CONTACT RESISTANCE OF ALUMINUM

The electrode-workpiece and faying surface contact resistances were me asured for a variety of aluminum alloys having various surface finishe s. The measured value of each contact resistance was dependent upon th e imposed current and the surface roughness, R-a, of both the workpiec e and the electrode. With sufficiently high currents, often much less than those suggested in various spot welding specifications, electrica l breakdown of the contact could occur. An increase in electrode tip l ife can be associated with higher workpiece surface roughness, R-a. Lo wer electrode-workpiece contact resistance is also associated with hig her values of R-a. Although faying surface contact resistance measurem ents have not proven successful in identifying long weld tip life beha vior, possibly because such determinations often are made with high cu rrents that can produce surface changes, measurements of contact resis tance made with a low current, and that are associated with high value s of R-a, may provide a means for quality control in the spot welding of aluminum alloys. This current should be kept to a value that preven ts the potential drop across the faying surfaces from exceeding 0.15 V to avoid changes in the surface characteristics of the aluminum.