Post-build thermomechanical processing of wire arc additively manufactured stainless steel for improved mechanical properties and reduction of crystallographic texture

Abstract

Wire-arc additive manufacturing (WAAM) was used to fabricate 304 L stainless steel plates for the purpose of characterizing texture and anisotropic behavior in the as-built parts, followed by post-build rolling and heat-treating to modify mechanical properties and reduce texture. The plates were made using a robotic controlled gas metal arc (GMA) welding system designed for high deposition rates and were characterized using uniaxial tensile testing, macro- and micro-hardness, and optical microscopy. In addition, resonant ultrasound spectroscopy (RUS) was used for the first time of WAAM plates to measure the anisotropic elastic modulus, shear modulus and Poisson’s ratio. Post processing consisted of mechanical rolling the walls to 25% and 40% strains followed by heat treating at 650 °C, 850 °C and 1050 °C. Results show that anisotropy of mechanical and physical properties is present in the starting microstructure and persists in the rolled plates prior to heat treating, with elastic modulus in the build direction of 50% of the polycrystalline wrought value, and shear modulus and Poisson’s ratio of 200% of the polycrystalline value. Anisotropy also persists for all samples stress relieved at 650 °C, and in samples annealed at 1050 °C without any prior rolling strain. Heat treating has shown the reduction in texture and anisotropy only in the rolled plates. Complete elimination of texture and anisotropy was observed for WAAM plates rolled and annealed at 1050⁰C, and for plates rolled to 40% and recrystallized at 850 °C. Recrystallization of the rolled plates at temperatures 850 °C or higher increases yield strength, ultimate strength and elongation to failure relative to the as-built parts, with properties that equal or exceed typical wrought or cast alloys of similar composition. These results demonstrate that WAAM can be used to build custom sized semi-finished parts and blanks that can later be mechanically formed, and heat treated into high quality finished components.