Analysis of the development status of aluminum alloy die-casting process and mold technology

After the 1990s, China's die-casting industry has achieved amazing development and has developed into an emerging industry. At present, the aluminum alloy die-casting process has become one of the most widely used processes in automotive aluminum alloy forming processes, accounting for 49% of various automotive forming process methods.
There are about 3,000 die-casting companies in China. The output of die-casting parts increased from 266,000 tons in 1995 to 870,000 tons in 2005. The annual growth rate remains above 20%. Among them, aluminum alloy die-casting parts account for 3/3 of all die-casting parts output. 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 capabilities, the types and application fields of die-casting products continue to expand, and its die-casting equipment, die-casting molds and die-casting processes have undergone tremendous changes. Die-cast aluminum alloy Since its commercial production in 1914, die-cast aluminum alloy has developed rapidly with the development of the automobile industry and the invention of the cold chamber die-casting machine.

Die-cast 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-cast 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-cast aluminum alloys is often accompanied by a decrease in the performance of the casting process. The high-pressure and rapid solidification characteristics of die-casting make this contradiction more prominent in some aspects. Therefore, it is difficult for general die-casting parts to undergo solution heat treatment, which restricts To improve the mechanical properties of die-cast aluminum alloys, although oxygenated die-casting and vacuum die-casting are effective ways to improve the mechanical properties of alloys, widespread adoption is still difficult, so the development of new die-cast aluminum alloys has been ongoing.
Advanced die-casting technology The die-casting process of early horizontal cold chamber die-casting machines had only one speed to send molten metal into the mold, and the injection speed was only 1m-2m/s. Using this process, there were many pores inside the casting and the structure was loose. It was soon improved to 2-stage injection, which simply decomposed the injection process into two stages: slow speed and fast speed, but the fast speed was only 3m/s. Later, in order to To increase the density of die castings, a pressure increase stage is added after the slow and fast speeds, becoming three stages: slow injection, fast injection and pressurization. This is the classic three-stage injection.

Compared with ordinary die casting method, vacuum die casting method has the following characteristics:
01 Porosity is greatly reduced
02 Vacuum die-casting castings have high hardness and fine microstructure
03 Vacuum die castings have higher mechanical properties

Recently, vacuum die casting mainly focuses on extracting gas from the mold cavity, and there are two main forms:
① Extract air directly from the mold; ② Place the mold in a vacuum box to evacuate air. When using vacuum die casting, the design of the exhaust channel position and exhaust channel area of the mold is crucial.
There is a "critical area" in the exhaust duct, which is related to the amount of gas extracted from the cavity, the exhaust time and the filling time. During the filling process of molten metal, the molten metal should be filled in a dispersed spray state. The size of the runner also has a great influence on the effect of oxygenated die-casting. The appropriate runner size can not only allow the molten metal to fill the mold in a turbulent flow, but also prevent the temperature of the molten metal from dropping too fast. The highly dispersed distribution of oxides will not have an adverse effect on the casting, but can increase the hardness of the casting and refine the structure after heat treatment. Oxygen die casting can be used for Al, Mg and Zn alloys that react with oxygen.
At present, oxygenated die casting can be used to produce various aluminum alloy castings, such as: hydraulic transmission housings, heat exchangers for heaters, hydraulic transmission valve bodies, computer brackets, etc., which require heat treatment or assembly welding and require high air tightness and For die castings used at higher temperatures, oxygenated die casting has technical and economic advantages. Semi-solid die-casting technology Semi-solid die-casting is a technology that stirs liquid metal when it solidifies, obtains a slurry with a solid component of about 50% or even higher at a certain cooling rate, and then uses the slurry for die-casting.
Semi-solid die-casting technology currently has two forming processes: flow forming process and thixoforming process. The former is to feed liquid metal into a specially designed injection molding barrel, where shear is applied by a spiral device to cool it into a semi-solid slurry, which is then die-cast. The latter is to feed solid metal particles or chips into a spiral injection molding machine, and turn the metal particles into slurry under heating and shearing conditions before die-casting. The key to the semi-solid die-casting forming process is the effective preparation of semi-solid alloy slurry, precise control of the proportion of solid-liquid components and the research and development of automated control of the semi-solid forming process.

In order to realize automated production of semi-solid forming, American scientists believe that the following technologies need to be vigorously developed:
01 Adaptable and flexible bar transportation
02 Precision die casting lubrication and maintenance
03 Controllable casting cooling system
04 Plasma degassing and treatment

Electromagnetic pump low-pressure casting is a newly emerging low-pressure casting process. Compared with gas-based low-pressure casting technology, it is completely different in terms of pressurization methods. It uses non-contact electromagnetic force to directly act on liquid metal, which greatly reduces oxidation and aspiration problems caused by impure compressed air and excessive partial pressure of oxygen in compressed air, and achieves smooth transportation of liquid aluminum. And filling can prevent secondary pollution caused by turbulence. In addition, the electromagnetic pump system is completely controlled by computer numerical control, and the process execution is very accurate and repeatable, giving aluminum alloy castings obvious advantages in terms of yield, mechanical properties, surface quality and metal utilization. As research on this technology continues to deepen, the process is becoming more and more mature.
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. In particular, cold chamber die-casting machines have evolved from the original fully hydraulic The type mechanism was changed to a toggle type closing mechanism, and automatic loading, automatic spraying, automatic picking up, automatic edge cutting, etc. were also added. Electrical appliances were also changed from ordinary power supply control to computer control, and the control level was greatly improved. Some It has reached or is close to the international level and is moving towards large-scale, automation and unitization.

During this period, new domestic die-casting machine companies have emerged one after another, among which Hong Kong Lijin Company is a typical representative. The company has developed a number of domestic leading die-casting machine models. For example, the horizontal cold chamber die-casting machine has a maximum air pressure injection speed of 6m /s (1997) and 8m/s (early 2000), magnesium alloy hot chamber die-casting machine (early 2000), uniformly accelerated injection system (2002), maximum air injection speed 10m/s and multi-stage die-casting system (2004 June), real-time control of the injection system (August 2004) and a large die-casting machine with a clamping force of 30,000kN (July 2004), etc.
Small and medium-sized die-casting machines are still dominated by domestic equipment. The gap between domestic die-casting machines and foreign advanced die-casting equipment is mainly reflected in the following aspects:
01 The overall structural design is backward
02 Serious oil leakage
03 Poor reliability is the most prominent flaw of domestic die-casting machines.
04 Incomplete varieties and specifications, poor supporting capabilities

China's mold industry has developed rapidly. From 1996 to 2004, the average annual growth rate of mold output was 14%. In 2003, the output value of die-casting molds was 3.8 billion yuan. At present, China's domestic mold market satisfaction rate is only about 80%, of which medium and low-end molds are the main ones. Large and complex precision molds cannot meet the needs of national economic development in terms of production technology, mold quality and life, and production capacity. need. Research and development direction The consumption of automobiles, motorcycle industries and automobile accessories and the demand for supporting products provide a broad market for the production of die-casting parts. The application of die-casting aluminum alloys in automobiles will also continue to expand.
In the future research and development of die-casting technology, the deepening of aluminum alloy die-casting will still be a main direction for the development of die-casting technology. In order to adapt to market demand, the following issues should be further solved in the future:
01 Promote the application of new high-strength, high-wear-resistant die-casting alloys, research colorable die-casting alloys, and new die-casting alloys for castings with special safety requirements.
02 Develop die-cast aluminum alloys with stable performance and easy-to-control composition
03 Simplify alloy composition and reduce alloy grades to provide a foundation for green production
04 Further improve new die-casting processes (vacuum die-casting, oxygenated die-casting, semi-solid die-casting, squeeze casting, etc.)
05 Improve the ability to respond quickly to the market and implement concurrent engineering (CE) and rapid prototyping manufacturing (RPM) technology
06 Carry out research and development of CAD/CAM/CAE systems
07 Develop and apply more die-cast aluminum alloy automotive parts

Die-casting aluminum alloy is the main body of non-ferrous alloy die-casting, and AI-Si series alloys are the main body of die-casting aluminum alloy. The development of the die-casting industry has a long way to go. It involves the research and development of high-performance alloys and the development of process methods. Practice has shown that , it is a wise choice to combine enterprises with relevant universities and research units, and the relevant results we have achieved have been confirmed.
To achieve the revitalization of the overall die-casting industry, it is also very important to improve the performance of die-casting machines and correspondingly improve the design and manufacturing level of die-casting molds. In particular, many foreign technical software are introduced and applied. What method is used to verify them? It is worth further research.