Selecting Welding Electrodes for Spot Welding
Spot welding is the name always attached with the resistance welding process. This has got the significance as the spot welding electrodes are subjected to the most severe service of any current carrying member of the resistance welding process. Through the weld area, these electrodes transmit both the welding current and the welding force. If you compare electrodes being used in the projection welding, flash or upset butt welding, they have an area of contact much greater than the weld area. Hence in such welding scenarios the current and force densities would be considerably less. However the spot welding electrodes are required to carry high current densities and this can be accomplished with effective water cooling.
Due to their significance in the spot welding process, it is important to select the proper electrode material as it assures correct current carrying capability. Resistance to electrode pick-up and sticking are other main factors which affect spot welding material selection. Both these factors depend upon the overheating of the electrode and work piece. If you are wise in selecting the proper electrode material, the severity of these conditions can be reduced. The main intention of these alterations is to arrive at the ideal interface temperature between the electrode and the work piece.
While selecting the spot welding electrodes, you will find many alloys with each having its own advantages depending on the application. Usually the alloys differ in the electrical conductivity, hardness or wear resistance and annealing or softening temperature of these materials. Even though the ideal electrode material can be defined as something with the compressive strength of steel and the electrical conductivity of silver, nothing of that sort is available presently. Hence the welding application would give more importance to high conductivity than electrode life and long service dressings.
If you compare copper alloys and its features, it can be seen that the increased wear resistance or hardness of alloyed copper is accompanied always by lower conductivity. This means that while selecting electrodes for spot welding, you need to go for the ones with least conductivity which would produce good welds on a given application. Also you need to consider the annealing or softening temperatures of electrode materials which are significant for the long life of the electrodes. It is also the function of the electrodes to conduct the current and to withstand the high forces so that a uniform contact area is maintained and a proper relationship is established between current, time and force. For achieving satisfactory welds, you need to take care in the selection of proper electrode.