laser is turned off. The shape of the surface acoustic wave remains relatively constant, but the arrival is delayed as heating proceeds. This lack of dispersion is expected as the heating times are long and the thermal field is thus relatively constant throughout the surface wave penetration depth. Figure 5 (b) shows similar results for a CW laser heating power of 46 W. The basic features of the waveforms at 30 W and 46 W are similar but the delay in the SAW is more pronounced with the larger heating power due to the increase in temperature in the laser-heated region. Figures 5 (c) and 5(d) show the evolution of the displacement field throughout these experiments for CW laser heating powers of 30 W and 46 W, respectively. The abscissa gives the experiment time, where t h = 0 represents the time at which the heating laser is turned on and the heating laser is turned off at t h = 10 s, while the ordinate gives the time after the excitation laser pulse. The color bar represents the out of plane displacement of the surface. In these images, the SAW arrival has the negative-most amplitude shown in red. In both images, the surface wave arrival shows a marked delay associated with the onset of surface heating. This is followed by a more gradual change between t h = 2 s and t h = 10 s as heat diffuses through the sample and the sample temperature begins to approach the steady state. Finally, a rapid decrease in the arrival time is observed when the heating laser is turned off. Interestingly, the 46 W heating power not only causes more of a surface wave delay but the shape of the arrival over heating time is quite different than the 30 W heating laser case, and there is not a simple linear scaling between them. Furthermore, optical microscopy revealed no visible markings left on the surface from the 30 W heating power experiments, while there were clear discolorations and surface texture changes, indicative of surface melting, resulting from the 46 W heating power experiments. The frequency content of the surface acoustic waves generated in these experiments extends to 32 MHz. At this frequency and with the slow heating time of 10 s, the SAW is minimally dispersive with the increasing temperatures as seen in Fig. 5 , while the delay of the SAW is sensitive to the heating power or surface state. The arrival time of the negative peak of the surface wave was determined as a function of heating time for different laser powers. The surface wave delay was then determined by subtracting the room temperature arrival time. Figure 6 (a) shows the real-time transient surface wave delays for heating powers of 30, 34, and 40 W. Note that at these lower heating powers, all the curves show a similar shape. If these curves are divided by the heating laser power to determine the normalized surface wave delay (in units of ns/W), all the curves collapse to a single curve as shown in Fig. 6 (b). This indicates that in this regime, the surface wave delay is a simple linear function of the heating power. Optical microscopy confirmed that there were no visible changes to the surface subsequent to heating in this power range. At higher heating powers the response is quite different, however: experiments performed at 46, 48, and 56 W resulted in visible markings and surface texture changes. The delay curves for these heating powers are plotted in Fig. 6 (c) together with the results from the 30 W experiment during which no surface changes were observed. The SAW delay is significantly more pronounced and the normalized surface wave delay plots in Fig. 6 (d) show a distinct change in shape. While all the curves follow the 30 W heating curve for early heating times, deviations begin at later times, with the deviations occurring earlier for higher power. It is hypothesized that the nonlinearity in the curve is associated with phase change and the presence of surface melt. Incremental heating experiments were performed in which the laser power was fixed at 48 W but the illumination time was varied between 0.5 s and 10.0 s in 0.5 s intervals. The sample surface was optically observed after each interval, and the sample was translated between measurements such that a new region was illuminated. Figure 7 (a) shows a subset of the surface wave delay data, again demonstrating that surface waves can be very effective for the characterization of the transient thermal field. Figure 7 (b) shows a zoom-in of the first 5.0 s of normalized surface wave delay for heating laser powers of 30 W and 48 W. The curves are nearly identical up until about 2.0 s of illumination, after which the 48 W curve shows a significantly higher delay. Optical micrographs of the surface are shown in Fig. 7 (c). There is no detectable surface discoloration before 2.0 s of heating. However, there is a clear indication of a melt region starting at 2.0 s and this region continues to grow at longer heating times. Note that in the modeling results, a one-dimensional illumination model was used to elucidate the effects of surface heating and melting on surface wave propagation. Thus a quantitative comparison with experimental results, in which the waves propagate through a region heated with a Gaussian laser source, is not possible. Nevertheless, there is qualitative agreement between the two with a rapid delay in the surface wave velocity at the onset of illumination and further decrease in surface wave velocity associated with melting. The surface acoustic wave delay curves, such as those shown in Figs. 6 and 7 , may prove useful in monitoring laser-induced thermal processes, particularly in more complex cases involving phase transformation. Changes in these curves indicate variations in the heating laser parameters or laser-material interaction. These experimental results are limited to relatively slow heating, where the temperature is uniform over the surface wave penetration depth. A higher repetition rate pulsed laser can be used to probe more rapid thermal processes and potentially obtain quantitative information about the heated zone, including temperature distribution and melt pool depth. Conclusion Laser-based ultrasonics is a non-contact technique that can be used to monitor transient laser-induced heating and melting processes. Numerical simulations were performed to show that, for a spatially uniform heating beam, laser-induced surface acoustic waves are strongly influenced by surface heating conditions, are dispersive in the case of rapid heating, and that an abrupt velocity reduction happens upon the onset of surface melting. The complementary experimental results monitor the transient change of surface wave travel time associated with high power laser surface heating. Qualitative agreement between theory and experiment is observed: both show a rapid delay in the surface wave velocity at the onset of illumination and further decrease in surface wave velocity associated with melting. This technique may ultimately find application in the mapping of transient laser-induced thermal fields and melt zones, providing critical information for real-time process control in advanced manufacturing systems including those relying on laser powder bed fusion. Declarations Acknowledgements This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344 and was supported by the LLNL-LDRD Program under Project No. 19-ERD-008. The document release number is LLNL-JRNL-831155-DRAFT. R.E.M. thanks the National GEM Consortium for supporting the preparation of this manuscript. Author contributions T.W.M., D.M.S., and R.EM. suggested the experiments. R.EM. constructed the analytical models, performed the experiments, analyzed the data, and prepared figures. J.W.T., D.M.S., and T.W.M. acquired funding for this work. J.W.T. and K.J.H. provided resources for relevant work. R.E.M and T.W.M. wrote the manuscript. All authors reviewed the manuscript. Data availability The datasets generated during and/or analyzed during the current study are available from the corresponding author on reasonable request. Competing interests The authors declare no competing interests. References Von Allmen, M. & Blatter, A. Laser-beam interactions with materials: physical principles and applications . (Springer, 1995). Ready, J. F. Effects of high-power laser radiation . (Academic Press, 1971). Ye, J. et al. Energy Coupling Mechanisms and Scaling Behavior Associated with Laser Powder Bed Fusion Additive Manufacturing. Adv. Eng. Mater. 21 , 1900185 (2019). Khairallah, S. A. et al. Controlling interdependent meso-nanosecond dynamics and defect generation in metal 3D printing. Science 368 , 660–665 (2020). Mercelis, P. & Kruth, J. Residual stresses in selective laser sintering and selective laser melting. Rapid Prototyping Journal 12 , 254–265 (2006). Thijs, L., Verhaeghe, F., Craeghs, T., Humbeeck, J. V. & Kruth, J.-P. A study of the microstructural evolution during selective laser melting of Ti–6Al–4V. Acta Materialia 58 , 3303–3312 (2010). Matthews, M. J. et al. Denudation of metal powder layers in laser powder bed fusion processes. Acta Materialia 114 , 33–42 (2016). Khairallah, S. A., Anderson, A. T., Rubenchik, A. & King, W. E. Laser powder-bed fusion additive manufacturing: Physics of complex melt flow and formation mechanisms of pores, spatter, and denudation zones. Acta Materialia 108 , 36–45 (2016). Grasso, M. & Colosimo, B. M. Process defects and in situ monitoring methods in metal powder bed fusion: a review. Meas. Sci. Technol. 28 , 044005 (2017). Kiss, A. M. et al. Laser‐Induced Keyhole Defect Dynamics during Metal Additive Manufacturing. Adv. Eng. Mater. 21 , 1900455 (2019). King, W. E. et al. Laser powder bed fusion additive manufacturing of metals; physics, computational, and materials challenges. Applied Physics Reviews 2 , 041304 (2015). Everton, S. K., Hirsch, M., Stravroulakis, P., Leach, R. K. & Clare, A. T. Review of in-situ process monitoring and in-situ metrology for metal additive manufacturing. Materials & Design 95 , 431–445 (2016). Rieder, H., Dillhöfer, A., Spies, M., Bamberg, J. & Hess, T. Ultrasonic online monitoring of additive manufacturing processes based on selective laser melting. in 184–191 (2015). doi: 10.1063/1.4914609 . Kouprianoff, D., Yadroitsava, I., du Plessis, A., Luwes, N. & Yadroitsev, I. Monitoring of Laser Powder Bed Fusion by Acoustic Emission: Investigation of Single Tracks and Layers. Front. Mech. Eng. 7 , 678076 (2021). Shevchik, S. A., Masinelli, G., Kenel, C., Leinenbach, C. & Wasmer, K. Deep Learning for In Situ and Real-Time Quality Monitoring in Additive Manufacturing Using Acoustic Emission. IEEE Trans. Ind. Inf. 15 , 5194–5203 (2019). Dalong Liu & Ebbini, E. S. Real-Time 2-D Temperature Imaging Using Ultrasound. IEEE Trans. Biomed. Eng. 57 , 12–16 (2010). Lai, P. et al. Real-Time Monitoring of High-Intensity Focused Ultrasound Lesion Formation Using Acousto-Optic Sensing. Ultrasound in Medicine & Biology 37 , 239–252 (2011). Gillespie, J. et al. In situ characterization of laser-generated melt pools using synchronized ultrasound and high-speed X-ray imaging. The Journal of the Acoustical Society of America 150 , 2409–2420 (2021). Yang, T. et al. In-situ monitoring and ex-situ elasticity mapping of laser induced metal melting pool using ultrasound: Numerical and experimental approaches. Journal of Manufacturing Processes 71 , 178–186 (2021). Klein, M. & Sears, J. Laser ultrasonic inspection of laser cladded 316LSS and TI-6-4. in International Congress on Applications of Lasers & Electro-Optics 1006 (Laser Institute of America, 2004). doi: 10.2351/1.5060183 . Manzo, A. J. & Helvajian, H. Pulsed laser ultrasonic excitation and heterodyne detection for in situ process control in laser 3D manufacturing. Journal of Laser Applications 29 , 012012 (2017). Manzo, A. J. & Helvajian, H. Utility of optical heterodyne displacement sensing and laser ultrasonics as in situ process control diagnostic for additive manufacturing. Opt. Eng. 57 , 1 (2018). Davis, G. et al. Laser ultrasonic inspection of additive manufactured components. Int J Adv Manuf Technol 102 , 2571–2579 (2019). Smith, R. J. et al. Spatially resolved acoustic spectroscopy for selective laser melting. Journal of Materials Processing Technology 236 , 93–102 (2016). Yamada, H., Kosugi, A. & Ihara, I. Noncontact Monitoring of Surface Temperature Distribution by Laser Ultrasound Scanning. Jpn. J. Appl. Phys. 50 , 07HC06 (2011). Shi, Y.-A. et al. A Method for Reconstructing Internal Temperature Distribution in Solid Structure. J Nondestruct Eval 39 , 48 (2020). Queheillalt, D. T. & Wadley, H. N. G. Laser ultrasonic sensing of the melting and solidification of cadmium telluride. Journal of Crystal Growth 225 , 34–44 (2001). Kil-Mo Koo, Dong-Gyu Jeong, Jong-Ho Choi, & Duck-Young Ko. A new measurement system of very high temperature in atomic pile using ultrasonic delay time. in Proceedings of IEEE Region 10 International Conference on Electrical and Electronic Technology. TENCON 2001 (Cat. No.01CH37239) vol. 2 860–863 (IEEE, 2001). Hinterlechner, I. et al. In-situ monitoring of phase transformation in Ti-6Al-6V-2Sn using laser ultrasonics. Nondestructive Testing and Evaluation 33 , 130–138 (2018). Kruger, S. E. & Damm, E. B. Monitoring austenite decomposition by ultrasonic velocity. Materials Science and Engineering: A 425 , 238–243 (2006). Shinbine, A., Garcin, T. & Sinclair, C. In-situ laser ultrasonic measurement of the hcp to bcc transformation in commercially pure titanium. Materials Characterization 117 , 57–64 (2016). Saunders, N., Guo, U. K. Z., Li, X., Miodownik, A. P. & Schillé, J.-Ph. Using JMatPro to model materials properties and behavior. JOM 55 , 60–65 (2003). Rose, J. L. Ultrasonic waves in solid media . (Cambridge University Press, 1999). Ogi, H. et al. Titanium’s high-temperature elastic constants through the hcp–bcc phase transformation. Acta Materialia 52 , 2075–2080 (2004). Singh, R. K. & Narayan, J. A novel method for simulating laser-solid interactions in semiconductors and layered structures. Materials Science and Engineering: B 3 , 217–230 (1989). Welsch, G., Boyer, R. & Collings, E. W. Materials properties handbook: titanium alloys . (ASM International, 1994). Smithells, C. J., Gale, W. F. & Totemeier, T. C. Smithells metals reference book . (Elsevier Butterworth-Heinemann, 2004). Mills, K. C. Recommended values of thermophysical properties for selected commercial alloys . (Woodhead, 2002). Casas, J., Kéita, N. M. & Steinemann, S. G. Sound Velocity in Liquid Titanium, Vanadium and Chromium. Physics and Chemistry of Liquids 14 , 155–158 (1984). Wood, R. F. & Giles, G. E. Macroscopic theory of pulsed-laser annealing. I. Thermal transport and melting. Phys. Rev. B 23 , 2923–2942 (1981). Cheng, A., Murray, T. W. & Achenbach, J. D. Simulation of laser-generated ultrasonic waves in layered plates. The Journal of the Acoustical Society of America 110 , 848–855 (2001). Spicer, J. B., McKie, A. D. W. & Wagner, J. W. Quantitative theory for laser ultrasonic waves in a thin plate. Appl. Phys. Lett. 57 , 1882–1884 (1990). McDonald, F. A. On the precursor in laser‐generated ultrasound waveforms in metals. Appl. Phys. Lett. 56 , 230–232 (1990). Sanderson, T., Ume, C. & Jarzynski, J. Laser generated ultrasound: a thermoelastic analysis of the source. Ultrasonics 35 , 115–124 (1997). Murray, T. W., Krishnaswamy, S. & Achenbach, J. D. Laser generation of ultrasound in films and coatings. Appl. Phys. Lett. 74 , 3561–3563 (1999). Bertoni, H. L. & Tamir, T. Unified theory of Rayleigh-angle phenomena for acoustic beams at liquid-solid interfaces. Appl. Phys. 2 , 157–172 (1973). Biot, M. A. The interaction of Rayleigh and Stoneley waves in the ocean bottom*. Bulletin of the Seismological Society of America 42 , 81–93 (1952). Meegan, G. D., Hamilton, M. F., Il’inskii, Yu. A. & Zabolotskaya, E. A. Nonlinear Stoneley and Scholte waves. The Journal of the Acoustical Society of America 106 , 1712–1723 (1999). Murray, T. W., Balogun, O., Steen, T. L., Basu, S. N. & Sarin, V. K. Inspection of compositionally graded mullite coatings using laser based ultrasonics. International Journal of Refractory Metals and Hard Materials 23 , 322–329 (2005). Ing, R. K. & Monchalin, J. ‐P. Broadband optical detection of ultrasound by two‐wave mixing in a photorefractive crystal. Appl. Phys. Lett. 59 , 3233–3235 (1991). Blouin, A. & Monchalin, J. Detection of ultrasonic motion of a scattering surface by two‐wave mixing in a photorefractive GaAs crystal. Appl. Phys. Lett. 65 , 932–934 (1994). Delaye, P. et al. Detection of ultrasonic motion of a scattering surface by photorefractive InP:Fe under an applied dc field. J. Opt. Soc. Am. B 14 , 1723 (1997). Additional Declarations No competing interests reported. Supplementary Files ScientificReportsAppendixREM.docx Cite Share Download PDF Status: Under Review Version 1 posted Editorial decision: Major revision 25 Apr, 2022 Reviews received at journal 15 Apr, 2022 Reviewers agreed at journal 12 Apr, 2022 Reviewers agreed at journal 04 Apr, 2022 Reviewers invited by journal 22 Mar, 2022 Editor assigned by journal 22 Mar, 2022 Editor invited by journal 17 Feb, 2022 Submission checks completed at journal 17 Feb, 2022 First submitted to journal 10 Feb, 2022 You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-1347814","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":84573925,"identity":"5b4b09d3-03f7-4cfc-9f14-db79e50cdb05","order_by":0,"name":"Rosa E. Morales","email":"","orcid":"","institution":"University of Colorado","correspondingAuthor":false,"submittingAuthor":false,"prefix":"","firstName":"Rosa","middleName":"E.","lastName":"Morales","suffix":""},{"id":84573926,"identity":"a293add0-9bc7-4223-9dd3-bc2bbb27c57d","order_by":1,"name":"Kathryn J. Harke","email":"","orcid":"","institution":"Lawrence Livermore National Laboratory","correspondingAuthor":false,"submittingAuthor":false,"prefix":"","firstName":"Kathryn","middleName":"J.","lastName":"Harke","suffix":""},{"id":84573927,"identity":"77faac4c-9276-4dd8-9fc8-8014c6b6b662","order_by":2,"name":"Joseph W. Tringe","email":"","orcid":"","institution":"Lawrence Livermore National Laboratory","correspondingAuthor":false,"submittingAuthor":false,"prefix":"","firstName":"Joseph","middleName":"W.","lastName":"Tringe","suffix":""},{"id":84573928,"identity":"d7a5f86f-8666-4cd4-a605-ef66759dd388","order_by":3,"name":"David M. Stobbe","email":"","orcid":"","institution":"Lawrence Livermore National Laboratory","correspondingAuthor":false,"submittingAuthor":false,"prefix":"","firstName":"David","middleName":"M.","lastName":"Stobbe","suffix":""},{"id":84573929,"identity":"d3031a2f-b49d-48f7-87c6-f7139e133c5b","order_by":4,"name":"Todd W. Murray","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA6UlEQVRIiWNgGAWjYBAC+wMMBhKMbWA24wMGBgsQwwCImXFqYWMAaTkHZjMDlUoQq+UfhC1BnBb25o03GLcdzjdvP2NW8aNCQs68/fDGBwwV1okNuLTwHCu2AGqxnHMmx+xmzxkJY5kzacUGDGfScWuRyDGTAGoxkGDIMbvN2CaROIMBJNJ2mICWNqAW/jdmxWAtQAbQd8RoATKYwVrAIg14tID8ktiWDtTyrFgS5BcQwyDhWLoxTi2gEPvYZg10WPLGDz8qbORAjAcfaqxlcWkBgwQwyWGALkIQsD8gTt0oGAWjYBSMOAAAJSlQ/JFWR+IAAAAASUVORK5CYII=","orcid":"","institution":"University of Colorado","correspondingAuthor":true,"submittingAuthor":false,"prefix":"","firstName":"Todd","middleName":"W.","lastName":"Murray","suffix":""}],"badges":[],"createdAt":"2022-02-10 20:29:10","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-1347814/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-1347814/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":18436327,"identity":"f591077d-388f-4c34-9752-e5d5c36852d3","added_by":"auto","created_at":"2022-02-21 14:45:28","extension":"jpg","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":135622,"visible":true,"origin":"","legend":"\u003cp\u003eRayleigh wave speed of Ti-6Al-4V as a function of temperature ranging from room temperature to the melting temperature of 1943 K.\u003c/p\u003e","description":"","filename":"FIG1.jpg","url":"https://assets-eu.researchsquare.com/files/rs-1347814/v1/80257260985df23e0092cde3.jpg"},{"id":18436346,"identity":"be9d15bc-c1c8-4c42-af43-34f14ab21e32","added_by":"auto","created_at":"2022-02-21 14:48:28","extension":"jpg","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":727326,"visible":true,"origin":"","legend":"\u003cp\u003e(a) Surface temperature (solid line) and melt size (dotted line) as a function of heating time for a 6 kW/cm\u003csup\u003e2\u003c/sup\u003e laser power density incident on Ti-6Al-4V. (b) Calculated temperature field as a function of heating time and depth for the 6 kW/cm\u003csup\u003e2\u003c/sup\u003e power density. (c) and (d) Results for a higher laser power density of 250 kW/cm\u003csup\u003e2\u003c/sup\u003e.\u003c/p\u003e","description":"","filename":"FIG2.jpg","url":"https://assets-eu.researchsquare.com/files/rs-1347814/v1/5a279db784c873596d8fd23c.jpg"},{"id":18436330,"identity":"ae05dffa-deec-4770-8b62-37d45dd5bc98","added_by":"auto","created_at":"2022-02-21 14:45:28","extension":"jpg","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":813489,"visible":true,"origin":"","legend":"\u003cp\u003e(a) Calculated laser ultrasonic signals with a source to detector distance of 1.0 mm at room temperature and heating times of 150.0, 300.0, 450.0, 508.5, 517.0, and 525.5 ms for a heating laser power density of 6 kW/cm\u003csup\u003e2\u003c/sup\u003e. (b) Temporal evolution of the displacement field for the 6 kW/cm\u003csup\u003e2 \u003c/sup\u003epower density. (c) Calculated laser ultrasonic signals from the 250 kW/cm\u003csup\u003e2\u003c/sup\u003e heating laser power density at room temperature and heating times of 100, 200, 300, 330, 410, and 490 μs. (d) Temporal evolution of the displacement field for the 250 kW/cm\u003csup\u003e2\u003c/sup\u003e power density.\u003c/p\u003e","description":"","filename":"FIG3.jpg","url":"https://assets-eu.researchsquare.com/files/rs-1347814/v1/d4351c4c1f7161a4e5558958.jpg"},{"id":18436189,"identity":"e994b20f-c13f-4ac2-9d3d-bf0733cfcbd0","added_by":"auto","created_at":"2022-02-21 14:42:28","extension":"jpg","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":405641,"visible":true,"origin":"","legend":"\u003cp\u003eSchematic of the experimental setup depicting the three laser beams on the sample surface. The following abbreviations are used: PBS – polarizing beam splitter, λ/2 – half-wave plate, λ/4 – quarter-wave plate, L1, L2, L3 – focusing lenses.\u003c/p\u003e","description":"","filename":"FIG4.jpg","url":"https://assets-eu.researchsquare.com/files/rs-1347814/v1/3ccc30d03c67797c3c15d06e.jpg"},{"id":18436188,"identity":"11812ba6-b815-4b15-bbed-99fb1aa36dfb","added_by":"auto","created_at":"2022-02-21 14:42:28","extension":"jpg","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":961013,"visible":true,"origin":"","legend":"\u003cp\u003eLaser ultrasonic signals in Ti-6Al-4V samples with a source to detector distance of 4.0 mm at room temperature, heating times of 1, 3, 5, and 10 s, and cooling times of 1 and 3 s for CW laser heating powers of (a) 30 W and (b) 46 W. Temporal evolution of the displacement fields, where experiment time of 0 s indicates the time at which the heating laser is turned on, for CW laser heating powers of (c) 30 W and (d) 46 W.\u003c/p\u003e","description":"","filename":"FIG5.jpg","url":"https://assets-eu.researchsquare.com/files/rs-1347814/v1/ca0cc07869bd2dfeac8e00ed.jpg"},{"id":18436347,"identity":"fa7311ba-a999-47cb-8c30-985d945d27c7","added_by":"auto","created_at":"2022-02-21 14:48:28","extension":"jpg","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":738439,"visible":true,"origin":"","legend":"\u003cp\u003e(a) Real-time transient surface wave delays and (b) normalized surface wave delays, in units of ns/W, for heating powers of 30, 34, and 40 W. (c) Real-time transient surface wave delays and (d) normalized surface wave delays for heating laser powers of 30, 46, 48, and 56 W.\u003c/p\u003e","description":"","filename":"FIG6.jpg","url":"https://assets-eu.researchsquare.com/files/rs-1347814/v1/c9bdd054d32c81880c5f1dae.jpg"},{"id":18436182,"identity":"b48c19f8-fada-4261-ac47-bc4a8c9dcd37","added_by":"auto","created_at":"2022-02-21 14:42:28","extension":"jpg","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":1233360,"visible":true,"origin":"","legend":"\u003cp\u003e(a) Subset of surface wave delay data from the incremental heating experiment at a fixed laser power of 48 W. (b) Zoom-in of the normalized surface wave delay for heating laser powers of 30 W and 48 W. (c) Optical micrographs of the sample surface after the indicated times of heating.\u0026nbsp;\u003c/p\u003e","description":"","filename":"FIG7.jpg","url":"https://assets-eu.researchsquare.com/files/rs-1347814/v1/3965272e6a6d02a0b2295829.jpg"},{"id":18436348,"identity":"9495cb00-5e92-4b56-96c6-95adef4cda2b","added_by":"auto","created_at":"2022-02-21 14:48:31","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":944665,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-1347814/v1/d6e43481-f939-4bd3-8f68-d13b81e9fa51.pdf"},{"id":18436185,"identity":"80aa0f4e-dd3a-49c4-abe3-42d8d060979e","added_by":"auto","created_at":"2022-02-21 14:42:28","extension":"docx","order_by":9,"title":"","display":"","copyAsset":false,"role":"supplement","size":25502,"visible":true,"origin":"","legend":"","description":"","filename":"ScientificReportsAppendixREM.docx","url":"https://assets-eu.researchsquare.com/files/rs-1347814/v1/f785cd653396fa5c1b061e47.docx"}],"financialInterests":"No competing interests reported.","formattedTitle":"Real-Time Laser Ultrasonic Monitoring of Laser-Induced Thermal Processes","fulltext":[{"header":"Introduction","content":"\u003cp\u003eLasers have seen widespread use in materials processing and manufacturing applications including laser cutting, welding, ablation, drilling, surface texturing, and advanced manufacturing\u003csup\u003e\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u003c/sup\u003e. In these applications, laser energy is absorbed by a material leading to local heating, melting, and vaporization, and \u003cem\u003ein situ\u003c/em\u003e control of these laser-induced processes is critical to ensure the integrity of the final product\u003csup\u003e\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u003c/sup\u003e. For example, in laser powder bed fusion an object is built in consecutive layers by laser melting of powder mechanically distributed over the build surface\u003csup\u003e\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e,\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e\u003c/sup\u003e. Intra- and inter-layer integrity is dependent upon the rapid heating, melting, and solidification processes during which defects and material discontinuities are likely to form\u003csup\u003e\u003cspan additionalcitationids=\"CR4 CR5\" citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e\u003c/sup\u003e. If the heating laser power is too high for a given scan speed, it can lead to evaporation of the material from a molten pool, a subsequent recoil force, and collapse of the melt pool, resulting in porosity in the component. Alternatively, a low laser heating power can lead to an incomplete melting and solidification process and lack of fusion defects\u003csup\u003e\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e,\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e\u003c/sup\u003e. The optimal laser parameters, defined as those required to produce a part with the lowest number of processing defects, are difficult to determine \u003cem\u003ea priori\u003c/em\u003e due to the complexity of the process and additional variables including powder quality and machine to machine variations in laser characteristics\u003csup\u003e\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eThe success of laser-based manufacturing techniques, such as powder bed fusion, hinges on the ability of the operator to set and control process variables such as laser power and speed\u003csup\u003e\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e,\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e\u003c/sup\u003e. Nondestructive evaluation can aid in this process and provide \u003cem\u003ein situ\u003c/em\u003e feedback on the build quality such that process variables can be adjusted in real time\u003csup\u003e\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e\u003c/sup\u003e. Several nondestructive evaluation methods such as thermal imaging, optical imaging, and conventional ultrasonics have been employed to monitor laser powder bed fusion builds\u003csup\u003e\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e,\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e\u003c/sup\u003e. These techniques offer remote access to the high temperature laser-material interaction site. Thermal imaging can provide information regarding the surface temperature distribution, and optical imaging can be used to ascertain surface morphology changes\u003csup\u003e\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e\u003c/sup\u003e. Conventional acoustic emission and ultrasound can potentially provide additional data regarding the build process\u003csup\u003e\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e\u003c/sup\u003e. In the case of acoustic emission, the sound generated during the laser heating process is detected using a microphone or transducer and analyzed to infer information about the process\u003csup\u003e\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e\u003c/sup\u003e. Using signal processing techniques, including those employing machine learning, it is possible to categorize the laser-material interaction into different regimes\u003csup\u003e\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eIn addition, the physical, mechanical, optical, and thermal properties of materials are functions of temperature. It follows that the velocity of ultrasonic waves is also temperature-dependent with an increase in temperature generally leading to a softening of the material and lower longitudinal, shear, and surface acoustic wave velocities. Ultrasound thermography is a well-known technique used to determine the temperature of a uniformly heated sample or to map out temperature distributions below the surface\u003csup\u003e\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e\u003c/sup\u003e. Ultrasound thermography has attracted much attention in the medical ultrasonics community to continuously monitor and provide feedback to thermal treatment processes in biological tissue such as high intensity focused ultrasound therapy\u003csup\u003e\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e,\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e\u003c/sup\u003e. Some of the advantages of conventional ultrasonic techniques for process monitoring are that they are not limited to surface temperature determination and that the transducers can be placed remote from the process zone. For additive manufacturing, they do require physical access to the build surface, but the contact location can be somewhat removed from the high temperature environment. The dependence of bulk wave propagation on temperature, mechanical properties, and phase state of the material has, for example, been used to infer the temperature in the processing zone, and to predict the melt pool size based on monitoring bulk waves reflected and scattered from the melt pool\u003csup\u003e\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e,\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eLaser-based ultrasonic techniques are well suited for the real-time monitoring and nondestructive evaluation of laser-induced thermal processes. The laser detection probe beam and laser source exciting the ultrasonic waves can be separated from the high temperature manufacturing process environment using optically transparent windows or other means. Laser ultrasonic techniques have been used to evaluate advanced manufacturing builds by using surface acoustic waves and bulk waves to detect surface and near subsurface defects \u003cem\u003eex situ\u003c/em\u003e\u003csup\u003e\u003cspan additionalcitationids=\"CR21 CR22\" citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e\u003c/sup\u003e. Surface acoustic waves and bulk waves have also been used to evaluate material microstructure and grain size\u003csup\u003e\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e\u003c/sup\u003e, to infer the surface temperature in laser-induced thermal processes\u003csup\u003e\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e,\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e\u003c/sup\u003e, to predict internal temperature distributions based on waves propagating over multiple paths\u003csup\u003e\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e\u003c/sup\u003e, and to observe melting and solidification during crystal growth\u003csup\u003e\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e\u003c/sup\u003e. Finally, laser ultrasonic methods have been used for high temperature measurements of materials properties\u003csup\u003e\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e\u003c/sup\u003e and for phase transformation studies in metals\u003csup\u003e\u003cspan additionalcitationids=\"CR30\" citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eIn this work, laser-based ultrasound is used to monitor laser-induced heating and melting processes. Numerical simulations of a spatially-uniform heating beam show that laser-induced surface acoustic waves are strongly influenced by surface heating conditions. They are dispersive in the case of rapid heating where the thermally-induced mechanical property change is on the same spatial scale as the wavelength of the surface acoustic waves, and show an abrupt velocity reduction upon the onset of surface melting. Furthermore, using a laser line source for surface acoustic wave excitation and a point interferometric detector, experimental results demonstrate the transient change of surface wave travel time associated with high power laser surface heating with a Gaussian beam placed between the source and receiver positions. In agreement with the numerical simulations, a deviation in the response is observed when the heating laser power is sufficient to cause local surface melting. The proposed technique may find effective application in the mapping of transient laser-induced thermal fields and melt zones, providing critical information for real-time process control.\u003c/p\u003e"},{"header":"Background And Theory","content":"\u003cp\u003eThe thermal and elastic properties of materials are dependent on temperature, with an increase in temperature generally resulting in a decrease in both material density and stiffness. A uniformly heated sample of a titanium alloy (Ti-6Al-4V) that is assumed to be homogeneous and isotropic is considered first. The temperature-dependent density, elastic modulus, and Poisson\u0026rsquo;s ratio are obtained from the software \u003cem\u003eJMatPro\u003c/em\u003e\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e32\u003c/span\u003e\u003c/sup\u003e and are used to determine the longitudinal, \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({c}_{L}\\)\u003c/span\u003e\u003c/span\u003e, and shear, \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({c}_{T}\\)\u003c/span\u003e\u003c/span\u003e, wave speeds as a function of temperature. These values are, in turn, used to calculate the Rayleigh wave speed using the characteristic\u0026nbsp;equation\u003csup\u003e33\u003c/sup\u003e. The result is shown in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003e where the Rayleigh wave speed is seen to decrease from 3000 m/s at room temperature (293 K) to 1880 m/s at the melting temperature of 1943 K. The decrease is relatively monotonic outside of a small region between 1100 and 1275 K, the temperature range at which Ti-6Al-4V undergoes an hcp (\u0026alpha;) \u0026rarr; bcc (\u0026beta;) phase transformation\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e34\u003c/span\u003e\u003c/sup\u003e. In the case of uniform heating, the thermal field and elastic properties are not depth-dependent and the Rayleigh waves propagate without dispersion.\u003c/p\u003e\n\u003cp\u003eRayleigh wave propagation becomes more complex in the case of transient heating, such as that produced by a high power laser source, as the thermal field and temperature-dependent elastic properties are functions of both time and space. Furthermore, the thermal properties are also temperature-dependent, and, at sufficiently high heating powers, the material will undergo a phase transformation resulting in surface melt. Here, the elastic displacement response generated by a nanosecond pulsed laser source incident upon a surface that is being heated by a spatially uniform continuous wave (CW) laser with a step-function time dependence is calculated. First, the temperature field produced by a CW laser is calculated using the implicit finite difference method presented by Singh and Narayan\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e35\u003c/span\u003e\u003c/sup\u003e. Temperature-dependent thermal properties and density are included for both the solid and liquid Ti-6Al-4V phases\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e36\u003c/span\u003e\u0026ndash;\u003cspan class=\"CitationRef\"\u003e39\u003c/span\u003e\u003c/sup\u003e. These properties are summarized in Appendix A. The model is used to determine the temperature as a function of time (\u003cem\u003et\u003c/em\u003e) at each depth (\u003cem\u003ed\u003c/em\u003e) below the surface. Figure\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003e(a) shows the surface temperature (at \u003cem\u003ed\u0026thinsp;=\u0026thinsp;0\u003c/em\u003e) as a function of time where the heating laser is turned on at \u003cem\u003et\u0026thinsp;=\u0026thinsp;0\u003c/em\u003e and a heating laser power density of 6 kW/cm\u003csup\u003e2\u003c/sup\u003e. The surface temperature rises until it reaches the melting temperature of 1943 K at a time of 465 ms, where it briefly remains until the net heat absorbed exceeds the latent heat of the phase change\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e35\u003c/span\u003e,\u003cspan class=\"CitationRef\"\u003e40\u003c/span\u003e\u003c/sup\u003e. The melt front then begins to propagate into the material and, as shown on the right axis of Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003e(a), proceeds rapidly to a depth of over 35 \u0026micro;m. Figure\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003e(b) shows the full extent of the calculated thermal data, with the color bar indicating the temperature rise at each depth and heating time.\u003c/p\u003e\n\u003cp\u003eSurface acoustic waves are confined to propagate in the near surface region with a penetration depth on the order of the wavelength. For example, for a frequency of 30 MHz, surface waves in Ti-6Al-4V will be sensitive to mechanical property changes that occur over a depth of approximately 100 \u0026micro;m. In Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003e(b), the temperature rise is somewhat uniform over the near surface region, and thus it is expected that the mechanical property changes will also be relatively constant over the penetration depth, resulting in a surface acoustic wave delay that is frequency independent. The surface temperature for a significantly higher heating power density of 250 kW/cm\u003csup\u003e2\u003c/sup\u003e is shown in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003e(c). Here, surface melting occurs at about 300 \u0026micro;s and there are marked thermal gradients in the near surface region within the 500 \u0026micro;s time window as shown in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003e(d). These thermal gradients can cause dispersion of surface waves since the higher frequency waves, with a shorter wavelength, will be more influenced by the near surface region while lower frequency waves will penetrate further into the cooler substrate.\u003c/p\u003e\n\u003cp\u003eThis model allows for the calculation of temperature as a function of depth and melt front position at any time after the heating laser is turned on. Next, the pulsed laser excitation and interferometric detection of the CW laser-heated surface at a given time is modeled. The material near the surface is discretized into 400 layers, with a layer thickness of 0.6 \u0026micro;m. The elastic properties of each layer are calculated from the mean temperature of the layer, and the elastic wave propagation problem is then reduced to an analogous problem of wave propagation in a homogeneous, isotropic layered media\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e41\u003c/span\u003e\u003c/sup\u003e. When surface melting occurs, the thickness of the surface layer is set as the thickness of the melt pool and the density\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e38\u003c/span\u003e\u003c/sup\u003e (3920 kg/m\u003csup\u003e3\u003c/sup\u003e) and longitudinal wave velocity\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e39\u003c/span\u003e\u003c/sup\u003e (4407 m/s) of liquid Ti-6Al-4V are used. Laser generation of ultrasound in plates\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e42\u003c/span\u003e\u0026ndash;\u003cspan class=\"CitationRef\"\u003e44\u003c/span\u003e\u003c/sup\u003e and in multi-layer plates on a semi-infinite substrate\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e45\u003c/span\u003e\u003c/sup\u003e have been previously addressed by others. The approach presented by Cheng \u003cem\u003eet al\u003c/em\u003e. is followed in which the excitation laser source is represented as an equivalent elastic boundary source (Gaussian in space with a 10 ns pulse width), and the transfer matrix technique is used to enforce the continuity of stress and displacement across all homogeneous and isotropic layer boundaries\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e41\u003c/span\u003e\u003c/sup\u003e. The problem is solved in cylindrical coordinates using the integral transform technique where a Hankel transform of the elastic wave equation is taken with respect to the radial coordinate (\u003cem\u003er\u003c/em\u003e) and a Laplace transform is taken with respect to time. The normal surface displacement as a function of time at a given \u003cem\u003er\u003c/em\u003e is found through numerical inversion of the Hankel-Laplace transforms.\u003c/p\u003e\n\u003cp\u003eThe excitation laser spot size was set to 100 \u0026micro;m full width at half maximum (FWHM) with a detection location at \u003cem\u003er\u003c/em\u003e\u0026thinsp;=\u0026thinsp;1.0 mm. The normal surface displacement as a function of time is given in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003e(a) for a heating power of 6 kW/cm\u003csup\u003e2\u003c/sup\u003e. The top curve shows the room temperature response in the absence of CW laser surface heating. A small wave amplitude arrival corresponding to the surface skimming longitudinal wave is seen at 0.16 \u0026micro;s followed by the larger surface acoustic wave (SAW) amplitude arrival at about 0.30 \u0026micro;s. The other curves show the displacement response at various times after the CW heating laser is turned on. For the signals between \u003cem\u003et\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0 and \u003cem\u003et\u003c/em\u003e\u0026thinsp;=\u0026thinsp;450 ms, the shape of the surface acoustic wave remains relatively uniform, but the arrival is delayed as heating proceeds. During the last three time steps: \u003cem\u003et\u003c/em\u003e\u0026thinsp;=\u0026thinsp;508.5, 517.0, and 525.6 ms, melting has occurred with melt depths of 6.0, 16.8 and 27.0 \u0026micro;m, respectively. More prominent dispersion is seen when the surface waves traverse the molten layer; the higher frequency, short wavelength, components are delayed due to the strong interaction with the melt layer. Note that in this case, the longitudinal wave velocity in the molten liquid is higher than the shear wave velocity in the substrate so the surface waves are not leaky\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e46\u003c/span\u003e\u003c/sup\u003e. In general, the velocity of waves propagating on a liquid-covered half space transition from the Rayleigh wave velocity at zero thickness to the Scholte wave velocity when the thickness of the liquid is large with respect to the wavelength\u003csup\u003e47\u0026thinsp;\u0026minus;\u0026thinsp;48\u003c/sup\u003e.\u003c/p\u003e\n\u003cp\u003eFigure\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003e(b) shows the evolution of the displacement field calculated throughout the heating time. Here the abscissa gives the time after the heating laser is turned on while the ordinate gives the time after the excitation laser pulse. The color bar represents the normal displacement of the surface. In this image, the SAW arrival has the largest negative amplitude and is shown in red. The initial pronounced change in the SAW arrival time is associated with the rapid rise in the near surface temperature as seen in Figs.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003e(a) and 2(b). The SAW arrival time is then relatively constant between 75 and 100 ms during which the \u0026alpha; \u0026rarr; \u0026beta; phase transformation in Ti-6Al-4V occurs. After this transition region, the arrival of the SAW continues to be delayed with heating time in a monotonic fashion until approximately 508 ms when a sharp break in the curve associated with surface melting is observed. The SAW signal sensitivity to the presence of melt makes it an attractive option for sensing melt depth.\u003c/p\u003e\n\u003cp\u003eFigure\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003e(c) shows the normal displacement of the surface for a higher heating power density, 250 kW/cm\u003csup\u003e2\u003c/sup\u003e. The excitation source characteristics are the same as those given above. In this case, however, the heating takes place much more rapidly and surface melting starts at about 305 \u0026micro;s. Such rapid heating leads to strong near-surface thermal gradients (see Figs.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003e(c) and 2(d)) which, in turn, lead to sharp changes in the mechanical properties within the wavelength range of the broadband surface acoustic wave. At heating times between \u003cem\u003et\u0026thinsp;=\u003c/em\u003e\u0026thinsp;0 and \u003cem\u003et\u003c/em\u003e\u0026thinsp;=\u0026thinsp;300 \u0026micro;s, a significant degree of surface acoustic wave dispersion is evident, with the higher frequency components that probe the near-surface temperature delayed with respect to the lower frequencies that penetrate further into the cooler bulk of the material. This effect is more pronounced at later times (\u003cem\u003et\u0026thinsp;\u0026gt;\u003c/em\u003e\u0026thinsp;300 \u0026micro;s) where the higher frequency SAW components are also delayed by the presence of surface melt. Figure\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003e(d) shows the temporal evolution of the displacement field with surface heating. While the dispersion is certainly more pronounced than in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003e(b), the onset of surface melting is not as evident. Note that the dispersion of the SAWs in a multilayer system can be used to back out the depth-dependent mechanical properties using a model-based inversion approach\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e49\u003c/span\u003e\u003c/sup\u003e. For the heating case, depth-dependent mechanical properties could ultimately be related to the subsurface temperature profile.\u003c/p\u003e"},{"header":"Experimental Setup","content":"\u003cp\u003eA laser ultrasonic system was used to study surface acoustic wave propagation through a laser-heated region of Ti-6Al-4V samples. A schematic of this experimental configuration is shown in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003e. A pulsed Nd:YAG laser operating at the fundamental frequency (\u0026lambda;\u0026thinsp;=\u0026thinsp;1064 nm) and a repetition rate of 15 Hz is used to generate broad band surface acoustic waves. The generation laser is collimated and focused to a line on the sample surface using a cylindrical lens. At the sample surface, the line source was approximately 15 mm in length and had a Gaussian FWHM of 60 \u0026micro;m. The laser energy at the sample was 1.6 mJ, sufficiently low that generation remained in the thermoelastic regime.\u003c/p\u003e\n\u003cp\u003eThe displacement normal to the surface associated with the laser-generated acoustic wave was detected using a photorefractive crystal (PRC) based interferometer using a Bismuth Silicon Oxide (BSO) PRC\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e50\u003c/span\u003e\u0026ndash;\u003cspan class=\"CitationRef\"\u003e52\u003c/span\u003e\u003c/sup\u003e. The detection laser was a 300 mW frequency doubled Nd:YAG laser operating at 532 nm. The laser output was directed to a beamsplitter where it was divided into reference and signal beams. The reference beam was sent directly to the PRC, while the signal beam was focused onto the specimen surface and, upon reflection, was sent to the PRC where it interferes with the reference beam and creates an index grating in the crystal. A portion of the reference beam diffracts from the grating in the two-wave mixing process and interferes with the signal beam at the photodetector. Polarization optics after the PRC were used to ensure that the diffracted reference beam and transmitted signal beam were in quadrature, optimizing the detection sensitivity. The output of the photodetector was sent to a digital oscilloscope with a 200 MHz bandwidth limit and subsequently transferred to a computer and filtered using a 40 MHz second order low pass Butterworth filter. The distance between the SAW excitation and detection was set to 4.0 mm.\u003c/p\u003e\n\u003cp\u003eA fiber-coupled 60 W continuous wave Nd:YAG laser operating at a wavelength of 1064 nm was used to heat the sample surface. The laser output was collimated and sent through a spherical lens to the surface. The Gaussian spot size at the surface was 644 \u0026micro;m and the heating laser was positioned directly in between the SAW excitation laser line and the detection point using translation stages. The samples were polished Ti-6Al-4V disks with a diameter of 25 mm and a height of 13 mm. A LabVIEW code was used to control the heating laser power and to acquire laser ultrasonic signals during the heating process at a data acquisition rate of 15 Hz (corresponding to the excitation laser repetition rate). At a given heating laser power, data acquisition commenced at a heating time \u003cem\u003et\u003c/em\u003e\u003csub\u003e\u003cem\u003eh\u003c/em\u003e\u003c/sub\u003e \u003cem\u003e=\u003c/em\u003e -3 s, and single shot laser ultrasonic signals were acquired continuously throughout the experiment. At \u003cem\u003et\u003c/em\u003e\u003csub\u003e\u003cem\u003eh\u003c/em\u003e\u003c/sub\u003e \u003cem\u003e=\u003c/em\u003e 0 s the heating laser was switched on and the sample was illuminated with a constant power for 10 s, after which the heating laser was turned off. To monitor sample cooling, data acquisition continued for 5 s after the heating laser was turned off. The sample was then allowed to cool to room temperature and translated to a new position. The experimental procedure was repeated a total of 10 times at each heating power and the laser ultrasonic signals collected at each time, with respect to heating laser turn on at \u003cem\u003et\u003c/em\u003e\u003csub\u003e\u003cem\u003eh\u003c/em\u003e\u003c/sub\u003e \u003cem\u003e=\u003c/em\u003e 0, were averaged to improve the signal-to-noise ratio. After each experiment, the sample surface was inspected using an optical microscope for signs of surface melting and discoloration.\u003c/p\u003e"},{"header":"Results And Discussion","content":"\u003cp\u003eFigure\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e5\u003c/span\u003e(a) shows the ultrasonic signals detected at several heating times, with the prominent feature (negative dip) corresponding to the arrival of the surface acoustic wave. The top waveform shows the response in the absence of surface heating (\u003cem\u003et\u003c/em\u003e\u003csub\u003e\u003cem\u003eh\u003c/em\u003e\u003c/sub\u003e \u003cem\u003e\u0026lt;\u003c/em\u003e 0) and the negative peak in the surface wave arrival is seen at about 1.33 \u0026micro;s. The next four curves show the displacement responses at various times after the heating laser is turned on, and the last two curves show the response at 1 and 3 s after the heating laser is turned off. The shape of the surface acoustic wave remains relatively constant, but the arrival is delayed as heating proceeds. This lack of dispersion is expected as the heating times are long and the thermal field is thus relatively constant throughout the surface wave penetration depth. Figure\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e5\u003c/span\u003e(b) shows similar results for a CW laser heating power of 46 W. The basic features of the waveforms at 30 W and 46 W are similar but the delay in the SAW is more pronounced with the larger heating power due to the increase in temperature in the laser-heated region.\u003c/p\u003e\n\u003cp\u003eFigures\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e5\u003c/span\u003e(c) and 5(d) show the evolution of the displacement field throughout these experiments for CW laser heating powers of 30 W and 46 W, respectively. The abscissa gives the experiment time, where \u003cem\u003et\u003c/em\u003e\u003csub\u003e\u003cem\u003eh\u003c/em\u003e\u003c/sub\u003e \u003cem\u003e=\u003c/em\u003e 0 represents the time at which the heating laser is turned on and the heating laser is turned off at \u003cem\u003et\u003c/em\u003e\u003csub\u003e\u003cem\u003eh\u003c/em\u003e\u003c/sub\u003e \u003cem\u003e=\u003c/em\u003e 10 s, while the ordinate gives the time after the excitation laser pulse. The color bar represents the out of plane displacement of the surface. In these images, the SAW arrival has the negative-most amplitude shown in red. In both images, the surface wave arrival shows a marked delay associated with the onset of surface heating. This is followed by a more gradual change between \u003cem\u003et\u003c/em\u003e\u003csub\u003e\u003cem\u003eh\u003c/em\u003e\u003c/sub\u003e \u003cem\u003e=\u003c/em\u003e 2 s and \u003cem\u003et\u003c/em\u003e\u003csub\u003e\u003cem\u003eh\u003c/em\u003e\u003c/sub\u003e \u003cem\u003e=\u003c/em\u003e 10 s as heat diffuses through the sample and the sample temperature begins to approach the steady state. Finally, a rapid decrease in the arrival time is observed when the heating laser is turned off. Interestingly, the 46 W heating power not only causes more of a surface wave delay but the shape of the arrival over heating time is quite different than the 30 W heating laser case, and there is not a simple linear scaling between them. Furthermore, optical microscopy revealed no visible markings left on the surface from the 30 W heating power experiments, while there were clear discolorations and surface texture changes, indicative of surface melting, resulting from the 46 W heating power experiments.\u003c/p\u003e\n\u003cp\u003eThe frequency content of the surface acoustic waves generated in these experiments extends to 32 MHz. At this frequency and with the slow heating time of 10 s, the SAW is minimally dispersive with the increasing temperatures as seen in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e5\u003c/span\u003e, while the delay of the SAW is sensitive to the heating power or surface state. The arrival time of the negative peak of the surface wave was determined as a function of heating time for different laser powers. The surface wave delay was then determined by subtracting the room temperature arrival time. Figure\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e6\u003c/span\u003e(a) shows the real-time transient surface wave delays for heating powers of 30, 34, and 40 W. Note that at these lower heating powers, all the curves show a similar shape. If these curves are divided by the heating laser power to determine the normalized surface wave delay (in units of ns/W), all the curves collapse to a single curve as shown in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e6\u003c/span\u003e(b). This indicates that in this regime, the surface wave delay is a simple linear function of the heating power. Optical microscopy confirmed that there were no visible changes to the surface subsequent to heating in this power range. At higher heating powers the response is quite different, however: experiments performed at 46, 48, and 56 W resulted in visible markings and surface texture changes. The delay curves for these heating powers are plotted in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e6\u003c/span\u003e(c) together with the results from the 30 W experiment during which no surface changes were observed. The SAW delay is significantly more pronounced and the normalized surface wave delay plots in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e6\u003c/span\u003e(d) show a distinct change in shape. While all the curves follow the 30 W heating curve for early heating times, deviations begin at later times, with the deviations occurring earlier for higher power. It is hypothesized that the nonlinearity in the curve is associated with phase change and the presence of surface melt.\u003c/p\u003e\n\u003cp\u003eIncremental heating experiments were performed in which the laser power was fixed at 48 W but the illumination time was varied between 0.5 s and 10.0 s in 0.5 s intervals. The sample surface was optically observed after each interval, and the sample was translated between measurements such that a new region was illuminated. Figure\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e7\u003c/span\u003e(a) shows a subset of the surface wave delay data, again demonstrating that surface waves can be very effective for the characterization of the transient thermal field. Figure\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e7\u003c/span\u003e(b) shows a zoom-in of the first 5.0 s of normalized surface wave delay for heating laser powers of 30 W and 48 W. The curves are nearly identical up until about 2.0 s of illumination, after which the 48 W curve shows a significantly higher delay. Optical micrographs of the surface are shown in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e7\u003c/span\u003e(c). There is no detectable surface discoloration before 2.0 s of heating. However, there is a clear indication of a melt region starting at 2.0 s and this region continues to grow at longer heating times.\u003c/p\u003e\n\u003cp\u003eNote that in the modeling results, a one-dimensional illumination model was used to elucidate the effects of surface heating and melting on surface wave propagation. Thus a quantitative comparison with experimental results, in which the waves propagate through a region heated with a Gaussian laser source, is not possible. Nevertheless, there is qualitative agreement between the two with a rapid delay in the surface wave velocity at the onset of illumination and further decrease in surface wave velocity associated with melting. The surface acoustic wave delay curves, such as those shown in Figs.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e6\u003c/span\u003e and \u003cspan class=\"InternalRef\"\u003e7\u003c/span\u003e, may prove useful in monitoring laser-induced thermal processes, particularly in more complex cases involving phase transformation. Changes in these curves indicate variations in the heating laser parameters or laser-material interaction. These experimental results are limited to relatively slow heating, where the temperature is uniform over the surface wave penetration depth. A higher repetition rate pulsed laser can be used to probe more rapid thermal processes and potentially obtain quantitative information about the heated zone, including temperature distribution and melt pool depth.\u003c/p\u003e"},{"header":"Conclusion","content":"\u003cp\u003eLaser-based ultrasonics is a non-contact technique that can be used to monitor transient laser-induced heating and melting processes. Numerical simulations were performed to show that, for a spatially uniform heating beam, laser-induced surface acoustic waves are strongly influenced by surface heating conditions, are dispersive in the case of rapid heating, and that an abrupt velocity reduction happens upon the onset of surface melting. The complementary experimental results monitor the transient change of surface wave travel time associated with high power laser surface heating. Qualitative agreement between theory and experiment is observed: both show a rapid delay in the surface wave velocity at the onset of illumination and further decrease in surface wave velocity associated with melting. This technique may ultimately find application in the mapping of transient laser-induced thermal fields and melt zones, providing critical information for real-time process control in advanced manufacturing systems including those relying on laser powder bed fusion.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eAcknowledgements\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344 and was supported by the LLNL-LDRD Program under Project No. 19-ERD-008. The document release number is LLNL-JRNL-831155-DRAFT. R.E.M. thanks the National GEM Consortium for supporting the preparation of this manuscript.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthor contributions\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eT.W.M., D.M.S., and R.EM. suggested the experiments. R.EM. constructed the analytical models, performed the experiments, analyzed the data, and prepared figures. J.W.T., D.M.S., and T.W.M. acquired funding for this work. J.W.T. and K.J.H. provided resources for relevant work. R.E.M and T.W.M. wrote the manuscript. All authors reviewed the manuscript.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eData availability\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe datasets generated during and/or analyzed during the current study are available from the corresponding author on reasonable request.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCompeting interests\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors declare no competing interests.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n \u003cli\u003eVon Allmen, M. \u0026amp; Blatter, A. \u003cem\u003eLaser-beam interactions with materials: physical principles and applications\u003c/em\u003e. (Springer, 1995).\u003c/li\u003e\n \u003cli\u003eReady, J. F. \u003cem\u003eEffects of high-power laser radiation\u003c/em\u003e. (Academic Press, 1971).\u003c/li\u003e\n \u003cli\u003eYe, J. \u003cem\u003eet al.\u003c/em\u003e Energy Coupling Mechanisms and Scaling Behavior Associated with Laser Powder Bed Fusion Additive Manufacturing. \u003cem\u003eAdv. Eng. Mater.\u003c/em\u003e \u003cstrong\u003e21\u003c/strong\u003e, 1900185 (2019).\u003c/li\u003e\n \u003cli\u003eKhairallah, S. A. \u003cem\u003eet al.\u003c/em\u003e Controlling interdependent meso-nanosecond dynamics and defect generation in metal 3D printing. \u003cem\u003eScience\u003c/em\u003e \u003cstrong\u003e368\u003c/strong\u003e, 660\u0026ndash;665 (2020).\u003c/li\u003e\n \u003cli\u003eMercelis, P. \u0026amp; Kruth, J. Residual stresses in selective laser sintering and selective laser melting. \u003cem\u003eRapid Prototyping Journal\u003c/em\u003e \u003cstrong\u003e12\u003c/strong\u003e, 254\u0026ndash;265 (2006).\u003c/li\u003e\n \u003cli\u003eThijs, L., Verhaeghe, F., Craeghs, T., Humbeeck, J. V. \u0026amp; Kruth, J.-P. A study of the microstructural evolution during selective laser melting of Ti\u0026ndash;6Al\u0026ndash;4V. \u003cem\u003eActa Materialia\u003c/em\u003e \u003cstrong\u003e58\u003c/strong\u003e, 3303\u0026ndash;3312 (2010).\u003c/li\u003e\n \u003cli\u003eMatthews, M. J. \u003cem\u003eet al.\u003c/em\u003e Denudation of metal powder layers in laser powder bed fusion processes. \u003cem\u003eActa Materialia\u003c/em\u003e \u003cstrong\u003e114\u003c/strong\u003e, 33\u0026ndash;42 (2016).\u003c/li\u003e\n \u003cli\u003eKhairallah, S. A., Anderson, A. T., Rubenchik, A. \u0026amp; King, W. E. Laser powder-bed fusion additive manufacturing: Physics of complex melt flow and formation mechanisms of pores, spatter, and denudation zones. \u003cem\u003eActa Materialia\u003c/em\u003e \u003cstrong\u003e108\u003c/strong\u003e, 36\u0026ndash;45 (2016).\u003c/li\u003e\n \u003cli\u003eGrasso, M. \u0026amp; Colosimo, B. M. Process defects and \u003cem\u003ein situ\u003c/em\u003e monitoring methods in metal powder bed fusion: a review. \u003cem\u003eMeas. Sci. Technol.\u003c/em\u003e \u003cstrong\u003e28\u003c/strong\u003e, 044005 (2017).\u003c/li\u003e\n \u003cli\u003eKiss, A. M. \u003cem\u003eet al.\u003c/em\u003e Laser‐Induced Keyhole Defect Dynamics during Metal Additive Manufacturing. \u003cem\u003eAdv. Eng. Mater.\u003c/em\u003e \u003cstrong\u003e21\u003c/strong\u003e, 1900455 (2019).\u003c/li\u003e\n \u003cli\u003eKing, W. E. \u003cem\u003eet al.\u003c/em\u003e Laser powder bed fusion additive manufacturing of metals; physics, computational, and materials challenges. \u003cem\u003eApplied Physics Reviews\u003c/em\u003e \u003cstrong\u003e2\u003c/strong\u003e, 041304 (2015).\u003c/li\u003e\n \u003cli\u003eEverton, S. K., Hirsch, M., Stravroulakis, P., Leach, R. K. \u0026amp; Clare, A. T. Review of in-situ process monitoring and in-situ metrology for metal additive manufacturing. \u003cem\u003eMaterials \u0026amp; Design\u003c/em\u003e \u003cstrong\u003e95\u003c/strong\u003e, 431\u0026ndash;445 (2016).\u003c/li\u003e\n \u003cli\u003eRieder, H., Dillh\u0026ouml;fer, A., Spies, M., Bamberg, J. \u0026amp; Hess, T. Ultrasonic online monitoring of additive manufacturing processes based on selective laser melting. in 184\u0026ndash;191 (2015). doi:\u003ca href=\"https://doi.org/10.1063/1.4914609\"\u003e10.1063/1.4914609\u003c/a\u003e.\u003c/li\u003e\n \u003cli\u003eKouprianoff, D., Yadroitsava, I., du Plessis, A., Luwes, N. \u0026amp; Yadroitsev, I. Monitoring of Laser Powder Bed Fusion by Acoustic Emission: Investigation of Single Tracks and Layers. \u003cem\u003eFront. Mech. Eng.\u003c/em\u003e \u003cstrong\u003e7\u003c/strong\u003e, 678076 (2021).\u003c/li\u003e\n \u003cli\u003eShevchik, S. A., Masinelli, G., Kenel, C., Leinenbach, C. \u0026amp; Wasmer, K. Deep Learning for \u003cem\u003eIn Situ\u003c/em\u003e and Real-Time Quality Monitoring in Additive Manufacturing Using Acoustic Emission. \u003cem\u003eIEEE Trans. Ind. Inf.\u003c/em\u003e \u003cstrong\u003e15\u003c/strong\u003e, 5194\u0026ndash;5203 (2019).\u003c/li\u003e\n \u003cli\u003eDalong Liu \u0026amp; Ebbini, E. S. Real-Time 2-D Temperature Imaging Using Ultrasound. \u003cem\u003eIEEE Trans. Biomed. Eng.\u003c/em\u003e \u003cstrong\u003e57\u003c/strong\u003e, 12\u0026ndash;16 (2010).\u003c/li\u003e\n \u003cli\u003eLai, P. \u003cem\u003eet al.\u003c/em\u003e Real-Time Monitoring of High-Intensity Focused Ultrasound Lesion Formation Using Acousto-Optic Sensing. \u003cem\u003eUltrasound in Medicine \u0026amp; Biology\u003c/em\u003e \u003cstrong\u003e37\u003c/strong\u003e, 239\u0026ndash;252 (2011).\u003c/li\u003e\n \u003cli\u003eGillespie, J. \u003cem\u003eet al.\u003c/em\u003e \u003cem\u003eIn situ\u003c/em\u003e characterization of laser-generated melt pools using synchronized ultrasound and high-speed X-ray imaging. \u003cem\u003eThe Journal of the Acoustical Society of America\u003c/em\u003e \u003cstrong\u003e150\u003c/strong\u003e, 2409\u0026ndash;2420 (2021).\u003c/li\u003e\n \u003cli\u003eYang, T. \u003cem\u003eet al.\u003c/em\u003e In-situ monitoring and ex-situ elasticity mapping of laser induced metal melting pool using ultrasound: Numerical and experimental approaches. \u003cem\u003eJournal of Manufacturing Processes\u003c/em\u003e \u003cstrong\u003e71\u003c/strong\u003e, 178\u0026ndash;186 (2021).\u003c/li\u003e\n \u003cli\u003eKlein, M. \u0026amp; Sears, J. Laser ultrasonic inspection of laser cladded 316LSS and TI-6-4. in \u003cem\u003eInternational Congress on Applications of Lasers \u0026amp; Electro-Optics\u003c/em\u003e 1006 (Laser Institute of America, 2004). doi:\u003ca href=\"https://doi.org/10.2351/1.5060183\"\u003e10.2351/1.5060183\u003c/a\u003e.\u003c/li\u003e\n \u003cli\u003eManzo, A. J. \u0026amp; Helvajian, H. Pulsed laser ultrasonic excitation and heterodyne detection for \u003cem\u003ein situ\u003c/em\u003e process control in laser 3D manufacturing. \u003cem\u003eJournal of Laser Applications\u003c/em\u003e \u003cstrong\u003e29\u003c/strong\u003e, 012012 (2017).\u003c/li\u003e\n \u003cli\u003eManzo, A. J. \u0026amp; Helvajian, H. Utility of optical heterodyne displacement sensing and laser ultrasonics as in situ process control diagnostic for additive manufacturing. \u003cem\u003eOpt. Eng.\u003c/em\u003e \u003cstrong\u003e57\u003c/strong\u003e, 1 (2018).\u003c/li\u003e\n \u003cli\u003eDavis, G. \u003cem\u003eet al.\u003c/em\u003e Laser ultrasonic inspection of additive manufactured components. \u003cem\u003eInt J Adv Manuf Technol\u003c/em\u003e \u003cstrong\u003e102\u003c/strong\u003e, 2571\u0026ndash;2579 (2019).\u003c/li\u003e\n \u003cli\u003eSmith, R. J. \u003cem\u003eet al.\u003c/em\u003e Spatially resolved acoustic spectroscopy for selective laser melting. \u003cem\u003eJournal of Materials Processing Technology\u003c/em\u003e \u003cstrong\u003e236\u003c/strong\u003e, 93\u0026ndash;102 (2016).\u003c/li\u003e\n \u003cli\u003eYamada, H., Kosugi, A. \u0026amp; Ihara, I. Noncontact Monitoring of Surface Temperature Distribution by Laser Ultrasound Scanning. \u003cem\u003eJpn. J. Appl. Phys.\u003c/em\u003e \u003cstrong\u003e50\u003c/strong\u003e, 07HC06 (2011).\u003c/li\u003e\n \u003cli\u003eShi, Y.-A. \u003cem\u003eet al.\u003c/em\u003e A Method for Reconstructing Internal Temperature Distribution in Solid Structure. \u003cem\u003eJ Nondestruct Eval\u003c/em\u003e \u003cstrong\u003e39\u003c/strong\u003e, 48 (2020).\u003c/li\u003e\n \u003cli\u003eQueheillalt, D. T. \u0026amp; Wadley, H. N. G. Laser ultrasonic sensing of the melting and solidification of cadmium telluride. \u003cem\u003eJournal of Crystal Growth\u003c/em\u003e \u003cstrong\u003e225\u003c/strong\u003e, 34\u0026ndash;44 (2001).\u003c/li\u003e\n \u003cli\u003eKil-Mo Koo, Dong-Gyu Jeong, Jong-Ho Choi, \u0026amp; Duck-Young Ko. A new measurement system of very high temperature in atomic pile using ultrasonic delay time. in \u003cem\u003eProceedings of IEEE Region 10 International Conference on Electrical and Electronic Technology. TENCON 2001 (Cat. No.01CH37239)\u003c/em\u003e vol. 2 860\u0026ndash;863 (IEEE, 2001).\u003c/li\u003e\n \u003cli\u003eHinterlechner, I. \u003cem\u003eet al.\u003c/em\u003e In-situ monitoring of phase transformation in Ti-6Al-6V-2Sn using laser ultrasonics. \u003cem\u003eNondestructive Testing and Evaluation\u003c/em\u003e \u003cstrong\u003e33\u003c/strong\u003e, 130\u0026ndash;138 (2018).\u003c/li\u003e\n \u003cli\u003eKruger, S. E. \u0026amp; Damm, E. B. Monitoring austenite decomposition by ultrasonic velocity. \u003cem\u003eMaterials Science and Engineering: A\u003c/em\u003e \u003cstrong\u003e425\u003c/strong\u003e, 238\u0026ndash;243 (2006).\u003c/li\u003e\n \u003cli\u003eShinbine, A., Garcin, T. \u0026amp; Sinclair, C. In-situ laser ultrasonic measurement of the hcp to bcc transformation in commercially pure titanium. \u003cem\u003eMaterials Characterization\u003c/em\u003e \u003cstrong\u003e117\u003c/strong\u003e, 57\u0026ndash;64 (2016).\u003c/li\u003e\n \u003cli\u003eSaunders, N., Guo, U. K. Z., Li, X., Miodownik, A. P. \u0026amp; Schill\u0026eacute;, J.-Ph. Using JMatPro to model materials properties and behavior. \u003cem\u003eJOM\u003c/em\u003e \u003cstrong\u003e55\u003c/strong\u003e, 60\u0026ndash;65 (2003).\u003c/li\u003e\n \u003cli\u003eRose, J. L. \u003cem\u003eUltrasonic waves in solid media\u003c/em\u003e. (Cambridge University Press, 1999).\u003c/li\u003e\n \u003cli\u003eOgi, H. \u003cem\u003eet al.\u003c/em\u003e Titanium\u0026rsquo;s high-temperature elastic constants through the hcp\u0026ndash;bcc phase transformation. \u003cem\u003eActa Materialia\u003c/em\u003e \u003cstrong\u003e52\u003c/strong\u003e, 2075\u0026ndash;2080 (2004).\u003c/li\u003e\n \u003cli\u003eSingh, R. K. \u0026amp; Narayan, J. A novel method for simulating laser-solid interactions in semiconductors and layered structures. \u003cem\u003eMaterials Science and Engineering: B\u003c/em\u003e \u003cstrong\u003e3\u003c/strong\u003e, 217\u0026ndash;230 (1989).\u003c/li\u003e\n \u003cli\u003eWelsch, G.,\u0026nbsp;Boyer, R. \u0026amp; Collings, E. W.\u003cem\u003e\u0026nbsp;Materials properties handbook: titanium alloys\u003c/em\u003e. (ASM International, 1994).\u003c/li\u003e\n \u003cli\u003eSmithells, C. J., Gale, W. F. \u0026amp; Totemeier, T. C. \u003cem\u003eSmithells metals reference book\u003c/em\u003e. (Elsevier Butterworth-Heinemann, 2004).\u003c/li\u003e\n \u003cli\u003eMills, K. C. \u003cem\u003eRecommended values of thermophysical properties for selected commercial alloys\u003c/em\u003e. (Woodhead, 2002).\u003c/li\u003e\n \u003cli\u003eCasas, J., K\u0026eacute;ita, N. M. \u0026amp; Steinemann, S. G. Sound Velocity in Liquid Titanium, Vanadium and Chromium. \u003cem\u003ePhysics and Chemistry of Liquids\u003c/em\u003e \u003cstrong\u003e14\u003c/strong\u003e, 155\u0026ndash;158 (1984).\u003c/li\u003e\n \u003cli\u003eWood, R. F. \u0026amp; Giles, G. E. Macroscopic theory of pulsed-laser annealing. I. Thermal transport and melting. \u003cem\u003ePhys. Rev. B\u003c/em\u003e \u003cstrong\u003e23\u003c/strong\u003e, 2923\u0026ndash;2942 (1981).\u003c/li\u003e\n \u003cli\u003eCheng, A., Murray, T. W. \u0026amp; Achenbach, J. D. Simulation of laser-generated ultrasonic waves in layered plates. \u003cem\u003eThe Journal of the Acoustical Society of America\u003c/em\u003e \u003cstrong\u003e110\u003c/strong\u003e, 848\u0026ndash;855 (2001).\u003c/li\u003e\n \u003cli\u003eSpicer, J. B., McKie, A. D. W. \u0026amp; Wagner, J. W. Quantitative theory for laser ultrasonic waves in a thin plate. \u003cem\u003eAppl. Phys. Lett.\u003c/em\u003e \u003cstrong\u003e57\u003c/strong\u003e, 1882\u0026ndash;1884 (1990).\u003c/li\u003e\n \u003cli\u003eMcDonald, F. A. On the precursor in laser‐generated ultrasound waveforms in metals. \u003cem\u003eAppl. Phys. Lett.\u003c/em\u003e \u003cstrong\u003e56\u003c/strong\u003e, 230\u0026ndash;232 (1990).\u003c/li\u003e\n \u003cli\u003eSanderson, T., Ume, C. \u0026amp; Jarzynski, J. Laser generated ultrasound: a thermoelastic analysis of the source. \u003cem\u003eUltrasonics\u003c/em\u003e \u003cstrong\u003e35\u003c/strong\u003e, 115\u0026ndash;124 (1997).\u003c/li\u003e\n \u003cli\u003eMurray, T. W., Krishnaswamy, S. \u0026amp; Achenbach, J. D. Laser generation of ultrasound in films and coatings. \u003cem\u003eAppl. Phys. Lett.\u003c/em\u003e \u003cstrong\u003e74\u003c/strong\u003e, 3561\u0026ndash;3563 (1999).\u003c/li\u003e\n \u003cli\u003eBertoni, H. L. \u0026amp; Tamir, T. Unified theory of Rayleigh-angle phenomena for acoustic beams at liquid-solid interfaces. \u003cem\u003eAppl. Phys.\u003c/em\u003e \u003cstrong\u003e2\u003c/strong\u003e, 157\u0026ndash;172 (1973).\u003c/li\u003e\n \u003cli\u003eBiot, M. A. The interaction of Rayleigh and Stoneley waves in the ocean bottom*. \u003cem\u003eBulletin of the Seismological Society of America\u003c/em\u003e \u003cstrong\u003e42\u003c/strong\u003e, 81\u0026ndash;93 (1952).\u003c/li\u003e\n \u003cli\u003eMeegan, G. D., Hamilton, M. F., Il\u0026rsquo;inskii, Yu. A. \u0026amp; Zabolotskaya, E. A. Nonlinear Stoneley and Scholte waves. \u003cem\u003eThe Journal of the Acoustical Society of America\u003c/em\u003e \u003cstrong\u003e106\u003c/strong\u003e, 1712\u0026ndash;1723 (1999).\u003c/li\u003e\n \u003cli\u003eMurray, T. W., Balogun, O., Steen, T. L., Basu, S. N. \u0026amp; Sarin, V. K. Inspection of compositionally graded mullite coatings using laser based ultrasonics. \u003cem\u003eInternational Journal of Refractory Metals and Hard Materials\u003c/em\u003e \u003cstrong\u003e23\u003c/strong\u003e, 322\u0026ndash;329 (2005).\u003c/li\u003e\n \u003cli\u003eIng, R. K. \u0026amp; Monchalin, J. ‐P. Broadband optical detection of ultrasound by two‐wave mixing in a photorefractive crystal. \u003cem\u003eAppl. Phys. Lett.\u003c/em\u003e \u003cstrong\u003e59\u003c/strong\u003e, 3233\u0026ndash;3235 (1991).\u003c/li\u003e\n \u003cli\u003eBlouin, A. \u0026amp; Monchalin, J. Detection of ultrasonic motion of a scattering surface by two‐wave mixing in a photorefractive GaAs crystal. \u003cem\u003eAppl. Phys. Lett.\u003c/em\u003e \u003cstrong\u003e65\u003c/strong\u003e, 932\u0026ndash;934 (1994).\u003c/li\u003e\n \u003cli\u003eDelaye, P. \u003cem\u003eet al.\u003c/em\u003e Detection of ultrasonic motion of a scattering surface by photorefractive InP:Fe under an applied dc field. \u003cem\u003eJ. Opt. Soc. Am. B\u0026nbsp;\u003c/em\u003e\u003cstrong\u003e14\u003c/strong\u003e, 1723 (1997).\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"scientific-reports","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"scirep","sideBox":"Learn more about [Scientific Reports](http://www.nature.com/srep/)","snPcode":"","submissionUrl":"","title":"Scientific Reports","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Scientific Reports","inReviewEnabled":true,"inReviewRevisionsEnabled":true},"keywords":"","lastPublishedDoi":"10.21203/rs.3.rs-1347814/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-1347814/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eIntra- and inter-layer integrity of components fabricated with advanced manufacturing techniques, such as laser powder bed fusion, is dependent upon rapid heating, melting, and solidification processes. There is a need for new techniques to provide \u003cem\u003ein situ\u003c/em\u003e feedback of these processes. Here a laser-based ultrasonic technique to probe thermal effects induced by a high-power continuous wave laser in titanium samples is described. Numerical simulations were performed to show that, for a spatially uniform heating beam, laser-induced surface acoustic waves are strongly influenced by surface heating conditions, are dispersive in the case of rapid heating, and that an abrupt velocity reduction happens upon the onset of surface melting. Furthermore, laser-based ultrasound experimental results which monitor the transient change of surface wave travel time associated with high power laser surface heating are provided. A pulsed laser is used to generate high frequency surface acoustic waves that propagate through the laser-heated region and are detected using a photorefractive crystal-based interferometer. Qualitative agreement is observed between theory and experiment with both showing a rapid delay in the surface wave velocity at the onset of illumination and further decrease in surface wave velocity associated with melting. It is demonstrated that changes in the surface wave velocity can be used to track local heating and detect the onset of surface melting in real time.\u003c/p\u003e","manuscriptTitle":"Real-Time Laser Ultrasonic Monitoring of Laser-Induced Thermal Processes","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2022-02-21 14:42:26","doi":"10.21203/rs.3.rs-1347814/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Major revision","date":"2022-04-25T19:55:16+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2022-04-15T10:40:24+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"c10c5a74-2683-420d-91fa-0148d10bcb5e","date":"2022-04-12T18:12:23+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"e801637f-769f-4fae-8015-70698325187c","date":"2022-04-04T14:08:47+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2022-03-22T09:16:52+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2022-03-22T09:12:42+00:00","index":"","fulltext":""},{"type":"editorInvited","content":"","date":"2022-02-17T13:05:35+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2022-02-17T12:58:14+00:00","index":"","fulltext":""},{"type":"submitted","content":"Scientific Reports","date":"2022-02-10T20:27:55+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"scientific-reports","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"scirep","sideBox":"Learn more about [Scientific Reports](http://www.nature.com/srep/)","snPcode":"","submissionUrl":"","title":"Scientific Reports","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Scientific Reports","inReviewEnabled":true,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"30280ed7-a764-4123-af69-c9e8c26eb5ce","owner":[],"postedDate":"February 21st, 2022","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"under-review","subjectAreas":[],"tags":[],"updatedAt":"2022-05-30T16:44:17+00:00","versionOfRecord":[],"versionCreatedAt":"2022-02-21 14:42:26","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-1347814","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-1347814","identity":"rs-1347814","version":["v1"]},"buildId":"rHA-KDH7Qsr4HCuvH75dn","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}