Microstructure refinement and corrosion resistance improvement mechanisms of a novel Al-Si-Fe-Mg-Cu-Zn alloy prepared by ultrasonic vibration-assisted rheological die-casting process

Highlights

• Compared with TDC and ACSR R-DC alloys, the ACSR+UV R-DC alloy had a smaller α-Al, eutectic Si, and a Fe-rich β phase.

• Pitting corrosion of the alloy mainly originated from the interface between the β-Al₅FeSi and the α₂-Al.

• The corrosion resistance of the ACSR+UV R-DC alloy was better than that of the T-DC and ACSR R-DC alloys.

• Microstructure refinement mechanism of the ACSR+UV R-DC alloy was discussed.

• Corrosion resistance improvement mechanism of the ACSR+UV R-DC alloy was discussed.

Abstract

The Al-Si-Fe-Mg-Cu-Zn alloy was used to manufacture large, thin-walled parts for 5 G base stations by using the ultrasonic vibration-assisted air-cooled stirring rod rheological die-casting (ACSR + UV R-DC) process. Investigations were performed on the microstructure and corrosion resistance of the ACSR + UV R-DC alloy which was then compared with traditionally die-casting (T-DC) and ACSR R-DC alloys. The results reported that the ACSR + UV R-DC process not only refined the primary α-Al (α₁-Al), the eutectic silicon, and the secondary α-Al (α₂-Al), but also significantly improved the morphology and distribution of the β-Al₅FeSi phase. Pitting corrosion of the alloy mainly originated from the interface between the β-Al₅FeSi and the α₂-Al. At the initial corrosion stage, the α₁-Al and the interface between the α₂-Al and the eutectic silicon were not corroded. As the corrosion time increased, the pitting corrosion continued to spread in the eutectic region and connected to form one large corrosion area. Moreover, the corrosion resistance of the ACSR + UV R-DC alloy was better than that of the T-DC and the ACSR R-DC alloys. This was due to the refinement of the eutectic silicon and iron-rich phases, and the reduction of the potential difference between the iron-rich β-phase and the aluminum matrix.

Introduction

Die-casting is the most high-efficiency and cost-effective technology for producing Al-Si-Fe alloys. As the casting size increases and the wall thickness decreases, the difficulty in forming the alloy increases dramatically. Large thin-walled parts produced by the traditional die-casting (T-DC) technology have many casting defects, poor thermal conductivity and lower mechanical properties [[1], [2], [3]]. This makes it difficult for parts to meet application requirements and therefore they need to be upgraded. Recently, semisolid rheological die-casting (R-DC) has been an advanced and effective near-net-shape process that solves the above problems and produces high-performance castings [[4], [5], [6]]. The hypoeutectic Al-Si-Fe alloy has a wide temperature window for semisolid forming, and the solid fraction increases uniformly with temperature decreasing. Therefore, the Al-Si-Fe alloy is suitable for R-DC. Compared with T-DC, R-DC can refine the grains, improve the morphology and distribution of the secondary phase, reduce casting defects, increase the density and strengthen mechanical properties [7,8].

The air-cooled stirring rod (ACSR) process is an advanced and efficient process for preparing the semisolid slurry of aluminum alloys. The ACSR process uses mechanical stirring and a cold, aerated stirring rod to rapidly cool the melt to the semisolid temperature range to promote nucleation [9,10]. At present, the ACSR process has been combined with die-casting machines to facilitate the mass production of thin-walled parts using R-DC. However, for the R-DC of large thin-walled parts, the mass of the semisolid slurry in a single preparation often needs to exceed 40 kg. Although the ACSR process can rapidly cool the bulk alloy melt, the melt away from the stirring rod does not receive forced stirring, so a single ACSR process does not always prepare a semisolid slurry with a uniform and fine microstructure. This indicates that the combination of the ACSR process and other processes for the preparation of large-volume slurry may be an effective idea. Ultrasonic vibration (UV) is environmentally friendly, energy saving, and low-cost, which makes it a feasible process [11,12]. Although the UV process has low slurry preparation efficiency and is usually not suitable for large-volume slurry preparation, it causes acoustic cavitation and acoustic streaming in the melt to refine the grains, improve the uniformity of microstructure, and remove gas [[13], [14], [15]]. Therefore, the use of the ultrasonic vibration-assisted air-cooled stirring rod (ACSR + UV) process to prepare large-volume slurry is not only expected to solve the problem of poor uniformity of microstructure in the rapid preparation of large-volume slurry, but also to prepare large thin-wall parts with fine grains and high mechanical properties.

In Al-Si-Fe alloys, the addition of elemental Fe is beneficial to the demoulding of die castings. Fe often exists in the eutectic structure in the form of the lath β-AlFeSi phase [16]. The hard and brittle β phase forms a non-coherent phase boundary with the aluminum matrix, which easily breaks the matrix and reduces the mechanical properties of the alloy [17,18]. For Al-Si-Fe alloys, obtaining a fine and uniformly distributed β phase is the key to improving the alloy’s properties. The size and distribution of the β phase can be optimized by adding alloying elements (Mn, Ni, etc.) [17,19], adding rare earth elements (La, Pr, Er, Tb, etc.) [20,21], applying UV treatment [22,23], increasing the cooling rate [24] or using rheo-casting [25]. The β-phase not merely affects the mechanical performances of Al-Si-Fe alloys, but also affects the corrosion behavior [10]. Thus, it is feasible to use ACSR + UV R-DC process to refine and homogenize the iron-rich phase for improvement in the mechanical properties and corrosion resistance of the alloy. However, there is little research on the corrosion behavior of rheologically casted Al-Si-Fe alloys. Zhu and Zanella [26] studied and compared the corrosion resistance of the anodized layers in an Al-Si alloy formed by rheo-casting and liquid-casting in 3 wt.% NaCl solution. The results showed that longitudinal macro-segregation affected the corrosion resistance, and the near-to-vent zone had poor corrosion resistance due to the high eutectic content. The presence of the liquid segregation layer had no significant effect on the corrosion resistance of the oxide layers in the rheological casting alloy and the liquid casting alloy. Park et al. [27] compared the corrosion resisting property of the rheological and low-pressure casting Al-7.2Si-0.4 Mg alloys and believed that the enhancement of the corrosion resistance of the rheo-casting alloys was mainly attributed to the reduction in the proportion of eutectic silicon to eutectic aluminum. Mingo et al. [28] researched the effect of heat treatment on the corrosion resisting property of the A356 aluminum alloy. The results indicated that heat treatment refined the intermetallic compounds and eutectic silicon, but these changes did not greatly improve the corrosion resistance of the alloy. Regretfully, these reports did not clarify the unique role of the β-AlFeSi in the corrosion process of the rheo-casting Al-Si-Fe alloys. Therefore, it is necessary to conduct a deeper investigation on the corrosion behavior and corrosion mechanism of R-DC Al-Si-Fe alloys.

In this research, the Al-Si-Fe-Mg-Cu-Zn alloy was developed, and was used to produce large thin-walled heat dissipation shells for 5 G base stations using the ACSR + UV R-DC process. Systematic investigations were performed on the microstructure and corrosion resistance of the ACSR + UV R-DC alloy, and the as-produced alloy was compared with the alloys prepared by the T-DC and ACSR processes. Finally, the mechanisms for the microstructure refinement and the enhancement of corrosion resistance performances of the ACSR + UV R-DC alloy were analyzed.