On the Impact of Non-Optimal Parameters in Additive Manufacturing - From Microstructure to Mechanical Properties

The additive manufacturing with its layerwise built-up comprises a variety of tempting properties such as an unprecedented freedom of design and tailorable mechanical properties. Amongst others, the laser-based powder bed fusion of metals (PBF-LB/M) as one representative additive manufacturing process permits the production of highly complex parts requiring a minimum of post-processing effort. As the additive manufacturing techniques are still rather new, there are still several open questions concerning the process-microstructure-properties relationship. In the present work, the influence of various process divergences representing a non-ideal process condition on the microstructure and mechanical properties of iron- and aluminum-based alloys were investigated. Due to the large number of different process divergences, this work focused exemplarily on three non-ideal processes, i.e., process interruptions, the use of non-spherical powders, and a varying process gas flow. Using high-resolution electron microscopy analysis methods as well as X-ray tomography, deep insights into the microstructure and defect-formation mechanisms were obtained. The influence of microstructure and defects on the damage evolution under mechanical loading is analyzed and a model idea for a robust production is derived and discussed with existing concepts. By the application of different process divergences on the aluminum- and iron-based alloys, a varying sensitivity was found between these material grades. It was originally thought, that the higher ductility of the iron-based alloys will lead to a higher defect tolerance and, thereby, a higher robustness against a non-ideal process condition. However, the results in the present ork show that the thermo-physical properties of an alloy may have a higher influence on a robust mechanical response under monotonic and cyclic loading than the existance of a high ductility and, thus, a high defect tolerance. The results of the process divergences for the aluminum-based feedstock show that the microstructure and, thereby, the mechanical properties are highly unaffected which is thought to be a result of the high thermal conductivity in aluminum. In contrast, the iron-based alloys seem to be highly vulnerable for a change within the process. However, based on the findings, several influences could be found being crucial for a robust production in PBF-LB/M. Next to the thermal conductivity, the alloying concept with its thermal and metallurgical properties such as oxygen affinity and the evolution of intermetallic phases should be considered. This may help to identify newly suitable alloys for the application in PBF-LB/M process.