Processing–Structure–Properties Relationship in Metal Additive Manufacturing

The last three decades have seen the transition of additive manufacturing, from applications exclusively in rapid-prototyping to an emerging production method in the manufacturing industry that is rapidly gaining more relevance. Within additive manufacturing methods, selective laser melting (SLM) is one of the most widely used and mature technologies and is the focus of this thesis. In particular, this work aims at characterizing novel microstructures and/or alloys produced with SLM and to understand how the process parameters influence the microstructure and properties. Hitherto, the most prevalent material selection approach for SLM has been the use and optimization of well-known alloys, such as steels, Ni- and Ti-based alloys, among others. Favorable microstructures are usually achieved with a combination of appropriate parameters and post-processing techniques. Another approach, especially interesting from a research perspective, is the exploration of materials and microstructures suited for the inherent characteristics of SLM. In alignment with the latter strategy, three types of materials are successfully produced and analyzed in this work: the amorphous Zr-based AMZ4 alloy, 316L stainless steel with strong preferential orientation (i.e., similar orientation of the crystalline structure of the grains) and the intermetallic MnAl(C) with strong preferential orientation. The latter contains a ferromagnetic phase with potential applications as a permanent magnet. SLM was found to be an effective method to produce the amorphous phase in the Zr59.3Cu28.8Al10.4Nb1.5 system (AMZ4). The laser power and oxygen impurities were found to have a central role in the formation of crystalline particles in the amorphous matrix. These crystalline particles and the oxygen impurities reduced the thermal stability of the alloy in comparison to specimens fabricated by suction casting. For the more conventional 316L stainless steel, it was demonstrated that the scan strategy can be used to influence the type of texture, with a notable effect on the mechanical properties. In the case of MnAl(C), it was established that the high temperature polymorph – ε-phase, can be retained during the printing process. This phase can be subsequently transformed to the ferromagnetic τ-phase with annealing procedures. It was observed that a strong preferred orientation of the ε-phase can be achieved, although it did not translate into a strong texture in the τ-phase (after the heat treatments). The research methodology used in this thesis and the findings regarding the processing–structure–properties relationship in SLM provide an important reference for future studies of novel materials and microstructures produced by additive manufacturing