Automotive die castings: Lightweight structural components…

automotive lightweight

The automotive industry uses a large number of lightweight structural components and also consumes a large number of lightweight alloy materials. With the rapid changes in the automotive industry and the further increase in the demand for lightweight due to energy saving and emission reduction, the demand and development of lightweight alloy structural components have also undergone great changes. In the paper, the impact and demand for structural load-bearing components under the new development trend of the automotive industry will be discussed in terms of each key subsystem of the vehicle, different structural materials, and types of forming processes, respectively.

Demand for lightweight structural components in automotive subsystems

Drive systems

The powertrain systems of conventional internal combustion engines are the main area of application for lightweight structural components, which are functionally complex and often accompanied by high temperature and pressure service conditions, making them particularly suitable for production by casting processes. Typical components are engine blocks and heads, gearbox housings, pistons, oil pans, etc. As conventional fuel vehicles will remain the main mode of transport until at least 2025, there will still be a great demand for these lightweight structural components. With the accelerated development of new energy vehicles, traditional car companies have invested heavily in new energy vehicle platforms, resulting in stagnation or a slowdown in traditional internal combustion engine powertrains. Therefore, subsequent internal combustion engine powertrains will only continue with existing platforms or improvements based on existing platforms. After this, it won’t be easy to launch completely new internal combustion engine powertrain platforms.

The launch of a new electrified drive platform will see the introduction of many new components, such as motor housings, battery pack housings, transmission housings, and other components that do not exist in the traditional internal combustion engine approach powertrains and will require new design and manufacture. A high rate of replacement characterizes these new lightweight structural components. While replacing traditional internal combustion engine powertrains may take 5-8 years, replacing these new energy components may only take 2-3 years, or even less. This poses a huge challenge to new components’ design, production, and quality control. For example, there are currently various solutions for the power pack shell, such as aluminum alloy extrusion profile welding, high-strength steel plate stamping, aluminum alloy die-casting one-piece forming, etc. These materials and processes are also constantly being upgraded to meet the demand for continuous upgrading of the power pack.


Body and cover systems

Since the beginning of the 21st century, the lightweight of bodywork and coverings has been the main protagonist in the wave of automotive lightweighting. Starting from the introduction of the all-aluminum body by Audi, the body’s lightweight has been synonymous with the lightweight of automobiles. With the growing emphasis on energy saving and emission reduction in all countries, the trend toward lightweight will continue. With the launch of new energy vehicles and intelligent driving vehicles, vehicle lightweighting has risen to a new level, especially as the problem of the self-weight of the power battery system is difficult to solve within a short period, so lightweight has again taken on the difficult task of increasing range. From the perspective of the major new energy car companies, most of the newly launched models are all-aluminum, such as the Tesla Model S and X, and the ES8 and ES6 from Azera, or at least a steel and aluminum hybrid body, mainly due to the huge contribution of lightweight alloy components to weight reduction.

In terms of body components, the major car manufacturers, especially in the mid-to-high-end segment, have generally accepted high-pressure cast structural parts as a solution for the integration/lightweight of body components. A typical component is the shock tower, which is traditionally stamped and welded to a die-cast integrated design, with a significant reduction in components and a clear, lightweight effect. In addition, the four-door and two-cover systems using aluminum alloy stamping and magnesium alloy die-casting integrated forming have also been applied on a large scale. The price bottleneck problem will be solved with the further advancement of lightweight requirements and the increased production of related materials and component suppliers.

Cover components are the most demanding parts in terms of appearance and are currently mainly formed from sheet steel. The design of all-aluminum bodies introduces 6 x x x aluminum sheets as automotive cover parts. These products are still mainly used in high-end models for technical reasons, such as the natural aging of aluminum alloys and the difficulty of controlling the spring back of stamping, and also for the reason that there are few high-end suppliers with high prices.

Chassis and suspension system

Chassis and suspension systems use many lightweight alloy components, and the proportion of lightweight materials used is also the highest among all car subsystems. Representative components include wheel hubs, control arms, steering knuckles, and subframes, which have been used extensively by the components industry for nearly half a century. Chassis system design is heavily influenced by the new energy direction, mainly because of the integration of the battery pack, subframe and motor system in the chassis subsystem. The current chassis system of the EV platform uses a large number of aluminum structural components, which make a significant contribution to the lightweight of the chassis. In the suspension system, wheel hubs and steering knuckles, typical of lightweight components, have not changed significantly during the development of electrification and intelligence.

Interior and exterior trim and other systems

In the interior and exterior subsystems of the vehicle, the use of traditional lightweight construction materials is relatively small because there are few load-bearing components. Representative components are the steering wheel skeleton, the instrument panel skeleton, the seated skeleton, and other bracket-type components. As the interior components are usually in good service condition and do not have harsh corrosion resistance requirements, magnesium alloy components have a certain penetration rate, especially the steering wheel skeleton has a very high penetration rate, more than 80%. With the development of lightweight trends, these components in lightweight materials will have a more extensive demand, but also can provide good support for automotive lightweight.


Demand and outlook for lightweight structural materials

The main body of aluminum alloy structural parts in the automobile industry is castings; for example, engine parts, transmission parts, and chassis parts are all made of cast aluminum alloy parts as the main supply system, and deformed aluminum such as calendering and extrusion is mainly used in the production of radiators, aluminum doors and coverings, crash beams and other parts in automobile parts. Currently, aluminum castings account for about 75% of the aluminum alloy parts for vehicles. Deformed products, especially calendered aluminum sheets and die-cast structural parts for bodywork, are new growth points with good development space.

Aluminium alloy

The average amount of aluminum used in the automotive industry is 150-180 kg, and the all-aluminum body and new energy models can reach more than 400 kg. The annual growth rate of aluminum alloy application in automobiles is more than 8%, and by 2025, the average aluminum use per vehicle is expected to reach more than 220 kg.

For example, die-casting engine blocks and low-pressure casting wheels have already formed a relevant industry chain.

From the material type point of view, chassis parts are used A356 (American grade) or AlSi7Mg (German grade), these parts are used low pressure/gravity casting process, and parts need T6 heat treatment. The most used alloys for these parts are A380 (US grades), AlSi9Cu3Fe (German grades), and ADC12 (Japanese grades), all of which are sub-eutectic aluminum-silicon systems with a copper mass fraction of between 2% and 4% to improve yield strength to a certain extent and an iron mass fraction of between 0.8% and 1.3%, to solve the problem of mold sticking during die casting. The elongation requirements for such parts components are not high, generally below 3%, and the alloying elements are less demanding to control and can be produced using recycled aluminum. Body structure parts components using Al-Si-Mn system, mainly AlSi10MnMg (German grades) and A365 (American grades), these parts are characterized by high elongation, strict alloy range requirements, can not be produced using recycled aluminum, usually need more than 10% to meet the connection requirements of body parts.

Since 2010, there has been a wave of vacuum die-casting of structural body parts, producing large vacuum die-cast thin-walled parts for the structural body parts of high-end models, the main material for these parts is AlSi10-MnMg material; the main feature is the need for the ultra-high vacuum to meet the T7 (solution + over-aging) heat treatment and riveting performance. It should be stressed that the emergence of structural body parts has provided a huge driving force for the demand and development of aluminum alloy materials, mainly because existing vacuum die castings require heat treatment to achieve high elongation for riveting with surrounding body parts, hence the low yield of such parts, few high-quality suppliers and high prices. The international research and development institutions of major aluminum alloys and car companies are developing die-cast aluminum alloys that can be riveted without heat treatment for body parts, such as Maximal-59 and Castduct-18 from Rheinland, C611, and A152 from Alcoa, A379 from General Motors R&D Centre, etc. There are already non-heat-treated alloys available for mass production. With the emergence of non-heat-treated high-toughness alloys, the cost and performance of structural body parts have a greater advantage. With the rapid growth of large and complex structural components for new energy models, this field will achieve rapid development.

In addition, the fastest growth of aluminum alloys in automobiles is the application of calendered products in the four-door and two-cover components. Taking the 6××× series aluminum sheet as an example, it has now gained more than 50% share in the hood cover. From a material point of view, aluminum sheets for automotive use mainly include 5××× series with good formability (e.g., 5182 and 5754). These aluminum alloys have no heat treatment strengthening characteristics and are mainly used for the production of complex formed parts for inner door panels; the other type of 6××× series aluminum sheets (e.g., 6016 and 6061) are suitable for forming the outer panels of four doors and two covers due to their good age-hardening characteristics. As the production and coating process of the outer plate of the car is more complex, it needs to perfectly match the natural aging characteristics of the 6××× series alloy in a solid solution or pre-aging state (generally valid for half a year). Therefore, the 6××× series alloy has higher requirements for the aluminum plate manufacturers and manufacturers using it.

Magnesium alloy

The average amount of magnesium used in the automotive industry is within 10 kg. From 2000 onwards, governments and R & D institutions have invested a lot of resources in the research and development of magnesium alloys and the promotion of products, especially China as a large country of magnesium alloy resources, has been hoping to increase further the amount of magnesium alloy in the automotive industry. However, the amount of magnesium alloy for automotive use has not seen the expected significant growth; the main magnesium combined grades are still mainly AZ91D and AM50, and the main magnesium alloy products for steering wheel skeleton, dashboard skeleton, seat skeleton, and other interior components. One of the main reasons for limiting the large-scale application of magnesium alloy is still determined by the characteristics of magnesium alloy; the poor corrosion resistance of magnesium alloy, especially galvanic corrosion, is the biggest resistance to the application of magnesium alloy in the non-interior bearing parts system.

The development trend of lightweight and integration of automotive components, which can play the advantage of good fluidity of magnesium alloy material and easy forming of large and complex structural parts, will lead to new large-scale application of magnesium alloy, for example, the inner panel of the door and the inner panel of the luggage compartment back cover, using die-casting magnesium alloy can achieve the optimal lightweight and structural optimization effect. At present, China has the most magnesium alloy mineral and smelting resources and also has the largest cargo die-casting equipment downstream production enterprises; in this field, China has formed the world’s most complete industrial chain, can realize from the original magnesium to high-end magnesium alloy die casting of the whole process of production and manufacturing.

Light structural components forming process needs and prospects

Lightweight structural components of the forming process, including casting class and deformation class two, at present, automotive calendering class and extrusion class parts of the process and materials are relatively stable, and automotive casting class parts are designed for a wide variety of process types, products, new technology is also emerging. The following will be from the high-pressure casting and metal casting two processes for automotive lightweight structural components of the forming process status and prospects for discussion.

High-pressure casting

At present, automotive aluminum alloy parts use die-casting process. Its main products include traditional high-pressure casting (engine block, transmission housing, and other shell-type components), less than 5 kPa ultra-vacuum die-casting (shock tower, longitudinal beam, and other body structure parts), and all kinds of die-casting process variants (such as ultra-low-speed die-casting, extrusion die-casting, semi-solid die-casting, etc.). With the increase in new energy components and smart driving vehicle components, the requirements for the die-casting process are increasing, such as the die-casting of pure aluminum motor rotors and the welding of heat dissipating electric control box shells. These new needs to the traditional die-casting enterprises put forward a new challenge, die-casting enterprises need to adjust the direction of research and development in time, for the requirements of the new part can be converted into the die-casting process, materials, molds, and other specific requirements, and through the optimization of the production of high-quality qualified castings.

With the high density of automotive components and mechanical performance requirements increasing, extrusion casting, semi-solid die-casting, and other new die-casting process components are also increasingly popular. The biggest feature of these parts is to achieve T6 (complete solution + aging) heat treatment; parts can reach the strength of the chassis class parts and has a high efficiency close to the die-casting; currently, in the chassis class bracket, parts have been replaced the traditional die-casting products appear. Compared with the traditional die-casting process, extrusion and semi-solid die-casting products need to enhance the quality and stability further, broaden the process’s scope, improve the product’s cost performance, and expand the supply system.

Metal casting

Metal casting mainly includes low-pressure casting, gravity or gravity tilting casting, differential pressure casting process, etc. These processes correspond to the typical automotive parts for wheel hubs, cylinder heads, and steering knuckles. In terms of the low-pressure casting process, this is currently the most suitable process for wheels, with over 90% of OEM wheels being produced using this process. However, it is the trend in automotive design toward larger diameter wheels, e.g., before 2000, most wheels were 15-16″; now, the trend is towards 17-19″ wheels, which poses a huge challenge to traditional wheel manufacturing. The gravity tilting process is more common for producing wheels for aftermarket and Japanese cars. This process, combined with the spinning process, enables the sequential solidification of wheels and is more suitable for producing large-sized wide tread wheels with good appearance, mechanical properties, and good market prospects.

The differential pressure casting process is now widely used in steering knuckle components, mainly because of the high-quality stability and fatigue performance requirements of these parts; the differential pressure process through cooling and low and medium pressure solidification pressure to solve such requirements, but the differential pressure casting equipment and molds have high requirements, only suitable for steering knuckle such thick and large parts, medium size parts are not suitable for larger wheels and subframes, etc. Components such as wheels and subframes are not suitable. At present, new structural components for new energy and autonomous vehicles are experimenting with various new forming processes, such as battery pack shell parts, which are produced mainly by profile welding as the volume is not very large. Still, various manufacturers are experimenting with casting as an efficient and low-cost process solution. There are already low-pressure casting and ultra-vacuum die-casting production of battery pack shells, but none are in mass production. The market and technology need to test which specific program is more suitable. In addition, gravity or gravity tilting process motor shell has entered the mass production stage, mainly because the parts usually have a cooling water circuit. The metal-type process is more suitable for producing core-containing parts.

Summing up

As the electrification and intelligence of vehicles advance, new challenges are posed to traditional lightweight alloys and their forming processes, and a wealth of new opportunities arise.


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