The Expanding Application Field Of Aluminum Alloy Die Casting

Publish Time: 05/11 2021 Author: Site Editor Visit: 15951

Since 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 automotive aluminum alloy forming process, accounting for 49% of the various automotive forming process methods.

There are about 3,000 die-casting enterprises in China. The output of die-casting parts has increased from 266,000 tons in 1995 to 870,000 tons in 2005, and the annual growth rate has remained above 20%. Among them, aluminum alloy die-casting parts account for 3/ 4 and above. The types of die casting products in China are diversified, including automobiles, motorcycles, telecommunications, 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 die-cast aluminum alloy 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-cast aluminum alloys can be divided into low- and medium-strength (such as China's Y102) and high-strength (such as China's Y112) according to their performance. At present, the die-cast aluminum alloys used in industrial applications 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 the reduction of casting process performance. Due to the characteristics of high-pressure rapid solidification of die-casting, this contradiction is 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 oxygen-filled die-casting and vacuum die-casting are effective ways to improve the mechanical properties of alloys, it is still difficult to use them widely. Therefore, the development and research of new die-cast aluminum alloys has been going on.

Advanced Die Casting Technology

The die-casting process of the early horizontal cold-chamber die-casting machine has only one speed to push the molten metal into the mold, and the injection speed is only 1m~2m/s. With this process, there are many pores inside the casting and the structure is loose. It will soon be improved to two-stage injection. The injection process is simply broken down into two stages, slow and fast, but the fast speed is only 3m/s. Increase the density of die castings, at slow and fast speeds

After that, a pressure increase stage was added, which became three stages of slow injection, fast injection and pressurization. This is the classic three-stage injection. In the middle of the 1960s, this three-stage injection has been generally pushed away, and the speed of the fast injection stage has been increased to 5m/s. Over the next 40 years, leading die-casting machine manufacturers around the world conducted research and experimentation on the injection process, and developed some new processes, such as the parabolic injection system in the 70s, the flashless die-casting system in the 80s, and the 90s The flashless injection system, some of which re-decompose each stage from the three-stage injection, which is an extension of the continuous development of this classic three-stage injection. Now the injection speed and pressure have been changed from the original manual handwheel adjustment control to computer control. In recent years, in order to solve the problems of pores and shrinkage holes in the die-casting parts, people can produce high-strength, high-density, weldable, heat-treatable, and twistable die-casting parts with various high requirements. In addition to continuing to improve vacuum die-casting, it has also developed New technologies such as squeeze casting and semi-solid die-casting are introduced, and they are generally called "high-density die-casting method".

Vacuum die casting technology

The vacuum die-casting method is to evacuate or partially evacuate the gas in the cavity to reduce the air pressure in the cavity to facilitate the filling and the removal of the gas in the alloy melt, so that the alloy melt fills the cavity under the action of pressure, and Solidify under pressure to obtain dense die castings. Compared with the ordinary die-casting method, the vacuum die-casting method has the following characteristics:

The porosity is greatly reduced;
The castings of vacuum die casting have high hardness and microstructure small
The mechanical properties of vacuum die castings are relatively high.

Recently, vacuum die casting is mainly used to remove the gas in the cavity, and there are two main forms:

Pump air directly from the mold;
Place the mold in a vacuum box to pump air.

When vacuum die-casting is used, the design of the position of the exhaust duct and the area of the exhaust duct 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 in the cavity, the pumping time and the filling time. When the area of the exhaust duct is larger than the critical area, the vacuum die-casting effect is obvious; on the contrary, it is not obvious. The choice of the vacuum system is also very important. It is required that the vacuum in the cavity can be maintained until the filling is completed before the vacuum pump is turned off.

Oxygenated Die Casting Technology

Most of the gas in the pores of die castings is N2 and H2, and there is almost no O2. The main reason is that O2 reacts with active metals to form solid oxides, which provides a theoretical basis for oxygen-filled die-casting technology. Oxygen-filled die-casting is to fill the cavity with oxygen before die-casting to replace the air in it. When the molten metal enters the cavity, a part of the oxygen is discharged from the exhaust groove, and the remaining oxygen reacts with the molten metal to generate dispersed oxide particles, which form an instant vacuum in the mold, thereby obtaining a porosity-free die casting. In the process of oxygen-filled die-casting, the vacuum in the cavity is produced by a chemical reaction. In order to ensure safety in production, the oxygenation amount should be strictly controlled, and the cavity pressure should be reduced to match the oxygenation pressure. Combining the vacuum die-casting and oxygenation process to make the cavity in a negative pressure state, better results can be obtained. In the molten metal filling process, the molten metal should be filled in a dispersed spray state. The size of the runner size also has a great influence on the effect of oxygen-filled die-casting. The proper runner size can not only satisfy the molten metal filling the mold in the form of turbulent flow, but also prevent the molten metal temperature from falling too fast. The highly dispersed distribution of oxides will not adversely affect the castings, but can increase the hardness of the castings and refine the structure after heat treatment. Oxygen-filled die casting can be used for Al, Mg and Zn alloys that react with oxygen. At present, various aluminum alloy castings can be produced by oxygen-filled die casting, such as: hydraulic transmission housing, heater heat exchanger, hydraulic transmission valve body, computer bracket, etc.

For die-casting parts that require heat treatment or group welding, high air-tightness and use at higher temperatures, oxygen-filled die-casting has technical and economic advantages.

Semi-solid die casting technology

Semi-solid die-casting is a technique in which liquid metal is stirred during solidification, and a slurry of about 50% or higher solid phase composition is obtained at a certain cooling rate, and then the slurry is used for die-casting. Semi-solid die casting technology currently has two forming processes: rheological forming process and thixoforming process. The former is to send the liquid metal into a specially designed injection molding machine barrel, which is sheared by a screw device to cool it into a semi-solid slurry, and then die-cast. The latter is to send the solid metal particles or scraps into the spiral injection molding machine, and the metal particles are converted into slurry under the conditions of heating and shearing and then die-casting. The key to the semi-solid die-casting forming process is the research and development of the effective preparation of semi-solid alloy slurry, the precise control of the solid-liquid composition ratio, and the automated control of the semi-solid forming process. In order to realize the automated production of semi-solid forming, American scientists believe that the following technologies need to be vigorously developed:

Bar transportation with adaptability and flexibility;
Precision die-casting lubrication and maintenance;
Controllable casting cooling system;
Plasma degassing and treatment.

Extrusion die casting technology

Extrusion die casting is also called "liquid metal molding". The casting has good compactness, high mechanical properties, and no pouring riser. Some enterprises in our country have applied it in actual production. Squeeze die casting technology has excellent technological advantages. It can replace traditional die casting, squeeze casting, low pressure casting, vacuum die casting processes, and is compatible with rheological casting processes such as differential pressure casting, continuous casting and continuous forging, and semi-solid processing. Experts believe that extrusion die-casting technology is a cutting-edge new technology, spanning multiple process areas, rich in connotation, strong innovation, and extremely challenging.

Electromagnetic pump low pressure casting

Electromagnetic pump low-pressure casting is a newly emerging low-pressure casting process. Compared with gas-type low-pressure casting technology, the pressure method is completely different. It uses non-contact electromagnetic force to directly act on the liquid metal, which greatly reduces the oxidation and inhalation problems caused by the impure compressed air and the high partial pressure of oxygen in the compressed air, and realizes the smooth transportation of the aluminum liquid. And filling type can prevent secondary pollution caused by turbulence. In addition, the electromagnetic pump system completely adopts computer digital control, the process execution is very accurate, and the repeatability is good, so that aluminum alloy castings have obvious advantages in terms of yield, mechanical properties, surface quality and metal utilization. With the continuous in-depth research of this technology, the process has become 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 machine have been improved to varying degrees, especially the cold chamber die-casting machine, which has been changed from the original fully hydraulic closing mechanism to Toggle type closing mechanism, and also adds automatic loading, automatic spraying, automatic pickup, automatic cutting edge, etc., the electrical appliances have also been changed from ordinary power control to computer control, and the control level has been greatly improved, and some have reached or approached At the international level, it is marching towards large-scale, automated and modularization. During this period, new domestic die-casting machine companies have emerged one after another. Hong Kong Lijin Company is a typical representative. The company has developed a number of domestic leading die-casting models. For example, the horizontal cold-chamber die-casting machine has a maximum air injection speed of 6m. /s (1997) and 8m/s (early 2000), magnesium alloy hot chamber die casting machine (early 2000), uniform acceleration 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 30000kN (July 2004), etc. In recent years, Shanghai Die Casting Machine Factory, Guannan Die Casting Machine Factory and other key enterprises have developed horizontal cold chamber die casting machines with a maximum air injection speed of over 8m/s and large die casting machines with a clamping force of over 10000kN; 2005 Guangdong Sunwill Yili Precision Press Technology Co., Ltd. put into production will produce 10000kN~30000kN large-scale die-casting machines. It can be seen that China is forming a powerful die-casting mechanism with independent intellectual property rights Manufacturing industry.

The total number of die-casting machines in China is 12,000, of which domestic die-casting machines account for about 85%, and imported die-casting machines account for about 15%. In the past two years, the annual sales volume of die-casting machines in China has been more than 1,800, of which die-casting machines of 10000kN and above accounted for 2%, 8000kN~9000kN die-casting machines accounted for 5%, 5000kN~7000kN die-casting machines accounted for 13%, and 3500kN~4000kN die-casting machines accounted for 20%, and die casting machines of 3000kN and below account for 60%. Among the die-casting machines below 3000kN, the hot-chamber die-casting machine accounts for about 30%. Small and medium-sized die-casting machines are still dominated by domestically-made equipment. The gap between domestic die-casting machines and foreign advanced die-casting equipment is mainly manifested in the following aspects:

The overall structure design is backward;
Serious oil leakage;
Poor reliability: This is the most prominent defect of domestic die-casting machines. It is understood that the mean time between failures of domestic die-casting machines is less than 3000 hours, which is not even up to the level of foreign countries in the 1950s and 1960s. And foreign countries generally exceed 20,000 hours;
Insufficient varieties and specifications, poor supporting capacity: Although the horizontal cold chamber die-casting machine has basically become a series, there are still some broken gears. For example, there are no products between 16000kN and 28000kN. The hot chamber die casting machine also lacks products above 4000kN.

The Development Of Die Casting Mold

The earliest die-casting mold core materials used 45% steel, cast steel and forged steel, etc., due to its poor high temperature impact resistance, so the service life was short at that time. With the development of science and technology, major changes have taken place in die-casting core materials. Nowadays, high-temperature, high-strength 3Cr2N8VH13 hot forging steel is used as the core material. In recent years, imported 8407 materials have been used, which greatly improves the service life of the mold. ,
Especially in recent years, most domestic factories have adopted computer design and simulation filling technology, which greatly improves the production quality of die-casting molds and greatly shortens the production period. China's mold industry is developing 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, the satisfaction rate of China's domestic molds to the market is only about 80%. Among them, medium and low-end molds are mainly used. Large and complex precision molds cannot meet the development of the national economy in terms of production technology, mold quality and life, and production capacity. need.

Research And Development Direction

The consumption of automobile and 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-cast aluminum alloy 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 of die-casting technology development. In order to adapt to market demand, the following issues should be further resolved in the future:

Promote the application of new high-strength, high-wear-resistant die-casting alloys, and study colorable die-casting alloys and new die-casting alloys used in castings with special safety requirements;
Develop die-cast aluminum alloy with stable performance and easy-to-control composition;
Simplify alloy composition, reduce alloy grades, and provide a basis for realizing green production;
Further improve the new die-casting process (vacuum die-casting, oxygen-filled die-casting, semi-solid die-casting, squeeze casting, etc.);
Improve the ability to respond quickly to the market, and implement concurrent engineering (CE) and rapid prototyping manufacturing (RPM);
Carry out the research and development of CAD/CAM/CAE system;
Develop and apply more die-cast aluminum alloy auto parts.

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