Resistance welding comes in many forms. Spot, seam, and projection are commonly used resistance weld processes. This article is going to focus on spot welding.
If you are concerned with a consistent quality of spot welding production then you need to understand that the three key principles are the time, the electrical current, and the pressure (electrode force - mechanical pressure). These three factors are closely related and need to be balanced in order for the operator to produce a quality bond between two pieces of metal with consistency. Electrodes conduct the current to the two pieces of metal to be fused as they are forged together.
The weld cycle must first build an adequate amount of heat through the electrical current and the time the current is applied to raise a small volume of molten metal from the work pieces to be bond. This molten metal is then allowed to cool while under the pressure of the electrodes until it has set at the maximum strength that the metal will allow to hold the parts together. This is referred to as the hold time on the welder.
If one of these parameters is changed without the proper adjustments to another, it may result in a poor bond.
Example: If the current is increased without an adequate amount of pressure it will result in an excessive weld splash. Conversely, a change in pressure without the corresponding changes in time and current may result in poor welds also.
The kva ratings of resistance welders are greatly impacted by various aspects of the machine set-up. When a part is very large and cannot be fit into a short set of spot welder arms - the arms may have to be extended or changed in order to accommodate the depth of the part. A spot weld weld machine with long arms for a deep entry of a product to be bonded will have a much greater electrical impedance than the same machine kva with a short set of arms. The resistance welder efficiency is greatly reduced by the electrical impedance.
Electrical impedance is the measure of the opposition that an electrical circuit presents to the passage of a current when a voltage is applied. In terms of resistance welding it is the increased resistance due to the greater distance and obstacles that the current needs to flow through to get to its destination (the welding electrodes).
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