Laser Scan Strategies for in-situ Precipitation Hardening of Stainless Steel During Powder Bed Fusion

Abstract

In this work, we demonstrate how in-situ microstructure control could be used as an alternative to post-processing heat treatment to optimise the properties of additively manufactured (AM) components. This processing strategy improves both the cost and sustainability of AM due to the significant time and energy savings associated with the replacement of heat treatments. We demonstrate our strategy on 17-4 precipitation hardening stainless steel produced via laser powder bed fusion (LPBF). We show how the intrinsic heat treatment (IHT) produced by the cyclic reheating associated with layer-wise metal consolidation can be tailored to promote precipitation hardening to variable extents using different energy densities. By analysing the kinetics of precipitation, we define and map an “IHT window” surrounding each melt pool as a function of laser energy density, wherein in-situ precipitation hardening occurs. We further show how overlapping between the larger IHT windows at high energy densities leads to increased precipitation hardening. Our work highlights the nuances of precipitation hardening during LPBF and provides guidelines to achieve “print and use” design strategies for precipitation hardened alloys without requiring further heat treatment. This advanced microstructural control strategy has the potential to accelerate the adoption of AM technologies in industry by reducing production costs, time, and the energy embodied in as-manufactured parts.

Keywords: L-PBF, Microstructure hull/Microstructure design, Precipitation hardening, EDXS, Differential scanning calorimetry (DSC)