Microstructural evolution and nanoindentation study of magnetic pulse welded Nitinol and Aluminium sheets

Abstract

The present investigation involved joining of commercially available Aluminium (Al) and Nitinol (NiTi; 51 at.% Ni) sheets using magnetic pulse welding technique under discharge voltages ranging from 12 to 16 kV and standoff distances from 1.5 mm to 2.25 mm. Primarily, scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) analyses have ensured the mixing of parent metals at the transition zone (max similar to 2 mu m wide) in wavy pattern caused by localized heating, especially for the samples welded with 2 mm standoff distance with 14 kV and 14.5 kV discharge voltages. In-depth transmission electron microscopy (TEM) analysis of the transition zone of 14 kV_2mm welded sample has further confirmed the intermixing of (Al, Ni, and Ti) at the transition zone. The selected area electron diffraction pattern from the intermixed layer has indicated nanometric (max. similar to 50 nm) intermetallic (Ni4Ti3) precipitates coherent with the NiTi (B2) phase along with amorphous structure. The refinement of the Al grains adjacent to the intermixed layer has been substantiated with TEM as well as electron back scattered diffraction (EBSD) technique. The nano-indentation study on the welded sample across the weld zone has shown variation in hardness values and pseudo-elastic behaviour of the NiTi, being a primary effect of the stand-off distance. Acceptable joint strength in terms of improved nano-hardness has been found for 2 mm standoff distance, irrespective of the welding voltage, imparted by fine Ni4Ti3 precipitates. Besides, significant amount of pseudo-elasticity of NiTi has been retained at the weld bead as well as at the surrounding location (only 50 mu m away from the weld bead) for the aforesaid welded samples. The pseudo-elastic behaviour of Nitinol is associated with the martensite to austenite phase transformation during the welding process, which has been substantiated with the thermal (Differential scanning calorimetry, DSC) and x-ray diffraction analyses.