Decreasing already low expulsion rates
The BMW Group’s MINI plant at Oxford already had a low 3.7% expulsion rate. But the goal of UK government’s WeldZero project is zero weld defects. With funding from WeldZero, MINI Plant Oxford and The Welding Institute systematically worked to bring the plant’s expulsion rate even lower for each of the 6000 resistance spot welds (RSWs) found on every MINI model.
MINI Plant Oxford uses state-of-the-art, high-end robots and welding guns, with integrated/adaptive controls for all spot welds — so it has no issues regarding poor quality, undersized, or low-strength welds. The only remaining issue is weld splash.
In the on-going study, data analysis is used to identify:
- Weld expulsion occurrences
- Their root causes
- Data patterns for each root cause
To inform production engineers of appropriate corrective actions.
Initial corrective measures for weld splash
- Reducing the weld gun air pressure, which was originally and intentionally “over-pressured” in the mistaken belief that overpressure would compensate for the differences in distances from the air supply to the weld gun. Lowering the air pressure, it turns out, better equalized the pressure rate for all weld guns, reducing weld expulsions while reducing air supply energy costs by 25%.
- Monitoring cooling water flow to the weld guns to discover blockages or drops in flow levels. Disruptions to the cooling water flow caused the welding electrodes to overheat and experience excessive electrode wear — leading to weld splash.
- Welding process data analysis of welding voltage, welding current, welding force, and measured resistance, which was then correlated to the welding robots having the highest occurrence of weld expulsions.
Additional weld expulsion factors found by data analysis
Then TWI and BMW performed a data analysis of the remaining weld expulsions to determine their leading factors:
- Panel mismatch: including poor part shape and springback, but also issues such as parts being pushed out of position by other parts.
- Effect of gaps: for higher strength and thicker components — or ones with three or four sheet stack-ups — gaps between panels can destabilize the welding process, leading to weld expulsion.
- Electrode edge distance: panel mismatch can cause the welding electrode to be too near to the edge of a panel, causing the welding zone to “break out” of the panel edge — with heavy expulsion as the result.
- Poor weld spot orientation: caused by panel mismatch or shape defect and resulting in the electrode being other than its ideal 90-degree orientation. This situation leads to weld expulsion, as well as electrode deterioration which, in turn, can cause more expulsions.
- Heavily-worn electrode tips: the amount of zinc alloying on the electrodes influences expulsion rates.
- Poorly-designed water cooling of weld guns: blockages or tight bends in the cooling channels restricted water flow, causing the electrode to overheat and wear out too quickly, leading to weld expulsions.
Determining the importance of each factor in weld splash
TWI then set up a robotic welding cell to simulate the BMW production processes in their own laboratory. They could then identify the process tolerances for each of the above-listed factors to determine the severity of each factor required to cause a weld expulsion.
They also identified weld process data signatures from the welder timers to diagnose the cause of expulsion for each case.
The “smoking gun”: electrode tip condition
The study determined the sensitivity of each expulsion factor was dependent on the electrode’s current condition. Electrode tip wear also influenced the expulsions’ data signatures.