Spot Welding
Questions and Answers
The resistance made when the electrode touches the workpiece is created by:
• Composition of the workpiece
• Surface condition of the workpiece – rough, smooth, clean, dirty
• Conductivity or resistivity of workpiece surface
• Composition of the electrode
• Surface condition of the electrode – rough, smooth, clean, dirty
• Conductivity or resistivity of the electrode surface
• Pressure or force of the electrode on the workpiece
Sandblasting has a dramatic affect upon the surface condition of the workpiece. Depending upon the media used for sand blasting the surface resistance can be a problem?
The weld gun pressure is used for two main reasons to hold the parts together tightly during the weld and to provide forging action during the weld and cool down phase. The hold together includes containing the weld nugget and flash from the faying surfaces and the electrode contact areas. Any adjustments to pressure would be made to facilitate these functions and improve or lengthen the life of the electrodes.
One must always remember that for every action there can be reactions. A change in the pressure can change the heat being generated and thus cause the nugget formation to increase or decrease. This is discussed in an article in this blog titled:
HOW WILL TOO MUCH PRESSURE AFFECT RESISTANCE WELDING?
This article demonstrates that increasing the pressure will decrease the contact resistance. Therefor with the same current flow the total heat in the part will decrease. If the heat is decreased the nugget size will not grow and it is likely going to decrease in size. If the goal was a smaller nugget increasing pressure might work.
If one were to decrease the voltage at the same time by dropping the transformer tap or adjusting the control settings, this would reduce the current flow. Now we have lowered the total current and the resistance is lower due to the increased pressure.
Joules law states that heat is generated by:
Lowering the current and resistance at the same time would greatly decrease the heat being generated at the faying surface. Lowering the voltage when the pressure is increased is not a good parallel adjustment. Increasing the voltage might be a better move if the nugget needs to be maintained after the pressure change.
Adjustments to pressure may be necessary to maintain proper containment of the parts. If the pressure changes affect the weld nugget then changes to the current would be necessary to compensate. To drive more current more voltage would be required.
If indentation is excessive maybe less pressure might be applied. If the nugget is not affected all is good. If the nugget is out of tolerance after the pressure adjustment, then adjustment to the current might be needed to maintain proper nugget size.
References: RWMA – Resistance Welding Manual 4th Edition
With no input on the process or materials many assumptions will be made in this analysis. First the control and transformer are of adequate size and are functioning properly. Second the material being welded is weldable and the parts have good fit up. The force system of the welder is in good working order.
Now we can address common causes for low weld strength:
Low weld strengths can be caused by material of low strength. This will not be considered. To have high weld strengths you need a good nugget generally of modest size as prescribed by published weld schedules. Smaller nuggets produce lower strengths. Anything that retards nugget formation or growth will reduce weld strength.
Another article in this blog describes the factors that affect nugget size and growth:
WHY DO I HAVE LOW WELD STRENGTH?
Reference: RWMA - Resistance Welding Manual 4th edition
That is an interesting question. The answer could be found in the equipment manual. The pendant could be faulty. Equipment design and function is beyond the scope of this blog.
Contact the manufacturer, local distributor or supplier for assistance.
One should always attempt to make a resistance weld under steady state conditions. If the desired pressure, current and time were applied consistently each weld, one can expect every weld to be the same weld after weld. If one of those three input varies then one could expect that the welds might vary. Rising pressure, loose cables, electrode issues, misalignment, faulty cooling or many other issues can and do affect the welding results.
If one has a robust process that is running well but you are pushing the limits on the pressure build up, remember to keep a close watch on the weld results. You may be taking advantage of the lag in the weld gun reaction and initiation speed. The machine does not react instantly and takes several more cycles before the electrodes are actually closed. You should always attempt to reach 90% of desired force before the weld is initiated.
However:
If one has something that works
It is a Robust process
It is monitored closely
NEVER ARGUE WITH SUCCESS!
Maintaining the proper pressure during a welding operation is very important to obtaining good welds and a long running operation.
Many articles have been written in this blog on the importance of pressure and force. They include:
Why is the control of pressure important in resistance welding?
How are force and pressure related?
How does pressure affect resistance?
HOW DO FORCE AND PRESSURE CONTAIN EXPULSION?
Reference: RWMA - Resistance Welding Manual 4th Edition
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