On the correlation between the Laser Powder Bed Fusion process parameters and residual stress in the AISI 316L stainless steel

Additive manufacturing is an innovative manufacturing technique, allowing a virtually infinite number of designs, as there are no constraints given by conventional manufacturing methods. Novel techniques require study and additive manufacturing is no exception. This work sets itself to investigate the effect of process parameters in parts produced through Laser Powder Bed Fusion (L-PBF). Samples were manufactured by keeping hatch distance and laser power constant whereas scanning speed, volumetric energy density (VED) were varied along with two different scanning strategies (30° and 60° shift by layer rotation). Outcomes were analyzed to test the effects on density, porosity, mechanical properties, surface roughness and residual stress. The results yielded from this study suggest that as confirmed by literature increasing VED makes for more dense parts, along with low scanning speed by facilitating melting of the metal powder. The results obtained with the Archimedes method were compared to the ones obtained through image analysis, showing a similar trend yet with differences which have been addressed. Surface roughness appeared to be higher on top surfaces than lateral surfaces. High VED and low scanning speed seemed to be correlated with higher surface roughness. Scanning strategy had an influence at low scanning speeds, when the 60° scanning strategy returned smoother surfaces compared to its 30° counterpart, while at higher scanning speeds the opposite occurred. Mechanical properties proved to be influenced by process parameters as well. It emerged that low scanning speeds and high VED are extremely beneficial in mechanical properties and elongation at break, which appeared to be ductile. Scanning strategy did not provide significative differences, although the 60° type provided higher YS and UTS values, probably due to finer crystalline structures. The benefit of using lower scanning speed and higher VED was proven in residual stress measurements: as samples manufactured within this range also presented lower residuals stress magnitude. This is true regardless of the scanning strategy used, although the difference was slightly more evident in the 60° scanning case. A linear correlation with porosity was also proven. Finally, only a slight difference of the effect of the scanning strategy on residual stress was noted. Despite identifying a general trend with the 30° scanning strategy having lower residual stresses compared to the 60° strategy, the difference in magnitude was not considered significant and therefore no clear benefit has emerged from preferring one strategy over the other. Finally, optical microscope analysis was also performed to determine porosity and observe pores. Samples manufactured at high VED and low scanning speed showed the presence of gas pores, while from samples with low VED and high scanning speed emerged the presence of Lack of Fusion (LOF) pores.