Mid Sweden University

Multi-material additive manufacturing offers new opportunities for tailoring local material performance, but reliable design principles for joining dissimilar materials with large thermophysical mismatch remain limited. Binder-jetted WC-Co substrates were combined with electron beam powder bed fusion deposited 316L stainless steel to investigate structure − property relationships governing interfacial integrity. Microstructural characterization revealed bidirectional elemental diffusion (Fe, Cr, Ni, Co, and W) across the interface and formation of a chemically graded transition region with a thickness of approximately 144–204 µm depending on processing parameters. X-ray diffraction confirmed retention of the primary phases together with minor secondary phase formation, while nanoindentation mapping demonstrated a continuous gradient in hardness and elastic modulus across the interface rather than a mechanically weak layer. Despite the formation of a mechanically continuous interface, cracking occurred within the WC-Co substrate, which is consistent with thermally induced stresses arising from the mismatch in coefficients of thermal expansion between WC-Co and 316L. These results suggest that interfacial reactions can enhance interfacial integrity even in systems with large property mismatch, shifting failure away from the interface and into the weaker constituent. The findings provide design insight for engineering robust interfaces in multi-material additive manufacturing systems combining ductile alloys with brittle composites.

This work investigates microstructural and mechanical transitions in multi-material assemblies of American iron and steel institute 316L stainless steel and V4E tool steel, fabricated using a dual-hopper layer alternation strategy via electron beam powder bed fusion (PBF-EB). Two types of layered transitions are produced: alternating transition and alternating incremental transition (AIT). The assemblies are characterized using electron backscattered diffraction and high-speed nanoindentation (HSN), with mechanical phases extracted via Gaussian mixture modeling applied to nanoindentation maps. In the alternating layer sample, localized mechanical features are identified within the elongated austenite grains of 316L, and V4E is observed growing along grain boundaries at the interphase. In the AIT sample, similar interfacial features are observed, but the gradual addition of V4E layers generates a continuous mechanical gradient. Nanoindentation maps reveal increasing hardness and elastic modulus along the transition, ultimately reaching homogenized mechanical behavior in the V4E region. The study demonstrates the feasibility of producing compositionally graded steel assemblies with robust interfaces using PBF-EB. It also emphasizes the value of HSN combined with statistical clustering to resolve micromechanical heterogeneities, enabling more precise design and optimization of advanced multi-material systems.

In this study, AISI 316 L stainless steel sample was fabricated by electron beam powder bed fusion (PBF-EB) additive manufacturing, achieving a build height of 262 mm. The microstructural characteristics and mechanical properties of the tall sample were investigated. Electron backscatter diffraction (EBSD) reveals predominantly columnar crystal structures with significant texture formation along the build direction. The width of the grains reaches up to 165 μm and the height up to 213 μm. Nanoindentation experiments demonstrate anisotropic mechanical properties, with average hardness and elastic modulus varying between 3.41 and 5.07 GPa and 182–232 GPa, respectively. Grain orientation analysis shows that 〈111〉 and 〈101〉 crystallographic directions exhibit higher hardness and elastic modulus compared to 〈001〉 orientation. The research elucidates the complex relationships between microstructural features, crystallographic orientation, and mechanical performance in PBF-EB processed stainless steel, providing insights into material design and process optimization. 

This study introduces an innovative approach for optimizing process development in electron beam powder bed fusion (PBF-EB) of non-flowing chemically reduced tungsten powder. Both line- and spot melting strategies were employed, with gradient-based variations of key processing parameters—beam current and scanning speed (line melting)/dwell time (spot melting)—applied across the XZ and XY planes on prismatic specimens. This method allowed mapping the transition from porous to swelling material within a single specimen and exposed the effects of changing gradient directions. Scripts were developed to analyze swelling and porosity from stacked backscattered electron data, providing valuable insights into material density and defect distribution. Optimal parameters for line melting (1400 W, 115 mm/s) and spot melting (1400 W, 4.5 ms dwell time) were identified, resulting in high-density samples. Solid samples were achieved with Archimedes densities of 99.8% and 99.9% respectively. Microscopical analysis verified parameter windows with dense, swelling-free material, selected for further builds and detailed characterization. Microstructural and compositional analysis was conducted using SEM and EBSD, while local micromechanical properties were assessed through micro hardness. Scaling up line melting was deemed infeasible due to warping, while spot melting was scaled to a melting area of 50 mm × 50 mm. 

The ability to control process parameters over time and build space in electron beam powder bed fusion (PBF-EB) opens up unprecedented opportunities to tailor the process and use materials of a different nature in the same build. The present investigation explored the various methods used to adapt the PBF-EB process for the production of functionally graded materials (FGMs). In this way, two pre-alloyed powders—a stainless steel (SS) powder and a highly alloyed cold work tool steel (TS) powder—were combined during processing in an S20 Arcam machine. Feasibility experiments were first carried out in a downscaled build setup, in which a single powder container was installed on top of the rake system. In the container, one powder was placed on top of the other (SS/TS) so that the gradient materials were produced as the powders were spread and intermixed during the build. The process was later scaled up to an industrial machine setup, where a similar approach was implemented using two configurations of powder disposal: SS/SS + TS/TS and TS/TS + SS/SS. Each configuration had an intermediate layer of powder blend. The FGMs obtained were characterized in terms of their microstructure and local and macromechanical properties. For the microstructural analysis, optical microscopy, scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDX) were performed on the polished cross-sections. This provided evidence of gradual microstructural and compositional transitions in the samples, with a shift from SS to TS and vice versa. Nanoindentation experiments confirmed that there was a consequent gradient in the hardness, stiffness, and wear ratio from the softer and ductile SS to the harder and stiff TS. Scratch experiments revealed gradual evolution in the sliding wear behavior of the printed materials. A “progressive spring” and a “hardness-tailored punching tool” were fabricated as demonstrators. The results obtained demonstrate the great potential to gradually tailor the composition, microstructure, mechanical properties, and wear resistance by combining different powders, and they suggest that any PBF-EB system can be repurposed to build gradient materials without hardware modification. Potential applications include the tooling industry, where hard and wear-resistant materials are needed for the surfaces of tools, with tougher and more ductile materials used in the cores of tools.

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.

Electron beam powder bed fusion (PBF-EB) is an additivemanufacturing (AM) method based on layer-by-layer melting of apowder bed. The technology is industrialized in certain applicationsbut still considered as immature and is not as widely used as laserbeam-based systems (PBF-LB). PBF-EB can offer several benefits overPBF-LB such as process cleanliness, thermal efficiency, fast beam speed,higher power and energy transfer, low residual stresses in built partsand a good signal environment for process monitoring. This can beadded on top of the general benefits of AM such as geometricalfreedom, manufacturing efficiency, easy design revisions, short leadtimes and so on. This suggests that PBF-EB holds potential as atechnology for the sustainable production of materials andcomponents. This thesis investigates how PBF-EB can be furtherdeveloped to create new and unique materials features. This isachieved by introducing innovative methods for material processingand by further developing the PBF-EB process itself. The thesisintroduces a charge-free heating method for PBF-EB and the resultssuggest an enhanced processability of difficult-to-process materialsand powders. A method for building multi-materials in PBF-EB isintroduced and demonstrated by the manufacturing of direct andlamellar transitions between different alloys. Methods for processmonitoring and powder bed resistivity evaluation are proposed andxiidemonstrated. It is concluded that the results presented in this thesisenabled new PBF-EB processing modes, increased the knowledge ofthe process, and introduced a new material group by demonstratingthat ceramics can be processed at high temperatures (~1600C).

Electron beam powder bed fusion (PBF-EB) is a known metal additive manufacturing (AM) technology. Processing non-conducting powders such as ceramics has so far been considered as not feasible because of the inherent problems with Coulomb repulsion due to insufficient electrical conductivity. In this study, a method for functionalizing ceramic powder is proposed where particles are electroless coated by a ~ 1 µm Ni layer to decrease the surface resistivity. The feasibility of the suggested approach is tested on Al2O3 powder, and the results show that the coated ceramic powder has a decreased surface resistivity, which enables processing by PBF-EB. Heating and melting parameters were investigated and samples were manufactured at ~ 1600 °C. Sintered and melted powders were analyzed by microscopy and micromechanically tested by nanoindentation. Calculations, visual observation and SEM–EDX suggest that the Ni coating is evaporated during the process, which suggests that the process could be feasible for the manufacturing of pure ceramic parts.

Metal additive manufacturing technologies, such as electron beam powder bed fusion (PBF-EB), rely on layer heating to overcome the so-called “smoke” phenomenon. When scaled up for industrial manufacturing, PBF-EB becomes less productive due to the lengthy preheating process. Currently, only the electron beam (EB) is used for preheating in PBF-EB, resulting in increased manufacturing times, energy consumption, and in some cases limiting the applicability of the technology. In this study, a new preheating approach is suggested that incorporates a near-infrared radiation (NIR) emitter inside an PBF-EB system. The NIR unit eliminates the need for EB heating, reducing build time and powder charging. Successful builds using 316 L and Ti6Al4V precursor powders validate the feasibility of the proposed approach. The produced samples exhibit similar properties to those obtained by the standard PBF-EB process. The introduction of NIR technology also reduced build cost and increased the service intervals of the electron gun. 

The use of an electron beam (EB) as a heating source in EB-based powder bed fusion (PBF-EB) has several limitations, such as reduced powder recyclability, short machine service intervals, difficulties with heating large areas and the limited processability of charge-sensitive powders. Near-infrared (NIR) heating was recently introduced as a feasible replacement and/or complement to EB heating in PBF-EB. This work further investigates the feasibility of using NIR to eliminate the need for a build platform as well as to enable easier repairing of parts in PBF-EB. NIR-assisted Ti-6Al-4V builds were successfully carried out by starting from a loose powder bed without using a build platform. The results do not only confirm that it is possible to eliminate the build platform by the aid of NIR, but also that it can be beneficial for the process cleanliness and improve the surface quality of built parts. Furthermore, a 430 stainless-steel (SS) component could be repaired by positioning it in a loose 316L SS powder bed using a fully NIR-heated PBF-EB process.