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Since the advent of pressure casting in the 1940s, the development of precision machining as a metal part approaching the final shape has been in the ascendant. New progress has been made in die casting equipment and its control, die casting process and die casting alloys. At the same time, the market needs to mass produce complex thin-walled and beautiful metal parts to meet the increasingly high requirements of die-casting parts in the automotive industry, electronic communication and household appliances, toys and other industries. However, due to some inherent problems of die castings, the potential of the alloy has not been fully utilized. The die-casting industry is also faced with the task of further improving the level of technology and management to ensure the high quality and low cost of castings.
The current market is a dynamic market. To be invincible in competition, companies must be able to adjust their business strategies in a timely manner, based on advanced technology and management. Technological innovation is about to become the focus of corporate competition in the 21st century. Only by mastering key technologies in their own hands and adopting advanced management systems can they improve their responsiveness to dynamic and volatile markets and improve their competitiveness.
At present, the die-casting industry in Guangdong Province and even the whole country is still far from the international advanced level. Since the reform and opening up, there have been hundreds of enterprises related to die casting that have moved from Hong Kong to Guangdong. Therefore, in the past decade or so, the Guangdong die-casting industry has experienced rapid development [1]. According to incomplete statistics, there are more than 600 die-casting manufacturers of a certain scale in the province, and there are more than 8 manufacturers with annual output of 3,000-5,000 tons of die-casting parts, and more than 10 manufacturers of 1,000-3,000 tons, 500~1. There are dozens of 000 t. The annual output of zinc alloy die-casting parts is close to 100,000 tons, and aluminum alloy die-casting parts are about 40,000 tons. There are more than 2,000 die casting machines in the province, and the largest is the Italian die casting machine with a clamping force of 21 000 kN. There are 7 die casting machine manufacturers in the province and more than 10 die casting peripheral equipment manufacturers. The annual output of die casting machines is about 600 sets, most of which are hot chamber machines. There are mainly the following problems: the die-casting equipment is mainly small, and the control system is relatively backward; the die-casting parts are mainly zinc alloy, mostly non-stressed parts such as household appliances and toys, and the proportion of parts such as automobiles and motorcycles is relatively small. Mold manufacturing is a weak link, and there are many mold manufacturers, but most of them are small-scale, the equipment is relatively backward, and the production cycle is long. Only a single mold factory began to implement CAD/CAM technology. There is still a long way to go to make China's die-casting industry reach the world's advanced level. It is necessary to promote the innovation of die casting technology; develop new injection systems and control systems; improve the intrinsic quality of die castings; develop new die casting technology; research new die casting alloy materials; and implement modern management.
1 Development of new die casting equipment and its control system
Pressure casting is a process in which molten metal fills a cavity at a very high speed under high pressure conditions and is a complex dynamic thermodynamic process. On the one hand, die casting can produce metal castings with complex thin walls, beautiful surfaces and high precision. On the other hand, the general die casting process is difficult to achieve laminar filling state and is involved in gas and inclusions, and the dense, heat treatable workpiece is not obtained, which affects its mechanical properties. If the method of reducing the filling speed is used to improve the filling state of the molten metal, the advantages brought by the above-mentioned die-casting production are inevitably sacrificed, and it is not necessarily effective for the complicated thin-walled parts. The production of high-quality, non-porous thin-walled die-casting parts is our goal, which is one of the chips to win in the competition of other processes in the die-casting process.
In order to improve competitiveness, this indicator of thin wall is also constantly pushing up, it is a very flexible indicator. For example, in the 1950s, the thin-walled body of the automobile industry was 2 mm, which is now 0.7 mm, and will be 0.5 mm by the year 2000. The thin wall of the zinc die casting is 2 mm in the 1960s, 1 mm in the 1970s, 0. 7 mm in the 1980s, and 0.3 to 0.5 mm in the 1990s. Aluminium die castings have a similar development, from 0. 5 to 1 mm in the 1990s.
Dense thin-walled castings are conditioned on sufficient metal pressure and short filling times, that is, filling at a high in-gate flow rate and sufficient metal pressure in a very short period of time. This requires the die casting machine to produce high injection speeds while producing high metal pressures. That is, according to the casting process requirements, the die casting machine must be able to provide a certain amount of injection energy in order to achieve high energy filling. To obtain a dense casting with a wall thickness of 0.75 mm or less, a die casting machine with an injection energy of 550 kW·h or more is required.
The high-energy filling type can also adjust various process parameters to obtain high-quality die-casting parts. The high energy filling type is the development direction of the injection molding system of the die casting machine. In order to achieve high filling energy, a die casting machine should minimize the friction loss and local resistance as well as the energy loss caused by inertial force--pressure drop.
In addition, the advancement of the die casting machine is mainly reflected in the stability and reproducibility of the production process, that is, each shot is as close as possible to the preset ideal shot curve. However, there are many variables in the die casting process that affect the stability of the die casting process [2]. Static factors such as the working state of the die casting machine and the mold, and the pressure of the accumulator are unchangeable after the process equipment is selected.
Some variables caused by dynamic factors and human factors, such as the amount of metal injected each time and its temperature, mold temperature, viscosity of hydraulic oil, paint quality of paint, artificially set parameters, etc., must be carried out through the injection control system. Corrected. The average injection time takes 20 to 80 ms, and the required filling time for thin-walled aluminum alloy or magnesium alloy parts is shorter, 5 to 12 ms [3 ]. The injection control system must be able to take only one injection time. The injection curve is controlled by about 10%, that is, a response time of 2 to 8 ms. This puts severe demands on the electronic circuit system, that is, the electronic circuit control system must react within a few microseconds. Reproducibility requirements are high, it is necessary to install a metal leading edge sensor [4]. When the metal liquid seals the sensor, the leading edge of the molten metal is accurately identified, the information is fed back to the electronic control device, and the electronic instrument is recalculated to obtain a stable optimal injection curve. The development of advanced sensors is driving the development of die casting machines and die casting processes. The microstructure and properties of die-casting parts depend on the thermophysical conditions in the die-cast cavity and its adjacent areas, so it is important to develop heat detectors and sensors close to the cavity.
2 Develop new die casting technology
2. 1 semi-solid alloy die casting
Semi-solid alloys have two characteristics compared to all-liquid and all-solid alloys. First, after the solid component of the semi-solid alloy exceeds 50%, the viscosity increases sharply as the solid component increases. For unstirred alloys, the viscosity increases sharply with the increase of solid components after the solid component exceeds about 15%. Second, the strongly stirred alloy has solvating properties.
It is because of this special physical property that this highly stirred semi-solid alloy has excellent casting properties, and its fluidity is quite good in the case of a relatively high solid content, compared to a full liquid alloy. There is no serious decline. In addition, the feeding performance is also better, on the one hand, its own shrinkage has been reduced, on the other hand, it can also be fed by the liquid 2 solid and the same flow. Further, since this alloy is solvating, a die casting and extrusion process which is subjected to a large shearing action during molding is suitable. Semi-solid die casting has its own unique features compared to all-liquid alloys.
First, it is inconvenient to operate all-liquid alloys, which is an obstacle to improving working conditions and improving mechanization and automation. Semi-solid alloys only exhibit fluid-like properties during molding, and can be like solids before molding. Handling, which is very beneficial for the organization's highly mechanized and automated production. Second, when the semi-solid alloy is die-casted, the heat state of the mold is greatly improved. One reason is that the temperature of the alloy itself is reduced, and the heat contained is less. The heat that the semi-solid metal needs to eject from die casting to complete solidification is only about half of the metal in the superheated state. Another reason is that the semi-solid alloy enters the cavity in different modes, does not flow, and has a low degree of turbulence. It can basically achieve full wall thickness filling, the thermal shock to the mold is very low, and the life of the mold is improved. Therefore, semi-solid die casting has great application prospects for high melting point alloys.
Semi-solid alloys also provide advantages for the manufacture of metal matrix composites by casting methods. It is a relatively simple method to prepare a composite material by adding non-coated non-metal particles during the intense stirring of the semi-solid alloy. Since the semi-solid alloy has a solid primary crystal, the floating or agglomeration of the non-metallic particles can be prevented, and the distribution is relatively uniform. In addition, the non-metallic particles are subjected to intense stirring and friction, and the surface is activated to be tightly bonded to the matrix alloy.
Although the concept of semi-solid alloy casting was proposed by MC Flemings et al. in the early 1970s [5], how to effectively prepare and shape semi-solid alloy slurry has been a problem so far [6,7]. Application is limited, material selection is limited, and process specifications are very strict. In particular, the solid and liquid fractions should be accurately controlled, and the deviation should be within ±3 %. In order to meet the process requirements of semi-solid die casting, process equipment design is a key. H. Peng et al. [6] proposed a flow-forming process (Rheomolding). The principle is to feed liquid metal into a specially designed injection molding barrel, which is sheared by a rotating screw to cool it into a semi-solid slurry. After the process requirements are met, die-casting is performed, and one machine completes two processes to produce slurry and die-casting. Temperature control is a key. N. Bradley et al. [8] proposed a thixoforming process in which solid magnesium metal particles or debris are fed into a screw compression molding machine. In the case of heating and shearing, the metal becomes The slurry is die casted. This process has a more chopping process.
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