Brazing of Ti-Ni shape memory alloy with stainless steel

Ti-Ni shape memory alloy (NT-L) was brazed to SUS304 by using BAg-8, BAg-8+0.5%Ni and BAg-8+3.0%Ni filler metals. The joint strength and reaction layer formation were investigated. In the case of BAg-8, Fe and Cr from SUS304 diffused to NT-L, and Ti from NT-L diffused to SUS304, which caused the formation of (Fe, Ni)(2)Ti and (Fe, Ni)Ti reaction layers at the bondline. This layer was not formed at the bondline of the similar material joints. Obvious reaction layer was not formed at relatively low brazing temperature. In such conditions, the joint strength was increased with higher brazing temperature or longer holding time. Once the Fe-Ti based reaction layers mere formed, joint strength reached to maximum about 275 MPa that was less than 50% of based metal strength, and was not affected by brazing condition. Fracture occurred around reaction layers consisted of Fe-Ti intermetallic compounds. In the case of Ni added BAg-8 filler, diffusion of Ti and Fe was surppressed, resulting in no Fe-Ti intermetalliic compounds formation at the bondline. In the NT-L side, Ni in filler metal diffused to NT-L/filler interface to form Ni3Ti layer. In the SUS304 side, fee solid solution was formed, and any intermetallic compounds were not formed. Fracture occurred along the boundary between Ni3Ti layer and NiTi base metal. Due to the absence of brittle Fe-Ti based compound layers, joint strength increased to about 400 MPa.