Surfaces after chemical post-processing treatments of electron beam melting (EBM) produced Ti-6Al-4V have been studied. Targeted chemical treatment allowed the study of variation in surface quality with material removal depth. Characterization of surface and defect morphologies were made, comparing two chemical post-processing methods, Hirtisation® and chemical milling with different milling depths. Surface topography was characterized using white light interferometry and subsurface defect distribution was studied using X-ray computed tomography (XCT). The morphology of the surface at different milling depths was compared to the sub-surface information from XCT scans of the as-built material. Furthermore, Hot Isostatic Pressing (HIP) treated material was documented for comparison. Results show that post-processed surfaces contain a number of different defects of mixed morphology, position and origin. Post-processing deteriorates the surface quality with increased removal depth due to the presence of sub-surface defects. The position of sub-surface defects in relation to the material surface coincides with the depth at which contour-hatch interactions are likely to have occurred during the EBM building process. The distribution of this sub-surface defect population is anisotropic in the building (horizontal) plane and reasons for this are explored. Hirtisation® produces surfaces morphologically different from chemically milled surfaces. This difference was found to contribute to Hirtisation® producing surfaces with higher roughness (Sa) than chemically milled surfaces at comparable removal depth. HIP did remove all detectable sub-surface defects but microstructural artefacts indicating healed porosity were found.

In this work, the fatigue behaviour of Ti6Al4V manufactured using electron beam melting, its dependency on porosity, distance from the base plate and build layer height were investigated. XCT scans of the fatigue sample gauge lengths were correlated to SEM investigations of the fracture surfaces. A comparison between the top and bottom halves of the builds in terms of defect population and fatigue behaviour was also made. Larger pores were detected in samples with a larger build layer height and lower position in the build chamber. Results also indicate that part geometry and pore location, specifically closeness to the surface, are important factors regarding the initiation location of fatigue fractures at 1 % strain. Furthermore, a fatigue critical lack of fusion defect was undetectable in the XCT scan.

The fatigue behavior of additively manufactured (AM) structural parts is sensitive to the surface and near-surface material conditions. Chemical post-processing surface treatments can be used to improve the surface condition of AM components, including complex geometries with surfaces difficult to access. In this work, surfaces of electron beam powder bed fusion (EB-PBF) produced Ti–6Al–4V were subject to two different chemical post-processing surface treatments, chemical milling and Hirtisation. As-built and machined surfaces, as well as hot isostatic pressing (HIP), treated conditions were also investigated. Fatigue testing was carried out in four-point bending. The investigation focused on the relationship between fracture mechanisms and fatigue life through fractographic study. It was found that a majority of fractures were initiated at internal surface-near defects or defects on the surface. Chemical post-processing was found to smoothen the surface but to leave a surface waviness. Material removal during post-processing could open up internal defects to the treated surface. In HIP-treated specimens, fractures initiated at defects open to the surface. Despite post-processing increasing the mean life of fatigue specimens, no significant improvements in the lowest tested life were observed for any specimen condition.

Electron beam melting is a powder bed fusion (PBF) additive manufacturing (AM) method for metals offering opportunities for the reduction of material waste and freedom of design, but unfortunately also suffering from material defects from production. The stochastic nature of defect formation leads to a scatter in the fatigue performance of the material, preventing wider use of this production method for fatigue critical components. In this work, fatigue test data from electron beam melted Ti-6Al-4V specimens machined from as-built material are compared to deterministic fatigue crack growth calculations and probabilistically modeled fatigue life. X-ray computed tomography (XCT) data evaluated using extreme value statistics are used as the model input. Results show that the probabilistic model is able to provide a good conservative life estimate, as well as accurate predictive scatter bands. It is also shown that the use of XCT-data as the model input is feasible, requiring little investigated material volume for model calibration.

In this study a fatigue life model was developed and used to predict the fatigue life of electron beam powder bed fusion  Ti-6Al-4V specimens produced through powder bed fusion. To focus on the impact of surface quality, fatigue testing was carried out in four point bending fatigue on as-built, machined and chemically milled specimens. X-ray computed combined with extreme value statistics provided information about the distribution of internal defects near the surface and white light interferometry was used for surface roughness mesurements. Together the two methods provided input to a crack propagation fatigue life model. It was found that local information from high resolution XCT scans can improve model accuracy in capturing the effect of surface treatments. Together with surface roughness effects this information can be used to accurately capture general trends in fatigue behavior.

This study investigates the use of X-ray computed tomography and machining to various depths for predicting the fatigue life of Ti-6Al-4V specimens additively manufactured using electron beam melting. Four point bend testing in fatigue was carried out on specimens machined to various machining depths.  The results show promising tendencies in capturing the variance of life observed, indicating that the defect distribution at specific machining depths can accurately predict the fatigue life of the specimens. It was also shown that the selection of machining depth is an important factor in producing material safe to use in fatigue critical applications. The findings also provide insights into the use of X-ray computed tomography and machining as effective methods for predicting fatigue life in Ti-6Al-4V specimens additively manufactured using electron beam melting.