For the four-layer resistance spot welds, there are three interfaces in the spot welds, namely two side interfaces and one center interface. The nugget diameters along the two side interfaces are equal. I am thinking which one is larger between the nugget diameter along side interface and the nugget diameter along the center interface. Furthermore, at the beginning of the resistance spot welding, which interface forms weld nugget firstly, side interface or center interface?
Though I have no practical backing for my answer, but seems the middle nugget should form first, as this is the region offering maximum electrical resistance and minimum cooling rate (assuming of course, sufficient pitch among the spots) since this region is farthest from the cooled and high thermal conductivity electrodes. The nugget diameters should depend on electrode diameter and sheet thickness. Assuming all parameters to be same and symmetrical, the nugget diameter for the middle one should be maximum. Destructive sectioning on a couple of jobs should provide the required insight and I too would like to see the results in this respect.
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