Aluminum alloy die-casting
At present, the main die-casting aluminum alloys for industrial application are the following series: Al-Si, Al-Mg, Al-Si-Cu, Al-Si-Mg, Al-Si-Cu-Mg, Al-Zn and so on. The improvement of mechanical properties of die-casting aluminum alloy is often accompanied by the reduction of casting process performance, pressure casting because of its high-pressure rapid solidification characteristics make this contradiction in some aspects more prominent, so the general die casting is difficult to solid solution heat treatment, which restricts the improvement of mechanical properties of die-casting aluminum alloy, although oxygen-filled die-casting, vacuum die-casting, etc. is an effective way to improve the mechanical properties of the alloy, but there is still some difficulty in the widespread adoption Therefore, the development of new die-casting aluminum alloy has been in progress. Advanced die-casting technology early horizontal cold chamber die-casting machine die-casting process only one speed pressure feeding metal liquid into the mold, the speed of the injection is only 1m ~ 2m / s. Using this process, the castings internal porosity, loose organization, soon improved for 2-stage injection, the injection process is simply broken down into slow and fast 2 stages, but the fast speed is only 3m / s, later in order to increase the die-casting densities, after the slow and fast density, after the slow speed and fast added a pressure boosting stage, become a slow injection, fast injection and boosting 3 stages, which is the classic 3-stage injection.
The vacuum die-casting method has the following characteristics compared with the common die-casting method:
(1) The porosity is greatly reduced;
(2) High hardness and fine microstructure of vacuum die-casting castings;
(3) the mechanical properties of vacuum die casting is high.
Recently, vacuum die-casting to remove the gas in the cavity, there are two main forms:
(1) direct extraction from the mold;
(2) place the mold in the vacuum box pumping. When using vacuum die-casting, the location of the exhaust channel of the mold and the design of the exhaust channel area is crucial.
Exhaust channel there is a “critical area”, and the amount of gas pumped out of the cavity, pumping time and filling time related.
During the filling process, the metal should be filled in a diffuse jet. The size of the sprue size also has a large impact on the effect of oxygen die-casting, the appropriate sprue size can meet the metal liquid to turbulent flow form filled with the mold, but also can avoid the metal liquid temperature drop too fast. The high dispersion distribution of the oxide will not adversely affect the casting, but can improve the hardness of the casting, and make the organization of the heat treatment refinement. Oxygen-filled die-casting can be used for Al, Mg and Zn alloys that react with oxygen. At present, the use of oxygen-filled die-casting can produce a variety of aluminum alloy castings, such as: hydraulic transmission housing, heat exchangers for heaters, hydraulic transmission valve body, computer brackets, etc. For heat treatment or group welding, requiring high gas tightness and in the higher temperature die-casting, oxygen-filled die-casting has technical and economic advantages. Semi-solid die-casting technology semi-solid die-casting is in the liquid metal solidification when stirring, in a certain cooling rate to obtain about 50% or even higher solid phase components of the slurry, and then use the slurry for die-casting technology. Semi-solid die-casting technology currently has two forming processes: rheological forming process and thixotropic forming process. The former is to feed the liquid metal into a specially designed injection molding barrel, which is cooled to a semi-solid slurry by shear applied by a screw device, and then die-cast. In the latter, solid metal particles or scraps are fed into the screw injection molding machine, and the metal particles are heated and subjected to shear to turn into slurry and then die-cast. The key to the semi-solid die-casting forming process is the effective production of semi-solid alloy slurry, precise control of the ratio of solid-liquid components and the research and development of automatic control of the semi-solid forming process.
In order to realize the automatic production of semi-solid forming, U.S. scientists believe that the following technologies need to be vigorously developed:
(1) with self-adaptive, flexible bar transport; (2) precision die-casting lubrication and maintenance; (3) controlled casting cooling system; (4) plasma degassing and processing.
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, in terms of pressurization is completely different. It adopts non-contact electromagnetic force to act directly on the liquid metal, which greatly reduces the problems of oxidation and aspiration caused by impure compressed air and high partial pressure of oxygen in the compressed air, realizes the smooth transportation and filling of aluminum liquid, and prevents secondary pollution caused by turbulent flow. In addition, the electromagnetic pump system completely adopts computer digital control, and the process execution is very accurate and repeatable, so that the aluminum alloy castings have obvious advantages in terms of yield, mechanical properties, surface quality and metal utilization rate. This technology with the research continues to deepen, the process is also more and more mature.
The development of die-casting equipment through the development of recent years, China die-casting machine design level, technical parameters, performance indicators, mechanical structure and manufacturing quality have different degrees of improvement, especially the cold chamber die-casting machine, from the original full hydraulic closing mechanism to the curved elbow type closing mechanism, but also increased the automatic loading, automatic spraying, automatic pick-up, automatic cutting material edge, etc., electrical appliances are also changed from ordinary power control to Computer control, control level greatly improved, some have reached or close to the international level, is to large-scale, automation and unitization. During this period, the new domestic die-casting machine enterprises have emerged, of which Hong Kong Li Jin company is a typical representative, the company developed a number of leading domestic die-casting machine type, for example, horizontal cold chamber die-casting machine maximum empty injection speed 6m / s (1997) and 8m / s (early 2000), magnesium alloy hot chamber die-casting machine (early 2000) uniform acceleration injection system (2002), the maximum empty die casting speed 10m/s and multi-stage die casting system (June 2004), real-time control die casting system (August 2004) and clamping force 30000kN large die casting machine (July 2004), etc.
Small and medium-sized die-casting machine is still dominated by domestic equipment. The gap between domestic die-casting machine and foreign advanced die-casting equipment is mainly in the following aspects:
(1) the overall structure design backward; (2) serious oil leakage;
(3) poor reliability: This is the most prominent defect of domestic die-casting machine, it is understood that the average trouble-free operation time of domestic die-casting machine is less than 3000 hours, even less than the level of foreign 50s and 60s. And foreign countries generally more than 20000 hours;
(4) variety specifications are not complete, supporting capacity is poor: Although in the horizontal cold chamber die-casting machine has basically become a series, but there are still individual breaks, such as from 16000kN to 28000kN between no products. Hot chamber die-casting machine also lack of 4000kN above the product. The development of die-casting mould the earliest die-casting mould core material selection is 45? steel, cast steel and forged steel, etc., because of its high temperature impact resistance is poor, so then the service life is also shorter. With the development of science and technology, die-casting die core material has also undergone significant changes, now are using high temperature, high strength 3Cr2N8VH13 hot forging steel as the core material, in recent years and the use of imported 8407 material, so that the service life of the mold greatly improved, especially in recent years most of the domestic factories have adopted the computer design and simulation filling technology, so that die-casting die production quality greatly improved, production period greatly shortened.
China’s mold industry is developing rapidly, from 1996 to 2004 the average annual growth rate of 14% mold production, 2003 die-casting mold output value for the year 3.8 billion yuan. At present, China’s domestic mold to the market to meet the rate of only about 80%, which is mainly in the low-end mold, large, complex precision mold, in the production technology, mold quality and life and production capacity are unable to meet the needs of the national economic development. Research and development direction of the automobile and motorcycle industry, as well as the consumption of automotive accessories and ancillary products demand for die-casting production provides a broad market, die-casting aluminum alloy in the automotive applications will also continue to expand.
In the future of die-casting technology research and development, aluminum alloy die-casting deepening will still be a major direction of die-casting technology development. In order to meet the market demand, the future should further solve the following problems:
(1) promote the application of new high strength, high wear resistance of die-casting alloy, research can be colored die-casting alloy and for special security requirements of the casting and other aspects of the new die-casting alloy;
(2) the development of stable performance, easy to control the composition of die-casting aluminum alloy;
(3) simplify the composition of the alloy, reduce the number of alloy grades, to provide the basis for green production;
(4) further improve the new die-casting process (vacuum die-casting, oxygen-filled die-casting, semi-solid die-casting, extrusion casting, etc.);
(5) improve the rapid response to the market, the implementation of parallel engineering (CE) and rapid prototyping technology (RPM);
(6) Develop research and development of CAD/CAM/CAE systems;
(7) Develop and apply more die-casting aluminum alloy auto parts.
Die-casting aluminum alloy is the main body of non-ferrous alloy die-casting, and AI-Si series alloy is the main body of die-casting aluminum alloy, the development of die-casting industry is a long way to go, it involves the research and development of high performance alloy and the development of the process way, practice shows that the combination of the enterprise as the center combined with the relevant universities and research units is a wise choice, we have achieved the relevant results have been confirmed.