Effect of Defocusing Variation on Laser Wire Additive Manufacturing Morphology and Plasma Plume Change

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This experiment analyzed the plasma plume geometry, melting pool spatter, and melting pool length, along with observing bead geometry and porosity. The following conclusions are drawn: Variations in defocusing amount caused changes in energy distribution, which affected the stability of the LWAM process and led to changes in the surface accuracy of the deposited layer and the melt pool morphology. Increasing the defocusing amount enlarged the spot area, diminished laser power density, and effectively reduced spattering during LWAM. Surface roughness increased with defocusing, transforming the hump morphology from an initial smooth fish-scale pattern to a periodic undulating pattern. Larger defocusing decreases the melt pool area and keyhole depth, this reduction aids gas escape from the keyhole, thus mitigating porosity defects in LWAM. Physical sciences/Optics and photonics/Lasers leds and light sources/Fibre lasers Physical sciences/Engineering/Mechanical engineering Laser Wire Additive Manufacturing Defocusing amount Plasma plume Bead geometry Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Introduction Metal Additive Manufacturing (MAM) technology has received significant attention from countries around the world due to a series of advantages such as moldless free-form, personalized customization, and a high degree of automation. As a type of MAM, laser wire additive manufacturing (LWAM) has attracted wide attention due to its advantages of high molding efficiency, fast cooling rate, low heat input deformation, and pollution-free production process. As the current energy source for laser additive manufacturing, focused Gaussian beams are widely used in metal laser AM devices. In contrast, the concentrated energy distribution, high thermal gradient, and instability of the molten pool can lead to unsatisfactory molding quality of additive parts during processing, resulting in defects such as cracks, pores, and undercuts. Additionally, LWAM deposited specimens exhibit poor wettability and spreading, which collectively diminish the mechanical properties and fatigue life of the metal components 1–3 。 Youyu Su et al. 4 concluded that pore defects deposited by laser directed energy deposition is the biggest factor affecting the overall performance of the component. Hu Bin et al. 5 used a specific wide-band shaped laser spot for deposition and found that no porosity existed within the deposited specimens. Some authors suggest that changes in the amount of defocusing can effect deposition molding control. Farui Du et al. 6 showed that if the height increment is too low the laser is in negative defocus and the beam diameter will gradually increase, leading to surface defects in thin-walled parts and the manufacturing process cannot continue. Zhaoyang Liu et al. 7 found that a defocusing amount of -3 mm is favorable for the continuity of epitaxial growth of columnar dendrites deposited by laser powder deposition of single-crystal high-temperature alloys. Junhui Li et al. 8 deposited laser-fed powder additive manufacturing specimens without the presence of porosity by varying the amount of defocus to increase the laser spot diameter. Christian Bernauer et al. 9 experimented with different processes of 316L laser fusion by varying the focal position of the laser beam to obtain a 2.75 mm diameter annular laser spot and found that the volume of the deposited weld channel increased significantly at higher melt pool temperatures, however, the overall hardness of the melt pool was rather lower. Yanhua Zhao et al. 10 , on the other hand, argued that the variation of the defocusing amount can change the convergence and divergence of the beam, and large defocusing amount has a more uniform energy density distribution. G.F. Dong et al. 11 optimized the process parameters for laser fed powder melting of TiNiTa functional coatings by orthogonal tests and concluded that the optimum process parameters were found at 10 mm defocus. The highly concentrated laser beam irradiated on the material surface during the LWAM process leads to rapid melting and evaporation of the substrate, generating large amounts of metal vapor and plasma plumes. As a physical phenomenon inherent to the laser additive process, the characteristic behavior of the plasma plume is closely related to the deposition process. Wencong Qiu et al. 12 carried out the observation of the welding process at different laser powers and found that the shape of the plasma plume for the heat conduction welding process was much smaller than penetration welding and the plasma plume fluctuations were smoother. Jie Li et al. 13 investigated the plasma plume in laser welding by using an image processing method, and concluded that the plasma plume was generated due to the intersection of the front wall surface of the keyhole and the laser beam. Min Li et al. 14 used a multi-imaging method to observe the kinetic behavior of the plasma plume and keyhole simultaneously. The inclination angle of the plume is the largest when the laser-induced vapor was close to the orifice, and the inclination angle of the plume gradually decreased as the laser-induced vapor moved toward the bottom of the front wall of the locking orifice. At the present stage, studies are mostly focused on analyzing the effect of the change of focusing amount on the molten pool morphology and plasma plume in welding, however, there are relatively few specific studies on the change of focusing amount on the plasma plume in the additive manufacturing process of LWAM. Therefore, it is of great significance to explore the transient plasma plume fluctuation and the change mechanism of the deposited layer morphology during the LWAM deposition process under different process parameters, which is important for the optimization of the selection of the process parameters for the LWAM enhancement. Materials and Methods The LWAM experiment utilized the HY-PTBZGQ-1000 fiber laser additive manufacturing system, as illustrated in Fig. 1 (a). This system comprises the MFSC-1000X fiber laser, the DI-2000L oscillation laser head, the QL-1000 wire feeder, the CNC coordinate table, and the control system. The fiber laser has a maximum power of 1000 W, operates at a wavelength of 1080 nm, the focal length of the laser head is 200mm and the minimum spot diameter is 0.19 mm. In this experiment, 0.8mm diameter 316L stainless steel wire was chosen for LWAM on a 50 mm*50 mm*8 mm 316 stainless steel substrate. Refer to Table 1 for the material composition details: Before conducting the experiment, the oxidized layer of the deposited substrate was removed using a grinder and cleaned with alcohol. The wire feeder was positioned in front of the laser, and the wire feeding angle was 60°. The spacing between the laser and the welding wire is set at 0.2 mm on the substrate plane. And Ar gas flow rate was set of 15 L/min for protective gas during LWAM. The German BASLER camera was used for high-speed video recording of the plasma plume in the process of LWAM, and the camera shooting frequency of 340 HZ. The camera was placed horizontally on the right side of the deposition track, and the plasma plume photographic site after adjusting the aperture and focus is shown in Fig. 1 (b): The process parameters of the laser wire melting process are laser power 450 W, scanning speed 2 mm/s, wire feeding speed 6 mm/s. The variation values of the defocusing amount were 0 mm, 2.5 mm, 5 mm, 7.5 mm, and 10 mm. Each group of process experiments was repeated three times, and the values of the morphology parameters were the average of the three measurements. Table 1 Material composition of welding wire and substrate. C Si Mn S P Cr Ni Mo N Cu 316L wire 0.022 0.42 1.89 0.007 0.009 19.12 12.62 2.62 0 0.34 316 substrate 0.043 0.46 1.12 0.003 0.027 18.15 10.01 2.1 0.031 0 First the LWAM specimens were treated with a stainless steel pickling passivation paste to eliminate the oxidized surface coating. Subsequently, the topography of the bead surface was documented using a Canon 70D DSLR camera, while the KEYENCE laser topography meter LJ-X8000A performed three-dimensional topography scanning under different laser oscillation modes. Following this, specimens were line-cutted along the hump position, and the cutting surfaces were subjected to metallographic mosaic. The specimens were then ground, polished, and corroded in 316L special corrosion solution to observe the molten pool. The corrosion solution had a ratio of hydrofluoric acid: hydrochloric acid: nitric acid: water = 1:2:3:4, which allowed for clear observation of the demarcation line of the melting pool post-corrosion. Furthermore, metallographic observation of the deposited specimen sections was carried out using an optical microscope (OM), and data extraction from the images was conducted using Image J software. Experimental results Plasma plume variation For easier observation, the plasma plume fluctuation photos of the LWAM deposition process in each group of defocused amount were first selected, and the six photos with the largest plasma plume fluctuation were collected respectively. The plasma plume fluctuation of the LWAM process in different defocused cases was summarized as shown in Fig. 2 : As shown in Fig. 2 , the change in the amount of defocus has a large impact on the plasma plume shape, the plasma plume was generated in the laser irradiation position at the front end of the melting pool located near the side of the wire, and the overall shape presented an eruption shape that bottom narrow and top wide. By observing the transient melting pool (the highlighted part at the bottom of the plasma) and the plasma fluctuation, it can be found that the length of the melting pool and the size of the plasma plume were relatively stable when the defocusing amount of the laser beam was 0 mm. When the defocusing amount was 2.5 mm, the plasma plume fluctuated with a stable melting pool length, and the plasma plume appeared to have a significant downward pressure at the top and a slight deflection to both sides. The melting pool length and plasma volume thickness fluctuated drastically between 5 mm and 7.5 mm, and the length of the melting pool was shorter when the plasma plume was vertically upward and longer when the plasma plume was deflected. The melting pool length and the plasma plume tend to be stabilized when the defocusing amount was 10 mm. By observing the transfer condition of the melting pool: the transition between the melting wire and the melting pool was a stable liquid bridge transition, and the transition of the melting pool was not affected by changing the value of the amount of defocusing. By observing the molten pool spattering: when the defocusing amount was 0 mm and 2.5 mm, there was a large amount of spattering in the process of paraxial fusion, and when the laser defocusing amount was raised above 5 mm, the spattering decreased to no spattering, which proves that the change of defocus can effectively eliminate the spattering. A clear and visible plasma plume contour can be obtained by using PS software to colorize and sharpen the plasma plume photos. The thickness (H), area (S) and melt pool length (L) features of the plasma plume are extracted in ImageJ software. The continuous photo shooting of 15 photographs was selected to measure the plasma plume and melting pool length under different defocusing amounts in the LWAM process, and the shooting interval is 15ms for each photo. Based on the measurement results, the mean values and fluctuations of the plasma plume and melting pool sizes for different cases of defocusing are calculated, and the error bar graphs are plotted as shown in Fig. 3 : Based on the mean value analysis shown in Fig. 3 (a) and Fig. 3 (b), the plasma plume has the lowest thickness and the smallest area when the defocusing amount was 0 mm, and the plume thickness and area show the first increasing and then decreasing trend with the increase of defocusing amount. The average thickness of the plasma plume was largest when the defocusing amount was 5 mm, and the average area of the plasma plume was largest when the defocusing amount was 2.5 mm. Based on the analysis of the error bars shown in Fig. 3 (a) and Fig. 3 (b), the plasma plume fluctuation also showed a first increasing and then decreasing trend, when the amount of defocusing was 0mm and 10mm, the plasma plume fluctuation was not obvious, and when the amount of defocusing was from 2.5mm to 7.5mm, the melting pool fluctuation is larger. According to the analysis of the average length of the melting pool shown in Fig. 3 (c), the length of the melting pool was the shortest when the amount of defocusing was 0 mm, and when the amount of defocusing was raised to 2.5 mm, the length of the melting pool showed a sudden change and steep increase. With the further increase in the amount of defocusing, the average length of the molten pool showed a linear decreasing trend; when the amount of defocusing was raised to 10mm, the length of the molten pool was the shortest. Surface morphology of the deposited layer The topographic picture of the deposited layer surface and the 3D topography measured by the laser scanner are shown in Fig. 4 (a). The top line roughness measurements were taken for the specimens deposited with different amounts of defocusing and the variations are shown in Fig. 4 (b): As shown in Fig. 4 (a) , the variation of the defocusing amount affects the surface accuracy and morphology of the deposited layer. The surface flatness of the deposited layer decreased with the increase of the defocusing amount, the surface of the deposited layer was very smooth with the distribution of fine fish scale pattern when the defocusing amount was 0 mm and 2.5 mm, and then it turned into a slight hump protrusion when the defocusing amount was increased to 5 mm. When the defocusing amount was increased to 7.5 mm, the overall surface of the deposited layer was relatively worse, and the overall morphology was a periodic fluctuating hump with unfused defects on both sides of the deposited layer. When the defocusing amount was increased to 10 mm, the deposited layer could not spread, and the unfused defects became more serious. Moreover, the wire was sticking at the end of the welding channel. Combined with the roughness measured in Fig. 4 (b), it showed that the surface roughness of the deposited layer gradually increased with the increase of laser defocus due to the gradual increase of the height difference formed by the top fish scale pattern and the hump. Molten pool cross-section morphology The cross-sectional morphology of the LWAM deposition specimens with different defocusing amounts is shown in Fig. 5 . As shown in Fig. 5 , by comparing the top morphology of the molten pool, it is found that the curvature of the top of the molten pool was small when the defocusing amount was less than 5 mm. When the defocusing amount was 7.5 mm, the contour of bead geometry showed a hemispherical contour, and when the defocusing amount was 10 mm, the contour of bead geometry showed a spherical shape. When observing the bottom profile of the molten pool cross-section, it appeared sharp protrusions when the amount of devitrification was 0-2.5mm, and the protrusions at the bottom of the molten pool gradually weakened with the increase of defocusing amount. The protrusion of the molten pool bottom tends to be horizontal when the amount of defocusing was 5mm. As the amount of devitrification continued to increase, the bottom of the molten pool showed no transition step protrusion. Simultaneously, when the defocusing amount was 0-2.5mm, there were pores at the bottom of the molten pool, which are mainly distributed in the protruding parts. Image J software was used to extract the metallographic dimensions of the cross-section of the bead in Fig. 5 . The average values of molten width (W), molten height (H), molten depth (D), left and right wetting angles, molten pool dilution rate, molten pool area (S), and porosity of the deposited layer (P) for different defocusing cases are plotted as shown in Fig. 6 : As shown in Fig. 6 (a) for the variation of the bead geometry, the molten width showed a decreasing trend with the increase of the laser defocusing amount, which decreased slowly at the defocusing amount of 0–5 mm and decreased rapidly at the amount of 5–10 mm. The change of molten height is opposite to that of molten width, showing an upward trend with a sudden increase at 5 mm. The changes in molten depth and dilution rate also show an overall decreasing trend with the increase of defocusing amount, but the inflection point of the plunge was located at the defocusing amount of 7.5mm. As shown in Fig. 6 (b), the change in the wetting angle of the deposited layer was not obvious when the defocusing amount was located between 0–5 mm, and when the value of the defocusing amount was higher than 5 mm, there is a significant decreasing trend. The wetting angle was greater than 90° when the defocusing amount value was less than 7.5 mm, whereas when the defocusing amount value increased to 10 mm, there were unfused defects on both sides of the sedimentary layer with wetting angle less than 90°. As shown in Fig. 6 (c), the dilution rate showed a decreasing trend with the increase of the laser defocusing amount. As shown in Fig. 6 (d) the variation of melt pool area showed a linear decreasing trend with the variation of defocusing amount, the larger defocusing amount was the smaller molten pool area was. Discussion The molten pool heat of the LWAM process depends on the intensity of Fresnel absorption inside the molten pool, which in turn depends on the input intensity of the laser Gaussian beam and the reflection of the beam on the inner wall of keyhole. The intensity of the laser energy input is, in turn, dependent on the laser beam divergence and the spot size of the laser irradiation. Combined with the laser divergence and the spot size change caused by the defocusing amount shift, the laser energy density can be calculated by the following Eq. 1 5 : $$PD=P/(\pi *{\omega ^2})$$ 1 $$\omega (z)={\omega _0}\sqrt {1+{{(\frac{z}{{{z_r}}})}^2}}$$ 2 $${z_r}=\frac{{\pi {\omega _0}^{2}}}{\lambda }$$ 3 In the above equation, PD is the laser energy density, P is the laser output power, ω is the laser spot size radius, ω0 is the minimum laser spot radius at focus, z is the defocusing amount, Zr is the Rayleigh length, and λ is the laser wavelength. The variation of laser spot size and power density with the defocusing amount is shown in Fig. 7 : As shown in Fig. 7 , the laser irradiation spot size was determined by the defocusing amount, the increased defocusing amount leads to the increase of spot area and the decrease of laser power density. The increase in spot area improved the keyhole opening area of the molten pool, which was helpful for gas escaping from the keyhole to reduce the porosity defects in the LWAM process. At the same time, the reduction of laser irradiation energy density weakened the recoil pressure formed by the melting pool evaporation, which helped to eliminate the melting pool spattering. As shown in Fig. 2 , the spattering in the melting pool was serious when the defocusing amount was less than 2.5mm, and the spattering was obviously weakened when increasing the defocusing amount. In practice, when the laser light travels through the plasma plume, there is a diminishment of the laser energy due to the inverse bremsstrahlung absorption of the plasma plume. According to the Beer-Lambert law of laser energy attenuation 15 , the thicker the plasma plume is, the stronger the laser attenuation is. When the plasma plume is too thick, the energy absorption in the melting pool is reduced, and the energy absorption in the melting pool affects the evaporation of the internal solution, which in turn weakens the plasma plume growth, hence leading to periodic fluctuations in the plasma plume. The larger the plasma plume thickness, the more intense the plasma plume fluctuation. As shown in Fig. 3 , the plasma plume fluctuation was the most intense when the defocusing amount was 5 mm and 7.5 mm, and the plasma plume thickness was the largest. The interaction schematic of the laser and melting pool under defocusing is shown in Fig. 8 . As shown in Fig. 8 , the laser is reflected downward after irradiating the inner wall of the keyhole. The smaller the defocusing amount is, the more concentrated the energy converging at the center of the keyhole. The highly concentrated laser beam is reflected many times by the inner wall of the keyhole, which enhances the Fresnel absorption of the melting pool. The continuous absorption of energy by the molten pool further deepens the depth of the keyhole, which makes it difficult for gas escape during LWAM, and it is more likely to be sealed inside the molten pool to form porosity defects. As the defocus increases the spot focus converges above the substrate and the reduction in reflection leads to a decrease in the Fresnel absorption efficiency of the laser energy by the keyhole wall. The energy absorbed by the melting pool is not sufficient to prompt a complete spreading of the liquid pool, which leads to an increase in the molten height and a decrease in the molten width of the bead. In summary, based on the surface accuracy of the deposited layer, it is showed that an increase in the defocusing amount led to a decrease in the surface accuracy of the deposited layer, and that excessive defocusing led to unfused defects in the deposited layer. Depending on the porosity defects in the molten pool and the spattering during deposition, an increase in the defocusing amount can help to reduce the porosity defects and decrease the spattering during the LWAM process. At the same time, by observing the metallography of the deposited layer cross-section, when the defocusing amount was 5 mm, the deposited layer cross-section has a large melting width size and the dilution rate of the melting pool was close to 0.5, which can be effectively fused with the substrate. Therefore, the defocusing amount of the LWAM should be selected as 5 mm to ensure the maximum molding accuracy. Conclusion (1) The increase of defocusing amount leads to the increase of spot area and the decrease of laser power density, which reduces the recoil pressure formed by the evaporation of the melting pool and can effectively improve the spattering situation during the deposition process. (2) The larger the plasma plume thickness is, the more violent the plasma plume fluctuation is. The plasma plume thickness, area and fluctuation showed a first increase and then decrease trend with the increase of defocusing amount. (3) Variations of defocusing amount effect the morphology of the deposited layer. With the increase of laser defocusing amount the surface roughness gradually increased, and the hump morphology changed from the initial smooth fish scale pattern to periodic fluctuations, when the defocusing amount up to 10 mm the deposited layer did not spread smoothly and there were serious unfused defects. (4) The increase of defocusing amount affects the energy absorption of molten pool, the larger defocusing amount is the smaller the molten pool area is. The reduction of keyhole depth is helpful for the gas escape from keyhole during the LWAM process. Declarations Acknowledgments The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by the National Natural Science Foundation of China (51375146); Ministry of Industry and Information Technology of the People’s Republic of China (2018ZNZX01-02) and Natural Science Foundation of Henan Province (17A460012). Author Contributions C. Y. wrote the paper and analyzed the experimental results, Professor H. H. designed the experiment. P. Z., R. W., and J. H. completed the experiment. Competing Interests The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article. 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Manuf. Processes 75 (593-604 (2022). https://doi.org/10.1016/j.jmapro.2022.01.032. M. Li et al., "Correlation between plume fluctuation and keyhole dynamics during fiber laser keyhole welding," J. Laser Appl. 32 (2), 022010 (2020). https://doi.org/10.2351/1.5138219. G. E. Bean et al., "Effect of laser focus shift on surface quality and density of inconel 718 parts produced via selective laser melting," Addit. Manuf. 22 (207-215 (2018). https://doi.org/10.1016/j.addma.2018.04.024. Additional Declarations No competing interests reported. Cite Share Download PDF Status: Posted Version 1 posted 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. 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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-4458930","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Article","associatedPublications":[],"authors":[{"id":309994796,"identity":"aab361ef-a498-4032-83bb-4342297faff0","order_by":0,"name":"Chenxiao Yan","email":"","orcid":"","institution":"Henan University of Science and Technology","correspondingAuthor":false,"prefix":"","firstName":"Chenxiao","middleName":"","lastName":"Yan","suffix":""},{"id":309994797,"identity":"25be2b5e-7616-498a-af17-0eca1599759f","order_by":1,"name":"Hongbiao 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Technology","correspondingAuthor":false,"prefix":"","firstName":"Peng","middleName":"","lastName":"Zhang","suffix":""},{"id":309994799,"identity":"b92fe557-9568-4630-891f-602a4d9b6ae1","order_by":3,"name":"Rui Wang","email":"","orcid":"","institution":"Henan University of Science and Technology","correspondingAuthor":false,"prefix":"","firstName":"Rui","middleName":"","lastName":"Wang","suffix":""},{"id":309994800,"identity":"77cdef12-3e7d-48b2-bdd4-6e0dad1bcf37","order_by":4,"name":"Jiayang Hu","email":"","orcid":"","institution":"Henan University of Science and Technology","correspondingAuthor":false,"prefix":"","firstName":"Jiayang","middleName":"","lastName":"Hu","suffix":""}],"badges":[],"createdAt":"2024-05-22 07:23:49","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-4458930/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-4458930/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":57768427,"identity":"ad5df6cd-0c29-4b3a-ae34-2f0b80e4a4a4","added_by":"auto","created_at":"2024-06-05 11:34:36","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":104788,"visible":true,"origin":"","legend":"\u003cp\u003eLaser wire additive manufacturing equipment.\u003cstrong\u003e(a). \u003c/strong\u003eLaser additive manufacturing system. \u003cstrong\u003e(b).\u003c/strong\u003e Plasma plume recording condition.\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-4458930/v1/e8902a16c936a202ceb5faf9.png"},{"id":57767704,"identity":"40eb7446-c99a-41f2-a406-e1a660fdbf05","added_by":"auto","created_at":"2024-06-05 11:26:36","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":428778,"visible":true,"origin":"","legend":"\u003cp\u003eFluctuation of plasma plume with different defocusing amounts.\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-4458930/v1/e74b33e0e2786b126cfb784a.png"},{"id":57767700,"identity":"53897384-6bef-41e6-8765-546b849f5198","added_by":"auto","created_at":"2024-06-05 11:26:36","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":72453,"visible":true,"origin":"","legend":"\u003cp\u003ePlasma plume and melting pool size for different defocusing conditions.\u003cstrong\u003e (a). \u003c/strong\u003eVariation of plasma plume thickness.\u003cstrong\u003e (b).\u003c/strong\u003e Variation of plasma plume area.\u003cstrong\u003e (c).\u003c/strong\u003e Variation of melting pool length.\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-4458930/v1/78920d7a0d7ead66ce0fde6e.png"},{"id":57768430,"identity":"318b8aa6-4ded-4196-bd8a-3c98e400c95e","added_by":"auto","created_at":"2024-06-05 11:34:39","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":387179,"visible":true,"origin":"","legend":"\u003cp\u003eSurface appearance of different defocused LWAM deposited layers.\u003cstrong\u003e (a).\u003c/strong\u003e Deposited layer morphology. \u003cstrong\u003e(b).\u003c/strong\u003e Top roughness.\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-4458930/v1/fecf7972e26077025f20f864.png"},{"id":57767702,"identity":"d9bbafcd-9cb7-48c7-982b-a2ae038760e8","added_by":"auto","created_at":"2024-06-05 11:26:36","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":89470,"visible":true,"origin":"","legend":"\u003cp\u003eMetallographic cross-section of the deposited layer.\u003c/p\u003e","description":"","filename":"5.png","url":"https://assets-eu.researchsquare.com/files/rs-4458930/v1/9c80654239c2e30309bf9a3d.png"},{"id":57767707,"identity":"8aaed892-4eb0-4a57-81d3-e60882209912","added_by":"auto","created_at":"2024-06-05 11:26:36","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":170414,"visible":true,"origin":"","legend":"\u003cp\u003eDimensions of the bead geometry for different defocusing amounts.\u003cstrong\u003e (a).\u003c/strong\u003e Variation of width (W), molten height (H), molten depth (D). \u003cstrong\u003e(b).\u003c/strong\u003e Variation of left and right wetting angle.\u003cstrong\u003e (c).\u003c/strong\u003eVariation of dilution rate. \u003cstrong\u003e(d).\u003c/strong\u003e Variation of molten pool area and pore area.\u003c/p\u003e","description":"","filename":"6.png","url":"https://assets-eu.researchsquare.com/files/rs-4458930/v1/b97c71a6150545bef98276cb.png"},{"id":57769057,"identity":"ac02e107-0d8f-4080-aac8-31d10bc54d7d","added_by":"auto","created_at":"2024-06-05 11:42:36","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":13254,"visible":true,"origin":"","legend":"\u003cp\u003eSpot size and power density at different defocusing amounts.\u003c/p\u003e","description":"","filename":"7.png","url":"https://assets-eu.researchsquare.com/files/rs-4458930/v1/707fa5cccd32a03049bcb53f.png"},{"id":57768429,"identity":"cb23f7e2-1e39-4c1e-863e-9ad503ac6dbd","added_by":"auto","created_at":"2024-06-05 11:34:37","extension":"png","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":20759,"visible":true,"origin":"","legend":"\u003cp\u003eThe interaction schematic of the laser and melting pool.\u003c/p\u003e","description":"","filename":"8.png","url":"https://assets-eu.researchsquare.com/files/rs-4458930/v1/421028a4482b8308cb99f9f9.png"},{"id":63246038,"identity":"60a4a446-af73-4f66-9db5-eb33e3b7b974","added_by":"auto","created_at":"2024-08-26 05:50:48","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":2007966,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-4458930/v1/7fe7dd5d-722a-4bf0-8141-487b2edcad59.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"Effect of Defocusing Variation on Laser Wire Additive Manufacturing Morphology and Plasma Plume Change","fulltext":[{"header":"Introduction","content":"\u003cp\u003eMetal Additive Manufacturing (MAM) technology has received significant attention from countries around the world due to a series of advantages such as moldless free-form, personalized customization, and a high degree of automation. As a type of MAM, laser wire additive manufacturing (LWAM) has attracted wide attention due to its advantages of high molding efficiency, fast cooling rate, low heat input deformation, and pollution-free production process. As the current energy source for laser additive manufacturing, focused Gaussian beams are widely used in metal laser AM devices. In contrast, the concentrated energy distribution, high thermal gradient, and instability of the molten pool can lead to unsatisfactory molding quality of additive parts during processing, resulting in defects such as cracks, pores, and undercuts. Additionally, LWAM deposited specimens exhibit poor wettability and spreading, which collectively diminish the mechanical properties and fatigue life of the metal components\u003csup\u003e1\u0026ndash;3\u003c/sup\u003e。\u003c/p\u003e \u003cp\u003eYouyu Su et al.\u003csup\u003e4\u003c/sup\u003e concluded that pore defects deposited by laser directed energy deposition is the biggest factor affecting the overall performance of the component. Hu Bin et al.\u003csup\u003e5\u003c/sup\u003e used a specific wide-band shaped laser spot for deposition and found that no porosity existed within the deposited specimens. Some authors suggest that changes in the amount of defocusing can effect deposition molding control. Farui Du et al.\u003csup\u003e6\u003c/sup\u003e showed that if the height increment is too low the laser is in negative defocus and the beam diameter will gradually increase, leading to surface defects in thin-walled parts and the manufacturing process cannot continue. Zhaoyang Liu et al.\u003csup\u003e7\u003c/sup\u003e found that a defocusing amount of -3 mm is favorable for the continuity of epitaxial growth of columnar dendrites deposited by laser powder deposition of single-crystal high-temperature alloys. Junhui Li et al.\u003csup\u003e8\u003c/sup\u003e deposited laser-fed powder additive manufacturing specimens without the presence of porosity by varying the amount of defocus to increase the laser spot diameter. Christian Bernauer et al.\u003csup\u003e9\u003c/sup\u003e experimented with different processes of 316L laser fusion by varying the focal position of the laser beam to obtain a 2.75 mm diameter annular laser spot and found that the volume of the deposited weld channel increased significantly at higher melt pool temperatures, however, the overall hardness of the melt pool was rather lower. Yanhua Zhao et al.\u003csup\u003e10\u003c/sup\u003e, on the other hand, argued that the variation of the defocusing amount can change the convergence and divergence of the beam, and large defocusing amount has a more uniform energy density distribution. G.F. Dong et al.\u003csup\u003e11\u003c/sup\u003e optimized the process parameters for laser fed powder melting of TiNiTa functional coatings by orthogonal tests and concluded that the optimum process parameters were found at 10 mm defocus.\u003c/p\u003e \u003cp\u003eThe highly concentrated laser beam irradiated on the material surface during the LWAM process leads to rapid melting and evaporation of the substrate, generating large amounts of metal vapor and plasma plumes. As a physical phenomenon inherent to the laser additive process, the characteristic behavior of the plasma plume is closely related to the deposition process. Wencong Qiu et al.\u003csup\u003e12\u003c/sup\u003e carried out the observation of the welding process at different laser powers and found that the shape of the plasma plume for the heat conduction welding process was much smaller than penetration welding and the plasma plume fluctuations were smoother. Jie Li et al.\u003csup\u003e13\u003c/sup\u003e investigated the plasma plume in laser welding by using an image processing method, and concluded that the plasma plume was generated due to the intersection of the front wall surface of the keyhole and the laser beam. Min Li et al.\u003csup\u003e14\u003c/sup\u003e used a multi-imaging method to observe the kinetic behavior of the plasma plume and keyhole simultaneously. The inclination angle of the plume is the largest when the laser-induced vapor was close to the orifice, and the inclination angle of the plume gradually decreased as the laser-induced vapor moved toward the bottom of the front wall of the locking orifice.\u003c/p\u003e \u003cp\u003eAt the present stage, studies are mostly focused on analyzing the effect of the change of focusing amount on the molten pool morphology and plasma plume in welding, however, there are relatively few specific studies on the change of focusing amount on the plasma plume in the additive manufacturing process of LWAM. Therefore, it is of great significance to explore the transient plasma plume fluctuation and the change mechanism of the deposited layer morphology during the LWAM deposition process under different process parameters, which is important for the optimization of the selection of the process parameters for the LWAM enhancement.\u003c/p\u003e"},{"header":"Materials and Methods","content":"\u003cp\u003eThe LWAM experiment utilized the HY-PTBZGQ-1000 fiber laser additive manufacturing system, as illustrated in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e(a). This system comprises the MFSC-1000X fiber laser, the DI-2000L oscillation laser head, the QL-1000 wire feeder, the CNC coordinate table, and the control system. The fiber laser has a maximum power of 1000 W, operates at a wavelength of 1080 nm, the focal length of the laser head is 200mm and the minimum spot diameter is 0.19 mm. In this experiment, 0.8mm diameter 316L stainless steel wire was chosen for LWAM on a 50 mm*50 mm*8 mm 316 stainless steel substrate. Refer to Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e for the material composition details:\u003c/p\u003e \u003cp\u003eBefore conducting the experiment, the oxidized layer of the deposited substrate was removed using a grinder and cleaned with alcohol. The wire feeder was positioned in front of the laser, and the wire feeding angle was 60\u0026deg;. The spacing between the laser and the welding wire is set at 0.2 mm on the substrate plane. And Ar gas flow rate was set of 15 L/min for protective gas during LWAM.\u003c/p\u003e \u003cp\u003eThe German BASLER camera was used for high-speed video recording of the plasma plume in the process of LWAM, and the camera shooting frequency of 340 HZ. The camera was placed horizontally on the right side of the deposition track, and the plasma plume photographic site after adjusting the aperture and focus is shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e(b):\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe process parameters of the laser wire melting process are laser power 450 W, scanning speed 2 mm/s, wire feeding speed 6 mm/s. The variation values of the defocusing amount were 0 mm, 2.5 mm, 5 mm, 7.5 mm, and 10 mm. Each group of process experiments was repeated three times, and the values of the morphology parameters were the average of the three measurements.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eMaterial composition of welding wire and substrate.\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"11\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c7\" colnum=\"7\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c8\" colnum=\"8\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c9\" colnum=\"9\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c10\" colnum=\"10\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c11\" colnum=\"11\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eC\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eSi\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eMn\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eS\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c6\"\u003e \u003cp\u003eP\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c7\"\u003e \u003cp\u003eCr\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c8\"\u003e \u003cp\u003eNi\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c9\"\u003e \u003cp\u003eMo\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c10\"\u003e \u003cp\u003eN\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c11\"\u003e \u003cp\u003eCu\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e316L wire\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e0.022\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.42\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e1.89\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e0.007\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e0.009\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e19.12\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c8\"\u003e \u003cp\u003e12.62\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c9\"\u003e \u003cp\u003e2.62\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c10\"\u003e \u003cp\u003e0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c11\"\u003e \u003cp\u003e0.34\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e316 substrate\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e0.043\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.46\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e1.12\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e0.003\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e0.027\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e18.15\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c8\"\u003e \u003cp\u003e10.01\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c9\"\u003e \u003cp\u003e2.1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c10\"\u003e \u003cp\u003e0.031\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c11\"\u003e \u003cp\u003e0\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003eFirst the LWAM specimens were treated with a stainless steel pickling passivation paste to eliminate the oxidized surface coating. Subsequently, the topography of the bead surface was documented using a Canon 70D DSLR camera, while the KEYENCE laser topography meter LJ-X8000A performed three-dimensional topography scanning under different laser oscillation modes. Following this, specimens were line-cutted along the hump position, and the cutting surfaces were subjected to metallographic mosaic. The specimens were then ground, polished, and corroded in 316L special corrosion solution to observe the molten pool. The corrosion solution had a ratio of hydrofluoric acid: hydrochloric acid: nitric acid: water\u0026thinsp;=\u0026thinsp;1:2:3:4, which allowed for clear observation of the demarcation line of the melting pool post-corrosion. Furthermore, metallographic observation of the deposited specimen sections was carried out using an optical microscope (OM), and data extraction from the images was conducted using Image J software.\u003c/p\u003e"},{"header":"Experimental results","content":"\u003cdiv id=\"Sec4\" class=\"Section2\"\u003e \u003ch2\u003ePlasma plume variation\u003c/h2\u003e \u003cp\u003eFor easier observation, the plasma plume fluctuation photos of the LWAM deposition process in each group of defocused amount were first selected, and the six photos with the largest plasma plume fluctuation were collected respectively. The plasma plume fluctuation of the LWAM process in different defocused cases was summarized as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e:\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eAs shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e, the change in the amount of defocus has a large impact on the plasma plume shape, the plasma plume was generated in the laser irradiation position at the front end of the melting pool located near the side of the wire, and the overall shape presented an eruption shape that bottom narrow and top wide. By observing the transient melting pool (the highlighted part at the bottom of the plasma) and the plasma fluctuation, it can be found that the length of the melting pool and the size of the plasma plume were relatively stable when the defocusing amount of the laser beam was 0 mm. When the defocusing amount was 2.5 mm, the plasma plume fluctuated with a stable melting pool length, and the plasma plume appeared to have a significant downward pressure at the top and a slight deflection to both sides. The melting pool length and plasma volume thickness fluctuated drastically between 5 mm and 7.5 mm, and the length of the melting pool was shorter when the plasma plume was vertically upward and longer when the plasma plume was deflected. The melting pool length and the plasma plume tend to be stabilized when the defocusing amount was 10 mm. By observing the transfer condition of the melting pool: the transition between the melting wire and the melting pool was a stable liquid bridge transition, and the transition of the melting pool was not affected by changing the value of the amount of defocusing. By observing the molten pool spattering: when the defocusing amount was 0 mm and 2.5 mm, there was a large amount of spattering in the process of paraxial fusion, and when the laser defocusing amount was raised above 5 mm, the spattering decreased to no spattering, which proves that the change of defocus can effectively eliminate the spattering.\u003c/p\u003e \u003cp\u003eA clear and visible plasma plume contour can be obtained by using PS software to colorize and sharpen the plasma plume photos. The thickness (H), area (S) and melt pool length (L) features of the plasma plume are extracted in ImageJ software. The continuous photo shooting of 15 photographs was selected to measure the plasma plume and melting pool length under different defocusing amounts in the LWAM process, and the shooting interval is 15ms for each photo. Based on the measurement results, the mean values and fluctuations of the plasma plume and melting pool sizes for different cases of defocusing are calculated, and the error bar graphs are plotted as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e:\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eBased on the mean value analysis shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e(a) and Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e(b), the plasma plume has the lowest thickness and the smallest area when the defocusing amount was 0 mm, and the plume thickness and area show the first increasing and then decreasing trend with the increase of defocusing amount. The average thickness of the plasma plume was largest when the defocusing amount was 5 mm, and the average area of the plasma plume was largest when the defocusing amount was 2.5 mm. Based on the analysis of the error bars shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e(a) and Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e(b), the plasma plume fluctuation also showed a first increasing and then decreasing trend, when the amount of defocusing was 0mm and 10mm, the plasma plume fluctuation was not obvious, and when the amount of defocusing was from 2.5mm to 7.5mm, the melting pool fluctuation is larger. According to the analysis of the average length of the melting pool shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e(c), the length of the melting pool was the shortest when the amount of defocusing was 0 mm, and when the amount of defocusing was raised to 2.5 mm, the length of the melting pool showed a sudden change and steep increase. With the further increase in the amount of defocusing, the average length of the molten pool showed a linear decreasing trend; when the amount of defocusing was raised to 10mm, the length of the molten pool was the shortest.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec5\" class=\"Section2\"\u003e \u003ch2\u003eSurface morphology of the deposited layer\u003c/h2\u003e \u003cp\u003eThe topographic picture of the deposited layer surface and the 3D topography measured by the laser scanner are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e(a). The top line roughness measurements were taken for the specimens deposited with different amounts of defocusing and the variations are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e(b):\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eAs shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e\u003cb\u003e(a)\u003c/b\u003e, the variation of the defocusing amount affects the surface accuracy and morphology of the deposited layer. The surface flatness of the deposited layer decreased with the increase of the defocusing amount, the surface of the deposited layer was very smooth with the distribution of fine fish scale pattern when the defocusing amount was 0 mm and 2.5 mm, and then it turned into a slight hump protrusion when the defocusing amount was increased to 5 mm. When the defocusing amount was increased to 7.5 mm, the overall surface of the deposited layer was relatively worse, and the overall morphology was a periodic fluctuating hump with unfused defects on both sides of the deposited layer. When the defocusing amount was increased to 10 mm, the deposited layer could not spread, and the unfused defects became more serious. Moreover, the wire was sticking at the end of the welding channel. Combined with the roughness measured in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e(b), it showed that the surface roughness of the deposited layer gradually increased with the increase of laser defocus due to the gradual increase of the height difference formed by the top fish scale pattern and the hump.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec6\" class=\"Section2\"\u003e \u003ch2\u003eMolten pool cross-section morphology\u003c/h2\u003e \u003cp\u003eThe cross-sectional morphology of the LWAM deposition specimens with different defocusing amounts is shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eAs shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e, by comparing the top morphology of the molten pool, it is found that the curvature of the top of the molten pool was small when the defocusing amount was less than 5 mm. When the defocusing amount was 7.5 mm, the contour of bead geometry showed a hemispherical contour, and when the defocusing amount was 10 mm, the contour of bead geometry showed a spherical shape. When observing the bottom profile of the molten pool cross-section, it appeared sharp protrusions when the amount of devitrification was 0-2.5mm, and the protrusions at the bottom of the molten pool gradually weakened with the increase of defocusing amount. The protrusion of the molten pool bottom tends to be horizontal when the amount of defocusing was 5mm. As the amount of devitrification continued to increase, the bottom of the molten pool showed no transition step protrusion. Simultaneously, when the defocusing amount was 0-2.5mm, there were pores at the bottom of the molten pool, which are mainly distributed in the protruding parts.\u003c/p\u003e \u003cp\u003eImage J software was used to extract the metallographic dimensions of the cross-section of the bead in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e. The average values of molten width (W), molten height (H), molten depth (D), left and right wetting angles, molten pool dilution rate, molten pool area (S), and porosity of the deposited layer (P) for different defocusing cases are plotted as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e:\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eAs shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e(a) for the variation of the bead geometry, the molten width showed a decreasing trend with the increase of the laser defocusing amount, which decreased slowly at the defocusing amount of 0\u0026ndash;5 mm and decreased rapidly at the amount of 5\u0026ndash;10 mm. The change of molten height is opposite to that of molten width, showing an upward trend with a sudden increase at 5 mm. The changes in molten depth and dilution rate also show an overall decreasing trend with the increase of defocusing amount, but the inflection point of the plunge was located at the defocusing amount of 7.5mm. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e(b), the change in the wetting angle of the deposited layer was not obvious when the defocusing amount was located between 0\u0026ndash;5 mm, and when the value of the defocusing amount was higher than 5 mm, there is a significant decreasing trend. The wetting angle was greater than 90\u0026deg; when the defocusing amount value was less than 7.5 mm, whereas when the defocusing amount value increased to 10 mm, there were unfused defects on both sides of the sedimentary layer with wetting angle less than 90\u0026deg;. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e(c), the dilution rate showed a decreasing trend with the increase of the laser defocusing amount. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e(d) the variation of melt pool area showed a linear decreasing trend with the variation of defocusing amount, the larger defocusing amount was the smaller molten pool area was.\u003c/p\u003e \u003c/div\u003e"},{"header":"Discussion","content":"\u003cp\u003eThe molten pool heat of the LWAM process depends on the intensity of Fresnel absorption inside the molten pool, which in turn depends on the input intensity of the laser Gaussian beam and the reflection of the beam on the inner wall of keyhole. The intensity of the laser energy input is, in turn, dependent on the laser beam divergence and the spot size of the laser irradiation. Combined with the laser divergence and the spot size change caused by the defocusing amount shift, the laser energy density can be calculated by the following Eq.\u0026nbsp;1\u003csup\u003e5\u003c/sup\u003e:\u003cdiv id=\"Equ1\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ1\" name=\"EquationSource\"\u003e\n$$PD=P/(\\pi *{\\omega ^2})$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e1\u003c/div\u003e\u003c/div\u003e\u003cdiv id=\"Equ2\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ2\" name=\"EquationSource\"\u003e\n$$\\omega (z)={\\omega _0}\\sqrt {1+{{(\\frac{z}{{{z_r}}})}^2}}$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e2\u003c/div\u003e\u003c/div\u003e\u003cdiv id=\"Equ3\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ3\" name=\"EquationSource\"\u003e\n$${z_r}=\\frac{{\\pi {\\omega _0}^{2}}}{\\lambda }$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e3\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003eIn the above equation, PD is the laser energy density, P is the laser output power, ω is the laser spot size radius, ω0 is the minimum laser spot radius at focus, z is the defocusing amount, Zr is the Rayleigh length, and λ is the laser wavelength. The variation of laser spot size and power density with the defocusing amount is shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e:\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eAs shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e, the laser irradiation spot size was determined by the defocusing amount, the increased defocusing amount leads to the increase of spot area and the decrease of laser power density. The increase in spot area improved the keyhole opening area of the molten pool, which was helpful for gas escaping from the keyhole to reduce the porosity defects in the LWAM process. At the same time, the reduction of laser irradiation energy density weakened the recoil pressure formed by the melting pool evaporation, which helped to eliminate the melting pool spattering. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e, the spattering in the melting pool was serious when the defocusing amount was less than 2.5mm, and the spattering was obviously weakened when increasing the defocusing amount.\u003c/p\u003e \u003cp\u003eIn practice, when the laser light travels through the plasma plume, there is a diminishment of the laser energy due to the inverse bremsstrahlung absorption of the plasma plume. According to the Beer-Lambert law of laser energy attenuation\u003csup\u003e15\u003c/sup\u003e, the thicker the plasma plume is, the stronger the laser attenuation is. When the plasma plume is too thick, the energy absorption in the melting pool is reduced, and the energy absorption in the melting pool affects the evaporation of the internal solution, which in turn weakens the plasma plume growth, hence leading to periodic fluctuations in the plasma plume. The larger the plasma plume thickness, the more intense the plasma plume fluctuation. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e, the plasma plume fluctuation was the most intense when the defocusing amount was 5 mm and 7.5 mm, and the plasma plume thickness was the largest.\u003c/p\u003e \u003cp\u003eThe interaction schematic of the laser and melting pool under defocusing is shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eAs shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e, the laser is reflected downward after irradiating the inner wall of the keyhole. The smaller the defocusing amount is, the more concentrated the energy converging at the center of the keyhole. The highly concentrated laser beam is reflected many times by the inner wall of the keyhole, which enhances the Fresnel absorption of the melting pool. The continuous absorption of energy by the molten pool further deepens the depth of the keyhole, which makes it difficult for gas escape during LWAM, and it is more likely to be sealed inside the molten pool to form porosity defects. As the defocus increases the spot focus converges above the substrate and the reduction in reflection leads to a decrease in the Fresnel absorption efficiency of the laser energy by the keyhole wall. The energy absorbed by the melting pool is not sufficient to prompt a complete spreading of the liquid pool, which leads to an increase in the molten height and a decrease in the molten width of the bead.\u003c/p\u003e \u003cp\u003eIn summary, based on the surface accuracy of the deposited layer, it is showed that an increase in the defocusing amount led to a decrease in the surface accuracy of the deposited layer, and that excessive defocusing led to unfused defects in the deposited layer. Depending on the porosity defects in the molten pool and the spattering during deposition, an increase in the defocusing amount can help to reduce the porosity defects and decrease the spattering during the LWAM process. At the same time, by observing the metallography of the deposited layer cross-section, when the defocusing amount was 5 mm, the deposited layer cross-section has a large melting width size and the dilution rate of the melting pool was close to 0.5, which can be effectively fused with the substrate. Therefore, the defocusing amount of the LWAM should be selected as 5 mm to ensure the maximum molding accuracy.\u003c/p\u003e"},{"header":"Conclusion","content":"\u003cp\u003e(1) The increase of defocusing amount leads to the increase of spot area and the decrease of laser power density, which reduces the recoil pressure formed by the evaporation of the melting pool and can effectively improve the spattering situation during the deposition process.\u003c/p\u003e\n\u003cp\u003e(2) The larger the plasma plume thickness is, the more violent the plasma plume fluctuation is. The plasma plume thickness, area and fluctuation showed a first increase and then decrease trend with the increase of defocusing amount.\u003c/p\u003e\n\u003cp\u003e(3) Variations of defocusing amount effect the morphology of the deposited layer. With the increase of laser defocusing amount the surface roughness gradually increased, and the hump morphology changed from the initial smooth fish scale pattern to periodic fluctuations, when the defocusing amount up to 10 mm the deposited layer did not spread smoothly and there were serious unfused defects.\u003c/p\u003e\n\u003cp\u003e(4) The increase of defocusing amount affects the energy absorption of molten pool, the larger defocusing amount is the smaller the molten pool area is. The reduction of keyhole depth is helpful for the gas escape from keyhole during the LWAM process.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eAcknowledgments\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by the National Natural Science Foundation of China (51375146); Ministry of Industry and Information Technology of the People\u0026rsquo;s Republic of China (2018ZNZX01-02) and Natural Science Foundation of Henan Province (17A460012).\u003cstrong\u003e\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthor Contributions\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eC. Y. wrote the paper and analyzed the experimental results, Professor H. H. designed the experiment. P. Z., R. W., and J. H. completed the experiment.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCompeting Interests\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eData availability\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe datasets used and/or analysed during the current study available from the corresponding author on reasonable request.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n \u003cli\u003eT. U. Tumkur et al., \u0026quot;Nondiffractive beam shaping for enhanced optothermal control in metal additive manufacturing,\u0026quot; \u003cem\u003eSci. Adv.\u0026nbsp;\u003c/em\u003e\u003cstrong\u003e7\u003c/strong\u003e(38), (2021). https://doi.org/10.1126/sciadv.abg9358.\u003c/li\u003e\n \u003cli\u003eW. 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Bean et al., \u0026quot;Effect of laser focus shift on surface quality and density of inconel 718 parts produced via selective laser melting,\u0026quot; \u003cem\u003eAddit. Manuf.\u0026nbsp;\u003c/em\u003e\u003cstrong\u003e22\u003c/strong\u003e(207-215 (2018). https://doi.org/10.1016/j.addma.2018.04.024.\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"Laser Wire Additive Manufacturing, Defocusing amount, Plasma plume, Bead geometry","lastPublishedDoi":"10.21203/rs.3.rs-4458930/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-4458930/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eTo examine the impact of defocusing amount on the deposition accuracy in the Laser Wire Additive Manufacturing (LWAM) process, a one-factor variable experiment was designed. This experiment analyzed the plasma plume geometry, melting pool spatter, and melting pool length, along with observing bead geometry and porosity. The following conclusions are drawn: Variations in defocusing amount caused changes in energy distribution, which affected the stability of the LWAM process and led to changes in the surface accuracy of the deposited layer and the melt pool morphology. Increasing the defocusing amount enlarged the spot area, diminished laser power density, and effectively reduced spattering during LWAM. Surface roughness increased with defocusing, transforming the hump morphology from an initial smooth fish-scale pattern to a periodic undulating pattern. Larger defocusing decreases the melt pool area and keyhole depth, this reduction aids gas escape from the keyhole, thus mitigating porosity defects in LWAM.\u003c/p\u003e","manuscriptTitle":"Effect of Defocusing Variation on Laser Wire Additive Manufacturing Morphology and Plasma Plume Change","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2024-06-05 11:26:31","doi":"10.21203/rs.3.rs-4458930/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"22c304a9-fa84-45d9-bc6c-54863e66851e","owner":[],"postedDate":"June 5th, 2024","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[{"id":32755804,"name":"Physical sciences/Optics and photonics/Lasers leds and light sources/Fibre lasers"},{"id":32755805,"name":"Physical sciences/Engineering/Mechanical engineering"}],"tags":[],"updatedAt":"2024-08-26T05:42:40+00:00","versionOfRecord":[],"versionCreatedAt":"2024-06-05 11:26:31","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-4458930","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-4458930","identity":"rs-4458930","version":["v1"]},"buildId":"qtupq5eGEP_6zYnWcrvyt","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}