Die casting, as a high productivity and low-cost near net forming process, has been widely used in fields such as communication, automotive, and 3C. Some researchers believe that the main problem currently restricting the further expansion of die-casting technology is the porosity of castings and the strength issues they bring. In the traditional die-casting process, the liquid melt fills the mold cavity in a turbulent state, causing the gas inside the cavity to be unable to be discharged in time and rolled into the alloy, forming porosity defects, reducing the effective bearing area of the casting and causing stress concentration, thereby reducing the mechanical properties of the casting. Some people also believe that internal pores are a serious hidden danger of sudden failure during product service.
Compared with the high-temperature liquid melt used in traditional die casting, the semi-solid slurry used in semi-solid die casting with a higher solid phase ratio has higher apparent viscosity and laminar flow characteristics. It flows smoothly during high-speed filling, is less prone to gas entrapment, and the solidification shrinkage of the slurry is smaller than that of traditional liquid metals, which can reduce or eliminate defects such as porosity and shrinkage, and improve the mechanical properties of castings. In addition, due to the lower temperature of the slurry entering the pressure chamber compared to traditional liquid metal, it significantly reduces the thermal impact on the die-casting mold cavity and extends the service life of the mold. Therefore, semi-solid die casting combines the advantages of semi-solid forming and traditional die casting, and has broad application prospects in industrial applications. Semi solid die casting is mainly divided into thixotropic die casting and rheological die casting. Due to the low efficiency of secondary heating, rheological die casting has become a research hotspot in the field of semi-solid machining, and its industrial application has received special attention.
The preparation of high-quality semi-solid slurry is a prerequisite and key to the development of rheological die casting technology. In recent years, various semi-solid slurry preparation technologies have been proposed both domestically and internationally. Different researchers have developed double helix shearing technology, which prepares semi-solid slurries by stirring the melt at high shear rates using a pair of high-speed rotating screws; Developed RSF pulping technology, using entropy exchange materials as cooling media to absorb heat from metal melts and prepare semi-solid slurries; The SEED pulping process was proposed, which involves eccentric rotation of the preparation crucible under low superheat casting conditions to generate effective shear in the melt, suppress the growth of primary phase dendrites, and thus prepare a semi-solid slurry; Developed GISS technology, introducing inert gas during melt solidification and utilizing bubble disturbance to prepare semi-solid slurry; The development of a vibration inclined plate process for preparing semi-solid slurry is believed to be the combined effect of nucleation thermodynamic conditions and vibration shear collision; Developed LSPSF pulping process, pouring alloy melt into the inlet of a rotating conveying pipe. Under the combined action of gravity and shear/cooling of the inner wall of the rotating conveying pipe, the alloy transforms from a molten state to a semi-solid slurry with a certain solid phase ratio; SCP technology has been developed to pour superheated melt into a vertical serpentine channel for cooling, and to prepare semi-solid slurry by utilizing the disturbance caused by its own gravity. These processes enrich the preparation technology of semi-solid slurry and promote the development and application of rheological die-casting technology. In order to prepare high-quality semi-solid pulp more stably, continuously, and efficiently, and to meet the promotion of rheological die casting industrialization and break through foreign pulp technology patent protection, it is necessary to develop some new simple, efficient, practical, and low-cost pulp making technologies.
In view of this, an Air Cool Stirring Rod (ACSR) pulping process was proposed to achieve continuous and rapid preparation of large volume semi-solid slurry, and closely connected with the die-casting machine to form a rheological die-casting process that integrates slurry preparation, transportation, and forming. This study mainly introduces the industrialization status of ACSR rheological die casting process, and uses Al-8Si alloy and Al-6Si alloy (Sr modified) as raw materials, combined with the production of shell parts for 4G/5G communication base stations and new energy vehicles, and compares it with traditional die casting to study the influence of ACSR rheological die casting process on the microstructure and properties of alloys.
Graphic and textual results
The experimental materials are Al-6Si, Al-8Si high thermal conductivity alloys, and Al-7Si-4Cu-0.2Cd high-strength and tough aluminum alloy. The chemical composition is shown in Table 1. Using SETARAM TGA-92 high-temperature comprehensive thermal analyzer to perform differential thermal analysis on the heating process of the alloy, the liquidus and solidus temperatures of Al-8Si alloy were obtained to be 623 ℃ and 565 ℃, respectively. The liquidus and solidus temperatures of Al-6Si alloy were 635 ℃ and 570 ℃, respectively. The liquidus and solidus temperatures of Al-7Si-4Cu-0.2Cd alloy were 607 ℃ and 518 ℃, respectively. Figure 1 shows the schematic diagram and physical image of the equipment for uniform solidification control technology of aluminum alloy, mainly composed of high-pressure gas supply device, air duct, stirring rod, aluminum alloy melt, crucible, and thermocouple. The size of the stirring rod is determined based on the volume of the aluminum alloy melt and the size of the ladle. Currently, the aluminum alloy ACSR pulping device has been connected to various tonnage die-casting machines, with specific matching die-casting machine locking forces of 4000, 8500, 12500, 16000, 20000, 30000, and 40000 kN. In addition, the ACSR pulping device can be combined with a vacuum mechanism to prepare high-quality semi-solid pulp in a vacuum environment.
With the increasing integration of signal electrical components in 5G wireless base stations, communication equipment is developing towards ultra-thin and lightweight, high heat dissipation, high mechanical performance, and high corrosion resistance. By combining ACSR pulping technology with die-casting technology, a new high-performance large-scale thin-walled die-casting production line has been established, which integrates uniform solidification control processing, conveying, die-casting, and part extraction of aluminum alloys, is fully automatic and efficient, and can meet the application requirements of communication equipment structural components. At present, this technology has been applied to important high-quality die-casting structural components such as 4G/5G wireless base station heat dissipation shells, filters, shielding boxes, and installation brackets. Figure 3 shows several typical large thin-walled die-casting parts for high-performance communication of Al-8Si and Al-6Si aluminum alloys produced using this technology.
It can be seen that there are many dendritic α - Al in the structure of traditional die castings, and many shrinkage and porosity defects can be observed. However, a large number of small nearly spherical primary α - Al grains can be observed in the microstructure of ACSR rheological die castings, and internal defects are significantly reduced. In addition, secondary α - Al grains and eutectic Si in ACSR rheological die castings have also been refined to a certain extent. According to relevant research, the Fe rich phase in ACSR rheological die cast aluminum alloy is uniformly distributed in the eutectic structure, and its average size is smaller than that of traditional die cast alloys.
conclusion
(1) We have developed a stable and efficient integrated process for semi-solid slurry preparation and die-casting forming, namely ACSR rheological die-casting process. We have established multiple new high-performance rheological die-casting production lines that integrate uniform solidification control of aluminum alloy slurry preparation, transportation, die-casting, and component extraction, fully automatic, and efficient. This has achieved the industrialization of high-quality large-scale thin-walled aluminum alloy structural components through rheological die-casting.
(2) Compared to traditional die casting, aluminum alloy castings produced by ACSR rheological die casting process have better surface quality and lower porosity, with a flatness of only 0.20-0.25mm, surface roughness reduced to 3.2 μ m, and porosity reduced to 0.9%.
(3) Compared with traditional die cast alloys, ACSR rheological die cast aluminum alloys have superior mechanical and thermal conductivity properties, with tensile strength and elongation increased by 20%~22% and 34%~75%, respectively, and thermal conductivity increased by 7.5%~10.1%.
Author of this article:
Qi Mingfan, Zhu Guoming, Kang Yonglin
School of Materials Science and Engineering, Beijing University of Science and Technology
Wang Jicheng, Zhang Ying, Li Gunan
Zhuhai Runxingtai Electric Appliance Co., Ltd
Zhang Guangjin
Shandong Detai Machinery Manufacturing Group Co., Ltd
This article comes from: "Special Casting and Nonferrous Alloys" magazine, "Die Casting Weekly" strategic partner
