The question posed is about welded parts that fail - pop open while sitting on the workbench for a few minutes. The parts were probably distorted and stressed during welding. As they cool the shape tries to return to its original state. This puts stress on the weld joints. Poor joints could fail. There are two general areas to discuss as the cause of the weak joints. One is the work piece material composition. The other is the general weld procedure and resultant nugget.
Copper is a very conductive material and therefore may require one to use modified procedures from the ordinary. It can be resistance welded to similar materials. There is so much difference in the conductivity between copper and steel that this would be considered a difficult weld to make. The steel will want to heat up before the copper. The weld charts call for Class 1 on the steel side and Class 13 or 14 on the copper side. There probably will not be a weld nugget formed. The bond would be described as a stick weld.
Push-pull welds are generally series welds which make two welds at a time. In some cases only one weld is required and there may be cosmetic reasons to keep marks off the part. In this case the second pair electrodes are flat and large. They become electrical contact pickup points. The same dual transformer arrangement is set up with the proper polarity but one set of electrodes are flat not contoured. This is an indirect push-pull weld.
Indirect welds use similar machine and power set ups to series welds. The difference is that one pole off the transformer is connected to a pick up conductor rather than an electrode. Therefore the current returns to the transformer through this location but no weld is made. This process eliminates marking at the pickup point and the need for electrodes on the other side of the part when only when weld is required.
Another variation of the series weld is the push-pull weld. In this process the electrodes are connected to separate secondary circuits on both sides of the weldment. The polarities are arranged as shown for proper current flow. Shunting current is still present but now in both workpieces. The voltage between the electrodes increases the weld current to shunting current ratio when compared to other series weld set ups. Therefore you get more power delivered to the weld.