Influence of PEO and mechanical keying on the strength of AA 5052 alloy/polypropylene friction stir spot welded joints

A study has been carried out of the effect of plasma electrolytic oxidation (PEO) on the strength of AA 5052 alloy/polypropylene joints prepared using friction stir spot welding (FSSW). The joint strengths were determined using lap-shear tests and failure modes were investigated using scanning electron microscopy. Comparisons were made between control joints prepared with the alloy in the as-rolled condition or the as-rolled condition with a mechanical key and with PEO-treated alloy, with or without a mechanical key. Mechanical keying alone, provided by infiltration of polymer into holes of either 3 or 4.5 mm diameter drilled in the alloy, yielded enhancements of the joint strength by a factor of 1.8 and 3.8, respectively, compared with the as-rolled alloy. In contrast, PEO pre-treatment provided a much greater improvement in strength, by a factor of 21.3, with no significant influence of the presence of a mechanical key. The fracture of PEO-treated joints involved a mixture of de-bonding at the polymer/coating interface, cohesive failure within the coating and ductile failure of the polymer. The last resulted from formation of gas bubbles within the polypropylene due to thermal degradation of the polymer during FSSW. In contrast, in the absence of PEO, the fracture path of the as-rolled and also the as-rolled and mechanically keyed joints passed along the alloy/polypropylene interface and through gas bubbles within the polymer. The enhanced strength of the PEO-treated joints resulted from the strong polypropylene/coating bond resulting from flow of molten polymer into the coating porosity.