Aluminum has a tendency to form voids or cracks in the weld nugget as it solidifies. These voids are generally not desirable. In many cases this condition can be improved with good force follow up or application of a secondary forging force near the end of the weld cycle.
Squeeze is the first segment of the weld schedule. It’s function is to apply force on the work pieces to bring them together. This pressure under the face of the electrode in turn makes good electrical contact and controls and contains the resultant weld nugget during the weld and hold portion of the weld cycle. If current is applied prematurely before the proper force has been reached these functions will be compromised and expulsion and poor nuggets should be expected.
Squeeze time is the first sequence of a weld schedule. It is designed to allow the electrodes to close and build up full force on the parts before current is applied. If squeeze time is too long it does not hurt the welding operation but it does increase the cost of the total operation by increasing the total time of the weld schedule and thus fewer parts per shift.
Part fit up is very important in determining the quality of resistance welds. All weld schedules call out a force value with the purpose of holding the part together, containing the weld nugget and performing a little forging as the weld cools. If the part does not fit up properly then part of this force is used to force or form the part into place and is not available to contain and forge the weld nugget.
Cracks appearing in the parent material around the weld nugget can be caused by several issues. They are:
High Weld Force – can fracture the parent material with excessive forging action.
Insufficient Cooling – Over heating can cause excessive stresses from expansion and contraction along with the forging action.
Metallurgy of Material Welded – Some materials become brittle at the temperatures experienced in resistance welding.
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