Effect of plasma electrolytic oxidation on joining of AA 5052 aluminium alloy to polypropylene using friction stir spot welding

The effect of a plasma electrolytic oxidation (PEO) pre-treatment on joining of AA 5052 aluminium alloy and polypropylene by friction stir spot welding (FSSW) is investigated using lap tensile shear tests. Two surface conditions of the AA 5052 alloy are compared, one with a PEO pre-treatment in a silicate-based electrolyte, another without any pre-treatment. The resultant specimens are examined by high resolution scanning electron microscopy, thermogravimetric analysis and attenuated total reflectance-infrared spectroscopy. The PEO treatment generated a thermally-insulating, porous ceramic coating, which has a highly porous, rough surface that is favourable for incorporating polypropylene melted by FSSW. The pre-treatment significantly increased the lap tensile shear strength, by about a factor of three, in comparison with the untreated alloy, suggesting that open pores in the coating filled by polypropylene provide strong micromechanical interlocking and covalent bonding between the coated alloy and the polymer.