The effect of mechanical loading on the contact resistance of coated aluminium

Measurements of contact resistance, related to resistance spot welding, were made using pre-treated (coated) and abraded aluminium alloy strip. With conventional domed electrodes, the contact resistance was much larger at the sheet-to-sheet (faying) contact than at the electrode-sheet contact. The effect is believed to be associated with the role of sliding in breaking down contact resistance of sheet with an insulating surface film. When the coating was removed the difference between faying surface and electrode-sheet contact was much smaller. Macroscopic shear stresses are developed by electrode-sheet contact whereas no shear stresses are present at the faying surface. The hypothesis is supported by experiments made with asymmetrical electrode pairs which give rise to varying shear stresses in faying surface contact. Some implications for the control of spot welding of different aluminium surfaces are discussed.