Friction stir welding (FSW) is a relatively new solid-state joining process. This joining technique is energy efficient, environment friendly, and versatile. In particular, it can be used to join high-strength aerospace aluminum alloys and other metallic alloys that are hard to weld by conventional fusion welding. FSW is considered to be the most significant development in metal joining in a decade. Recently, friction stir processing (FSP) was developed for microstructural modification of metallic materials. In this review article, the current state of understanding and development of the FSW and FSP are addressed. Particular emphasis has been given to: (a) mechanisms responsible for the formation of welds and microstructural refinement, and (b) effects of FSW/FSP parameters on resultant microstructure and final mechanical properties. While the bulk of the information is related to aluminum alloys, important results are now available for other metals and alloys. At this stage, the technology diffusion has significantly outpaced the fundamental understanding of microstructural evolution and microstructure–property relationships.

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Friction Stir Welding and Processing

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For different tool plunge downforces, the optimum FSW conditions of aluminum die casting alloy were examined. The higher the tool plunge downforce is, the wider the range of the optimum FSW conditions is. The following three different types of defects are formed, depending on the FSW conditions. (1) A large mass of flash due to the excess heat input; (2) cavity or groove-like defects caused by insufficient heat input; and (3) cavity caused by the abnormal stirring. As for the abnormal stirring, it is clearly seen that the shape of the top part on the advancing side in the stir zone is completely different. For this type of defect, the effect of the tool plunge downforce is small, though the size of the defect due to insufficient heat input significantly decreases with the increasing downforce.

Three defect types in friction stir welding of aluminum die casting alloy

In order to determine the effect of the carbon content and the transformation on the mechanical properties and microstructures of the FSW carbon steel joints, three types of carbon steels with different carbon contents (IF steel, S12C, S35C) were friction stir welded under various welding conditions. Compared with IF steel, the microstructures and mechanical properties of the carbon steel joints are significantly affected by the welding conditions. The strength of the S12C steel joints increases with the increasing welding speed (decreasing the heat input), while the strength of the S35C steel joints shows a peak near 200 mm/min. This can be explained by the relationship between the peak temperature and the A 1 and A 3 points. When friction stir welding is performed in the ferrite–austenite two-phase region, the microstructure is refined and the highest strength is then achieved.

Kazuhiro Nakata

Papers

Three defect types in friction stir welding of aluminum die casting alloy

Friction stir welding of carbon steels

Friction stir lap joining aluminum and magnesium alloys

Microstructural characterization and mechanical properties in friction stir welding of aluminum and titanium dissimilar alloys

Joining of metal to plastic using friction lap welding