Automotive lightweight is unstoppable, and integrated die-casting is ready for development

Automobile lightweight is the process of reducing the curb weight of a car as much as possible while ensuring its strength and safety performance. This aims to enhance the car's power, reduce fuel consumption, and decrease exhaust emissions. According to a report from the World Aluminium Association, a 10% reduction in the overall weight of a car can lead to a 6%-8% improvement in fuel efficiency. Research by Volkswagen suggests that for every 100-kilogram reduction in the curb weight, carbon dioxide emissions per kilometer can decrease by 8-11 grams, and fuel consumption per hundred kilometers can be reduced by 0.3-0.5 liters. Therefore, in the current context of increasing pressure to reduce emissions from automobiles, lightweight plays a highly positive role in conserving energy, reducing emissions, and achieving sustainable development goals.

1.1. Fuel vehicle consumption reduction target is high, vehicle weight reduction can effectively improve fuel efficiency

Lightweighting has become a key focus for the development of fuel vehicles, with a target of a 10% reduction in the overall vehicle lightweighting coefficient for gasoline-powered cars by 2025. Given the goals for reducing fuel consumption in gasoline-powered vehicles, the vigorous development of automotive lightweight technologies and the continuous establishment of a technical development and application system for automotive lightweighting become crucial. The 'Energy-saving and New Energy Vehicle Technology Roadmap 2.0' discards the traditional approach of using the overall vehicle equipment mass and the quantity of lightweight materials as measurement standards. Instead, it introduces the overall vehicle lightweighting coefficient as a basis for assessing the level of vehicle lightweighting. It proposes that by 2025, 2030, and 2035, China's gasoline-powered passenger cars should achieve a 10%, 18%, and 25% reduction in the overall vehicle lightweighting coefficient, while freight trucks, tractors, and buses should achieve reductions of 5%, 10%, and 15%, respectively.

1.2. The weight of the three electric vehicle (EV) power systems is substantial, and the demand to increase driving range is driving the development of lightweight.

In 2022, the penetration rate of new energy vehicles (NEVs) increased rapidly year-on-year, with the national stock of NEVs growing significantly to 13.1 million vehicles. According to statistics from the Ministry of Public Security, the national stock of NEVs reached 13.1 million in 2022, accounting for 4.10% of the total number of vehicles. Deducting the scrapped and deregistered vehicles, this represents an increase of 5.26 million from 2021, a growth of 67.13%. Among these, the stock of pure electric vehicles (EVs) reached 10.45 million, constituting 79.78% of the total NEV stock. In 2022, the national registration of new NEVs was 5.35 million, accounting for 23.05% of the total registered vehicles, an increase of 2.4 million compared to the previous year, marking an 81.48% growth. The number of newly registered NEVs has shown a high-speed growth trend, rising from 1.07 million in 2018 to 5.35 million in 2022.

Compared to traditional fuel vehicles, the demand for weight reduction in new energy vehicles (NEVs) is more urgent:

• 1) The overall weight of NEVs is higher than that of traditional fuel vehicles. Unlike fuel vehicles equipped with engines and transmissions, the power of new energy vehicles comes from three major systems: batteries, electric drive, and electronic control, accounting for 30-40% of the total vehicle weight. At the current level of battery density, the power system of NEVs is significantly heavier than that of fuel vehicles. Especially with the continuous advancement of networking and intelligence, the overall vehicle weight will further increase after incorporating related configurations.
• 2) The driving range of NEVs is a core concern for users, and reducing the overall vehicle weight to improve the driving range will further enhance the competitiveness of new energy vehicles. Generally, the driving range of NEVs depends on factors such as battery capacity, motor efficiency, temperature, and operating conditions. Although the driving range of newly launched models has increased, the actual driving range often differs significantly from the officially announced range, even showing a significant decrease in winter, severely affecting consumer purchasing decisions. Research from the National New Energy Vehicle Technology Innovation Center indicates that reducing the overall vehicle weight can significantly increase the driving range, with an increase of approximately 2.5 km for every 10 kg reduction in vehicle weight. Therefore, the China Society of Automotive Engineers proposes a reduction in the lightweight coefficient for pure electric passenger vehicles by 15%, 25%, and 35% by 2025, 2030, and 2035, respectively.

2. Material, structural, and process lightweight are the three main focuses for reducing the weight of automobiles.

To meet the technical requirements of lightweight casting in the automotive industry, breakthroughs are currently being made in three aspects: materials, structural design, and processes, according to cutting-edge industry technology:

• 1) Material Lightweighting: Choose materials with guaranteed structural strength and relatively low density to replace traditional steel materials, including high-strength steel, aluminum alloy, magnesium alloy, carbon fiber, and other materials. This is done to achieve overall weight reduction by lowering material density and quantity. In terms of weight reduction effectiveness, carbon fiber is the best; in terms of cost, high-strength steel is relatively lower.
• 2) Structural Lightweighting: Without affecting the basic state of the vehicle body, advanced optimization design methods and techniques are employed to optimize the structural parameters of the vehicle body. This involves removing redundant parts of components while achieving walling, hollowing, downsizing, and compounding to improve material utilization. Structural lightweighting, depending on the type of design variables and optimization problems, can be categorized into topology optimization, dimension optimization, shape optimization, and morphology optimization. Generally, the use of new materials and processes significantly increases development costs, while structural optimization, as it does not involve new materials, can reduce costs while achieving weight reduction. It is one of the most commonly used methods for body lightweighting.
• 3) Process Lightweighting: Laser welding technology is the most commonly used method by automotive manufacturers. Its principle involves using rolled plates of unequal thickness. Through real-time computer control and adjustment of the spacing of rolling mills, continuously changing sheets with pre-determined thickness along the rolling direction are obtained. In addition, there are other techniques such as hydraulic forming, hot forming, roll forming technology, low (difference) pressure casting forming technology, and various automotive lightweight connection technologies. It's worth mentioning that Tesla's integrated die-casting technology leads innovation in both manufacturing processes and materials.

2.1. Material Lightweighting: High-strength steel is currently mainstream, and aluminum alloy shows promising potential.

Among the three major lightweighting methods, material lightweighting serves as the foundation. Based on the use of lightweight materials, the overall vehicle weight is reduced through optimization of structure and upgraded processes. In the development of lightweight materials, the 'Energy-saving and New Energy Vehicle Technology Roadmap 2.0' indicates that China's independent development and application system for lightweight technologies will focus on perfecting the application of high-strength steel in the short term, directing efforts toward establishing a lightweight alloy application system in the medium term, and aspiring to form a multi-material hybrid application system in the long term.

2.1.1. High-Strength Steel: High yield and tensile strength, currently widely applied with potential growth in aluminum alloy.

According to the International Iron and Steel Association's USL-AB project, steel types can be classified based on their mechanical properties into low-strength steel (soft steel), high-strength steel, and ultra-high-strength steel. Low-strength steel has a tensile strength (Rm) of <270MPa and a yield strength (Re) of <210MPa. Ultra-high-strength steel has a tensile strength (Rm) of >700MPa and a yield strength (Re) of >550MPa. High-strength steel falls between these two. Low-strength steel includes IF steel and soft steel; ordinary high-strength steel includes carbon-manganese steel, BH steel, high-strength IF steel, and HSLA steel, among others; advanced high-strength steel (AHSS) includes dual-phase steel (DP steel), transformation-induced plasticity steel (TRIP steel), complex phase steel (CP steel), and martensitic steel (MS steel).

Steel accounts for a large portion of the vehicle's weight, approximately 55-60% of the vehicle's total weight. According to the Automotive Materials Network, in the case of a modern car, steel constitutes 55%-60% of the vehicle's weight, cast iron 5%-12%, non-ferrous metals 6%-10%, plastics 8%-12%, rubber 4%, glass 3%, and other materials (paint, various liquids, etc.) 6%-12%. It is evident that steel usage in cars is significant, and the application of high-strength steel plates can reduce the weight of stamped parts, saving energy and reducing the cost of stamped products. High-strength steel plates used for automotive parts can have a tensile strength of 600-800MPa, while the corresponding tensile strength of ordinary cold-rolled soft steel plates is only 300MPa. Currently, the world's largest steel company, Arcelor, has developed the hot-stamped steel plate USIBOR1500. This galvanized plate has a coating mass of 120-160g/m2, and after quenching, it exhibits significant mechanical properties with a strength value of up to 1600MPa.

High-strength steel can be applied to various parts of a vehicle, including:

• 1) Automotive panels: Components such as roofs and doors require deformation rigidity and dent resistance. They mainly use bake-hardening steel plates (BH steel) with a tensile strength of 340-390MPa. The yield strength of BH steel increases during baking and painting, allowing for improved dent resistance and thinner steel plates. Some models already use 440MPa-grade BH-type high-strength steel plates. The body structure of the new Mazda 2 uses high-strength steel plates with grades of 440, 590, 780, and 980MPa.
• 2) Vehicle frames: With the improvement of front and side impact safety standards, structural components and reinforcements mainly use 590MPa high-strength steel plates. Some manufacturers also use 780MPa and 980MPa high-strength steel plates. Some manufacturers even use the method of stamping 390MPa and 440MPa high-strength steel plates, then subjecting the strengthened parts to high-frequency heating and quenching to achieve a local tensile strength of 1200MPa. Meanwhile, cooling is applied during the stamping and heating of the steel plate to achieve an overall tensile strength of 1470MPa. In addition, laser welding methods are employed to combine steel plates of different thicknesses and materials, making the material configuration suitable for the required quality and usage.
• 3) Automobile chassis: The material used for the automobile chassis has evolved from traditional 440MPa hot-rolled plates to 780MPa, achieving a maximum weight reduction of 30%. In recent years, the proportion of high-strength steel plates used in the chassis has been rapidly increasing. In the future, the proportion of high-strength steel plates and the application of even higher-strength steel plates are expected to further increase.

Multiple projects confirm that high-strength steel can achieve lightweighting without increasing costs. According to the magazine 'Rolling Steel,' to promote the application of high-strength steel in automobiles, the International Iron and Steel Association has organized several projects, including the UltraLight Steel Auto Body (ULSAB), Advanced Concept Vehicle UltraLight Steel Auto Body Program (ULSAB-AVC), and Future Steel Vehicle (FSV).

• 1) ULSAB Project: The main goals are to reduce body weight, enhance structural strength, improve safety, simplify manufacturing processes, and lower production costs. The ULSAB body weighs 203kg, a 25% reduction compared to benchmark vehicles, with a 91% application rate of high-strength steel. The application rates for cold stamping are 42.8%, laser-welded panels are 44.9%, and hydraulic forming is 9.3%.
• 2) ULSAB-AVC Project: Lightweighting of the body is achieved through overall vehicle design, with a 97% application rate of high-strength steel. In terms of forming technology, over 30% of parts use laser-welded panels, and over 20% of parts use hydraulic forming technology.
• 3) FSV Project: Indicates that advanced high-strength steel can meet the requirements for a five-star collision safety rating and reduce the total emissions of vehicles throughout their entire usage cycle, achieving lightweighting without increasing costs.

2.1.2. Aluminum Alloy: High cost-effectiveness in weight reduction, substantial potential for medium to long-term growth.

Aluminum alloy demonstrates significant advantages in weight reduction, performance improvement, and recyclability. Aluminum alloy, the most abundant green metal in the earth's crust, is not only lightweight and high in strength but also easy to shape, has excellent energy absorption, corrosion resistance, and high recyclability value. Additionally, aluminum alloy, while reducing weight, enhances braking performance, provides better handling, improved driving comfort, and outstanding power for vehicles. According to Automotive Materials Network, specific advantages include:

1) Excellent weight reduction, energy-saving, and emission reduction effects.

• ① Weight reduction effect: According to the report of the Aluminum Association in the United States, for every 1kg of aluminum used in a car, a weight reduction effect of 2.2kg is achieved, leading to a reduction of 20kg in tailpipe emissions during the service life. Due to its lower density compared to steel, aluminum used in car components can achieve secondary lightweighting. According to research by the Aluminum Association of the United States, the first-time lightweighting effect of typical parts in a car using aluminum can reach 30%-40% (replacing ordinary steel with high-strength steel can reduce weight by about 11%), and the secondary lightweighting effect can be increased to 50%.
• ② Emission reduction effect: The fuel consumption of a car is somewhat related to the overall vehicle weight. Generally, the larger the vehicle's mass, the higher its fuel consumption. Carbon dioxide emissions are also positively correlated with fuel consumption. Therefore, by reducing the overall vehicle weight, car fuel consumption can be reduced, resulting in a decrease in carbon dioxide emissions.

2) Improved driving performance, safety, comfort, and stability.

• ① Driving performance: The use of aluminum alloy can reduce the weight of the car, thereby reducing the acceleration time per 100 kilometers and improving driving performance. According to a study by the Aluminum Association of the United States, if aluminum alloy achieves a 25% weight reduction effect in a car, the time for the car to accelerate to 96.56 km/h can be shortened by 4 seconds.
• ② Safety performance: Under the same design requirements, aluminum alloy's ability to absorb collisions is superior to that of steel. Therefore, when a car collides, compared to steel, aluminum alloy material is more likely to form wrinkles and deform, absorbing an additional 50%-70% of impact force, thereby improving car safety.
• ③ Comfort and stability: The use of aluminum alloy in a car typically lowers the overall center of gravity, improving the comfort and stability of car driving.

3) High assembly efficiency and ease of recycling.

Aluminum alloy requires fewer spot welds for the overall vehicle body, shortening processing steps. Moreover, it is not prone to rust, eliminating the need for anti-rust treatment, significantly improving the efficiency of car assembly. Additionally, due to its low melting point, low corrosion rate, and mild corrosion during usage, aluminum alloy is easy to recycle.

4) Lower cost compared to carbon fiber and high self-sufficiency in raw materials.

The price of aluminum alloy is only slightly higher than that of high-strength steel and much lower than that of carbon fiber composite materials. The high chemical stability of aluminum alloy makes it less susceptible to corrosion compared to magnesium alloy, limiting its extensive application in the automotive field. Therefore, overall, aluminum alloy is an ideal material for automotive lightweighting at present. In addition, China is the world's largest producer of alumina and electrolytic aluminum, with a high self-sufficiency rate in raw materials.

Aluminum alloy is one of the optimal materials for lightweighting, with considerable potential for medium to long-term growth. Compared to high-strength steel, aluminum alloy has a more pronounced weight reduction effect due to its lower density. Moreover, it does not face issues such as corrosion susceptibility, high processing costs, and the high price of carbon fiber raw materials, which makes recycling more challenging. Furthermore, the excellent metallic properties of aluminum alloy allow for better integration of structural and process lightweighting, achieving comprehensive weight reduction goals. The 'Energy-saving and New Energy Vehicle Technology Roadmap' outlines phased goals for lightweighting in China, with aluminum alloy usage reaching 250kg and 350kg per vehicle by 2025 and 2030, respectively, far surpassing high-strength steel. As the trend toward lightweighting deepens, materials and technologies for lightweighting continue to advance, and aluminum alloy is poised to become the primary material in the automotive market, with clear advantages in long-term growth.

2.1.3. Engineering Plastics: Extending from Interior and Exterior Components to Functional Structural Components, with Broad Development Prospects.

Plastics come in various types and are essential materials for automotive lightweighting. With the emphasis on energy efficiency, emission reduction, and the rise of new energy vehicles, automotive lightweighting has become an industry trend, leading to an increasing use of plastics in automobiles. Based on the different usage characteristics of various plastics, they are generally classified into three types: general plastics, engineering plastics, and specialty plastics. The primary role of engineering plastics in automobiles is to achieve lightweighting, thereby promoting fuel efficiency at high speeds. Developed countries consider the quantity of plastics used in cars as a crucial indicator of automotive design and manufacturing proficiency, with Germany having the highest plastic consumption, accounting for 15% of the overall material usage. According to automotive engineers, replacing some metals with plastics in the high-voltage electrical components of new energy vehicles can reduce weight by around 30% while meeting performance requirements. Currently, using plastic lightweighting in a pure electric vehicle can reduce weight by approximately 100kg, achieving energy savings and emission reduction. The application of engineering plastics in the automotive field has expanded from interior and exterior components to structural and functional parts. According to the Automotive Materials Network, automotive plastics offer many advantages over traditional materials, primarily in terms of being lightweight, providing excellent aesthetic decoration effects, offering various practical application functions, exhibiting good physical and chemical properties, being easy to process and mold, saving energy, and being sustainable.

• 1) Applications in automotive exterior components aim to substitute steel with plastic to reduce the vehicle's weight. Major applications include bumpers, mudguards, wheel covers, radiator grilles, and air deflectors.
• 2) Applications in automotive interior components focus on safety, environmental protection, and comfort. Major applications include dashboards, door inner panels, auxiliary dashboards, glove compartments, and rear seat back panels.
• 3) Structural and functional components primarily use high-strength engineering plastics. Major applications include fuel tanks, radiator water chambers, air filter housings, fan blades, etc.

2.1.4. Carbon Fiber: Greatest Weight Reduction, Hindered by High Costs and Recycling Challenges

Japan and the United States are leading in the development of carbon fibers, while China is accelerating its pace, with the domestication rate reaching 47% in 2021. China's carbon fiber industry started in the 1960s, nearly simultaneously with countries like Japan and the United States. However, due to insufficient knowledge reserves and issues related to intellectual property rights, its development has been slow. Additionally, countries like Japan and the United States have monopolized core carbon fiber technologies, resulting in China's overall lag in carbon fiber production technology and equipment. Since 2000, the country has increased its support for independent innovation in the carbon fiber field, designating it as a key research and development project. With strong support from national policies, the domestic carbon fiber industry has made significant breakthroughs in technology, rapidly increased industrialization levels, expanded application areas, and formed carbon fiber clusters, mainly in Jiangsu, Shandong, Jilin, and other regions. Major domestic companies include Jilin Chemical Fiber, CFEC Shenyang, Zhongcai Sci-Tech, Guangwei Composites, among others. According to Forward Industry Research Institute, in 2021, mainland China's carbon fiber production capacity surpassed the United States for the first time, becoming the world's largest capacity country, with a capacity of 63,400 tons, accounting for over 30% of the global total capacity, and a production volume of 24,300 tons, a YoY growth of 30.03%.

The application of carbon fiber in the automotive sector is hindered by material costs, processing technology, and material recycling challenges. According to the Automotive Lightweighting Technology Innovation Strategic Alliance, key obstacles preventing the widespread use of carbon fiber materials in the field of new energy vehicles include:

• 1) High material costs: Carbon fiber materials are more expensive to process compared to traditional steel and aluminum alloys. The material cost for the body of a car using carbon fiber can exceed tens of thousands of yuan, with some vehicles using high-quality carbon fiber materials costing over 20,000 yuan.
• 2) Processing technology limitations result in poor puncture resistance of carbon fiber products. Compared to traditional materials, carbon fiber materials have weaker shear resistance, requiring overlap molding in specific applications to ensure post-application quality and effectiveness. This production process gives carbon fiber materials good impact resistance but poor puncture resistance. In cases of excessive force, components may fracture, and once this problem occurs, repair is impossible, requiring replacement.
• 3) Material recycling: Carbon fiber reinforced plastics (CFRP) do not naturally degrade. Incineration or landfilling were common early methods of disposal, but incinerating CFRP waste generates a large amount of toxic and harmful gases, affecting the natural environment. Additionally, the residue from incineration or landfilling causes secondary soil pollution. Current major recycling methods include mechanical recycling, thermal recycling, and solvent recycling, but large-scale and industrialized CFRP recycling has not been established domestically.

2.1.5. Magnesium Alloys: Second Only to Carbon Fiber in Weight Reduction, Corrosion Issues and Manufacturing Challenges Await Breakthroughs

Magnesium alloy exhibits significant performance advantages and finds applications in various areas such as automotive shells, brackets, armrest structures, and automotive display systems. Being the lightest metal material, magnesium alloy boasts features like low density, high strength, excellent heat dissipation, and superior seismic noise reduction performance. The density of die-cast magnesium alloy is only 2/3 of aluminum alloy and 1/4 of steel, with both specific strength and specific stiffness surpassing those of steel and aluminum alloy and far exceeding engineering plastics. Due to its excellent characteristics, magnesium alloy can be used in automotive shells, brackets, armrest structures, and automotive display systems, with relatively high attention and acceptance from market customers for body components such as lamp heat dissipation brackets, dashboard brackets, steering brackets, central control skeletons, and in-vehicle display screen frames.

The forming methods of magnesium alloy materials include casting processing and plastic forming, with manufacturing processes also restricting the widespread application of magnesium in the automotive field.

• 1) Casting Forming: This is currently the main method, and die-casting is the most widely used method for forming magnesium alloy. Recently developed new technologies for magnesium alloy die-casting include oxygen-filled die-casting and vacuum die-casting. Oxygen-filled die-casting is widely used in the production of magnesium alloy components for automobiles, while vacuum die-casting can produce AM60B magnesium alloy automotive steering wheels and hubs. Although casting is the main process for magnesium alloy forming, the defects of castings limit the improvement of magnesium alloy performance and restrict its widespread application.
• 2) Plastic Forming: Using plastic forming methods for magnesium alloy can effectively mitigate the impact of casting defects. Heat treatment reinforcement and deformation reinforcement are often employed to significantly improve the alloy's performance. However, due to magnesium's hexagonal close-packed structure, deformation is more challenging compared to materials like steel, aluminum, and copper. Directly applying the plastic forming methods used for aluminum alloy often results in a low yield rate for magnesium alloy materials, leading to high costs for plastic forming and affecting the application of magnesium alloy in various fields. Therefore, accelerating the development of magnesium alloy plastic forming methods is a current research focus and development trend.

2.2. Structural Lightweighting: Various Methods Have Their Advantages, and Topology Optimization is Highly Valuable for Pre-optimization

According to 'Research and Progress on Automotive Structural Lightweighting,' structural lightweighting refers to the development and design of components through parameter optimization (dimensions, shapes, positions, thickness, etc.), morphological optimization, and topology optimization. The goal is to reduce weight while maintaining or increasing stiffness and strength.

• 1) Dimensional Optimization: This is the earliest and most mature automotive structural lightweighting technology. The principle of dimensional optimization is to establish an optimization mathematical model by taking the dimensions of components as design variables while satisfying the constraints of stiffness, strength, and modal characteristics under typical working conditions. The objective is to minimize the mass of automotive structures. Wang Jifeng's article 'Application of Parameter Optimization Technology in Lightweight Design of Automotive Frames' optimized the frame thickness for heavy-duty truck frames. After optimization, the thickness of certain frame beams decreased, and the total frame mass reduced from 691kg to 654kg, achieving a significant 5.4% weight reduction.
• 2) Shape Optimization: Without changing the existing topology pattern, shape optimization involves using the geometric shape of components as design variables to achieve more uniform stress distribution and more efficient material utilization. The main principle of shape optimization is to alter the geometric shape of the structure under given conditions of structure type, layout, and material to achieve more uniform stress distribution and more efficient material utilization, thus achieving automotive structural lightweighting. Generally, shape optimization is divided into parametric shape optimization (for structures with regular shapes, the geometric shape of the structure is parameterized, and the parameters are then optimized) and non-parametric shape optimization (for structures with irregular shapes, the shape variables are used as design variables, and the structure is optimized in terms of shape).
• 3) Morphological Optimization: Morphological optimization can meet strength and frequency requirements while reducing structural mass, especially suitable for shell structures. The principle of morphological optimization is to improve the stiffness and modal characteristics of sheet metal components by using variables such as the shape, position, and quantity of reinforcement ribs and concave-convex structures. Since morphological optimization does not remove material, it can meet strength, frequency, and other requirements while reducing structural mass. This method allows flexible setting of the types of plane ribs, including height, width, and angle, to meet process requirements, making it especially suitable for shell structures.
• 4) Topology Optimization: This method is highly valuable for optimization design when the structural layout is not yet determined. Unlike dimensional and shape optimization, topology optimization is performed in the conceptual design phase, optimizing design when the layout is not yet determined. It is an organic combination of finite element analysis and mathematical optimization methods. The principle is to find the optimal structural material distribution scheme for an object subjected to a single load or multiple loads within a specified design space, satisfying constraints and design goals. This method allows the structure's stiffness to reach the maximum or meet specified requirements for output displacement, stress, etc. Its advantages include avoiding blind design, improving the efficiency of structural design, producing qualified products with less material, and achieving engineering goals such as structural lightweighting.

2.3. Process Lightweighting: Synergistic Development of Various Lightweighting Processes, Wide Application of Hot Forming in the Past Decade

As one of the three main approaches to automotive lightweighting, process lightweighting can effectively help achieve energy savings and weight reduction at the manufacturing level. Lightweighting technology in automobiles aims to integrate lightweight structural design with various lightweight materials and process technologies, considering the characteristics of the adopted lightweight materials, the requirements of lightweight structural design, and the manufacturing technology used to control product costs, all while maintaining or enhancing the performance, safety, and cost-effectiveness of automobiles. Process lightweighting, based on overall lightweight design for automobiles, comprehensively considers the characteristics of adopted lightweight materials, requirements for lightweight structural design, and product cost control in choosing manufacturing technology.

1) Laser Welding: Enables welding of different materials

Laser welding technology involves using advanced laser techniques and equipment to automatically assemble and weld a certain number of materials, such as steel and aluminum alloys, with different materials, thicknesses, and coatings, to form a single integrated sheet. These sheets are then stamped to create components that meet the specific requirements for different components based on their functions, material properties, thickness, and corrosion resistance. Laser welding techniques used in automotive body welding mainly include linear welding, angular welding, curved welding, and multi-part assembly welding. This process uses laser equipment to weld materials with different properties into welded sheets, which are then stamped to produce the final required components, making modern cars both lightweight and energy-efficient.

2) Hydraulic Forming: Shapes complex, high-precision, hollow components in a single forming step

Hydraulic forming uses liquid as a transmission medium. Under the joint action of liquid pressure and the mold cavity, standard pipes or sheets are shaped into structurally complex, single-piece components. This process replaces traditional welding or casting methods, saving processes and maximizing material efficiency. The hydraulic forming technology for high-strength steel can achieve weight reduction and rational space utilization while maintaining safety performance indicators. Hydraulic forming can be divided into sheet hydraulic deep drawing, tube hydraulic bulging, and shell hydraulic forming. According to the difference in the pressure borne by the liquid in the mold cavity, it can be further divided into high-pressure forming and low-pressure forming.

3) Hot Forming: The preferred technology over the past decade

Hot forming technology involves heating sheet metal to the austenite temperature, then hot forming it in the mold. After cooling with water, high-strength martensitic structures are obtained while maintaining the part's good shape. Hot forming addresses drawbacks such as cracking, springback, and wrinkling in the cold forming process. Parts manufactured using this method meet the characteristics of lightweight and high strength, contributing to the lightweighting of automobiles. Over the past decade, hot forming technology has rapidly become the preferred manufacturing technology in the automotive industry.

The three automotive process lightweight technologies have different advantages. In the future, the choice of technology can be based on the characteristics and requirements of different components of the automobile. The biggest advantage of laser welding technology is its ability to weld blanks with different thicknesses, materials, strengths, stamping performance, and surface treatment conditions together before stamping. Hydraulic forming technology excels in shaping complex, high-precision, hollow components in a single step and is suitable for various hollow components with axisymmetric changes in the automotive field, such as exhaust pipes, engines, and subframe main pipes. It has the advantages of increasing the strength and stiffness of formed parts, reducing the number of molds, and lowering production costs. Hot forming technology is suitable for components with high requirements for comfort, strength, and safety. Typical hot-stamped parts include front and rear door side impact bars, front and rear bumper crossbeams, A/B pillars, floor channel, roof reinforcement beams, and suspension fixed brackets. Hot forming achieves the goal of reducing vehicle weight without compromising safety.

4) Pressure Casting: Diverse casting methods, each with its pros and cons

Pressure casting is a casting method that fills the mold cavity with liquid or semi-solid metal or alloy, or liquid metal or alloy containing reinforcing phases, under pressure at a relatively high speed to solidify and form the casting. Pressure casting can be divided into low-pressure die casting, high-pressure die casting, vacuum high-pressure die casting, differential pressure die casting, extrusion casting, etc.

5) Joining Technology: One of the key technologies for lightweight, composite connections complement each other

Joining technology is one of the key technologies for the development of lightweight manufacturing technology. It is related to many aspects such as the performance, weight, processing technology, assembly, safety, and recycling of the connected structure. Traditional joining technologies mainly include resistance spot welding and inert gas shielded welding/reactive gas shielded welding (MIG/MAG). However, with the increasing need for lightweight design of materials, new joining technologies such as laser welding, riveting and self-piercing riveting, bonding, and composite connections have gradually developed and been applied more widely. Mechanical joining technologies include press welding, clinching, self-piercing riveting, blind riveting, and folding. The advantages of using mechanical joining technology instead of resistance spot welding are that it can be used for various material combinations or laminated materials, allows for coated surfaces, does not require heating (low deformation, does not change material properties), and does not require pretreatment and processing. Bonding technology refers to using suitable adhesives as process materials, adopting appropriate joint forms, and using reasonable bonding processes to achieve the purpose of connection. Adhesive connections produce continuous connections, resulting in a more uniform stress distribution. Compared with spot welding and mechanical connections, which are local and intermittent connections, adhesive connections improve connection stiffness.

3. Integration of Die Casting Aids in Cost Reduction and Efficiency Improvement in Vehicle Manufacturing

3.1. Integration of Die Casting for Cost Reduction and Efficiency Improvement, with Equipment Cost, Mold Manufacturing Difficulty, and Material Requirements as Major Barriers

The integration of die casting combines traditional stamping and welding processes in automobile production into a die-casting process, greatly simplifying the manufacturing process. Traditional automobile manufacturing consists of four major processes: stamping, welding, painting, and final assembly. Stamping involves pressing metal sheets into various components needed for the vehicle body, followed by welding or riveting to produce large aluminum parts. In contrast, integrated die casting uses large-tonnage die-casting machines to combine stamping and welding into a single die-casting step, merging the first two steps into one. This process highly integrates multiple individual and dispersed components, directly casting large parts.

Compared to the traditional 'Stamping + Welding' model, the integrated die casting model demonstrates advantages in several aspects:

1) Triple cost reduction in production line, materials, and labor.

• a) Construction cost and footprint of the production line are reduced. The traditional automotive manufacturing process involves numerous individually processed components, each requiring machinery, molds, and peripherals such as robotic arms, conveyors, and fixtures. The complete production line is large, occupies significant space, and incurs high costs. In contrast, integrated die casting highly consolidates multiple individual and dispersed components, requiring only a large die-casting machine, minimal auxiliary equipment, and molds. This eliminates the need for heat treatment, molding, passivation equipment, etc. As a result, the construction cost and footprint of the production line are significantly reduced. After adopting a large die-casting machine, the factory's footprint is reduced by 30%.
• b) Improved material utilization. Traditional stamping processes inevitably generate scrap materials during the stamping process, and due to the complex usage of materials, different components usually correspond to different types and material grades. In traditional stamping-welding processes, the plate utilization rate is typically only 60%-70%. However, integrated die casting treats liquid metal on a one-to-one basis as equivalent to the material used in casting, resulting in a higher material utilization rate. Additionally, since only a single aluminum alloy is used, the car body can be directly melted and recycled after disassembly, with a recycling rate exceeding 90%.
• c) Reduced labor costs. Traditional car body manufacturing involves welding processes with numerous welding points, requiring a large number of welding technicians. Currently, mainstream welding plants in the country typically employ 200-300 workers. With the adoption of integrated die-casting technology, the reduced number of welding points requires fewer skilled workers, reducing the workforce to at least 30-40 people.

2) Simplified process flow and increased production efficiency.

The traditional manufacturing process for car bodies mainly consists of four stages: stamping, welding, painting, and final assembly. The main car manufacturers purchase various structural components manufactured by suppliers nationwide through stamping and die-casting, assembling them (including welding, riveting, and gluing) to form the car's body-in-white assembly. In contrast, the integrated die-casting process reduces the workload of stamping and welding and eliminates many gluing process steps, resulting in a significant increase in production efficiency. For example, the Tesla Model Y's rear floor uses integrated die-casting, where all parts are die-cast in one step, applying new alloy materials. The rear floor assembly, cast in one piece, no longer requires heat treatment, reducing manufacturing time from 1-2 hours in traditional processes to 3-5 minutes.

3) Reduction in body weight, decreasing the amount of installed batteries.

According to Cheqian Information, the cost reduction of batteries is 6.6 times that of the cost increase from switching to aluminum body materials from steel. The next-generation integrated die-cast chassis from Tesla is expected to reduce vehicle weight by 10%, corresponding to a 14% increase in range. For example, using an 80 kWh battery capacity for a typical electric vehicle, adopting an integrated die-casting body for weight reduction while maintaining the range can lead to a reduction of about 10 kWh in battery capacity. Calculated based on a cost of 100 dollars/kWh for lithium iron phosphate battery packs, this can reduce costs by 1,000 dollars.

All these advancements in lightweight technologies, structural optimization, manufacturing processes, and die casting integration collectively aim to enhance efficiency, reduce costs, and contribute to the development of lighter and more sustainable vehicles in the automotive industry.