Effect of Ultrasonic Impact Treatment on the Stress-Controlled Fatigue Performance of Additively Manufactured Ti-6Al-4V Alloy

Abstract

Additive Manufacturing (AM) for metals, is based on melting or sintering and then solidification of metal powder particles to fabricate 3D objects directly out of Computer Aided Design (CAD) data. Due to its strong capabilities to fabricate components with complex geometries, AM largely relaxes the manufacturing constraints opening the design space to optimize components for performance maximization and extensive weight minimization. These features have made AM a strong manufacturing candidate for several applications in the aerospace and defense industries. However, it is found that during the AM process several powder particles are left un-melted leading to high porosity and poor component surface finish. These surface pores act as potential crack initiation sites during cyclic loading and thus significantly compromise the component’s tensile and fatigue performances, leading to poor mechanical properties compared to traditional cast or wrought metals. In this paper, we study the effect of Ultrasonic Impact Treatment (UIT) on the stress-controlled fatigue performance of additively manufactured Ti–6Al–4V alloy. Here, severe plastic deformations are developed on the surface of the metal by multiple sliding impacts generated by the UIT system. Specimens of Ti–6Al–4V alloy were manufactured by Direct Metal Laser Sintering (DMLS) and were subject to the UIT. The treatment was applied to statically preloaded specimens at 30 N and a controlled scanning speed of 1000 mm/min. Structure investigations were performed by testing as-built specimens as well as treated specimens by UIT in low-cycle fatigue under tensile cyclic loading of 400 MPa using 810 Materials Testing System (MTS). Fatigue life, surface defects and roughness were examined to observe the effects of the UIT on the metal’s surface. It is found that the UIT treatment has improved the fatigue life of the treated specimens by 192% compared to the as-built specimens at the same dynamic stress level. Also, microscopic observations indicate a better surface finish of the treated specimens.