Influence of laser offsets on microstructure and tensile properties of laser welded Ti-3Al-6Mo-2Fe-2Zr and TA15 dissimilar joints

Abstract

The influence of laser offsets on the composition, microstructure, and tensile properties of laser welded Ti-3Al-6Mo-2Fe-2Zr and TA15 dissimilar joints was investigated. Laser offsets of −0.1 mm, 0 mm, and 0.1 mm were adopted for the welding process with laser beam shifting from Ti-3Al-6Mo-2Fe-2Zr to centerline and TA15, respectively. The microstructure evolution in the HAZ was extensively discussed using numerical simulation. In the HAZ of Ti-3Al-6Mo-2Fe-2Zr alloy, a single-β-phase region was formed because both the αp and αs phase had transformed into β phase on heating, and the following β → αp, αs transformation was suppressed by the fast cooling rate and high content of β-stabilizing elements. The α → β transformation also occurred on heating in the HAZ of TA15 alloy, followed by the β → α’ transformation during the cooling period. For different laser offsets, the microstructure of fusion zone was composed of columnar β phase and α’ martensites. The α’ martensites were dispersed inhomogeneously in the different regions of fusion zone. The region closest to TA15 alloy presented larger amount and size of α’ martensites than the regions closest to Ti-3Al-6Mo-2Fe-2Zr. As the laser offset changed from −0.1 mm to 0.1 mm, the amount and size of α’ martensites increased as a result of higher [Al]eq and lower [Mo]eq. The joints with laser offsets of −0.1 mm and 0 mm fractured in the tensile deformation stage. The joints with laser offset of 0.1 mm exhibited the highest strength and plastic deformation capacity because of the strengthening effects of α’ martensites.