The Benefits of Die Casting Technology in Manufacturing

In 2020, China Automobile's "Energy Saving and New Energy Vehicle Technology Roadmap 2.0" was released. In 2025, 2030, and 2035, the average fuel consumption of new passenger cars will reach 4.6, 3.2, and 2.0 L/100 km, and the energy saving and emission reduction standards will continue to be upgraded. According to the data of "Buhler China", every 10% reduction in vehicle weight can save up to 5% to 10% of fuel, and every 100 kg reduction in vehicle weight can reduce fuel consumption by 0.3 to 0.6 L per 100 kilometers and reduce CO2 emissions by 8.5 g/ km, the average cruising range of pure electric vehicles will increase by 5% to 8% for every 10% reduction in vehicle quality. Therefore, the lightweight of automobiles has become a key development technology in the field of energy saving and emission reduction. The main technical way to achieve the lightweight of new energy vehicles is the integrated die-casting of aluminum alloy bodies.
1
Current Situation of Aluminum Alloy Integrated Die Casting
In July 2019, Tesla issued a new patent "Multi-directional Body Integral Forming Casting Machine for Automobile Frames and Related Casting Methods". In this method, a set of fixed molds is placed in the center, and four sets of movable molds are placed in four different directions. The movable molds are respectively attached to the fixed molds in the center through hydraulic equipment to form a closed cavity together. The aluminum alloy liquid is pressed into the cavity of the mold from the sprues on the four moving molds respectively, and the aluminum alloy flows and solidifies in the cavity, and finally forms a large-scale integrated die-casting structural part.

Tesla is the founder of aluminum alloy integrated die-casting for large-scale automobile parts. The layout of integrated die-casting can be divided into three stages. The first stage: Model S/X adopts an all-aluminum body, according to the traditional stamping and welding process Complete vehicle development; the second stage: Model Y uses a 6 000 t die-casting machine to produce the rear floor, reducing the weight of the lower body by 10%, and reducing the manufacturing cost by 40%; the third stage: integrated die-casting lower body, front and rear Body integration (the number of parts is reduced from 171 to 2, and the number of welding points is reduced by more than 1 600) and 4 680 battery cells CTC (the battery pack is integrated into the car body and directly connected to the seat), the weight of the vehicle is reduced by 10%, The cruising range has increased by 14%, the number of parts has decreased by 370, the cost has decreased by 7%, and the unit investment has decreased by 8%. It is currently in mass production at the Austin, Texas plant.

In 2020, Tesla began to cooperate with Italian die-casting equipment manufacturer Idra to use the 6,000-ton die-casting unit Giga Press to produce the Model Y rear floor assembly by integral die-casting. According to Tesla's layout, two sets of Idra IDRA6000t integrated die-casting equipment from Italy have been installed in the Fremont plant in California, three sets of LK DCC6000 series integrated die-casting equipment have been installed in Shanghai, and one IDRA 8000t-class integrated die-casting equipment has been installed in the Texas plant. For die-casting equipment, the planned production capacity of the Berlin factory is 500,000 vehicles, and 8 integrated die-casting equipment will be arranged. With four factories and 14 integrated die-casting equipment, Tesla has laid out integrated die-casting technology as a standard process.

Volkswagen plans to start applying integrated die-casting on the SSP (Scalable Systems Platform) platform. According to Online EV, in May 2022, Volkswagen's integrated aluminum die-casting rear body prototype will roll off the production line at the Kassel factory. The parts are produced by a 4 400 t die-casting machine, which integrates more than 30 parts and reduces the weight by about 10 kg. The Trinity is one of the first models based on the SSP platform.

According to Auto-evolution News, Volvo announced an investment of SEK 20 billion to modernize its Torslanda factory, including integrated die-casting technology. It will also establish an aluminum casting plant with an annual capacity of 55,000 tons in Torslanda, and eventually plans to introduce integrated die-casting technology in all its factories. It is expected to achieve mass production of integrated die-casting cars in 2025.

In January 2022, Mercedes-Benz released a new concept car VISIONEQXX. The rear part of the body and the top of the shock-absorbing tower at the front use the same bionic engineering structural components as Tesla. The entire body consists of three parts: one die-casting casting at the front and the rear, and a structural battery pack in the middle. Such a design can reduce the weight of the vehicle body by 15% to 20%. The energy consumption of the VISION EQXX can reach below 10 kW h/100 km, and the actual electric mileage exceeds 1,000 km.

Among the new domestic forces, in December 2021, NIO announced at the ET5 press conference that it will start using an integrated casting process. The ET5 uses an ultra-high-strength steel-aluminum hybrid body, which reduces the weight of the rear floor of the body by 30% and increases the trunk space by 7 liters. , the torsional rigidity of the vehicle is as high as 34 000 N·m/deg.

At the 2021 performance conference, Xiaopeng Motors announced that it will release two new platforms and its first model in 2023, and use a new ultra-large integrated die-casting process. Xiaopeng Motors Wuhan Industrial Base Project is expected to be put into operation in October 2022, including an integrated die-casting workshop. Xpeng Motors Wuhan Plant will also introduce more than one set of super-large die-casting islands and automated production lines.

On February 25, 2022, the 7200t integrated super-large die-casting rear cabin jointly developed by Gaohe Automobile and Tuopu Group was successfully rolled off the assembly line. The alloy material of Shanghai Jiao Tong University is used in this part, which achieves a weight reduction of 15%~20%, and the entire development cycle is shortened by one-third. This die-casting machine is the largest known integrated aluminum alloy die-casting in the field of auto parts. Many domestic suppliers have accelerated the pace of introducing large-scale die-casting equipment and the development of heat-free materials and entered the integrated die-casting industry.

2 Barriers to integrated die-casting technology

At present, the barriers to integrated die-casting technology are mainly reflected in four aspects: large-scale die-casting machines, heat-free aluminum alloy material formula, die-casting mold design, and vacuum die-casting process.

2.1 Large die-casting machine

The large-scale die-casting machine industry has high barriers to entry and high requirements for theory, experience, and manufacturing processes. At the same time, the development cycle of large-scale die-casting machines is very long, with large initial investment and high cost (one die-casting machine requires an average of 100 million yuan). Integrated die-casting requires that the clamping force of the die-casting machine be at least greater than 6000 t (the clamping force of traditional high-pressure die-casting is usually below 5000 t). In October 2021, Buhler, Switzerland launched the Carat 840/920 ultra-large two-platen die-casting machine (Figure 2), with a clamping force of up to 92,000 kN. Two sets of Carat 840 die-casting islands were delivered to Volvo's Torslanda factory to realize integrated die-casting. The OL6200CS die-casting machine developed by Italy’s IDRA (a subsidiary of LK) is 19.5 m long, 5.9 m wide, and 5.32 m high, which is equivalent to a badminton court. The effective size of the platform is 2.35 m long and wide. The space of the workbench can place any parts whose length, width, and height are within 2 m, and can provide a maximum clamping force of 6218 t. According to the patent applied by the Swiss Buhler company in 2011, it can form a cavity between the casting molds, inject aluminum liquid into the cavity, and generate negative pressure in the mold cavity after vacuuming, which can effectively eliminate the turbulence caused by casting. Porosity, which helps to improve the internal compactness of die-cast parts. LK, a domestic die-casting machine factory, has a share of more than 40% in the global die-casting machine market. It took the lead in developing a 6,000-ton die-casting machine (Figure 3) and has now developed a 12,000-ton die-casting machine. In April 2021, Haitian Metals launched the world's first HDC8800 t die-casting machine in Chongqing Milison, which can realize the integration of large structural parts such as body and chassis including new energy vehicles. Yizumi's 7000t die-casting machine will also be assembled soon.

2.2 High-pressure casting alloy without heat treatment

Heat treatment-free alloy refers to the special alloy formula, after the die-casting is completed, the casting can obtain ideal mechanical properties without heat treatment and can avoid deformation of the workpiece during heat treatment (high-temperature solution and aging treatment). As of 2021, domestic and foreign companies, including Alcoa, Rheinland, Tesla, Shanghai Jiaotong University, NIO, and Lizhong Group, are developing and promoting heat-free materials, which make integrated die-casting possible.

Alcoa's product is THE EZCAST series C611 heat-free AI-Si die-casting aluminum alloy, which was successfully developed by Alcoa in the 1990s and applied to the all-aluminum body of Audi A8. Shuaiyichi Group signed an exclusive agency agreement with Alcoa to represent Alcoa's high-strength, heat-free cast aluminum alloys. In addition, Germany's Rheinmetall has also developed Al-Si series and Al-Mg series heat-free high-strength and tough alloys, such as Castasil-37 die-casting aluminum alloys; Canadian Aluminum Corporation has developed die-casting aluminum alloys such as Aural-2 and Aural-3 Material. The LDHM-02 heat-free alloy material developed by Lizhong Group has obtained relevant patents and has been supplied in batches. According to the latest relevant patent "A High-Strength and Tough Heat-Free Aluminum Alloy Material and Its Preparation Method" in the application of a subsidiary of Lizhong Group, the aluminum alloy material developed by the company has a yield strength of 130 MPa, a tensile strength of 250 MPa, and an elongation rate of 250 MPa. More than 10%, to meet the needs of customers, Lizhong Group has reached strategic cooperation with Wencan. In December 2021, the Light Alloy Center of Shanghai Jiaotong University and Human Horizons Gaohe Automobile reached a strategic cooperation, the world's first Tech CastTM low-carbon aluminum alloy for super large castings, which will be adopted in large quantities in subsequent models of Gaohe Automobile.

The composition and process of heat-free alloy materials are complex and have high technical barriers. Among them, the composition design of alloy materials is the core technical barrier to the development of heat-free alloys. Commonly used die-casting aluminum alloys are The Al-Si series, Al-Mg series, Al-Si-Cu series, and Al-Si-Mg series. The main component distribution affects the mechanical properties of the alloy such as strength and hardness, and at the same time affects the fluidity, solidification, etc. casting performance.

SJTU-A-Mg-Si-Mn alloy is a heat treatment-free die-casting alloy developed by Shanghai Jiaotong University. Its purpose is to increase the yield strength while maintaining the good toughness of the material. The current Al-Si series and Al-Mg series alloys generally have medium strength and toughness. With the continuous improvement of the integration and lightweight requirements of aluminum alloy die-casting structural parts, the development of new die-casting alloys should be aimed at increasing the yield strength or Toughness, while having good casting fluidity direction.

2.3 Die-casting mold

Integrated die-casting has higher requirements on mold strength and toughness, and requires impact toughness and tempering stability, good thermal conductivity and fatigue resistance, small thermal expansion coefficient, and

high-temperature oxidation resistance. Die-casting requires high-speed filling and high-speed solidification, and the requirements for die-casting mold temperature, vacuum degree, forming scheme, process parameters, and post-processing are higher. Integrated die-casting requires a large mold, the mold is more complex, and the mold design is more difficult. The mold design needs to design the exhaust and slag collection system, and CAE simulation technology needs to be used to simulate the filling process. The poor exhaust will cause pores in the parts and waste products. When the part is cooled after die casting, the size will shrink. It is necessary to accurately simulate the cooling process of the casting, set a feeding riser at the final cooling part, and avoid thermal islands, otherwise, shrinkage cavities, porosity, and crack defects will occur in the part during the cooling shrinkage process. Mold design manufacturers must have the ability to simulate and analyze die-casting mold filling. Only a few domestic integrated die-casting molds can do it, such as Guangzhou Cavity, Ningbo Zhenzhi, Feiwang, Beilun Saiweida, Xiangshan Heli, Beilun Zhenzhi, etc. A set of molds can support 60,000 to 80,000 sets of production capacity per year, and the unit price of each set of molds is close to 10 million yuan. The first set of 6 800 t ultra-large integrated aluminum alloy die-casting molds in China was successfully trial-manufactured in Hongtu Technology. The mold is independently developed by Guangzhou Cavity Mold, and the mold weight exceeds 140 t. Guangdong Hongtu's integrated structural parts for automobile chassis adopt self-developed high-strength, toughness, and heat-free aluminum alloy materials. The casting size is about 1.7 m long, 1.5 m wide, and 0.7 m high, and the pouring weight is about 100 kg. It is the largest aluminum alloy at present. One of the high strength and toughness die castings. Wencan Group's first batch of 9 000 t large-scale integrated rear floor die-casting auto parts was successfully rolled off the production line in Tianjin, and the molds were provided by Ningbo Saiweida. Currently, Saivida and Heli Technology are developing 12 000-20 000 t molds for integrated body structural parts.

2.4 Die casting process

Integrated die-casting is actually a high-vacuum control system added to the vacuum die-casting process, which requires high-precision sensors to control the vacuuming process. The technological process is mold closing, pouring, vacuum opening, cavity vacuuming and exhausting, injection, mold opening, pick-up, spraying, mold closing again, etc. During the die-casting process, the vacuum tank, pouring exhaust valve, and cavity exhaust valve are controlled by high-precision vacuum sensors, and four contact points are triggered by parameter setting: pouring vacuum start, pouring vacuum end, cavity vacuum start, and The cavity is vacuumed. The high-speed filling of die-casting can easily lead to the inability to completely discharge the gas in the pressure chamber or cavity. The gas involved in the molten metal will exist in the casting in the form of pores, which cannot be welded and reduces the mechanical properties. Therefore, the integrated die-casting must be equipped with cavity vacuuming. system. The die-casting process is very important to produce qualified parts. For different part structures, the die-casting process parameters need to be debugged and explored for a long time. The correct injection mode and die-casting parameters are conducive to reducing the defects of castings. Most of the gas in die-casting comes from the metal liquid in the pre-filling stage in the pressure chamber, it is necessary to repeatedly optimize the slow injection parameters to avoid the inclusion of gas in the molten metal during the die-casting process. For alloy smelting and dehydrogenation treatment, it is necessary to use high-purity electrolytic metal ingredients in the melting process to avoid contamination by metal impurities; during smelting, in order to prevent oxidation and segregation of the molten metal, it needs to be melted quickly, and the molten aluminum needs to be dehydrogenated and slag-removed after melting For treatment, nitrogen or argon gas is generally used to remove hydrogen, and the hydrogen removal time is 10~15 minutes, which is equivalent to AlSi10MnMg. The holding time should not be too long to avoid the burning of the modification Sr. The melting temperature is (730±10) °C and must not exceed 780 °C. The pouring method generally adopts bottom injection to avoid inclusion defects in castings. The spraying time, spraying angle and range, spraying amount, blowing angle, and type selection of the release agent (small gas generation, good volatility) of the release agent needs to be verified. There should be no residual moisture in the cavity before mold closing. The die-casting temperature is 700~710 ℃, about 20 ℃ higher than that of AlSi10MnMg. The mold needs to use a special mold temperature controller to control the mold temperature, and the mold temperature is generally controlled at 120~180 ℃. During the die-casting process, the vacuum pumping system should be started in time. After the punch seals the sprue, the vacuum pumping should be started immediately. Before the pressure chamber is full, the vacuum degree must be met. The vacuum pumping time should be extended as much as possible, and the vacuum valve should be closed as late as possible. The quality of integrated die-casting parts is very important, and the vacuum valve basically depends on imports. The pass rate of Tesla's earliest integrated die-casting was 65%~72%, and the gross profit rate could reach 30%. After the casting quality standard was revised in February 2022, the pass rate reached more than 80%.

 

3 Conclusion
The aluminum alloy integrated die-casting process is a major revolutionary technology in the manufacture of automotive structural parts. In the past 50 years, the automotive body manufacturing process has always been dominated by robot welding after sheet metal stamping. The integrated die-casting technology has brought about major changes in the automotive body manufacturing process. Die-casting machines will replace welding robots as the core equipment for car manufacturing in the new energy field. Under the promotion of the "dual carbon" goal, the integrated die-casting technology has obvious advantages in terms of production efficiency, cost reduction, and weight reduction. Aluminum alloy integrated die-casting design and manufacturing is an all-around integrated technology with high technical barriers, including software development and application, component structure design, heat-free material development, die-casting forming process, die-casting unit capacity, and large-scale mold design. innovation. In particular, mold design and manufacturing should not only consider the fluidity of casting materials, heat balance, mold life, part forming process, and quality assurance but also consider the optimization of subsequent processing technology of parts. "Integrated die-casting molding process and equipment" has been included in the national key research and development plan. The continuous development of the new energy vehicle industry, will greatly promote the development of aluminum alloy integrated die-casting technology, and aluminum alloy integrated die-casting will become the preferred manufacturing process for new energy vehicle companies.