Ductilizing additively manufactured paramagnetic alloys via auxiliary magnetic field: Beyond the porosity effect

Introducing auxiliary magnetic field (MF) in metal additive manufacturing (AM) offers great potential to tailor microstructures and optimize mechanical properties, where the performance enhancement is commonly attributed to porosity reduction. For paramagnetic alloys with broad structural and functional applications, the mechanisms by which MF regulates the grain morphology and mechanical performance improvement, remain unexplored. Here, we study the MF-assisted AM of paramagnetic alloys by using 316L stainless steel as a representative, and reveal MF-enabled new mechanisms for the formation of tailored grain morphology and damage delocalization, achieving elongation improvement by 25 % with strength increase. Through comprehensive comparison, the performance enhancement in the proposed method outperforms other field-assisted AM and post-processing techniques. Mechanistically, the MF decreases the inclination angle of crescent grains and refines the width of columnar grains without altering the grain size, preferential texture, or average initial geometrically necessary dislocation (GND) density. The change in grain inclination angle originates from the altered melt pool flow as revealed by the melt pool simulation, which promotes dendrite deflection. Moreover, the ductilization mechanisms are revealed via in-situ tensile tests and crystal plasticity simulation: (i) reduced fraction of columnar grains susceptible to damage localization and (ii) damage delocalization through a tailored spatial arrangement of soft-to-hard crescent grains, whereas the AM-featured initial GNDs contribute negligibly. Regarding the signature columnar-to-crescent grain structures commonly existing in AM-ed metals, we unveil a new fundamental role of MF in governing grain morphology change and performance optimization, which is feasible for broad paramagnetic alloys with similar grain structures.