Abstract
The use of post-processing heat treatments is often considered a necessary approach to relax high-magnitude residual stresses (RS) formed during the layerwise additive manufacturing laser powder bed fusion (LPBF). In this work, three heat treatment strategies using temperatures of 450 °C, 800 °C, and 900 °C are applied to austenitic stainless steel 316L samples manufactured by LPBF. These temperatures encompass the suggested lower and upper bounds of heat treatment temperatures of conventionally processed 316L. The relaxation of the RS is characterized by neutron diffraction (ND), and the associated changes of the microstructure are analyzed using electron backscattered diffraction (EBSD) and scanning electron microscopy (SEM). The lower bound heat treatment variant of 450 °C for 4 hours exhibited high tensile and compressive RS. When applying subsequent heat treatments, we show that stress gradients are still observed after applying 800 °C for 1 hour but almost completely vanish when applying 900 °C for 1 hour. The observed near complete relaxation of the RS appears to be closely related to the evolution of the characteristic subgrain solidification cellular microstructure.
Highlights
ADDITIVE manufacturing (AM) has been the focus of many studies over the past two decades
The analysis of stress relaxation in shot-peened 316 Almen strips reported in Reference 32 shows that a relaxation of about 38 pct is resulting from a heat treatment at 900 °C
This is in contradiction with the results for laser powder bed fusion (LPBF) 316L reported in References 23,24,26
Summary
ADDITIVE manufacturing (AM) has been the focus of many studies over the past two decades. The TGM is a solid-state mechanisms and does not require the material to be molten compared to the cool-down phase model.[4,5] In the cool-down phase model, the contraction of the solidifying material is constrained by the surrounding solidified material, leading to misfit and, acting as source for the formation of RS. Both these mechanisms tend to generate tensile RS at the top deposited region, balanced by surrounding compressive RS (beneath or adjacent). The effects of TGM and cool-down model (referred to as solidification shrinkage) were further investigated in Reference 6 to understand their individual contribution to the formation of METALLURGICAL AND MATERIALS TRANSACTIONS A
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