Pulsed laser directed energy deposition of super duplex stainless steel: engineering microstructure and improving mechanical properties

Alloys developed by fusion-based additive manufacturing often suffer from the coarse columnar grain structure and their effect on properties. This work involves the practical application of pulsed laser in laser-based directed energy deposition (DED-LB) of super duplex stainless steel which led to engineering the microstructure, improving the mechanical properties, and changing the dominant texture. Pulsed laser DED-LB (here P-DED) with laser spot sizes of 1 and 2 mm and different frequencies were used. Refine-grained ferritic steels containing porosity were produced when using a small laser spot size. Ferrite-to-austenite (alpha -> gamma) transformation was constrained to the grain boundaries under the effect of small excitation overlaps. Using broader laser, higher energy input, and ultra-short pulse intervals encouraged gamma nucleation, promoted the density, and decreased the content of undesirable oxides that are typically formed during the conventional DED-LB (here C-DED). The local ferritization under the fusion lines of C-DED was avoided by P-DED. Directionally solidified alpha, extending into several layers, was inhibited by P-DED with optimum overlap. Enhanced supercooling resulted in an in situ grain refinement and columnar-to-equiaxed morphological transition. Defect-free microstructure and effective distribution of interphase boundary surface by P-DED, with a laser spot size of 2 mm and similar to 99% excitation overlap, largely improved the toughness and elongation (with acceptable strength). Pulse-induced convection and isotropic heat flow during P-DED with smaller laser spot size subsided the trend of preferred orientation. However, an alignment of < 001 > (alpha) with deposition direction during P-DED with the broader laser preserved the typical {001} < 100 > solidification texture and, consequently, the transformation texture.