Microstructure and mechanical properties of dissimilar NiTi/Ti6Al4V joints via back-heating assisted friction stir welding

Abstract To confirm the feasibility of dissimilar NiTi/Ti6Al4V joints fabricated via friction stir welding, the conventional friction stir welding and back-heating assisted friction stir welding were conducted. Results revealed that during conventional friction stir welding process, it was a challenge to obtain a sound joint due to cracking and tunnel defects that caused by insufficient plastic fluidity and welding stress. However, at travel speed of 23.5 mm/min with constant rotation speed of 475 rpm, the defect-free joint was obtained when preheating temperature at 200 ℃ during back-heating assisted friction stir welding. The microhardness of the special area at where the NiTi (Ti6Al4V) materials debris flowed into Ti6Al4V (NiTi) matrix (vice versa) was higher than that of base materials NiTi (310 ± 50 HV) and Ti6Al4V (330 ± 30 HV), reached 710 HV. The thermal-mechanical coupling of reserve deformation and frictional heat in the friction stir welding process promoted the formation of Ti2Ni intermetallic compounds (IMCs) and it dramatically grew coarse. It was considered that brittle Ti2Ni IMCs was the main factor to deteriorate the mechanical properties of the back-heating assisted friction stir welded joint except for defects.