Study on the properties of high-strength die cast aluminum alloys

High-strength properties of die cast aluminum alloys play a key role in expanding the application areas of die castings

Title: Microstructure and Mechanical Properties of High-Strength Al-Si-Mg-Cu Alloy for Die Casting

Abstract: A new type of Al-Si-Mg-Cu alloy for die casting was designed, developed, and subjected to natural aging and artificial aging treatments at various temperatures. The influence of alloying elements and aging treatments on the alloy’s mechanical properties was analyzed through mechanical testing and metallographic observation. The results showed that increasing the magnesium content in the die cast aluminum alloys, along with the addition of copper, zinc, titanium, and other elements, followed by Sr modification and aging treatment, led to significant improvements in tensile strength and elongation after fracture. The as-cast tensile strength of the optimized alloy exceeded 310 MPa, with a maximum elongation of 5.9%. Under fixed alloy composition, artificial aging enhanced both tensile strength and elongation. However, when aging temperatures were raised within the range of 140 to 170°C, both tensile strength and elongation exhibited a decreasing trend. After a low-temperature aging treatment at 140°C for 8 hours, the material exhibited excellent comprehensive mechanical properties, with an average tensile strength of 377.08 MPa and an average elongation of 3.42%.

In recent years, due to advancements in die casting technology and the demand for automotive lightweighting, more and more automotive components are being produced using aluminum alloy die casting. These components include automotive engine cylinder blocks, camshaft brackets, wheel hubs, transmission casings, and more. With the continuous development of die casting processes, there is a growing need for die-casting alloys with excellent comprehensive properties. The earliest applied die-casting aluminum alloy was the Al-Si binary eutectic alloy, which exhibited good fluidity, air tightness, low shrinkage, and reduced susceptibility to thermal cracking. It also demonstrated favorable mechanical, physical, and corrosion-resistant properties, making it suitable for casting thin-walled parts and components that require compactness but are not under high stress, such as automotive instrument attachments, covers, casings, and cylinder bodies with cooling fins. For castings with high machining requirements, Al-Si-Mg series, Al-Si-Cu series, and Al-Si-Cu-Mg series alloys are generally used. Among these, the Al-Si series alloy is the most commonly used, with main alloying elements including Si, Mg, Cu, Ti, Zn, Fe, and possibly Cr, Zr, V, Sr, B, Sb, and RE elements. Each element has independent and interactive effects on the alloy properties.

In ordinary die casting, due to the rapid filling of the mold cavity by the molten metal, gases within the cavity have minimal opportunity to escape and become entrapped in the casting, resulting in the formation of numerous pores. These gases, upon subsequent solution treatment, are prone to thermal expansion, leading to swelling and deformation of the casting. This prevents effective solution treatment and limits performance improvement. However, by subjecting die castings to acceptable lower-temperature aging treatments, strength can be enhanced, stress relieved, dimensions stabilized, and material properties optimized.

In this study, a novel Al-Si-Mg-Cu alloy for die casting was designed, developed, and subjected to natural aging and artificial aging treatments. The influence of alloying elements and aging treatments on the mechanical properties of the castings was analyzed, aiming to provide reference for its application.

01 Experimental Content

1.1 Test Material and Chemical Composition

The nominal composition of the new die cast aluminum alloys (in mass fractions, hereinafter the same) is: 9% Si, 0.5% Mg, 0.5% Mn, 0.5% Cu, 0.5% Zn, 0.15% Ti, 0.02% Sr, Fe: ≤0.3%, with the balance being Al. Alloying elements or metals added during alloy melting include Al-5Ti-B, Al-10Sr, Mg-10Al, pure zinc, pure copper, etc., and were melted in an induction resistance furnace. When the alloy liquid temperature reached 720°C, nitrogen gas was introduced for degassing treatment. To prevent iron pickup during melting, a graphite crucible was used. Considering the volatilization of certain metal elements during alloying and the actual yield of other metal elements, the measured chemical composition closely matched the design values, as determined by direct-reading spectrometer, as shown in Table 1.

Study on the Properties of A High-Strength Aluminum Alloy for Die Casting1

Table 1 Chemical composition of newdie cast aluminum alloys (%)

1.2 Preparation of die-casting specimens

Die casting specimen with reference to the standard design, specimen see Figure 1, specimen size see Figure 2, diameter of 6.4 mm, 50 mm pitch, total length of 170 mm. horizontal cold chamber die casting machine, punch diameter of 40 mm. die casting process parameters: pouring temperature of 660 ~ 670 ℃, mold temperature of 150 ℃.

Study on the Properties of A High-Strength Aluminum Alloy for Die Casting12


Fig. 1 Diagram of die-casting specimen

Study on the Properties of A High-Strength Aluminum Alloy for Die Casting13

Fig. 2 Dimensions of round test bars for die casting mechanical properties

1.3 Aging Treatment

To compare the effects of natural aging and artificial aging on the alloy’s mechanical properties, 5 specimens were chosen for each treatment. Natural aging was conducted for 90 days, and artificial aging was carried out at temperatures of 140°C, 155°C, and 170°C for 8 hours each.

1.4 Mechanical Property Testing

Tensile testing was conducted on a universal testing machine, and elongation was measured using an extensometer. The tests were conducted at room temperature with a loading rate of 5 mm/min, and each group consisted of 5 specimens for which the average values were calculated.

1.5 Microstructural Observation

Identical regions of the specimens were selected for metallographic analysis. The specimens were sequentially polished using sandpapers of grit sizes 400, 800, 1200, and 2000. Subsequently, they were polished using 2.5μm and 1.5μm diamond pastes. Finally, they were immersed in a corrosion solution consisting of 1% HF, 1.5% HCl, and 2.5% HNO3 for 10 to 20 seconds, followed by thorough cleaning with ethanol. Prepared metallographic samples were observed under a metallographic microscope and scanning electron microscope (SEM) to study their microstructure. Elemental distribution in localized regions was determined using energy-dispersive X-ray spectroscopy (EDS) analysis.

02 test results and discussion

2.1 Alloy microstructure

2.1.1 Effect of alloy elements on microstructure


Fig. 3 Photographs of metallographic organization of YL104 alloy and Al-Si-Mg-Cu alloy

Figure 3 shows the organization comparison between YL104 die-casting alloy and Al-Si-Mg-Cu alloy, it can be seen that the white organization in the two alloys is α-Al matrix, and the gray organization between the dendrites is the eutectic organization, but the α-Al matrix in the Al-Si-Mg-Cu alloy is much more rounded, and the size is obviously smaller, and the smallest size can be up to 3 μm. in which a part of the coarse α-Al can be found in Figure 3c. This part of coarse dendrites should be formed in the press chamber, where they solidify, grow up, and are finally pressed into the cavity by the punch. Due to the limitation of magnification, there are still some black unknown tissues that need to be analyzed more minutely.

According to the Al-Si-Cu phase diagram, the phase composition of the alloy after solidification is [α-Al+Si+Mg2Si+Al5Mg8Cu2Si6] when the Mg content is 0.5% and the Cu content is ≤0.5%. Due to the low content of alloying elements such as Mg, Cu, Zn, etc., most of the added alloying elements such as Mg, Cu, Zn, etc., are dissolved in Al matrix, so it is difficult to detect the characteristic phases formed by the above elements in microstructure photos. It is difficult to find the characteristic phases formed by the above elements. Through EDS analysis, it is found that Mg, Cu, Zn, Si and other alloying elements are dissolved in the α-Al matrix, which plays the role of solid solution strengthening, see Figure 4.


Fig. 4 Microstructure and EDS analysis of Al-Si-Mg-Cu alloys

Further analysis using SEM revealed the presence of dark-colored fibrous structures and blocky bright gray structures, as shown in Figure 5. EDS analysis (with mass fractions of 82.2% for Al and 16.2% for Si) indicated that the fibrous structure corresponds to eutectic silicon (Si). Due to the addition of Sr during the alloy melting phase, the originally elongated plate-like eutectic Si transformed into a fine fibrous structure due to Sr modification. The size of the refined eutectic silicon was around 2μm. The addition of Sr significantly weakened the cracking effect of eutectic Si on the matrix. The bright gray structures correspond to the iron (Fe) phase.

In aluminum-silicon alloys, iron phases primarily exist as the Chinese character-shaped α-AlFeSi and the needle-like β-AlFeSi phases. However, based on SEM observations, the present Fe phase appeared as blocky, suggesting that it is not one of the above-mentioned phases. According to the research by Lin Chong et al., the addition of Mn to Al-Si alloys results in the formation of blocky Fe phase Al4(Fe,Mn)Si2, which transforms into blocky Al5(Fe,Mn)Si after T6 heat treatment. In the current study, the alloy had a Si content of 9%. Based on EDS spectra analysis (with molar fractions of 74.52% for Al, 4.49% for Fe, 8.23% for Mn, and 12.11% for Si), coupled with the morphology of the iron phase, it was identified as Al5(Fe,Mn)Si. The presence of Mn improved the morphology, leading to a significant reduction in its crack-promoting effect on the matrix. In conclusion, the microstructure of the as-cast alloy is primarily composed of refined α-Al, fibrous eutectic Si, and blocky Fe phase.


Fig. 5 SEM photos and EDS analysis of Al-Si-Mg-Cu alloy

2.1.2 Effect of aging treatment on microstructure


Fig. 6 Microstructure and elemental distribution of Al-Si-Mg-Cu alloy after aging at 140°C: (a) Metallographic photo (b) SEM photo (c) Al elemental distribution (d) Si elemental distribution (e) Mg elemental distribution

Figure 6 depicts the microstructure and elemental distribution of the Al-Si-Mg-Cu alloy after aging at 140°C. It can be observed that the primary microstructure after aging consists mainly of primary α-Al and eutectic Si phases. A comparison with Figure 3b indicates that aging has minimal impact on the morphology and size of primary α-Al; both exhibit fine α-Al dendrites along with a few coarse α-Al dendrites. SEM analysis (Figure 6b) reveals the presence of dispersed black regions within the α-Al matrix. EDS analysis (Figure 6c, Figure 6e) identifies these regions as primarily composed of Al and Mg. In conjunction with previous research by Hu Zhiqi et al., it can be inferred that the β-Al8Mg5 phase, formed in Al-Mg-Si alloys, is re-dissolved into the aluminum matrix through aging treatment, contributing to solid-solution strengthening.

EDS analysis of the aged microstructure is shown in Figure 7. From Figure 7a, a grayish ring-like feature is visible within the eutectic Si regions, mainly composed of Si and Mg. This is identified as the Mg2Si phase. Irregular-shaped gray regions are also present in the upper-left corner and central region of Figure 7a. EDS analysis (Figure 7b) coupled with the elemental distribution reveals that these regions consist of Al, Mg, Si, and Cu, with a molar ratio of Mg to Cu at 4:1. This leads to the identification of the Al5Mg8Cu2Si6 phase. During die casting, elements such as Mg and Cu, due to their low content, can fully dissolve into the matrix. Through aging treatment, these dissolved strengthening phases precipitate, contributing to dispersion strengthening. Zinc (Zn), with high solubility in aluminum, mainly serves as a solid-solution strengthening element.

In conclusion, the microstructure of the aged alloy primarily consists of fine α-Al, uniformly distributed passive eutectic Si, dispersed Mg2Si phases, and a small amount of Al5Mg8Cu2Si6 phases.


Figure 7: Microstructure and EDS Analysis of Al-Si-Mg-Cu Alloy After Aging (a) SEM Image (b) EDS Analysis Spectrum (c) Distribution of Cu Element (d) Distribution of Mg Element (e) Distribution of Si Element (f) Distribution of Al Element

2.2 Mechanical Properties

Tensile testing was performed on as-cast, naturally aged, and artificially aged specimens of the optimized alloy using an electronic universal testing machine, and the results are shown in Figure 8. It can be observed that the average tensile strength of the as-cast alloy is 306.316 MPa, with an average elongation of 4.262%, which is an improvement of 39.1% and 100% compared to YL104 alloy. This improvement can be attributed to the higher content of Mg, Cu, and Zn, as well as the addition of small amounts of titanium and strontium in the Al-Si-Mg-Cu multi-component alloy compared to YL104 alloy.

Most of the Mg, Cu, and Zn dissolve in the α-Al matrix. During the die casting process, the rapid cooling rate prevents complete precipitation of the alloying elements dissolved in the α-Al matrix, leading to solid-solution strengthening within the matrix. When the Mg and Cu contents are increased from 0.3% to 0.5%, Mg can react with Si to form the Mg2Si phase. Additionally, when the Cu/Mg mass ratio is below 2.5, the strengthening phase Al5Mg8Cu2Si6 forms. This strengthening phase precipitates during aging treatment, contributing to dispersion strengthening and enhancing the alloy’s strength.

The addition of titanium forms TiAl3, providing heterogeneous nucleation sites and refining the grain size when present at 0.15%. The addition of 0.02% strontium for modification treatment transforms the coarse needle-like eutectic Si into fine fibrous structures, reducing eutectic Si’s propensity for cracking the matrix and greatly improving the alloy’s ductility.

The presence of Fe can alleviate sticking during die casting, but it is detrimental to the alloy’s mechanical properties. By controlling the Fe content to within 0.3% and introducing 0.5% Mn, the iron phase’s shape transforms from needle-like or plate-like to small blocks with minimal matrix-cracking effects. The addition of Mn significantly reduces the negative impact of the iron phase, thus enhancing the alloy’s performance.


Figure 8: Mechanical Properties of the Alloy After Aging

2.3.2 Effects of Aging Treatment on Alloy Performance

After natural aging, the average tensile strength of the specimens was 321.077 MPa, with an average elongation of 2.842%. Upon artificial aging at different temperatures, it was found that the specimens aged at 140°C exhibited the highest average tensile strength, reaching 377.081 MPa, with an average elongation of 3.42%. Tensile strength decreased with increasing artificial aging temperature, and elongation also decreased as the aging temperature increased.

The main phases in the Al-Si-Mg-Cu alloy after solidification are [α-Al + Si + Mg2Si + Al5Mg8Cu2Si6]. Aging treatment had little effect on the morphology and size of the α-Al phase, but it promoted atomic mobility and diffusion, improving the alleviation of local composition segregation and stress caused by rapid local cooling during casting. It also relieved residual stresses within the casting, reducing local stress concentration. This simultaneous increase in alloy strength and maintenance of elongation stability was observed. Furthermore, aging treatment resulted in the precipitation of Mg2Si and a small amount of Al5Mg8Cu2Si6 phases. These strengthening phases dispersed at grain boundaries, contributing to dispersion strengthening and increased strength.

With increasing aging temperature, more precipitation of strengthening phases occurred, leading to higher artificial aging strength compared to natural aging. However, when the temperature exceeded a certain point, phase coarsening and aggregation could occur, disrupting the integrity of the matrix and causing a decrease in strength. Therefore, strength decreased with rising temperature. An increase in precipitate phases led to greater resistance during deformation, causing reduced alloy ductility and a subsequent decrease in elongation with increasing aging temperature.


The microstructure of the new die-casting aluminum alloy primarily consists of α-Al and fibrous eutectic Si. The α-Al grains are fine, with a minimum size of 3μm, and the eutectic Si morphology changes to fibrous, with a size less than 2μm.

Compared to YL104 alloy, the alloy exhibits a 39.1% increase in tensile strength and a twofold increase in elongation, showcasing significantly improved mechanical properties.

After aging, small amounts of Mg2Si and Al5Mg8Cu2Si6 phases are detected in the metallographic structure. Concurrently, stress concentration within the casting is alleviated, resulting in increased strength alongside relatively stable elongation.

The tensile strength and elongation of the alloy decrease with increasing artificial aging temperature beyond 140°C. Therefore, the suitable aging temperature is 140°C, which results in a 23.2% increase in tensile strength (reaching 377 MPa) compared to the as-cast state, albeit with a decrease in elongation from 4.26% to 3.42%.


Ningbo Ruiyu Metal Products Co., Ltd. is located in the southern part of Yinzhou District, Ningbo City, Zhejiang Province, China. We are situated 30 kilometers away from Ningbo Port, and our factory covers an area of 3,000 square meters. With a workforce of over 30 employees, our team possesses more than 20 years of experience in the die-casting industry.

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