In this study, development of visualization method for material flow phenomena and improvement for mechanical properties of multi-materials which are aluminum/aluminum and aluminum/steel lap joint by friction stir welding were investigated. In the case of aluminum/aluminum overlap joint, the main factor determining mechanical properties was the effective sheet thickness by the deformed surface and interface rather than the softening material. In literature, the effective sheet thickness is defined as the value obtained by subtracting the hook height from the material thickness. But if both sheets were bonded even if the hooking occurs, hooking is good for the peak load since the effective thickness is rather thicker. Thus the crucial factor was the position of the un-bonded line rather than the height of the hook. A method for improving peak load of joint was to create thicker the effective sheet thickness by deforming the un-bonded line toward the hard material.
In the case of aluminum/steel overlap joint, intermetallic compounds such as FexAly are usually formed in the welded zone by resistance spot welding and friction stir welding. Previous work has focused on preventing the formation of intermetallic compounds. However, in this study, we explore an entirely different approach to solve the above problem. It is based on inducing tensile fractures, since neither intermetallic compound formation nor interface characteristics are important factors for determining fracture positions and their strength in this case. Based on relationships between the shear and tensile stress, we proposed some simple formulae for determining the tensile or shear fractures. It was confirmed the match between proposed formulae number with literatures and results of this study. And the joining mechanism of diffusion bonding and mechanical bonding areas were identified.
Additionally, a visualization method for material flow phenomena of FSW was developed and introduced in this study. The results revealed that the threaded probe was significantly correlated to typical onion ring structure and the onion structure formed as soon as it touched the probe. This result is different from the results so far. On the other hand, the remnant of original interface between top and bottom plates after FSLW and asymmetrical flow around rotating tool were significantly correlated to the formation of void defect in low heat input condition.