Microstructural transition across interlayer ultrasonic impact peened interfaces in hybrid additive manufacturing of 316L stainless steel

While additive manufacturing generally involves building components layer by layer, hybrid additive manufacturing in this context refers to a process that combines printing with interlayer cold working. Although cold working induces plastic deformation, the subsequent microstructural evolution in the newly printed layers is largely dictated by the thermal input of the printing process. Hence, the objective of this study was to characterize the microstructural evolution at the interface of interlayer ultrasonic impact peened 316 L stainless steel manufactured by laser powder bed fusion. Ultrasonic treatment was applied intensively over five consecutive layers, and the resulting interface was mapped through microstructural analysis and hardness testing across multiple length scales. The results revealed a succession from highly deformed grains in treated regions to recrystallized equiaxed structures, and finally remelted columnar features in the subsequent layers. This transition was driven by the rapid cooling rate and steep thermal gradient of the printing process.