Key points in the production process of magnesium alloys

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Key points in the production of magnesium alloys: With the rise and development of magnesium alloy die-casting, exploring the characteristics of this casting process as well as the key points for safe operation is beneficial for achieving safe and high-quality production. Given the different characteristics of magnesium alloys, special protective measures and equipment should be employed. Some factories still use traditional conventional cold-chamber die-casting machines to produce magnesium alloy die-cast parts, which poses potential dangers and risks during production. 　　1. Die-casting process The die-casting process for magnesium alloys is similar to that of other alloys, but due to the unique properties of magnesium alloys, there are differences in terms of pressure, speed, temperature, and the use of coatings. 　　1.1 Pressure Magnesium alloy die casting is carried out in two forms: hot-chamber and cold-chamber types, and the pressure used during casting varies accordingly. The injection pressure in hot-chamber machines is around 40 MPa, while that in cold-chamber machines is higher, typically ranging from 40 to 70 MPa. Another important factor is the time required to build up the pressure. Due to the low latent heat of fusion of magnesium alloys, they solidify much faster inside the mold compared to aluminum alloys. If the pressure is not built up quickly enough, the molten metal in the gate and cavity will have already solidified, rendering the pressure application ineffective. Therefore, the time needed to build up pressure is an important indicator of the performance of a magnesium alloy die casting machine. Most die casting machines take more than 60 ms to build up pressure at this point, by which time the magnesium alloy in the gate has already solidified, preventing the pressure from reaching the mold cavity. An excellent injection system usually manages to build up pressure in less than 20 ms. 1.2 Speed Magnesium alloys have low density (only 2/3 that of aluminum alloys), which results in low inertia. At the same time, since magnesium alloys also solidify very quickly, it is necessary to fill the entire mold cavity before the metal solidifies; therefore, the injection speed for magnesium alloys must be high. The injection speed for hot-chamber magnesium alloys can reach 6 m/s, while the speed of cold-chamber die-casting machines is even higher, at 8 m/s. A high shot speed also results in a high gate speed. For example, the gate velocity for die-casting molds made of zinc alloys and aluminum alloys is typically between 40 m/s and 60 m/s; otherwise, mold ablation may occur. The gate velocity for thin-walled magnesium alloy castings often exceeds 80 m/s. Due to the low thermal conductivity of magnesium alloys and their poor wettability with mold steel, mold ablation is not as severe as it is with aluminum alloys. 　　1.3 Temperature Temperature is a thermal factor in the die-casting process. To ensure favorable filling conditions, maintain the quality of the cast parts, and control and preserve thermal stability, it is necessary to adopt appropriate temperature specifications, primarily referring to the pouring temperature of the alloy and the temperature of the mold. 　　The material pot of a hot-chamber die-casting machine is located inside the furnace, which results in minimal heat loss during injection. As a result, the temperature required for magnesium alloy casting using hot-chamber die-casting is lower, usually around 640°C. The temperature of cold-chamber die-casting machines is higher, usually around 680°C. One thing worth noting regarding magnesium alloy die casting is that if the shaping of the product is not ideal, improvements can be made in other aspects such as the injection speed and mold temperature. It is not advisable to simply increase the pouring temperature of the alloy, as the protective gas used in magnesium alloy furnaces loses its effectiveness at high temperatures (above 710°C). 　　An inappropriate mold temperature can lead to unstable casting dimensions and may cause defects such as sticking, warping, and underfilling. To heat the molds, some die-casting factories commonly use a torch for heating. This method is not very suitable for magnesium alloys, as it is difficult to maintain a uniform temperature in the mold, and the mold’s lifespan is also reduced due to cracks that can occur as a result of exposure to excessive cooling or heating. Furthermore, due to the low latent heat of magnesium alloys, in production, especially when manufacturing thin-walled parts, it is necessary to continuously heat the mold in order to maintain favorable filling conditions and stable production, thus keeping the mold temperature steady. In the die-casting of magnesium alloys, the most ideal heating method is to use a mold temperature controller to heat the mold with hot oil. The hot oil flows continuously through the pipes inside the mold, heating it from within and helping to maintain a stable equilibrium state for the mold. Since heat transfer oil can not only heat but also cool the mold like water, it functions as a heat exchanger to keep the mold temperature within a certain range. With this heating method, the temperature of the mold remains stable and uniform, which effectively extends the mold’s lifespan, improves product quality, and also stabilizes the production rhythm. During the die-casting of magnesium alloys, the temperature of the mold is generally maintained between 180°C and 280°C. 　　1.4 Coatings The function of coatings is to provide an effective isolation layer between the die-casting alloy and the mold, preventing the molten metal from directly contacting the cavity. They also help maintain the fluidity of the molten metal, as well as cool the mold and reduce the tendency for sticking. Compared to aluminum alloys, magnesium alloys are lighter in terms of mold compatibility and weldability. However, since the injection speed of magnesium alloys is higher than that of aluminum alloys, weldation may occur when hot metal impacts certain components of the mold at high speeds. Using the appropriate mold coating can reduce this trend. The most commonly used lubricants are water-based ones. Due to the low heat capacity of magnesium alloys, it is not necessary to use lubricants as a cooling medium, and their usage time should be kept as short as possible, typically 50% of that for aluminum alloys. To reduce the water content, higher-concentration coatings are usually used. 　　2. Safe Operation 　　Due to the flammability and explosiveness of magnesium, it is essential to pay close attention to safety during production. The following outlines the key points regarding personal protection, equipment safety measures, and accident handling. 　　2.1 Personal Protection Personal protection is a fundamental requirement for working with magnesium alloys. Generally, the following protective equipment constitutes the essential safeguards for those involved in magnesium alloy die-casting: (1) Work clothes (2) Safety helmet (3) Protective face shield (4) Heat-resistant asbestos gloves (5) Fire-resistant clothing and pants (with a heat resistance of over 700°C) (6) Safety shoes Operators must wear the above-mentioned protective equipment before starting work. Those who do not wear such protection should not approach the work area and must not carry out any operations. 　　2.2 Equipment Safety Protection Magnesium alloy furnaces must not use nickel-containing stainless steel, and the same applies to melting tools as well. It is preferable for the furnace to have a double-layer structure; in the event that the inner crucible breaks, the magnesium melt can flow into the space between the inner and outer layers. At the same time, an alarm is triggered to stop heating, preventing the melted magnesium from leaking out and causing danger. It is necessary to regularly check the furnace for signs of rust; if rust is present, it should be removed promptly. In the process of magnesium alloy die-casting, it is crucial to maintain a dry and clean working environment. Before each startup, the mold should be preheated to over 150°C. Do not apply too much coating during spraying, to avoid water accumulation in the cavity, which could pose a danger. Also, try to avoid using water cooling for the punch and die cooling. The punch can be cooled by air cooling, while the heating and cooling of the mold are generally done using high-temperature resistant oil. The speed of die-casting punches for magnesium alloys is also higher than that for aluminum alloys; to prevent flying debris from injuring people, a baffle to catch such debris is sometimes installed at the parting line of the mold. 　　2.3 Handling of accidents involving magnesium alloys Fire extinguishing agents suitable for magnesium alloys include dry sand, covering materials, and Class D fire extinguishers. These devices should be placed in easily accessible locations so that they can be used promptly in emergency situations. Dry sand and covering materials should be stored in containers to prevent moisture exposure, and regular inspections should be carried out. 　　For the combustion of small amounts of magnesium, it is possible to… 3. Disasters and safety issues that can arise from inferior magnesium alloy die-casting equipment. Due to their flammability and explosiveness, and because high pressure and high speed are involved during the injection process, high standards are required for die-casting equipment; inferior equipment poses potential risks. 　　In die-casting operations, the molten magnesium alloy is injected into the mold cavity at a speed of 70–100 meters per second (at the gate) to achieve molding. Since molten magnesium is flammable and explosive, it burns violently in contact with oxygen and explodes when exposed to water. It also reacts violently with rust, moist concrete, silicon-containing refractory materials, etc., and is difficult to extinguish in case of a fire. Once a fire breaks out, it cannot be extinguished easily; therefore, extremely high demands are placed on the performance, reliability, and safety of the die-casting equipment used. Inferior equipment can very easily lead to disaster-related accidents. During the process of exploring its application, many serious safety accidents caused by equipment issues have occurred both domestically and internationally. Examples are as follows: In December 1998, a crack and leakage accident occurred in a magnesium melting furnace at a die-casting factory in Hyogo Prefecture, Japan. A large amount of molten magnesium flowed onto the machine frames and the floor, which led to a fire; the entire factory was destroyed and many people were injured or killed. 　　In March 1999, an fire broke out at a factory in Mie Prefecture, Japan, due to a leak of molten magnesium. The magnesium flowed out through corrosion holes in the crucible and reacted with the heat-insulating refractory materials inside the furnace, causing the workshop to be destroyed. 　　Between 1996 and 2001, Shaanxi Northwest Forest Machinery Factory used old-fashioned conventional die-casting machines and simple homemade furnaces to produce magnesium alloy housings, resulting in several major safety accidents. A sudden explosion occurred when a batch of hot magnesium melt was injected into the mold cavity; this explosion damaged the mold locking mechanism, and a fireball was ejected, injuring the plant’s chief process engineer and other people who were on site at the time ； Once, corrosion and perforation of the furnace crucible led to a leak and a fire; a blinding white light filled the area as the flames spread rapidly. The factory urgently used trucks to transport fine sand and covered all the machines and furnaces with it, which helped to bring the fire under control initially. 　　Under normal circumstances, it is still possible to take some measures to extinguish a small-scale initial fire in the magnesium melt; however, once a large fire breaks out, spreads, or an explosion occurs, it becomes impossible to control and extinguish the fire, resulting in severe losses in terms of lives and property. 　　To ensure safe production as well as reliability under long-term use and in harsh operating conditions, the quality requirements for die-casting equipment made of magnesium alloys are extremely high. Unlike ordinary mechanical equipment, inferior devices produced by manufacturers lacking strong comprehensive technical capabilities can easily lead to serious accidents. The list is as follows: 1) When low-quality crucibles are used in high-temperature conditions of over 6500°C (with the outer layer exposed to temperatures above 7000°C) during prolonged production, the outer layer tends to oxidize rapidly. The inner layer, in turn, suffers from corrosion due to both the magnesium melt and the SF6 protective gas; this corrosion progresses swiftly, leading to perforations. Once these occur, the molten magnesium leaks out, causing fires and explosions that can result in serious disasters. Using traditional, conventional protective methods only exacerbates the process of corrosion. 　　The crucibles of high-quality equipment are made from specially developed composite materials; the inner layer is corrosion-resistant, while the outer layer is heat-resistant and resistant to high-temperature oxidation, which helps to prevent serious perforation accidents. 　　2) High-quality die-casting equipment features precise and stable control of the protective gas; the composition and flow rate of the gas are well maintained. It also comes equipped with special automatic protection mechanisms for situations such as sudden power outages or accidents, ensuring extremely high levels of safety. Low-quality equipment has inaccurate control of gas composition and flow rate; low concentrations and flows can easily cause fires in the furnace, while high concentrations and flows can lead to rapid corrosion of the furnace, as well as leaks of magnesium liquid that result in fires and explosions. Moreover, there is a lack of reliable automatic protection measures for special situations. 　　3) High-quality equipment uses specially developed refractory materials that do not react with molten magnesium, while lower-quality materials use ordinary refractory materials that are prone to reacting violently with molten magnesium in the event of a leak, leading to fires and explosions. 　　4) High-quality equipment is made from specially developed heat-treated steel imported from Germany; it can resist corrosion caused by high-temperature magnesium alloys, and maintains excellent high-temperature properties such as hardness, tensile strength, yield strength, toughness, creep resistance, and temper stability at temperatures ranging from 650 to 700°C. Low-quality equipment is made from ordinary hot-work steel; it cannot withstand the corrosion caused by high-temperature magnesium alloy liquids and finds it difficult to maintain good performance at high temperatures. When it deforms, cracks, or leaks, it can easily cause splashing of high-temperature magnesium alloy liquids under high pressure (400–800 kgf/cm2) and high speed (70–100 m/s), leading to injuries or explosions and fires. 　　5) High-quality equipment enables precise, scientific, and reliable control of the injection system, whereas low-quality equipment has an immature design; it uses components and materials of poor quality that are unreliable, leading to safety issues and problems with the quality of the cast products. High-quality equipment uses top international brand hydraulic components, ensuring extremely reliable operation with no oil leakage. Low-quality equipment uses inferior hydraulic components, resulting in unreliable operation and significant oil leakage. The leaked oil mixes with the moisture from the release agents, forming an oily-water mixture that accumulates around the machine; upon contact with high-temperature magnesium melt, this mixture can easily explode. 　　In terms of the overall performance and quality of the equipment, there is a significant difference between the two. Poor quality of the crucible and unstable composition of the protective gas can both affect the composition and properties of the alloy, leading to a decline in the internal quality of the die-cast parts. This includes an increased tendency to develop cold or hot cracks, a decline in overall mechanical properties, and corrosion resistance that does not meet the required standards. Unstable performance of the injection system can also lead to a loose internal structure of the castings, unstable mechanical properties of the die-cast products, and various other die-casting defects. 　　Poor-quality equipment produces inferior and unstable magnesium alloy products, which will erode the confidence of end-users of these alloys. It will destroy the magnesium alloy application market that has just begun to develop as a result of the hard work of all those involved in our industry. It will pull China’s magnesium alloy processing industry, which has made progress and possesses significant international competitive advantages, back into a path of low-quality, low-price competition. This will ultimately reduce the technical quality standards of the industry, as well as undermine the trust of international customers and result in a loss of competitiveness on the global stage. 　　4. Conclusion: Magnesium alloy die-cast parts also have a broad market and great potential for application in the automotive industry. Due to the significant benefits that magnesium alloy die-cast parts bring in terms of vehicle lightweighting, higher speeds, and improved fuel efficiency, they have received considerable attention from automobile manufacturers; as a result, the production of magnesium alloy castings is increasing at a rate of 20-25% per year. Some typical components, such as steering wheels and clutch housings, are seeing a trend toward being replaced by magnesium alloy die-cast parts. In China, due to the relatively short history of magnesium alloy die-casting, domestic manufacturers are still cautious when entering this field. In fact, with high-quality and reliable magnesium alloy die-casting equipment, along with appropriate casting processes and safety measures, it is quite easy to achieve success in this area. Moreover, magnesium alloy die-casting is a field with great prospects