Microstructure, mechanical properties and intermetallic compounds formation in electron beam welding of Nitinol to Ti6Al4V

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Abstract
Dissimilar welding of Nickel and Titanium alloys is crucial for high performance applications, yet possess several challenges because of disparate metallurgical properties. This paper explores autogenous Electron Beam Welding (EBW) of Nitinol with Ti6Al4V in a butt joint configuration. Unlike, other autogenous dissimilar fusion welding of titanium and nickel alloys, no macroscopic cracks were observed during or after solidification. Optical microscopy and Scanning Electron Microscopy revealed distinct microstructures at the titanium and nickel interfaces, and within the weld zone. Microstructure at the interface of titanium was markedly different from the microstructure at nickel alloy interface. Composition analysis via Energy Dispersive X-Ray Spectroscopy and X-Ray Diffraction detected Ti2Ni and NiTi intermetallic compounds, with higher Ti2Ni concentrations near the titanium interface. Tensile test yielded an average strength of 160 ± 5 MPa and 0.92% percent elongation, with fracture occurring at the titanium interface in regions of high Ti2Ni concentration. Fracture surface of the tensile test specimen mostly highlighted brittle failure mode. The peak hardness value was also observed near the titanium interface. This study demonstrates the feasibility of autogenous EBW for dissimilar nickel-titanium alloy joints without interlayers, providing a foundation for future research and potential improvements through interlayer applications.

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