Mechanical properties and microstructural study of homogeneous and heterogeneous laser welds in alpha, beta, and alpha plus beta titanium alloys

Heterogeneous welding has been investigated for three different couples of titanium alloys: alpha/alpha + beta, alpha/beta, and alpha + beta/beta. Plates of 100 x 60 mm and 1.6 or 1.8 mm thick were welded with a Yb:YAG laser. Tensile tests show that the resistance of the heterogeneous welded specimens was generally controlled by those of the weakest material except for the alpha + beta/beta where the ultimate tensile strength was approximately equal to the average value of both materials. In every case, the elongation of the welded sample was found to be smaller than that of the base metals. The rupture generally took place outside the weld metal and was found to be most of the time located in the alloy having the lowest mechanical properties. Nevertheless, a few large-size porosities detected by tomography in the alpha + beta/beta couple could explain why rupture for these samples occurred in the weld bead. For each couple, the porosities were situated at the board between the heat affected zone and the molten zone. EBSD maps and EDX enabled the observation of different microstructures, which could be correlated to the heterogeneous composition and diffusion into the melted bath. When the stable microstructure of one of the couple alloys is the beta phase, the molten zone of the bead consists of an alternative disposition of thin layers made of large equiaxed beta grains and nano-martensite alpha'. That is explained by the weak diffusion of the alloying elements.