A Study of Anodising Behaviour of Al-Si Components Produced by Rheocasting

Abstract

This paper aims to investigate the anodising behaviour of Al-Si components produced by rheocasting, to understand the effect of the surface liquid segregation (SLS) on the anodising response. The material investigated was EN AC 42000 Al-alloy with an addition of 150 ppm Sr. The component was rheocast and conventionally liquid cast for benchmarking. The RheoMetalTM process was used to prepare slurry and subsequently cast using a vertical pressure die casting machine. Prior to anodising, mechanical grinding was used as pre-treatment method for selected samples as comparison with components in the as-cast state. Anodising was performed on the components using a constant controlled voltage at 25 V, in 1 M H2SO4, at room temperature. The duration of anodising was varied from 30 mins to 120 mins to examine the relationship between oxide layer thickness and the anodising time. The oxide layer was investigated and characterised. The results demonstrated that the presence of the SLS layer, which was enriched with alloying elements, had a significant influence on the anodising behaviour of the cast component. The oxide layer thickness of the components produced by rheocasting and fully liquid casting was measured and compared. The relations between the oxide layer thickness and anodising time, as well as the casting methods are presented and discussed in this paper.

Anodizing Al–Si Foundry Alloys: A Critical Review

“…Moreover, the Si interparticle spacing is greater, favoring the propagation of the anodic front among the eutectic Si particles. Similar results were found in the machined rheocast substrates studied by Zhu et al [4,36,52] The machining operations remove the surface liquid segregation enriched with Fe-rich compounds that are formed during the casting process. Due to the low amount of Fe-rich intermetallics embedded in the oxide layer, the machined substrate results slightly thicker, harder, and more resistant to corrosion after anodizing.…”

“…[26,49] The partial or total dissolution of Fe-rich intermetallics during anodizing causes the formation of voids and defects in the oxide layer (Figure 8), decreasing its mechanical performance. [4,36,50,51] Zhu et al [4,36,52] investigated the anodizing response of Fe-rich particles in a rheocast AlSi7Mg0.4 alloy. It was observed how the π-Al 8 FeMg 3 Si 6 phase, with respect to the α-Al 15 (FeMn) 3 Si 2 phase, is easier dissolved during anodizing due to a higher oxidation rate.…”

“…[5,59] The analogous phenomenon affects rheocast components, where the hardness and corrosion protection of the anodic layer are reduced due to the formation of a surface segregation layer enriched with solute elements. [41,52,60,61] The "externally solidified crystals" are large bright α-Al crystals solidified during the melt transfer from the holding furnace to the shot sleeve of a cold chamber diecasting machine, or inside the shot sleeve, and injected into the die cavity during the filling phase. [59,62,63] Due to the greater dimensions with respect to the grains of the Al-matrix, these defects increase the heterogeneity of the surface and can create preferential areas for the growth of the oxide front with respect to the surrounding material.…”

“…Some studies on rheocast and diecast hypoeutectic Al-Si alloys were carried out to investigate the influence of machining operations on the subsequent anodizing treatment. [5,52,59,61] Regardless of anodizing parameters and investigated alloys, machined substrates show generally a thicker anodic layer after anodizing than the as-cast ones [5,37] (Figure 10a). Caliari et al [5,37,59] explained this result considering the effects of milling operation on the diecast substrate before anodizing.…”

“…Zhu et al [ 4,36,52 ] investigated the anodizing response of Fe‐rich particles in a rheocast AlSi7Mg0.4 alloy. It was observed how the π‐Al 8 FeMg 3 Si 6 phase, with respect to the α‐Al 15 (FeMn) 3 Si 2 phase, is easier dissolved during anodizing due to a higher oxidation rate.…”