As a high-productivity, low-cost near-net-shape process, die-casting is widely used in communications, automobiles, 3C and other fields. Some researchers believe that main problem currently restricting further expansion of application of die-casting technology is porosity of castings and resulting strength issues. In traditional die-casting process, liquid melt fills mold cavity in a turbulent state, so that gas in the cavity cannot be discharged in time and is involved in alloy, forming pore defects, reducing effective bearing area of casting and causing stress concentration, thus reducing mechanical properties of casting. Some people also believe that internal pores are a serious hidden danger of sudden failure during product service.
Compared with high-temperature liquid melt used in traditional die-casting, semi-solid slurry with a higher solid phase ratio used in semi-solid die-casting has higher apparent viscosity and laminar flow characteristics, flows smoothly during high-speed filling process, and is not easy to entrain air. Solidification shrinkage of slurry is smaller than that of traditional liquid metal. It can reduce or eliminate defects such as pores and shrinkage, and improve mechanical properties of castings. In addition, since temperature of slurry entering pressure chamber is lower than that of traditional liquid metal, thermal shock to die-casting mold cavity is greatly reduced and service life of mold is extended. Therefore, semi-solid die casting takes into account 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 low secondary heating efficiency of thixotropic die-casting, rheological die-casting has become a research hotspot in the field of semi-solid processing, and its industrial application has received special attention.
Preparation of high-quality semi-solid slurry is premise and key to development of rheological die-casting technology. In recent years, a variety of semi-solid slurry preparation technologies have been proposed at home and abroad. Different researchers have developed double-helix shear technology, which uses a pair of high-speed rotating screws to stir melt at high shear rates to prepare semi-solid slurry; developed RSF pulping technology, using entropy exchange materials as cooling media to absorb heat from metal melt to prepare semi-solid slurry; SEED slurry process was proposed. Under low superheat pouring conditions, preparation crucible was eccentrically rotated to produce effective shearing in melt, inhibiting growth of primary phase dendrites, thereby preparing a semi-solid slurry; developed GISS technology, which introduces inert gas when melt solidifies, and uses bubble disturbance to prepare semi-solid slurry; vibrating tilted plate process was developed to prepare semi-solid slurry, which is believed to be combined effect of nucleation thermodynamic conditions and vibration shear collision; LSPSF slurrying process was developed. Alloy melt is poured into the entrance of rotating conveyor pipe. Under combined effects of gravity and shearing/cooling of inner wall of rotating conveyor pipe, alloy is transformed from molten state into a semi-solid slurry with a certain solid phase ratio; SCP technology was developed, in which superheated melt is poured into a vertical serpentine channel to cool down, and disturbance caused by its own gravity is used to prepare a semi-solid slurry. These processes enrich semi-solid slurry preparation technology, promote development and application of rheological die-casting processes. In order to prepare high-quality semi-solid slurry more stably, continuously and efficiently, cater to industrial promotion of rheological die-casting and break through patent protection of foreign pulping processes, it is necessary to develop some new simple, efficient, practical and low-cost pulping technologies.
In view of this, an air-cooled stirring rod slurry process (Air-Cooled Stirring Rod, ACSR) is proposed to achieve continuous and rapid preparation of large-volume semi-solid slurry, and is closely connected with die-casting machine to form a rheological die-casting process that integrates slurry preparation, transportation and forming. This study mainly introduces industrialization status of ACSR rheological die-casting process. At the same time, Al-8Si alloy and Al-6Si alloy (Sr modification) are used as raw materials, combined with production of shell parts for 4G/5G communication base stations and new energy vehicles, compared with traditional die-casting, impact of ACSR rheological die-casting process on alloy structure and properties is studied. .

Test materials are Al-6Si, Al-8Si high thermal conductivity alloy, Al-7Si-4Cu-0.2Cd high-strength and tough aluminum alloy. Chemical composition is shown in Table 1. Using SETARAM TGA-92 high temperature comprehensive thermal analyzer to perform differential thermal analysis on heating process of alloy, it can be obtained that liquidus temperature and solidus temperature of Al-8Si alloy are 623℃ and 565℃ respectively, liquidus and solidus temperature of Al-6Si alloy are 635℃ and 570℃ respectively, liquidus and solidus temperatures of Al-7Si-4Cu-0.2Cd alloy are 607℃ and 518℃ respectively. Figure 1 shows equipment schematic and physical diagram of aluminum alloy uniform solidification control technology. It mainly consists of a high-pressure gas supply device, an air guide pipe, a stirring rod, an aluminum alloy melt, a crucible and a thermocouple. Size of stirring rod is determined based on volume of aluminum alloy melt and size of scooping spoon. At present, aluminum alloy ACSR slurrying device has been connected with various tonnage die-casting machines. Specific supporting die-casting machine clamping forces are 4000, 8500, 12500, 16000, 20000, 30000 and 40000kN. In addition, ACSR pulping device can be combined with a vacuum mechanism to prepare high-quality semi-solid slurry in a vacuum environment.

Table 1 Chemical composition of alloy (%)

 

Figure 1 Schematic diagram and physical picture of equipment for preparing semi-solid slurry using aluminum alloy uniform solidification control technology

 

Figure 2 Aluminum alloy ACSR rheological die casting process
With increase in integration of signal electrical devices in 5G wireless base stations, communication equipment is developing in direction of ultra-thin and ultra-light, high heat dissipation, high mechanical properties and high corrosion resistance. Combining ACSR pulping technology with die-casting technology, a brand-new, fully automatic, efficient and high-performance large-scale thin-walled die-casting production line integrating uniform solidification control processing, transportation, die-casting and picking-up of aluminum alloys has been established to meet application requirements of communication equipment structural parts. At present, this technology has been applied to important high-quality die-cast structural parts such as 4G/5G wireless base station cooling casings, filters, shielding boxes and mounting brackets. Figure 3 shows several typical large thin-walled die castings of Al-8Si and Al-6Si aluminum alloys for high-performance communications produced using this technology.

 

(a) 5G communication heat sink 1 Al-8Si

 

(b) 5G communication heat sink 2 Al-6Si

 

(c) 5G communication heat sink 3 Al-6Si
Figure 3 Several typical Al-8Si and Al-6Si aluminum alloy high-quality large thin-walled rheological die castings for 5G communications

 

(a) New energy vehicle end cover

 

(b) Car steering gear connecting pipe

 

Figure 4 Several typical high-quality automotive ACSR rheological die castings

Table 2 Comparison of surface quality and porosity of aluminum alloy 5G communication casings produced by traditional die casting and rheological die casting

 

(a) Al-8Si alloy, traditional die casting    (b) Al-8Si alloy, ACSR rheological die casting

 

(c) Al-6Si alloy, traditional die casting      (d)Al-6Si alloy, ACSR rheological die casting

 

(e) Al-7Si-4Cu-0.2Cd alloy, traditional die casting   (f)Al-7Si-4Cu-0.2Cd alloy, ACSR rheological die casting
Figure 5 Microstructure of conventional die-casting and ACSR rheological die-casting Al-8Si, Al-6Si and Al-7Si-4Cu-0.2Cd alloy samples
It can be seen that there are more dendritic α-Al in structure of traditional die castings, more shrinkage cavities and shrinkage porosity defects can be observed. However, a large number of small, nearly spherical primary α-Al grains can be observed in structure 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, it can be seen that Fe-rich phase in ACSR rheological die-casting aluminum alloy is evenly distributed in eutectic structure, and average size is smaller than that of traditional die-casting alloy.

Table 3 Mechanical and thermal properties of ACSR rheological die-casting and traditional die-casting Al-8Si, Al-6Si and Al-7Si-4Cu-0.2Cd alloys

(1) Developed a stable and efficient integrated process of semi-solid slurry preparation and die-casting, namely ACSR rheological die-casting process, and established a number of new aluminum alloy uniform solidification controlled slurry, transportation, die-casting and part-picking integrated, fully automatic, efficient and high-performance rheological die-casting production lines, realizing industrialization of rheological die-casting of high-quality large-scale thin-walled aluminum alloy structural parts.
(2) Compared with traditional die-casting, aluminum alloy castings produced by ACSR rheological die-casting process have better surface quality and lower porosity. Flatness is only 0.20~0.25mm, surface roughness is reduced to 3.2μm, and porosity is reduced to 0.9%.
(3) Compared with traditional die-casting alloys, ACSR rheological die-casting aluminum alloys have better mechanical properties and thermal conductivity. Its tensile strength and elongation are increased by 20%~22% and 34%~75% respectively, thermal conductivity is increased by 7.5%~10.1%.