THE EFFECT OF MECHANICAL LOADING ON THE CONTACT RESISTANCE OF COATED ALUMINUM

Measurements of contact resistance, related to resistance spot welding , were made using pre-treated (coated) and abraded aluminium alloy str ip. With conventional domed electrodes, the contact resistance was muc h larger at the sheet-to-sheet (faying) contact than at the electrode- sheet contact. The effect is believed to be associated with the role o f sliding in breaking down contact resistance of sheet with an insulat ing surface film. When the coating was removed the difference between faying surface and electrode-sheet contact was much smaller. Macroscop ic shear stresses are developed by electrode-sheet contact whereas no shear stresses are present at the faying surface. The hypothesis is su pported by experiments made with asymmetrical electrode pairs which gi ve rise to varying shear stresses in faying surface contact. Some impl ications for the control of spot welding of different aluminium surfac es are discussed. (C) 1997 Elsevier Science S.A.