Livingston & Company WELDWISE 2400, User Manual

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0428-INS-400 Rev. E
1-8
Law, Current is not constant either. This means that the I-squared version Joule's Law will not reveal
the amount of heat generated at the workpiece unless the resistance at the tips is known.
Simply put, to determine how much heat is being generated at the workpiece using Joule’s Law,
current, voltage or resistance must be measured at the workpiece. Although a weld controller may be
programmed to deliver 20 KA at 10 Volts, if there is significant resistance in the secondary weld loop,
the heat will go there and not to the workpiece. Likewise, if the electrodes are worn or the workpiece is
dirty, resistance and current density will be affected. In such a situation, a controller might indicate 10
Volts at the secondary, however there might actually be only 5 Volts at the weld tips. Such a disparity
could easily cause bad welds.
Identifying and Correcting Weld Problems
A simple rule to remember is that quality usually equals consistency: welds that are always made
within the specified weld lobe will consistently be of high quality. The question is, how can you
determine if welds are being made consistently within the lobe? If a weld control is programmed to
deliver a certain amount of current at a certain amount of force, how can you ensure that the right
amount of current and force was delivered at the tips? The amount of current coming out of the
transformer may be correct, but is the current density at the workpiece where it should be? How do you
know if the weld is good? The most common method of answering these questions is through
destructive testing. It's hard to dispute the quality of a weld after it has been pulled apart and inspected.
However, destructive testing produces a lot of scrap metal, and while it will reveal whether the weld is
good or bad, it cannot explain the specific details of why or how a weld turned out the way it did.
Resistance weld monitoring provides a way to see what is happening while each weld is being made.
Critical parameters, such as resistance and current density, can be observed and measured
at the
workpiece during the weld process. The next chapter will discuss how this process works.
The following is an abbreviated guide of commonly encountered welding problems and their possible
causes, adapted from documents published by the Resistance Welder Manufacturers' Association and
reprinted with permission.