Abstract:Recently, an additive manufacturing method, a supplement to the traditional manufacturing industry, has gained a lot of attentions due to the advantages of rapid preparation of complex parts. With the improvement of additive manufacturing technology, the range of additive manufacturing raw materials has been expanded from the organic materials with low melting points to the metal materials with the high melting points, which extends its application range to the fields of aerospace and automobile manufacturing. The selective laser melting (SLM) is a mainstream technology of additive manufacturing with the metal materials. However, the supply of high quality metal powder has become an obstacle for the SLM development. Currently, the amount of stainless steel powder, such as 316L, used in molds accounts for about 20% of the global 3D printing metal materials. In this study, the 316L stainless steel powder with the low oxygen content, fine particle size and good circularity was prepared by the vacuum gas atomization. The chemical composition and physical properties, including the angle of repose, the bulk density and the tap density, of the 316L stainless steel powder all meet the requirements of SLM process. In addition, the effects of selective laser melting (SLM) process parameters, including the laser power and scanning speed, on the melt channel, microstructure and mechanical properties of SLM forming products were investigated in detail. The formation reasons of various defects were also analyzed. The results could be summarized as follows. Under the conditions of the low laser power or the high scanning speed, due to the insufficient laser energy, the powder in the molten pool is not completely melted, resulting in the increasing viscosity of the molten pool and the poor wettability between the molten pool and substrate. Therefore, a large number of spherical particles appear in the melt channel, and in the severe cases, the melt channel could be even broken. In addition, there is a large amount of voids between the spherical particles, which further deteriorates the melting condition of the next layer of powder. It is the root cause of disordered distribution of the melt channels and the generation of holes and cracks in the longitudinal and horizontal sections of SLM forming product. During the stretching process, the above defects are the source of cracks causing the fracture of SLM forming product, which further leads to a decrease in the mechanical properties of SLM forming product, such as the tensile strength, yield strength, elongation at break and relatively density. Under the conditions of the high laser power and low scanning speed, the Marangoni flow inside the molten pool enhanced and exhibits instability. In addition, the rapid heating or cooling processes of molten pool enhances the thermal stress in the SLM forming product. The above phenomenon leads to the fact that the defects, including holes and cracks, could also be found in the SLM forming product. And these defects are generally present at the junction of adjacent melt channel in the same or different powder layers. Furthermore, under the experimental conditions of this work, when the laser power is 350 W and the scanning speed is 1750 mm/s, the mechanical properties of SLM forming products are best, which includes a tensile strength of 731 MPa, a yield strength of 638 MPa, an elongation of 40.0% and a relatively density of 96.27%.