Additive manufacturing (AM) using powder bed fusion is becoming a mature technology that offers great possibilities and design freedom for manufacturing of near net shape components. However, for many gas turbine and aerospace applications, machining is still required, which motivates further research on the machinability and work piece integrity of additive-manufactured superalloys. In this work, turning tests have been performed on components made with both Powder Bed Fusion for Laser Beam (PBF-LB) and Electron Beam (PBF-EB) in as-built and heat-treated conditions. The two AM processes and the respective heat-treatments have generated different microstructural features that have a great impact on both the tool wear and the work piece surface integrity. The results show that the PBF-EB components have relatively lower geometrical accuracy, a rough surface topography, a coarse microstructure with hard precipitates and low residual stresses after printing. Turning of the PBF-EB material results in high cutting tool wear, which induces moderate tensile surface stresses that are balanced by deep compressive stresses and a superficial deformed surface that is greater for the heat-treated material. In comparison, the PBF-LB components have a higher geometrical accuracy, a relatively smooth topography and a fine microstructure, but with high tensile stresses after printing. Machining of PBF-LB material resulted in higher tool wear for the heat-treated material, increase of 49%, and significantly higher tensile surface stresses followed by shallower compressive stresses below the surface compared to the PBF-EB materials, but with no superficially deformed surface. It is further observed an 87% higher tool wear for PBF-EB in as-built condition and 43% in the heat-treated condition compared to the PBF-LB material. These results show that the selection of cutting tools and cutting settings are critical, which requires the development of suitable machining parameters that are designed for the microstructure of the material.

Additive manufacturing (AM), of metals is gaining popularity as an alternative to conventional manufacturing techniques such as casting and forging. Metal-AM allows for the production of complex part geometries with reduced material waste and shorter lead times. The aerospace industry has been quick to adopt this technology; however, the fatigue performance of  metal-AM is a critical consideration for ensuring safety.

One of the challenges of AM metal is limited knowledge in its ability to withstand various loading conditions, from static loads to complex multiaxial thermo-mechanical fatigue loads. Defects in AM materials, such as rough surfaces, pores, and lack-of-fusion between build layers, act as local stress concentrators and crack initiation sites in the material. Some defects can be reduced through careful build process optimization and post-processing treatments, but it is generally not considered possible to eliminate all defects. Therefore, it is necessary to estimate the fatigue performance of AM-produced critical components containing defects.

The aim of the thesis is to investigate the relationship between defect characteristics and fatigue behaviour in AM-produced metal. The AM-material studied is electron beam powder bed fusion (EB-PBF) produced Ti-6Al-4V. Defect distributions, both on the surface and further inside the material, are statistically analysed and a simple fracture mechanical model for predicting fatigue life is developed. Post-production treatments, such as machining, chemical surface treatments and hot isostatic pressing (HIP), are also examined to determine their impact on defects and fatigue behaviour.

The thesis consists of six scientific papers. In the first three papers (1-3), fatigue behaviour and material characteristics are studied using mechanical testing and materials characterisation techniques such as optical microscopy, scanning electron microscopy, interferometry, and X-ray computed tomography (XCT). Internal defects are documented using XCT and compared with fatigue crack initiations (paper 1). Surface roughness and morphology of post-production treated EB-PBF material are analysed using interferometry and microscopy, and its connection to the surface near distribution of internal defects is examined (paper 2). Material that has been surface treated and subjected to Hot Isostatic Pressing (HIP) was tested in four-point bending fatigue followed by a fractographic study (paper 3).

The final three papers (4-6) of the thesis aim to take the material characteristics investigated in the first three papers as input for a crack-propagation-based fracture mechanics model to predict fatigue life using statistical analysis of the observed surface quality and defect distribution. These papers include modelling based on information about internal defects, as studied in the first paper, applied in a tension-compression cyclic load case (paper 4);  an exploration of surface morphology and four-point fatigue testing combined with surface adjacent XCT to use as input for a surface-sensitive fatigue life model (paper 5); and an estimation of the impact of surface machining depth on the material's fatigue behaviour using the experience gained from all previous work (paper 6).

It was found that the severity of the impact of a defect on the fatigue behaviour of the material largely depends on its characteristics and position relative to the surface. Production and post-processing of the material also play a role in the severity of this impact. The thesis also concludes that probabilistic statistical analysis can be used to accurately predict the life of the studied material under the conditions tested for.

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.

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.

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.

Layer by layer manufacturing (additive manufacturing, AM) of metals is emerging as an alternative to conventional subtractive manufacturing with the goal of enabling near net-shape production of complex part geometries with reduced material waste and shorter lead times. Recently this field has experienced rapid growth through industrial adaptation but has simultaneously encountered challenges. One such challenge is the ability of AM metal to withstand loading conditions ranging from static loads to complex multiaxial thermo-mechanical fatigue loads. This makes fatigue performance of AM materials a key consideration for the implementation of AM in production. This is especially true for AM in the aerospace industry where safety standards are strict.

Defects in metal AM materials include rough surfaces, pores and lack-of-fusion (LOF) between build layers. These defects are detrimental to fatigue as they act as local stress concentrators that can give rise to cracks in the material.  Some defects can be avoided by careful build process optimization and/or post-processing but fully eliminating all defects is not possible. Because of this, a need arises for the capability to estimate the fatigue performance of AM produced critical components containing defects.

The aim of the thesis is to increase understanding regarding the connection between defect characteristics and the fatigue behaviour in AM produced Ti-6Al-4V. Defect distributions are statistically analysed for use in a simple fracture mechanical model for fatigue life prediction. Other study areas include the impact of post-production treatments such as chemical surface treatments and hot isostatic pressing (HIP) on defects and fatigue behaviour.

The thesis constitutes three scientific papers. The AM technique studied in these papers is Electron Beam Melting (EBM) in which an electron beam selectively melts pre-alloyed metal powder. In paper 1, defects were studied using X-ray computed tomography (XCT) and fatigue crack initiation was related to the observed defect distribution. In paper 2, XCT data was used to relate the surface morphology and roughness of post-production treated EBM material to the surface near defect distribution. The connection between this distribution and manufacturing parameter has also been explored. Paper 3 builds on and extends the work presented in paper 1 by including further fatigue testing as well as a method for predicting fatigue life using statistical analysis of the observed defect distribution.

The impact of a defect on the fatigue behaviour of the material was found to largely depend on its characteristics and position relative to the surface. Production and post-processing of the material was found to play a role in the severity of this impact. Finally, it was found that a probabilistic statistical analysis can be used to accurately predict the life of the studied material at the tested conditions.

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.

In this work, the microstructures of Ti-6Al-4V manufactured by different additive manufacturing (AM) processes have been characterized and compared. The microstructural features that were characterized are the α lath thickness, grain boundary α (GB-α) thickness, prior β grain size and α colony size. In addition, the microhardnesses were also measured and compared. The microstructure of shaped metal deposited (SMD) Ti-6Al-4V material showed the smallest variations in α lath size, whereas the material manufactured with laser metal wire deposition-0 (LMwD-0) showed the largest variation. The prior β grain size was found to be smaller in material manufactured with powder bed fusion (PBF) as compared with corresponding material manufactured with the directed energy deposition (DED) processes. Parallel bands were only observed in materials manufactured with DED processes while being non-present in material manufactured with PBF processes.