On the influence of in situ sound wave superposition on the microstructure of laser welded 7000 aluminum alloys

The use of ultra-high-strength aluminum alloys plays a key role in future lightweight construction in the field of electromobility. In particular, high-strength 7000 series aluminum alloys are excellent candidates for lightweight structures due to their excellent properties, i.e. their superior specific strength. In terms of production engineering research, however, approaches are lacking allowing for direct tailoring of the local properties of components in a fashion that is established in steel technology, i.e. press-hardening of high-strength steels. This is where the research project ALLEGRO - High-performance components made of aluminum alloys by resource-optimized process technologies comes to the fore. The project is aiming at development of new efficient processes for integrated- shaping and heat treatment of 7000 series aluminum alloys. Realization of a spatially resolved property gradation, which should be maintained upon further processing steps such as joining, i.e. by fusion welding, is a key milestone. Typically, strength of aluminum alloys in the heat-affected zone is significantly reduced in laser beam welding due to the concentrated heat input and related local microstructure evolution (Enz et al., 2014, 2016). One approach for improving the mechanical-technological weld properties in this context is the superposition of mechanically induced sound waves to the laser beam welding process. The sound waves induced by piezoshaker promote various mechanical, metallurgical, fluidic and thermodynamic effects, eventually resulting in pronounced dendrite shearing and enhanced nucleation during solidification and consequently a much finer grain structure in the weld. Furthermore, vibration enhances the solidification speed and, hence, strongly affects the time for the growth of nucleated dendritic grains (Dong et al., 2012; Eskin, 2001). Positive effects on the weld seam have already been demonstrated in other research projects for the aluminum alloys AA-5083 (Woizeschke et al., 2017) and AA-6082 (Radel and Woizeschke, 2018) as well as high-strength steels (Volkers et al., 2017). The main objective of the present study is to investigate the influences of different heat treatment strategies and a sound wave superimposed laser beam welding process on the microstructure variations and mechanical properties of an AA-7075 welded joint.