Examination of Ti2Ni formation during dissimilar friction stir welding of thin sheet NiTi and Ti-6Al-4V

Examination of Ti2Ni formation during dissimilar friction stir welding of thin sheet NiTi and Ti-6Al-4V | 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 Examination of Ti2Ni formation during dissimilar friction stir welding of thin sheet NiTi and Ti-6Al-4V Brayden Terry, Austen Shelton, Alvin Strauss This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-6271208/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 Solid-state dissimilar joining of NiTi and Ti-6Al-4V is a currently underexplored field with high potential to expand applications for each alloy in the aerospace industry. Friction stir welding of butted 1 mm thick sheets of NiTi and Ti-6Al-4V was tested under a matrix of welding parameters. Six of eight tested parameter conditions joined but each showed degraded mechanical properties. Higher traverse speed conditions joined more successfully into testable samples. Upper and lower quartiles for tensile strength varied between 140 to 60 MPa with the lower rotation speed showing higher median values. Weld degradation is attributed to the formation of an up to 10 µm wide Ti2Ni intermetallic compound layer at the weld interface. Higher rotation speeds showed a thicker intermetallic layer. The Ti2Ni layer showed equiaxed grains on the order of 2-3µm in diameter. It is theorized that this layer grew from pre-existing Ti-6Al-4V via nickel diffusion from the NiTi due to in-process heating. Accelerated property mapping nanoindentation shows that the Ti2Ni layer has a greater microhardness and reduced elastic modulus (12.42 GPa and 150.06 GPa) than the stir zone of the NiTi (5.10 GPa and 97.65 GPa) and Ti-6Al-4V (5.93 GPa and 137.2 GPa). Crack propagation along this brittle, high stiffness intermetallic layer is proposed as the cause of failure in the welded samples. Friction Stir Welding Dissimilar Friction Stir Welding NiTi Shape Memory Alloy Titanium Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 Figure 10 Introduction Titanium alloys are consistently used in the aerospace industry due to their high specific strength, elevated temperature capabilities, and excellent corrosion resistance. These properties allow titanium to replace components that were historically made of steel, aluminum, or nickel alloys. Among titanium alloys, the most commonly used for aerospace applications is Ti-6Al-4V, also known as Ti-64 or Grade 5 Titanium [1]. Ti-64 is an α-β alloy that possesses the increased ductility of α-alloys, such as commercially pure Ti, while also containing a small weight percentage of stabilized β-phase, providing an increase in strength while sacrificing little in the way of ductility. This makes it an attractive option for the aerospace industry in many functions. Nitinol is the colloquial name for a family of nickel titanium shape memory alloys (SMA) of which the most common is the near-equiatomic alloy NiTi. Aerospace applications of nitinol and other SMA’s have been a topic of research for well over two decades now. NiTi SMA’s are interesting for aerospace due their superb mechanical properties and high corrosion resistance along with the unique possibilities due to the shape memory effect and superelastic effect. Large scale applications, such as smart, morphing wing surfaces and temperature-controlled cowls, have seen limited use. The primary uses of SMA components have been as low-shock actuators, vibration control, and temperature sensors [2, 3, 4]. NiTi joining is a notoriously difficult process due to the high temperature and mechanical sensitivity of nitinol along with atmospheric embrittlement. Any changes to its thermo-mechanical history are reflected in changes to the shape memory effect and superelastic properties. This is especially worth considering in joining processes, such as welding, where property changes can be drastic due to highly localized heat sources. Titanium welding also sees difficulties due to titanium’s extreme reactivity to atmospheric gases under high temperatures. In part to combat these difficulties, research around friction stir welding (FSW) these alloys has shown growth in recent years [5-8]. Among the capabilities of FSW is joining dissimilar materials without the creation of a weld pool and resulting solidification defects. Dissimilar joining of NiTi to Ti-64 will provide new opportunities to aerospace manufacturers in their designs. Current research on joining NiTi with Ti-64 is largely dominated by laser welding [9-13]. Difficulties encountered in these efforts show that joints between NiTi and Ti64 are frequently brittle and crack during formation or cooling due to material property mismatch and formation of intermetallic compounds of which the main culprit is often Ti 2 Ni. Dissimilar friction stir welding (DFSW) research on NiTi and Ti-64 is currently extremely limited with a pair of investigations into general friction welding of these alloys and only one published article that utilized FSW to join them [14-16]. Rehman et. al. performed two separate studies that used rotational and linear friction welding, respectively, to produce successful joints between the alloys in question [14, 15]. Both joints experienced reduced strength and elongation compared to the base materials. Authors attributed these reductions to the formation of Ti 2 Ni layers at the interface of the joint. Deng et. al. performed successful DFSW of NiTi and Ti64 using back-heating assisted FSW [16]. They identified the extensive formation of Ti 2 Ni within the weld and attributed these formations as causes of failure during mechanical testing. Joints strength was also considerably lower than the base material as with the previous research. Clearly, limiting intermetallic formation is of key importance when creating a successful joint between NiTi and Ti-64. This study takes care to examine the formation of intermetallic compounds in the context of varied weld parameters and resulting mechanical properties. Methods and Materials 2.1 Materials The materials joined were near-equiatomic nitinol and the alpha-beta titanium alloy, Ti-6Al-4V. The nitinol and titanium sheets had dimensions of 25.4 mm (1.0”) wide and 152.4 mm (6.0”) long with a thickness of 1 mm (0.039”). The NiTi had an austenitic finish temperature of 45 °C. Table 1 shows the expected mechanical properties of the base materials. Table 1 Literature values for material properties of NiTi and Ti-6Al-4V. Values vary depending on material phase and heat treatment [17]. Material Elastic Modulus (GPa) Yield Strength 0.2% Offset (MPa) Ultimate Tensile Strength (MPa) Elongation at Failure (%) Melting Point (°C) NiTi (austenite) 83 195-690 895 25-50 1300 NiTi (martensite) 28-41 70-140 1900 5-10 1300 Ti-6Al-4V 106-114 828-1075 897-1205 10-18 1649 2.2 Joint Fabrication Sheets were joined via FSW in a butted configuration. In the dissimilar welds, the Ti-6Al-4V was placed on the advancing side (AS) of the rotating tool. This is in accordance with the findings that the stronger material should generally be placed on the AS in DFSW [18]. Preliminary testing led to a range of weld parameters that allowed for successful joining. The primary variable parameters were rotation speed and traverse speed as shown in Table 2. Each of the four traverse speeds was combined with each rotation speed. Three replicates were attempted for each condition chosen. Each successful replicate was then cut using wire EDM across the joint into five dogbone tensile specimens and three rectangular specimens for further characterization methods. Run order among all replicates was randomized to minimize environmental error factors. Table 2 Variable parameters chosen for testing. Each combination of parameters was performed three times to test viability in joining. Traverse Speed Rotation Speed 38.1 mm/min (1.5 in/min) 1600 rpm 50.8 mm/min (2.0 in/min) 63.5 mm/min (2.5 in/min) 1400 rpm 76.2 mm/min (3.0 in/min) Fixed parameters for the weld were a tool tilt of 1.5 degrees and a tool plunge depth at 0.3048 mm (0.012”). Tool plunge speed upon weld initiation was 0.508 mm/min (0.02 in/min). This relatively slow plunge speed was used to allow additional material heating during the plunge to promote plasticization at the beginning of the weld. The tool was a tungsten carbide (WC) rod with a 12.7 mm (0.5”) diameter. The tool face was flat and pinless with no scrolling or additional features. 2.3 Mechanical Testing To evaluate the strength and ductility of the dissimilar joints, tensile testing was performed according to ASTM E8 [19]. This utilized the five dogbone specimens cut from successful weld replicates. These specimens were pulled to failure at a rate of 0.56 mm/min to determine ultimate tensile strength (UTS) and elongation at failure. These values were compared amongst each other condition that joined as well as each base material to help determine which welding parameters promote successful joining. 2.4 Electron Microscopy Characterization specimens from welds that successfully joined at the four extremes of the test regime were mounted in epoxy then ground flat. They were then polished in steps of increasing fineness to a mirror finish along the transverse cross section of the weld, ending with 1µm polycrystalline diamond paste. Backscatter electron images (BSE) were taken of the weld interfaces using a FEI FEG Quanta 650 environmental scanning electron microscope (ESEM). A working distance of 10 mm was used with an accelerating voltage of 15 kV and a spot size of 3.5. Multiple BSE images were taken in a grid to accommodate the entirety of each weld interface and stitched together into one large composite image for comparison and analysis. Further microscopy was accomplished with a FEI Tecnai G2 Osiris scanning/transmission electron microscope (S/TEM). Bright field imaging and energy dispersive spectroscopy (EDS) were performed at 200 kV on a small slice of joined material taken across the weld interface from a welded specimen. This slice was prepared using a FEI Helios NanoLab G3 CX DualBeam focused ion beam and scanning electron microscope (FIB-SEM). 2.5 X-Ray Diffraction Weld specimens and base material specimens were analyzed with X-ray diffraction (XRD) using a Rigaku Smart Lab X-ray diffractometer with a Cu Kα X-ray source. Peak comparisons were made between phases identified in base materials and the weld zones of joined specimens in order to identify any new phases that may have formed. 2.6 Nanoindentation and Property Mapping Accelerated Property Mapping (XPM) nanoindentation was performed on a polished weld sample joined at 1600 rpm and 50.8 mm/min. A Hysitron TI 980 TriboIndenter with a diamond Berkovich tip was used to test four locations at or near the weld interface. The sample was tested to have an average surface roughness at or below 50 nm before indentation. Indents were performed in a square 6 x 6 point grid with 5 µm between each point, resulting in a 25 x 25 µm grid. Indents were performed with trapezoidal loading pattern at a maximum load of 2000 µN. Each indent’s load-displacement curve was analyzed to report the reduced elastic modulus (GPa) and the hardness (GPa) of the test locations. These were then compiled into heat maps to compare the reduced modulus and hardness of desired locations and materials. Results and Discussion 3.1 Weld Quality As seen in Table 3, welds were successfully produced in six of the eight conditions attempted, though lower tool traversal speeds showed difficulty in consistent joining. Welds that failed often did so due to post-weld shrinkage cracking or inconsistent material mixing due to high material stiffness, leaving unjoined sections along the weld. Table 3 Most weld parameters were successful with lower traverse speeds leading to joining difficulty more often. Traverse Speed 1600 rpm 1400 rpm 38.1 mm/min (1.5 in/min) Did Not Join Success 50.8 mm/min (2.0 in/min) Success Did Not Join 63.5 mm/min (2.5 in/min) Success Success 76.2 mm/min (3.0 in/min) Success Success Examples of welds chosen for further examination are seen in Figure 1. As shown, the weld surfaces are clean with minor amounts of flashing. There is some oxidation at the beginning of the welds, but this diminishes as the welds progress. A centerline on the joint surface can be seen between the two materials, though this does not indicate a crack beneath the surface. Rather, it is a result of the more ductile NiTi being smeared over the surface of the less ductile Ti64, along with reflectivity differences between the materials, resulting in a textural and visual effect following the joint line of the weld. 3.2 Mechanical Testing The mechanical properties of the welded joints showed clear degradation compared to base material properties. Figure 2 shows the collected ultimate tensile strengths for all samples joined at1400 rpm and 1600 rpm. Weld strengths showed large deviations from the average with some specimens as low as 10.2 MPa and as high as 165.9 MPa. For the range of parameters examined, there is not a clear trend relating strength to changes in traversal speed when rotational speed is held constant nor to changes in rotational speed when traversal speed is constant. The joints demonstrated a significant lack of ductility as a majority of tested specimens failing before 0.5% strain. Figure 3 shows examples of engineering stress-strain curves for the samples created under parameter maximums and minimums. The specimens typically failed before showing any form of yield stress plateau as commonly seen in NiTi tensile behavior. Each specimen tested failed along the joint line, demonstrating brittle failure of the joined material. The failures began at the bottom of the welds and propagated upwards along the joint line as stress concentration increased along the weld seam. Poor mechanical properties in the joined material and brittle failure have been seen in friction welding of NiTi and Ti64 and attributed to formation of Ti 2 Ni along the joint. Rehman et. al. reported the formation of Ti 2 Ni in both rotational and linear friction welding, and Deng et. al. found that Ti 2 Ni formed during back-heating assisted FSW [14, 15, 20]. The formation of Ti 2 Ni is a unifying factor in the reduced mechanical performance of NiTi to Ti-64 friction welding studies so far. 3.3 Interface Examination and Intermetallic Compound Identification BSE images of weld interfaces of samples joined under parameter maximums and minimums were collected as seen in Figure 4. This was done to examine the interfaces of the joints based upon the reduced mechanical properties of the welds and literature knowledge of intermetallic compound (IMC) formation during friction joining of these alloys. In each sample, the weld zone (WZ) shows a sharp interface between the two materials with some evidence of mixing within the stir zone of the tool. This interfacial mixing is limited and decreases further from the surface of the weld where tool interaction was highest. Also visible at the interface is a third phase that formed along the joint during FSW. XRD was performed to determine the material phases found within the weld area. In each sample, peaks were found for alpha titanium and the B2 austenitic NiTi phase as expected. Strong peaks for Ti 2 Ni were also found, identifying the IMC formed at the joint interface. Results from one of the welds examined are presented in Figure 5. The Ti 2 Ni IMC layers examined showed slight changes in size based on weld parameters used. It was observed that the welds performed at 1400 rpm showed layer widths between 7-1 µm with sporadic areas up to 15 µm in width interspersed along the interface. Welds joined at 1600 rpm showed IMC layer widths between 10-1 µm with some areas growing up to 14 µm in width. In each weld examined, the IMC layer width was larger nearer to the surface of the weld. This is likely due to higher heat at the joints surface leading to faster diffusion rates. It is therefore proposed that the higher rotational speed led to a greater IMC layer width due to an increase in heat input in the work piece. This finding can be compared to the work of Deng et al. who found that back-heating assisted FSW at 450 rpm led to an Ti 2 Ni layer thickness of 1µm in the SZ [20]. They also reported NiTi debris swept into the Ti-64 leading to 9µm diameter regions of Ti 2 Ni forming. This debris fracturing behavior was not observed in this study, though there was some bulk movement of NiTi, as seen in Figure 4c). Closer examination of the weld interface, shown in Figure 6, reveals the grain structure of Ti 2 Ni forming at the joint, along with crack propagation on this interface due to the newly formed IMC layer. It has been shown by Bastin and Rieck that Ni is the faster diffusional component in the Ti-Ni system [21]. This indicates that it is likely that the nickel from the NiTi side of the joint diffused into the Ti-64 to create Ti 2­ Ni. This is further evidenced by the Ti 2 Ni grains growing from within the Ti-64 matrix. As the IMC grains form and enlarge, they consume the Ti-64, leaving the newly formed grains surrounded by residual amounts of the titanium alloy. This does not discount the fact that any titanium diffusing in the opposite direction into the NiTi would also cause local stabilization of the Ti 2 Ni phase, which is one of the common precipitates to form in Ti-rich nitinol. Imaging the IMC layer using TEM reveals the interfaces between Ti 2 Ni and the bulk materials, along with the individual Ti 2 Ni grains that have formed within the joint. Figure 7 illustrates that these grains range from 2 to 3 µm in diameter, suggesting that the IMC layer typically consists of only a few grains, depending on its thickness. A similar trend is observed in Figure 6a, where the IMC grain sizes vary but generally remain below 3 µm. TEM-EDS mapping, along with mass quantification shown in Table 4, reveals a strong presence of nickel and titanium along with minor amounts of aluminum. Vanadium was present in negligible quantities. The presence of aluminum in the interface further supports the theory that nickel diffusion into and consumption of Ti64 are driving the formation of the intermetallic layer. Table 4 Mass quantification results of EDS spots taken within the intermetallic layer. Element Ti Ni Al V Mass Percent 57.946 39.979 1.928 0.146 3.4 Mechanical Properties of the Ti 2 Ni Layer Highspeed nanoindentation was performed using XPM to determine the hardness and reduced elastic moduli of both the workpiece materials and the Ti 2 Ni seen at the interface. At selected locations shown in Figure 8, tests were performed on areas of NiTi and Ti-64 within the WZ as well as performed on the weld interface to capture the Ti 2 Ni properties in comparison to the bulk materials. Figure 9 shows the hardness in GPa for each material. The average hardness values of the base materials were 5.10 GPa (SD = 0.6) for NiTi and 5.93 GPa (SD = 0.5) for Ti-64. The IMC layer was found to be over twice as hard at 12.42 GPa (SD = 1.7). These findings coincide with Deng et. al. who reported an increase in Vickers microhardness in the weld interface due to Ti 2 Ni formation [20]. Seen in Figure 10, the NiTi and Ti-64 respectively have reduced elastic moduli of 97.65 GPa (SD = 4.99) and 137.2 GPa (SD = 7.76). In comparison, the Ti 2 Ni has a higher modulus of 150.06 GPa (SD = 11.06). This modulus mismatch between the layered materials will result in a stress concentration within the higher stiffness Ti 2 Ni. It also corroborates the sudden and brittle failure of the tensile test specimen along the weld joint. This points to the Ti 2 Ni layer as the primary cause for reduced mechanical performance of the dissimilar joints due to loss of strain accommodation at the weld interface. Conclusion Dissimilar friction stir welding is a currently underexplored method for joining NiTi and Ti-6Al-4V. Developing processes that can bond these materials that are regularly used in both aerospace and biomedical industries will open new design spaces for each. This study demonstrates that friction stir welding can successfully join 1mm thick butted sheets of NiTi and Ti-6Al-4V within the parameters examined. However, they are difficult to join at lower traverse speeds due to increased heat input leading to shrinkage cracking. Even among successful welds mechanical properties were severely diminished due to formation of a Ti 2 Ni intermetallic compound layer at the weld interface. Most specimens failed before 0.5% strain and tensile strength only reached a maximum of 165.9 MPa with sample averages between 120 and 64 MPa. Formation of the Ti 2 Ni layer is primarily attributed to nickel diffusion from NiTi into Ti-6Al-4V at the mixed interface. This diffusion stabilizes the transformation of bulk titanium from the Ti-6Al-4V into Ti 2 Ni as evidenced by retained amounts of the base alloy surrounding newly formed grains of the intermetallic compound. Nanoindentation measurements taken of the newly formed Ti 2 Ni and the stirred materials found in the weld zone show a severe mismatch of mechanical properties. The Ti 2 Ni is twice as hard and has a reduced elastic modulus higher than both the base materials in the weld zone. These differences lead to stress concentration, crack propagation, and eventual brittle failure of the joints within the intermetallic layer. Therefore, future investigations of friction stir welding of NiTi and Ti-6Al-4V should determine methods for minimizing the formation of Ti 2 Ni at the weld interface. Declarations Acknowledgements and Funding The authors thank the NASA Tennessee Space Grant Consortium for their support and funding. The authors thank Skyline Manufacturing in Nashville, TN for their assistance in specimen preparation via wire EDM. Data Availability Statement Not Applicable Author Contributions Brayden Terry – welding, sample creation, data collection and analysis, figure creation, writing Austen Shelton – figure creation, concept review, manuscript review and editing Alvin Strauss – manuscript review and editing, supervision, funding acquisition Declaration of Competing Interests The authors have no competing interests to declare. References Williams JC, Boyer RR. Opportunities and Issues in the Application of Titanium Alloys for Aerospace Components. Metals. 2020; 10(6):705. doi:10.3390/met10060705 Chaudari R, Vora JJ, Parikh DM. A Review on Applications of Nitinol Shape Memory Alloy. In: Parwani AK, Ramkumar PL, Abhishek K, Yadav SK, editors. Recent Adv in Mech Infrastruct. Proceedings of ICRAM; 2020 Aug 21-23; Ahmedabad, India. Singapore: Springer; 2021. p. 123-132. Sharma N, Raj T, Jangra K. Applications of nickel-titanium alloy. Journal of Engineering and Technology. 2015;5(1):1-7. Costanza G, Tata ME. Shape Memory Alloys for Aerospace, Recent Developments, and New Applications: A Short Review. Materials. 2020; 13(8):1856. doi: 10.3390/ma13081856 Datta S, Bhattacharjee R, Biswas P. Present status and future trend of friction stir-based fabrication of NiTinol: a review. Weld World 67, 269–307 (2023). doi: 10.1007/s40194-022-01384-4 Mani Prabu SS, Aravindan S, Ghosh S, Palani IA. Solid-state welding of nitinol shape memory alloys: A review. Mater Today Commun. 2023;35:105728. doi: 10.1016/j.mtcomm.2023.105728 Russell MJ, Blignault C, Horrex NL, Wiesner CS . Recent Developments in the Friction Stir Welding of Titanium Alloys. Weld World 52 , 12–15 (2008). doi:10.1007/BF03266662 Gangwar K, Ramulu M. Friction stir welding of titanium alloys: A review. Materials & Design. 2018;141:230-255. doi: 10.1016/j.matdes.2017.12.033. Rehman AU, Usmani YS, Babu NK, Talari MK, Al-khalifa H. An Overview on Dissimilar Metals Joining Techniques for Nitinol Shape Memory Alloy to Titanium Alloy Ti-6Al-4V. Miranda RM, Assunção E, Silva RJ, Oliveira JP, Quintino L. Fiber laser welding of NiTi to Ti-6Al-4V. The International Journal of Advanced Manufacturing Technology. 2015;81:1533-1538. doi: 10.1007/s00170-015-7307-8 Zoeram AS, Mousavi SA. Laser welding of Ti–6Al–4V to Nitinol. Materials & Design. 2014;61:185-90. doi: 10.1016/j.matdes.2014.04.078 Yuhua C, Yuqing M, Weiwei L, Peng H. Investigation of welding crack in micro laser welded NiTiNb shape memory alloy and Ti6Al4V alloy dissimilar metals joints. Optics & Laser Technology. 2017;91:197-202. doi: 10.1016/j.optlastec.2016.12.028 Datta S, Raza MS, Kumar S, Saha P. Exploring the possibility of dissimilar welding of NiTi to Ti using Yb-fiber laser. Advances in Materials and Processing Technologies. 2018;4(4):614-25. Doi: 10.1080/2374068X.2018.1486533 Rehman AU, Babu NK, Talari MK, Usmani YS, Al-Khalefah H. Microstructure and mechanical properties of dissimilar friction welding Ti-6Al-4V alloy to Nitinol. Metals. 2021;11(1):109. Doi: 10.3390/met11010109 Rehman AU, Kishore Babu N, Talari MK, Usmani Y, Alkhalefah H. Characterisation of Microstructure and Mechanical Properties of Linear Friction Welded α+β Titanium Alloy to Nitinol. Applied Sciences. 2021; 11(22):10680. Doi:10.3390/app112210680 Deng H, Yuhua C, Yanlin J, Yong P, Timing Z, Shanlin W, Limeng Y. Microstructure and mechanical properties of dissimilar NiTi/Ti6Al4V joints via -backheating assisted friction stir welding. Journal of Manufacturing Processes. 2021; 64:379-391. doi: 10.1016/j.jmapro.2021.01.024 Cardarelli F, Materials Handbook: A Concise Desktop Reference, 2nd edn. Springer, London, 2008. pp 140, 310. Simar A, Avettand-Fenoel MN. State of the art about dissimilar metal friction stir welding. Science and Technology of Welding and Joining. 2017;22(5):389-403. doi: 10.1080/13621718.2016.1251712 American Society for Testing and Materials (ASTM). Standard Test Methods for Tension Testing of Metallic Materials. West Conshohocken (PA): ASTM; 2022. Standard E8/E8M-22. Deng H, Yuhua C, Yanlin J, Yong P, Timing Z, Shanlin W, Limeng Y. Microstructure and mechanical properties of dissimilar NiTi/Ti6Al4V joints via -backheating assisted friction stir welding. Journal of Manufacturing Processes. 2021; 64:379-391. doi: 10.1016/j.jmapro.2021.01.024 Bastin GF and Rieck GD. Diffusion in the titanium-nickel system: II. Calculations of chemical and intrinsic diffusion coefficients. Metall Trans 5, 1827–1831 (1974). doi:10.1007/BF02644147 Cite Share Download PDF Status: Under Review Version 1 posted Reviewers agreed at journal 03 Apr, 2025 Reviewers invited by journal 03 Apr, 2025 Editor invited by journal 01 Apr, 2025 Editor assigned by journal 28 Mar, 2025 First submitted to journal 26 Mar, 2025 You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-6271208","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":437946368,"identity":"b11c55c0-093f-471e-b671-09d7a2d7b344","order_by":0,"name":"Brayden Terry","email":"data:image/png;base64,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","orcid":"https://orcid.org/0009-0003-8396-5042","institution":"Vanderbilt University","correspondingAuthor":true,"prefix":"","firstName":"Brayden","middleName":"","lastName":"Terry","suffix":""},{"id":437946369,"identity":"6b18af6b-870c-4300-8e14-45a69019ac60","order_by":1,"name":"Austen Shelton","email":"","orcid":"","institution":"Vanderbilt University","correspondingAuthor":false,"prefix":"","firstName":"Austen","middleName":"","lastName":"Shelton","suffix":""},{"id":437946370,"identity":"a77f5ab7-e571-481a-a367-ee4d867f5eb3","order_by":2,"name":"Alvin Strauss","email":"","orcid":"","institution":"Vanderbilt University","correspondingAuthor":false,"prefix":"","firstName":"Alvin","middleName":"","lastName":"Strauss","suffix":""}],"badges":[],"createdAt":"2025-03-20 16:01:34","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-6271208/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-6271208/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":81290112,"identity":"fb20001a-4f84-4691-b4ed-7069c3f128f1","added_by":"auto","created_at":"2025-04-24 11:46:30","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":62389,"visible":true,"origin":"","legend":"\u003cp\u003eSuccessful welds from samples created with the extremes of tool rotation speed and traverse speed: a) 1400 rpm, 38.1 mm/min traverse, b) 1400 rpm, 76.2 mm/min traverse, c) 1600 rpm, 50.8 mm/min traverse, d) 1600 rpm, 76.2 mm/min traverse\u003c/p\u003e","description":"","filename":"floatimage1.png","url":"https://assets-eu.researchsquare.com/files/rs-6271208/v1/a641aa8af7d746adaf8548b8.png"},{"id":81290632,"identity":"52bf198d-72c5-4118-be01-99ff06ca87f1","added_by":"auto","created_at":"2025-04-24 11:54:30","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":38987,"visible":true,"origin":"","legend":"\u003cp\u003eSummarized ultimate tensile strength results for each traverse speed joined at 1400 and 1600 RPM\u003c/p\u003e","description":"","filename":"floatimage2.png","url":"https://assets-eu.researchsquare.com/files/rs-6271208/v1/be88545bdffcb259cb27b99a.png"},{"id":81290152,"identity":"b10f0597-5dad-44b4-9157-f13228821b6e","added_by":"auto","created_at":"2025-04-24 11:46:31","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":93817,"visible":true,"origin":"","legend":"\u003cp\u003eStress-strain diagrams from specimens in welds performed at the highest and lowest traverse speeds joined for both rotation speeds\u003c/p\u003e","description":"","filename":"floatimage3.png","url":"https://assets-eu.researchsquare.com/files/rs-6271208/v1/6ebfcb537e073840fdb8dcd9.png"},{"id":81290633,"identity":"3c7a85fc-a29b-4ae7-b2c0-f207b4a10fc7","added_by":"auto","created_at":"2025-04-24 11:54:30","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":74874,"visible":true,"origin":"","legend":"\u003cp\u003eSEM images showing the weld interfaces cross-sections for welds performed at successful parameter maximums and minimums: a) 1400 rpm, 38.1 mm/min, b) 1400 rpm, 76.2 mm/min, c) 1600 rpm, 50.8 mm/min, d) 1600 rpm, 76.2 rpm\u003c/p\u003e","description":"","filename":"floatimage4.png","url":"https://assets-eu.researchsquare.com/files/rs-6271208/v1/934b6029a4d40125f095c131.png"},{"id":81290634,"identity":"dc302d0d-a696-4718-90e6-6097c2c45d82","added_by":"auto","created_at":"2025-04-24 11:54:30","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":27280,"visible":true,"origin":"","legend":"\u003cp\u003eX-ray diffraction pattern observed in a joint formed at 1400 rpm and 38.1 mm/min. It shows the peaks expected from both base materials as well as peaks for Ti\u003csub\u003e2\u003c/sub\u003eNi\u003c/p\u003e","description":"","filename":"floatimage5.png","url":"https://assets-eu.researchsquare.com/files/rs-6271208/v1/52de0b10c17f1c37e269f5ac.png"},{"id":81290136,"identity":"1866f0c9-3115-42a3-aa39-8d9cec5d7270","added_by":"auto","created_at":"2025-04-24 11:46:30","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":42914,"visible":true,"origin":"","legend":"\u003cp\u003eCloseup images of a) Ti\u003csub\u003e2\u003c/sub\u003eNi IMC layer at the weld interface in the middle of the weld and b) crack propagation along the joint via the IMC layer located near the bottom of the weld\u003c/p\u003e","description":"","filename":"floatimage6.png","url":"https://assets-eu.researchsquare.com/files/rs-6271208/v1/c9543061fdc64aefc64e640f.png"},{"id":81290638,"identity":"f270db3c-1b61-48b3-bfd1-535feceb7032","added_by":"auto","created_at":"2025-04-24 11:54:31","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":58308,"visible":true,"origin":"","legend":"\u003cp\u003eTEM cross-section taken from the weld interface of a sample joined at 1400 rpm and 38.1 mm/min and EDS mapping of the IMC layer found in the weld joint. NiTi is on the left of the specimen and Ti-64 is on the right. The inner layer is comprised of Ti\u003csub\u003e2\u003c/sub\u003eNi and some residual Ti-64 between the IMC grains. A TEM-EDS spectrum is show from an area taken within the IMC layer\u003c/p\u003e","description":"","filename":"floatimage7.png","url":"https://assets-eu.researchsquare.com/files/rs-6271208/v1/b7ea437d4a8b2aecf1fde84e.png"},{"id":81290118,"identity":"3ca6820c-9e53-43bc-a8f6-36f92744506e","added_by":"auto","created_at":"2025-04-24 11:46:30","extension":"png","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":43376,"visible":true,"origin":"","legend":"\u003cp\u003eSEM imaging of areas selected for XPM. Specimen shown was produced at 1600 rpm and 50.8 mm/min. Image has been rotated 90° counter-clockwise relative to XPM maps in order to match the orientation Figure 4\u003c/p\u003e","description":"","filename":"floatimage8.png","url":"https://assets-eu.researchsquare.com/files/rs-6271208/v1/46ef293222b74a02f41a5002.png"},{"id":81290140,"identity":"51db8b02-84d5-4aba-b86a-7a71fde639d4","added_by":"auto","created_at":"2025-04-24 11:46:30","extension":"png","order_by":9,"title":"Figure 9","display":"","copyAsset":false,"role":"figure","size":63908,"visible":true,"origin":"","legend":"\u003cp\u003eSections a) and b) are XPM hardness maps of the weld interface in two locations showing the high hardness of the IMC layer, c) XPM hardness mapping of bulk Ti-64 in the WZ, d) XPM hardness mapping of bulk NiTi in the WZ\u003c/p\u003e","description":"","filename":"floatimage9.png","url":"https://assets-eu.researchsquare.com/files/rs-6271208/v1/8206f9e1fd994ae3927c4124.png"},{"id":81290636,"identity":"741b059a-8dce-4ad2-9ff0-204d5ff88053","added_by":"auto","created_at":"2025-04-24 11:54:30","extension":"png","order_by":10,"title":"Figure 10","display":"","copyAsset":false,"role":"figure","size":67283,"visible":true,"origin":"","legend":"\u003cp\u003eSections a) and b) are XPM elastic modulus mapping in two locations of the weld interface showing elastic moduli differences between bulk materials and the IMC layer, c) XPM elastic modulus mapping of bulk Ti-64 in the WZ, d) XPM elastic modulus mapping of bulk NiTi in the WZ\u003c/p\u003e","description":"","filename":"floatimage10.png","url":"https://assets-eu.researchsquare.com/files/rs-6271208/v1/3b26daac50e680d32b6a8f34.png"},{"id":81291313,"identity":"77f53b73-9da1-430a-92e0-48cf8189f540","added_by":"auto","created_at":"2025-04-24 12:02:36","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1116227,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-6271208/v1/da222115-d135-486e-9f62-372a9a6fbfba.pdf"}],"financialInterests":"","formattedTitle":"Examination of Ti2Ni formation during dissimilar friction stir welding of thin sheet NiTi and Ti-6Al-4V","fulltext":[{"header":"Introduction","content":"\u003cp\u003eTitanium alloys are consistently used in the aerospace industry due to their high specific strength, elevated temperature capabilities, and excellent corrosion resistance. These properties allow titanium to replace components that were historically made of steel, aluminum, or nickel alloys. Among titanium alloys, the most commonly used for aerospace applications is Ti-6Al-4V, also known as Ti-64 or Grade 5 Titanium [1]. Ti-64 is an \u0026alpha;-\u0026beta; alloy that possesses the increased ductility of \u0026alpha;-alloys, such as commercially pure Ti, while also containing a small weight percentage of stabilized \u0026beta;-phase, providing an increase in strength while sacrificing little in the way of ductility. This makes it an attractive option for the aerospace industry in many functions.\u003c/p\u003e\n\u003cp\u003eNitinol is the colloquial name for a family of nickel titanium shape memory alloys (SMA) of which the most common is the near-equiatomic alloy NiTi. Aerospace applications of nitinol and other SMA\u0026rsquo;s have been a topic of research for well over two decades now. NiTi SMA\u0026rsquo;s are interesting for aerospace due their superb mechanical properties and high corrosion resistance along with the unique possibilities due to the shape memory effect and superelastic effect. Large scale applications, such as smart, morphing wing surfaces and temperature-controlled cowls, have seen limited use. The primary uses of SMA components have been as low-shock actuators, vibration control, and temperature sensors [2, 3, 4].\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eNiTi joining is a notoriously difficult process due to the high temperature and mechanical sensitivity of nitinol along with atmospheric embrittlement. Any changes to its thermo-mechanical history are reflected in changes to the shape memory effect and superelastic properties. This is especially worth considering in joining processes, such as welding, where property changes can be drastic due to highly localized heat sources. Titanium welding also sees difficulties due to titanium\u0026rsquo;s extreme reactivity to atmospheric gases under high temperatures. In part to combat these difficulties, research around friction stir welding (FSW) these alloys has shown growth in recent years [5-8].\u003c/p\u003e\n\u003cp\u003eAmong the capabilities of FSW is joining dissimilar materials without the creation of a weld pool and resulting solidification defects. Dissimilar joining of NiTi to Ti-64 will provide new opportunities to aerospace manufacturers in their designs. Current research on joining NiTi with Ti-64 is largely dominated by laser welding [9-13]. Difficulties encountered in these efforts show that joints between NiTi and Ti64 are frequently brittle and crack during formation or cooling due to material property mismatch and formation of intermetallic compounds of which the main culprit is often Ti\u003csub\u003e2\u003c/sub\u003eNi. Dissimilar friction stir welding (DFSW) research on NiTi and Ti-64 is currently extremely limited with a pair of investigations into general friction welding of these alloys and only one published article that utilized FSW to join them [14-16].\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eRehman et. al. performed two separate studies that used rotational and linear friction welding, respectively, to produce successful joints between the alloys in question [14, 15]. Both joints experienced reduced strength and elongation compared to the base materials. Authors attributed these reductions to the formation of Ti\u003csub\u003e2\u003c/sub\u003eNi layers at the interface of the joint. Deng et. al. performed successful DFSW of NiTi and Ti64 using back-heating assisted FSW [16]. They identified the extensive formation of Ti\u003csub\u003e2\u003c/sub\u003eNi within the weld and attributed these formations as causes of failure during mechanical testing. Joints strength was also considerably lower than the base material as with the previous research. Clearly, limiting intermetallic formation is of key importance when creating a successful joint between NiTi and Ti-64. This study takes care to examine the formation of intermetallic compounds in the context of varied weld parameters and resulting mechanical properties.\u003c/p\u003e"},{"header":"Methods and Materials","content":"\u003cp\u003e\u003cstrong\u003e\u003cem\u003e2.1 Materials\u003c/em\u003e\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe materials joined were near-equiatomic nitinol and the alpha-beta titanium alloy, Ti-6Al-4V. The nitinol and titanium sheets had dimensions of 25.4 mm (1.0\u0026rdquo;) wide and 152.4 mm (6.0\u0026rdquo;) long with a thickness of 1 mm (0.039\u0026rdquo;). The NiTi had an austenitic finish temperature of 45 \u0026deg;C. Table 1 shows the expected mechanical properties of the base materials.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eTable 1\u003c/strong\u003e Literature values for material properties of NiTi and Ti-6Al-4V. Values vary depending on material phase and heat treatment [17].\u003c/p\u003e\n\u003ctable border=\"1\" cellspacing=\"0\" cellpadding=\"0\"\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd style=\"width: 110px;\"\u003e\n \u003cp\u003eMaterial\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 105px;\"\u003e\n \u003cp\u003eElastic Modulus (GPa)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 104px;\"\u003e\n \u003cp\u003eYield Strength 0.2% Offset (MPa)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 105px;\"\u003e\n \u003cp\u003eUltimate Tensile Strength (MPa)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 108px;\"\u003e\n \u003cp\u003eElongation at Failure (%)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 90px;\"\u003e\n \u003cp\u003eMelting Point (\u0026deg;C)\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd style=\"width: 110px;\"\u003e\n \u003cp\u003eNiTi (austenite)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 105px;\"\u003e\n \u003cp\u003e83\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 104px;\"\u003e\n \u003cp\u003e195-690\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 105px;\"\u003e\n \u003cp\u003e895\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 108px;\"\u003e\n \u003cp\u003e25-50\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 90px;\"\u003e\n \u003cp\u003e1300\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd style=\"width: 110px;\"\u003e\n \u003cp\u003eNiTi (martensite)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 105px;\"\u003e\n \u003cp\u003e28-41\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 104px;\"\u003e\n \u003cp\u003e70-140\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 105px;\"\u003e\n \u003cp\u003e1900\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 108px;\"\u003e\n \u003cp\u003e5-10\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 90px;\"\u003e\n \u003cp\u003e1300\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd style=\"width: 110px;\"\u003e\n \u003cp\u003eTi-6Al-4V\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 105px;\"\u003e\n \u003cp\u003e106-114\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 104px;\"\u003e\n \u003cp\u003e828-1075\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 105px;\"\u003e\n \u003cp\u003e897-1205\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 108px;\"\u003e\n \u003cp\u003e10-18\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 90px;\"\u003e\n \u003cp\u003e1649\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n\u003c/table\u003e\u003cstrong\u003e\u003cem\u003e\u003cstrong\u003e\u003cem\u003e2.2 Joint Fabrication\u003c/em\u003e\u003c/strong\u003e\u003c/em\u003e\u003c/strong\u003e\n\u003cp\u003eSheets were joined via FSW in a butted configuration. In the dissimilar welds, the Ti-6Al-4V was placed on the advancing side (AS) of the rotating tool. This is in accordance with the findings that the stronger material should generally be placed on the AS in DFSW [18].\u003c/p\u003e\n\u003cp\u003ePreliminary testing led to a range of weld parameters that allowed for successful joining. The primary variable parameters were rotation speed and traverse speed as shown in Table 2. Each of the four traverse speeds was combined with each rotation speed. Three replicates were attempted for each condition chosen. Each successful replicate was then cut using wire EDM across the joint into five dogbone tensile specimens and three rectangular specimens for further characterization methods. Run order among all replicates was randomized to minimize environmental error factors.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eTable 2\u003c/strong\u003e Variable parameters chosen for testing. Each combination of parameters was performed three times to test viability in joining.\u003c/p\u003e\n\u003ctable border=\"1\" cellspacing=\"0\" cellpadding=\"0\"\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd style=\"width: 186px;\"\u003e\n \u003cp\u003eTraverse Speed\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 120px;\"\u003e\n \u003cp\u003eRotation Speed\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd style=\"width: 186px;\"\u003e\n \u003cp\u003e38.1 mm/min (1.5 in/min)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd rowspan=\"2\" style=\"width: 120px;\"\u003e\n \u003cp\u003e1600 rpm\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd style=\"width: 186px;\"\u003e\n \u003cp\u003e50.8 mm/min (2.0 in/min)\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd style=\"width: 186px;\"\u003e\n \u003cp\u003e63.5 mm/min (2.5 in/min)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd rowspan=\"2\" style=\"width: 120px;\"\u003e\n \u003cp\u003e1400 rpm\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd style=\"width: 186px;\"\u003e\n \u003cp\u003e76.2 mm/min (3.0 in/min)\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n\u003c/table\u003e\n\u003cp\u003eFixed parameters for the weld were a tool tilt of 1.5 degrees and a tool plunge depth at 0.3048 mm (0.012\u0026rdquo;). Tool plunge speed upon weld initiation was 0.508 mm/min (0.02 in/min). This relatively slow plunge speed was used to allow additional material heating during the plunge to promote plasticization at the beginning of the weld. The tool was a tungsten carbide (WC) rod with a 12.7 mm (0.5\u0026rdquo;) diameter. The tool face was flat and pinless with no scrolling or additional features.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e\u003cem\u003e2.3 Mechanical Testing\u003c/em\u003e\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eTo evaluate the strength and ductility of the dissimilar joints, tensile testing was performed according to ASTM E8 [19]. This utilized the five dogbone specimens cut from successful weld replicates. These specimens were pulled to failure at a rate of 0.56 mm/min to determine ultimate tensile strength (UTS) and elongation at failure. These values were compared amongst each other condition that joined as well as each base material to help determine which welding parameters promote successful joining.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e\u003cem\u003e2.4 Electron Microscopy\u003c/em\u003e\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eCharacterization specimens from welds that successfully joined at the four extremes of the test regime were mounted in epoxy then ground flat. They were then polished in steps of increasing fineness to a mirror finish along the transverse cross section of the weld, ending with 1\u0026micro;m polycrystalline diamond paste. Backscatter electron images (BSE) were taken of the weld interfaces using a FEI FEG Quanta 650 environmental scanning electron microscope (ESEM). A working distance of 10 mm was used with an accelerating voltage of 15 kV and a spot size of 3.5. Multiple BSE images were taken in a grid to accommodate the entirety of each weld interface and stitched together into one large composite image for comparison and analysis.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eFurther microscopy was accomplished with a FEI Tecnai G2 Osiris scanning/transmission electron microscope (S/TEM). Bright field imaging and energy dispersive spectroscopy (EDS) were performed at 200 kV on a small slice of joined material taken across the weld interface from a welded specimen. This slice was prepared using a FEI Helios NanoLab G3 CX DualBeam focused ion beam and scanning electron microscope (FIB-SEM).\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e\u003cem\u003e2.5 X-Ray Diffraction\u003c/em\u003e\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eWeld specimens and base material specimens were analyzed with X-ray diffraction (XRD) using a Rigaku Smart Lab X-ray diffractometer with a Cu K\u0026alpha; X-ray source. Peak comparisons were made between phases identified in base materials and the weld zones of joined specimens in order to identify any new phases that may have formed.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e\u003cem\u003e2.6 Nanoindentation and Property Mapping\u003c/em\u003e\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eAccelerated Property Mapping (XPM) nanoindentation was performed on a polished weld sample joined at 1600 rpm and 50.8 mm/min. \u0026nbsp;A Hysitron TI 980 TriboIndenter with a diamond Berkovich tip was used to test four locations at or near the weld interface. The sample was tested to have an average surface roughness at or below 50 nm before indentation. Indents were performed in a square 6 x 6 point grid with 5 \u0026micro;m between each point, resulting in a 25 x 25 \u0026micro;m grid. Indents were performed with trapezoidal loading pattern at a maximum load of 2000 \u0026micro;N. Each indent\u0026rsquo;s load-displacement curve was analyzed to report the reduced elastic modulus (GPa) and the hardness (GPa) of the test locations. These were then compiled into heat maps to compare the reduced modulus and hardness of desired locations and materials.\u003c/p\u003e"},{"header":"Results and Discussion","content":"\u003cp\u003e\u003cstrong\u003e\u003cem\u003e3.1 Weld Quality\u003c/em\u003e\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eAs seen in Table 3, welds were successfully produced in six of the eight conditions attempted, though lower tool traversal speeds showed difficulty in consistent joining. Welds that failed often did so due to post-weld shrinkage cracking or inconsistent material mixing due to high material stiffness, leaving unjoined sections along the weld.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eTable 3\u003c/strong\u003e Most weld parameters were successful with lower traverse speeds leading to joining difficulty more often.\u003c/p\u003e\n\u003cdiv\u003e\n \u003ctable border=\"1\" cellspacing=\"0\" cellpadding=\"0\" width=\"390\"\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd style=\"width: 185px;\"\u003e\n \u003cp\u003eTraverse Speed\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 103px;\"\u003e\n \u003cp\u003e1600 rpm\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 102px;\"\u003e\n \u003cp\u003e1400 rpm\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd style=\"width: 185px;\"\u003e\n \u003cp\u003e38.1 mm/min (1.5 in/min)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 103px;\"\u003e\n \u003cp\u003eDid Not Join\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 102px;\"\u003e\n \u003cp\u003eSuccess\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd style=\"width: 185px;\"\u003e\n \u003cp\u003e50.8 mm/min (2.0 in/min)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 103px;\"\u003e\n \u003cp\u003eSuccess\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 102px;\"\u003e\n \u003cp\u003eDid Not Join\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd style=\"width: 185px;\"\u003e\n \u003cp\u003e63.5 mm/min (2.5 in/min)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 103px;\"\u003e\n \u003cp\u003eSuccess\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 102px;\"\u003e\n \u003cp\u003eSuccess\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd style=\"width: 185px;\"\u003e\n \u003cp\u003e76.2 mm/min (3.0 in/min)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 103px;\"\u003e\n \u003cp\u003eSuccess\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 102px;\"\u003e\n \u003cp\u003eSuccess\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\u003eExamples of welds chosen for further examination are seen in Figure 1. As shown, the weld surfaces are clean with minor amounts of flashing. There is some oxidation at the beginning of the welds, but this diminishes as the welds progress. A centerline on the joint surface can be seen between the two materials, though this does not indicate a crack beneath the surface. Rather, it is a result of the more ductile NiTi being smeared over the surface of the less ductile Ti64, along with reflectivity differences between the materials, resulting in a textural and visual effect following the joint line of the weld.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e\u003cem\u003e3.2 Mechanical Testing\u003c/em\u003e\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe mechanical properties of the welded joints showed clear degradation compared to base material properties. Figure 2 shows the collected ultimate tensile strengths for all samples joined at1400 rpm and 1600 rpm. Weld strengths showed large deviations from the average with some specimens as low as 10.2 MPa and as high as 165.9 MPa. For the range of parameters examined, there is not a clear trend relating strength to changes in traversal speed when rotational speed is held constant nor to changes in rotational speed when traversal speed is constant.\u003c/p\u003e\n\u003cp\u003eThe joints demonstrated a significant lack of ductility as a majority of tested specimens failing before 0.5% strain. Figure 3 shows examples of engineering stress-strain curves for the samples created under parameter maximums and minimums. The specimens typically failed before showing any form of yield stress plateau as commonly seen in NiTi tensile behavior. Each specimen tested failed along the joint line, demonstrating brittle failure of the joined material. The failures began at the bottom of the welds and propagated upwards along the joint line as stress concentration increased along the weld seam. Poor mechanical properties in the joined material and brittle failure have been seen in friction welding of NiTi and Ti64 and attributed to formation of Ti\u003csub\u003e2\u003c/sub\u003eNi along the joint. Rehman et. al. reported the formation of Ti\u003csub\u003e2\u003c/sub\u003eNi in both rotational and linear friction welding, and Deng et. al. found that Ti\u003csub\u003e2\u003c/sub\u003eNi formed during back-heating assisted FSW [14, 15, 20]. The formation of Ti\u003csub\u003e2\u003c/sub\u003eNi is a unifying factor in the reduced mechanical performance of NiTi to Ti-64 friction welding studies so far.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e\u003cem\u003e3.3 Interface Examination and Intermetallic Compound Identification\u003c/em\u003e\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eBSE images of weld interfaces of samples joined under parameter maximums and minimums were collected as seen in Figure 4. This was done to examine the interfaces of the joints based upon the reduced mechanical properties of the welds and literature knowledge of intermetallic compound (IMC) formation during friction joining of these alloys. In each sample, the weld zone (WZ) shows a sharp interface between the two materials with some evidence of mixing within the stir zone of the tool. This interfacial mixing is limited and decreases further from the surface of the weld where tool interaction was highest.\u003c/p\u003e\n\u003cp\u003eAlso visible at the interface is a third phase that formed along the joint during FSW. XRD was performed to determine the material phases found within the weld area. In each sample, peaks were found for alpha titanium and the B2 austenitic NiTi phase as expected. Strong peaks for Ti\u003csub\u003e2\u003c/sub\u003eNi were also found, identifying the IMC formed at the joint interface. Results from one of the welds examined are presented in Figure 5.\u003c/p\u003e\n\u003cp\u003eThe Ti\u003csub\u003e2\u003c/sub\u003eNi IMC layers examined showed slight changes in size based on weld parameters used. It was observed that the welds performed at 1400 rpm showed layer widths between 7-1 \u0026micro;m with sporadic areas up to 15 \u0026micro;m in width interspersed along the interface. Welds joined at 1600 rpm showed IMC layer widths between 10-1 \u0026micro;m with some areas growing up to 14 \u0026micro;m in width. In each weld examined, the IMC layer width was larger nearer to the surface of the weld. This is likely due to higher heat at the joints surface leading to faster diffusion rates. It is therefore proposed that the higher rotational speed led to a greater IMC layer width due to an increase in heat input in the work piece.\u003c/p\u003e\n\u003cp\u003eThis finding can be compared to the work of Deng et al. who found that back-heating assisted FSW at 450 rpm led to an Ti\u003csub\u003e2\u003c/sub\u003eNi layer thickness of 1\u0026micro;m in the SZ [20]. They also reported NiTi debris swept into the Ti-64 leading to 9\u0026micro;m diameter regions of Ti\u003csub\u003e2\u003c/sub\u003eNi forming. This debris fracturing behavior was not observed in this study, though there was some bulk movement of NiTi, as seen in Figure 4c).\u003c/p\u003e\n\u003cp\u003eCloser examination of the weld interface, shown in Figure 6, reveals the grain structure of Ti\u003csub\u003e2\u003c/sub\u003eNi forming at the joint, along with crack propagation on this interface due to the newly formed IMC layer. It has been shown by Bastin and Rieck that Ni is the faster diffusional component in the Ti-Ni system [21]. This indicates that it is likely that the nickel from the NiTi side of the joint diffused into the Ti-64 to create Ti\u003csub\u003e2\u0026shy;\u003c/sub\u003eNi. This is further evidenced by the Ti\u003csub\u003e2\u003c/sub\u003eNi grains growing from within the Ti-64 matrix. As the IMC grains form and enlarge, they consume the Ti-64, leaving the newly formed grains surrounded by residual amounts of the titanium alloy. This does not discount the fact that any titanium diffusing in the opposite direction into the NiTi would also cause local stabilization of the Ti\u003csub\u003e2\u003c/sub\u003eNi phase, which is one of the common precipitates to form in Ti-rich nitinol.\u003c/p\u003e\n\u003cp\u003eImaging the IMC layer using TEM reveals the interfaces between Ti\u003csub\u003e2\u003c/sub\u003eNi and the bulk materials, along with the individual Ti\u003csub\u003e2\u003c/sub\u003eNi grains that have formed within the joint. Figure 7 illustrates that these grains range from 2 to 3 \u0026micro;m in diameter, suggesting that the IMC layer typically consists of only a few grains, depending on its thickness. A similar trend is observed in Figure 6a, where the IMC grain sizes vary but generally remain below 3 \u0026micro;m. TEM-EDS mapping, along with mass quantification shown in Table 4, reveals a strong presence of nickel and titanium along with minor amounts of aluminum. Vanadium was present in negligible quantities. The presence of aluminum in the interface further supports the theory that nickel diffusion into and consumption of Ti64 are driving the formation of the intermetallic layer.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eTable 4\u0026nbsp;\u003c/strong\u003eMass quantification results of EDS spots taken within the intermetallic layer.\u003c/p\u003e\n\u003cdiv\u003e\n \u003ctable border=\"1\" cellspacing=\"0\" cellpadding=\"0\"\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd style=\"width: 105px;\"\u003e\n \u003cp\u003eElement\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 91px;\"\u003e\n \u003cp\u003eTi\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 91px;\"\u003e\n \u003cp\u003eNi\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 81px;\"\u003e\n \u003cp\u003eAl\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 81px;\"\u003e\n \u003cp\u003eV\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd style=\"width: 105px;\"\u003e\n \u003cp\u003eMass Percent\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 91px;\"\u003e\n \u003cp\u003e57.946\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 91px;\"\u003e\n \u003cp\u003e39.979\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 81px;\"\u003e\n \u003cp\u003e1.928\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 81px;\"\u003e\n \u003cp\u003e0.146\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\u003e\u003cstrong\u003e\u003cem\u003e3.4 Mechanical Properties of the Ti\u003csub\u003e2\u003c/sub\u003eNi Layer\u003c/em\u003e\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eHighspeed nanoindentation was performed using XPM to determine the hardness and reduced elastic moduli of both the workpiece materials and the Ti\u003csub\u003e2\u003c/sub\u003eNi seen at the interface. At selected locations shown in Figure 8, tests were performed on areas of NiTi and Ti-64 within the WZ as well as performed on the weld interface to capture the Ti\u003csub\u003e2\u003c/sub\u003eNi properties in comparison to the bulk materials. Figure 9 shows the hardness in GPa for each material. The average hardness values of the base materials were 5.10 GPa (SD = 0.6) for NiTi and 5.93 GPa (SD = 0.5) for Ti-64. The IMC layer was found to be over twice as hard at 12.42 GPa (SD = 1.7). These findings coincide with Deng et. al. who reported an increase in Vickers microhardness in the weld interface due to Ti\u003csub\u003e2\u003c/sub\u003eNi formation [20].\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eSeen in Figure 10, the NiTi and Ti-64 respectively have reduced elastic moduli of 97.65 GPa (SD = 4.99) and 137.2 GPa (SD = 7.76). In comparison, the Ti\u003csub\u003e2\u003c/sub\u003eNi has a higher modulus of 150.06 GPa (SD = 11.06). This modulus mismatch between the layered materials will result in a stress concentration within the higher stiffness Ti\u003csub\u003e2\u003c/sub\u003eNi. It also corroborates the sudden and brittle failure of the tensile test specimen along the weld joint. This points to the Ti\u003csub\u003e2\u003c/sub\u003eNi layer as the primary cause for reduced mechanical performance of the dissimilar joints due to loss of strain accommodation at the weld interface.\u003c/p\u003e"},{"header":"Conclusion","content":"\u003cp\u003eDissimilar friction stir welding is a currently underexplored method for joining NiTi and Ti-6Al-4V. Developing processes that can bond these materials that are regularly used in both aerospace and biomedical industries will open new design spaces for each. This study demonstrates that friction stir welding can successfully join 1mm thick butted sheets of NiTi and Ti-6Al-4V within the parameters examined. However, they are difficult to join at lower traverse speeds due to increased heat input leading to shrinkage cracking. Even among successful welds mechanical properties were severely diminished due to formation of a Ti\u003csub\u003e2\u003c/sub\u003eNi intermetallic compound layer at the weld interface. Most specimens failed before 0.5% strain and tensile strength only reached a maximum of 165.9 MPa with sample averages between 120 and 64 MPa. Formation of the Ti\u003csub\u003e2\u003c/sub\u003eNi layer is primarily attributed to nickel diffusion from NiTi into Ti-6Al-4V at the mixed interface. This diffusion stabilizes the transformation of bulk titanium from the Ti-6Al-4V into Ti\u003csub\u003e2\u003c/sub\u003eNi as evidenced by retained amounts of the base alloy surrounding newly formed grains of the intermetallic compound. Nanoindentation measurements taken of the newly formed Ti\u003csub\u003e2\u003c/sub\u003eNi and the stirred materials found in the weld zone show a severe mismatch of mechanical properties. The Ti\u003csub\u003e2\u003c/sub\u003eNi is twice as hard and has a reduced elastic modulus higher than both the base materials in the weld zone. These differences lead to stress concentration, crack propagation, and eventual brittle failure of the joints within the intermetallic layer. Therefore, future investigations of friction stir welding of NiTi and Ti-6Al-4V should determine methods for minimizing the formation of Ti\u003csub\u003e2\u003c/sub\u003eNi at the weld interface.\u003c/p\u003e\n"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eAcknowledgements and Funding\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors thank the NASA Tennessee Space Grant Consortium for their support and funding.\u003c/p\u003e\n\u003cp\u003eThe authors thank Skyline Manufacturing in Nashville, TN for their assistance in specimen preparation via wire EDM.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eData Availability Statement\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eNot Applicable\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthor Contributions\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eBrayden Terry \u0026ndash; welding, sample creation, data collection and analysis, figure creation, writing\u003c/p\u003e\n\u003cp\u003eAusten Shelton \u0026ndash; figure creation, concept review, manuscript review and editing\u003c/p\u003e\n\u003cp\u003eAlvin Strauss \u0026ndash; manuscript review and editing, supervision, funding acquisition\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eDeclaration of Competing Interests\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors have no competing interests to declare.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eWilliams JC, Boyer RR. Opportunities and Issues in the Application of Titanium Alloys for Aerospace Components. Metals. 2020; 10(6):705. doi:10.3390/met10060705\u003c/li\u003e\n\u003cli\u003eChaudari R, Vora JJ, Parikh DM. A Review on Applications of Nitinol Shape Memory Alloy. In: Parwani AK, Ramkumar PL, Abhishek K, Yadav SK, editors. Recent Adv in Mech Infrastruct. Proceedings of ICRAM; 2020 Aug 21-23; Ahmedabad, India. Singapore: Springer; 2021. p. 123-132.\u003c/li\u003e\n\u003cli\u003eSharma N, Raj T, Jangra K. Applications of nickel-titanium alloy. Journal of Engineering and Technology. 2015;5(1):1-7.\u003c/li\u003e\n\u003cli\u003eCostanza G, Tata ME. Shape Memory Alloys for Aerospace, Recent Developments, and New Applications: A Short Review. Materials. 2020; 13(8):1856. doi: 10.3390/ma13081856\u003c/li\u003e\n\u003cli\u003eDatta S, Bhattacharjee R, Biswas P. Present status and future trend of friction stir-based fabrication of NiTinol: a review. Weld World 67, 269\u0026ndash;307 (2023). doi: 10.1007/s40194-022-01384-4\u003c/li\u003e\n\u003cli\u003eMani Prabu SS, Aravindan S, Ghosh S, Palani IA. Solid-state welding of nitinol shape memory alloys: A review. Mater Today Commun. 2023;35:105728. doi: 10.1016/j.mtcomm.2023.105728\u003c/li\u003e\n\u003cli\u003eRussell MJ, Blignault C, Horrex NL, Wiesner CS\u003cem\u003e.\u003c/em\u003e Recent Developments in the Friction Stir Welding of Titanium Alloys. \u003cem\u003eWeld World\u003c/em\u003e \u003cstrong\u003e52\u003c/strong\u003e, 12\u0026ndash;15 (2008). doi:10.1007/BF03266662\u003c/li\u003e\n\u003cli\u003eGangwar K, Ramulu M. Friction stir welding of titanium alloys: A review. Materials \u0026amp; Design. 2018;141:230-255. doi: 10.1016/j.matdes.2017.12.033.\u003c/li\u003e\n\u003cli\u003eRehman AU, Usmani YS, Babu NK, Talari MK, Al-khalifa H. An Overview on Dissimilar Metals Joining Techniques for Nitinol Shape Memory Alloy to Titanium Alloy Ti-6Al-4V.\u003c/li\u003e\n\u003cli\u003eMiranda RM, Assun\u0026ccedil;\u0026atilde;o E, Silva RJ, Oliveira JP, Quintino L. Fiber laser welding of NiTi to Ti-6Al-4V. The International Journal of Advanced Manufacturing Technology. 2015;81:1533-1538. doi: 10.1007/s00170-015-7307-8\u003c/li\u003e\n\u003cli\u003eZoeram AS, Mousavi SA. Laser welding of Ti\u0026ndash;6Al\u0026ndash;4V to Nitinol. Materials \u0026amp; Design. 2014;61:185-90. doi: 10.1016/j.matdes.2014.04.078\u003c/li\u003e\n\u003cli\u003eYuhua C, Yuqing M, Weiwei L, Peng H. Investigation of welding crack in micro laser welded NiTiNb shape memory alloy and Ti6Al4V alloy dissimilar metals joints. Optics \u0026amp; Laser Technology. 2017;91:197-202. doi: 10.1016/j.optlastec.2016.12.028\u003c/li\u003e\n\u003cli\u003eDatta S, Raza MS, Kumar S, Saha P. Exploring the possibility of dissimilar welding of NiTi to Ti using Yb-fiber laser. Advances in Materials and Processing Technologies. 2018;4(4):614-25. Doi: 10.1080/2374068X.2018.1486533\u003c/li\u003e\n\u003cli\u003eRehman AU, Babu NK, Talari MK, Usmani YS, Al-Khalefah H. Microstructure and mechanical properties of dissimilar friction welding Ti-6Al-4V alloy to Nitinol. Metals. 2021;11(1):109. Doi: 10.3390/met11010109\u003c/li\u003e\n\u003cli\u003eRehman AU, Kishore Babu N, Talari MK, Usmani Y, Alkhalefah H. Characterisation of Microstructure and Mechanical Properties of Linear Friction Welded \u0026alpha;+\u0026beta; Titanium Alloy to Nitinol. Applied Sciences. 2021; 11(22):10680. Doi:10.3390/app112210680\u003c/li\u003e\n\u003cli\u003eDeng H, Yuhua C, Yanlin J, Yong P, Timing Z, Shanlin W, Limeng Y. Microstructure and mechanical properties of dissimilar NiTi/Ti6Al4V joints via -backheating assisted friction stir welding. Journal of Manufacturing Processes. 2021; 64:379-391. doi: 10.1016/j.jmapro.2021.01.024\u003c/li\u003e\n\u003cli\u003eCardarelli F, \u003cem\u003eMaterials Handbook: A Concise Desktop Reference, 2nd edn. \u003c/em\u003eSpringer, London, 2008. pp 140, 310.\u003c/li\u003e\n\u003cli\u003eSimar A, Avettand-Fenoel MN. State of the art about dissimilar metal friction stir welding. Science and Technology of Welding and Joining. 2017;22(5):389-403. doi: 10.1080/13621718.2016.1251712\u003c/li\u003e\n\u003cli\u003eAmerican Society for Testing and Materials (ASTM). Standard Test Methods for Tension Testing of Metallic Materials. West Conshohocken (PA): ASTM; 2022. Standard E8/E8M-22.\u003c/li\u003e\n\u003cli\u003eDeng H, Yuhua C, Yanlin J, Yong P, Timing Z, Shanlin W, Limeng Y. Microstructure and mechanical properties of dissimilar NiTi/Ti6Al4V joints via -backheating assisted friction stir welding. Journal of Manufacturing Processes. 2021; 64:379-391. doi: 10.1016/j.jmapro.2021.01.024\u003c/li\u003e\n\u003cli\u003eBastin GF and Rieck GD. Diffusion in the titanium-nickel system: II. Calculations of chemical and intrinsic diffusion coefficients. Metall Trans 5, 1827\u0026ndash;1831 (1974). doi:10.1007/BF02644147\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"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":"Friction Stir Welding, Dissimilar Friction Stir Welding, NiTi, Shape Memory Alloy, Titanium","lastPublishedDoi":"10.21203/rs.3.rs-6271208/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-6271208/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"Solid-state dissimilar joining of NiTi and Ti-6Al-4V is a currently underexplored field with high potential to expand applications for each alloy in the aerospace industry. Friction stir welding of butted 1 mm thick sheets of NiTi and Ti-6Al-4V was tested under a matrix of welding parameters. Six of eight tested parameter conditions joined but each showed degraded mechanical properties. Higher traverse speed conditions joined more successfully into testable samples. Upper and lower quartiles for tensile strength varied between 140 to 60 MPa with the lower rotation speed showing higher median values. Weld degradation is attributed to the formation of an up to 10 µm wide Ti2Ni intermetallic compound layer at the weld interface. Higher rotation speeds showed a thicker intermetallic layer. The Ti2Ni layer showed equiaxed grains on the order of 2-3µm in diameter. It is theorized that this layer grew from pre-existing Ti-6Al-4V via nickel diffusion from the NiTi due to in-process heating. Accelerated property mapping nanoindentation shows that the Ti2Ni layer has a greater microhardness and reduced elastic modulus (12.42 GPa and 150.06 GPa) than the stir zone of the NiTi (5.10 GPa and 97.65 GPa) and Ti-6Al-4V (5.93 GPa and 137.2 GPa). Crack propagation along this brittle, high stiffness intermetallic layer is proposed as the cause of failure in the welded samples.","manuscriptTitle":"Examination of Ti2Ni formation during dissimilar friction stir welding of thin sheet NiTi and Ti-6Al-4V","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-04-24 11:46:25","doi":"10.21203/rs.3.rs-6271208/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"reviewerAgreed","content":"","date":"2025-04-03T23:46:08+00:00","index":0,"fulltext":""},{"type":"reviewersInvited","content":"","date":"2025-04-03T12:23:33+00:00","index":"","fulltext":""},{"type":"editorInvited","content":"Welding in the World","date":"2025-04-01T14:07:19+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2025-03-28T15:03:44+00:00","index":"","fulltext":""},{"type":"submitted","content":"Welding in the World","date":"2025-03-27T03:59:24+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":"4d5a86fd-eaa1-4e69-a149-23f6384a5063","owner":[],"postedDate":"April 24th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"under-review","subjectAreas":[],"tags":[],"updatedAt":"2025-08-27T15:28:24+00:00","versionOfRecord":[],"versionCreatedAt":"2025-04-24 11:46:25","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-6271208","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-6271208","identity":"rs-6271208","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}