Diffusion in Electrodes Used for Resistance Spot Welding of Galvannealed Steel
A study was carried out on Zr-Cr bearing copper electrodes used for resistance spot welding of galvannealed steel strips. One electrode exhibited a series of well-defined layers in which Zn diffused to form β- and γ-brasses; an external layer containing iron was detected in this electrode. Another electrode that exhibited a high degree of damage did not exhibited continuous Zn-diffusion layers in all places, moreover, the Fe-containing layer was either removed, or had it grown to a high extent in some places; the occurrence of Cu-rich particles embedded within the Fe containing layer was observed. Multiple cracks were observed within the γ brass layer in both electrodes. The difference in the observed behaviour of the electrodes can be attributed to a difference in the characteristics of the galvannealed coating of the strips, as the first electrode was used to weld strips in which the layer corresponding to the phase was well developed, whereas the second electrode was used to weld strips with only an incipient layer. It can be concluded that growth of the phase changes the thermophysical properties of the zinc coating, affecting the temperature profile during spot welding.
Andreas Öchsner, Graeme E. Murch, Ali Shokuhfar and João M.P.Q. Delgado
M. de la Garza et al., "Diffusion in Electrodes Used for Resistance Spot Welding of Galvannealed Steel", Defect and Diffusion Forum, Vols. 297-301, pp. 300-307, 2010