Surface transformation hardening of Ti-5Al-2.5Sn alloy by pulsed Nd:YAG laser: an experimental study

Abstract

In this study, surface hardening of Ti-5Al-2.5Sn alloy was performed using a pulsed Nd:YAG laser. The effect of laser parameters including laser frequency, laser pulse width, laser speed, and focal point position was investigated on the microstructure, geometrical dimensions of hardened zone (hardened width, hardened depth, and entry angle of hardened zone), and micro-hardness. In the specimens with higher cooling rate, martensitic phase (ά) is formed in the hardened zone, while in the specimens with lower cooling rate, combination of equaxied and acicular alpha phase is formed in the hardened zone. The laser speed has a significant effect on the overlapping factor compared to other laser variables. By increasing the laser speed, the cooling rate increase and the pulse overlapping decreases in the hardened zone. High cooling rate leads to the refinement of prior beta grain size in this zone. Combination of equiaxed and acicular alpha phase, as well as acicular martensitic phase, is observed in the transition zone. The martensitic phase is a dominant phase in location near the hardened zone. A laser power of 165 W, lase frequency of 14 Hz, pulse width of 22 ms, laser speed of 105 mm/min, and focal point position of + 3.5 mm are optimum parameters for obtaining the best laser surface hardening results. The presence of martensitic phase in the optimum sample causes an increase in the hardness about 36% as compare to the base metal.