Mr. Nissar Ahmed

Stainless steel 316L has been an extensively investigated metallic material for laser powder bed fusion (L-PBF) in the past few decades due to its high corrosion resistance. However, there are challenges related to producing L-PBF parts with minimal defects, attaining mechanical properties comparable with traditional process and dependency on time consuming post process treatments. The selection of L-PBF process parameters is crucial to overcome these challenges. This paper reviews the research carried out on L-PBF process parameter optimization for fabrication of 316L steel components for maximizing part densifications and attaining desired microstructure morphologies in parts. A brief work on numerical simulation approach for process parameter optimization for high densifications is also included in this paper.

The layer-by-layer process of additive manufacturing (AM) is known to give rise to high thermal gradients in the built body resulting in the accumulation of high residual stresses. In the current study, a numerical investigation is conducted on the effect of residual stresses on the mechanical properties of IN718 triply periodic minimal surface (TPMS) lattices fabricated using the selective laser melting (SLM) process for different relative densities. The AM simulation of four different sheet- and ligament-based TPMS topologies, namely, Schwarz Primitive, Schoen Gyroid, Schoen IWP-S, and IWP-L, are performed using a sequentially coupled thermomechanical finite element model to evaluate the thermal histories and residual stress evolution throughout the SLM process. The finite element results are utilized to obtain the effective mechanical properties, such as elastic modulus, yield strength, and specific energy absorption (SEA), of the TPMS lattices while accounting for the residual stress field arising from the SLM process. The mechanical properties are correlated to relative density using the Gibson–Ashby power laws and reveal that the effect of the residual stresses on the elastic modulus of the as-built TPMS samples can be significant, especially for the Schwarz Primitive and Schoen-IWP-L TPMS topologies, when compared to the results without accounting for residual stresses. However, the effect of the residual stresses is less significant on yield strength and SEA of the TPMS samples. The work demonstrates a methodology for numerical simulations of the SLM process to quantify the influence of inherited residual stresses on the effective mechanical properties of complex TPMS topologies

 Fabrication of complex geometries is now being enabled using the SLM process which are not possible with conventional manufacturing methods for e.g.: precision thin-walled metal structures below 0.5 mm thickness have difficulties fabricating with traditional machining methods due to the low rigidity and the cutting force [1]. However, despite the freedom of design for AM process, there are still obstacles in its wide-scale adoption due to numerous AM parameters involved that must be understood properly in order to manufacture high quality parts [2]. The layer-by-layer nature of SLM process sets up extremely large thermal gradients in build parts that result in the formation of residual stresses and may lead to dimensional inaccuracies, part distortions and crack formation [[3],[4],[5]]. High-performance thin-wall structures made of superalloys are often needed for aerospace structures and medical implants, thin-wall sections are also used in heat exchangers to help heat flow. However, the criticalities involved in SLM process, especially the challenges associated with controlling distortion due to the thermal stress during the fabrication needs to be addressed [6].