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Spot Welding - Welding Conditions
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There are several elements to achieving a good weld. The welding pressure, welding current, and welding time have a great impact on the quality. They are called as “Three Major Welding Conditions”. In addition, the shape of the electrode tip also has a great impact on the current supply and cooling processes, which helps determine the end quality of the weld. With the electrode shape included, there are “Four Major Welding Conditions”.
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PRESSURE CHANGE AND NUGGET DIAMETER
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Welding pressure minimizes the occurrence of “welding spatter” while supplying the current to the parts being joined. It also stabilizes the strength of the welded parts by holding pressure even after the current is applied. If the welding current and time remain the same and only the welding pressure is changed, the higher the pressure applied, the smaller the nugget will be and lower the strength of the welded parts. The reason for this phenomenon can be explained this way:
When two work pieces are held together by the welding tips, the current will run through the pressurized part, in other words a slightly wider area of work pieces around the tips. When the pressure is high, the surface area of which the current runs becomes larger. Under these conditions, the current does not fully concentrate (lowering of current density) and the temperature does not become high enough to melt the parts being welded thoroughly. As a result, the nugget becomes smaller.
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The larger the welding current becomes, the bigger the diameter of nugget becomes. Increasing the current generates “spatter”, but by applying a high welding pressure to work pieces, “spatter” is minimized because the electrified surface becomes larger. In other words, spatter occurs when high current is applied in combination with low welding pressure. As such, current and pressure have a correlation.
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Welding time impacts the size and quality of the weld nugget. A long welding time generates a large amount of heat resulting in a large nugget.
The amount of heat dissipation around the welding area and tips increases as the welding time gets longer. After a certain variable amount of time, the temperature of welding area saturates. Increasing welding time beyond that point doesn’t increase the weld nugget size, but it does increase pressure dents from the weld tips and heat deflection, resulting in weld quality issues.
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Electrode Tip Diameter and Current Density:
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The major functions of the welding electrodes are: |
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Allows electrical current to pass through the parts being welded | |
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Applies pressure to the weld area to prevent spatter | |
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Acts as a heat sink to cool down the parts after welding | |
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Electrodes must have high conductivity, wear-resistance, and thermal conductivity. | |
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After making repeated welds, the electrode contact tips become larger, reducing current density and heat transfer. This results in a smaller weld nugget size and a low weld strength. | | | | 
