Laser welding of Ti-5Al-5V-5Mo-3Cr

Ti-5Al-5V-5Mo-3Cr butt joints were welded using a 4 kW continuous wave Nd:YAG laser. The effect of welding speed and defocusing distance on the weld quality was investigated. Welds with full penetration were achieved at a defocusing distance ranging from -1 to 0 mm and welding speeds from 2.25 to 6.0 m min(-1). Underfill and porosity were the two main defects most frequently observed; however, within the optimum process window, these defects could be maintained to meet aerospace specification tolerances. The fusion zone consisted entirely of retained beta with a refined dendritic morphology. Compared with the bimodal alpha+beta microstructure of the base metal, dissolution of the alpha phase in the heat affected zone and the presence of entirely metastable/retained beta phase in the fusion zone were observed, which led to a significant decrease in the HAZ and FZ hardness. To determine the tensile properties of the welds, an automated three-dimensional deformation measurement system was used to measure the local strain in the weld region. The yield strength and ultimate tensile strength increased with welding speed, achieving a maximum joint efficiency of 75%, albeit with a concomitant reduction in the ductility.