Abstract

Butt joints of Ti-6Al-4V alloy sheets with two thicknesses (3.175 and 5.08 mm) are laser welded using a high power continuous wave Nd:YAG laser welding machine with or without the use of filler wires. Laser processing parameters investigated include laser power (2 – 4 kW), welding speed (0.75 – 7.5 m/min), joint gap (0 – 0.6 mm) and defocusing distance (-1 mm and -2 mm). Two post-weld heat treatments (i.e. stress relief annealing, solution treatment and aging) are investigated and compared with the as-welded condition. Although some welding defects such as underfill and porosity have been observed, high quality welds can be obtained using a high power Nd:YAG laser. Underfill defect is found to increase but the porosity area decreases with increasing welding speed. The use of the filler wire decreases the underfill defect. However, the porosity area increases with increasing joint gap. The hardness is maximum in the fusion zone (FZ) followed by a sharp decrease in the heat affected zone (HAZ) to the base metal (BM). The low temperature annealing process increases the hardness of the fusion zone but the combination of the high temperature solution treatment followed by aging decreases the fusion zone hardness almost to the base metal values. Tensile testing shows that the joint efficiency varies between 95 and 105 %. The porosity and underfill are the two main defects for failure in the FZ and HAZ, respectively. The local mechanical properties determined by digital image correlation (DIC) technique show that the elastic modulus and yield stress are maximum in the FZ and minimum in the HAZ. The lowest plastic strain at fracture is obtained in the FZ.