Figure 10, shows the cross section of a typical ‘good’ spot weld. Due to the local heat and pressure applied, a small indentation is left in the sheet surface each side of the weld where the electrodes contact.

Figure 10,  Cross section of a 'good' spot weld
Figure 10, Cross section of a 'good' spot weld

Spot welding is useful as it can produce high quality welds at a relatively low cost. However, inspection and quality assurance for this joining method are more problematic than is the case for other welding techniques. For years the only way to tell if a spot weld was good or not was by destructive testing. There are a number of difficulties associated with the ultrasonic testing of spot welds:

  • The need to resolve echoes from thin plates
  • The fact that cold or ‘stuck’ welds may transmit ultrasound
  • The bad coupling caused by the cup shaped depression at the weld.

In repair body shops destructive testing is clearly out of the question. Furthermore, the body shop does not have the experience of the original car manufacturing company.

Body shops are also likely to see a different type of car every day and therefore have to make spot welds to order. Also, with the advent of new lightweight materials such as high and ultra-high strength steel, which are rapidly increasing in their usage, experience with welding parameters is reduced further still. However, there is escalating concern about how to deliver good quality repairs that return vehicles to their pre-accident condition.

There are four variables to consider with spot welding; pressure, weld time, current and electrode tip diameter. If any of these settings are incorrect, the spot weld will be defective.

There are six possible basic outcomes:

1. Good spot weld

4. Stick weld (or cold weld)

2. Burnt (or oversized) weld

5. Loose weld (no fusion has occurred).

3. Undersized nugget

6. Flaws (porosity)

Figure 11, shows an example of a few defective spot welds.

Figure 11, Some cross sections of defective spot welds
Figure 11, Some cross sections of defective spot welds:

  1. Undersized;
  2. Pore;
  3. Stuck (plates separated during sectioning);
  4. Oversized with cracks and small pores.

In ultrasonic testing, each of the above outcomes results in a distinctive A-scan. The A-scan is a display of the received pulse amplitude along the y-axis and the time of flight of the pulse along the x-axis. For a good spot weld, the amplitudes of the echoes drop relatively quickly because the weld structure is coarse grained and therefore has a high sound attenuation characteristic. The echo intervals correspond to the total thickness of the welded sheets.

The Spot Track consortium are investigating the various parameters of the echo sequence, such as its extent, the signal attenuation, the amplitude and position of intermediate echoes to allow good and defective spot welds to be differentiated. The Spot Track device will give an instant pass or fail indication without the need for expert interpretation.