Microstructure and mechanical properties of laser-welded joints of Ti-22Al-25Nb/Ti-6Al-4V dissimilar titanium alloys

Laser welding was applied to join 2-mm-thick dissimilar titanium alloys, Ti-22Al-25Nb (at. %) and Ti-6Al-4V (wt. %). Defect-free joints were obtained. The microstructures and mechanical properties of the welded joints were investigated. The results showed that the fusion zone mainly consisted of B2 phase and martensite α′ phase. The heat-affected zone microstructures of Ti-22Al-25Nb and Ti-6Al-4V were different because of the different base metal. The heat-affected zone of Ti-22Al-25Nb mainly consisted of B2 phase, O phase, and α2 phase, whereas the heat-affected zone of Ti-6Al-4V mainly consisted of α phase, β phase, and martensite α′ phase. EDS (Energy Dispersive Spectroscopy) results showed that the element distribution of each titanium alloy heat-affected zone and fusion zone was similar without an obvious change. Due to severe agitation, the element distribution was even. Different microstructures in different heat-affected zones resulted in an unsymmetrical hardness distribution. The hardness of fusion zone was the lowest of around 300 HV when the hardness of Ti-6Al-4V heat-affected zone near fusion zone was the highest of around 400 HV due to the influence of martensite α′ phase. In tensile tests performed at room temperature and 650 °C, the tensile strength of the joint reached 1057 and 363 MPa, respectively. Both these joints fractured at the fusion zone.