Effect of Process Parameters on Surface Roughness of NiTi Alloys Produced by Selected Laser Melting

Since the surface quality of the NiTi parts prepared by selected laser melting (SLM) technology is generally difficult to meet the requirements of the application due to the high surface roughness, the work aims to investigate the effects of laser power, scanning speed and hatch space on the surface roughness of NiTi alloy samples during the process of SLM NiTi parts. The pre-alloy NiTi powders were atomized by the electrode induction melting gas atomization technique (ALD, Germany) under the protection by argon gas. NiTi powders were observed by scanning electron microscopy (SEM). The particle size distribution (PSD) was measured by laser scattering particle size analyzer (HORIBA LA-960S, Japan). For experimental design, the model was created based on the Taguchi design model of the L16 orthogonal array. The range of laser power, scanning speed and hatch spacing was 20-50 W, 200-500 mm/s and 0.05-0.08 μm. The scanning strategy of laser rotation 67° between two consecutive layers was applied to produce NiTi parts, with a fixed layer thickness of 30 μm. NiTi samples of 6 mm× 6 mm× 6 mm were produced by SLM technology (EOS M100, Germany). The surface roughness value and surface morphology were measured by laser confocal microscope (Olympus LEXT OSLS4100, Japan). The surface roughness signal-to-noise (S/N) ratio was calculated by the equation. The S/N ratio was a logarithmic function used as an objective function for optimization, which was conductive to data analysis and prediction of optimal results. Samples are successfully prepared by SLM technology. The results showed that, at the laser power of 20 W and 30 W, the sample surface had high fluctuation due to the powder, which could not melt sufficiently to the unstable melt track during the process of SLM NiTi parts, with a maximum surface roughness value of 7.8 μm. When the value of laser power reached 50 W, the sample with low surface roughness value was obtained which was attributed to the stable melt track, with a minimum surface roughness value of 1.3 μm. The sample surface roughness value increased with the increase of scanning speed, at the same laser power, which was attributed to the time of NiTi powder melting increasing at low scanning speed. The decreasing of hatch spacing could remelt the adjacent laser track to improve the surface morphology of NiTi parts. However, the surface roughness affected was not obvious when the laser track was not unstable. The rank order of the process parameters on the surface roughness is laser power, scanning speed, hatching spacing after statistical methods are used to analyze the surface roughness signal-to-noise ratio. According to the mathematical model, the optimal combination of process parameters is laser power of 50 W, scanning speed of 200 mm/s and hatch space of 0.07 mm. The surface roughness value (1.38 μm) of the sample prepared by the final optimized process parameters combination is close to the predicted value (1.43 μm) by fitting equation. The difference of the surface roughness value is only 9.97%. The model provides an accurate guide for experiments for the study of SLM NiTi parts. © 2024 Chongqing Wujiu Periodicals Press. All rights reserved.