Anodizing of Al alloy diecastings - Alloy design, manufacturing process and material properties

The current goal on manufacturing sustainable and lightweight automotive components to reduce fuel consumption and greenhouse gas emissions has significantly boosted the use of diecast Al-Si alloys. To enhance their surface mechanical properties, electrical insulation, and corrosion resistance, an anodizing process can be performed. During this process, a layer of aluminum oxide grows from the casting surface into the substrate, induced by the action of electrical current. However, the high content of alloying elements in diecast Al-Si alloys makes these alloys challenging to anodize. An exhaustive knowledge of the primary factors affecting the anodizing response of Al-Si alloy diecastings, as well as the identification of the key variable necessary for optimizing their anodizing response, has not been acquired yet. Therefore, this research aims to compensate this knowledge gap by examining the influence of the alloy’s chemical composition, surface topography and anodizing parameters on the anodizing behavior of diecast components. Industrial components were produced through high-pressure diecasting with different alloy’s chemical compositions. Milling and blasting operations were then performed on the castings’ surface to assess the influence of distinct surface topographies on the anodizing behavior. The casting were anodized under varying parameters to investigate the effect of the electrolytic temperature and the sulfuric acid concentration on the growth of the oxide structure. Additionally, hardness, wear, and scratch tests were performed to examine the mechanical properties of anodized surfaces. Impedance spectroscopy measurements were also carried out to assess the corrosion protection offered by different sealing processes. The results of this research indicate that the surface topography and near-surface microstructure are the primary factors influencing the growth of the oxide structure. The addition of Cu is detrimental to the thickening of the anodic layer, while a high Si content is not harmful for the anodizing response as long as the eutectic Si particles have a refined and fibrous morphology. During anodizing, α-Alx(Fe,Mn,Cr)ySiz and Al12(Mn,Fe)3Si2 phases dissolve, leading to the formation of voids and cracks in the anodic layer. A rough and irregular surface topography inhibits the thickening of the anodic layer, thereby reducing the wear and scratch resistance. As a result, milled surfaces exhibit a greater anodizing response and enhanced surface mechanical properties compared to blasted surfaces. Anodizing at a low temperature (-4.5°C) with a low concentration of sulfuric acid (168 g/L) maximizes the thickening of the anodic layer and improves the surface mechanical properties. Conversely, a low anodizing temperature (-4.5°C) combined with a high concentration of sulfuric acid (217 g/L) negatively affects the thickness and scratch resistance of the oxide layer. Finally, cold sealing in a NiF2 solution is more beneficial than hydrothermal sealing for improving corrosion protection.