Abstract
Friction stir welding of the dissimilar metals steel and aluminum involves slight scraping of the joint surface on the steel side using a tool to be metallurgically joined. However, excessive contact between the tool and workpiece during the welding generates excessive heat, which significantly reduces the joint strength. Therefore, we focused on a joint method, which uses the anchor effect of the protrusions formed by an additive manufacturing method, to establish a new dissimilar metal friction stir welding technique without involving cutting with a tool. In this study, we used a laser additive manufacturing system to form a 1-mm high inverted truncated conical protrusion on the steel side joint surface of maraging steel and investigated the effect of the optimal protrusion shape for maximizing the joint strength. For the root diameter of the protrusion of at least 1.1 mm, the protrusion did not break owing to plastic flow during friction stir welding. Numerical analysis showed that the joint strength increased as the gap between protrusions decreased; however, when the gap was 0.29 mm or less, the deformation behavior under tension changed, and the joint strength decreased as the gap decreased. Numerical analysis also showed that the joint strength increased with increasing tilt angle of the inverted truncated cone, but decreased with increasing tilt angle of ≥70°. Additionally, the joint strength obtained by the experiment was approximately 40% lower than the joint strength obtained by numerical analysis owing to the insufficient plastic flow during friction stir welding, and inability of sufficiently filling the gaps between the protrusions with aluminum, which resulted in defects. However, the correlation between tensile strength and protrusion shape predicted by numerical analysis showed the same trend as the experimental results, suggesting usefulness of the numerical analysis in optimizing the anchor shape.
