Aluminum alloy die casting technology and development milestones

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 more widely used processes in the automotive aluminum forming process, accounting for 49% of various automobile forming process methods.

There are about 3,000 die-casting enterprises in China, and the output of die-casting products has increased from 266,000 tons in 1995 to 870,000 tons in 2005. The annual growth rate has remained above 20%, of which aluminum alloy die castings account for 3 per cent of all die castings. 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, and toy lights. With the improvement of technology 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. Since the die-casting aluminum alloy die-casting aluminum alloy was put into commercial production in 1914, with the development of the automobile industry and the invention of the cold chamber die casting machine, rapid development has been achieved.

Die-casting aluminum alloys are divided into medium-low strength (such as China's Y102) and high-strength (such as China's Y112). At present, die-cast aluminum alloys for industrial applications mainly include the following major series: Al-Si, Al-Mg, Al-Si-Cu, Al-Si-Mg, Al-Si-Cu-Mg, and Al-Zn. The improvement of the mechanical properties of die-cast aluminum alloys is often accompanied by a decrease in the casting process performance. Pressure casting due to the characteristics of high pressure rapid solidification makes this contradiction more prominent in some aspects, so the general die castings difficult to carry out solution heat treatment, which restricts Although the mechanical properties of die-casting aluminum alloys are improved, although oxygen-filled die-casting and vacuum die-casting are effective ways to improve the mechanical properties of the alloys, there are still some difficulties in their wide adoption. Therefore, the development of new die-cast aluminum alloys has been ongoing. Advanced Die Casting Technology In the early die casting process of the horizontal cold chamber die casting machine, only one speed was used to transfer the molten metal into the die, and the shot speed was only 1 m~2 m/s. Using this process, the casting has many pores inside it and the structure is loose. Soon after, it is modified into a two-stage shot, and the shot process is simply decomposed into two stages of slow speed and rapid speed, but the rapid speed is only 3m/s. Later, in order to To increase the density of the die castings, a phase of pressure increase is added after the slow speed and rapid speed, and the three stages of slow injection, rapid injection and pressurization become the classic three-stage injection.

In the middle of the 1960s, this type of 3-stage injection has been generally pushed away, and the speed of the rapid injection stage has been increased to 5m/s. Over the following 40 years, the leading die casting machine manufacturers in the world have conducted experiments on the injection process and developed new processes such as the 70's parabolic injection system, the 80's flashless die casting system, and the 1990s. The flashless injection system, some of which are re-decomposed from each stage of the three-phase shot, is an extension of this classic 3-stage shot. 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 blowholes and shrinkage holes in the die castings, people have been able to produce high-strength, high-tightness, weldable, heat-treatable, distortable and other high-demand die castings, in addition to continuing to improve vacuum die-casting. New techniques such as squeeze casting and semi-solid die-casting are referred to as "high-density die casting." Vacuum die casting technology Vacuum die casting method is to evacuate or partly evacuate the gas in the cavity to reduce the gas pressure in the cavity so as to facilitate the filling and the elimination of gas in the alloy melt, so that the alloy melt fills the cavity under the pressure , and solidified under pressure to obtain a dense die casting.

Vacuum die-casting method has the following characteristics compared with common die-casting method: (1) Porosity rate is greatly reduced; (2) Vacuum die-casting casting has high hardness and fine microstructure; (3) Vacuum die casting has higher mechanical properties. Recently, vacuum die-casting is mainly used to extract the gas in the cavity. There are mainly two forms: (1) directly extract gas from the mold; (2) set the mold to evacuate in a vacuum box.

With vacuum die casting, the design of the exhaust port and exhaust area of ​​the mold is critical. There is a "critical area" in the exhaust passage, which is related to the amount of gas extracted in the cavity, the extraction time, and the filling time.

When the area of ​​the exhaust passage is larger than the critical area, the effect of vacuum die-casting is obvious; otherwise, it is not obvious. The choice of the vacuum system is also very important. It is required that the vacuum inside the cavity be maintained until the filling is completed before the vacuum pump is turned off. Oxygen-filled die-casting technology Most of the gas in the die-casting cavity is N2 and H2. There is almost no O2. The main reason is that O2 reacts with the active metal to form a solid oxide, which provides a theoretical basis for the oxygen-filled die-casting technology. Oxygen die casting is the filling of oxygen into the cavity before die casting to replace the air. When the molten metal enters the cavity, part of the oxygen is discharged from the exhaust tank, and the residual oxygen reacts with the molten metal to form dispersed oxide particles, forming an instantaneous vacuum in the mold, thereby obtaining a non-porous die casting. During the oxygen-filled die-casting process, the vacuum inside the cavity is generated by a chemical reaction. In order to ensure safety during production, the oxygen content should be strictly controlled and the cavity pressure should be reduced to match the oxygenation pressure. Combining the vacuum die-casting with the oxygenating process, the cavity is in a negative pressure state, and better results can be obtained.

In the molten metal filling process, the molten metal should be filled with dispersion. The size of the runner also has a great influence on the effect of oxygen-filled die-casting. The appropriate runner size not only satisfies the molten metal in a turbulent manner but also prevents the temperature of the molten metal from falling too fast. The highly dispersed distribution of the oxides does not adversely affect the casting, but instead increases the hardness of the casting and refines the microstructure after heat treatment. Oxygenated die casting can be used for Al, Mg, and Zn alloys that react with oxygen. At present, various kinds of aluminum alloy castings can be produced by using oxygen-filled die-casting, such as: hydraulic transmission housings, heat exchangers for heaters, hydraulic transmission valve bodies, computer brackets, etc. For heat treatment or group welding, high airtightness is required and Castings used at higher temperatures, oxygen-filled die casting have technical and economical advantages. Semi-Solid Die-Casting Technology Semi-solid die casting is a technique in which a solid metal component is agitated when the liquid metal is solidified, and a solid content component of approximately 50% or more is obtained at a certain cooling rate, and then is subjected to die casting using a slurry. Semi-solid die casting technology currently has two forming processes: a rheology process and a thixotropic process. The former is to send 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 the solid metal pellets or chips sent into the spiral injection molding machine, after heating and under shear conditions to make the metal particles into a slurry after the injection molding. The key to the semi-solid die-casting forming process is the effective research and development of semi-solid alloy slurry, accurate control of the proportion of solid-liquid components and automatic control of the semi-solid forming process.

To realize the automated production of semi-solid forming, American scientists believe that it is necessary to develop the following technologies: (1) Bar transport with self-adaptability and flexibility; (2) Precision die-cast lubrication and maintenance; (3) Controllable Casting cooling system; (4) Plasma degassing and treatment.

Squeeze Die Casting Technology Squeeze die casting is also called "liquid metal molding." Its castings have good compactness, high mechanical properties, and no pouring riser. Some enterprises in China have applied it to actual production. Extrusion die casting technology has excellent technical advantages. It can replace the traditional die casting, squeeze casting, low pressure casting, vacuum die casting process, and is compatible with differential pressure casting, continuous casting and forging, semi-solid flow rheocasting process. Experts believe that extrusion die-casting technology is a cutting-edge new technology that spans multiple process areas and is rich in content, innovative, and challenging.

Electromagnetic pump Low-pressure casting electromagnetic pump Low-pressure casting is a newly emerged low-pressure casting process. Compared with the gas-type low-pressure casting technology, it is completely different in the pressure mode. The non-contact electromagnetic force acts directly on the liquid metal, greatly reducing the problems of oxidation and inhalation caused by the impure oxygen in the compressed air and the high oxygen partial pressure in the compressed air, thereby realizing the smooth transportation of the aluminum liquid. And filling, can prevent secondary pollution due to turbulence. In addition, the electromagnetic pump system is completely controlled by computer numerical control. The process 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. As the technology continues to deepen, the technology becomes more and more mature.

The development of die-casting equipment through the development of recent years, China's die-casting machine design level, technical parameters, performance indicators, mechanical structure and manufacturing quality have all been improved to varying degrees, especially the cold chamber die-casting machine, from the original full hydraulic The type structure changed to an elbow-type fitting mechanism. At the same time, it also added automatic charging, automatic spraying, automatic picking, automatic cutting, etc. Electrical appliances were also changed from ordinary power supply control to computer control. The level of control was greatly improved. Already at or near the international level, it is moving toward large-scale, automated and unitized.

In the meantime, new domestic die-casting machine companies have emerged one after another, among which Hong Kong Lijin is a typical representative. The company has developed a number of leading domestic die-casting machines. For example, the horizontal cold chamber die casting machine has a large air injection speed of 6m. /s (1997) and 8m/s (early 2000), magnesium alloy die casting machine (early 2000) uniform acceleration injection system (2002), large 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 30000 kN (July 2004).

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 large air injection speed of 8 m/s or more and large die casting machines with a clamping force of 10,000 kN or more; The production of Guangdong Shunwei Yili Precision Technology Co., Ltd. will produce 10000kN ~ 30000kN large-scale die-casting machine. It can be seen that China is forming a powerful die casting machine manufacturing industry with independent intellectual property rights. China currently has 12,000 die casting machines in total, of which domestically produced 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 is more than 1,800, of which 10000kN and above die casting machines accounted for 2%, 8000kN~9000kN die casting machines accounted for 5%, 5000kN~7000kN die casting machines accounted for 13%, 3500kN~4000kN die casting machines accounted for 20%, 3000kN and below die casting machines account for 60%. In the die casting machine below 3000kN, the hot chamber die casting machine accounts for about 30%.

Small and medium-sized die-casting machines still use domestic equipment. The gap between domestic die-casting machines and foreign advanced die-casting equipment is mainly reflected in the following aspects: (1) The overall structural design is backward; (2) The oil leakage is serious; (3) The reliability is poor: This is a prominent defect of the domestic die-casting machine. It is understood that the average fault-free running time of domestic die-casting machines is less than 3,000 hours, or even less than the level of foreign 50 and 60 years. And foreign countries generally exceed 20,000 hours; (4) Variety specifications are not complete, supporting ability is poor: Although in the horizontal cold chamber die casting machine has basically become a series, but there are still some off the file, if there is no product from 16000kN to 28000kN. Hot chamber die casting machines also lack 4000kN or more products. The development of die-casting molds The earlier die-casting mold core materials used were 45-gauge steel, cast steel, and forged steel. Due to the poor high-temperature impact resistance, the service life at that time was also short. With the development of science and technology, major changes have also taken place in die-casting core materials. Now, high-temperature, high-strength 3Cr2N8VH13 hot forged steels have been used as core materials. In recent years, imported 8407 materials have been used to greatly increase the service life of the molds. In recent years, especially in most domestic plants, computer design and simulation filling techniques have been used to greatly increase the production quality of die-casting molds and greatly shorten the production period.

China's mold industry has developed rapidly. The average annual growth rate of mold production from 1996 to 2004 was 14%. In 2003, the annual 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 the main low-grade mold, large-scale, sophisticated precision molds, in the production technology, mold quality and life expectancy and production capacity can not meet the national economic development need. Research and development The demand for automotive and motorcycle industry and automotive accessories and the demand for ancillary products have provided a broad market for die casting production. The application of die-cast aluminum alloys to 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 major direction for the development of die-casting technology. In order to meet market demand, the following issues should be further addressed in the future: (1) Promote the use of new high-strength, high-abrasion die-cast alloys, study of die-castable alloys, and new die castings for castings with special safety requirements Alloys; (2) Development of die-cast aluminum alloys with stable performance and easy-to-control composition; (3) Simplification of alloy composition and reduction of winning prizes to provide a basis for green production; (4) Further improvement of new die-casting processes (vacuum die-casting, (Oxygen die-casting, semi-solid die casting, squeeze casting, etc.); (5) Improve rapid response to the market, promote concurrent engineering (CE) and rapid prototyping (RPM); (6) Develop CAD/CAM/CAE systems Research and development; (7) develop and apply more die-cast aluminum automotive parts.