Microstructure engineering of additively manufactured materials by powder bed fusion-laser beam: Cases for ferritic stainless steels and medium entropy alloys

Abstract: Powder bed fusion laser beam (PBF-LB) is a powder bed fusion additive manufacturing process that is one of the most established and widely used techniques for manufacturing near net shape components. The unique characteristics of PBF-LB, such as the high cooling rates and local directional heat transfer, lead to the formation of unique microstructures. These microstructures can be tailored to achieve desired properties by controlling the process parameters. This thesis studies the influence of PBF-LB processing conditions on the microstructure and resulting properties of two different types of alloys: high alloyed ferritic stainless steels and medium entropy alloys. Due to the rapid melting and solidification, the resulting microstructure typically consists of columnar grains due to the conditions favourable for the epitaxial growth. Ways to address this include, by either manipulating the process parameters to limit the favourable conditions for epitaxial growth, or by utilizing the alloy design approach to promote the columnar to equiaxed transition. The first part of the thesis focuses on ferritic stainless steels. The influence of inoculation on heterogeneous nucleation and as-printed microstructure is studied in ferritic stainless steels with and without inoculants. The results show that inoculation can promote equiaxed grain growth and significantly reduce the epitaxial growth by altering the solidification conditions. The second part of the thesis focuses on medium entropy alloys. The influence of interstitial solid solution strengthening on the mechanical properties of CoCrNi-N medium entropy alloys in as-printed and heat-treated conditions is studied. The results show that interstitial solid solution strengthening can improve the mechanical properties of the alloys and also provide significant microstructural stability by delaying the onset of recrystallization and grain growth as compared to the nitrogen free CoCrNi. In addition, the influence of processing conditions, specifically the high cooling rates on the as-printed microstructure, mechanical properties, and TRIP behaviour of a Co45Cr25(FeNi)30 metastable medium entropy alloy is investigated. The results show that processing conditions can significantly affect the microstructure and phase stability which in turn influence the resulting deformation behaviour of the alloy. The results of this thesis demonstrate that PBF-LB is a versatile process that can be used to produce high-performance materials with tailored microstructure and properties. The findings of this research will be valuable to researchers and engineers who are interested in developing novel alloys and materials by and for additive manufacturing.