Based on actual production, this article summarizes the common problems, causes and improvement methods in the stir friction welding process for your reference.

1-Surface defects

(1) Surface grooves

l Problem description: Surface grooves, also known as plowing defects, usually appear on the upper surface of the weld, tending towards the advancing edge of the weld, and are groove-shaped.

l Cause: The thermoplastic metal flow around the weld is insufficient, and the plastic metal of the weld cannot fully fill the instantaneous cavity left by the stirring needle during its movement.

l Improvement method: Increase the shoulder diameter, increase the pressure, and reduce the welding speed.

Figure 1: Grooves

（2）Edge burrs

l Problem description: Flash burrs appear on the outer edge of the weld and are wavy in shape.

l Cause: Improper matching of rotation speed and welding speed, excessive downward pressure.

l Improvement method: Optimize welding parameters and reduce the amount of pressure.

Figure 2: Edge burrs

（3）Surface peeling

l Problem description: Surface peeling or threading appears on the surface of the weld in the form of skin or thread.

l Cause: A large amount of heat generated by metal friction accumulates in the surface metal of the weld, causing the local metal on the surface to reach a molten state.

l Improvement method: Optimize welding parameters, reduce rotation speed, and increase welding speed.

Figure 3: Surface peeling

(4) Back weld nodule

l Problem description: Weld bead forms on the back side of the weld.

l Cause: Improper control of the depth and pressure of the stirring head during welding.

l Improvement method: Optimize the depth and pressure settings of the mixing head.

2-Internal defects

(1) Incomplete penetration

l Problem description: A "crack-like" defect occurs when the weld bottom is not connected or is not completely connected.

l Cause: The stirring needle is not long enough, resulting in insufficient stirring of the material at the bottom of the weld.

l Improvement method: Select the appropriate stirring needle length to ensure that the material on the weld thickness can be fully stirred.

Figure 4: Incomplete penetration

(2) Holes

l Problem description: A "crack-like" defect occurs when the weld bottom is not connected or is not completely connected.

l Cause: During the welding process, due to insufficient friction heat input, insufficient material reaches the plasticized state, resulting in insufficient material flow and incomplete closure of the weld area.

l Improvement methods: Optimize welding parameters, improve stirring needle design, and control welding speed and rotation speed.

Figure 5: Holes

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