Electron beam powder bed fusion processing of stainless steels

Sammanfattning: 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.