High cycle fatigue strength of hot isostatically pressed and chemically etched laser powder bed fusion produced Ti-6Al-4V

Abstract

Metal additive manufacturing (AM) parts are likely to fail in fatigue when subjected to cyclic loading due to inherent manufacturing defects, including a rough surface finish, internal pores, high residual stress, and martensitic microstructure. In particular, a rough surface finish is understood to limit fatigue strength most severely since surface aberrations act as severe stress raisers where stresses are typically the highest. As such, post-surface finishing processes are recommended to reduce surface roughness. Conversely, one main advantage of AM is the ability to produce complex geometries without the need for machining. This makes it impractical to use mechanical surface finishing techniques. One solution is to use chemical etching techniques that are not bound by geometry constraints. This study further investigates the effect chemical etching, with an HF-HNO3 solution, has on the fatigue strength of laser powder bed fusion-produced Ti-6Al-4V when immersed for different time intervals (5, 15 and 90 min) in the chemical solution. The reduction in surface roughness and the increase in fatigue strength determined the effectiveness of the different chemical etching times. The results show that a 90-minute etching interval provides the highest surface roughness reduction of 75.9 % and fatigue limit increase of 46 % compared to the as-built surface roughness. However, the internal pores limited the fatigue strength increase because the subsurface pores were brought to the surface by the etching process. Combining hot isostatic pressure (HIP) with chemical etching resulted in a significant 92 % increase in the fatigue limit that is comparable to a wrought defect-free specimen.