Assessment of Microstructural Evolution and Mechanical Properties of Laser Metal Deposited 316L Stainless Steel

Laser metal deposition (LMD) method is considered as a promising additive manufacturing technique owing to its advanced features. This includes printing complex shaped structures via three-dimensional computer-aided-designs, laser cladding and rapid prototyping. In the present study, detailed microstructure-property correlation is pursued for a block of 316L stainless steel alloy, manufactured by LMD. Presence of fully austenitic phase with corresponding residual stresses in the as-built structure is observed through x-ray diffraction analysis. At the first hand, detailed microstructural analysis is done at the bottom and top sections of the block along the build direction. Presence of hatch structures, variation in the layer thickness along with non-uniformity in the grain size and morphology are noted at these sections. An enhancement in the layer thickness and residual stress along with coarsened grain size is noted to exist towards the top section. This is particularly reflected in achieving highest localized hardness at the bottom section, which progressively decreases towards the top, as characterized through nano-indentation. The LMD-manufactured 316L SS alloy however, exhibits strong indentation size effect along with higher global hardness and strengths with respect to the as-cast alloys. This is primarily related to the complex thermal history of the AM technique. Standard solution heat treatment of the LMD-manufactured 316L SS alloy although coarsens the grain size, minimizes the microstructural non-uniformities and residual stress. Such increment in grain size however, leads to reduction in hardness at the macro, micro- and nano-levels, in comparison to the as-built counterparts. Tensile tests of the solutionized 316L alloys reveal a reduction in yield and ultimate tensile strength along with increased ductility and toughness, thereby suggesting solution treatment as a useful measure to optimize the combination of properties. Importantly, the properties are noted to supersede those of the wrought alloys.