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This study investigated the microstructure, residual stress, and tensile properties of directed energy deposited stainless steel 316L (SS316L) under thermal annealing. Microstructure characterization shows the as-printed sample has laser-generated patterns where dendritic structure is observed at the edge of the patterns and cellular structure dominates the interior region. The thermal annealing at 983 and 1093°C effectively removes the dendritic/cellular structures. Synchrotron x-ray diffraction reveals that the asprinted SS316L exhibits compressive residual stress of - 197.4 MPa, which is greatly relieved to - 63.8 MPa after annealing at 1093°C. The room temperature tensile testing indicates that the yield strength and ultimate tensile strength drop from 378 MPa and 602 MPa in the as-printed sample to 268 MPa and 446 MPa in the annealed samples (1093°C), respectively. Our study provides insights into the relationship among microstructure, residual stress, and tensile properties of laser additive manufactured SS316L.
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INTRODUCTION
Additive manufacturing (AM) is a building up process in which materials are added layer upon layer.1 With the rapid growth of AM technology, many materials with superior properties can be additively manufactured. For example, Inconel 718 cylinders were fabricated by selective laser melting (SLM), and the performance of the parts is comparable to their wrought counterparts.2 Qiu et al.3 built Ti6Al4V parts by direct laser deposition (DLD), and the result shows a high laser power and low powder feed rate can minimize porosities. Zhang et al.4 fabricated Inconel-stainless steel bimetallic structures to combine high strength and corrosion resistance of Inconel with excellent deformability of stainless steel. Ti6Al4V/TiC functionally graded composite was developed with increased microhardness and enhanced wear resistance.6
DED is an AM process that uses a heat source (usually a laser) to melt and deposit wire or powder feedstock on a substrate to produce a part. Laserengineered net shaping (LENS), a DED method, was originally developed in the Sandia National Laboratories to fabricate metal parts directly from computer-aided design (CAD) models. This process has been widely used to fabricate metallic parts and repair damaged components.6-9 De Lima and Sankare6 studied the porosity, microstructure, tensile behavior, and flexural performance of 316L stainless steel (SS316L). Cui et al.8 coated AlC°CrFeNi high-entropy alloys on stainless steel and characterized bonding conditions, crystal structure, and hardness. Zhang et...