Influence of laser power and inter-layer dwell time on thermal history, hardness, and hydrogen uptake in laser-based directed energy deposition of 316L

Abstract

The mechanical properties and hydrogen uptake behavior of metallic components fabricated by means of laser-based directed energy deposition (DED-LB/M) are strongly influenced by the process parameters applied during fabrication. In this work, the effect of laser power (400 W and 600 W) and inter-layer dwell time (0 s and 60 s) on the inter-layer thermal history, hardness distribution, and electrolytic charging behavior of 316L austenitic stainless steel wall structures fabricated by powder-based DED-LB/M is investigated. Using infrared thermography and Vickers hardness measurements, a direct correlation between inter-layer dwell time, heat accumulation and hardness could be established, with dwell-free conditions leading to continuously increasing inter-layer temperatures and pronounced hardness gradients along the build direction. Furthermore, by means of electrolytic hydrogen charging, surface condition was identified as the dominant factor influencing hydrogen charging behavior. For polished specimens, no systematic influence of laser power or inter-layer dwell time was detected, indicating that process-parameter-induced microstructural differences are not resolvable under the applied charging conditions. For as-built specimens, two distinct groups of potential-time curves were identified correlating with the inter-layer dwell time, which could not be attributed to surface roughness alone but is likely related to differences in surface oxide formation and partially fused powder particle adhesion.