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Spotweld Testing in Car Body Manufacturing

Car body components are joint by electro resistance spot welding and have to be inspected in the manufacturing with ultrasound.

During the welding process, copper electrodes compress two or more plates applying a high electrical current. The electrical resistance generates a liquid phase, which after the cooling forms a lens.

Lens of a spotweld – Metallographic cut

The following defect types can occur:
  • No lens (lack of fusion)
  • Lens diameter too small
  • Stick joint, if only the zinc coating has been melted
The testing is done in pulse-echo-technique using broadband 20 MHz ultrasonic probes. A rubber membrane and a water delay in the probe housing enable a flexible sound coupling over the compressed spotweld surfaces.

Spotweld probe Probe for spotweld testing

Different plate thicknesses require different lens diameters. For this purpose a probe with an oscillator diameter DS, which corresponds to the nominal lens diameter DL, is applied.

After manually positioning the probe, the instrument UT Mate of the company ScanMaster automatically evaluates the spotweld quality.

ScanMaster Spotweld testing in the car body manufacturing

A good spotweld lens with DL ≥ DS generates an echo sequence from the backwall of the last plate.

A-scan of a good spotweld
(Click image to enlarge)

In the example above a spotweld of 2 plates with the wall thickness 1 mm was tested. The nominal lens diameter is 4 mm.

The echo sequence contains clear backwall echoes. The measured sound attenuation 2.9 dB/mm is an indication for the coarse-grained structure in the lens. During the liquid phase in the melting process the spot is compressed, which leads to a determined residual thickness of 1.87 mm.

Based on these criteria the spot is “Good”.

If the lens diameter is too small, the A-Scan presents intermediate echoes of the backwall of the first plate.

A long undamped echo sequence indicates that no coarse grained lens exists and that only the zinc coating for corrosion protection has been melted. The resulting stick joint is fine-grained and does not yield a significant sound attenuation.

Summarized, the dedicated software automatically evaluates the spotweld quality as follows:

Signal Result
Short damped echo sequence of the last plate and sufficient compression depth Spotweld OK
Intermediate echoes of the first plate Lens too small: NOK
Long undamped echo sequence of the last plate Stick joint: NOK
Only backwall echo sequence of the first plate No joint: NOK

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