High-Performance Lightweight Materials Hitch a Ride on Cars’ “Weight Reduction” Imperative

By Qu Xing, China Auto Lightweight Technology Innovation Strategic Alliance

In the automotive sector, “lightweight” refers to maximal reduction of overall automobile mass while ensuring the strength and safety of the automobile, to improve dynamic characteristics, reduce fuel consumption and minimize polluting emissions. At the same time, lighter weight can also bring about “secondary weight reduction” because in this way the power system, the transmission system and the braking system of lightweight vehicles can be smaller. Lighter weight has already become an important development trend in the auto industry.
Lightweight auto technology includes optimized structural design, use of lightweight materials and advanced manufacturing processes.

Lightweight materials

1. High-strength steel

Two main categories of steel are used in automobiles. One is structural alloyed steel. It is mainly used in main structures such as engines, transmission systems and suspension systems. The other is high-strength steel, used mainly in body interior/exterior panels and body structures.
With the rapid increase of demand for high-strength steel in making automobiles, steel mills in China have accelerated their development of advanced high-strength steel for automobiles. Based on requirements for physical properties for automotive components, dual phase (DP) steel, transformation-induced plasticity (TRIP) steel, martensitic stainless (MS) steel and preheating shaped (PHS) steel series products have been formed.

2. Aluminum alloys

Aluminum alloys are excellent lightweight materials. Their use as structural materials to replace iron and steel can reduce vehicle weight remarkably. The main uses of aluminum alloys in automobiles today are in cast parts, forged parts, extruded parts and boards. The development of aluminum alloys for automobiles centers mainly on increasing strength and reducing cost.
Many commercial vehicle wheels use aluminum alloy forged parts today. The ultimate effect of aluminum alloy in reducing a vehicle’s unsprung weight is evident in improved fuel efficiency and reduced emissions, not to mention the improved ride.
The typical application of aluminum alloy extruded parts is in front/rear bumpers of passenger buses. Bumper weight can be reduced 40-50%. Aluminum alloy extruded profiles have gained large-scale applications in special vehicles such as multi-functional gull wing opening type vans, refrigerator wagons, civil-use vans, mail wagons, generator cars, emergency rescue vehicles, military readiness vehicles and vehicles for catering commercial aircraft.
Aluminum alloy extruded parts are already common in new-energy commercial vehicles made in China. Manufacturers of big new-energy passenger buses all vigorously promote weight reduction by using aluminum alloys. Some big new-energy passenger buses with full aluminum frames are already on the market. Weight reduction can reach 40%, and the continuous-running mileage is increased by around 20 km.
Aluminum alloy boards can be used in engine housings, front mudguards, roofs, doors, luggage box covers, floor structures and even full aluminum frames.
Flexible packaging aluminum foils used in the power cells of some new-energy vehicles typically have a complex three-layer structure of nylon/aluminum foil/polymer. Rigid packaging aluminum shells are made of aluminum alloy boards. Such shells require high strength, special welding behavior and special forming characteristics.

3. Magnesium alloys

North America and Europe are the main regions that use considerable quantities of magnesium alloys in automobiles. More than 60 types of magnesium alloy automobile components are already being used or under development in Europe. Typical consumption of magnesium alloys per car is 9.3-20.3 kg. More than 100 types of magnesium alloy automobile components are already being used or under development in North America. Typical consumption of magnesium alloys per car there is 5.8-26.3 kg. Consumption of magnesium alloys in China is still very limited: the average consumption per car is less than 1.5 kg. Major applications are in housings such as gearboxes, and in other cast components. Magnesium alloy cast steering wheel skeletons are the most mature example.

4. Composites

The main plastics used in automobiles are general-purpose plastics, engineering plastics, plastic alloys and polymer or resin (structural/functional) composites. The most popular variety of polymer composites is fiber reinforced resin composites (also called FRP).
Reinforced materials used in automobile components today typically have glass fiber varieties and carbon fiber varieties. Glass fiber reinforced composites are usually used in body parts, engine housings and tail boards of passenger buses and bumpers, mudguards and foot pedals of commercial vehicles, as well as helmets. The use of carbon fiber reinforced composites (also called CFRP) can reduce auto body mass by 30-60%. Due to the high cost of carbon fiber and the high manufacturing cost of CFRP components, however, their use in autos is still limited to F1 racing cars, high-grade passenger buses and small-batch models.

Diversified development becomes a trend

With the development of lightweight auto materials and related technologies, the proportions of aluminum, magnesium and plastics in a vehicle’s makeup have increased, obviously displacing iron/steel materials. But high-strength steel has evident advantages in collision behavior, processing process, cost and green production, compared with aluminum alloys and magnesium alloys. So high-strength steel will remain the preferred material for reducing vehicle weight while increasing collision safety for some time. Third-generation automobile steel varieties (including medium manganese steel and quenched & partitioned steel) are a major orientation in the development of high-strength steel.
Low density steel made by alloying with elements such as aluminum, silicon and manganese, and steel with a high elasticity modulus made by introducing high modulus particles (such as TiB2) into iron alloys both constitute important development trends of steel for use in lightweight autos. These two technologies are still at the stage of lab research and pilot testing. With the development of lightweight auto technologies, the use of iron/steel mixed with other materials can reduce weight further. The design and manufacture of steel/fiber reinforced composites mixed structures and steel/aluminum mixed structures has already become a development trend.
Consumption of aluminum alloys in making automobiles will increase. The per-car consumption of aluminum is expected to grow to around 190 kg in 2020. As technical progress reduces costs, more aluminum alloy components with complex structure and high-but-declining cost will be introduced into automobiles. The per-car consumption of aluminum is expected to reach 250 kg in 2025. Today’s technologies for aluminum alloys used in automobiles will be very well developed by 2030, so various cast parts and extruded parts with complicated structure and performance compliance will be in large-scale production. At that time 30% of all automobiles will have fully aluminum frames, and the per-car consumption of aluminum will exceed 350 kg.
With deepening research into magnesium alloys, many more magnesium alloy cast parts will go into new automobiles in the future. The application of deformed magnesium alloys will also increase gradually. New magnesium alloys, technology for forming magnesium alloys, magnesium alloy anticorrosion technology and other technologies related to magnesium alloy auto components have already become research focuses both here and abroad. Magnesium in passenger buses is expected to reach 45 kg per vehicle in 2030.
With the gradually falling price of carbon fiber and the maturing of composites manufacturing processes, some OEMs are developing carbon fiber components. Components made of carbon fiber composites will be more and more common. Carbon fiber composites will become mainstream auto materials by 2030. By that time, both the cost and the production cycle of carbon fiber composites will fall drastically, by around 33%.