This can happen for several reasons. If the materials are coated it is probably a weld schedule settings issue. If the materials are thin then sticking is probably caused by the wrong diameter electrode. It could also be caused by the wrong electrode material sometimes refered to as gyp-tips. It could also be a machine problem. If the TX does not have enough voltage then the tendency is for increasing the time so that the weld holds the parts together.
The spot weld nugget grows from the inside out due to the highest point of resistance in conjuntion with the pinch effect of the A/C current. The current, through what is called dynamic resistance, causes the weld to grow or spread along the interface of the faying surfaces. Then the weld heat, traveling down the temperature gradient, spreads out to the cold material as long as the current is flowing. If the surface temperature gets high enough the electrode can stick.Too much time will permit the nugget to make it to the surface and the heating of the surface will cause a bonding of the electrode. This is easier to accomplish when the surface is coated with low melting temperature alloys as in galvanised and hot dipped coatings.
Think speed to avoid this problem. The faster you can make the weld the
less likely you will have electrode sticking. This is another reason
to weld as quickly as possible with a higher current. If your machine, or
more specifically the transformer (TX) can not produce the current at a percent heat
in the seventy to eighty range then consider a different machine or use a smaller
electrode tip face.
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