Significance of tool offset and copper interlayer during friction stir welding of aluminum to titanium

Abstract

This investigation highlights the influence of tool offset on the microstructural evolution, phase formation, and hardness distribution during friction stir welding (FSW) of commercially pure aluminum (Al) to commercially pure titanium (Ti) with a copper (Cu) interlayer (200-μm thick). It was observed that tool offset position controls the mechanical mixing of materials in the weld nugget. The mechanical mixing also depends on the deformation, fragmentation, and distribution of each material in the weld nugget. The fragmentation of materials leads to the development of comparatively fine particles with variation in size and morphology. Insufficient mixing at higher tool offsets promotes the formation of root defects and produces inferior welds. On the other hand, when the tool offset is less than the optimum value, severe deformation and mechanical mixing lead to the formation of wormhole defects and evolution of intermetallic compounds in the weld. The spatial distribution of particles and intermetallics in the weld nugget leads to a large scatter in hardness values. Since mechanical mixing affects the morphology, phase evolution, and mechanical properties of the weld, tool offset is considered to be a very important parameter to be optimized for monitoring mechanical mixing and further development of the dissimilar weld with interlayer material.