Surface transformation hardening of Ti-5Al-2.5Sn alloy by pulsed Nd:YAG laser: an experimental study ORIGINAL ARTICLE First Online: 24 October 2018 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.
Keywords Transformation surface hardening Pulsed Nd:YAG laser Ti-5Al-2.5Sn alloy Microstructure Hardness Highlight
1. Laser surface transformation hardening of Ti-5Al-2.5Sn was performed successfully.
2. The laser speed has a significant effect on the overlapping factor compared to other laser parameters.
3. The hardened zone consists of the prior beta phase grains and the martensitic phase.
4. The microstructure in transition zone varies from the alpha phase (the base metal) to the prior beta phase (the hardened zone).
5. The micro-hardness value for optimum sample is almost 36.0% more than the base metal.
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