Common FSW Welding Defects in Cold Plates and How to Control Them

FSW Is Not Simply "Stir and Weld"

Friction Stir Welding seems intuitive: a high-speed rotating probe inserts into the butt joint of two plates, friction heat plasticizes the material, and the probe advances to form a weld. But anyone doing volume production knows FSW has an extremely narrow process window.

Three Common Defects

1. Kissing Bond

The most insidious defect — visually undetectable, but the weld interior has incompletely bonded interfaces. Caused by insufficient probe pressure or RPM, material not fully plasticized. Helium testing may pass, but fatigue life is significantly reduced.

2. Lazy S

The "S"-shaped pattern visible on the weld cross-section, caused by insufficient material flow at the probe tip end. Minor Lazy S typically does not affect strength, but severe cases reduce effective weld thickness.

3. Thermal Deformation

Although FSW is solid-phase welding with lower heat input than fusion welding, residual stresses still occur. Cold plate channel wall thickness is only 0.3-1mm — post-weld flatness deviation of 0.1mm can cause seal failure.

Bisite Control Methods

• Process parameter window: Each product gets a 3D window of RPM-feed-pressure, no trial-and-error tuning
• Real-time monitoring: Spindle torque and Z-axis pressure collected in real-time, auto-alarm on window deviation
• Post-weld straightening: Products with critical flatness ±0.05mm get a mechanical straightening operation
• 100% helium leak test: 2MPa helium pressure, leak rate <= 1x10⁻⁹ Pa·m³/s

Good FSW makes good cold plates. This is not a theory problem — it is a process discipline problem.