Effect of tool material on joint quality in friction stir welding of aluminum-steel tailor welded blanks

Friction stir welding (FSW) enables high-strength joints between dissimilar materials such as aluminum and steel and is particularly suited for hybrid tailor welded blanks in deep drawing. A special joining configuration, developed at the Material Testing Institute in Stuttgart, allows joining a high-strength aluminum alloy to thinner steel in a combined butt and overlap joint. Despite its proven advantages, this welding process shows inconsistent outcomes regarding formability and joint strength. This research examines the influence of tool material on weld quality and process robustness. H13 steel, ceramics (Si 3 N 4 and SiAlON) and TiAlN-coated tungsten carbide tools were evaluated in welds joining 1 mm steel to 2 mm aluminum sheets. Material accumulation was quantified by 3D scanning and continuous weighing. Weld seam integrity was assessed by X-ray imaging, metallography and tensile testing with regard to aluminum-steel intermixing. The findings show that during friction stir welding of aluminum and steel in a combined butt and overlap joint, both steel and ceramic tools predominantly degrade through the adhesion of workpiece material. The H13 tool exhibits steel/aluminum buildup detaching after 1.5-2.5 m, altering geometry and intermixing. Fluctuations in material accretion lead to varying process conditions over successive welds or even within one weld. By contrast, the TiAlN-coated WC tool exhibits significantly less buildup and therefore more uniform weld seam quality. Moreover, a relatively high degree of aluminum-steel intermixing consistently correlates with superior weld strength and formability. The study highlights how tool degradation and intermixing affect weld quality and emphasizes the role of tool materials for robust industrial applications. • During friction stir welding of aluminum and steel in a combined butt and overlap joint, degradation in the form of adhering workpiece material predominates for both steel and ceramic tools. • Material adhering to the tool around the pin alters its geometry, leading to an increased incorporation of steel particles into the aluminum matrix. • The amount of material buildup, particularly for the steel and ‘as fired’ ceramic tools, as well as on the ground ceramic tools, is subject to fluctuations, leading to varying process conditions throughout successive welds. • The TiAlN-coated WC tool demonstrates significantly less wear and produces the most uniform weld seam quality, compared to the other tool variants examined. • Irrespective of the tool used, a relatively high degree of aluminum-steel intermixing indicates particularly good weld seam strength and formability. Welds with very low intermixing withstand lower forces and strains, failing at the aluminum-steel interface during tensile testing.