1. Introduction:
Die casting alloys include the commonly used aluminum, magnesium, zinc, and copper alloys, as well as the earlier tin and lead, and the less commonly used steel and iron. Pure copper is very difficult to cast, and its products are prone to surface cracking, porosity problems, and the formation of internal cavities. General die-casting manufacturers by adding a small number of elements (including beryllium, silicon, nickel, tin, zinc, chromium and silver) can improve the castability of copper, used for die-casting production bearings, bushings, gears, fittings, valve body, chemical with a variety of parts, decorative parts, and motor rotor.
In view of the requirement for a narrow condensation range and high beta-phase content, copper die casting alloys are mostly referred to as brass alloy die casting (usually with lead or silicon) and copper alloy die casting, the latter being used for electrically conductive products, such as rotors for electric motors. Copper die casting is usually for very complex geometry and large quantities of copper alloy products.
2. The characteristics of copper die casting and vacuum application
Die cast copper alloys have the following key features:
- Excellent corrosion resistance
- Highly efficient electrical conductivity
- Excellent low friction
- Good elongation and tensile strength
- Suitable for low to medium hardness
This makes copper a versatile alloy among metal die casting alloys.
Brass die-casting components
Here is an example of a die-cast copper motor rotor: since the American inventor Tesla invented the AC motor in the late 19th century, this simple structure, using alternating current, without rectification, spark-free motor is widely used in industrial and household appliances.
As early as the beginning of the last century businessmen are envisioning: copper’s higher conductivity, copper instead of aluminum will improve the electrical efficiency of induction motors. Copper conductivity is higher than aluminum about 40%, cast copper rotor can make the total loss of the motor significantly reduced, thereby improving the overall efficiency of the motor, the temperature rise is also greatly reduced, made of copper motor rotor will be motor applications to bring a great enhancement, only due to the melting point of copper is too high, mobility is too poor, etc., has been bound to the development of the motor cast copper rotor.
In recent years, along with the rapid development of die-casting technology and mold technology, the mass production of die-cast copper rotors has been realized. In many types of electric motors, die-cast copper rotors have mature and stable applications.
With other die-casting process is the most significant difference is, copper die-casting fluidity is poor, injection time to be short; Pouring temperature is high, to 900-1200 ° C; With other die-casting process is faced with the same is the common porosity problem, brought by filling incomplete, surface defects, the internal loose, airtightness and so on defects. The introduction of vacuum exhaust has also become a die-casting enterprises to consider the main process to solve the measures.
A die-casting motor rotor die-casting mold cavity volume is 3.8l, put inlays (silicon steel sheet iterative piece of core) after the volume: 2.2l; melting cup filling rate of 70%; punch sealing the position of the sprue mouth to start the slow compression injection, the slow compression injection time is 0.22s; turn to fast compression injection, the fast compression injection filling time is 0.08s. It can be seen that, the time of the slow exhaust is short, and need to be greater instantaneous exhaust capacity as well as the whole process of the exhausting.
If the HV300 system is used, the vacuum tank is 300 liters, and the instant on vacuum is 13 mbar; the double exhaust plate is used to exhaust, the total area of exhaust is 300 mm2, and the exhaust time takes 0.273s.
3.Vacuum copper die-casting mold points
As we all know, compared with aluminum die casting mold life of ten thousand times or more than one hundred thousand times, copper die casting mold is thousands of times, so copper die casting process is more challenging. The high temperature and poor fluidity of copper alloy is the root cause of the complexity and high cost of copper die casting.
3.1 Mold materials
In the higher pouring temperature, brass 900-1000 ° C and copper 1000-1200 ° C, die-casting molds need to be higher requirements: with high temperature resistance and heat preservation; taking into account the poor fluidity of copper, the area of the gate relative to aluminum alloy molds to be slightly larger;
Mold materials are usually INCONEL alloy 617 and alloy 625, and Haynes alloy 230; INCONEL alloy 617 is a 22% Cr, 12.5% Co alloy solid solution strengthened with 9% Mo; alloy 625 has 21.5% Cr, 9% Mo and 3.65% Nb;
Haynes alloy 230 is used as an optional material for copper die casting molds and has slightly higher yield strength and ductility than alloy 617, with high weldability and repairability. The life of H13 as a die steel is greatly limited by decarburization and softening of common die steels at high surface temperatures.
3.2 Temperature control
In the die casting of high melting point copper melts, high temperatures, high heat of fusion, high latent heat, and high thermal conductivity all combine to maximize, resulting in a large temperature difference between the cooler interior and the mold surface. The resulting thermal shock and thermal fatigue cause rapid mold failure.
When using molds at higher temperatures, temperature control will be critical to enable the mold material to maintain the necessary strength even after prolonged exposure to high temperatures. A mold thermostat is its initial preheating measure, with copper alloys requiring more than 200°C compared to the 150 to 180°C preheating insulation setting for aluminum alloys.
For mold preheating temperatures of 350-500°C, the capabilities of oil heaters are clearly inadequate. It is possible to reduce the degree of temperature difference by preheating the mold insert (resistance heater) to control the mold failure caused by thermal shock.
Melt cups require the same preheating and holding. The addition of cooling water is also a means of maintaining a balanced mold temperature, e.g. ensuring a mold operating temperature of 600-650°C, but excessive cooling is counterproductive. A dry powdered release agent should be used before each press shot.
3.3 Mold Design
In the copper die casting process, mold design is a fundamental aspect of the overall manufacturing process, and all attributes of the mold affect the final product. The quantity of product to be produced, the size and/or weight of the casting, the shape and complexity of the product, the amount and quality of finishing required, the required surface finish, the internal requirements specified (e.g., sealing), the type and level of treatment and inspection to be performed, the amount of dimensional accuracy of a single part that can be varied, and the castability characteristics of the copper alloy specified or selected are initial conditions for mold design.
Usually, the way the copper alloy flows depends on the designer’s understanding and interpretation of the copper die casting technology and process, including parting surface setup, gate setup, barrel and runner setup, inlet direction determination, and holding and cooling channel arrangement, and finally vacuum exhaust setup, including the corresponding mold sealing.
That is to say, for high-pressure copper die casting, the copper melt will flow under high external pressure, and all the details involved in entering the cavity to the completion of mold filling should be carefully considered. Vacuum venting is always the last aspect to be considered in mold design.
4.Copper die-casting vacuum system
Copper Die Casting Vacuum Principle: Vacuum technology for copper die casting has been adopted by several manufacturing companies. The usual drawback is that it is prone to porosity problems, which are due to air being trapped in the casting during the die casting process. Over the years, vacuum die casting has proven to be the best way to eliminate porosity problems.
Typically, the cavities of steel molds must form a relatively hermetic environment. The vacuum vent is located near the top or side of the mold; due to the introduction of the vacuum, the molten copper is injected into the mold cavity by means of the kinetic energy of the ejection under a back pressure below atmospheric pressure; along with the ejection time, the molten copper continues to be filled with the mold, and the pressure inside the mold cavity decreases until the filling is finished and the ejection process is completed. This process is repeated periodically.
Key benefits of vacuum copper die casting technology include:
- Die castings have good welding properties.
- Die castings have high mechanical strength.
- Suitable for high volume production.
- Minimize porosity.
- The product is dense and has a uniform electrical conductivity.
- The product is highly airtight.
- Improvement of the product’s filling degree.
- Ensure product precision.
Disadvantages of Vacuum Copper Die Casting The technology has the following major disadvantages:
Initial investment costs are relatively high, including molds and equipment, and sealing. Setting up the vacuum die casting process is more tedious, not only to meet the copper alloy die castability, but also to consider the feasibility of exhaust. Affect the vacuum exhaust efficiency of many factors, in addition to the vacuum machine and exhaust components, die casting machine, molds, alloy quality, alloy temperature, spraying technology and so on should be considered uniformly.
Sealing must be considered for press injection components and molds, including parting surfaces, melt cups and punches, ejector pins, and even slides. This will ensure the efficiency and reliability of vacuum venting. In order to obtain high repeatability and accuracy of the product, it is important to control the pressure and speed profile of the die casting machine to be stable.
The following factors must be optimized: die coating, punch lubrication, copper melt alloy quality and die temperature control.
Achieving an optimized copper die casting vacuum process can be a delicate and challenging process. Each process should be optimized in order to obtain a high process vacuum.
Vacuum venting program: Vacuum venting needs to be set up in conjunction with the compression injection process, including the selection of the vacuum machine and the determination of the venting element. Despite the high exhaust efficiency of vacuum valve, combined with the characteristics of high pouring temperature of copper alloy and the cost of exhaust element, the exhaust block is often the most suitable choice. The poor fluidity of copper alloy, high filling rate of molten cup, its low speed in 0.25-0.5m/s, high speed from 0.8 eventually reach 3m/s, to the vacuum exhaust brought relatively little opportunity, so the exhaust element often need a larger exhaust area.
Considering that the temperature of gas inside the cavity is very high, even reaching more than 500-600°C in an instant, which brings trouble to the sealing of the vacuum system, it is usually considered to set up a gas water cooling device in the pipeline between the mold and the vacuum machine to reduce the temperature of the gas and improve the life of the vacuum seal.
5. conclusion
Copper die casting for the solution of complex shape, large batch of copper products provides a wider and more effective means of production. One of the brass die-casting products used in decorative parts and wear parts has been very mature, and brass die-casting transmission gearshift system, control rods and water pump turbine parts, etc.; Copper die casting used in motor rotor more broad prospects, rotor copper weight from a few grams to dozens of kilograms, and its permanent magnet performance, life and cost is better than the use of the current motor permanent magnets.
With the growing market rate of electric vehicles and hybrid vehicles, copper die casting motor rotor will have a greater prospect. As the vacuum copper die casting service process is becoming more mature and widely used, will certainly provide strong support and guarantee for high quality copper die casting products.
