Interlayer machining effects on microstructure and residual stress in directed energy deposition of stainless steel 316L

Abstract

Powder-based directed energy deposition is one of the major additive manufacturing processes for producing and repairing large-size and high-value metallic components. Due to the rough surface finish and low dimensional accuracy, DED products require post-build machining. Accordingly, the effects of post-machining on build properties is an emerging and important field of research. To date, most of the reported machining cases have been performed after the entire build’s completion, even though interlayer machining may be applied due to design modification or to achieve improved deposition control on localized regions of high curvature. Therefore, in this work, for the first time, the influence of interlayer machining on the processing–structure–properties relationships in powder-based directed energy deposition of stainless steel 316L is investigated. Four types of single-track builds are manufactured on stainless steel 316L substrates: single-layer, double-layer, machined single-layer, and double-layer with interlayer machining. The effects of interlayer machining on the microstructure and residual stress before and after the second layer’s deposition are studied via metallographic imaging and neutron diffraction. In single-layer samples, due to induced plastic strains and heat generated during the machining operation, the microstructure reveals dynamic recrystallization, exhibited by smaller, more equiaxed grains. In the double-layer samples, interlayer machining leads to considerable variation in the microstructure, with grains oriented almost parallel to the scan direction. The interlayer machining also results in greater tensile residual stresses near the interface between the two deposited layers. Findings of this work reveal that interlayer machining has significant effects that should be considered in the design and control of processing–structure–properties–performance relationships in directed energy deposition.