The effects of surface finish on the fatigue performance of electron beam melted Ti–6Al–4V

Abstract

Additive manufacturing (AM) is an advanced manufacturing technique whose uptake within the aerospace industry is being limited by the lack of understanding of the effects of surface finish on the fatigue properties of AM built components. It is known that the surface profile of as-built AM parts is significantly rougher than its traditionally manufactured equivalent, promoting stress raising features that have an adverse effect on fatigue life.This paper will focus on developing a deeper understanding of the fatigue properties of the widely-used alpha-beta titanium alloy, Ti–6Al–4V, as manufactured through electron beam melting (EBM) and the role that surface roughness plays. Results have been generated on as-built and machined & polished EBM test coupons, supported by microstructural and fractographic analysis, X-ray computed tomography (XCT), advanced surface profilometry and hardness testing.Results have shown that as-built EBM Ti–6Al–4V samples have a significantly inferior fatigue life than machined & polished samples, despite the material in each of the surface finish types being hot isostatically pressed (HIP). It has been revealed that while surface parameters, MR2 and Ra, provide no correlation to high cycle fatigue life, it is shown that as the Rp surface parameter decreases, the respective number of cycles to failure increases. It can be concluded that a samples’ surface roughness is a highly contributing factor in influencing fatigue performance, yet it should not be considered in isolation since other features, including porosity, also play an important role.