Process Development and Metallurgical Characterization of Wire-based Direct Energy Deposition for Aluminium-Magnesium Alloys

Abstract

Additive manufacturing is a production technology based on the layered addition of material to produce a component, typically using powder or wire raw material. Wire­based additive manufacturing stands out for its potential to realize high material depo­sition rates of several kilograms, rather than a few hundred grams per hour, compared to powder-based processes. The melt to solidification behaviour is important, since it determines the resulting solidification microstructure and chemical composition. Thus, it affects the mechanical properties of the processed component. Besides the physical prop­erties of the processed material, the melt to solidification behaviour is dependent on the process temperature and cooling rates, which can be controlled by targeted adjustment of the process energy input. Due to their excellent physical and mechanical properties, Al-Mg alloys are among the most used light-weight construction materials in the transportation industry. They are used for large-scale components and could be processed economically with wire-based high-throughput laser metal deposition. However, the wire-based laser metal deposition process of Al alloys is poorly understood so far. Complex challenges such as the high reflectivity of Al, pore and crack development, or loss of volatile elements impede its in­dustrial implementation. The aim of this work is to methodically address these challenges in order to enable the pro­cessability of Al-Mg alloys by wire-based laser metal deposition. Special attention is paid to the process energy to solidification microstructure relationship, since the microstruc­ture of non-heat-treatable Al-Mg alloys strongly determines the mechanical properties of the generated structure. For this purpose, theoretical considerations on important thermophysical properties are made and relevant process energies in wire-based laser metal deposition are identified and discussed with regard to their influence on the process temperature. Moreover, fre­quently observed defects during the processing of Al-Mg alloys are identified and discussed. Knowledge gained is used to develop two approaches enabling the processing of defect-free Al-Mg wall-like structures in wire-based laser metal deposition. The first approach is based on an externally generated affection of the process temperature by pre-heating the substrate during deposition. This allows for high deposition veloci­ties and showed positive effects on the fusion characteristics. However, metallographic characterization and microhardness testing revealed the development of grain coarsening along the height of the deposited wall-like structures, which is also accompanied with a decrease of microhardness. The second approach is based on the use of a low deposition velocity to achieve sta­ble melting properties combined with a systematic adjustment of the laser beam irradi­ance, which is identified to an important process energy that strongly affects the process temperature and melt-to-solidification behaviour during deposition. To investigate the process-to-part-property relationships, the deposition of wall-like structures using differ­ent laser beam irradiances is analysed by a thermal monitoring system. The wall-like structures are characterized in order to link the results of thermal analysis to distinct microstructural features, which affect the mechanical properties of the structures. Ho­mogeneous microstructure evolutions along the height of the deposited structures are produced. Moreover, it is found that a targeted control of the laser beam irradiance can significantly affect the process temperature and enables the achievement of distinctly different solidification microstructures and mechanical properties.