Asymmetric local strain, microstructure and superelasticity of friction stir welded Nitinol alloy

Thus far, the joining of Nitinol alloys has been studied extensively using laser welding. Here, we studied the weldability of powder metallurgy (PM) Ti–51 at %Ni alloy by friction stir welding (FSW) via microstructural, superelasticity and mechanical property evaluations. The FSW welds were produced by a constant welding speed and different rotational speeds of 250 rpm, 300 rpm, and 350 rpm utilizing the cemented carbide tool. No Ti3Ni4 precipitate was observed after FSW welding, which agreed with XRD analysis. Additionally, it was found that FSW triggered the dynamically re-crystallized fine grains in which tangles of dislocations existed. The crystallographic orientations in the weld zone revealed locally high texture evolution along with diverse Schmid factors measured for active slip systems of Nitinol in different locations of the welds. These heterogeneous crystallographic orientations, dislocation density and Schmid factor variations caused a gradient in local mechanical properties across the welds during the uniaxial tensile test performed by digital image correlation (DIC). The 350 rpm weld exhibited the excellent mechanical properties, yielding at ~765 MPa (~37% higher than the base metal and all previously reported Nitinol welds) and reaching an ultimate strength of ~870 MPa and 8.1% elongation. However, the superelasticity of the FSW-welded joints curtailed upon cyclic loading-unloading; hence it is suggested that post-heat treatment is essential to recovering their functionality.