Poor heat balance in a resistance weld is when the variables cause the weld nugget to form in a location other than the desired mating/faying surfaces. This can be caused by welding two different materials, varying thicknesses, or different surface coatings. Different electrode material, size, shape or contact face or perpendicularity to the part. Water cooling or lack of same can cause variations also. The first step is to examine the part and determine what is causing the nugget to form in this undesired location.
Misaligned electrodes could be because they are not perpendicular to the work piece. Or the two opposing electrodes do not line up with each other as shown below. The first condition can cause the electrode to make contact on the edge of the face rather than the desired face area. This puts a large amount of current through a smaller area. This leads to overheating and expulsion. It also can cause skidding of the electrode on the work surface.
The location of the weld nugget is determined by the locating the resistance center of the part being welded. The “Relative Resistivity Diagram” explains this graphically.
Every component and contact point in the weld has a resistance and contributes to the weld nugget location.
By definition - If the upper and lower halves of the resistances are equal, and RI is the largest Resistance then the weld will occur in the center. If there is any imbalance then the nugget will move up or down toward the hottest spot. This is called heat balance.
In most manufacturing operations nuggets are tested by pulling a test part or test coupon apart after welding. This is frequently done with a combination of hammer, pliers and chisel. Then the resultant pulled weld nugget is measured with a set of calipers in two directions, 90 degrees apart. The two numbers are added together and divided by two for an average. This is your result to be compared to your quality standard. If the nugget is oblong or odd shape you still make the same measurement and divide by two getting the average over the varying dimensions.
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.
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