Impact of beam oscillation and power modulation on the intermixing behavior of dissimilar Titanium / Niobium / Nitinol joints during micro electron beam welding

Impact of beam oscillation and power modulation on the intermixing behavior of dissimilar Titanium / Niobium / Nitinol joints during micro electron beam welding | 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 Impact of beam oscillation and power modulation on the intermixing behavior of dissimilar Titanium / Niobium / Nitinol joints during micro electron beam welding Michael Wiegand, Johannes-Seneca Wolfgang Fritz Loose, Martin Kahlmeyer, and 2 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-5245806/v1 This work is licensed under a CC BY 4.0 License Status: Under Review Version 1 posted 5 You are reading this latest preprint version Abstract Nickel-titanium shape memory alloys (NiTi) as well as titanium alloys (Ti) are essential materials in various modern medical technology applications. Combining them in functionally-graded components would allow the fabrication of highly innovative products with major economic and technical advantages. While dissimilar fusion welding of these materials is not feasible due to the formation of brittle intermetallic compounds, recent studies have shown that niobium (Nb) is a very promising filler material to overcome this limitation while simultaneously maintaining the biocompatibility of welded components. The present study seeks to expand the current knowledge regarding dissimilar fusion welding of the material combination NiTi / Nb / Ti by investigating micro electron beam welding in a butt-joint configuration. In addition to adapted power modulation, a novel approach of utilizing the process-inherent fast beam oscillation is applied to optimize the melting and intermixing behavior of the comparatively high-melting Nb. Furthermore, two different dimensions of the filler material measuring 0.2 and 0.4 mm in thickness are implemented and compared with regard to the microstructural evolution in the weld metal. It is demonstrated that the welding experiments are associated with major challenges due to the considerable differences in melting temperature and thermal conductivity of the base and filler materials. Nevertheless, the welded joints exhibit excellent mechanical properties under quasi-static tensile load, which can be attributed to a reduced formation of Ti 2 Ni intermetallic compounds. Ultimate tensile strengths of up to 673 MPa can be achieved, proving that micro electron beam welding is a suitable process to produce high-quality dissimilar NiTi / Nb / Ti joints. Electron beam welding dissimilar welding medical technology intermetallic compounds microjoining Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 1 Introduction and state-of-the-art The dissimilar welding of metals allows individual material qualities to be combined in functionally-graded components. Thus, drastic improvements can be achieved in terms of cost-effectiveness, functionality, and manufacturing conditions [1, 2]. Especially hybrid components made of nickel-titanium shape memory alloys (NiTi) and titanium alloys (Ti) offer great potential in this respect. Due to their excellent mechanical properties, good corrosion resistance as well as outstanding biocompatibility, both materials are used in similar application areas, e.g., aerospace, energy industry, and medical technology [3–9]. However, as NiTi is a high-priced and difficult-to-machine material, there is growing interest in local substitution with Ti using a robust joining process. While similar welding of the mentioned materials is generally possible, e.g., by means of beam welding processes, dissimilar fusion welding of NiTi to Ti is restricted by their chemical incompatibilities. The materials are characterized by low solubility in solid state and the formation of the intermetallic compounds (IMC) Ti 2 Ni and TiNi 3 [10]. These IMC are characterized by a brittle, ceramic-like behavior and significantly impair both the weldability and the mechanical properties of fabricated joints. Another fundamental challenge in dissimilar fusion welding arises from differing thermophysical properties, such as melting temperatures, thermal conductivities, and thermal expansion rates, as these further increase the susceptibility of solidification cracking due to increased stress conditions during cooling and the emergence of residual stresses [11, 12]. Owing to the aforementioned limitations, previous studies on autogenous beam welding of NiTi to Ti have achieved very poor joining properties. Accordingly, no defect-free or mechanically resilient samples could be welded using either pulsed Nd:YAG laser [11] or Yb fiber laser [12, 13]. All welded joints are characterized by severe solidification cracking, caused specifically by the formation of Ti 2 Ni IMC and the resulting embrittlement in the weld metal. A promising approach to improve both, chemical compatibility and mechanical performance during dissimilar fusion welding, is the implementation of filler materials. As a result, Teshome et al. were able to achieve significantly improved joining properties by using zirconium [14], cobalt ADDIN 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UiOiJKb3VybmFsQXJ0aWNsZSIsIlNob3J0VGl0bGUiOiJUZXNob21lLCBQZW5nIGV0IGFsLiAyMDIyIOKAkyBNaWNyb3N0cnVjdHVyZSIsIlNob3J0VGl0bGVVcGRhdGVUeXBlIjowLCJTb3VyY2VPZkJpYmxpb2dyYXBoaWNJbmZvcm1hdGlvbiI6IkNyb3NzUmVmIiwiU3RhdGljSWRzIjpbImM4NjkyZDA2LTc1OGUtNGM1YS1hNDUyLTUwYjUxZDNmMGIyNyJdLCJUYWJsZU9mQ29udGVudHNDb21wbGV4aXR5IjowLCJUYWJsZU9mQ29udGVudHNTb3VyY2VUZXh0Rm9ybWF0IjowLCJUYXNrcyI6W10sIlRpdGxlIjoiTWljcm9zdHJ1Y3R1cmUsIE1hY3Jvc2VncmVnYXRpb24sIGFuZCBNZWNoYW5pY2FsIFByb3BlcnRpZXMgb2YgTmlUaSB0byBUaTZBbDRWIERpc3NpbWlsYXIgTGFzZXIgV2VsZHMgVXNpbmcgQ28gSW50ZXJsYXllciIsIlRyYW5zbGF0b3JzIjpbXSwiVm9sdW1lIjoiMzEiLCJZZWFyIjoiMjAyMiIsIlllYXJSZXNvbHZlZCI6IjIwMjIiLCJDcmVhdGVkQnkiOiJfTWljaGFlbCBXaWVnYW5kIiwiQ3JlYXRlZE9uIjoiMjAyMy0wMy0zMVQwOToxNzoxOSIsIk1vZGlmaWVkQnkiOiJfVWswMjE5MDciLCJJZCI6IjFjMjA1OTljLTQ4YWEtNGViZC1iY2RjLTZjZmE3NTk5ODg4ZSIsIk1vZGlmaWVkT24iOiIyMDI0LTA2LTE0VDE1OjI3OjEyIiwiUHJvamVjdCI6eyIkcmVmIjoiNSJ9fSwiVXNlTnVtYmVyaW5nVHlwZU9mUGFyZW50RG9jdW1lbnQiOmZhbHNlfV0sIkZvcm1hdHRlZFRleHQiOnsiJGlkIjoiMTYiLCJDb3VudCI6MSwiVGV4dFVuaXRzIjpbeyIkaWQiOiIxNyIsIkZvbnRTdHlsZSI6eyIkaWQiOiIxOCIsIk5ldXRyYWwiOnRydWV9LCJSZWFkaW5nT3JkZXIiOjEsIlRleHQiOiJbMTVdIn1dfSwiVGFnIjoiQ2l0YXZpUGxhY2Vob2xkZXIjOGM0Njc4YTgtMTMxZi00OWQxLWExZTQtMzZjODk0NWVkZDU4IiwiVGV4dCI6IlsxNV0iLCJXQUlWZXJzaW9uIjoiNi4yLjAuMTIifQ==} [15], and palladium [16] for dissimilar welding of NiTi to Ti. In the latter, comparatively high tensile strengths of up to 520 MPa were achieved, representing a major increase compared to autogenous beam welding of NiTi to Ti. Furthermore, the implementation of a copper foil can also improve the weld seam quality of NiTi / Ti joints, resulting in tensile strengths of up to 300 MPa [11]. However, it should be noted that in all studies mentioned above, the weld seams invariably represent a weak point under mechanical load. In addition to the aforementioned materials, several studies have been carried out using niobium (Nb) as filler material for dissimilar NiTi to Ti fusion welding. Due to its good chemical compatibility with the Ti base material and its equally excellent biocompatibility, Nb is particularly interesting for the material combination [17, 18]. Consequently, there is great potential for dissimilar joints to be directly adopted in medical technology applications if the fusion welding of mechanical resilient parts can be realized in a process-safe manner. In the investigation by Oliveira et al. [19], it was demonstrated that Nb can be used as a diffusion barrier between Ti and NiTi and, due to a reduced IMC formation, tensile strengths of up to 300 MPa can be achieved. Moreover, Wiegand et al. [20] have shown that Nb can also be successfully implemented as an intermediate layer during multi-pass micro electron beam welding, thus, achieving complete metallurgical separation of the base materials. In comparison with hafnium and tantalum, which were also considered in this publication, niobium was found to have the best joining properties with NiTi as well as Ti. During quasi-static tensile tests, the corresponding specimens failed reproducibly in the un-molten filler material at tensile strengths of 453 MPa, confirming the great compatibility of the materials. However, as the filler material has already reached its strength limit, further improvements in joint strength are restricted with this approach. In a very recent study by Wiegand et al. [21], it was proven that controlled intermixing of Nb into a combined NiTi / Nb / Ti butt-joint by means of pulsed laser beam welding can overcome the limitation of the comparatively poor mechanical properties of Nb. However, this method requires very specific parameter optimization to ensure that a high volume proportion of Nb is mixed into the joint weld metal in order to reduce the formation of IMC. The associated specimens fail in the Ti base material and achieve ultimate tensile strengths (UTS) of up to 679 MPa as well as elongations at break of 18.9 %. As these welded joints can fully utilize the superelastic plateau of NiTi, the results represent a significant improvement of the current state-of-the-art in the field of dissimilar fusion welding of NiTi to Ti. The present study complements the aforementioned investigations of the material combination Ti / Nb / NiTi and investigates the feasibility of continuous micro electron beam welding in butt-welding configuration. Due to the high-vacuum atmosphere of up to 10 -5 mbar in the working chamber, the welding process ensures optimum protection of the highly reactive materials [20]. In addition, the various process parameters allow precise control of the melt pool, which makes this process predestined for welding of dissimilar materials [20, 22]. Comparatively high welding speeds of 40 mm/s and specifically adapted beam currents are used to achieve a focused energy input that allows fusion of the comparatively high-melting Nb. Furthermore, a novel approach of utilizing the process-inherent fast beam oscillation is applied to tailor the energy input in the welding area in order to optimize the melting and intermixing behavior of the refractory metal within the weld metal. The microstructural evolution of the weld seams is compared using scanning electron microscopy (SEM) equipped with a back-scattered electron detector (BSD). In order to provide precise details on the chemical distribution in the weld metal, supplementary energy-dispersive X-ray spectroscopy (EDS) mappings are carried out with regard to the elements Ti, Nb and nickel (Ni). Finally, the findings regarding the microstructural evolution are correlated with the resulting mechanical properties by means of tensile tests and nanoindentation analyses. 2 Materials and methods The welding experiments were performed using thin sheet geometries measuring 12.5 x 25 x 0.25 mm³ of superelastic NiTi as well as commercially pure titanium (cp-Ti, grade 4). The chemical composition of the applied materials is depicted in Table 1. Table 1 Chemical composition of cp-Ti grade 4 (a) and NiTi (b) as specified by the material suppliers a) Element (wt.-%) Ti C N Fe O bal. 0.011 0.010 0.12 0.31 b) Element (wt.-%) Ni Ti C O Fe other single trace elements 55.94 bal. 0.0275 0.019 0.014 < 0.01 As filler material, pure Nb (99.9%) strips with the dimensions 12.5 x 0.25 mm 2 and a thickness of 0.2 mm and 0.4 mm respectively were implemented and positioned between the joint edges of the base materials. Table 2 summarizes important thermophysical and mechanical properties of the investigated materials. Table 2 Selected thermophysical and mechanical properties of the respective materials used in this investigation, adopted from [21]. Values are given for a temperature of 20°C. The mechanical properties (*) were determined by quasi-static tensile tests of the experimental materials Materials Melting point [°C] Ultimate tensile strength * [MPa] Elongation at break * [%] Thermal conductivity [Wm -1 K -1 ] Coefficient of thermal expansion [10 -6 K -1 ] Superelastic plateau * [MPa] cp-Titanium (grade 4) 1660 ~ 679 ~ 23 18 8.6 / NiTi (55.8 wt.-% Nickel) 1310 ~ 1357 ~ 12 18 11 ~ 400 - 500 Niobium (99.9 %) 2468 ~ 470 ~ 5 52 7.1 / All geometries were produced using electrical discharge machining (EDM) to ensure high dimensional accuracy and thermally unaffected material properties. In order to remove the process-related residues as well as to create optimal surface conditions, the joining edges of the base materials were ground using P2500 grade silicon-carbide paper after EDM. Due to the very small geometry of the Nb strips, mechanical processing was restricted, which is why they were treated with a chemical solution to remove any copper-containing deposits caused by EDM that could potentially affect the welding result. The solution was produced under the following conditions: Aqueous phase (1 L) containing 0,2 mol/L H 2 O 2 , 0,3 mol/L HCL and 0,5 mol/L NaCl. Prior to the welding process, the materials were cleaned using acetone and then dried in air. The welding tests were conducted using a micro electron beam welding machine (SEM108, pro-beam GmbH & Co. KGaA, Gilching, Germany as well as JSC Selmi, Sumy, Ukraine). The welding machine is equipped with a tungsten ribbon cathode and allows beam currents of up to 20 mA at an acceleration voltage of 60 kV, resulting in a maximum beam power of 1,2 kW. Furthermore, it is possible to achieve very small beam diameters of approx. 30 μm at a beam current of 1 mA. Thanks to the integrated beam control system, high deflection frequencies of up to 5 MHz can be achieved. The vacuum pumps generate a vacuum atmosphere of up to 10 -5 mbar in the working chamber, ensuring that even highly reactive materials are optimally protected from environmental influences during the welding process. To provide reproducible welding conditions with virtually no gap between the materials, the specimens were clamped in a special fixture featuring lateral and vertical clamping options, see Fig. 1(a). Furthermore, the fixture allows two samples to be clamped individually and, thus, the fabrication of two separate weld seams during one evacuation cycle. The filler material was placed manually and held in position by the applied lateral pressure. It should be emphasized that special attention was paid to ensure a precise alignment of the base and filler materials, as even minor deviations in the welding configuration can have a significant impact on the welding result. During welding, the electron beam was guided along the center of Nb, ensuring that the focused energy was being transferred into the Nb foil. Fig. 1(b) shows a photograph of the clamping configuration. Table 3 summarizes the parameters used within the scope of this study. For parameter I, the specimens were produced using a Nb strip with a thickness of 0.2 mm and a continuous electron beam without oscillation. The associated high welding speed of 40 mm/s was selected to minimize the impact of the high thermal conductivity of the filler metal and, thus, to generate small and easier-to-control weld seams. Furthermore, the beam current was systematically adjusted to provide the highest possible energy input while maintaining macroscopic defect-free weld seams. A more detailed explanation of this is provided in the subsequent chapter. For parameter II, identical parameters are used and supplemented by a beam oscillation transverse to the welding trajectory in order to tailor the energy input and ultimately to compare deviations in the intermixing behavior. Finally, parameter III involves the utilization of a Nb strip with an increased thickness to increase the proportion of Nb in the weld metal, which is expedient based on the findings of Wiegand et al. [21]. Preliminary tests on this filler material dimension have shown that it is required to adjust the parameters to a higher beam current as well as higher oscillation frequency and amplitude in order to counteract the high melting temperature and thermal conductivity of the refractory metal. It shall be mentioned in this context, that the welding tests to determine the optimal beam current in this configuration were associated either with insufficient penetration depth with burn-through defects. The beam current of 2.5 mA was finally chosen because of comparatively low defect formations, which is explained in more detail in the following chapter. Table 3 : Overview of the welding parameters used in this study Parameter Thickness of niobium [mm] Beam current [mA] Oscillation frequency [Hz] Oscillation amplitude [mm] Acceleration voltage [kV] Welding speed [mm/s] Focus Position I 0.2 1.6 - - 60 40 surface II 0.2 1.6 5000 0.1 III 0.4 2.5 10000 0.4 The welded samples were embedded in cold-curing epoxy resin, ground to the center of the weld seams using silicon-carbide paper, and subsequently polished using a diamond suspension with a grit size of 0.1 µm. Following this, the samples were etched using a hydrofluoric-acid-free etching solution according to Keller for 120 s at room temperature and analyzed using optical light microscopy (DM2700, Leica Microsystems GmbH, Wetzlar, Germany). Further in-depth analyses were carried out on a field-emission scanning electron microscope (Zeiss REM Ultra Plus, Carl Zeiss AG, Oberkochen, Germany) operating at an acceleration voltage of 15 kV. The material contrast in the weld metal was highlighted using BSD imaging in order to provide insights into the phase distributions and dendritic formations. Furthermore, the chemical composition in the weld area was determined quantitatively regarding the mass fraction by means of EDS (Bruker XFlash 6160, Bruker Corporation, MA). Nanoindentation mappings were performed using a NHT 3 testing machine (Anton Paar GmbH, Graz, Austria) with a maximum load of 10 mN and loading and unloading rates of 100 mN/s. In order to fulfill the high requirements of a plane surface finish, the samples were mechanically processed again using multiple short-time and pressure-optimized grinding and polishing steps. Since material was removed during this preparation, visible differences can be observed in the cross-section geometries compared to the images of the microstructural analyses. Tensile tests were carried out on the basis of DIN EN ISO 6892-1 with a universal testing machine (Z100, Zwick-Roell AG, Ulm, Germany) using bone-shaped tensile testing specimens, which were extracted from the center of the weld seams by means of EDM. Further information regarding the tensile specimen geometry can be found in [20]. The specimens were clamped using a preload of 5 N and subsequently tested with regard to their UTS and elongation at break. The mechanical properties were determined using three identically manufactured specimens for each set of parameters. 3 Results and discussion 3.1 Microstructural Evolution of micro electron beam welded Ti / Nb / NiTi joints Due to the high melting temperature of the pure Nb filler material, an appropriately high energy input is required to achieve sufficient melting. As mentioned in the previous chapter, a systematic parameter optimization was carried out regarding the applied beam current to provide the highest possible energy input while omitting the emergence of defects. As exemplified on representative weld seam surfaces in Fig. 2, an excessive increase of the beam current favors burn-through defects. The cause of this defect is most likely related to the drastically deviating melting temperatures of the materials. Although the beam was focused on and guided along the center of the Nb filler material, the comparatively high thermal conductivity as well as the two-dimensional heat transfer within the applied thin sheet geometries lead to a considerable energy absorption of the surrounding, lower-melting base materials. As a result, the excessive temperature results in heavy melting or even vaporization of Ti or NiTi respectively, which ultimately causes the weld pool to collapse. This demonstrates that controlling the energy input during dissimilar micro electron beam welding of materials with pronounced differences in their thermophysical properties is mandatory. The application of the three different parameter sets (cf. Table 3) resulted in strongly differing weld metal characteristics, as can be derived from Fig. 3. Although the samples welded with parameter I and II feature almost identical weld seam surface geometries, optical light microscopy analyses of the etched cross-sections reveal a considerable deviating melting behavior of the filler material. Without beam oscillation, a rather large, un-molten proportion of Nb remains in the weld metal, as highlighted by the red outline in Fig. 3(a). The locally applied intensity of the electron beam is apparently quickly dissipated due to the high thermal conductivity of the filler material and, thus, insufficient to fully melt the entire geometry. Given the fact that the beam current has already been optimized to the highest value for which defect-free welds can be produced, it can be concluded that complete melting of Nb cannot be achieved for the given welding configuration using conventional continuous electron beam welding without oscillation. In contrast, the cross-section of parameter II in Fig. 3(b) does not reveal any visible accumulations of Nb. The implemented oscillation transverse to the welding direction and the associated redistribution of the energy input to an extended area significantly improves the proportion of melted filler material. However, this also results in a wider weld seam, as can be seen from a comparison with the cross-section of parameter I. As can be observed in Fig. 3(c), the sample welded with parameter III is associated with a burn-through defect. As mentioned in the previous chapter, it was not feasible to achieve a sufficient bond between the materials and simultaneously produce defect-free weld seams using Nb with 0.4 mm thickness. This can be attributed to the increased volume of the refractory metal, which intensifies the impact of the high melting temperature and thermal conductivity. However, it is important to note that the burn-through defect was always located near the end crater and, thus, outside the area which is relevant for microstructural and mechanical characterization. Due to the location of the defect, it could also possibly be negated by the implementation of a slope-out process. To provide more detailed insights into the microstructural evolution as well as the chemical distribution in the respective weld metal of the three parameters, BSD and EDS analyses were performed. As can be derived from the distinct material contrast in Fig. 4(a), the weld metal of parameter I is characterized by an inhomogeneous phase distribution. Nb-rich dendritic structures, that are represented by the brighter material contrast, can be observed in a wide range of the weld metal, as highlighted by region 1 and depicted with higher magnification in Fig. 4(b). Nevertheless, a predominant proportion of the weld metal is characterized by darker material contrast, which represents an increased amount of Ti and Ni in the respective areas [21]. Furthermore, large regions without Nd-rich dendrites exist near the not liquefied Nb in the center of the weld metal, as highlighted by region 2. The associated EDS mapping in Fig. 4(c) provides a detailed view of the elemental distribution in the weld metal. It is evident that Nb is distributed throughout the entire weld metal but with a comparatively low mass fraction. In contrast, it can be seen that the previously detected dendrites have a considerably higher proportion of Nb. Furthermore, the elemental distribution of Ti as well as Ni also exhibits severe mixing gradients. Accordingly, the proportion of Ti is significantly higher in the range of the fusion line to the Ti base material compared to the remaining weld metal. From this, it can be concluded that the applied welding process provides insufficient time for an extensive intermixing, which can be attributed to the high welding speed and comparatively low energy input resulting in rapid cooling rates. As shown in Fig. 5(a), parameter II leads to a more uniform distribution of Nb-rich dendritic formations in the weld metal. Nevertheless, despite the improved melting behavior, the total proportion of Nb in the weld metal is still comparatively low. This can be attributed to the increased weld seam width compared to parameter I, which ultimately increases the proportion of molten and intermixed base materials. Furthermore, the existence of Nb accumulations, which were previously undiscovered in light microscopy analyses, is revealed. As can be seen in Fig. 5(b), region 1, these are dendritically solidified phases, which indicates an early solidification in the surrounding melt pool due to locally deviating melting temperatures. Consequently, the Nb filler material did not completely dissolve with the other materials. Furthermore, local deviations in the resulting microstructure occur in the area of the weld seam root. As region 2 emphasizes, multiple separate morphologic structures are present in this range. The EDS analyses in Fig. 5(c) show an increased concertation of Ni in the weld seam root, thus, explaining the varying microstructures. Irrespective of this area, the elemental distribution is improved compared to parameter I, indicating that the application of a high-frequent beam oscillation and the associated increase in melt pool dynamics improves the intermixing process during dissimilar welding. However, increased concentrations of the respective elements are still evident along the fusion lines of the base materials. BSD and EDS-Analyses of parameter III in Fig. 6(a)-(c) reveal drastic differences in the microstructural evolution of the weld metal compared to the previously discussed specimens. Despite the increased beam current and the specifically adjusted oscillation parameters, the resulting microstructure as well as elemental distribution in the weld metal is characterized by severe inhomogeneity. In addition to the aforementioned high affinity for defect formation, the increased volume of the implemented filler material evidently prevents uniform melting and intermixing on the given welding configuration. Furthermore, minor cracks are visible in the weld metal. According to the interdendritic crack pattern, which can be observed in Fig. 6(b), region 1, these can be identified as hot cracks, which arise from the presence of low-melting phases in between the high-melting Nb-rich dendrites. The surrounding microstructure in Fig. 6(b) shows a predominant formation of Nb-rich dendrites compared to the other parameters. As can be determined from the EDS mapping, the mass fraction of Nb in this area is over 60 % and, thus, in the desired range for achieving superior mechanical properties according to Wiegand et al. [21]. However, it should be emphasized that this microstructure exists only in a limited range due to the poor mixing in the weld metal. The Nb content and consequently the associated dendrite distribution decreases especially in the right-hand area of the weld metal, as exemplified by region 2. A further crucial observation can be derived from the Ni concentration, which decreases significantly towards the left area of the weld seam. Following the study by Oliveira et al. [19], in which the high melting temperature of Nb was utilized to act as a diffusion barrier, the large volume of Nb apparently restricts the material transport of the respective base materials. 3.2 Mechanical properties of micro electron beam welded Ti / Nb / NiTi joints As concluded in previous investigations on dissimilar welding of Ti to NiTi, the formation of IMC has a major effect on the mechanical properties of the joints [11, 12]. The associated high hardness peaks will ultimately act as weak points under stress, which leads to the conclusion that they should be avoided as far as possible [20]. In order to determine the IMC distribution of the joints welded within this study, detailed nanoindentation mappings were performed on the respective cross-sections, see Fig. 7(a)-(c). For parameter I and II, very high hardness values of up to 1000 HV can be detected along the fusion line to the Ti base material. Similar values were reported in previous studies on autogenous beam welding of NiTi to Ti and indicate a pronounced formation of Ti 2 Ni IMC [13]. According to the previously discussed EDS analyses, these areas are characterized by a high proportion of Ti as well as a suitable content of Ni, thus, confirming the potential for the respective stoichiometry to prevail. In good agreement with this finding, the remaining weld metals, which are characterized by a reduced amount of Ti, exhibit comparatively low hardness values. In comparison to parameter I and II, the nanoindentation mapping for the specimen welded with parameter III in Fig. 7(c) reveals no severe hardness peaks. Although individual measurements show hardness values of up to 742 HV, the majority of the weld metal remains comparatively soft. However, large regions with very low hardness values < 200 HV can be seen, which can be attributed to un-molten Nb concentrations. To correlate the findings of the microstructural analyses and hardness distribution to the mechanical performance, tensile tests for each of the three parameters were performed. As illustrated in Fig. 8(a), parameter I and II feature almost identical mechanical properties with an average UTS of 504 MPa and 519 MPa respectively. In consideration of the drastic differences in melting behavior, this implies that the presence of un-molten Nb in the weld metal of parameter I does not promote premature failure. Instead, it can be concluded that the hardness peaks, which were detected for both parameters, must be regarded as the primary initiator of early failure. In contrast, the application of parameter III exhibits significantly improved mechanical properties despite the aforementioned severe inhomogeneity regarding the chemical distribution in the weld metal. An average UTS of 663 MPa and elongation at break of 6.9 % can be achieved, which corresponds approximately to the Ti base material. A major difference to the other parameters lies in the fact that no critical hardness peaks were observed in the weld metal of parameter III, thus, substantiating the conclusion that the formation of IMC is responsible for premature failure in dissimilar welded joints. However, it should be mentioned that the drastic deviations in hardness distribution results in a metallurgical notch effect. In conjunction with the observed hot cracks, it can be assumed that the joint of parameter III will not withstand dynamic loading. Fig. 8(b) shows an engineering stress-strain diagram with one representative sample of each parameter. Similar to the results from Wiegand et al. [21], the specimens can fully utilize the superelastic transformation of NiTi, which is completed at approximately 500 MPa. Therefore, it can be confirmed that micro electron beam welding is a suitable process to achieve excellent mechanical properties during dissimilar welding of NiTi / Nb / Ti. Further information regarding the cause of failure can be determined based on SEM fracture surface analyses, see Fig. 9. The fracture pattern of parameter I shows a rather uniform morphology that represents a brittle cleavage, which can be attributed to the limited ductility due to the aforementioned IMC formation. The corresponding EDS mappings in a representative area confirm the presence of all alloying elements along the fracture surface. However, Nb appears to be less concentrated, which corresponds to the results of the cross-sectional EDS results. The fracture behavior is also in good agreement with previous studies, in which the specimens failed in the weld metal due to Ti 2 Ni IMC [16]. Parameter II exhibits a comparable fracture surface, which is consistent with the similar mechanical properties as well as hardness analyses. However, individual morphologies can be identified that are characterized by ductile tensile dimples. As the associated EDS analyses reveal, these represent Nb-accumulations, which were also previously discovered during the cross-section analyses. Parameter III shows a mixed fracture behavior, which is characterized by both brittle cleavage and ductile areas. As confirmed by the EDS mappings, the ductile fractured surface represents Nb-concentrations, which were also visible within the cross-section. In addition, several very large spherical pores can be observed. The development of these confirms the previously established conclusion that the lower-melting base materials can vaporize due to the excessive energy input. Due to the increased beam current and the high oscillation amplitude, which covers the entire width of the filler material, this phenomenon appears to be more pronounced in parameter III. Nevertheless, the associated tensile specimens showed superior mechanical properties. With regard to the nanoindentation mappings, which revealed significantly lower hardness peaks compared to parameter I and II, this again confirms that the formation of IMC has the biggest impact on the mechanical performance and represents the fundamental limitation in dissimilar welding of chemically incompatible materials. In summary, this study has shown that the fabrication of NiTi / Nb / Ti dissimilar joints by means of micro electron beam welding is associated with substantial challenges due to the strongly deviating thermophysical properties of the materials. Although the melting and intermixing behavior of the refractory metal can be improved by utilizing the high-frequency beam oscillation, the chemical composition within the weld seam remains inhomogeneous and the formation of highly critical Ti 2 Ni IMC cannot be fully avoided. However, it was also demonstrated that by increasing the proportion of Nb, high concentrations of those IMC can be prevented, which ultimately results in excellent mechanical properties under quasi-static mechanical load. Future investigations on the dissimilar beam welding of Ti / Nb / NiTi should therefore aim to improve the melting behavior of large Nb volumes by further optimizing the various process parameters or by geometrical adjustments. 4 Conclusion In the investigation at hand, NiTi was joined to cp-Ti by means of micro electron beam welding using Nb as filler material. Based on the experimental results, the following main conclusions can be drawn from this study: Owing to the high melting temperature and thermal conductivity of the Nb filler material, fully melting is not feasible on the given welding configuration without emitting excessive energy that would cause burn-through defects. By utilizing the process-inherent fast beam oscillation, the process energy can be coupled more evenly into the filler material, thus, significantly improving the melting behavior. Nevertheless, smaller accumulations of Nb remain in the weld metal, as was observed particularly along the fracture surface. Increasing the dimension of the applied Nb filler material promotes the affinity for weld seam irregularities, e.g., pores and hot cracks. Furthermore, it was not feasible to achieve uniform melting despite the utilization of a high beam current and specifically adjusted beam oscillation. Nanoindentation mappings revealed critical hardness peaks in the weld metal of the samples welded with the smaller dimension of Nb. These can be attributed to a pronounced formation of Ti 2 Ni IMC and are considered to be the major reason for the premature and brittle failure during the quasi-static tensile tests. Superior mechanical properties were achieved by using Nb with a higher thickness and specifically adapted oscillation parameters despite an inhomogeneous elemental distribution and severe welding irregularities. This can be attributed to the increased proportion of Nb in the weld metal, which leads to a reduced formation of highly critical Ti 2 Ni IMC. Ultimate tensile strengths of up to 673 MPa were achieved, which correspond approximately to the strength of the titanium base material. Thus, it can be confirmed that micro electron beam welding is a suitable welding process to produce high-quality NiTi / Nb / Ti joints. Due to the excellent mechanical properties under quasi-static load, the results offer high potential to be adopted in future industrial applications, e.g., medical technology parts. Declarations Funding No funding was obtained for this study. Acknowledgements The corresponding author thankfully acknowledges the support provided within the framework of “DVS IIW Young Professionals” by the German Welding Society. Furthermore, the authors would like to thank the technician employees and student assistants for their continuous support. Conflict of interests The authors declare that there exists no competing financial interest or personal relationships that could have appeared to influence the work reported in this paper. References Kah P, Shrestha M, Martikainen J (2013) Trends in Joining Dissimilar Metals by Welding. AMM 440:269–276. https://doi.org/10.4028/www.scientific.net/AMM.440.269 Quazi MM, Ishak M, Fazal MA et al. 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Materials & Design 228:111845. https://doi.org/10.1016/j.matdes.2023.111845 Murray JL (1981) The Nb−Ti (Niobium-Titanium) system. Bulletin of Alloy Phase Diagrams 2:55–61. https://doi.org/10.1007/BF02873704 Torkamany MJ, Malek Ghaini F, Poursalehi R (2014) Dissimilar pulsed Nd:YAG laser welding of pure niobium to Ti–6Al–4V. Materials & Design (1980-2015) 53:915–920. https://doi.org/10.1016/j.matdes.2013.07.094 Oliveira JP, Panton B, Zeng Z et al. (2016) Laser joining of NiTi to Ti6Al4V using a Niobium interlayer. Acta Materialia 105:9–15. https://doi.org/10.1016/j.actamat.2015.12.021 Wiegand M, Marks L, Sommer N et al. (2022) Dissimilar micro beam welding of titanium to Nitinol and stainless steel using biocompatible filler materials for medical applications. Weld World. https://doi.org/10.1007/s40194-022-01412-3 Wiegand M, Sommer N, Marks L et al. (2024) High-Strength Dissimilar Welds Between a NiTi Shape Memory Alloy and Titanium Obtained by Intermixing Niobium Using Pulsed Laser Beam Welding. Metall Mater Trans A 55:278–290. https://doi.org/10.1007/s11661-023-07248-w Hellberg S, Hummel J, Krooß P et al. (2020) Microstructural and mechanical properties of dissimilar nitinol and stainless steel wire joints produced by micro electron beam welding without filler material. Weld World 64:2159–2168. https://doi.org/10.1007/s40194-020-0 Cite Share Download PDF Status: Under Review Version 1 posted Reviewers agreed at journal 30 Jan, 2025 Reviewers invited by journal 30 Jan, 2025 Editor invited by journal 18 Oct, 2024 Editor assigned by journal 15 Oct, 2024 First submitted to journal 15 Oct, 2024 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. 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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-5245806","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":409045296,"identity":"2e1730e9-bcd8-4408-8a26-ac56749e8d97","order_by":0,"name":"Michael Wiegand","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA/klEQVRIie3PMWsCMRTA8XcInU67npOfoHAu16W1XyUhkCm4FA7BwZtust7s5FfI1PlJhlsOu564nHuHjEexarSlQyGnY6H5h7whvN8QAJfrj4bnmScAxByTh3BzDSnwh8BF8lV53r6C3CV+H/XHYNhZv0dVBbzXyxQixA9WEqEfLucz9tzdiPuQgOjLkhOEFW8gbana0xaVGxEFdD/yZOCH6KWqmXxOJ1SuiyggMHpaZG/akEMzgVpRWfonImiCAgxBO1G3evmS5HRe8NgQzsxfQiQrZid5ynS9G9NZrl67NbDHRaa2lY4HVgItc7309yuxg+92FzdcLpfrP3cEqM9i5KSefNQAAAAASUVORK5CYII=","orcid":"https://orcid.org/0000-0002-2747-3460","institution":"Kassel University Faculty 15 Mechanical Engineering: Universitat Kassel Fachbereich 15 Maschinenbau","correspondingAuthor":true,"prefix":"","firstName":"Michael","middleName":"","lastName":"Wiegand","suffix":""},{"id":409045297,"identity":"1d553972-99f3-4a8e-bd6a-6d81a440204f","order_by":1,"name":"Johannes-Seneca Wolfgang Fritz Loose","email":"","orcid":"","institution":"Kassel University Faculty 15 Mechanical Engineering: Universitat Kassel Fachbereich 15 Maschinenbau","correspondingAuthor":false,"prefix":"","firstName":"Johannes-Seneca","middleName":"Wolfgang Fritz","lastName":"Loose","suffix":""},{"id":409045298,"identity":"d0038e5a-c953-48dc-b8c9-90da6bfed4c4","order_by":2,"name":"Martin Kahlmeyer","email":"","orcid":"","institution":"Kassel University Faculty 15 Mechanical Engineering: Universitat Kassel Fachbereich 15 Maschinenbau","correspondingAuthor":false,"prefix":"","firstName":"Martin","middleName":"","lastName":"Kahlmeyer","suffix":""},{"id":409045299,"identity":"1a2da5f5-aa7d-41a8-a25f-4d83175c6f3e","order_by":3,"name":"Wenwen Song","email":"","orcid":"","institution":"Kassel University Faculty 15 Mechanical Engineering: Universitat Kassel Fachbereich 15 Maschinenbau","correspondingAuthor":false,"prefix":"","firstName":"Wenwen","middleName":"","lastName":"Song","suffix":""},{"id":409045300,"identity":"d4896b8a-fa6f-4e3f-9991-a178f242ac36","order_by":4,"name":"Stefan Böhm","email":"","orcid":"","institution":"Kassel University Faculty 15 Mechanical Engineering: Universitat Kassel Fachbereich 15 Maschinenbau","correspondingAuthor":false,"prefix":"","firstName":"Stefan","middleName":"","lastName":"Böhm","suffix":""}],"badges":[],"createdAt":"2024-10-11 11:50:01","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-5245806/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-5245806/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":75324801,"identity":"d1173377-4075-45c2-b299-91e581ff5a5d","added_by":"auto","created_at":"2025-02-03 11:07:59","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":3059819,"visible":true,"origin":"","legend":"\u003cp\u003e(a) 3d model of the fixture used for the welding experiments. (b) Photographic image of the welding samples with the filler material (0.2 mm Nb) fixed in position\u003c/p\u003e","description":"","filename":"Fig1.png","url":"https://assets-eu.researchsquare.com/files/rs-5245806/v1/25f61b816a853f21f655d382.png"},{"id":75324809,"identity":"71fc9d6e-88b1-4d7d-ab9d-e961f888feb4","added_by":"auto","created_at":"2025-02-03 11:08:00","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":1125182,"visible":true,"origin":"","legend":"\u003cp\u003eElectron-optical images of weld seam surfaces welded with parameter I using varying beam currents. Images obtained by the micro electron beam welding machine immediately after the welding process\u003c/p\u003e","description":"","filename":"Fig2.png","url":"https://assets-eu.researchsquare.com/files/rs-5245806/v1/7cea94b24e3695faf5a0a994.png"},{"id":75324803,"identity":"36a3ad40-a16e-4605-8a41-7260217accb5","added_by":"auto","created_at":"2025-02-03 11:08:00","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":4918889,"visible":true,"origin":"","legend":"\u003cp\u003eWeld seam surfaces and etched cross-sections of parameter I (a), II (b), and II (c)\u003c/p\u003e","description":"","filename":"Fig3.png","url":"https://assets-eu.researchsquare.com/files/rs-5245806/v1/0b875a9a2a070e1dacd90fb7.png"},{"id":75324800,"identity":"2e403bff-6ff3-44ce-9f47-5c270fb6342f","added_by":"auto","created_at":"2025-02-03 11:07:59","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":5142697,"visible":true,"origin":"","legend":"\u003cp\u003e(a) BSD image, highlighting the material contrast in the cross-section of parameter I. Regions 1 and 2 are depicted at higher magnification in (b). (c) EDS mappings of the identical cross-sectional area as shown in (a), scaled with respect to the mass fraction of Nb, Ti, and Ni.\u003c/p\u003e","description":"","filename":"Fig4.png","url":"https://assets-eu.researchsquare.com/files/rs-5245806/v1/cf45c9a58e41b8156fe06983.png"},{"id":75324829,"identity":"f66d1d85-b711-4b3d-a2fd-3d0559f202ce","added_by":"auto","created_at":"2025-02-03 11:08:01","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":5565466,"visible":true,"origin":"","legend":"\u003cp\u003e(a) BSD image, highlighting the material contrast in the cross-section of parameter II. Regions 1 and 2 are depicted at higher magnification in (b). (c) EDS mappings of the identical cross-sectional area as shown in (a), scaled with respect to the mass fraction of Nb, Ti, and Ni.\u003c/p\u003e","description":"","filename":"Fig5.png","url":"https://assets-eu.researchsquare.com/files/rs-5245806/v1/7d2e8a937e66e7d8f2110b5e.png"},{"id":75324802,"identity":"c8d784df-e890-4fcc-8401-023191820a47","added_by":"auto","created_at":"2025-02-03 11:08:00","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":5742290,"visible":true,"origin":"","legend":"\u003cp\u003e(a) BSD image, highlighting the material contrast in the cross-section of parameter III. Regions 1 and 2 are depicted at higher magnification in (b). (c) EDS mappings of the identical cross-sectional area as shown in (a), scaled with respect to the mass fraction of Nb, Ti, and Ni.\u003c/p\u003e","description":"","filename":"Fig6.png","url":"https://assets-eu.researchsquare.com/files/rs-5245806/v1/10eedca49d1dda1335b3e758.png"},{"id":75324804,"identity":"26fb2f46-6d8d-4c8c-995d-639bd3e84e17","added_by":"auto","created_at":"2025-02-03 11:08:00","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":5810214,"visible":true,"origin":"","legend":"\u003cp\u003eNanoindentation mappings of the cross-sectional area of parameter I (a), II (b), and III (c)\u003c/p\u003e","description":"","filename":"Fig7.png","url":"https://assets-eu.researchsquare.com/files/rs-5245806/v1/e691b47e7433aea6742958f1.png"},{"id":75324817,"identity":"bd5f4a24-c354-4802-9fb6-b08f7b369865","added_by":"auto","created_at":"2025-02-03 11:08:00","extension":"png","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":539425,"visible":true,"origin":"","legend":"\u003cp\u003e(a) Summarized results of the tensile tests on parameter I, II, and III. (b) Engineering-stress-strain diagram of one representative sample for each parameter.\u003c/p\u003e","description":"","filename":"Fig8.png","url":"https://assets-eu.researchsquare.com/files/rs-5245806/v1/b6b3dbc90500e2e0716515da.png"},{"id":75326628,"identity":"6ef21da7-cecf-474c-935b-e739b947d58b","added_by":"auto","created_at":"2025-02-03 11:24:00","extension":"png","order_by":9,"title":"Figure 9","display":"","copyAsset":false,"role":"figure","size":8681208,"visible":true,"origin":"","legend":"\u003cp\u003eSEM Fracture surface analyses of parameter I (a), II (b), and III (c), each supplemented by EDS mappings in areas of interest\u003c/p\u003e","description":"","filename":"Fig9.png","url":"https://assets-eu.researchsquare.com/files/rs-5245806/v1/debfc1b31722e40916bdb496.png"},{"id":75327196,"identity":"9f80b5c2-063d-47fd-9023-eaec2ca650f0","added_by":"auto","created_at":"2025-02-03 11:32:16","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":37453801,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-5245806/v1/6eed7e75-a39a-4e98-80d6-9ee6351c5fb7.pdf"}],"financialInterests":"","formattedTitle":"Impact of beam oscillation and power modulation on the intermixing behavior of dissimilar Titanium / Niobium / Nitinol joints during micro electron beam welding","fulltext":[{"header":"1 Introduction and state-of-the-art","content":"\u003cp\u003eThe dissimilar welding of metals allows individual material qualities to be combined in functionally-graded components. Thus, drastic improvements can be achieved in terms of cost-effectiveness, functionality, and manufacturing conditions [1, 2]. Especially hybrid components made of nickel-titanium shape memory alloys (NiTi) and titanium alloys (Ti) offer great potential in this respect. Due to their excellent mechanical properties, good corrosion resistance as well as outstanding biocompatibility, both materials are used in similar application areas, e.g., aerospace, energy industry, and medical technology [3\u0026ndash;9]. However, as NiTi is a high-priced and difficult-to-machine material, there is growing interest in local substitution with Ti using a robust joining process.\u003c/p\u003e\n\u003cp\u003eWhile similar welding of the mentioned materials is generally possible, e.g., by means of beam welding processes, dissimilar fusion welding of NiTi to Ti is restricted by their chemical incompatibilities. The materials are characterized by low solubility in solid state and the formation of the intermetallic compounds (IMC) Ti\u003csub\u003e2\u003c/sub\u003eNi and TiNi\u003csub\u003e3\u003c/sub\u003e [10]. These IMC are characterized by a brittle, ceramic-like behavior and significantly impair both the weldability and the mechanical properties of fabricated joints. Another fundamental challenge in dissimilar fusion welding arises from differing thermophysical properties, such as melting temperatures, thermal conductivities, and thermal expansion rates, as these further increase the susceptibility of solidification cracking due to increased stress conditions during cooling and the emergence of residual stresses [11, 12].\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eOwing to the aforementioned limitations, previous studies on autogenous beam welding of NiTi to Ti have achieved very poor joining properties. Accordingly, no defect-free or mechanically resilient samples could be welded using either pulsed Nd:YAG laser [11] or Yb fiber laser [12, 13]. All welded joints are characterized by severe solidification cracking, caused specifically by the formation of Ti\u003csub\u003e2\u003c/sub\u003eNi IMC and the resulting embrittlement in the weld metal. A promising approach to improve both, chemical compatibility and mechanical performance during dissimilar fusion welding, is the implementation of filler materials. As a result, Teshome et al. were able to achieve significantly improved joining properties by using zirconium [14], cobalt\n \u003c!--[if supportFields]\u003e\u003cspan style='mso-element:field-begin'\u003e\u003c/span\u003eADDIN 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style='mso-element:field-separator'\u003e\u003c/span\u003e\u003c![endif]--\u003e[15], and palladium [16] for dissimilar welding of NiTi to Ti. In the latter, comparatively high tensile strengths of up to 520 MPa were achieved, representing a major increase compared to autogenous beam welding of NiTi to Ti. Furthermore, the implementation of a copper foil can also improve the weld seam quality of NiTi / Ti joints, resulting in tensile strengths of up to 300 MPa [11]. However, it should be noted that in all studies mentioned above, the weld seams invariably represent a weak point under mechanical load.\u003c/p\u003e\n\u003cp\u003eIn addition to the aforementioned materials, several studies have been carried out using niobium (Nb) as filler material for dissimilar NiTi to Ti fusion welding. Due to its good chemical compatibility with the Ti base material and its equally excellent biocompatibility, Nb is particularly interesting for the material combination [17, 18]. Consequently, there is great potential for dissimilar joints to be directly adopted in medical technology applications if the fusion welding of mechanical resilient parts can be realized in a process-safe manner. In the investigation by Oliveira et al. [19], it was demonstrated that Nb can be used as a diffusion barrier between Ti and NiTi and, due to a reduced IMC formation, tensile strengths of up to 300 MPa can be achieved. Moreover, Wiegand et al. [20] have shown that Nb can also be successfully implemented as an intermediate layer during multi-pass micro electron beam welding, thus, achieving complete metallurgical separation of the base materials. In comparison with hafnium and tantalum, which were also considered in this publication, niobium was found to have the best joining properties with NiTi as well as Ti. During quasi-static tensile tests, the corresponding specimens failed reproducibly in the un-molten filler material at tensile strengths of 453\u0026nbsp;MPa, confirming the great compatibility of the materials. However, as the filler material has already reached its strength limit, further improvements in joint strength are restricted with this approach. In a very recent study by Wiegand et al. [21], it was proven that controlled intermixing of Nb into a combined NiTi / Nb / Ti butt-joint by means of pulsed laser beam welding can overcome the limitation of the comparatively poor mechanical properties of Nb. However, this method requires very specific parameter optimization to ensure that a high volume proportion of Nb is mixed into the joint weld metal in order to reduce the formation of IMC. The associated specimens fail in the Ti base material and achieve ultimate tensile strengths (UTS) of up to 679 MPa as well as elongations at break of 18.9 %. As these welded joints can fully utilize the superelastic plateau of NiTi, the results represent a significant improvement of the current state-of-the-art in the field of dissimilar fusion welding of NiTi to Ti.\u003c/p\u003e\n\u003cp\u003eThe present study complements the aforementioned investigations of the material combination Ti / Nb / NiTi and investigates the feasibility of continuous micro electron beam welding in butt-welding configuration. Due to the high-vacuum atmosphere\u0026nbsp;of up to\u0026nbsp;10\u003csup\u003e-5\u003c/sup\u003e mbar in the working chamber, the welding process ensures optimum protection of the highly reactive materials [20]. In addition, the various process parameters allow precise control of the melt pool, which makes this process predestined for welding of dissimilar materials [20, 22]. Comparatively high welding speeds of 40 mm/s and specifically adapted beam currents are used to achieve a focused energy input that allows fusion of the comparatively high-melting Nb. Furthermore, a novel approach of utilizing the process-inherent fast beam oscillation is applied to tailor the energy input in the welding area in order to optimize the melting and intermixing behavior of the refractory metal within the weld metal. The microstructural evolution of the weld seams is compared using scanning electron microscopy (SEM) equipped with a back-scattered electron detector (BSD). In order to provide precise details on the chemical distribution in the weld metal, supplementary energy-dispersive X-ray spectroscopy (EDS) mappings are carried out with regard to the elements Ti, Nb and nickel (Ni). Finally, the findings regarding the microstructural evolution are correlated with the resulting mechanical properties by means of tensile tests and nanoindentation analyses.\u003c/p\u003e"},{"header":"2 Materials and methods","content":"\u003cp\u003eThe welding experiments were performed using thin sheet geometries measuring 12.5 x 25 x 0.25 mm\u0026sup3; of superelastic NiTi as well as commercially pure titanium (cp-Ti, grade 4). The chemical composition of the applied materials is depicted in Table 1. \u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eTable\u0026nbsp;\u003c/strong\u003e\u003cstrong\u003e1\u003c/strong\u003e Chemical composition of cp-Ti grade 4 (a) and NiTi (b) as specified by the material suppliers\u003c/p\u003e\n\u003cp\u003ea)\u003c/p\u003e\n\u003ctable border=\"1\" cellspacing=\"0\" cellpadding=\"0\" width=\"246\"\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd colspan=\"5\" style=\"width: 246px;\"\u003e\n \u003cp\u003e\u003cstrong\u003eElement (wt.-%)\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd style=\"width: 49px;\"\u003e\n \u003cp\u003eTi\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 49px;\"\u003e\n \u003cp\u003eC\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 49px;\"\u003e\n \u003cp\u003eN\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 49px;\"\u003e\n \u003cp\u003eFe\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 49px;\"\u003e\n \u003cp\u003eO\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd style=\"width: 49px;\"\u003e\n \u003cp\u003ebal.\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 49px;\"\u003e\n \u003cp\u003e0.011\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 49px;\"\u003e\n \u003cp\u003e0.010\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 49px;\"\u003e\n \u003cp\u003e0.12\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 49px;\"\u003e\n \u003cp\u003e0.31\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n\u003c/table\u003e\n\u003cp\u003eb)\u003c/p\u003e\n\u003ctable border=\"1\" cellspacing=\"0\" cellpadding=\"0\" width=\"444\"\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd colspan=\"6\" style=\"width: 444px;\"\u003e\n \u003cp\u003e\u003cstrong\u003eElement (wt.-%)\u003c/strong\u003e\u003cstrong\u003e\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd style=\"width: 49px;\"\u003e\n \u003cp\u003eNi\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 49px;\"\u003e\n \u003cp\u003eTi\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 57px;\"\u003e\n \u003cp\u003eC\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 49px;\"\u003e\n \u003cp\u003eO\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 49px;\"\u003e\n \u003cp\u003eFe\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 191px;\"\u003e\n \u003cp\u003eother single trace elements\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd style=\"width: 49px;\"\u003e\n \u003cp\u003e55.94\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 49px;\"\u003e\n \u003cp\u003ebal.\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 57px;\"\u003e\n \u003cp\u003e0.0275\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 49px;\"\u003e\n \u003cp\u003e0.019\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 49px;\"\u003e\n \u003cp\u003e0.014\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 191px;\"\u003e\n \u003cp\u003e\u0026lt; 0.01\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n\u003c/table\u003e\n\u003cp\u003eAs filler material, pure Nb (99.9%) strips with the dimensions 12.5 x 0.25 mm\u003csup\u003e2\u003c/sup\u003e and a thickness of 0.2 mm and 0.4 mm respectively were implemented and positioned between the joint edges of the base materials. Table 2 summarizes important thermophysical and mechanical properties of the investigated materials.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eTable\u0026nbsp;\u003c/strong\u003e\u003cstrong\u003e2\u003c/strong\u003e Selected thermophysical and mechanical properties of the respective materials used in this investigation, adopted from [21]. Values are given for a temperature of 20\u0026deg;C. The mechanical properties (*) were determined by quasi-static tensile tests of the experimental materials\u0026nbsp;\u003c/p\u003e\n\u003cdiv align=\"center\"\u003e\n \u003ctable border=\"1\" cellspacing=\"0\" cellpadding=\"0\" width=\"100%\"\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd style=\"width: 18px;\"\u003e\n \u003cp\u003e\u003cstrong\u003eMaterials\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 13px;\"\u003e\n \u003cp\u003e\u003cstrong\u003eMelting point\u0026nbsp;\u003cbr\u003e\u0026nbsp;[\u0026deg;C]\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 13px;\"\u003e\n \u003cp\u003e\u003cstrong\u003eUltimate tensile strength *\u003cbr\u003e\u0026nbsp;[MPa]\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 13px;\"\u003e\n \u003cp\u003e\u003cstrong\u003eElongation\u0026nbsp;\u003cbr\u003e\u0026nbsp;at break *\u003cbr\u003e\u0026nbsp;[%]\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 14px;\"\u003e\n \u003cp\u003e\u003cstrong\u003eThermal conductivity [Wm\u003csup\u003e-1\u003c/sup\u003eK\u003csup\u003e-1\u003c/sup\u003e]\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 13px;\"\u003e\n \u003cp\u003e\u003cstrong\u003eCoefficient of thermal expansion\u0026nbsp;\u003cbr\u003e [10\u003csup\u003e-6\u003c/sup\u003eK\u003csup\u003e-1\u003c/sup\u003e]\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 13px;\"\u003e\n \u003cp\u003e\u003cstrong\u003eSuperelastic plateau *\u003cbr\u003e\u0026nbsp;[MPa]\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd style=\"width: 18px;\"\u003e\n \u003cp\u003ecp-Titanium\u0026nbsp;\u003cbr\u003e\u0026nbsp;(grade 4)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 13px;\"\u003e\n \u003cp\u003e1660\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 13px;\"\u003e\n \u003cp\u003e~ 679\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 13px;\"\u003e\n \u003cp\u003e~ 23\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 14px;\"\u003e\n \u003cp\u003e18\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 13px;\"\u003e\n \u003cp\u003e8.6\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 13px;\"\u003e\n \u003cp\u003e/\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd style=\"width: 18px;\"\u003e\n \u003cp\u003eNiTi\u003cbr\u003e\u0026nbsp;(55.8 wt.-% Nickel)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 13px;\"\u003e\n \u003cp\u003e1310\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 13px;\"\u003e\n \u003cp\u003e~ 1357\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 13px;\"\u003e\n \u003cp\u003e~ 12\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 14px;\"\u003e\n \u003cp\u003e18\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 13px;\"\u003e\n \u003cp\u003e11\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 13px;\"\u003e\n \u003cp\u003e~ 400 - 500\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd style=\"width: 18px;\"\u003e\n \u003cp\u003eNiobium\u003cbr\u003e\u0026nbsp;(99.9 %)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 13px;\"\u003e\n \u003cp\u003e2468\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 13px;\"\u003e\n \u003cp\u003e~ 470\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 13px;\"\u003e\n \u003cp\u003e~ 5\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 14px;\"\u003e\n \u003cp\u003e52\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 13px;\"\u003e\n \u003cp\u003e7.1\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 13px;\"\u003e\n \u003cp\u003e/\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n \u003c/table\u003e\n\u003c/div\u003e\n\u003cp\u003eAll geometries were produced using electrical discharge machining (EDM) to ensure high dimensional accuracy and thermally unaffected material properties. In order to remove the process-related residues as well as to create optimal surface conditions, the joining edges of the base materials were ground using P2500 grade silicon-carbide paper after EDM. Due to the very small geometry of the Nb strips, mechanical processing was restricted, which is why they were treated with a chemical solution to remove any copper-containing deposits caused by EDM that could potentially affect the welding result. The solution was produced under the following conditions: Aqueous phase (1 L) containing 0,2 mol/L H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e, 0,3 mol/L HCL and 0,5 mol/L NaCl. Prior to the welding process, the materials were cleaned using acetone and then dried in air.\u003c/p\u003e\n\u003cp\u003eThe welding tests were conducted using a micro electron beam welding machine (SEM108, pro-beam GmbH \u0026amp; Co. KGaA, Gilching, Germany as well as JSC Selmi, Sumy, Ukraine). The welding machine is equipped with a tungsten ribbon cathode and allows beam currents of up to 20 mA at an acceleration voltage of 60 kV, resulting in a maximum beam power of 1,2 kW. Furthermore, it is possible to achieve very small beam diameters of approx. 30 \u0026mu;m at a beam current of 1 mA. Thanks to the integrated beam control system, high deflection frequencies of up to 5 MHz can be achieved. The vacuum pumps generate a vacuum atmosphere of up to 10\u003csup\u003e-5\u003c/sup\u003e mbar in the working chamber, ensuring that even highly reactive materials are optimally protected from environmental influences during the welding process. To provide reproducible welding conditions with virtually no gap between the materials, the specimens were clamped in a special fixture featuring lateral and vertical clamping options, see Fig. 1(a). Furthermore, the fixture allows two samples to be clamped individually and, thus, the fabrication of two separate weld seams during one evacuation cycle. The filler material was placed manually and held in position by the applied lateral pressure. It should be emphasized that special attention was paid to ensure a precise alignment of the base and filler materials, as even minor deviations in the welding configuration can have a significant impact on the welding result. During welding, the electron beam was guided along the center of Nb, ensuring that the focused energy was being transferred into the Nb foil. Fig. 1(b) shows a photograph of the clamping configuration.\u003c/p\u003e\n\u003cp\u003eTable 3 summarizes the parameters used within the scope of this study. For parameter I, the specimens were produced using a Nb strip with a thickness of 0.2 mm and a continuous electron beam without oscillation. The associated high welding speed of 40 mm/s was selected to minimize the impact of the high thermal conductivity of the filler metal and, thus, to generate small and easier-to-control weld seams. Furthermore, the beam current was systematically adjusted to provide the highest possible energy input while maintaining macroscopic defect-free weld seams. A more detailed explanation of this is provided in the subsequent chapter. For parameter II, identical parameters are used and supplemented by a beam oscillation transverse to the welding trajectory in order to tailor the energy input and ultimately to compare deviations in the intermixing behavior. Finally, parameter III involves the utilization of a Nb strip with an increased thickness to increase the proportion of Nb in the weld metal, which is expedient based on the findings of Wiegand et al. [21]. Preliminary tests on this filler material dimension have shown that it is required to adjust the parameters to a higher beam current as well as higher oscillation frequency and amplitude in order to counteract the high melting temperature and thermal conductivity of the refractory metal. It shall be mentioned in this context, that the welding tests to determine the optimal beam current in this configuration were associated either with insufficient penetration depth with burn-through defects. The beam current of 2.5 mA was finally chosen because of comparatively low defect formations, which is explained in more detail in the following chapter.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eTable\u0026nbsp;\u003c/strong\u003e\u003cstrong\u003e3\u003c/strong\u003e\u003cstrong\u003e:\u0026nbsp;\u003c/strong\u003eOverview of the welding parameters used in this study\u003c/p\u003e\n\u003ctable border=\"1\" cellspacing=\"0\" cellpadding=\"0\"\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd\u003e\n \u003cp\u003e\u003cstrong\u003eParameter\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e\u003cstrong\u003eThickness \u0026nbsp;of niobium [mm]\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e\u003cstrong\u003eBeam current\u003cbr\u003e\u0026nbsp;[mA]\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e\u003cstrong\u003eOscillation frequency\u003cbr\u003e\u0026nbsp;[Hz]\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e\u003cstrong\u003eOscillation amplitude\u003cbr\u003e\u0026nbsp;[mm]\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e\u003cstrong\u003eAcceleration voltage\u003cbr\u003e\u0026nbsp;[kV]\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e\u003cstrong\u003eWelding\u0026nbsp;\u003cbr\u003e\u0026nbsp;speed\u0026nbsp;\u003cbr\u003e\u0026nbsp;[mm/s]\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e\u003cstrong\u003eFocus Position\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd\u003e\n \u003cp\u003eI\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e0.2\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e1.6\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e-\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e-\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd rowspan=\"3\"\u003e\n \u003cp\u003e60\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd rowspan=\"3\"\u003e\n \u003cp\u003e40\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd rowspan=\"3\"\u003e\n \u003cp\u003esurface\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd\u003e\n \u003cp\u003eII\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e0.2\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e1.6\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e5000\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e0.1\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd\u003e\n \u003cp\u003eIII\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e0.4\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e2.5\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e10000\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e0.4\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n\u003c/table\u003e\n\u003cp\u003eThe welded samples were embedded in cold-curing epoxy resin, ground to the center of the weld seams using silicon-carbide paper, and subsequently polished using a diamond suspension with a grit size of 0.1 \u0026micro;m. Following this, the samples were etched using a hydrofluoric-acid-free etching solution according to Keller for 120 s at room temperature and analyzed using optical light microscopy (DM2700, Leica Microsystems GmbH, Wetzlar, Germany). Further in-depth analyses were carried out on a field-emission scanning electron microscope (Zeiss REM Ultra Plus, Carl Zeiss AG, Oberkochen, Germany) operating at an acceleration voltage of 15 kV. The material contrast in the weld metal was highlighted using BSD imaging in order to provide insights into the phase distributions and dendritic formations. Furthermore, the chemical composition in the weld area was determined quantitatively regarding the mass fraction by means of EDS (Bruker XFlash 6160, Bruker Corporation, MA).\u003c/p\u003e\n\u003cp\u003eNanoindentation mappings were performed using a NHT\u003csup\u003e3\u003c/sup\u003e testing machine (Anton Paar GmbH, Graz, Austria) with a maximum load of 10 mN and loading and unloading rates of 100 mN/s. In order to fulfill the high requirements of a plane surface finish, the samples were mechanically processed again using multiple short-time and pressure-optimized grinding and polishing steps. Since material was removed during this preparation, visible differences can be observed in the cross-section geometries compared to the images of the microstructural analyses. Tensile tests were carried out on the basis of DIN EN ISO 6892-1 with a universal testing machine (Z100, Zwick-Roell AG, Ulm, Germany) using bone-shaped tensile testing specimens, which were extracted from the center of the weld seams by means of EDM. Further information regarding the tensile specimen geometry can be found in [20]. The specimens were clamped using a preload of 5 N and subsequently tested with regard to their UTS and elongation at break. The mechanical properties were determined using three identically manufactured specimens for each set of parameters.\u003c/p\u003e"},{"header":"3 Results and discussion","content":"\u003cp\u003e\u003cem\u003e3.1 Microstructural Evolution of micro electron beam welded Ti / Nb / NiTi joints\u003c/em\u003e\u003c/p\u003e\n\u003cp\u003eDue to the high melting temperature of the pure Nb filler material, an appropriately high energy input is required to achieve sufficient melting. As mentioned in the previous chapter, a systematic parameter optimization was carried out regarding the applied beam current to provide the highest possible energy input while omitting the emergence of defects. As exemplified on representative weld seam surfaces in Fig. 2, an excessive increase of the beam current favors burn-through defects. The cause of this defect is most likely related to the drastically deviating melting temperatures of the materials. Although the beam was focused on and guided along the center of the Nb filler material, the comparatively high thermal conductivity as well as the two-dimensional heat transfer within the applied thin sheet geometries lead to a considerable energy absorption of the surrounding, lower-melting base materials. As a result, the excessive temperature results in heavy melting or even vaporization of Ti or NiTi respectively, which ultimately causes the weld pool to collapse. This demonstrates that controlling the energy input during dissimilar micro electron beam welding of materials with pronounced differences in their thermophysical properties is mandatory.\u003c/p\u003e\n\u003cp\u003eThe application of the three different parameter sets (cf. Table 3) resulted in strongly differing weld metal characteristics, as can be derived from Fig. 3. Although the samples welded with parameter I and II feature almost identical weld seam surface geometries, optical light microscopy analyses of the etched cross-sections reveal a considerable deviating melting behavior of the filler material. Without beam oscillation, a rather large, un-molten proportion of Nb remains in the weld metal, as highlighted by the red outline in Fig. 3(a). The locally applied intensity of the electron beam is apparently quickly dissipated due to the high thermal conductivity of the filler material and, thus, insufficient to fully melt the entire geometry. Given the fact that the beam current has already been optimized to the highest value for which defect-free welds can be produced, it can be concluded that complete melting of Nb cannot be achieved for the given welding configuration using conventional continuous electron beam welding without oscillation.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eIn contrast, the cross-section of parameter II in Fig. 3(b) does not reveal any visible accumulations of Nb. The implemented oscillation transverse to the welding direction and the associated redistribution of the energy input to an extended area significantly improves the proportion of melted filler material. However, this also results in a wider weld seam, as can be seen from a comparison with the cross-section of parameter I.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eAs can be observed in Fig. 3(c), the sample welded with parameter III is associated with a burn-through defect. As mentioned in the previous chapter, it was not feasible to achieve a sufficient bond between the materials and simultaneously produce defect-free weld seams using Nb with 0.4 mm thickness. This can be attributed to the increased volume of the refractory metal, which intensifies the impact of the high melting temperature and thermal conductivity. However, it is important to note that the burn-through defect was always located near the end crater and, thus, outside the area which is relevant for microstructural and mechanical characterization. Due to the location of the defect, it could also possibly be negated by the implementation of a slope-out process.\u003c/p\u003e\n\u003cp\u003eTo provide more detailed insights into the microstructural evolution as well as the chemical distribution in the respective weld metal of the three parameters, BSD and EDS analyses were performed. As can be derived from the distinct material contrast in Fig. 4(a), the weld metal of parameter I is characterized by an inhomogeneous phase distribution. Nb-rich dendritic structures, that are represented by the brighter material contrast, can be observed in a wide range of the weld metal, as highlighted by region 1 and depicted with higher magnification in Fig. 4(b). Nevertheless, a predominant proportion of the weld metal is characterized by darker material contrast, which represents an increased amount of Ti and Ni in the respective areas [21]. Furthermore, large regions without Nd-rich dendrites exist near the not liquefied Nb in the center of the weld metal, as highlighted by region 2.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eThe associated EDS mapping in Fig. 4(c) provides a detailed view of the elemental distribution in the weld metal. It is evident that Nb is distributed throughout the entire weld metal but with a comparatively low mass fraction. In contrast, it can be seen that the previously detected dendrites have a considerably higher proportion of Nb. Furthermore, the elemental distribution of Ti as well as Ni also exhibits severe mixing gradients. Accordingly, the proportion of Ti is significantly higher in the range of the fusion line to the Ti base material compared to the remaining weld metal. From this, it can be concluded that the applied welding process provides insufficient time for an extensive intermixing, which can be attributed to the high welding speed and comparatively low energy input resulting in rapid cooling rates.\u003c/p\u003e\n\u003cp\u003eAs shown in Fig. 5(a), parameter II leads to a more uniform distribution of Nb-rich dendritic formations in the weld metal. Nevertheless, despite the improved melting behavior, the total proportion of Nb in the weld metal is still comparatively low. This can be attributed to the increased weld seam width compared to parameter I, which ultimately increases the proportion of molten and intermixed base materials. Furthermore, the existence of Nb accumulations, which were previously undiscovered in light microscopy analyses, is revealed. As can be seen in Fig. 5(b), region 1, these are dendritically solidified phases, which indicates an early solidification in the surrounding melt pool due to locally deviating melting temperatures. Consequently, the Nb filler material did not completely dissolve with the other materials. Furthermore, local deviations in the resulting microstructure occur in the area of the weld seam root. As region 2 emphasizes, multiple separate morphologic structures are present in this range.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eThe EDS analyses in Fig. 5(c) show an increased concertation of Ni in the weld seam root, thus, explaining the varying microstructures. Irrespective of this area, the elemental distribution is improved compared to parameter I, indicating that the application of a high-frequent beam oscillation and the associated increase in melt pool dynamics improves the intermixing process during dissimilar welding. However, increased concentrations of the respective elements are still evident along the fusion lines of the base materials.\u003c/p\u003e\n\u003cp\u003eBSD and EDS-Analyses of parameter III in Fig. 6(a)-(c) reveal drastic differences in the microstructural evolution of the weld metal compared to the previously discussed specimens. Despite the increased beam current and the specifically adjusted oscillation parameters, the resulting microstructure as well as elemental distribution in the weld metal is characterized by severe inhomogeneity. In addition to the aforementioned high affinity for defect formation, the increased volume of the implemented filler material evidently prevents uniform melting and intermixing on the given welding configuration. Furthermore, minor cracks are visible in the weld metal. According to the interdendritic crack pattern, which can be observed in Fig. 6(b), region 1, these can be identified as hot cracks, which arise from the presence of low-melting phases in between the high-melting Nb-rich dendrites. The surrounding microstructure in Fig. 6(b) shows a predominant formation of Nb-rich dendrites compared to the other parameters. As can be determined from the EDS mapping, the mass fraction of Nb in this area is over 60 % and, thus, in the desired range for achieving superior mechanical properties according to Wiegand et al. [21]. However, it should be emphasized that this microstructure exists only in a limited range due to the poor mixing in the weld metal. The Nb content and consequently the associated dendrite distribution decreases especially in the right-hand area of the weld metal, as exemplified by region 2. A further crucial observation can be derived from the Ni concentration, which decreases significantly towards the left area of the weld seam. Following the study by Oliveira et al. [19], in which the high melting temperature of Nb was utilized to act as a diffusion barrier, the large volume of Nb apparently restricts the material transport of the respective base materials.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cem\u003e3.2 Mechanical properties of micro electron beam welded Ti / Nb / NiTi joints\u003c/em\u003e\u003c/p\u003e\n\u003cp\u003eAs concluded in previous investigations on dissimilar welding of Ti to NiTi, the formation of IMC has a major effect on the mechanical properties of the joints [11, 12]. The associated high hardness peaks will ultimately act as weak points under stress, which leads to the conclusion that they should be avoided as far as possible [20]. In order to determine the IMC distribution of the joints welded within this study, detailed nanoindentation mappings were performed on the respective cross-sections, see Fig. 7(a)-(c). For parameter I and II, very high hardness values of up to 1000 HV can be detected along the fusion line to the Ti base material. Similar values were reported in previous studies on autogenous beam welding of NiTi to Ti and indicate a pronounced formation of Ti\u003csub\u003e2\u003c/sub\u003eNi IMC [13]. According to the previously discussed EDS analyses, these areas are characterized by a high proportion of Ti as well as a suitable content of Ni, thus, confirming the potential for the respective stoichiometry to prevail. In good agreement with this finding, the remaining weld metals, which are characterized by a reduced amount of Ti, exhibit comparatively low hardness values. In comparison to parameter I and II, the nanoindentation mapping for the specimen welded with parameter III in Fig. 7(c) reveals no severe hardness peaks. Although individual measurements show hardness values of up to 742 HV, the majority of the weld metal remains comparatively soft. However, large regions with very low hardness values \u0026lt; 200 HV can be seen, which can be attributed to un-molten Nb concentrations.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eTo correlate the findings of the microstructural analyses and hardness distribution to the mechanical performance, tensile tests for each of the three parameters were performed. As illustrated in Fig. 8(a), parameter I and II feature almost identical mechanical properties with an average UTS of 504 MPa and 519 MPa respectively. In consideration of the drastic differences in melting behavior, this implies that the presence of un-molten Nb in the weld metal of parameter I does not promote premature failure. Instead, it can be concluded that the hardness peaks, which were detected for both parameters, must be regarded as the primary initiator of early failure. In contrast, the application of parameter III exhibits significantly improved mechanical properties despite the aforementioned severe inhomogeneity regarding the chemical distribution in the weld metal. An average UTS of 663 MPa and elongation at break of 6.9 % can be achieved, which corresponds approximately to the Ti base material. A major difference to the other parameters lies in the fact that no critical hardness peaks were observed in the weld metal of parameter III, thus, substantiating the conclusion that the formation of IMC is responsible for premature failure in dissimilar welded joints. However, it should be mentioned that the drastic deviations in hardness distribution results in a metallurgical notch effect. In conjunction with the observed hot cracks, it can be assumed that the joint of parameter III will not withstand dynamic loading. Fig. 8(b) shows an engineering stress-strain diagram with one representative sample of each parameter. Similar to the results from Wiegand et al. [21], the specimens can fully utilize the superelastic transformation of NiTi, which is completed at approximately 500 MPa. Therefore, it can be confirmed that micro electron beam welding is a suitable process to achieve excellent mechanical properties during dissimilar welding of NiTi / Nb / Ti.\u003c/p\u003e\n\u003cp\u003eFurther information regarding the cause of failure can be determined based on SEM fracture surface analyses, see Fig. 9. The fracture pattern of parameter I shows a rather uniform morphology that represents a brittle cleavage, which can be attributed to the limited ductility due to the aforementioned IMC formation. The corresponding EDS mappings in a representative area confirm the presence of all alloying elements along the fracture surface. However, Nb appears to be less concentrated, which corresponds to the results of the cross-sectional EDS results. The fracture behavior is also in good agreement with previous studies, in which the specimens failed in the weld metal due to Ti\u003csub\u003e2\u003c/sub\u003eNi IMC [16].\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eParameter II exhibits a comparable fracture surface, which is consistent with the similar mechanical properties as well as hardness analyses. However, individual morphologies can be identified that are characterized by ductile tensile dimples. As the associated EDS analyses reveal, these represent Nb-accumulations, which were also previously discovered during the cross-section analyses.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eParameter III shows a mixed fracture behavior, which is characterized by both brittle cleavage and ductile areas. As confirmed by the EDS mappings, the ductile fractured surface represents Nb-concentrations, which were also visible within the cross-section. In addition, several very large spherical pores can be observed. The development of these confirms the previously established conclusion that the lower-melting base materials can vaporize due to the excessive energy input. Due to the increased beam current and the high oscillation amplitude, which covers the entire width of the filler material, this phenomenon appears to be more pronounced in parameter III. Nevertheless, the associated tensile specimens showed superior mechanical properties. With regard to the nanoindentation mappings, which revealed significantly lower hardness peaks compared to parameter I and II, this again confirms that the formation of IMC has the biggest impact on the mechanical performance and represents the fundamental limitation in dissimilar welding of chemically incompatible materials.\u003c/p\u003e\n\u003cp\u003eIn summary, this study has shown that the fabrication of NiTi / Nb / Ti dissimilar joints by means of micro electron beam welding is associated with substantial challenges due to the strongly deviating thermophysical properties of the materials. Although the melting and intermixing behavior of the refractory metal can be improved by utilizing the high-frequency beam oscillation, the chemical composition within the weld seam remains inhomogeneous and the formation of highly critical Ti\u003csub\u003e2\u003c/sub\u003eNi IMC cannot be fully avoided. However, it was also demonstrated that by increasing the proportion of Nb, high concentrations of those IMC can be prevented, which ultimately results in excellent mechanical properties under quasi-static mechanical load. Future investigations on the dissimilar beam welding of Ti / Nb / NiTi should therefore aim to improve the melting behavior of large Nb volumes by further optimizing the various process parameters or by geometrical adjustments.\u0026nbsp;\u003c/p\u003e"},{"header":"4 Conclusion","content":"\u003cp\u003eIn the investigation at hand, NiTi was joined to cp-Ti by means of micro electron beam welding using Nb as filler material. Based on the experimental results, the following main conclusions can be drawn from this study:\u003c/p\u003e\n\u003cul\u003e\n \u003cli\u003eOwing to the high melting temperature and thermal conductivity of the Nb filler material, fully melting is not feasible on the given welding configuration without emitting excessive energy that would cause burn-through defects.\u0026nbsp;\u003c/li\u003e\n \u003cli\u003eBy utilizing the process-inherent fast beam oscillation, the process energy can be coupled more evenly into the filler material, thus, significantly improving the melting behavior. Nevertheless, smaller accumulations of Nb remain in the weld metal, as was observed particularly along the fracture surface.\u003c/li\u003e\n \u003cli\u003eIncreasing the dimension of the applied Nb filler material promotes the affinity for weld seam irregularities, e.g., pores and hot cracks. Furthermore, it was not feasible to achieve uniform melting despite the utilization of a high beam current and specifically adjusted beam oscillation.\u0026nbsp;\u003c/li\u003e\n \u003cli\u003eNanoindentation mappings revealed critical hardness peaks in the weld metal of the samples welded with the smaller dimension of Nb. These can be attributed to a pronounced formation of Ti\u003csub\u003e2\u003c/sub\u003eNi IMC and are considered to be the major reason for the premature and brittle failure during the quasi-static tensile tests.\u003c/li\u003e\n \u003cli\u003eSuperior mechanical properties were achieved by using Nb with a higher thickness and specifically adapted oscillation parameters despite an inhomogeneous elemental distribution and severe welding irregularities. This can be attributed to the increased proportion of Nb in the weld metal, which leads to a reduced formation of highly critical Ti\u003csub\u003e2\u003c/sub\u003eNi IMC.\u003c/li\u003e\n \u003cli\u003eUltimate tensile strengths of up to 673 MPa were achieved, which correspond approximately to the strength of the titanium base material. Thus, it can be confirmed that micro electron beam welding is a suitable welding process to produce high-quality NiTi / Nb / Ti joints. Due to the excellent mechanical properties under quasi-static load, the results offer high potential to be adopted in future industrial applications, e.g., medical technology parts.\u003c/li\u003e\n\u003c/ul\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eFunding\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eNo funding was obtained for this study.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAcknowledgements\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe corresponding author thankfully acknowledges the support provided within the framework of \u0026ldquo;DVS IIW Young Professionals\u0026rdquo; by the German Welding Society. Furthermore, the authors would like to thank the technician employees and student assistants for their continuous support.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eConflict of interests\u003c/strong\u003e The authors declare that there exists no competing financial interest or personal relationships that could have appeared to influence the work reported in this paper.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eKah P, Shrestha M, Martikainen J (2013) Trends in Joining Dissimilar Metals by Welding. AMM 440:269\u0026ndash;276. https://doi.org/10.4028/www.scientific.net/AMM.440.269\u003c/li\u003e\n\u003cli\u003eQuazi MM, Ishak M, Fazal MA et al. (2020) Current research and development status of dissimilar materials laser welding of titanium and its alloys. Optics \u0026amp; Laser Technology 126:106090. https://doi.org/10.1016/j.optlastec.2020.106090\u003c/li\u003e\n\u003cli\u003eHartl DJ, Lagoudas DC (2007) Aerospace applications of shape memory alloys. Proceedings of the Institution of Mechanical Engineers, Part G: Journal of Aerospace Engineering 221:535\u0026ndash;552. https://doi.org/10.1243/09544100JAERO211\u003c/li\u003e\n\u003cli\u003eMohd Jani J, Leary M, Subic A et al. (2014) A review of shape memory alloy research, applications and opportunities. Materials \u0026amp; Design (1980-2015) 56:1078\u0026ndash;1113. https://doi.org/10.1016/j.matdes.2013.11.084\u003c/li\u003e\n\u003cli\u003ePetrini L, Migliavacca F (2011) Biomedical Applications of Shape Memory Alloys. Journal of Metallurgy 2011:1\u0026ndash;15. https://doi.org/10.1155/2011/501483\u003c/li\u003e\n\u003cli\u003eSaadat S, Salichs J, Noori M et al. (2002) An overview of vibration and seismic applications of NiTi shape memory alloy. Smart Mater Struct 11:218\u0026ndash;229. https://doi.org/10.1088/0964-1726/11/2/305\u003c/li\u003e\n\u003cli\u003eHermawan H, Ramdan D, P. Djuansjah JR (2011) Metals for Biomedical Applications. In: Fazel R (ed) Biomedical Engineering - From Theory to Applications. InTech\u003c/li\u003e\n\u003cli\u003ePeters M, Kumpfert J, Ward CH et al. (2003) Titanium Alloys for Aerospace Applications. Adv Eng Mater 5:419\u0026ndash;427. https://doi.org/10.1002/adem.200310095\u003c/li\u003e\n\u003cli\u003eVeiga C, Davim JP, Loureiro AJR (2012) Properties and applications of titanium alloys: A brief review. Rev. Adv. Mater. Sci.:133\u0026ndash;148\u003c/li\u003e\n\u003cli\u003eHu L, Xue Y, Shi F (2017) Intermetallic formation and mechanical properties of Ni-Ti diffusion couples. Materials \u0026amp; Design 130:175\u0026ndash;182. https://doi.org/10.1016/j.matdes.2017.05.055\u003c/li\u003e\n\u003cli\u003eShojaei Zoeram A, Akbari Mousavi SAA (2014) Laser welding of Ti\u0026ndash;6Al\u0026ndash;4V to Nitinol. Materials \u0026amp; Design 61:185\u0026ndash;190. https://doi.org/10.1016/j.matdes.2014.04.078\u003c/li\u003e\n\u003cli\u003eDatta S, Raza MS, Kumar S et al. (2018) Exploring the possibility of dissimilar welding of NiTi to Ti using Yb-fiber laser. Advances in Materials and Processing Technologies 4:614\u0026ndash;625. https://doi.org/10.1080/2374068X.2018.1486533\u003c/li\u003e\n\u003cli\u003eMiranda RM, Assun\u0026ccedil;\u0026atilde;o E, Silva RJC et al. (2015) Fiber laser welding of NiTi to Ti-6Al-4V. Int J Adv Manuf Technol 81:1533\u0026ndash;1538. https://doi.org/10.1007/s00170-015-7307-8\u003c/li\u003e\n\u003cli\u003eTeshome FB, Peng B, Oliveira JP et al. (2022) Dissimilar laser welding of NiTi to Ti6Al4V via Zr interlayer. Materials and Manufacturing Processes:1\u0026ndash;10. https://doi.org/10.1080/10426914.2022.2089897\u003c/li\u003e\n\u003cli\u003eTeshome FB, Peng B, Oliveira JP et al. (2022) Microstructure, Macrosegregation, and Mechanical Properties of NiTi to Ti6Al4V Dissimilar Laser Welds Using Co Interlayer. J of Materi Eng and Perform 31:9777\u0026ndash;9790. https://doi.org/10.1007/s11665-022-07064-0\u003c/li\u003e\n\u003cli\u003eTeshome FB, Peng B, Oliveira JP et al. (2023) Role of Pd interlayer on NiTi to Ti6Al4V laser welded joints: Microstructural evolution and strengthening mechanisms. Materials \u0026amp; Design 228:111845. https://doi.org/10.1016/j.matdes.2023.111845\u003c/li\u003e\n\u003cli\u003eMurray JL (1981) The Nb\u0026minus;Ti (Niobium-Titanium) system. Bulletin of Alloy Phase Diagrams 2:55\u0026ndash;61. https://doi.org/10.1007/BF02873704\u003c/li\u003e\n\u003cli\u003eTorkamany MJ, Malek Ghaini F, Poursalehi R (2014) Dissimilar pulsed Nd:YAG laser welding of pure niobium to Ti\u0026ndash;6Al\u0026ndash;4V. Materials \u0026amp; Design (1980-2015) 53:915\u0026ndash;920. https://doi.org/10.1016/j.matdes.2013.07.094\u003c/li\u003e\n\u003cli\u003eOliveira JP, Panton B, Zeng Z et al. (2016) Laser joining of NiTi to Ti6Al4V using a Niobium interlayer. Acta Materialia 105:9\u0026ndash;15. https://doi.org/10.1016/j.actamat.2015.12.021\u003c/li\u003e\n\u003cli\u003eWiegand M, Marks L, Sommer N et al. (2022) Dissimilar micro beam welding of titanium to Nitinol and stainless steel using biocompatible filler materials for medical applications. Weld World. https://doi.org/10.1007/s40194-022-01412-3\u003c/li\u003e\n\u003cli\u003eWiegand M, Sommer N, Marks L et al. (2024) High-Strength Dissimilar Welds Between a NiTi Shape Memory Alloy and Titanium Obtained by Intermixing Niobium Using Pulsed Laser Beam Welding. Metall Mater Trans A 55:278\u0026ndash;290. https://doi.org/10.1007/s11661-023-07248-w\u003c/li\u003e\n\u003cli\u003eHellberg S, Hummel J, Kroo\u0026szlig; P et al. (2020) Microstructural and mechanical properties of dissimilar nitinol and stainless steel wire joints produced by micro electron beam welding without filler material. Weld World 64:2159\u0026ndash;2168. https://doi.org/10.1007/s40194-020-0\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":true,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"welding-in-the-world","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"witw","sideBox":"Learn more about [Welding in the World](https://www.springer.com/journal/40194)","snPcode":"40194","submissionUrl":"https://www.editorialmanager.com/witw/","title":"Welding in the World","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"Electron beam welding, dissimilar welding, medical technology, intermetallic compounds, microjoining","lastPublishedDoi":"10.21203/rs.3.rs-5245806/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-5245806/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eNickel-titanium shape memory alloys (NiTi) as well as titanium alloys (Ti) are essential materials in various modern medical technology applications. Combining them in functionally-graded components would allow the fabrication of highly innovative products with major economic and technical advantages. While dissimilar fusion welding of these materials is not feasible due to the formation of brittle intermetallic compounds, recent studies have shown that niobium (Nb) is a very promising filler material to overcome this limitation while simultaneously maintaining the biocompatibility of welded components.\u003c/p\u003e\n\u003cp\u003eThe present study seeks to expand the current knowledge regarding dissimilar fusion welding of the material combination NiTi / Nb / Ti by investigating micro electron beam welding in a butt-joint configuration. In addition to adapted power modulation, a novel approach of utilizing the process-inherent fast beam oscillation is applied to optimize the melting and intermixing behavior of the comparatively high-melting Nb. Furthermore, two different dimensions of the filler material measuring 0.2 and 0.4 mm in thickness are implemented and compared with regard to the microstructural evolution in the weld metal. It is demonstrated that the welding experiments are associated with major challenges due to the considerable differences in melting temperature and thermal conductivity of the base and filler materials. Nevertheless, the welded joints exhibit excellent mechanical properties under quasi-static tensile load, which can be attributed to a reduced formation of Ti\u003csub\u003e2\u003c/sub\u003eNi intermetallic compounds. Ultimate tensile strengths of up to 673 MPa can be achieved, proving that micro electron beam welding is a suitable process to produce high-quality dissimilar NiTi / Nb / Ti joints. \u0026nbsp;\u003c/p\u003e","manuscriptTitle":"Impact of beam oscillation and power modulation on the intermixing behavior of dissimilar Titanium / Niobium / Nitinol joints during micro electron beam welding","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-02-03 11:07:54","doi":"10.21203/rs.3.rs-5245806/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"reviewerAgreed","content":"","date":"2025-01-30T10:51:11+00:00","index":0,"fulltext":""},{"type":"reviewersInvited","content":"","date":"2025-01-30T10:46:00+00:00","index":"","fulltext":""},{"type":"editorInvited","content":"Welding in the World","date":"2024-10-18T12:41:55+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2024-10-15T23:18:48+00:00","index":"","fulltext":""},{"type":"submitted","content":"Welding in the World","date":"2024-10-15T09:11:48+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"welding-in-the-world","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"witw","sideBox":"Learn more about [Welding in the World](https://www.springer.com/journal/40194)","snPcode":"40194","submissionUrl":"https://www.editorialmanager.com/witw/","title":"Welding in the World","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"2e1caef7-949f-480a-a7e7-79a9ce633729","owner":[],"postedDate":"February 3rd, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"under-review","subjectAreas":[],"tags":[],"updatedAt":"2025-04-03T00:38:39+00:00","versionOfRecord":[],"versionCreatedAt":"2025-02-03 11:07:54","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-5245806","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-5245806","identity":"rs-5245806","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}