Effect of HIP post-processing at 850 °C/200 MPa in the fatigue behavior of Ti-6Al-4V alloy fabricated by Selective Laser Melting

Abstract

Hot Isostatic Pressing(HIP) is a thermomechanical post-processing technique widely used in Additive Manufacturing parts to reduce internal defects, such as entrapped-gas-pores or lack-of-fusion, which have a great influence on the mechanical and fatigue properties of the material. In this paper, the effect of a non-conventional HIP-cycle on the fatigue behavior of a Ti-6Al-4V alloy manufactured by Selective Laser melting (SLM) is studied. The HIP-cycle examined in this study is carried out at pressure of 200 MPa and a temperature of 850 °C for 2 h. Moreover, the cooling process is faster than that obtained from conventional furnace cooling rates, with the aim to limit the microstructural coarsening effects that affect the fatigue behavior. For the study, an extensive experimental fatigue program was carried out which included a first batch of SLM specimens tested under as-built conditions, a second batch of SLM specimens subjected to the present HIP process, and a third batch of specimens of a reference wrought processed material obtained by rolling and annealing processes. The microstructure of the material, before and after HIPping, is analyzed and a fractographic analysis is carried out to study the mechanism of crack initiation and its relation to the fatigue behavior. The results show that the present HIP-process allows for very good material densification, a microstructure that shows minimal coarsening effects, and good fatigue properties comparable to the conventional wrought processed material.