**1. Introduction**

A356 alloy is an Al–Si casting alloy that contains ~7 wt.% Si, mostly developed for automotive powertrain components such as the engine block and automotive transmission cases. However, the solubility limitation of silicon (Si) in aluminum (Al) contributes to the precipitation of flake-shaped Si particles with sharp edges, which significantly affects the mechanical and electrochemical properties.

The coarse flake Si particles can cause the initiation of premature cracks during deformation. Consequently, this shape weakens the workability of the alloy at room temperature, thus reducing the ductility of the alloy [1]. The morphology of Si particles and its distribution play essential roles in the electrochemical properties of Al–Si alloy. The cathodic behavior of Si within the Al-rich matrix contributes to the occurrence of localized corrosion, with the formation of microgalvanic couples [2,3], which also leads to poor mechanical behavior, such as in stent applications [4]. The reduction in the area ratio of noble Si particles (cathode) to less-noble eutectic Al phase (anode) around Si particles largely improves pitting corrosion resistance. In other words, the reduction in the area ratio of cathode to anode (Ac/Aa) reduces the corrosion current density. In the Al–Si alloy, the size reduction of Si particles which as cathode, facilitates the re-passivation of protective film with improved stability [5–7].

Since the corrosion resistance and strength of alloys are mostly influenced by their microstructure [8–10], there are various methods to refine the microstructure of unmodified A356 alloy in order to improve the corrosion resistance and mechanical properties, such as heat treatment [11–13] and semisolid state processing to obtain a spherical microstructure using mechanical stirring, electromagnetic stirring, and controlled nucleation methods [14–16]. The cooling slope casting process is also considered as another simple method of the semisolid metal casting process to produce feedstock material, with a spheroid microstructure, which bears minimal equipment and operational costs [17–19]. The severe plastic deformation (SPD) technique effectively accommodates the combination of major grain refinement and Hall-Petch strengthening in bulk billets, where the working piece is subjected to extensive strains of cold or warm processing. Fundamentally, the SPD technique is eminently capable of effectively refining the structure of grain down to an ultrafine micrometer-scale or nano-scale due to the agglomeration of dislocation, which leads to the formation of subgrains. In particular, this technique enables various metals and metallic alloys that are brittle and ductile to achieve the structure of refined grain [20–22].

The equal channel angular pressing (ECAP) of SPD the technique is considered the most effective technique in fabricating bulk ultrafine-grained materials [23]. Some researchers have applied several methods of SPD to Al–Si alloy to improve the corrosion resistance of the alloy through microstructural refinement. In one study, [24,25] it was concluded that the improvement of the corrosion resistance and mechanical properties of pure aluminum and A356 alloy was due to microstructure refinement. In another study, the effect of ECAP processing on the corrosion resistance of Al—11wt.% Si [26], it was reported that the great refinement of both the matrix microstructure and the Si particles after severe plastic deformation leads to an improvement of the corrosion resistance.

In another research [27] on the Anticorrosion behavior of ultrafine-grained Al—26 wt.% Si alloy fabricated by ECAP, it was reported that the improvement of ECAPed Al—26 wt.% Si alloy corrosion resistance results from the homogeneous ultrafine structure, with the breakage of brittle large primary silicon crystals. SPD processing improves the corrosion resistance and mechanical properties of Al–Si alloy [28].

However, the reduction of both grain size and eutectic Si particles through the deformation process may alter the corrosion property of the alloy [10,29,30]. Addressing these issues, the present study aimed to refine Si particles and to produce material with a uniform microstructure through the combination of the cooling slope casting process and ECAP processing at room temperature. The effect of T6 heat treatment was studied to determine the microstructure changes after ECAP processing. This study further examined the effects of microstructure changes and its effect on strength as well as corrosion resistance for both ECAPed as-cast and ECAPed cooling slope-cast A356 alloy.
