Aluminum alloy die-casting technology and development mileage

After the 1990s, China's die-casting industry has achieved amazing development and has developed into a new industry. At present, the aluminum alloy die-casting process has become one of the most widely used processes in the aluminum alloy forming process for automobiles, accounting for 49% of the various automobile forming process methods.

There are about 3,000 die-casting enterprises in China. The output of die-casting parts increased from 266,000 tons in 1995 to 870,000 tons in 2005, with an annual growth rate of more than 20%. Among them, aluminum alloy die-casting parts account for 3/3 of the output of all die-casting parts. 4 or more. The types of die casting products in China are diversified, including automobiles, motorcycles, communications, home appliances, hardware products, power tools, IT, lighting, escalator steps, toy lights, etc. With the improvement of technical level and product development capability, the types and application fields of die-casting products have been continuously expanded, and the die-casting equipment, die-casting molds and die-casting processes have undergone tremendous changes. Die-casting aluminum alloy Since it was put into commercial production in 1914, it has developed rapidly with the development of the automobile industry and the invention of the cold-chamber die-casting machine.

Die-casting aluminum alloys are divided into two types according to their properties: medium and low strength (such as China's Y102) and high strength (such as China's Y112). At present, the die-casting aluminum alloys used in industry mainly include the following series: Al-Si, Al-Mg, Al-Si-Cu, Al-Si-Mg, Al-Si-Cu-Mg, Al-Zn, etc. The improvement of the mechanical properties of die-casting aluminum alloys is often accompanied by the reduction of the performance of the casting process. Due to the characteristics of high-pressure and rapid solidification of die-casting, this contradiction is more prominent in some aspects. To improve the mechanical properties of die-casting aluminum alloys, although oxygen-filled die-casting and vacuum die-casting are effective ways to improve the mechanical properties of alloys, it is still difficult to widely use them, so the development of new die-casting aluminum alloys has been in progress. Advanced die-casting technology The die-casting process of the early horizontal cold chamber die-casting machine has only one speed to press the molten metal into the mold, and the injection speed is only 1m-2m/s. With this process, there are many pores in the casting and loose organization. It was soon improved to 2-stage injection, and the injection process was simply decomposed into two stages: slow and fast, but the fast speed was only 3m/s. Later, in order to To increase the density of the die casting, a stage of pressure increase is added after the slow speed and the fast speed, which become three stages of slow injection, fast injection and pressurization, which is the classic 3-stage injection.

Compared with the ordinary die casting method, the vacuum die casting method has the following characteristics: (1) the porosity is greatly reduced; (2) the hardness of the vacuum die casting castings is high and the microstructure is small; (3) the mechanical properties of the vacuum die casting parts are higher. Recently, vacuum die casting is mainly to remove the gas in the cavity, and there are mainly two forms: (1) pumping directly from the mold; (2) pumping the mold in a vacuum box. When using vacuum die casting, the design of the exhaust channel position and exhaust channel area of ​​the mold is very important. There is a "critical area" in the exhaust duct, which is related to the amount of gas pumped out of the cavity, the pumping time and the filling time.

In the process of molten metal filling, the molten metal should be filled in a state of dispersive spraying. The size of the runner also has a great influence on the effect of oxygen-filled die casting. The appropriate runner size can not only satisfy the molten metal to fill the mold in turbulent flow, but also avoid the temperature of the molten metal from dropping too fast. The highly dispersed distribution of oxides will not adversely affect the casting, but can improve the hardness of the casting and refine the structure after heat treatment. Oxygenated die casting can be used for Al, Mg, and Zn alloys that react with oxygen. At present, oxygen-filled die casting can be used to produce various aluminum alloy castings, such as: hydraulic transmission casings, heat exchangers for heaters, hydraulic transmission valve bodies, brackets for computers, etc. For die castings used at higher temperatures, oxygen-filled die casting has technical and economic advantages. Semi-solid die-casting technology Semi-solid die-casting is a technology that stirs the liquid metal when it solidifies, obtains a slurry of about 50% or even higher solid phase components at a certain cooling rate, and then uses the slurry for die casting. There are currently two forming processes in semi-solid die casting technology: rheological forming process and thixoforming process. In the former, liquid metal is fed into a specially designed injection molding barrel, and sheared by a screw device to cool it into a semi-solid slurry, and then die-cast. The latter is to send solid metal particles or chips into a screw injection molding machine, and under the conditions of heating and shearing, the metal particles are turned into a slurry and then die-casting. The key to the semi-solid die-casting forming process is the research and development of effective preparation of semi-solid alloy slurry, precise control of the ratio of solid-liquid components and automatic control of semi-solid forming process. In order to realize the automatic production of semi-solid forming, American scientists believe that the following technologies need to be vigorously developed: (1) self-adaptive and flexible bar transportation; (2) precise die-casting lubrication and maintenance; (3) controllable casting cooling system; (4) plasma degassing and treatment.

Electromagnetic pump low-pressure casting Electromagnetic pump low-pressure casting is a new low-pressure casting process. Compared with gas-type low-pressure casting technology, it is completely different in terms of pressurization. It uses non-contact electromagnetic force to directly act on the liquid metal, which greatly reduces the problems of oxidation and suction caused by the impure compressed air and the high partial pressure of oxygen in the compressed air, and realizes the smooth transportation of aluminum liquid. And filling can prevent secondary pollution caused by turbulent flow. In addition, the electromagnetic pump system is completely controlled by computer numerical control, and the process execution is very accurate and repeatable, which makes the aluminum alloy castings have obvious advantages in terms of yield, mechanical properties, surface quality and metal utilization. With the continuous deepening of research, this technology has become more and more mature.

The development of die-casting equipment Through the development in recent years, the design level, technical parameters, performance indicators, mechanical structure and manufacturing quality of China's die-casting machines have been improved to varying degrees, especially the cold chamber die-casting machine. The type mechanism is changed to a toggle type closing mechanism, and at the same time, automatic loading, automatic spraying, automatic picking, automatic cutting edge, etc. are also added.