Effect of Process Parameters on Defects,Melt Pool Shape,Microstructure,and Tensile Behavior of 316L Stainless Steel Produced by Selective Laser Melting

Previous studies have revealed that laser power and energy density are significant factors affecting the quality of parts manufactured by selective laser melting(SLM).The normalized equivalent density E_(0)^(*) and dimensionless laser power q^(*),which can be regarded as a progress on the understanding of the corresponding dimensional quantities,are adopted in this study to examine the defects,melt pool shape,and primary dendrite spacing of the SLM-manufactured 316 L stainless steel,because it reflects the combined effect of process parameters and material features.It is found that the number of large defects decreases with increasing E_(0)^(*) due to enough heat input during the SLM process,but it will show an increasing trend when excessive heat input(i.e.,utilizing a high E_(0)^(*))is imported into the powder bed.The q^(*) plays an important role in controlling maximum temperature rising in the SLM process,and in turn,it affects the number of large defects.A large q^(*) value results in a low value of absolute frequency of large defects,whereas a maximum value of absolute frequency of large defects is achieved at a low q^(*) even if E_(0)^(*) is very high.The density of the built parts is greater at a higher q^(*) when E_(0)^(*)remains constant.Increasing the melt pool depth at relatively low value of E_(0)^(*) enhances the relative density of the parts.A narrow,deep melt pool can be easily generated at a high q^(*) when E_(0)^(*) is sumciently high,but it may increase melt pool instability and cause keyhole defects.It is revealed that a low E_(0)^(*) can lead to a high cooling rate,which results in a refined primary dendrite spacing.Relatively low E_(0)^(*) is emphasized in selecting the process parameters for the tensile test sample fabrication.It shows that excellent tensile properties,namely ultimate tensile strength,yield strength,and elongation to failure of 773 MPa,584 MPa,and 46%,respectively,can be achieved at a relatively low E_(0)^(*) without heat treatment.