Interaction between friction stir welding and mild steel backing plates: wear mechanisms and material adhesion

• Process of Friction Stir Welding significantly influences the surface and surface near areas in the steel Backing Plate during the welding of high strength EN AW 6082-T6 • The steel backing plates display deformation of the microstructure near the surface, delamination of near-surface layers, initiation of cracks that grow parallel to the surface and material adhesions from the base material (aluminum alloy) • High heat input leads to material adhesion with the formation of an intermetallic bond between the aluminum alloy and steel backing plate and the refinement of near-surface grains of the steel Friction stir welding (FSW) is a solid-state welding process particularly known for its high-quality weld seams and excellent mechanical properties. Characteristics of FSW are high mechanical process forces, which result in the necessity of clamping components and support structures for the joining zone, and also lead to significant thermomechanical load on the backing plate (BP). Phenomena such as surface fatigue, deformation and diffusion processes can lead to disturbances in the BP and its surface, subsequently affecting the properties of the weld seam (weld root). The mechanisms and influences on the BP remain unknown, without effective remedial measures. In this study, several BPs with a reproducible wear condition are produced by FSW. To cover a wide range of applications, both the sheet thickness of joining parts and tool geometry along with process parameters are varied. The resulting BPs are examined by metallographic analysis. Regardless of the process parameter variations, both adhesion of the base material (Al) to the BP and fatigue-based deformation of the surface-near grains of the steel BP occur. With high heat input (high n/v ratio), the formation of an intermetallic phase is detected. With regard to weld quality, coatings or surface modifications for BPs may need to be used in industrial applications to ensure a durable surface condition at the interface weld seam and BP.