Aluminum alloy has characteristics of low density, high specific strength, easy forming and corrosion resistance, and is considered to be an ideal material for lightweight automobiles. Die casting is one of main forming methods for preparing aluminum alloy parts. It has high production efficiency and can form a variety of complex thin-walled parts. However, ordinary die casting has characteristics of high speed and high pressure, which is easy to produce turbulence, causing gas to be involved and remaining in casting to form pores, thereby causing mechanical properties of casting to decrease, defects such as blistering will occur during subsequent heat treatment process. Vacuum die casting can remove gas in cavity before filling, reduce cavity gas pressure during filling, thereby eliminating or greatly reducing hole defects of die castings and improving performance of castings.
Adding Mg and Cu elements to Al-Si alloys at the same time can combine high corrosion resistance of Al-Si-Mg alloys with high strength and high heat resistance of Al-Si-Cu alloys, and have good comprehensive mechanical properties. In order to obtain excellent mechanical properties, Al alloys usually require T6 heat treatment. However, in actual production process, high-temperature solution treatment should be avoided to prevent blistering and dimensional deformation of aluminum alloy products. In addition, it is reported that use of high-temperature solution treatment almost doubles cost of final casting. Therefore, T5 treatment is essential for production of cast aluminum alloys. Researchers studied low-temperature aging process of AlSi7CuMnMg die-casting alloys and found that optimal aging process was 170℃×6h. Under this condition, tensile strength was 303MPa, yield strength was 183MPa, and elongation was 7.5%. By optimizing Cu content, a T5 heat-treated thixotropic casting Al-7Si-0.5Mg-0.5Cu alloy was developed. It was found that alloy had a tensile strength of 296MPa, a yield strength of 209MPa, and an elongation of 8.8%. Mechanical properties are comparable to those of some Al-7Si-Mg alloys after T6 heat treatment.
As the most common Al-Si-Mg die-casting alloy, there are few reports on addition of Cu elements to high vacuum die-casting Al-10Si-Mg-Mn alloys. Therefore, this study prepared Al-10Si-Mg-Mn alloys with different Cu contents by high vacuum die-casting, studied microstructure of cast and T5 alloys by optical microscopy (OM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Effects of Cu content and T5 heat treatment on microstructure of alloy were studied to provide a reference for optimizing addition of Cu in Al-10Si-Mg-Mn alloys.
Graphical results
Raw materials used for smelting alloys are Al-Si-Mg-Mn ingots and Al-50Cu and Al-50Mg master alloys. A TOYO BD-350T cold chamber die-casting machine equipped with vacuum equipment is used. Actual vacuum degree measured during die-casting process is less than 5kPa. During test, each material is weighed according to proportion (considering burnout). First, Al-Si-Mg-Mn alloy ingot is added to resistance furnace and heated until alloy is melted; when aluminum liquid temperature is stabilized at about 700℃, Al-50Cu master alloy is added and stirred evenly, and temperature is kept for 20 minutes; then Al-50Mg master alloy is added, modifier is Al-10Sr alloy, refiner is Al-5Ti-B alloy, mixture is stirred evenly and kept warm for 10 minutes; slag remover is added and mixture is left to stand for 10 minutes; high-purity argon is introduced, mixture is left to stand for 15 minutes, and slag is removed. Casting was carried out at 690℃, die casting mold was preheated to 180℃, high speed was 2m/s, and boost pressure was 80MPa. Measured composition of three groups of high vacuum die casting alloys is shown in Table 1. Actual die castings are shown in Figure 1, thickness of four tensile specimens is 2, 4, 6 and 8mm respectively.

Table 1 Chemical composition of test alloy (%)

 

Figure 1 Actual picture of die casting specimen

 

Figure 2 Dimensions of tensile specimen
(a) No. 1 (b) No. 2 (c) No. 3

 

Figure 3 Cast metallographic structure of high vacuum die casting Al-10Si-0.5Mn-0.4Mg-xCu alloy
It can be seen that cast structure of three groups of alloys is mainly composed of α-Al and Al-Si eutectic phases. There are two types of α-Al grains, marked as α1-Al and α2-Al. This is because die casting solidification process is a two-stage process. When molten aluminum liquid is poured into pressure chamber, relatively low temperature in pressure chamber can cool aluminum liquid to below liquidus temperature. At this time, α1-Al grains begin to nucleate and grow in pressure chamber, so it is also called pressure chamber pre-crystallization. These grains enter mold cavity together with unsolidified aluminum liquid. Because they have enough time to grow, final size is relatively coarse. During mold cavity filling process, due to very fast cooling rate, smaller and more rounded α2-Al grains are formed. Eutectic Si after Sr element modification is fibrous, and there is a polygonal Fe-rich phase in eutectic area. It is confirmed to be an α-Fe phase through EDS energy spectrum analysis.

 

(a) SEM No. 1 (b) SEM No. 2 (c) SEM No. 3

 

(d) EDS1 (e) EDS2 (f) EDS3
Figure 4 SEM morphology and EDS results of high vacuum die-cast Al-10Si-0.5Mn-0.4Mg-xCu alloy as-cast

Table 2 Statistical results of area fraction of intermetallic compounds in high vacuum die-cast Al-10Si-0.5Mn-0.4Mg-xCu alloy as-cast

 

Figure 5 Average hardness and aging time curve of high vacuum die-cast Al-10Si-0.5Mn-0.4Mg-xCu alloy
Figure 6 shows metallographic structure of Al-10Si-0.5Mn-0.4Mg-xCu alloy after T5 heat treatment. It can be seen that T5 state metallographic structure of three groups of alloys is still composed of α-Al, Al-Si eutectic area and α-Fe. Compared with cast alloy, morphology of α-Al, eutectic Si phase and α-Fe phase has not changed significantly. Figure 7 is a backscattered SEM image of alloy after T5 heat treatment. Dark gray, light gray and bright white intermetallic compounds can still be observed. Corresponding EDS analysis results show that intermetallic compounds are still α-Fe, Q and θ phases, and morphology has not changed. Table 3 shows area fractions of α-Fe, Q and θ phases in alloys 1 to 3 at peak aging. It can be seen that α-Fe phase is 1.13%, 1.09% and 1.11% respectively, Q phase is 0.89%, 0.82% and 0.86% respectively, θ phase is 0.74%, 1.66% and 2.64% respectively. Compared with results of cast alloy, area fraction of intermetallic compound has not changed significantly, indicating that T5 heat treatment will not change phase type and quantity.

 

(a) No. 1 (b) No. 2 (c) No. 3
Figure 6 High vacuum die-cast Al-10Si-0.5Mn-0.4Mg-xCu alloy T5 state metallographic structure
(a) No. 1 SEM (b) No. 2 SEM (c) No. 3 SEM

 

(d) EDS4 (e) EDS5 (f) EDS6

 

Figure 7 High vacuum die-cast Al-10Si-0.5Mn-0.4Mg-xCu alloy T5 state SEM morphology and EDS results

Table 3 Statistical results of area fraction of intermetallic compounds in high vacuum die-cast Al-10Si-0.5Mn-0.4Mg-xCu alloy T5 state

 

Figure 8 High vacuum die-cast Al-10Si-0.5Mn-0.4Mg-xCu alloy T5 state TEM bright field phase and corresponding SADP image

 

Figure 9 HRTEM image and corresponding FFT image

Table 4 Statistical results of T5 precipitation phases in high vacuum die-cast Al-10Si-0.5Mn-0.4Mg-xCu alloy
Conclusion
(1) As-cast microstructure of AlSi10CuMgMn alloy is composed of α-Al, eutectic Si and α-Fe, Q and θ phases. With increase of Cu content, amount of Q phase remains basically unchanged, and morphology is mainly lamellar; amount of θ phase gradually increases, and morphology changes from dispersed granular to aggregated block.
(2) T5 heat treatment has little effect on as-cast α-Al, eutectic Si, α-Fe, Q and θ phases. Main nano-precipitated phases during peak aging are β″ and θ′ phases. When Cu content is low, precipitation phase is mainly β″; when Cu content is high, precipitation phase is mainly θ′.