Effect of Prelube, Surface Coating and Substrate Materials on Initial Strength of Adhesive Joints Between Al Alloy and Steels

Growing usage of lightweight materials such as Al and Mg alloys in automotive body manufacture has come to a point that bonding of dissimilar materials is a realistic problem to address. A significant issue related to the bonding of dissimilar materials is that the differences in substrate surface conditions and substrate strengths often lead the bond to fail at strength far less than the bond strength established by adhesive manufacturer for a balanced joint. This research experimentally studied several factors potentially influencing initial strengths and debonding modes of adhesively-bonded Al-steel joints using single lap-shear coupons with comparison to like-substrate joints. Three commonly-used SLS coupon fabricating processes were investigated to determine which provided consistent bond strength and was efficient in making large quantities of coupons for the subsequent study. Next, the effect of prelube on the initial bond strength and debonding mode was investigated since the amount of prelube varies from sheet to sheet in automotive production. It was observed that even a very small change in the amount of prelube being applied on Al affected the initial bond strength. The more the prelube the weaker the bond became and the more adhesive failure occurred on the bonded Al surface. On the other hand, varying amount of the mill oil on the steel surface did not make much change to the bonding strength. Finally, various combinations of Al and steel substrates were studied to observe the effect of substrate materials on the initial bond strength and failure behavior. It revealed that the strength of joints between a relatively strong substrate and a relatively weak substrate fell below the strength of identical material joint made of the relatively strong substrate, and was closer to the strength of identical material joint made of the relatively weak substrate. For bonds having a high joint efficiency, adhesive failures were observed mostly on the surfaces of relatively weak substrates in the dissimilar material bonds due to large deformation in the weak substrate resulting in higher loading on that interface.