| This dissertation study has developed a fundamental understanding of the contact resistance behavior using the virtual contact volume concept and the equivalent contact resistivity definition. In this research, an integrated experimental-numerical approach was used to demonstrate the proposed equivalent contact resistivity versus temperature relationship. Such relationship was further used to study the expulsion behavior of the resistance spot welding aluminum alloy. A concept of using the constriction ring was demonstrated to be effective in preventing weld expulsion in resistance spot aluminum welds.; The a-spot model simulated a single ball contact to define the equivalent contact resistivity. For a specific loading range, the generic equivalent contact resistivity versus temperature relationship, consisting of a proportional term and a exponential term, was derived. The proportionality term represents the increasing contact resistivity with temperature reflecting the Wiedemann-Franz-Lorentz behavior. The exponential term represents the softening effect of the contact surface as temperature increases.; With the generic equivalent contact resistivity versus temperature relationship established, an a-spot welding which incorporated the contact pairs with the resistivity relationship leaving three factors as parametric variables and an experimental a-spot welding model were conducted. Both the numerical and experimental analyses show the same relationship that is defined by the theoretical hypothesis of a-spot model.; The parametric study was conducted to quantify the parametric factors based on comparisons on nugget size and electrical potential drops across the electrodes. For an ideal combination of parameters at the Cu/Al interface and the Al/Al faying surface, both the comparisons of weld nugget size and potential drops are satisfactory.; The mechanical analysis proposed a concept that expulsion would occur when the crack tip is within the solidus nugget zone at any instant during the welding process. With the application of the external constriction ring, expulsion can be prevented; and electrode indentation and workpiece distortion can be improved. |
