Impact of hot isostatic pressing on the microstructure of AISI 316L austenitic steel processed by laser direct energy deposition

This article deals with the microstructural analysis of austenitic steel AISI 316L processed by the direct energy deposition ( L -DED) process and further processed by hot isostatic pressing (HIP). The samples for the experiment were made from atomised powder with an average spherical particle size of 55 ± 30 µm. The powder was processed into blocks with dimensions of 33x33x18 mm, using a deposition layer thickness of 0.6 mm and a Zig-Zag deposition strategy. HIP processing was performed at a temperature of 1150 °C and a pressure of 100 MPa. The analysis of the samples consisted of X-ray phase diffraction analysis and scanning electron microscopy analysis using EDS, EBSD, SE and BSE detectors. X-ray phase analysis revealed that the HIP process causes coarsening of the crystallites size which changed from 26 nm after L -DED to 138 nm after HIP. Microstructural analysis shown that HIP processing led to coarsening of oxide inclusions which size change from 305 ± 76 nm for L -DED processed specimen to 342 ± 88 nm for HIP processed specimen. EDS analysis shown increase in Si content in the oxide inclusions during L -DED and HIP processing from 1.0 ± 0.5 wt. % to 6.8 ± 2.0 wt. %. EBSD analysis shown increase in the HAGB from 56.0 ± 4.9 % to 81.6 ± 6.1 % and reduce in LAGB from 13.7 ± 2.1 % to 1.1 ± 0.2 % in specimens evaluated Z direction. HIP led to formation of special Σ3 grain boundaries. This study confirms that HIP is possible use to tailor microstructure of L -DED processed austenitic steel AISI 316L, which was associated with Ostwald ripening of oxide inclusions and origin of special Σ3 high-angle grain boundaries during heat treatment caused by the stress-induced boundary motion. • HIP led to increase in HAGB and Σ3 CSL boundaries and decrease in LAGB. • HIP led to dissolving of cellular substructure and delta ferrite phase. • HIP led to coarsening of austenite grains, crystallite and oxide inclusions. • HIP led to homogenising distribution in the acting residual comprehensive stresses.