Aluminum alloy is considered an ideal material for lightweight automobiles due to its low density, high specific strength, easy forming, and corrosion resistance. Die casting is one of the main forming methods for preparing aluminum alloy parts, with high production efficiency and the ability to form various complex thin-walled parts. However, ordinary die casting has the characteristics of high speed and high pressure, which can easily generate turbulence, leading to gas entrapment and residual formation of pores in the casting, resulting in a decrease in the mechanical properties of the casting. In the subsequent heat treatment process, defects such as bubbles may also occur. Vacuum die casting can remove the gas in the cavity before filling, reduce the gas pressure in the cavity during filling, thereby eliminating or significantly reducing the porosity defects of the die casting and improving the casting performance.

Adding Mg and Cu elements simultaneously to Al Si alloys can achieve both the high corrosion resistance of Al Si Mg alloys and the high strength and heat resistance of Al Si Cu alloys, exhibiting excellent comprehensive mechanical properties. In order to obtain excellent mechanical properties, Al alloys usually require T6 heat treatment. However, in the actual production and manufacturing process, high-temperature solid solution treatment should be avoided to prevent foaming and dimensional deformation of aluminum alloy products. In addition, according to reports, using high-temperature solid solution treatment will almost double the cost of the final casting. Therefore, T5 treatment is crucial for the production of cast aluminum alloys. Researchers studied the low-temperature aging process of AlSi7CuMnMg die-casting alloy and found that the optimal aging process is 170 ℃ × 6h. Under these conditions, the tensile strength is 303MPa, the yield strength is 183MPa, and the elongation is 7.5%. By optimizing the content of Cu, a T5 heat treated thixotropic cast Al-7Si-0.5Mg-0.5Cu alloy was developed. It was found that the alloy had a tensile strength of 296 MPa, a yield strength of 209 MPa, and an elongation of 8.8%. Its mechanical properties were comparable to some T6 heat treated Al-7Si Mg alloys.
As the most common Al Si Mg die-casting alloy, there are few research reports on the addition of Cu element to high vacuum die-casting Al-10Si Mg Mn alloy. Therefore, this study prepared Al-10Si Mg Mn alloys with different Cu contents by high vacuum die casting, and studied the microstructure of as cast and T5 state alloys using optical microscopy (OM), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Study the effect of Cu content and T5 heat treatment on the microstructure of Al-10Si Mg Mn alloy, providing reference for optimizing the amount of Cu added in Al-10Si Mg Mn alloy.

Graphic and textual results

The raw materials used for melting alloys are Al Si Mg Mn ingots and Al-50Cu and Al-50Mg intermediate alloys. The TOYO BD-350T cold chamber die casting machine is equipped with vacuum equipment, and the actual measured vacuum degree during the die casting process is less than 5kPa. During the experiment, each material was weighed in proportion (considering burning loss). Firstly, Al Si Mg Mn alloy ingots were added to the resistance furnace and heated until the alloy melted; When the temperature of the aluminum liquid stabilizes at around 700 ℃, add Al-50Cu intermediate alloy and stir evenly, keeping it warm for 20 minutes; Then add Al-50Mg intermediate alloy, with Al-10Sr alloy as the modifier and Al-5Ti-B alloy as the refining agent, stir evenly, and maintain the temperature for 10 minutes; Add slag remover and let it stand for 10 minutes; Inject high-purity argon gas, let it stand for 15 minutes, and remove the slag. Pour at 690 ℃, preheat the die casting mold to 180 ℃, with a high-speed speed of 2m/s and a boost pressure of 80MPa. The measured composition of three sets of high vacuum die-casting alloys is shown in Table 1. The actual die-casting parts are shown in Figure 1, with four types of tensile specimens with thicknesses of 2, 4, 6, and 8mm, respectively.

It can be seen that the as cast microstructure of the three alloys is mainly composed of α - Al and Al Si eutectic phases. There are two types of α - Al grains, labeled as α 1-Al and α 2-Al, respectively. This is because the solidification process of die casting is a two-stage process. When the melted aluminum liquid is poured into the pressure chamber, the relatively low temperature inside the chamber can cool the aluminum liquid below the liquidus temperature. At this time, α 1-Al grains begin to nucleate and grow in the pressure chamber, which is also known as pressure chamber pre crystallization. These grains enter the mold cavity along with the un solidified aluminum liquid, as there is sufficient time for them to grow, resulting in a coarser final size. During the filling process of the mold cavity, due to the very fast cooling rate, smaller and rounder α 2-Al grains are formed. The eutectic Si after Sr element modification appears fibrous, and there are polygonal Fe rich phases in the eutectic region. EDS analysis confirms that it is an α - Fe phase.

Figure 6 shows the metallographic structure of Al-10Si-0.5Mn-0.4Mg xCu alloy after T5 heat treatment. It can be seen that the T5 microstructure of the three alloys is still composed of α - Al, Al Si eutectic regions, and α - Fe. Compared with the cast alloy, the morphology of α - Al, eutectic Si, and α - Fe phases has not changed significantly. Figure 7 shows the backscattered SEM image of the alloy after T5 heat treatment. Dark gray, light gray, and bright white intermetallic compounds can still be observed, and the corresponding EDS analysis results show that the intermetallic compounds are still α - Fe, Q, and θ phases, with no change in morphology. Table 3 shows the area fractions of α - Fe, Q, and θ phases in alloy 1-3 during peak aging. It can be seen that α - Fe phases are 1.13%, 1.09%, and 1.11%, respectively, while Q phases are 0.89%, 0.82%, and 0.86%, and θ phases are 0.74%, 1.66%, and 2.64%, respectively. Compared with the results of the as cast alloy, there is no significant change in the surface integral number of intermetallic compounds, indicating that T5 heat treatment will not change the type and quantity of phases.

conclusion
(1) The as cast microstructure of AlSi10CuMgMn alloy is composed of α - Al, eutectic Si and α - Fe, Q and θ phases. As the Cu content increases, the quantity of Q phase remains basically unchanged, and the morphology is mainly layered; The number of θ phases gradually increases, and the morphology changes from dispersed particles to aggregated blocks.
(2) The effect of T5 heat treatment on the as cast α - Al, eutectic Si, α - Fe, Q, and θ phases is minimal. The main nano precipitates during peak aging are β "and θ 'phases. When the Cu content is low, the precipitates are mainly β"; When the Cu content is high, the precipitate phase is mainly θ ′.

author

Liu Jinhui, Zhao Haidong
National Metal Materials Near Net Forming Engineering Technology Research Center of South China University of Technology
Gao Junmin, Li Shihua, Hou Xiaohua
Wen Can Group Co., Ltd
This article is from the magazine "Special Casting and Nonferrous Alloy" and the strategic partner of "Die Casting Weekly"