Structural parts generally refer to the components with functions of supporting and positioning, and their mechanical properties such as strength required for carrying capacity must be guaranteed. Automobile structural parts cover a wide range of components, including dial, sheet metal, strut, chassis, hub, support, bracket, etc. As the requirements of reducing weight and raising productivity increase, the practice of replacing steel with light alloy especially aluminum alloy, and of replacing gravity casting with die-casting, such as supports and brackets, have been widely developed and applied.
According to the load bearing requirements of automobile alloy structural parts, the internal structure of die-castings must be uniformly compacted and completely filled, and the porosity and shrinkage cavity rate must be well controlled. The following is a brief analysis and discussion on some existing applications.
I. Ultra-Low-Speed Die-Casting
It is also called laminar flow filling die-casting. The products feature thick wall and relatively simple structure. Under natural exhaust, the low speed of die-casting punch ranges from 0.1 to 0.25 m/s, high speed from 0.2 to 0.5m/s, and gate speed from 0.25 to 0.6m/s. In this way, it can reduce the inclusion of molten aluminum alloy in the pressure chamber flow, and reduce the steady injection of molten fluid during filling. The temperature of molten aluminum is required to reach above 700oC to enhance liquidity. It is required that the cinder ladle not be less than 30% of the product weight, and the time of holding pressure be long.
Under vacuum auxiliary exhaust, owing to slow injection speed and sufficient exhaust time, the exhaust area and the exhaust capacity of the system can be reduced by half compared with that under high-pressure die-casting. Under the same conditions, the further reduction of porosity is not obvious.
Compared with stoma, it may be more likely to form shrinkage cavity by means of ultra-low-speed die-casting due to its high melting temperature, large gate thickness and large casting wall thickness. Good control over mold temperature can greatly reduce the formation of shrinkage cavity.
Super-low-speed die-casting products feature high density and good mechanical properties. However, it is more likely to form shrinkage cavity and oxidation impurities. At the same time, its production cycle is a little long with relatively low productivity; it is difficult in edge removal with relatively large gate.
II. Vacuum Aided HP Die-Casting
In European and American manufacturers, there are a large number of successful cases in high-pressure die-casting automobile structural parts, which has been constantly tried at home. The basic condition is that auxiliary vacuum die-casting is introduced to solve the exhaust problem of structure parts with thick wall. The low speed of die-casting punch ranges from 0.2 to 0.3m/s, high speed from 1.8 to 2.3 m/s, and gate speed from 28 to 35 m/s. The temperature of molten aluminum is required to reach above 680oC. It should not be too fast at low speed to avoid inclusion of molten aluminum, and to indirectly provide adequate opportunities for exhaust. The mold temperature machines and water coolers will greatly reduce the impact of shrinkage cavity. The dynamic mold temperature is 180oC, while the static 150oC.
The faying surface of the mold is sealed with rubber strip, the punge is sealed with ferrule and the ejector pin of moving die is closely structured, All these help increase the possibility of high vacuum exhaust. Technically, the methods of mold vacuum exhaust, auxiliary pressure chamber exhaust and closed ejector plate exhaust can be adopted. Among them, the exhaust method of mold parting surface belongs to the major way, while the other two the auxiliary. The exhaust point of the pressure chamber will be closed before the punch reaches the exhaust point, while the parting surface exhaust and ejector plate exhaust are throughout the whole process. Vacuum valves are mechanical throughout the process.
III. Pore-Free Aided HP Die-Casting
Take the oxygenation on the top of the mold for example: after the mold is closed, the filling of oxygen begins and stops before the punch turns from low speed to high speed. At the filling stage, molten aluminum alloy particles and oxygen react violently at the moment, which consumes oxygen. And then the oxide is distributed evenly in the casting, thus the porosity of aluminum casting is greatly reduced. Due to the large amount of heat generated in an instant, the closed cavity should be open at the last filling in time, at which time air cannot enter the cavity.
The low speed of die casting punch ranges from 0.2 to 0.3m/s, high speed from 2.5 to 3.0 m/s, and gate speed from 50 to 60 m/s. It should be taken into account in the mold structure that the aluminum liquid is uniformly injected into smaller particles to increase the instantaneous reaction effect with oxygen. It is required that the temperature of molten aluminum should reach above 680oC. It should also not be too fast at low speed to avoid inclusion of molten aluminum, and to provide adequate opportunities for oxygen filling.
Similarly, due to the generation of a large amount of instantaneous heat, it is indispensable to use mold temperature machines and water coolers, especially in the places of material pipes, punch and diverging cones where high-pressure water coolers should be adopted to extend the service life of material pipes, punch and dies. And the dynamic mold temperature is 160oC, while the static 140oC.
Oxygenation can be applied to the inlet or top of the mold. The oxygenation at the feeding port ends before the feeding, then the top of the cavity opens. The time of oxygenation is relatively short. At the same time, the replacement rate of air and oxygen is poor due to the opening of the top of the cavity and the oxygen-filled part of the feeding port. The oxygenation at the top of the mold can be closed when the punch is at low speed, which requires a long time. At the same time, when it is filled with oxygen, only the feeding port is open, and the replacement rate of air and oxygen is high. Exhaust block can be used in exhaust and heat exhaust.
IV. PF-Vacuum Aided Die-Casting
Here, both oxygenation and vacuum exhaust are accomplished through vacuum exhaust blocks at the final mold filling part. The process of oxygenation followed by vacuum exhaust is as follows: oxygenation is completed before pressure injection and before the punch closes the feeding port. Vacuum exhaust then begins. Vacuum is used to remove the oxygen-dominated gas. The metal flow in the high-speed injection will burn off the oxygen in the cavity which has not been removed in time. During the whole process, compared with pure oxygenation, the reaction between aluminum and oxygen is reduced, and the generation of heat will be greatly reduced; thus the requirement of heat removal is small. Compared with pure vacuum exhaust, the cavity gas is mainly oxygen. With the process of injection, the uninterrupted reaction between aluminum and oxygen will further reduce the content of the cavity gas.
The low speed of die casting punch ranges from 0.2 to 0.3m/s, high speed from 2.2 to 2.5m/s, and gate speed from 45 to 50 m/s. The die structure changes little. It is required that the temperature of molten aluminum should reach above 680oC. Vacuum exhaust blocks are used in oxygen-filled and vacuum exhaust components.
V. Comparison of Aided Techniques in aluminium die casting parts
Above structures are respectively ultra-low-speed die-casting, vacuum die-casting and oxygen-filled die-casting.
The structural parts discussed above refer to the functional parts with simple structure (without core pulling), thick wall, the function of supporting, and the bracket type of automobile products. Traditional ultra-low-speed die-casting (laminar filling) has low productivity and difficulty in edge removal, which is not suitable for mass production. High pressure die-casting of auxiliary vacuum exhaust adopts vacuum valves, and does not occupy the cycle time of injection; thus it is simple, economical, safe and reliable. Although it is applied well in a few enterprises in China at present, it is still a mature mode of production in the whole market with the best application prospect. The process of oxygenation is relatively complex, but it also has a good effect on the application of some structural parts, which needs further research, improvement and popularization. Combined with the characteristics of both, the oxygen-filled and vacuum method is not more complicated than independent oxygen-filled method. As the oxygen-filled method matures, it is believed that the oxygen-filled vacuum method will be favored by the die-castings manufacturers.
The pure vacuum method can adopt many sealing methods such as mechanical vacuum valves to improve the degree of vacuum in the cavity and reduce the porosity. However, due to the extra heat generated during the process of oxygenation, mechanical exhaust valves should be avoided as much as possible to reduce maintenance work and time; while exhaust blocks should be adopted as much as possible, and no rubber strip should be added in the parting surface. The products with core-pulling sliders should be treated carefully in the process of oxygenation. Full attention should be paid to the process of alloy dehydrogenation in oxygen-filled die-casting. Control over mold temperature is especially important for oxygen-filled die-casting, and mold temperature machines and water coolers are indispensable.