The Non-Equilibrium of Resistance Welding Processes
By Jerry Gould on Tuesday, November 2nd, 2010
Resistance welding in nearly all it’s forms are known to be a fast, high productivity manufacturing technologies. Variations on single shot versions of resistance welding (spot, projection, upset) are known to employ welding times on the order of single digit to hundreds of milliseconds. The result of these short welding times (combined the copper electrodes that act as heat sinks) result in surprisingly fast thermal cycles. EWI spends a considerable amount of time analyzing the implied thermal cycles of these technologies in efforts to understand resulting microstructures and ultimately the performance of such welds. These thermal cycles have two significant components, including the peak temperature observed and the subsequent cooling rate. Peak temperatures typically define a starting condition for the transformations that create what is observed in the microstructure. For resistance spot welds, this is typically molten metal, while for solid projection and upset welding processes, this is a high temperature, generally fully solutionized phase (e.g. austenite in steels). Decomposition of this high temperature condition is then defined by the implicit cooling rate of the process. Cooling rates here are known to range from roughly 103–oC/sec up to 107–oC/sec, for processes ranging from resistance spot up to percussion welding. Such rapid cooling rates affect both solidification behavior and solid state transformations. With regard to solidification, the extreme end of the cooling rate range (characteristic of percussion and electro-spark deposition processes can actually suppress the concept of “local equilibrium”. Local equilibrium has long been used as a tool for assessing solidification behavior in welds. Resulting welds from process variations at the extreme end of the cooling rate range are characterized by minimal evidence of compositional segregation, but a fine dendritic structure. For cooling rates at the lower end of the range, solidification still demonstrates compositional segregation, but primary dendrite spacings are extremely fine (on the order of single digit microns). High cooling rates have long been known to result in non-equilibrium microstructures in metals. For the cooling rates characteristic of resistance welding processes, however, unique microstructures result that are not seen for any other process technology. These range from martensites in ultra-low carbon steels during spot welding to supersaturated solid solutions during either percussion or electro-spark processing. These examples demonstrate both the “non-eqiuilibrium” character of resistance welding processes, and suggest that new tools are required to understand both how these processes can be applied to a variety of material systems.