Effect of laser scanning speed on the microstructure and hardness of SS316L–IN625 FGM fabricated by laser additive manufacturing

Laser additive manufacturing (LAM) is an emerging technology used for fabricating functionally graded materials (FGMs). The effects of the laser scanning speed on the hardness, microstructure, and average secondary dendrite arm spacing of 316L stainless steel-Inconel 625 (SS316L-IN625) FGM were investigated. A continuous fiber laser with a maximum power of 1000 W was used. The compositional gradient of the layers was as follows: layer 1: 100% SS316L; layer 2: 75% SS316L + 25% IN625; layer 3: 50% SS316L + 50% IN625; layer 4: 25% SS316L + 75% IN625; and layer 5: 100% IN625. The laser scanning speed was studied at three levels: 120, 200, and 280 mm/min. The results reveal that the LAM process can be successfully utilized to fabricate SS316L/IN625 FGM parts. The as-produced FGM part, manufactured with a scanning speed of 280 mm/min, exhibited fine dendrites and fewer microstructural defects. The solidification morphology of as-produced FGM parts consisted of columnar and equiaxed grains. The findings reveal that increasing the laser scanning speed from 200 mm/min to 280 mm/min reduced the secondary dendrite arm spacing by 38.8% (from 5.44 ± 1 µm to 3.33 ± 0.76 µm). Additionally, this increase in scanning speed led to a 17.7% rise in microhardness, from 249 HV to 293 HV.