A novel in-situ field-assisted powder bed laser fusion using liquid metal enabling microstructure control and strength enhancement of austenitic steel

Abstract

The layer-by-layer powder bed additive manufacturing approach, which encapsulates the workpiece in powder during processing, imposes limitations on the integration of in-situ field assistance and enhances production costs. In this work, a novel laser powder bed fusion has been proposed in which the layer-wise accumulated powder bed is replaced by a thin powder layer floating on the liquid Sn. Such a liquid-metal-assisted laser powder bed fusion presents unique advantages: the characteristic thermal history of deposited materials due to high thermal conductivity and fluidity of liquid metals provides greater possibilities for microstructure modulation; the recyclable liquid metal also reduces the need for powder in the forming cylinder and reduces the number of times the powder is reused. Based on the normalized process diagram of liquid-metal-assisted laser powder bed fusion, forming experiments were carried out on the austenitic stainless steels, and the mechanisms underlying the regulation of fine-grain regions were investigated, along with an analysis of the microstructure of this region. Results indicated that the high cooling rate during liquid-metal-assisted laser powder bed fusion led to a finer microstructure and a heterogeneous grain structure ranging from submicron to micron scales in the austenitic stainless steels. The formed heterogeneous austenitic steel exhibits a yield strength surpassing 1.1 GPa and a tensile strength of 1.5 GPa, while retaining an average uniform elongation of 7 %. The in-situ heat treatment principles using liquid metal demonstrated in this work have significant applicability across various additive manufacturing processes and precipitation-hardening alloys.