This paper compares microstructure, mechanical properties, corrosion resistance of die-cast ADC12 aluminum alloy and semi-solid injection-molded AZ91D magnesium alloy for new energy vehicle electric drive housings. Results show that die-cast aluminum alloy exhibits a dendritic structure, while semi-solid magnesium alloy microstructure consists of nearly spherical α-Mg primary solid phase grains, post-solidified liquid phase grains, and a network of Mg₁₇Al₁₂ phases. Thick-walled parts exhibit a dense and defect-free microstructure. Regarding mechanical properties of bulk samples, yield strength of semi-solid AZ91D magnesium alloy approaches that of die-cast ADC12 aluminum alloy, while its elongation is slightly improved compared to aluminum alloy. In terms of corrosion resistance, semi-solid magnesium alloy outperforms die-cast aluminum alloy under neutral salt spray. Corrosion rate of die-cast ADC12 bulk sample is 0.546 mm/y, while that of semi-solid AZ91D is 0.325 mm/y.

New energy vehicles have become new engine of global economy. However, inclusion of power batteries and electric drive systems generally increases vehicle weight by 200–500 kg compared to comparable fuel vehicles, resulting in reduced range and handling, necessitating an urgent need for lightweighting. As a key component of electric drive system, reducing weight of its housing is crucial for overall vehicle lightweighting. Magnesium alloys have a density approximately one-third lower than aluminum alloys and have already been used in fuel-powered vehicles. For example, General Motors and Ford have successfully used magnesium alloys in transmission housings, achieving weight reduction. In new energy electric drive housing sector, with magnesium prices declining and stabilizing, magnesium alloys have attracted significant industry attention as a replacement for aluminum alloys.
Currently, aluminum alloy die-castings have long dominated automotive castings. ADC12 alloy, with its excellent casting properties and cost advantages, is widely used in components such as motor housings. Bo Bing et al. systematically characterized mechanical properties and die-casting process-microstructure relationship of ADC12 steering gear housings. While magnesium alloys can follow same aluminum die-casting process, semi-solid injection molding (thixomolding) is becoming preferred technology for large structural parts to balance safety, environmental protection, oxidation and burnout, and high density requirements. Gu et al. pointed out that solid phase fraction in semi-solid injection molding process significantly affects forming ability and defects, and that an appropriate solid phase distribution can improve performance of magnesium alloys. Okayasu et al. compared magnesium alloy cold-chamber die casting, hot-chamber die casting, and semi-solid injection molding on thin-walled parts, confirming that the latter method offers the best microstructure and properties. However, previous studies have mostly focused on standard specimens or thin-walled parts, lacking in-body sampling and systematic comparisons for thick-walled structures such as electric drive housings. Therefore, this paper systematically compares microstructure, mechanical, corrosion properties of identical electric drive housings using both semi-solid injection molding of magnesium alloys and high-pressure casting of aluminum alloys, providing a reference for practical application of magnesium alloys in electric drive housings.

Experimental materials were commercial aluminum alloy ADC12 and magnesium alloy AZ91D. Specimens used for analysis were taken from a casting of an electric drive housing for a new energy vehicle, as shown in Figure 1. Aluminum housing was produced using high-pressure casting and had a measured mass of 11.345 kg; magnesium housing was produced using semi-solid injection molding and had a measured weight of 7.975 kg. Under structural consistency conditions, magnesium alloys can significantly reduce weight by 29.7% by replacing aluminum alloys.

 

Figure 1: Photos of die-cast aluminum alloy and semi-solid injection-molded magnesium alloy electric drive housings.

Table 1: Weight comparison of aluminum alloy and magnesium alloy electric drive housings.
Location where electric drive housing was sampled for analysis is shown in Figure 2; wall thickness is 10 mm. Metallographic specimens of aluminum alloy were mechanically polished and etched with a 5% HF solution, while those of magnesium alloy were etched with a nitric acid solution. Specimens were then observed under an optical microscope. IPP (Imagine Pro Plus) image analysis software was used to quantitatively analyze solid fraction in semi-solid structure. Room-temperature tensile tests were conducted on an MTS universal testing machine using tensile specimens conforming to the GB/T 228.1-2021 standard at a rate of 1 mm/min. Neutral salt spray tests were conducted using a continuous spray of a 5 wt.% NaCl solution at a concentration of 50 g/L, a pH of 6.8, and a salt spray chamber temperature of 35℃.

 

Figure 2: Casting Sampling Location and Tensile Specimen

To clarify microstructural differences between two materials in thick-walled housings, samples were cut directly from electric drive housing body and microstructural observations were performed. Figure 3 shows microstructure of high-pressure cast ADC12 aluminum alloy, revealing a small number of microporous defects. ADC12 aluminum alloy is primarily composed of α-Al, Si, and Al2Cu phases, with aluminum grains exhibiting a dendritic morphology. While high cooling rate during die casting refines grains and secondary phases to some extent, randomly distributed needle-like eutectic silicon or AlFeSi phases along dendrites disrupt matrix continuity, negatively impacting strength and ductility.

 

Figure 3 Microstructure of Die-Cast Aluminum Alloy Electric Drive Housing (50x, 200x, 500x)
Figure 4 shows metallographic structure of AZ91D magnesium alloy produced by semi-solid injection molding. Structure exhibits a typical semi-solid morphology, containing a nearly spherical primary solid phase uniformly distributed within residual liquid phase solidification structure. Measured solid fraction is 10.5%, and continuous network-like Mg₁₇Al₁₂ precipitates are visible at grain boundaries. Thanks to laminar filling characteristics of semi-solid molding, thick-walled parts have a dense internal structure with no pores, shrinkage, or thermal cracking defects. This fine, uniform, and minimally defective structure provides alloy with excellent corrosion resistance.

 

Figure 4 Microstructure of Semi-Solid Magnesium Alloy Electric Drive Housing (50x, 200x, 500x)

Figure 5 and Table 2 present tensile test curves and corresponding mechanical properties of two types of electric drive housing samples, respectively. It can be seen that both die-cast aluminum alloys and semi-solid magnesium alloys experience a decrease in performance when thick-walled. This is primarily due to reduced solidification rate, which leads to microstructure coarsening, resulting in reduced grain refinement and second-phase strengthening. Specifically, yield strength of the die-cast ADC12 aluminum alloy in bulk samples is 147 MPa, similar to that of semi-solid AZ91D magnesium alloy. In terms of tensile strength, ADC12 exhibits superior strength, demonstrating a significant work-hardening effect. However, in terms of elongation, semi-solid AZ91D alloy exhibits superior performance, reaching 2.4% even in thick-walled samples. This demonstrates that microstructure uniformity and defect suppression achieved by semi-solid forming significantly enhance plasticity. Based on both yield strength and elongation, semi-solid AZ91D alloy is a viable alternative to ADC12.

 

Figure 5: Tensile curve of bulk samples

Table 2: Tensile mechanical properties of bulk samples

It is generally believed that magnesium alloys have inferior corrosion resistance to aluminum alloys, but their corrosion behavior is ultimately controlled by both alloy composition and microstructure. When content of Cu, Fe, and other elements in aluminum alloys is high, their corrosion resistance is significantly reduced. To objectively evaluate service corrosion resistance of two shell materials, neutral salt spray (NSS) tests were conducted on body samples (uncoated and sanded to a bare state). Results are shown in Figure 6. After 24 hours of salt spray exposure, surface of die-cast ADC12 aluminum alloy showed obvious corrosion and completely lost its metallic luster, while semi-solid AZ91D sample remained intact and retained its metallic luster. After 72 hours of salt spray exposure, aluminum alloy was covered with large areas of corrosion products, while magnesium alloy only darkened its surface color with no obvious corrosion products. After 168 hours of salt spray exposure, die-cast ADC12 aluminum alloy continued to show increasing corrosion and a thicker product layer. However, semi-solid AZ91D magnesium alloy only showed slight localized corrosion and good macroscopic integrity. Corresponding average corrosion rates measured by weight loss are listed in Table 3. Corrosion rate of die-cast ADC12 aluminum alloy is 0.546 mm/y, and corrosion rate of semi-solid AZ91D magnesium alloy is 0.325 mm/y. These results demonstrate that neutral salt spray corrosion resistance of semi-solid AZ91D magnesium alloy in thick-walled electric drive housings is not inferior to that of ADC12 aluminum alloy, but rather exhibits a lower corrosion rate and superior surface integrity.

 

Figure 6 Appearance of neutral salt spray test of upper body sample of electric drive housing

Table 3 7-day neutral salt spray corrosion rate of upper body sample of electric drive housing

Based on electric drive housing with same structure, this paper compares differences in microstructure, mechanical properties and corrosion resistance between high-pressure casting ADC12 aluminum alloy and semi-solid injection molding AZ91D magnesium alloy. Main conclusions are as follows:
(1) Microstructures of die-cast aluminum alloy and semi-solid magnesium alloy are significantly different. Die-cast ADC12 presents a dendritic morphology and contains a large amount of second phase distribution such as needle-like silicon; while microstructure of semi-solid AZ91D magnesium alloy contains a nearly spherical primary solid phase, and a continuous network of Mg₁₇Al₁₂ precipitation phase can be seen at grain boundaries. Microstructure under thick wall is dense and no obvious defects are observed.
(2) In terms of mechanical properties of electric drive housing body samples, both materials have decreased compared to standard values in thick wall state. Overall, yield strength of semi-solid AZ91D magnesium alloy is very close to that of die-cast ADC12 aluminum alloy, and elongation is slightly better. However, in terms of tensile strength, ADC12 is better.
(3) In terms of corrosion resistance of electric drive housing body samples, semi-solid AZ91D magnesium alloy is better than die-cast ADC12 aluminum alloy under neutral salt spray. Bare die-cast ADC12 aluminum alloy produces obvious corrosion products on the surface after 24 hours of salt spray, while bare semi-solid AZ91D magnesium alloy sample still does not show large-scale corrosion after 168 hours of salt spray, and surface condition is good.