Ultrasonic Impact Treatment (UIT) combined with powder bed fusion (PBF) process for precipitation hardened martensitic steels

Abstract

High-strength martensitic stainless steels such as 17–4 PH (SS 17–4 PH) generally exhibit poor ductility and strain-hardening rates. In this study, an Ultrasonic Impact Treatment (UIT) is combined with a powder bed fusion (PBF) manufacturing process with the objective of enhancing the mechanical properties of SS 17–4 PH. UIT is introduced as a surface peening step at regular intervals during the PBF process after depositing a set of predetermined number of layers. The coupon specimens extracted from the build are then subjected to a post-build heat treatment. Then the microstructural and the mechanical properties of these specimens are characterized. The characterizations reveal that the heavy plastic deformation induced by UIT drives microstructural relaxation and recrystallization under heat treatment, reducing defects extensively. The heavy plastic deformation also drives diffusional reversion of the austenite phase during deposition and an inhibition of the martensitic phase transformation during the post-build heat treatment. As a result, the UIT specimens subjected to post-build heat treatment exhibit remarkable enhancement in ductility along with high strength and strain-hardening rate compared to the non-UIT PBF specimens under as-built condition.