MICROSTRUCTURES AND PROPERTIES OF ANODIC OXIDE LAYERS ON MULTI-PHASE DIE-CASTING ALUMINIUM ALLOY A356.0 FORMED AT LOW TEMPERATURE

Abstract

To form a thick, uniform and high wear resistance anodic oxide layer on multi-phase die castings poses some of the most challenging problems in anodizing. In this work, the microstructures and properties of the anodic oxide layers formed on A356.0 aluminium alloy in H₂SO₄ electrolyte at low temperature were presented. The experimental results indicate that at anodizing temperature of (- 6^oC) ¸ (- 5^oC) and H₂SO₄ electrolyte concentration of 180 g/l, the thickness of the anodic oxide is more than 90 μm, the microhardness is high as 500 HV, the wear rate is high as that of the heat treated bearing steel. The thickness of the anodic oxide layer increases as the anodizing current density and the anodizing time increase then gradually decreases after reaching the critical value. The microhardness of the anodic oxide layer tends to decrease with increasing thickness. The investigation in the microstructures of the anodic oxide reveals that these properties are relate to the multi-phase microstructure of the alloy.