Effect of post-heat treatment cooling on microstructure and mechanical properties of selective laser melting manufactured austenitic 316L stainless steel

Abstract

Purpose This paper aims to investigate the influence of different post-annealing cooling conditions, i.e. furnace cooling (heat treatment (HT) 1 – slow cooling) and air cooling (HT 2 – fast cooling), on the microstructure and mechanical properties of selective laser melting (SLM) built austenitic 316L stainless steel (SS). Design/methodology/approach Three sets of 316L SS samples were fabricated using a machine standard scanning strategy. Each set consists of three tensile samples and a cubic sample for microstructural investigations. Two sets were subsequently subjected to annealing HT with different cooling conditions, i.e. HT 1 and HT 2, whereas one set was used in the as-built (AB) condition. The standard metallographic techniques of X-ray diffraction, scanning electron microscopy and electron back-scattered diffraction were used to investigate the microstructural variations induced by different cooling conditions. The resultant changes in mechanical properties were also investigated. Findings The phase change of SLM fabricated 316L was observed to be independent of the investigated cooling conditions and all samples consist of austenite phase only. Both HT 1 and HT 2 lead to dissolved characteristic melt pools of SLM. Noticeable increase in grain size of HT 1 and HT 2 samples was also observed. Compared with AB samples, the grain size of HT 1 and HT 2 was increased by 12.5% and 50%, respectively. A decreased hardness and strength, along with an increased ductility was also observed for HT 2 samples compared with HT 1 and AB samples. Originality/value From previous studies, it has been noticed that most investigations on HT of SLM fabricated 316L were mainly focused on the HT temperature or holding time. However, the post-HT cooling rate is also an equally important factor in deciding the microstructure and mechanical properties of heat-treated components. Therefore, this paper investigates the influence of different post-annealing cooling conditions on microstructure and mechanical properties of SLM fabricated 316L components. This study provides a foundation for considering the post-HT cooling rate as an influential parameter that controls the properties of heat-treated SLM components.

A Mathematical Dimensional Model for Predicting Bulk Density of Inconel 718 Parts Produced by Selective Laser Melting

“…Post-processing is also relevant in SLM. Waqar et al [ 15 ] found that microstructure and mechanical properties of SLMed SS316L components are influenced by different post-annealing conditions at which the cooling down takes place. Likewise, Gupta et al [ 16 ] found that higher angle layer rotation leads to a reduction in grain size, residual stress, and surface roughness.…”

Computational analysis of the effects of geometric irregularities and post-processing steps on the mechanical behavior of additively manufactured 316L stainless steel stents

“…In this study, the material behavior of the stent was considered isotropic and homogeneous assuming that the geometric effects (e.g. stent design, irregularities) outweigh the influence of material anisotropy with respect to the macroscopic mechanical behavior [27]. Although this approach is well established for L-PBF lattice structures [19,54,60,61], the direction-dependent properties of the stent should be further investigated (e.g.…”

“…Compared to L-PBF 316L in the as-built condition, a reduction in the yield and tensile strength can be achieved with a simultaneous increase in ductility due the homogenization of the microstructure (e.g. increase in grain size and misorientation), the reduction of the δ-ferrite phase and the reduction of residual stresses by adequate heat treatment [26][27][28].…”

Influence of High-Pressure Torsion on the Microstructure and Microhardness of Additively Manufactured 316L Stainless Steel

“…Therefore, as-fabricated AM metallic materials, including 316L SS often produce novel microstructures that result in mechanical and functional properties that are on par with, or even better than, the wrought/cast counterparts [27][28][29][30][31][32][33]. Certain post-processing techniques are also applied to improve the surface finish and integrity, such as by aging, finish machining, and blasting [34,35], as well as altering the mechanical properties of AM parts as required by post-heat treatment and/or cooling [36]. Thus, HPT processing is a viable technique to further enhance the mechanical properties of AM 316L SS [20][21][22][23].…”