Effects of laser brazing parameters on microstructure and properties of TiNi shape memory alloy and stainless steel joint

Laser brazing has been applied for joining TiNi shape memory alloy (SMA) and stainless steel (SS) using the silver-based filler metal with composition of 52%Ag, 22%Cu, 18%Zn and 8%Sn (wt.%) and effects of laser brazing parameters has been investigated. The filler metal has good wettability on both TiNi SMA and SS. The brazing seam metal consists of α-Ag solid solution, α′-Cu solid solution surrounded by the α-Ag and eutectic structures. Heat affected zones (HAZs) of TiNi SMA and SS are comprised of B2 and B′19B′19 phases and austenite phase, respectively. Increasing laser output power and brazing time resulted in the grain coarsening and reducing the microhardness value in the HAZs. The laser output power and brazing time have effects on the superelasticity (SE) and shape memory effect (SME) of the TiNi SMA HAZ. With increasing laser output power and brazing time from 50 W/10 s to 70 W/20 s, the residual strain (ɛu) at room temperature in the HAZ increased from 0.37% to 2.22% and the shape recovery rate (ψ) at 100 °C decreased from 91.6% to 62.1%. The deterioration of SE and SME in TiNi SMA HAZ could be attributed to the coarse-grained structures. It is favourable to decrease laser output power and brazing time for improving both SE and SME of TiNi SMA HAZ.