Effect of laser powder bed fusion gas flow rate on microstructure and mechanical properties of 316 L stainless steel

Abstract

The inert protective gas flow rate during laser powder bed fusion (LPBF) molding is an important environmental condition. The protective gas not only prevents parts from being oxidizing, but also significantly affects spatter particle removal and particle migration on the powder bed surface. There are relatively few systematic comparative studies reported on the effect of gas flow rate on the movement of two types of powders. Therefore, in this study, three flow rate environments, low, medium and high, were designed for multi-scale experimental analysis of the quality, microstructure, mechanical properties and defects of molded samples. It was observed that the movement of the powder that dominated the sample defects was different in the three environments as the gas flow velocity was adjusted. Under low flow conditions, spatter particles are the main contributor to the creation of defects such as porosity, resulting in poor surface quality (TOP: Sq = 14.023, Side: Sq = 22.006), elevated surface oxygen content (Wt. = 1.7 %), increased grain size, and i a higher number of pores and spatter particles. Consequently, this led to the lowest density, microhardness, and mechanical properties among the three sample groups. High flow rates increased the removal of spatter particles compared to low flow rates, resulting in the highest surface quality (top: Sq = 11.528, Side: Sq = 14.051) and the lowest surface oxygen content (Wt. = 0.88 %). However, under the influence of high flow rates, surface particle migration becomes an important factor in defects, leading to an increase in grain size, porosity, and unfused particles compared to medium flow rates. This leads to a decrease in density, microhardness and elongation. This study reveals, through multi-scale experimental characterization, the mechanism by which varying air flow rates affect powder movement, and hence the microstructure and properties of parts. These findings provide new insights for device development and coupled multi-physics field computational studies.