Mid Sweden University

Surface roughness of the additively manufactured (AM) parts can reduce the service life, especially under dynamic loadings, due to promoting fatigue by providing initiation sites for fatigue cracks. Therefore, improving the surface condition of AM specimens can be a solution to increase the fatigue strength. The stiffness method is a rapid fatigue test method that allows to obtain reliable fatigue limit values in a cheaper and easier way than traditional methods. Consequently, this work aims to investigate the effect of surface roughness on the fatigue behavior of stainless steel AISI 316L specimens using the stiffness method. These specimens were printed with both laser and electron beam powder bed fusion techniques, and different contour parameters were produced to obtain different surface roughness values. The competitional effect of the internal defects, which become surface defects after machining, and the surface roughness of the as-built specimens, is also addressed by the stiffness method results.

The use of additive manufacturing in metals by powder bed fusion via electron beam (PBF-EB) is increasing for fabricating high-quality parts meeting industrial standards. However, high surface roughness poses a consistent challenge in PBF-EB. This study investigates two novel approaches to optimise surface roughness for a given machine and powder combination. Using machine control software’s recently introduced research mode functionality, we develop customised beam control code to effectively explore a vast parameter space. Additionally, we explored the impact of beam travel direction and spot morphology on surface roughness. Line-melt-based contours were explored by specimen manufacturing with layer-wise parameter change, whilst spot-melting-based samples were built using a full factorial design of experiments with four factors at three levels. Initial sample characterisation was done using a stylus-based contact profilometer, followed by detailed evaluation using focus variation microscopy. Results reveal that increasing beam power and spot energy exacerbate surface roughness. We also find that a well-defined energy distribution at the spot's edge contributes to smoother surfaces. Whilst the influence of beam travel direction on surface roughness remains uncertain, our findings underscore the importance of parameter selection in achieving optimal results. By adjusting contouring parameters, we achieve a vertical roughness of Ra17.7 ± 0.9 (Sa 21.6), significantly lower than in the current literature. These findings advance our understanding of surface roughness optimisation in PBF-EB and offer practical insights for improving part quality in industrial applications. By harnessing tailored beam control strategies, manufacturers can enhance the capabilities of additive manufacturing technologies in producing metal components.

This study investigates the use of Electron Beam Powder Bed Fusion (PBF-EB) spot melting on SDSS 2507, a material known for its high tensile strength, corrosion resistance, and welding properties. Spot melting, a localized melting technique, utilize a stationary beam to create melt pools before rapidly repositioning to form new ones, offering precise control over material properties in additive manufacturing. We conducted a design of experiments with 32 samples and analysed them using SEM, XRD, EBSD, optical microscopy, nanoindentation and statistical modelling. Elevated PBF-EB processing temperatures significantly influence microstructure and phase composition. XRD analysis identified the presence of the detrimental sigma phase. EBSD analysis revealed a composition primarily consisting of austenite (63 %) and sigma phase (33.5 %), with residual ferrite (3.5 %). Statistical modelling demonstrated that a combination of spot-time, spot distance, focus offset, and layer thickness was the most reliable predictor for density while area energy proved to be the most accurate predictor for hardness, with R2adj values of 0.766 and 0.802, respectively. Our study confirms that PBF-EB is capable of processing SDSS 2507 through spot melting, resulting in high-density samples at high productivity rates. The presence of sigma phase shows a need for post build heat treatment to achieve the desired phase distribution. This research enhances the understanding of the process and also holds promise for industrial applications. 

This study focuses on adapting the Electron Beam Powder Bed Fusion (E-PBF) process for the manufacturing of parts from 2507 super duplex stainless steel, combining high strength and corrosion resistance, therefore presenting an interesting choice for E-PBF implementation. Samples manufactured using four sets of process parameters were characterized by scanning electron microscopy, tensile testing, X-ray diffraction analysis, energy dispersive X-ray spectroscopy and nanoindentation. The different E-PBF process temperatures investigated had the strongest influence on the phase characteristics and the resulting microstructure. The processability of the material was good, and high productivity rates were achieved, indicating good suitability for industrial transfer.

Additive manufacturing (AM) is still a relatively new technology. In contrast to traditional machining where material is removed from a blank, AM is used to fuse a feedstock material into complex shapes, layer by layer, starting from an empty workspace. AM enables the manufacture of complex part geometries and part variations with little to no extra manufacturing cost. Manufacturing of geometries which was not previously possible, are now available as design options such as bent internal channels, intricate lattice structures and designed surface porosity - all of which can be produced repeatably. Electron beam powder bed fusion (PBF-EB) is an AM method in which an electron beam is used to process a fine-grained powder into parts. Since its conception, PBF-EB has been hampered by the number of materials available for processing. The aim of this thesis is to explore the possibilities for processing stainless steels using PBF-EB. The work is focused on the development of parameters for efficient processing with the aim of achieving high-density as-built materials and an understanding of the relationship between process parameters and the resulting microstructure and other quality aspects of the parts. Two stainless steel powders, 316LN (austenitic) and super duplex 2507 (austenitic / ferritic), are processed via a wide range of process parameters into solid parts using various melting strategies. Density, microstructural features, and mechanical properties are evaluated and assessed before selecting a set of parameters that produce high-quality parts at a high processing rate. This work concludes that stainless steels are well suited for PBF-EB processing, with a wide processing window. The studies also show that the material properties are highly influenced by the processing parameters used. In the case of super duplex stainless steel 2507 the built parts require post-build heat treatment to achieve the desired microstructure, phase-composition and tensile properties, while 316LN can to a larger extent be used as-built, provided that proper build preparation and processing parameters are used.

Electron beam melting (EBM) is currently hampered by the low number of materials available for processing. This work presents an experimental study of process parameter development related to EBM processing of stainless steel alloy 316LN. Area energy (AE) input and beam deflection rate were varied to produce a wide array of samples in order to determine which combination of process parameters produced dense (>99%) material. Both microstructure and tensile properties were studied. The aim was to determine a process window which results in dense material. The range of AE which produced dense materials was found to be wider for 316LN than for many other reported materials, especially at lower beam deflection rates. Tensile and microstructural analysis showed that increasing the beam deflection rate, and consequently lowering the AE, resulted in material with a smaller grain size, lower ductility, lower yield strength, and a narrower window for producing material that is neither porous nor swelling.

Metal additive manufacturing (AM) is on its way to industrialization. One of the most promising techniques within this field, electron beam melting (EBM), is nowadays used mostly for the fabrication of high‐performance Ti‐based alloy components for the aerospace and medical industry. Among the industrial applications envisioned for the future of EBM, the fabrication of high carbon steels for the tooling industry is of great interest. In this context, the process windows for dense and crack‐free specimens for a highly alloyed (Cr–Mo–V) cold‐work steel powder are presented in this article. High‐solidification rates during EBM processing lead to very fine and homogeneous microstructures. The influence of process parameters on the resulting microstructure and the chemical composition is investigated. In addition, preliminary results show very promising mechanical properties regarding the as‐built and heat‐treated microstructure of the obtained material.

This work focuses on the possibility of processing stainless steel 316LN powder into lightweight structures using electron beam melting and investigates mechanical and microstructural properties in the material of processed components. Lattice structures conforming to ISO13314:2011 were manufactured using varying process parameters. Microstructure was examined using a scanning electron microscope. Compression testing was used to understand the effect of process parameters on the lattice mechanical properties, and nanoindentation was used to determine the material hardness. Lattices manufactured from 316L using EBM show smooth compression characteristics without collapsing layers and shear planes. The material has uniform hardness in strut shear planes, a microstructure resembling that of solid 316LN material but with significantly finer grain size, although slightly coarser sub-grain size. Grains appear to be growing along the lattice struts (e.g., along the heat transfer direction) and not in the build direction. Energy-dispersive x-ray spectroscopy analysis reveals boundary precipitates with increased levels of chromium, molybdenum and silicon. Studies clearly show that the 316LN grains in the material microstructure are elongated along the dominating heat transfer paths, which may or may not coincide with the build direction. Lattices made from a relatively ductile material, like 316LN, are much less susceptible to catastrophic collapse and show an extended range of elastic and plastic deformation. Tests indicate that EBM process for 316LN is stable allowing for both solid and lightweight (lattice) structures.

Additive manufacturing (AM) is a technology that inverts the procedure of traditional machining. Instead of starting with a billet of material and removing unwanted parts, the AM manufacturing process starts with an empty workspace and proceeds to fill this workspace with material where it is desired, often in a layer-by-layer fashion. Materials available for AM processing include polymers, concrete, metals, ceramics, paper, photopolymers, and resins. This thesis is concerned with electron beam melting (EBM), which is a powder bed fusion technology that uses an electron beam to selectively melt a feedstock of fine powder to form geometries based on a computer-aided design file input. There are significant differences between EBM and conventional machining. Apart from the process differences, the ability to manufacture extremely complex parts almost as easily as a square block of material gives engineers the freedom to disregard complexity as a cost-driving factor. The engineering benefits of AM also include manufacturing geometries which were previously almost impossible, such as curved internal channels and complex lattice structures. Lattices are lightweight structures comprising a network of thin beams built up by multiplication of a three-dimensional template cell, or unit cell. By altering the dimensions and type of the unit cell, one can tailor the properties of the lattice to give it the desired behavior. Lattices can be made stiff or elastic, brittle or ductile, and even anisotropic, with different properties in different directions. This thesis focuses on alleviating one of the problems with EBM and AM, namely the relatively few materials available for processing. The method is to take a closer look at the widely used stainless steel 316LN, and investigate the possibility of processing 316LN powder via the EBM process into both lattices and solid material. The results show that 316LN is suitable for EBM processing, and a processing window is presented. The results also show that some additional work is needed to optimize the process parameters for increased tensile strength if the EBM-processed material is to match the yield strength of additively laser-processed 316L material.