Common surface defects and countermeasures in friction stir welding Surface grooves Surface grooves, also known as plough groove defects, often appear on the upper surface of the weld seam, and tend to be grooved toward the advancing edge of the weld seam. The reason is that the thermoplastic metal around the welding seam is insufficient, and the plastic metal of the welding seam cannot fully fill the instantaneous cavity left during the course of the stirring pin, thereby forming a surface groove near the advancing edge of the welding seam. Control measures: increase the diameter of the shaft shoulder, increase the pressure, and reduce the welding speed.
Flash burr: Flash burr appears on the outer edge of the weld, which is wavy. 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 amount of depression is too large, and a large amount of flash will be formed.
Control measures: optimize welding parameters and reduce the amount of depression.
Restriction line finishing: surface peeling or threading appears on the surface of the weld seam as skin or thread. This defect is caused by a large amount of metal friction heat, accumulated in the surface metal of the weld, so that the local metal in the surface layer reaches the molten state, and gradually cools down in the form of skin or filament during welding.
Control measures: optimize welding parameters, reduce the rotation speed, and increase the welding speed.
Surface drum head: The surface drum head usually appears after FSW post-weld heat treatment, and the impurity drum located within 0.3mm of the weld surface. Welding of the weld seam is due to the inclusion of an oxide film on the surface of the weld seam, which is caused by the decomposition and expansion of the impurities due to the increase in temperature during the heat treatment.
Control measures: Clean oxide film or oil stain before welding.
Back weld: The back weld is characterized by a metal projecting from the weld. 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 protrudes, showing a weld-like shape.
Control measures: Ensure that the material to be welded fits the tooling well, ensure that the gap is as small as possible, and slightly reduce the length of the stirring pin.
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