Mid Sweden University

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.

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.