Aluminum surface treatment and process optimization: Boosting mechanical performance in aluminum/polypropylene composite friction stir lap joints

The effects of chemical surface treatment of aluminum sheet and tool rotational speed (in the range of 300–1100 rpm) were studied on the macro/microstructure and mechanical behavior of friction stir lap joints between aluminum-magnesium aluminum alloy and a polypropylene composite containing 20 wt.% talc and 10 wt.% elastomer. Macrostructural studies of the joints revealed the formation of macroscopic mechanical locks between the aluminum and polymer base sheets, characterized by aluminum pieces resembling anchors penetrating the polymer substrate. The size of the anchors decreased as the rotational speed increased, and their orientation changed from being parallel with the interface of the aluminum/composite sheets to being perpendicular, and then facing the opposite direction. The larger anchors, as well as those penetrating relatively perpendicular into the polymer composite substrate, provided the joints with the highest fracture load and absorbed energy up to peak load at the intermediate tool rotational speeds of 700 and 900 rpm. Microstructural analysis demonstrated that chemical surface treatment with a solution of HCl and FeCl₃ in distilled water significantly increased the surface roughness of the aluminum sheet (by a factor of ∼4) and created numerous microscopic voids on its surface. The molten polymer formed during welding penetrated into these voids, creating numerous microscopic mechanical locks. These locks substantially enhanced the tensile-shear performance of the joints, resulting in up to ∼80 % higher fracture load and ∼380 % higher absorbed energy compared to joints without surface treatment of the aluminum. The influence of the morphology of mechanical locks on the location and mode of joint fracture was also investigated.