Abstract
The current paper investigates the durability of the single-lap shear aluminum-composite friction spot joints and their behavior under harsh accelerated aging as well as natural weathering conditions. Four aluminum surface pre-treatments were selected to be performed on the joints based on previous investigations; these were sandblasting (SB), conversion coating (CC), phosphoric acid anodizing (PAA), and PAA with a subsequent application of primer (PAA-P). Most of the pre-treated specimens retained approximately 90% of their initial as-joined strength after accelerated aging experiments. In the case of the PAA pre-treatment, the joint showed a lower retained strength of about 60%. This was explained based on the penetration of humidity into the fine pores of the PAA pre-treated aluminum, reducing the adhesion between the aluminum and composite. Moreover, friction spot joints produced with three selected surface pre-treatments were held under outside natural weathering conditions for one year. PAA-P surface pre-treated specimens demonstrated the best performance with a retained strength of more than 80% after one year. It is believed that tight adhesion and chemical bonding reduced the penetration of humidity at the interface between the joining parts.
Highlights
Metal-composite hybrid structures have been gaining more attention lately from the transport industry
SB and phosphoric acid anodizing (PAA) treatment led to a rough aluminum surface, increasing the micromechanical interlocking between aluminum and the molten polymer
PAA with a subsequent application of primer (PAA-P) samples did not have any noticeable changes on the aluminum surface
Summary
Metal-composite hybrid structures have been gaining more attention lately from the transport industry. In addition to the humidity, ultraviolet (UV) radiation is another source of degradation for polymers and composites It is frequently reported [16,17] that photo-oxidation, as a result of UV radiation, changes the physical properties of polymers, such as discoloration and increase in the glass transition temperature, and reduces their mechanical performance. Davis et al explained for the first time the mechanisms of hydration inhibition by PAA pre-treatment [20] They pointed out that a very thin layer of AlPO4 is formed on top of the aluminum oxide. Bland et al used an epoxy-based primer containing strontium and chromium particles on a PAA pre-treated aluminum alloy prior to adhesive bonding [22] Their findings suggest that the primed joint had a better durability compared to PAA pre-treatment alone. Different microscopy and analytical techniques such as scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and X-ray photoelectron spectroscopy (XPS) were employed to analyze the surface of the joints as well as fracture surfaces after mechanical testing to evaluate the influence of the aging condition on the joints
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