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

Abstract

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 316L 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 sufficiently 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.

Optimization of Structural and Processing Parameters for Selective Laser Melting of Porous 316L Bone Scaffolds

“…In this study, it was decided that the test would be carried out using an energy density of between 40 and 120 J/mm 3 after combining the test equipment with the research results of many scholars [ 15 , 21 ]. The energy density was, in turn, determined by the laser power, scanning speed, powder layer thickness, and scanning spacing, so this study decided to choose a laser power of 100 W–160 W, a scanning spacing of 0.06–0.10 mm, a scanning speed of 500–700 mm/s, and a powder layer thickness of 0.04 mm for the construction of porous skeletal scaffolds, for each factor in the study.…”

“…Ma et al [ 14 ] concluded that higher scanning speeds can be used to obtain uniform and fine microstructures during the forming process due to the faster solidification rate. Jiang et al [ 15 ] and Anil et al [ 16 ] investigated the effect of energy density on the density and defects of SLM-formed parts and found that the use of suitable energy density can result in formed parts with excellent properties. Yadollahi et al [ 17 ] found that the construction process parameters play a decisive role in thermal cycling during the forming process, thus affecting the tissue structure and, hence, the mechanical strength under static and cyclic loading.…”

Finite Element Simulation of Stainless Steel Porous Scaffolds for Selective Laser Melting (SLM) and Its Experimental Investigation

“…The elastic modulus of stainless steel, titanium and titanium alloy is much higher than that of human bone, and it is easy to a produce "stress shielding" effect after implantation; this will affect the normal development of bone tissue near the implantation material [14]. Therefore, as a bone implant material, it is necessary to further improve the biological activity of stainless steel material to promote bone tissue regeneration [15][16][17]. Porous scaffolds also have the advantages of good penetration, adjustable pore size and porosity, and a similar elastic modulus to natural bone tissue, which can effectively prevent the generation of the stress shielding effect [16].…”

Selective Laser Melting and Mechanical Properties of Stainless Steels

“…Note that 420 martensitic stainless steel has also been of significant interest in SLM, since its properties are suitable for specific applications, due to its high strength and corrosion resistance [ 18 ]. Consequently, many studies have been published concerning the microstructure, mechanical properties, and roughness of these stainless steels, produced by SLM ([ 19 , 20 , 21 ] −316L, [ 22 , 23 , 24 ]−630, [ 25 , 26 , 27 ]−420). SS 440C is also a martensitic stainless steel, but has high carbon content; it is used in applications where high hardness and corrosion resistance are necessary, such as bearings, knives and automotive parts [ 28 ].…”