Additive Manufacturing of Ferritic Materials : A Journey from Stainless Steels to High-Entropy Alloys
Abstract: Design of new materials with complex geometries is an important part of new innovative solutions for technical applications. With the use of additive manufacturing (AM), the design possibilities are endless and geometries that are impossible to manufacture by conventional techniques are available. However, the number of alloys commercially available is limited and extensive research is needed to establish new materials with unique properties. An important group of materials is ferritic stainless steels which have a body centered cubic crystal structure. They are often used for their high strength, corrosion resistance or electrical properties at high temperatures. However, they are often less ductile than austenitic stainless steels and issues with cracking may arise during thermal cycling in the L-PBF process. In this thesis, two AM techniques, laser powder bed fusion (L-PBF) and binder jetting were used to produce components of two different ferritic stainless steels and of the AlCoCrFeNi high-entropy alloy (HEA). The main objective was to investigate the microstructural development, phase stabilities and mechanical properties in relation to conventional manufacturing routes. Furthermore, thermodynamic calculations were used to explain the phase stabilities and solidification. L-PBF enables manufacturing of the ferritic stainless steels SS441 and SS446 with excellent mechanical properties. It was shown that solid particles may form in the melt and act as heterogeneous nucleation points, resulting in effective grain refinement for SS441. Other secondary phases can form during the thermal cycling in the L-PBF process, enhancing the mechanical properties. An example is the formation of austenite in SS446. Furthermore, the formation of solid particles and segregated microstructure during solidification was predicted by thermodynamic calculations.The AlCoCrFeNi alloy could be produced with an intriguing hierarchical microstructure and excellent mechanical properties using binder jetting and post-treatments. The microstructure of the final component can also be controlled by pre-annealing of the feedstock powder. Thermodynamic calculations were used to design the phase composition of the alloy. A characteristic single-phase solid solution is only observed at very high temperatures close to the melting point. Hence, the AlCoCrFeNi alloy is not a thermodynamically true HEA, but is stabilized due to kinetic effects during manufacturing.
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