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Resistance Spot Weld Monitoring Ensures Quality and Provides Traceability

WM-100A weld monitoring system, spot weld monitoring

Product failure. Upset customers. Disappointed leadership and stakeholders. Product on stop shipment.

For the process manufacturing engineer, it’s a worst-case scenario. When this situation occurs, it requires swift attention and accurate resolution: do you know the fundamental underlying issue? Can you calmly and expertly identify the source of the problem and what to do to get back on track?

This is where process monitoring comes in. By observing the process, it is possible to discern good from bad product, and when bad occurs, specific defect signatures. In fact, process monitoring can help manufacturers avoid this situation altogether.

In this blog, we will specifically consider monitoring of resistance spot and seams welds.

Resistance welding is a process that uses current to heat metals and create a fusion bond.  The key parameters that define successful resistance welding are material composition, equipment performance, and process conditions. These are typically optimized on weld quality (often strength) and product throughput.

However, there are a number of production issues that may arise suddenly or build up slowly over time. Here are a few examples:

  • Operator places wrong part or right part in wrong orientation (sudden change)
  • A different grade or alloy of material is used (sudden change)
  • Electrodes wear (change over time)
  • Force changes (change over time)

All of these changes have the potential to introduce defects into products, limit throughput, or threaten the safety of equipment and personnel if a botched weld results in material expulsion.

The good news is that these process defects can be monitored and recorded for quality control. The data can identify the issue causing the defective welds, which will reduce downtime – or, even better, change the downtime type from unplanned to scheduled.

The spot weld monitoring data can also be used to develop better manual or automated workstations that can avoid weld inconsistencies. Plus, data collected with monitors can provide value after a product is sold in case of a recall or similar situation, as weld data can be correlated with serial numbers.

So, what does a resistance spot weld monitor actually monitor? Every resistance weld has to get three things right: current, force and time. If current is applied at the wrong time or with the wrong force, or if the wrong force is applied or it is applied for the wrong amount of time after the weld pulse, the joint will not be as strong as possible. So, a resistance spot welding monitor needs to account for both the electrical and mechanical process parameters – though in some cases simple checkers can be used for current only.

In addition to monitoring current, time, and force, some advanced monitors can measure voltage and displacement (e.g. weld collapse). Monitors can be set up to measure peak limits, RMS limits to monitor the average parameter value during a weld and set envelope limits to monitor the entire waveform of a parameter through a weld. While monitors built into advanced power supplies seem to be a simple solution, most rely on control circuitry for monitoring, so a welder could be outputting the wrong current without that change registering in any data. In short, it is highly recommended to ensure consistent welds and joint strength with an external monitor.

For more information about resistance weld monitoring, watch the webinar Resistance Weld Monitoring for Improved Quality and Throughput.

Category: Monitoring