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How can I monitor the quality of resistance spot welds?


Frequently Asked Questions

Welding process monitoring and control involves routine or continuous monitoring of process variables, particularly current, time and force. A wide range of commercial monitors are available and the basic requirement is a current meter and a force measuring device. Control of a constant welding current may be a feature of the weld timer and, once set, current is maintained even under conditions of variable mains voltage or circuit resistance.

In-process monitoring of weld quality relies on measuring a particular factor related to weld growth. Factors which are most suitable are dynamic resistance (the change in resistance of the weld measured between the electrodes) and weld expansion. These characteristics are shown in Fig.1, typical of welds in uncoated low carbon steel. Weld power has also been used as a monitor parameter. Many commercial systems exist which monitor the profile or the magnitude of the monitored parameter. However, these must be proven for a given application to ensure that there is sufficient correlation between monitor indication and actual weld quality, and that the uncertainty is low. Fig.2 illustrates this relationship. Some devices provide feedback control of current or weld time to maintain weld quality and these can work well for certain applications and when properly set up and maintained. Newer systems and development studies are based on more complex algorithms or the application of fuzzy logic to the record data. Used in conjunction with medium frequency inverter welders, the speed of data analysis and control allows significant improvement in the performance of such correction systems allowing weld size to be maintained under a range of process variations, including material type and thickness combinations, without setting up separate welding procedures.

Fig 1. Schematic instrumentation traces for weld resistance and expansion
Fig 1. Schematic instrumentation traces for weld resistance and expansion
Fig 2. Schematic diagram of a typical monitor count / nugget diameter relationship
Fig 2. Schematic diagram of a typical monitor count / nugget diameter relationship

In general, the usage of weld quality monitors is relatively low because of the need to set up and maintain allowable limits with confidence. However, the use of process monitoring either as a timer function or as an add-on unit can provide valuable information about deviations from the set process parameters. This can be used to identify problems with the secondary circuit, warning of a maintenance requirement for example. Monitored information may also be presented in statistical process control (SPC) format, to provide a record of process stability.

Newer spot welding systems are designed to require a minimum setting up procedure and are finding application particularly in prototype vehicle build. In this case, many different material combinations can be welded with a single setting, and the feedback control of weld time and current maintains weld quality. Monitoring is also common for miniature welding applications where the sophisticated power supplies have feedback control of current and voltage, and may also incorporate displacement and force monitoring capability.

Further information

Resistance welding of sheet metals - a guide to best practice


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