Die casting is a process of metal smelting and processing. It is a casting method in which molten alloy liquid fills cavity of a steel mold at high speed under action of high pressure and solidifies alloy liquid under pressure to form a casting. In the field of metal die-casting, magnesium alloys and aluminum alloys are widely used in many industrial fields due to their unique physical and chemical properties. Although they are similar in many ways, there are clear differences in die casting process, process, advantages and disadvantages, and application methods.

1. Hot chamber die casting: Magnesium alloys often use hot chamber die casting due to their low melting point and good fluidity. In this method, molten magnesium alloy is continuously fed into injection chamber of die-casting machine to maintain high temperature of molten metal, thereby ensuring fluidity of molten metal.
2. Cold chamber die casting: Although cold chamber die casting is less commonly used for magnesium alloys, this method can also be used in certain circumstances, such as when higher pressure or more complex mold design is required.
3. Vacuum die casting: In order to further improve quality and performance of magnesium alloy die castings, vacuum die casting technology is sometimes used. This method can eliminate pores and inclusions during die casting process, improve density and mechanical properties of casting.
Comparison table of magnesium alloy and aluminum alloy die casting methods

(1) Cold die casting machine means that injection chamber is not heated by melting;
(2) Melt is drawn from holding furnace manually or by an automatic scalding machine and injected into injection chamber. It is suitable for high temperature alloys such as aluminum, magnesium, copper and other alloys.

 

Cold chamber die casting machine structure
Cold chamber die-casting machines are mainly divided into two types: horizontal and vertical. Among them, cold-chamber horizontal die-casting machines are the most widely used worldwide. Large, thick-walled, stressed and die-casting parts with special requirements are often produced using this type of machine. Well-known die-casting machine manufacturers include BUHLER from Switzerland, FRECH from Germany, IDRA and ITALPRESSE from Italy, UBE, TOYO and TOSHIBA from Japan, and HPM from the United States.
Cold chamber die-casting machine mainly consists of three parts: molding system, injection system and injection process monitoring system. Although models of die-casting machines produced by each manufacturer are different, principles and structures of the first two are generally same, and main difference is reflected in injection process monitoring system. Main functions of this system include recording, evaluating and regulating die-casting process parameters, realizing human-machine dialogue, programming operations, having self-diagnosis, self-correction and fault alarm functions. In addition, it can interact with producers and users remotely via Internet.

 

The key to magnesium alloy cold chamber die casting machine is automatic pouring mechanism. At present, common automatic pouring mechanisms include vane pump type, air pressure pump type, gravity type and electromagnetic pump type. These mechanisms transport molten metal to injection chamber in different ways to achieve quantitative pouring. Among them, electromagnetic pump pouring system is favored because of its ability to accurately control pouring volume of molten metal (with an error of less than 2%).
Components such as dashboard of German Audi cars and right-angle bolsters of General Motors are produced on cold chamber die-casting machines. These parts have specific size, wall thickness and weight requirements, demonstrating widespread use of cold chamber die casting machines in production of magnesium alloy die castings. In addition, magnesium alloy die castings produced by cold chamber die casting machines also include car seat frames and car wheels.

It is directly immersed in molten soup and heated by molten soup, melt is directly drawn from molten soup during production. It is suitable for low alloys such as zinc, magnesium, lead and other alloys.

 

Hot chamber die casting machine structure
Magnesium alloy hot chamber die-casting machine is designed based on physical and chemical properties of magnesium alloy and characteristics of die-casting process. It is currently the most widely used magnesium alloy die-casting special equipment in magnesium alloy die-casting abroad.
Locking force of a hot chamber die-casting machine is generally below 7840kN, and die-casting production efficiency is about twice that of a cold chamber die-casting machine of same capacity. It is usually used to produce thin-walled die-casting parts with a small weight. For example, a hot chamber die-casting machine with a locking force of 9800kN can die-cast a bicycle frame with a single piece weight of 2.15kg, and production capacity is 70 pieces/hour. Computer magnesium alloy casing with an overall size of 610*610mm produced by American company WhiteMetalCasting is also produced using a large hot chamber die-casting machine. Current improvements to hot chamber die-casting machine mainly include: using energy storage to pressurize, injection speed of injection plunger can reach 6m/s, gooseneck and nozzle are induction heated to maintain optimal temperature; double furnaces are used for melting and heat preservation, thermal insulation devices and recirculation pipes are used to accurately maintain temperature of molten pool; wear parts are chromium plated to extend their service life. In terms of cold chamber die-casting machines, American Prince Company developed the first large-scale magnesium alloy cold-chamber die-casting machine with a locking force of 11176MN in 1986. In 1990, it produced a large-scale magnesium alloy die-casting machine with a locking force of 13172MN. This machine integrated magnesium alloy melting and die-casting, adopted a pick-up robot, making the entire unit a complete die-casting production unit. Maximum speed of injection plunger of magnesium alloy cold chamber die-casting machine manufactured by this company reaches 819m/s, change time of pressurization speed is controlled within 20ms, and minimum static pressure on molten metal is 419MPa.

 

Major foreign die-casting machine manufacturers, such as Germany's FRECH Company, Italy's IDRA Company and ITALPRESSE Company, Japan's TOYO Company, TOSHIBA Company, UBA Company and HISHINUMA Company, etc., all produce and provide magnesium alloy hot chamber die-casting machine series to the market. Magnesium alloy hot chamber die-casting machine consists of a molding mechanism, a punch gooseneck nozzle injection mechanism, a gas protection device for melting and heat-insulating crucible furnace, and an injection monitoring system. Main differences from ordinary hot chamber die-casting machines are: magnesium alloy hot-chamber die-casting machine requires a higher injection speed (generally above 8m/s); injection system is required to have a small hydraulic impact when injection is completed; working temperature of melting and holding crucible furnace is higher than that of zinc alloy hot chamber die-casting machine (about 660℃), temperature control is stricter; magnesium alloys have extremely strict control over content of impurity elements Fe and Cr. Therefore, crucible of magnesium alloy melting and holding furnace should be made of alloy materials with extremely low impurity elements C and Cr; since magnesium alloy melt is easy to oxidize and burn, magnesium alloy melting and insulating crucible furnace should be a closed structure and equipped with a protective gas input device to prevent oxidation of magnesium alloy melt.
Since magnesium alloy hot chamber die-casting machine uses a punch to directly press magnesium alloy melt into die-casting cavity through a closed gooseneck nozzle, injection pressure is small and there is usually no pressure increase, so it is generally not suitable for production of large, thick-walled, and high-load parts with higher requirements in automobiles and aerospace aircraft. In recent years, companies in United States, Japan, United Kingdom and other countries have successfully developed magnesium alloy semi-solid thixotropic injection molding machines. Magnesium alloy semi-solid thixotropic injection molding machine injects semi-solid magnesium alloy into die-casting mold under a certain pressure to form it. Its working principle is similar to that of an injection molding machine. It feeds prefabricated non-dendritic magnesium granules into a spiral feeding mechanism, where magnesium granules are heated to a semi-solid state, and semi-solid magnesium alloy slurry is sent into injection chamber through magnesium alloy slurry collection chamber at the other end of spiral feeding mechanism for injection molding.

 

Magnesium alloy semi-solid thixotropic injection molding machine can be used to produce magnesium alloy parts for automobiles, computers, mobile phones, household appliances, digital cameras, and video cameras that require lightness, fineness, and high specific strength. Castings have high strength, high reliability, close to finished product size, strong reproducibility, and almost no waste products, which can greatly reduce production cost of magnesium alloy castings. As of the end of 1998, there were about 100 magnesium alloy semi-solid thixotropic injection molding machines in production around the world. This casting forming method represents a development direction in production of magnesium alloy castings. With vigorous development of emerging industries such as smart home appliances, electronics, computers, and communications, there will be strong demand for magnesium alloys as green manufacturing materials. Magnesium alloy hot chamber die-casting machines will develop in direction of serialization, multi-function, fully automatic, and remote human-machine dialogue.
Magnesium alloy hot chamber die-casting machine is die-casting equipment specially designed for magnesium alloy and is widely used in the field of magnesium alloy die-casting. Its locking force is usually below 7840kN, and its production efficiency is twice that of a cold chamber die-casting machine with same capacity. It is especially suitable for producing light-weight, thin-walled die-casting parts. For example, a hot chamber die-casting machine with a locking force of 9800kN can produce 70 pieces of bicycle frames weighing 2.15kg per hour. In addition, products such as magnesium alloy casings for large computers are also produced using hot chamber die casting machines.
Improvements in hot chamber die-casting machines include use of accumulator pressurization, induction heating of goosenecks and nozzles, and double-furnace melting and insulation. In addition, wear parts will be chrome plated to improve service life. In terms of cold chamber die-casting machines, American Prince Company has developed a large-scale magnesium alloy cold-chamber die-casting machine, which has characteristics of high-speed injection, fast pressurization speed, and high static pressure of molten metal.
Many well-known die-casting machine manufacturers around the world, such as Germany's FRECH, Italy's IDRA and ITALPRESSE, Japan's TOYO, TOSHIBA, UBA and HISHINUMA, all provide magnesium alloy hot chamber die-casting machine series. These die-casting machines are composed of a molding mechanism, a punch gooseneck nozzle injection mechanism, a gas protection device for melting and heat-insulating crucible furnace, and an injection monitoring system. Compared with ordinary hot chamber die-casting machines, magnesium alloy hot-chamber die-casting machines require higher injection speeds, smaller hydraulic impact, higher operating temperatures, and stricter control of impurity elements. Magnesium alloy hot chamber die-casting machine is particularly suitable for producing thin-walled castings with high appearance requirements, such as housing parts such as home appliances, computers, communication equipment, daily hardware, instruments, etc. However, due to small injection pressure and usually no pressurization, it is not suitable for production of large, thick-walled, high-load automotive or aerospace aircraft parts.

Like other die-casting alloys, traditional die-casting technology causes magnesium alloy liquid to fill die-casting cavity in a high-speed turbulent and dispersive state, so that gas in cavity and gas generated by die-casting paint cannot be discharged smoothly. These gases either dissolve in die-cast alloy under high pressure, or form many high-pressure micropores dispersed in die-casting part. These gases and micropores dissolved under high pressure precipitate and expand at high temperatures, causing casting deformation and surface bubbling. Therefore, magnesium alloy die castings produced by traditional die casting methods, like die castings of other alloys, cannot be heat treated and strengthened, nor can they be used at higher temperatures. In order to eliminate this defect, improve inherent quality of die castings, and expand application range of die castings. In the past 20 years, some new die-casting methods have been researched and developed, including oxygenated die-casting, semi-solid metal rheology or thixotropic die-casting and squeeze casting, as well as several ups and downs vacuum die-casting, etc. Vacuum die casting eliminates or significantly reduces pores and dissolved gases in die casting by extracting gas in cavity during die casting process, thereby improving mechanical properties and surface quality of die casting. At present, AM60B magnesium alloy automotive wheel hubs have been successfully produced by vacuum die-casting on a cold chamber die-casting machine, and AM60B magnesium alloy automotive steering wheel parts have been produced on a hot-chamber die-casting machine with a locking force of 2940kN, and elongation of castings has been increased from 8% to 16%.

 

Die casting process flow chart
Oxygenated die casting is also called poreless die casting. In this method, oxygen or other active gases are filled into mold cavity before molten metal is filled to replace air in cavity. When the molten metal is filled, active gas reacts with filling metal liquid to generate metal oxide particles that are dispersed and distributed in die casting, thereby eliminating gas in die casting and allowing die casting to be heat treated and strengthened. Nippon Light Metal Co., Ltd. uses oxygen die-casting method to produce AZ91 magnesium alloy integral magnetic head bracket for computers, replacing original multi-layer laminated bracket, which not only reduces weight of bracket, but also achieves great economic benefits. Company also uses oxygen die-casting to mass-produce AM60 magnesium alloy car wheels and motorcycle wheels, which are 15% lighter than aluminum wheels. Oxygenated die-cast magnesium alloy parts can be heat-treated and strengthened like gravity-cast magnesium alloy parts, their mechanical properties are better than ordinary die-casting parts and gravity-casting parts. However, ordinary magnesium alloy die-casting parts are deformed during heat treatment and cannot be tested for mechanical properties.

 

Semi-solid rheological die casting has advantages of smooth mold filling, no metal splashing, less oxidation loss of molten metal, energy saving, safe operation, and reduction of defects such as holes in castings. Semi-solid rheological die-cast specimen of AZ91D magnesium alloy with a solid phase ratio of 40%-50% on a cold chamber die-casting machine eliminates pore defects and has a tensile strength of 140-200MPa. Magnesium alloy semi-solid die-casting method invented by American company Dow Chemical has been commercialized and has obtained three basic patents. Company launched second generation of semi-solid die-casting equipment in 1991. Its locking mechanism is same as that of an ordinary die-casting machine, while injection mechanism uses a spiral injection mechanism with an electric heating device. Granular magnesium alloy added to mechanism is spirally transported to a temperature-controlled heating zone protected by argon gas. In this zone, it is heated and sheared into a semi-solid state with a temperature of 580℃ and then enters accelerated injection zone with an injection speed of about 318m/s, cavity pressure is 34-41MPa, maximum can reach 136MPa, and cycle time is 20s. Compared with average porosity of ordinary die-casting parts, which is as high as 215%-310%, porosity of semi-solid die-casting parts is only 14%-118%. Another advantage of this method is that it reduces shrinkage of casting in mold. For some castings, zero draft angle can even be used, which significantly reduces demoulding resistance of casting and improves dimensional accuracy of casting. Magnesium alloy semi-solid die-casting parts that have been produced include automobile transmission housing covers, igniter housings, etc. Alloy used is AZ91D.

In addition, magnesium alloy matrix composites reinforced with particles such as silicon carbide have been researched and developed for many years. Although it has not yet reached stage of commercial application in the field of die-casting, impellers, bicycle cranks, automobile cylinder liners and other castings have been made by sand molding, precision casting and other methods, there is a trend of combining this composite material with semi-solid casting and applying it to the fields of die casting and squeeze casting.

Gravity casting refers to a process in which molten aluminum is poured into mold by gravity under action of earth's gravity. Gravity casting in a broad sense is divided into: sand mold casting, metal mold (steel mold) casting, lost foam casting, etc. Gravity casting in a narrow sense refers specifically to metal mold casting. Metal mold casting is divided into upright manual casting and tilt casting. The most commonly used metal mold (steel mold) casting is tilt casting. Mold is made of heat-resistant alloy steel, strength, size, and appearance of cast aluminum castings are higher than those of other casting processes. Molten aluminum for gravity casting is generally poured into gate manually, relying on weight of molten metal to fill cavity, exhaust, cool, and open mold to obtain sample. Process flow is generally: aluminum melting, casting filling, exhaust, cooling, mold opening, cleaning, heat treatment, and processing. When choosing which process to produce a product, choice is mainly based on wall thickness of workpiece. When wall thickness of product is greater than 8mm, die casting will cause a lot of pores to exist in wall, so products with thicker walls can be completed by gravity casting process.

 

 

Characteristics of aluminum alloy gravity castings are:
1. Surface finish of product is not high, and pits are easy to form after shot blasting.
2. Aluminum castings have few internal pores and can be heat treated.
3. Product has low density and slightly poor strength, but has high elongation.
4. Mold cost is low and mold has a long service life.
5. Production efficiency is low, thus increasing production cost.
6. Process is relatively simple and not suitable for producing thin-walled parts.
7. Special aluminum alloys with low fluidity can be used.

Pressure casting is divided into high pressure casting and low pressure casting. High pressure casting is what we usually call die casting. Liquid aluminum is poured into pressure chamber, mold cavity is filled at high speed by its pressure, and liquid aluminum is solidified under pressure to form an aluminum casting.
1. High-pressure die casting: A casting method in which liquid, or semi-solid metal, or alloy, or liquid metal or alloy containing a reinforcement phase, is filled into cavity of a die-casting mold under high pressure and at a high speed, and metal or alloy is solidified under pressure to form a casting. Commonly used pressure during die casting is 4-500MPa, and metal filling speed is 0.5-120m/s. Therefore, filling time of molten metal is very short, and mold cavity can be filled in about 0.01-0.2 seconds (depending on size of casting). Therefore, high pressure and high speed are fundamental differences between die casting and other casting methods, and are also important features.
Characteristics of aluminum alloy high-pressure castings are:
1. Surface finish of product is good, generally reaching Ra6.3 or even Ra1.6.
2. No heat treatment.
3. Product has high air tightness, high casting strength and surface hardness, but low elongation, and excessive wall thickness can easily produce pores.
4. Mold cost is high and service life is short.
5. High production efficiency.
6. It can produce thin-walled parts with small machining allowance.
7. Special aluminum alloy cannot be used.

 

2. Low-pressure casting: In a sealed crucible (or sealed tank), dry compressed air is introduced. Under action of gas pressure, molten metal enters sprue along riser pipe and rises, smoothly enters mold cavity through inner runner, maintains gas pressure on liquid surface in crucible until casting is completely solidified. Then release gas pressure on liquid surface, causing unsolidified metal liquid in liquid riser to flow back into crucible, then open mold and take out casting. This casting method has good feeding and dense casting structure. It is easy to cast large, thin-walled and complex castings without need for risers, and metal yield reaches 95%. No pollution, easy to realize automation. However, equipment cost is higher and production efficiency is lower. Generally used for casting non-ferrous alloys.
Characteristics of aluminum alloy low voltage parts are:
1. Product surface is worse than die-casting parts and stronger than gravity parts.
2. Aluminum castings have few internal pores and can be heat treated.
3. Product has high air tightness, strength and surface hardness of casting are higher than that of die casting, but elongation is lower than gravity.
4. Mold cost is low and service life is short.
5. Production efficiency is lower than gravity and far lower than die casting.
6. Process is complex and used to produce high-quality castings with high requirements.
7. Special aluminum alloys with low fluidity can be used.
Simply put, low pressure is a compromise between high pressure and gravity.

Aluminum alloys are more commonly used for cold chamber die casting due to their higher melting points and better stability. In this method, molten aluminum alloy is fed into injection chamber of die-casting machine and cooled in injection chamber, then metal is hydraulically forced into mold cavity through action of injection mechanism. Although hot chamber die casting is less commonly used for aluminum alloys, this method can also be used in some cases where rapid production is required or continuous casting is required.

So-called magnesium alloy die-casting process is a process that uses three major elements: machine, mold and alloy to unify pressure, speed and time.

 

BYD eight-in-one powertrain
1. Concept of magnesium alloy die-casting process: Magnesium alloy die-casting process is basically same as traditional aluminum alloy die-casting process. There are two main differences:
①Due to low heat capacity of magnesium alloy, injection system of die-casting machine is required to provide sufficient energy to meet rapid filling requirements;
② From perspective of safety and material loss, magnesium alloy pouring system needs to be equipped with a special gas protection furnace. Protective gas is used to prevent oxidation of molten pool surface. Furnace can ensure that magnesium liquid is maintained at a specific temperature and can be poured directly and quantitatively.
2. Magnesium alloy die-casting process flow: According to required pouring volume of die-casting part, inject it into injection chamber through a quantitative pump and a delivery pipe → select a reasonable fast injection speed according to low heat capacity requirements of magnesium alloy → compared with aluminum alloy die-casting, filling requirements of mold cavity are faster → after a relatively short solidification, mold can be opened and taken out.
3. Advantages of magnesium alloy die-casting process:
1. Because magnesium alloy has excellent flow properties, it can produce more complex and thinner-walled parts than aluminum alloy die-casting; 2. Magnesium alloy has good thermal conductivity and metal electromagnetic protection properties, and is more suitable for electronic industry products than other alloys; 3. Magnesium resources are inexhaustible; 4. Added materials can be recycled; 5. Sound insulation and vibration damping properties are good; 6. Specific gravity is about two-thirds of aluminum alloy.
4. Defects of magnesium alloy die-casting process:
Direct reasons: 1. Inappropriate shape of product; 2. Inappropriate die-casting machine or filling conditions; 3. Improper casting operation; 4. Improper mold and casting plan; 5. Inappropriate raw materials and melting technology; 6. Inappropriate operator.
Indirect reasons: 1. Unreasonable project portfolio and unrealistic project management; 2. Incomplete quality management (dimensional verification, process, operating standards, inspection standards, etc.); 3. Operator's negligence (insufficient education and training); 4 , Manager’s dereliction of duty (inadequate management education).

(1) Design mold: Aluminum alloy die-casting process requires shape and size of casting to comply with industrial standards, so designing mold is the first step. Design of mold should be based on actual needs, focusing on complete design and simple structure to ensure that mold can achieve correct and precise casting.
(2) Make mold: After completing mold design, make mold according to design drawing. Generally, precision of mold work is very high. Once there is a deviation, it will seriously affect casting. During mold making process, strict process requirements should match actual situation. Before production, process planning and production requirements need to be matched with parameters. Necessary quantitative management measures are a prerequisite for ensuring stable dimensional accuracy of molded aluminum alloy parts.
(3) Equip mold and adjust parameters: Produced mold needs to be installed on specific casting equipment for casting, and then parameters of equipment are adjusted to ensure the highest accuracy of aluminum alloy parts completed during casting process.
(4) Aluminum alloy material preparation: Aluminum alloy materials that meet requirements need to be prepared before casting. Commonly used ones include ADC12, A380, etc.
(5) Liquid aluminum injection: Aluminum alloy material is pre-melted and injected into mold. Precision measurement, temperature control and other steps are required before injection to ensure casting quality.
(6) Mold cooling: Molten aluminum in mold is rapidly cooled and formed in a short period of time. Cooling time and temperature are important factors affecting quality of workpiece.
(7) Demolding of aluminum alloy parts: After cooling of casting mold is completed, casting mold needs to be opened and die-cast aluminum alloy parts are taken out of mold.
(8) Surface treatment: After taking out aluminum alloy parts, a series of surface treatments such as trimming, finishing, painting or spraying are required to meet appearance and surface quality requirements of parts.
(9) Inspection: Inspection of aluminum alloy die-casting parts, including dimensional accuracy, appearance quality, defects, etc. After passing inspection, it can be determined whether aluminum alloy die casting is qualified.

Good product quality: castings have high dimensional accuracy, good surface finish, high strength and hardness. Strength is generally 25~30% higher than that of sand casting, but elongation is reduced by about 70%. Castings are dimensionally stable and have good interchangeability. It can die-cast aluminum thin-walled and complex castings. For example, current minimum wall thickness of zinc alloy die-casting can reach 0.3mm, and aluminum alloy die-casting can reach 0.5mm. High production efficiency: Machine has high productivity. For example, domestic JIII3 horizontal cold air die-casting aluminum machine can die-cast aluminum 600 to 700 times on average in eight hours, and small hot chamber die-casting aluminum machine can die-cast aluminum 3,000 to 7,000 times in eight hours on average. Excellent economic effect: Due to advantages of precise dimensions and smooth surface of die-cast aluminum parts. Generally, it is no longer mechanically processed but used directly, or processing amount is very small, so it not only improves metal utilization rate, but also reduces a lot of processing equipment and man-hours. Castings are cheap, can be combined with die-cast aluminum and other metal or non-metal materials to save assembly time and metal.

During die-casting, due to high speed of liquid metal filling cavity and unstable flow state, general aluminum die-casting method is used. Castings are prone to pores and cannot be heat treated. For castings with complex concavities, die casting is more difficult. Life of aluminum alloy die-casting molds is relatively low, with a lifespan of approximately 80,000 touches. It is not suitable for small batch production. Main reason is that manufacturing cost of die-cast aluminum molds is high and small batch production is uneconomical. Aluminum alloy die-casting is not easy to be anodized. Because many bubbles or sand holes are easily left after die-casting, appearance cannot be repaired well, and appearance problems cannot be covered after oxidation.
In summary, magnesium alloys and aluminum alloys each have their own advantages and applicable scenarios in die-casting process. When selecting a die-casting method, factors such as product requirements, production conditions, cost control, mold life should be comprehensively considered to select the most appropriate die-casting process and materials.