This work investigates the influence of a single tensile preload, applied prior to fatigue testing, on the fatigue strength of 316L stainless steel parts manufactured using laser-based powder bed fusion (PBF-LB) with a porosity of up to 4 %. The specimens were produced in both the horizontal and vertical build directions and were optionally preloaded to 85 % and 110 % of the yield strength before conducting the fatigue tests. The results indicate a clear tendency of improved fatigue life and fatigue limit with increasing overload in both cases. The fatigue limits increased by 25.8 % and 24.6 % for the horizontally and vertically built specimens, respectively. Extensive modelling and experiments confirmed that there was no significant alteration in the shape and size of the porosity before and after preloading. Therefore, the observed enhancement in fatigue performance was primarily attributed to the imposed local compressive residual stresses around the defects.

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.

This paper develops a hybrid experimental/simulation method for the first time to assess the thermal stresses generated during electron beam melting (EBM) at high temperatures. The bending and rupture of trusses supporting Inconel 625 alloy panels at similar to 1050 degrees C are experimentally measured for various scanning strategies. The generated thermal stresses and strains are thereafter simulated using the Finite-Element Method (FEM). It is shown that the thermal stresses on the trusses may reach the material UTS without causing failure. Failure is only reached after the part experiences a certain magnitude of plastic strain (similar to 0.33 +/- 0.01 here). As the most influential factor, the plastic strain increases with the scanning length. In addition, it is shown that continuous scanning is necessary since the interrupted chessboard strategy induces cracking at the overlapping regions. Therefore, the associated thermal deformation is to be minimized using a proper layer rotation according to the part length. Although this is similar to the literature reported for selective laser melting (SLM), the effect of scanning pattern is found to differ, as no significant difference in thermal stresses/strains is observed between bidirectional and unidirectional patterns from EBM.

Ceramic particles-based packed bed systems are attracting the interest from various high-temperature applications such as thermal energy storage, nuclear cooling reactors, and catalytic support structures. Considering that these systems work above 600 ◦C, thermal radiation becomes significant or even the major heat transfer mechanism. The use of coatings with different thermal and optical properties could represent a way to tune and enhance the thermodynamic performances of the packed bed systems. In this study, the thermal stability of several metallic (Inconel, Nitinol, and Stainless Steel) based coatings is investigated at both high temperature and cyclic thermal conditions. Consequently, the optical properties and their temperature dependence are measured. The results show that both Nitinol and Stainless Steel coatings have excellent thermal stability at temperatures as high as 1000 ◦C and after multiple thermal cycles. Contrarily, Inconel (particularly 625) based coatings show abundant coating degradation. The investigated coatings also offer a wide range of thermal emissivity (between0.6 and 0.9 in the temperature range of 400–1000 ◦C), and variable trends against increasing temperature. This work is a stepping-stone towards further detailed experimental and modelling studies on the heat transfer enhancement in different ceramic-based packed bed applications through using metallic coatings.

Metal AM processes produce significantly rough surfaces as compared to wrought or machined components. Currently, the as-EBM surfaces are extremely rough and consequently, the as-built components could not be directly used as commercial products. The state-of-the-art research has focused primarily on deeply machined (DM) samples. The mechanical properties of the EBM components as reported thus are not real representative of the process. This can largely dispute the main merits of the EBM process: print on demand and low buy-to-fly ratio. Hence, the research here was designed to bridge this research gap and systematically analyze the properties of net-shaped (NS) and near-net-shaped (NNS) components.

This thesis is divided into three levels, including the fundamental, validation and optimization level. The fundamental research work is performed on the commercialized processing parameters from Arcam, for Inconel 718 (IN718). The NNS samples made with and without the commercial multi-spot contour are tested for their mechanical properties. These samples achieved smoother surfaces and better tensile behavior. Thus, they were later compared with the NS and DM samples which were made with the same parameter settings but varied machining depth. The purpose of this work was to test the NS and NNS samples and analyze the cause of the failure. It was found that the premature failure of these samples was linked to the porous surface and subsurface region. Therefore, a machining protocol was set up according to the depth of the porous region which can theoretically improve the tensile strength of the machined samples to a level comparable to wrought material. The findings in case of IN718 were also validated for another superalloy, Inconel 625(IN625). The results again showed that the machining requirement of the as-EBM samples with contour is more than 2 mm. Therefore, a novel contouring strategy was later developed, optimized and tested in order to eliminate the machining requirement. To do so, the continuous contour with different energy, overlapping ratio and sequence to the hatching region was tested. The high-energy continuous contour applied after hatching was found to be the optimal strategy, which was able to maintain a comparable surface condition as the multi-spot contour whilst it could generate a denser subsurface region. Accordingly, the NNS samples with such contour achieved comparable tensile properties as DM samples.

It should be noted that all these research works were focused on the vertical samples (along the building direction) to avoid high thermal stresses in horizontal direction that seriously limit deformation free manufacturing of the samples. To deal with the challenge of thermal stresses and deformation, the last part of this research was designed to assess the thermal stresses and strains of EBM manufactured IN625 samples. The effects of different processing parameters and scanning strategies were tested to provide a set of guidelines about how to produce the horizontal and longer components. The results showed that one could effectively minimize the thermal stress/strain and deformation of the parts using a bidirectional scanning pattern with proper layer rotations angles to deliver shorter scans in each layer.

Additiva tillverkningsprocesser i metall ger avsevärt grövre ytor jämfört med smidda eller maskinbearbetade komponenter. Ytor tillverkade med as-EBM blir, med dagens teknik, extremt grova och följaktligen kan dessa komponenter inte användas direkt som kommersiella produkter. Den senaste forskningen har primärt fokuserat på djupt bearbetade (DM) prover. De mekaniska egenskaperna som rapporterats för dessa EBM-komponenter är således inte representativa för processen. Detta kan till stor del ifrågasätta de viktigaste fördelarna med EBM-processen: beställtryck och låg volymkvot mellan materialbit och komponent. Således var denna forskning utformad för att överbrygga detta forskningsgap samt systematiskt analysera egenskaperna hos slutformade (NS) och nära slutformade (NNS) komponenter.

Denna avhandling är indelad i tre nivåer bestående av grundläggande, validering och optimering. Det grundläggande forskningsarbetet har utförts på kommersialiserade processparametrar från Arcam och materialet Inconel 718 (IN718). NNS-proverna gjorda med och utan den kommersiella smältningsstrategin med flerpunktskontur har testats för utvärdering av mekaniska egenskaper. Dessa prover uppnådde jämnare ytor och bättre dragegenskaper. Således jämfördes de senare med NS- och DM-proverna vilka gjordes med samma parameterinställningar men olika bearbetningsdjup. Syftet med detta arbete var att testa NS- och NNS-proverna och analysera orsaken till brott. Det konstaterades för dessa provbitar att tidiga brott var kopplat till den porösa ytan och regionen nära ytan. Därför implementerades en bearbetningsprocedur upp i enlighet med djupet av den porösa regionen vilket teoretiskt kan förbättra draghållfastheten för de bearbetade proverna. Detta till en nivå som är jämförbar med smidda material. Utfallet med IN718 kunde även valideras för en annan superlegering, Inconel 625 (IN625). Resultaten visade återigen att bearbetningskravet för as-EBM-proverna med strategin för flerpunktskontur är mer än 2 mm. Därför utvecklades, optimerades och testades en ny kontureringsstrategi med syfte att eliminera bearbetningskravet. För att göra detta testades strategier med kontinuerlig kontur och olika energi, överlappningsförhållande och sekvens till regionen med fyllnadsläggning. Den kontinuerliga högenergi-konturen som applicerades efter fyllnadsläggning visade sig vara den optimala strategin som kunde bibehålla jämförbar kondition på ytan som strategin med multipunktskonturen, samtidigt som den generera en tätare struktur i regionen under ytan. Följaktligen uppnådde NNS-proverna med sådan kontur jämförbara dragegenskaper som DM-prover.

Det bör noteras att forskningen var fokuserad på vertikala prover (längs byggriktning) för att undvika höga termiska spänningar i horisontell riktning som kraftigt begränsar deformationsfri tillverkning av provbitar. För att hantera utmaningen med termiska spänningar och deformation utformades den sista delen, inom denna avhandling, för att bedöma termiska spänningar och töjningar av EBM-tillverkade provbitar i IN625. Effekten av olika processparametrar och skanningsstrategier utvärderades för att ta fram en uppsättning riktlinjer gällande tillverkning av horisontella och längre komponenter. Resultaten visade att termisk spänning, töjning och deformation kund minimeras genom att använda ett dubbelriktat skanningsmönster med rätt lagerrotationsvinklar för att ge kortare skanningar i varje lager.

Electron beam melting (EBM) is a powder bed fusion (PBF) additive manufacturing (AM) process for metal powder printing with wide applications in key industrial sectors, including automotive, healthcare, aerospace, etc. The high-temperature processing of this technique extensively sinters the powders on the surfaces and creates a poor and coarse surface finish. Differences between the surfaces from EBM in comparison with other AM processes make it difficult to answer which measurement method, with what measurement settings, and which evaluation parameters should be used for surface characterization. In this work, the performance of various optical methods for the measurement of areal topography of rough EBM-made metal surfaces was investigated. A specially prepared artefact allowing for the generation of different angles was designed and produced from a nickel-based alloy using EBM without any supporting structure for down-facing surfaces. The as-built up-facing and down-facing surfaces from the artefacts were measured in orthogonal to the build direction. Measurement system capability for as-EBM surfaces is presented along with areal surface texture analysis.

This work systematically analyzes and optimizes the process of electron beam melting for pure copper. It is shown that, for reliable manufacturing, the preheating temperature should be optimized to avoid porosity as well as part deformation. The electron beam should be fully focused to prevent shrinkage voids (correlated to negative defocusing) and material spattering (linked to positive defocusing). Smoother surfaces from lower hatch spacing (e.g., 100µm) can improve the density reliability, while longer overhangs are reached by a higher hatch spacing. A suitable starting contour strategy is also applied to mitigate border porosities, reduce side roughness and increase geometric precision.

This work systematically analyzes the influence of rough surfaces and porous subsurfaces in electron beam melting (EBM) printed components. Consequently, it applies various contouring strategies to improve the tensile properties of EBM printed Inconel 625 alloy parts. It is shown that no contouring (i.e., only hatching) creates a rough surface with numerous surface voids (as translated to surface notches). Although the commercially used multi-spot contouring can smoothen the surface to some extent (∼34 %), it fails to create a defect-free superficial region by leaving ∼25 % surface voids (translated to large surface notches) and ∼4 % subsurface porosity. These superficial defects form due to an interrupted shrinkage, occurring on the surface and in the contouring region. In contrast, optimal post-hatching high energy continuous contouring creates a thick and consistent post-hatching track that can successfully reconsolidate surface voids remaining from the hatching step. In comparison with the multi-spot contouring, this reduces the surface and subsurface porosity down to ∼10 % and ∼0.4 %, respectively, and hence increases the apparent stiffness by ∼140 %, tensile strength by ∼105 % and elongation by ∼260 %. This nearly reaches the mechanical properties of the conventionally machined parts (UTS ∼635 ± 20 MPa and elongation ∼50 ± 2 %).

Since electron beam (EB) is the main additive manufacturing (AM) tool in electron beam melting (EBM), EB spot size plays a significant role in the parts quality, surface roughness as well as the microstructure and corresponding properties. So far, the research on measuring EB spot size has been mainly based on printing with/without powder single tracks on a metal plate such as stainless steel. However, this method, due to material thermal properties as well as the melting phenomena, cannot reveal the actual value for the EB spot size. This research is carried out to establish a simple methodology on measuring the EB spot size in a more accurate way at a low cost. To do so, a ceramic surface coating was applied to the surface of a copper starting plate and a stainless steel starting plate respectively. Afterwards, the EB applied the tracks onto the coated starting plate and regular metal starting plate. The analysis showed that the EB tracks on ceramic coated stainless steel plates could be the best replica for the electron beam among those materials tested in this work.