Effect of Surface Conditions and Tool to Interface Spacing in Refill Friction Stir Spot Welding of Aluminum to Titanium

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Abstract
Optimizing the performance and reliability of welding techniques for dissimilar aluminum (Al) to titanium (Ti) is a promising way to establish new applications in aerospace industry. Due to structural weight reduction, lightweight materials can help to minimize fuel consumption and save emissions. Solid-state welding technologies allow short joining cycles and metallurgical changes, residual stresses and severe intermetallic compound formation can be reduced by limited thermal exposure. Besides temperature and plastic deformation, intimate contact plays an important role for diffusion. In this work, AlMgSi alloys with systematic variations of Mg and Si alloying elements, were welded to Ti6Al4V (Ti64) by refill Friction Stir Spot Welding. The focus lays on the effect of Ti64 sheet surface roughness, varied by different surface preparations. Additionally, the influence of the plunge depth, the distance between the tool and the Ti64 sheet surface is analyzed. It was found that a reduced tool to interface spacing has a beneficial influence on joint integrity. Grinding trenches allowed better bonding compared to the pit-like surface structure generated by sandblasting, which led to an increase in mechanical lap-shear properties. Knurling the grinded surfaces resulted in high standard deviation, as most likely not the whole interface area was bonded. However, the partially outstanding properties showed that a beneficial effect can be expected due to mechanical interlocking mechanisms, when sufficient diffusion is ensured.

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