As a rapidly developing green and environmentally friendly alloy, magnesium alloy has low density (1.75g/cm3-1.90g/cm3), high specific strength and specific stiffness, good dimensional stability, good electromagnetic shielding, excellent corrosion resistance, good shock absorption, excellent processing performance, easy to process and low processing cost, good mold filling fluidity and recyclability, etc., its price has been decreasing year by year in recent years, so it has become a substitute for structural materials such as steel, iron, aluminum and plastic. It is increasingly used in the fields of automobiles, electronics, home appliances, communications, instrumentation and aerospace. Judging from research results published so far, although there are many new molding methods, die-casting is still main method. Even new molding methods are derived from basic principles of die-casting.

 

Compared with aluminum alloy, magnesium alloy has smaller density, specific heat and latent heat of solidification, lower melting point, and does not react with iron during melting and die-casting. Therefore, melting consumes less energy, solidification speed is fast, injection cycle can be shortened by 20%-30%, and die-casting mold has a long life, generally up to more than 200,000 times. There are reports in United States that die-casting mold has a life span of 3 million times. However, magnesium alloy liquid is easy to oxidize and burn, and has a greater tendency of hot cracking than aluminum alloy during casting. Melting, pouring and die-casting mold temperature control are more complicated than aluminum alloy die-casting.
Magnesium alloys can be die cast using cold chamber or hot chamber die casting machines. 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, such as 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, with a production capacity of 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, induction heating of gooseneck and nozzle 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 increase 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 integrates magnesium alloy melting and die-casting, uses a pick-up robot to make 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.
Key technology of magnesium alloy cold chamber die-casting machine is automatic pouring mechanism. Currently, there are vane pump type, air pressure pump type, gravity type and electromagnetic pump type. Stainless steel pump with a pump pressure of 137kPa in automatic pouring molten pool transports molten metal to injection chamber through piping. Automatic pouring mechanism uses argon gas of a certain pressure to act on liquid surface of molten pool in the sealed crucible, quantitatively presses out magnesium alloy liquid through pump body immersed in molten pool. Quantitative range is 200-2000g. Gravity pouring system uses a lifting device to make liquid level of melting furnace pool higher than pouring liquid port, uses gravity to pour molten metal, and achieves quantitative pouring by controlling valve opening time; electromagnetic pump pouring system uses electromagnetic force to transport molten metal, which can accurately control pouring amount of molten metal, with an error of no more than 2%, and a wide adjustment range for pouring amount. Instrument panel of German Audi car has a length of 1440mm, a wall thickness of 3.15mm, and a weight of 4.12kg. It is die-cast on a cold chamber die-casting machine with an automatic pouring mechanism and a locking force of 24500kN. Size of right-angle bolster on General Motors is 1470×300mm, with an average wall thickness of 2mm and a weight of 1.18kg. It is die-cast with M60B magnesium alloy on a cold chamber die-casting machine with a locking force of 21560kN. Magnesium alloy die-casting parts produced by cold-chamber die-casting machine also include car seat frames and car hubs.

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 scope 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 mold 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. Elongation of castings increased from 8% to 16%.

 

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 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, 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℃, 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 the 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 using sand molding, precision casting and other methods. There is a trend of combining this composite material with semi-solid casting and applying it to fields of die casting and squeeze casting.
Currently, there are relatively many experimental studies on relationship between process parameters and mechanical properties of die-cast magnesium alloys in various countries around the world, but there are very few research results involving microscopic properties of die-casting processes. Therefore, if we can predict performance of magnesium alloy formed parts by quantitatively analyzing impact of die-casting process on structure and properties of magnesium alloys, it will be a basic research with great potential and application prospects.

What are methods of magnesium alloy die-casting process? What is process of magnesium alloy die-casting process? What are characteristics of magnesium alloy die-casting process? Are there any defects in magnesium alloy die-casting process? If so, how to solve defects of magnesium alloy die-casting process? So what exactly is magnesium alloy die-casting process?
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. Magnesium alloy die-casting process is a metal casting process. There are many methods for magnesium alloy processing, each with its own characteristics. So let’s talk about what magnesium alloy is. So-called magnesium alloy refers to an alloy based on magnesium and adding other elements.
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:
1. Due 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 for direct quantitative pouring.
Magnesium alloy die-casting process: According to required pouring volume of die-casting part, it is injected into injection chamber by a quantitative pump and a delivery pipe → a reasonable fast injection speed is selected 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 parts can be taken out.
Magnesium alloy die casting process method:

1. Cold forming: Magnesium alloy cold die-casting machine is shown in Figure 1 below:

 

Figure 1 Magnesium alloy cold die casting machine
(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.
2. Thermal forming: directly immersed in molten soup and heated by molten soup, and melt is directly drawn from molten soup during production. It is suitable for low alloys such as zinc, magnesium, lead and other alloys. Magnesium alloy hot die casting machine is shown in Figure 2 below

 

Figure 2 Magnesium alloy hot die casting machine
3. Injection molding
Comparison of magnesium alloy cold chamber machine and hot chamber machine in magnesium alloy die-casting process:

Equipment in magnesium alloy die-casting process:
1. Die-casting machine: Die-casting machine is key equipment for die-casting molding. It provides pressure and speed required for molding.
2. Melting furnace: dissolves magnesium ingots to provide clean and qualified raw materials for castings.
Heating magnesium soup temperature:
620~~650 degrees (thermal forming);
650~~680 degrees (cold forming).
3. Preheating furnace: Since magnesium soup reacts strongly with water, magnesium ingots must be preheated before being added to melting furnace. General preheating temperature is 150~~400 degrees.
4. Mold temperature controller: Mainly used to heat injection tube of mold. It can improve yield rate and extend life of mold.
5. Sprayer:
(1) Let molding surface be sprayed (especially inside molding hole);
(2) Do not spray too much to avoid separation and squeezing of mold surface, or moisture has not dried when mold is closed;
(3) After spraying, air dry moisture.
6. Manipulator: Automatically take out casting product and material head during each molding cycle and place them in designated position. Reduce deformed products when manually picking up products.
7. Mixer: Magnesium alloys generally ignite easily when heated to 350 degrees. Therefore, when heating, protective gas is introduced. Protective gases used include: sulfur dioxide and sulfur hexafluoride. If temperature of magnesium soup exceeds 700 degrees, protective gas will fail.
Die-casting operation process of magnesium alloy pressing process: see Figure 3

 

Figure 3 Magnesium alloy die-casting operation process

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. Additional materials can be recycled;
5. Good sound insulation and vibration damping properties;
6. Specific gravity is about two-thirds of aluminum alloy.
Factors to consider in design of magnesium alloy die-casting molds:
1. Die casting machine selection. Type of die casting machine used for production mainly depends on wall thickness of casting. In the process of researching issue of "Optimization of Magnesium Alloy Die-casting Process", Roland Fink analyzed economics of magnesium alloy die-casting, cold chamber die-casting and hot-chamber die-casting processes, proposed that in general, castings less than 1kg require a hot chamber die casting machine to ensure that thin-walled parts are filled, while for large parts, a cold chamber die casting machine is recommended.
2. Process parameters. In die-casting production process, selecting appropriate process parameters is a prerequisite for obtaining high-quality castings and maximizing productivity of die-casting machine, and is basis for correctly designing die-casting mold. During die-casting, there are many factors that affect filling and molding of alloy liquid, including injection pressure, injection speed, filling time, die-casting mold temperature, etc. Due to different wall thickness and complexity of die castings, selection of process parameters varies widely. Compared with aluminum and zinc alloys, magnesium alloy has better fluidity, so secondary injection speed can be greater. Punch speed of magnesium alloy is about 30% faster than that of aluminum alloy, and maximum even exceeds 10m/s. Since magnesium alloy casting properties such as fluidity are very sensitive to mold temperature and pouring temperature, magnesium alloy liquid is very easy to solidify during mold filling process. Mold temperature and pouring temperature must be accurately controlled, otherwise it will be easy to produce scrap products.
3. Gating system design. Gating system plays an important role in controlling and regulating direction of flow of molten metal, exhaust overflow conditions, temperature distribution of mold, pressure transmission, length of filling time, speed and flow state of molten metal passing through runner.
Gating system design is summarized as follows: Inner runner location: Since magnesium alloy solidifies faster than aluminum, zinc and other alloys in cavity, and magnesium alloy die-casting parts are generally thin-walled parts, location of ingate must be selected to avoid direct impact on the surface of mold cavity, ensure the shortest flow path of molten metal in mold cavity to prevent insufficient pouring and cold shut-off.
Filling speed: Generally speaking, due to thermodynamic properties of magnesium alloys, heat transfer rate from alloy to mold is very fast, solidification interval is large and fluidity is poor. Therefore, in order to avoid premature solidification of magnesium liquid in runner, magnesium liquid should be filled into mold cavity at high speed and smoothly. Generally, gate flow velocity is 90~100m/s. For some thin-walled magnesium alloy die castings, gate speed is even as high as 20m/s.
Gate size: In many cases, gate is removed by machining. Width of ingate should be less than 50% of wall thickness to avoid damage to casting during trimming. In order to obtain minimum gate thickness and ensure thin-wall requirements of magnesium die castings, gate width should be as large as possible to ensure a suitable gate cross-sectional area.
Filling time: It is closely related to gate speed and has a great influence on thin-walled castings with high surface quality requirements. Filling time is 0% less than that of aluminum alloy, usually 10-100ms.
Overflow tank design For thin-walled magnesium alloy die-casting parts, optimal overflow tank inlet area is about 20%-25% of cross-sectional area of inner runner.
Die-casting mold design: Since chemical, physical parameters and die-casting characteristics of magnesium alloys are very different from those of aluminum alloys, design principles of aluminum alloy die-casting molds cannot be fully applied to casting mold design.
Direct causes of magnesium alloy die-casting process defects:
1. Shape of product is inappropriate;
2. Die-casting machine or filling conditions are inappropriate;
3. Improper casting operations;
4. Improper mold and casting plans;
5. Improper raw materials and melting technology;
6. Operator is inappropriate.
Indirect causes of magnesium alloy die-casting process defects:
1. Unreasonable project portfolio and unrealistic project management;
2. Incomplete quality management (dimensional verification, process, operating standards, inspection standards, etc.);
3. Operator’s negligence (lack of education and training);
4. Manager’s dereliction of duty (inadequate management education).