Joining Aluminum Alloy and Steel by Friction Stir Forming from Steel Side

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A technique for fabricating structural joints by closed-die-type friction-stir forming (FSF) is introduced in this study. The process is as follows. First, a steel sheet with a prepared hole is placed on an aluminum alloy plate. A die with a through-hole cavity is then placed above it to press down the steel sheet tightly. Next, a rotating stepped cylindrical tool is inserted into the through-hole cavity. To enclose the die cavity, the upper side of the tool is then positioned such that it is nearly in full contact with the inner surface of the through-hole. Finally, the top part of the tool is allowed to penetrate into the aluminum alloy plate through the prepared hole of the steel sheet to cause the material to extrude backward. Consequently, the material fills the whole of the space between the tool and die to generate a hollow-rivet-like aluminum alloy structure fastening the steel sheet to the aluminum alloy plate. This technique enables easier alignment between the die and the prepared hole of the steel as compared with the conventional joining technique which uses FSF. In addition, the new technique uses a one-sided approach (i.e., from the side of the harder material with a higher melting temperature) to join dissimilar materials, a process which is difficult for conventional methods of friction-stir welding and forming.