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　　The common surface defects of friction stir welding include surface grooves, flash, surface peeling, bottom welding bumps, etc. This article will explain the causes and control measures of various surface defects in detail.

1. Surface groove

　　The surface groove is also called the plough groove defect. It often appears on the upper surface of the weld, and tends to be groove-shaped on the advancing edge of the weld. The reason is that due to insufficient flow of the thermoplastic metal around the weld, the plastic metal of the weld cannot fully fill the instantaneous cavity left by the stirring needle during the traveling process, thus forming a surface groove near the advancing edge of the weld.

　　Control measures: increase the diameter of the shaft shoulder, increase the pressure, and reduce the welding speed.

2. Flash burrs

　　Flash burrs appear on the outer edge of the weld and are wavy, and the flash on the return side is often larger than the forward side. This defect is due to the improper matching of the rotation speed and the welding speed. During the welding process, the downward pressure is too large, and a large amount of flash will be formed.

　　Control measures: Optimize welding parameters and reduce the amount of pressure.

3. Limit line finishing

　　The surface peeling or spilt appears on the surface of the weld in a skin-like or silk-like shape. This defect is caused by a large amount of metal frictional heat, which accumulates in the surface metal of the weld, so that the local metal of the surface reaches a molten state, and gradually cools down during the welding process to form a skin or wire distribution on the surface of the weld.

　　Control measures: optimize welding parameters, reduce speed, and increase welding speed.

4. Surface drum skin

　　Surface bulge usually appears after FSW post-weld heat treatment, and impurity bulges within 0.3mm of the weld surface. The weld bulge is caused by the decomposition and expansion of impurities due to the increase in temperature during the heat treatment process due to the inclusion of oxide film on the surface of the weld.

　　Control measures: clean the oxide film or oil stains before welding.

5. Welding on the back

　　Welding on the back is manifested by the protruding metal of the weld seam. The reason is that the gap between the top of the stirring pin and the bottom of the weld is too small, or there is a large gap at the bottom of the weld when the product is assembled. The bottom of the seam is protruding, showing a weld bead shape.

　　Control measures: ensure that the welded material fits well with the tooling, ensure that the gap is as small as possible, and slightly reduce the length of the stirring needle.