Effects of ultrasonic treatment on mechanical properties and microstructure of stainless steel 308L and Inconel 718 functionally graded materials fabricated via double-wire arc additive manufacturing

Functionally graded materials (FGMs) are composites comprising various materials that can overcome the limitations of homogeneous materials, such as abrupt property changes at material interfaces. This results in gradual changes in the microstructure and composition, thus limiting the performance optimization under complex operating conditions. In this study, a novel ultrasonic-assisted double wire arc additive manufacturing (U-DWAAM) process is performed to fabricate stainless steel 308 L (SS 308 L) and Inconel 718 (In 718) composites with and without ultrasonic treatment. The microstructure, chemical composition, tensile strength, and hardness of the fabricated FGMs are investigated with and without ultrasonic treatment. The results show that the microstructure of the FGMs without ultrasonic treatment is mainly composed of columnar grains. However, by performing ultrasonic treatment, the grain size reduced, the homogeneity improved, and the grains refined into equiaxed grains. The chemical composition shows a smooth transition from SS 308 L to In 718, with an increase in Ni, the main element in In 718, and a decrease in Fe, the main element in SS 308 L. Typical epitaxial grain growth is observed at interfaces with different compositions. For the specimens subjected to ultrasonic treatment, their tensile strength is slightly higher and elongation lower compared with those of the specimens without ultrasonic treatment. This is attributable to the change in the equiaxed grains and a decrease in the crystal size owing to the application of ultrasound in the columnar grains. The hardness increases gradually from the SS 308 L portion of the FGMs as the mixing ratio of In 718 increases. With and without ultrasonic treatment, the percentage increase in hardness is 7%, 9%, 10%, 12%, and 15%, compared with that of the same layer without ultrasonic treatment. The increase in hardness is due to microstructural changes (equiaxed grains in the columnar phase), solid solution strengthening, and precipitation enhancement owing to an increase in Ni, Mo, and Nb contents with the In 718 content.