With innovation and development of new alloy materials, die-casting technology with high product quality, good precision, and high productivity is widely used in manufacturing of various automotive parts. In context of increasing global environmental pollution, automobile lightweighting has become an important development trend in automotive field by reducing vehicle's overall quality as much as possible while ensuring vehicle's strength and safety performance, thereby improving vehicle's power, reducing fuel consumption, and reducing pollutant emissions. Due to high quality, high precision, high production efficiency of die-casting products, and ability to effectively achieve lightweight, types of automotive parts produced using die-casting process are gradually increasing. Application of lightweight alloy die-cast automobile parts can not only reduce weight of the entire vehicle and promote lightweight of automobile, but also shorten production time and reduce production cost, which is of great significance. This article introduces principles, characteristics, process flow and development status of die-casting technology in detail, analyzes common defects of die-casting parts and provides solutions; then application and development of aluminum and magnesium alloy die castings in automobiles are reviewed, typical aluminum and magnesium alloy die castings are introduced. Finally, development prospects of die castings in automotive industry are prospected.

Die-casting technology can be divided into scope of special casting. Its principle is to fill liquid molten metal or semi-liquid or semi-solid metal into mold cavity at a high speed under high pressure, and to solidify metal under pressure.

Pressure casting process mainly includes two basic methods: cold chamber die casting and hot chamber die casting. There is little or no need for metal cutting during production process of die casting process. Production efficiency is high and it is highly economical. During pressure casting process, pressure exerted on metal is generally 20 to 200MPa, initial speed of mold filling is 15 to 70m/s, and filling time usually does not exceed 0.2s. Due to unique filling and solidification methods of die-casting technology, die-casting technology has following unique advantages that other forming processes do not have.
(1) Able to produce thin-walled castings with complex shapes. Wall thickness of general castings is 1 to 6 mm. Wall thickness of small parts can be even thinner. Wall thickness of large castings can exceed 6 mm. Parts that cannot be produced by other casting methods can only be produced by die casting technology.
(2) Castings obtained by die casting have high precision, stable dimensions, smooth surfaces, and machining allowance is generally 0.2 to 0.5 mm.
(3) High efficiency, short production cycle, one operation takes no more than 3 minutes, and production speed is 300 pieces/min.
(4) Inlaid casting method used in pressure casting can save assembly process and simplify manufacturing process. Castings with special needs can be prepared using insert casting method.
(5) Surface of die castings can be coated. Castings can be die-cast to produce threads, lines, text, etc.; die-casting can be used to produce clear shapes such as peaks and valleys, unevenness, and narrow grooves.
However, pressure casting, like other forming processes, cannot be perfect, and there will also be some problems.
(1) Porosity and shrinkage will appear in castings. Process of filling mold cavity with molten metal and cooling it is very short. If gas in mold cavity cannot be discharged quickly, there will be pores or oxidized impurities in casting, which will affect quality of casting.
(2) Cost is higher than other forming methods. In die-casting production, die-casting machines and die-casting molds are relatively expensive, are not suitable for small batch production.
(3) Material of mold is mainly suitable for alloys with low melting points, and service life of mold is short. Die-casting molds often suffer from cracks, brittle fracture, corrosion and other phenomena during production process, which greatly shortens service life of mold.
With development of die-casting technology, above problems are constantly being solved.

Die-casting process is an organic combination of three major elements: die-casting machine, mold, and alloy materials. It is a unified process of conflicting factors such as pressure, speed, and temperature. There are basically 2 types of die casting processes: hot chamber die casting and cold chamber die casting.
Hot chamber die casting process is shown in Figure 1. During hot chamber die casting process, molten metal is held in a closed steel crucible under a protective gas. Valve allows a controlled volume of molten metal to enter a gooseneck that is immersed in molten metal. Plunger injects molten metal through a nozzle into cavity of mold. In order to prevent molten metal from freezing, nozzle is heated to 400 to 500℃ using gas, electric heating or induction heating. Nozzle is often kept filled with molten metal between shots to shorten cycle times. Pressure exerted on molten metal during injection process is lower than cold chamber die casting and limits size of parts made with hot chamber method. Main reason is that high operating temperatures of various components such as nozzles limit pressure that can be applied.

 

Figure 1 Schematic of hot chamber die casting
Cold chamber die casting process is shown in Figure 2. Molten metal liquid is delivered to injection column through a manual ladle, automatic ladle or pump. It is then rapidly injected into cavity via a plunger and solidified into a mesh-like part under high pressure. If used to form an undercut, core will be retracted. Finally, casting is ejected and part is trimmed by separating it from gating system and core. The entire cycle usually takes about 1 minute.

 

Figure 2 Schematic of cold chamber die casting

In recent years, die-casting machines and die-casting technology have developed rapidly. At present, die-casting technology is mainly developing in these three directions: (1) Die-casting machines and equipment are developing in direction of forward pressure casting automation. Die-casting machines are equipped with computer control parts, and automation programs have developed greatly; (2) Die-casting materials have been further developed, such as research and development of heat-free aluminum alloys, magnesium alloys and metal matrix composite materials; (3) Computer simulation technology and die-casting technology are closely integrated.

In die-casting process, choosing a suitable die-casting machine is top priority. When choosing a die-casting machine, you must consider not only specifications and price of die-casting machine, but also performance, quality, and stability of die-casting machine. Die-casting machines are divided into hot chamber die-casting machines and cold-chamber die-casting machines. According to structure and layout of pressure chamber, they are further divided into horizontal die-casting machines and vertical die-casting machines. Large die-casting machines began to develop in the 1950s and have developed most rapidly in recent years. At the beginning of 2021, Tesla officially announced that four vehicle factories, including Shanghai, have installed ultra-large die-casting machines. Before that, 4000t-level die-casting machines were large-scale die-casting machines. After that, 6000-9000t level die-casting machine was a large-scale die-casting machine. Currently, a 12000t-level ultra-large die-casting machine has been launched. As size and locking force of these die-casting machines increase, it becomes possible to die-cast engine cases, transmission cases, and other large, complex thin-walled parts, especially aluminum alloy parts. In addition to development of die-casting machine models, safety standards of die-casting machines have been continuously improved, and functions of die-casting machines have also been developed. Multi-slide high-speed die-casting machines are equipped with flexible unit equipment, intelligent manipulators, and separate automatic pouring, picking, and spraying devices, which effectively improves processing efficiency.

Quality of alloy materials also has a crucial impact on quality of die castings. According to characteristics of die-casting process, there are following basic requirements for die-casting alloys:
a. When superheat temperature is low, it must have good fluidity to facilitate filling of complex mold cavities to obtain die castings with good surface quality;
b. In order to avoid cracks in die castings, linear shrinkage and crack tendency are required to be small;
c. In order to prevent too many shrinkage cavities and porosity in die castings, crystallization temperature range of material should be small;
d. In order to prevent deformation when die casting is pushed out, material must have a certain high-temperature strength;
e. It has good processing performance and corrosion resistance. At present, the most common alloy materials include aluminum alloy and its composite materials, zinc alloy and its composite materials, magnesium alloy and its composite materials, copper alloy and its composite materials.
Automobile lightweighting is main direction of current development of automobile industry. Aluminum alloys and magnesium alloys with low density and easy die-casting have naturally become preferred materials for die-casting. Traditional High Performance and Hybrid Passivated Dual-Junction Cell (HPDC) parts cannot be heat treated because trapped air in pores expands and forms bubbles when heated to solution temperature. In recent years, many domestic and foreign companies have successively developed aluminum alloy heat treatment-free materials. Research and development of this material has significant promotion significance for die-casting large thin-walled parts. Quality of magnesium alloy die castings of same size is only two-thirds of aluminum castings, so magnesium alloy die castings have great development prospects. Some experts and scholars have improved its poor creep resistance at high temperatures by inhibiting formation of Mg17Al12 phase, pinning sliding of grain boundaries, and slowing down diffusion of solutes in magnesium matrix, making it suitable for use at high temperatures.

Development of computer simulation technology has led to theoretical breakthroughs to varying degrees in flow pattern of molten metal injected into mold cavity, solidification process in cavity, flow pressure of metal liquid in cavity, temperature field distribution of mold, temperature gradient of mold, deformation of mold, and stress analysis of lever system of die-casting machine. Simulation analysis software currently used mainly includes domestic Huazhu CAE and FT-Star and foreign ProCAST, FOLW-3D, Any-CAST, etc. Based on cycle temperature difference, a set of usage standards for fatigue life of die-casting molds was formulated. Through thermal load during ProCAST simulation test, and a verification test using H13 steel mold samples as object, application standard range of mold was finally obtained. For HPDC products, die-casting process parameters used are evaluated, which provides an effective method for parameter optimization of high-pressure casting.

Types of defects in die-casting parts mainly include shape and size problems, appearance defects, internal defects, material defects and other defects. This article analyzes causes of more common defects among five types of defects and gives prevention methods. See Table 1 for details.

Table 1 Common defect types and analysis of die castings

Die-casting technology was originally developed for automotive applications in order to produce parts that are lightweight, easy to operate and inexpensive. Die casting is widely used in materials such as aluminum and magnesium that are lighter than cast iron. With research and development of lightweight materials such as cast aluminum and magnesium alloys, application of these alloy castings in automobiles is gradually increasing, which largely meets growing needs of automobile fuel economy. The ever-increasing needs of economy. According to relevant data, every time vehicle weight is reduced by 10%, fuel efficiency can be increased by 6% to 8%, and fuel consumption will be reduced by 6% to 10%. Compared with steel, aluminum alloy castings can reduce weight of the entire vehicle by 30% to 50%, and magnesium alloy castings can reduce weight of the entire vehicle by 40% to 60%.

Based on advantages of aluminum alloys such as low density, corrosion resistance, good formability, and can absorb 50% more energy than steel in a collision, aluminum alloy die-casting technology has developed rapidly, and amount of aluminum castings used in automotive industry is also much higher than that of other non-ferrous metals.
As early as the early 20th century, some Western countries already had precedents for using aluminum alloys instead of cast iron to manufacture auto parts. At that time, bodies of Ford Model T cars and Ferrari 360 racing cars were made of aluminum alloys; in the 1940s, Fiat developed aluminum alloy intake pipes and cylinder heads, began small-scale production. In the 1950s, Australian companies introduced low-pressure casting technology to manufacture automotive aluminum castings. During same period, a German company conducted in-depth research and development of low-pressure casting technology so that it could be used to produce aluminum alloy parts with complex structures. General Motors also mass-produces aluminum castings such as cylinder heads, crankcases, and engine rear covers, applies them to automobiles. After the 1960s, development of high-pressure die-casting technology has led to an increasing proportion of lightweight materials such as aluminum alloys in vehicles. It has also established direction for modern automobile industry to generally use aluminum castings to reduce weight of the entire vehicle. By the 21st century, aluminum castings will be more widely used in global automotive industry. According to relevant data, global production capacity of aluminum castings is growing at an annual rate of about 6%. Today, amount of cast aluminum alloy used in automotive industry accounts for about 80% of the total aluminum used, and 65% of it is die castings. Since beginning of 21st century, consumption of aluminum alloy die-casting parts for bicycles has continued to rise year by year. Gap between consumption of aluminum alloy die-casting parts for bicycles in my country and developed countries such as Europe and United States has been continuously narrowing, as shown in Figure 3 below.

 

Figure 3 Quality comparison of aluminum castings for bicycles at home and abroad
As proportion of aluminum alloy die-casting parts in vehicles continues to increase, its application range in automobiles is also gradually expanding. According to use function, application range of aluminum alloy die-casting parts in automobiles can be divided into structural parts, stress-bearing parts, safety parts, and decorative parts. According to classification of automobile assembly, it can be divided into power system, transmission system, steering system, chassis assembly, body and others, see Table 2.

Table 2 Summary of application of aluminum alloy die-casting parts in various automobile assemblies
Castings such as engine blocks and oil pans are the most typical applications of aluminum alloy die castings in automotive applications. Alloy used to make cylinder must have good fluidity. A small amount of sodium and strontium is usually added to aluminum-silicon alloy, which will increase fluidity of metal. Nowadays, most automobile manufacturers use aluminum alloy die-cast cylinder blocks in automobiles. Although cost of doing so is higher, considering continuous development of automobile lightweight requirements, aluminum alloy die-cast cylinder blocks are gradually accepted by more and more automobile manufacturers. Oil pan is located in lower half of crankcase. Although aluminum alloy die-cast oil pan is softer than oil pan made by traditional stamping, it has better heat dissipation than traditional stamping parts and is lighter in weight.
Application of aluminum alloy castings not only significantly promotes lightweighting of traditional fuel vehicles, but also new energy vehicles. In 2019, Tesla introduced a large-scale die-casting machine to improve production and manufacturing of Model Y. 70 parts of rear floor assembly of Model Y are integrally die-cast, shortening manufacturing time to 3 to 5 minutes, completely overturning traditional vehicle manufacturing model dominated by stamping and welding (Figure 4). Tesla uses a one-piece die-cast rear floor assembly, which reduces weight of the entire vehicle by 30% and reduces manufacturing costs by 40%. This has had a huge impact on global manufacturing industry.

 

Figure 4 One-piece die-cast car rear floor

As early as 1925, Germany first used die-cast magnesium alloy pistons in automobiles. In following more than ten years, nearly 4 million magnesium casting pistons were applied to automobiles. Use of magnesium castings in automobiles continued to rise until peaking in 1971, when main automotive applications of magnesium castings were in air-cooled engines and transmissions. After that, with advancement of science and technology, power of engines became larger and larger, and air-cooled engines were converted to water-cooled engines. Magnesium alloy materials used at that time were no longer suitable for working environment at that time. When new magnesium alloy materials were developed, considering cost, magnesium castings were no longer optimal choice, but some models are still in use. 
Magnesium alloy is still the lightest among all metal structural materials currently used. Use of magnesium alloy can reduce weight of the entire vehicle by 15% to 20% on basis of aluminum alloy. In context of automobile lightweighting becoming main direction of automobile industry, magnesium is a promising metal material and has been focus of research and development in many countries. Casting has been primary manufacturing process for magnesium components, accounting for approximately 98% of magnesium structural applications. At present, there are more than 70 parts in automobiles made of magnesium alloy, of which more than 90% are die-casting parts. Among them, the most commonly used parts are instrument panel base and steering column, seat frame, steering wheel shaft, engine valve cover, and intake manifold. Table 3 summarizes applications of magnesium castings in automotive industry.

Table 3 Applications of magnesium alloy castings in automobiles
Magnesium alloy is an excellent material for manufacturing automobiles. Its excellent die-casting properties allow process to economically produce large, thin-walled and complex castings. It can not only reduce weight of the entire vehicle, reduce energy consumption, improve acceleration and braking performance, but also significantly reduce vibration and improve comfort. Most automotive magnesium castings are produced by cold chamber die casting, with typical parts including dashboard beams, seat frames, engine blocks and oil pans. Figure 5a shows one-piece magnesium die-cast instrument panel beam casting launched by General Motors in United States in 1996. Its thickness is only 4mm and its weight is only 12.3kg. Compared with steel parts, its weight is reduced by 32%, and it is based on an integrated die-casting design, which saves costs. With development of magnesium die-casting technology, it is now possible to reduce weight to a greater extent. Figure 5b shows a 6.9kg magnesium alloy die-casting part of Buick LaCrosse.

 

Figure 5 One-piece die-cast magnesium alloy casting

Excellent mold filling properties of aluminum and magnesium alloys enable them to be economically used to produce large, thin-walled and complex castings. Aluminum and magnesium alloy die castings are also developing in direction of large-scale, thin-walled and complex. One-piece die-cast instrument panel beam magnesium casting and Tesla's one-piece die-cast rear floor assembly launched by General Motors combine many simple parts into one complex part and are die-cast in one piece. In terms of technology, in order to solve the biggest shortcomings of conventional pressure casting of light metals, researchers have developed processes such as vacuum die casting, vacuum-assisted die casting, high vacuum die casting, and low pressure casting that can reduce porosity of castings. In the field of die-casting materials, overseas companies including Alcoa, Reinfeld and Tesla, as well as domestic companies such as Weilai Automobile, Hubei Xinjinyang, Lizhong Group, etc., have developed heat-treatment-free materials. Research and development of heat treatment-free materials has made large-scale integrated die-cast structural parts possible.

This article introduces pressure casting technology in detail from four aspects of die casting process principle, process flow, process characteristics and process development status, analyzes defects that often occur in die castings, and provides preventive measures. Development of application of aluminum and magnesium alloy die castings in automobiles was sorted out, application of aluminum and magnesium alloy die castings in automobile industry was summarized, typical applications of aluminum and magnesium alloy die castings in automobiles were introduced in detail. In context of "dual carbon", automobiles are developing in direction of energy conservation and environmental protection. Application of lightweight metal die-casting parts such as aluminum and magnesium alloys in automobiles is of significant significance to reduce quality of the entire vehicle, reduce production costs, and promote lightweight process of automobiles. Moreover, defect-free casting methods such as squeeze casting, semi-solid die casting, and vacuum die casting are being automated, production efficiency and quality will be further improved, and costs will be further reduced, which will further promote application of die castings in automobiles. At present, number of automobiles in China ranks first in the world, but consumption of aluminum and magnesium alloy die-casting parts for bicycles still lags behind that of developed countries. Therefore, application prospects of aluminum and magnesium alloy die-casting parts in automobile industry are very broad.