Laser welding of H-phase strengthened Ni-rich NiTi-20Zr high temperature shape memory alloy

Laser welding of a Ni-rich NiTi-20Zr (at.%) high temperature shape memory alloy was performed. The starting base material was aged for 3 h at 550 degrees C followed by air cooling prior to welding to induce H-phase precipi-tation. Advanced microstructure characterization encompassing scanning and transmission electron micros-copy, coupled with synchrotron X-ray diffraction, were used. Defect-free welds were obtained with a conduction welding mode. The weld thermal cycle altered the microstructure across the heat affected and fu-sion zones of the joints. The heat affected zone exhibited partial H-phase dissolution, causing a decrease in hardness. In the fusion zone, the H-phase fully dissolved, and the non-equilibrium rapid solidification condi-tions prevented the H-phase from re-precipitating during cooling, leading to a microstructure resembling that of an as-cast alloy with the same material composition. Mechanical testing revealed that the laser welded samples sustained stresses in the order of 500 MPa and exhibited stress-strain responses comparable to those of the unwelded base material. Thus, this initial study shows new possibilities for using advanced laser joining methods in these alloys. (c) 2021 The Author(s). Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Automated summary

Laser welding of a Ni-rich NiTi-20Zr high temperature shape memory alloy was successfully conducted, resulting in defect-free welds. The microstructure of the joints was altered by the weld thermal cycle, affecting hardness and mechanical properties. This study demonstrates the potential of advanced laser joining methods in these alloys.