Light Weight Materials in Automotive Industry

The growing concern about the oil prices and the environmental protection against emissions have made the auto makers move towards alternative lightweight materials like Al and Mg as a replacement for regular steel and cast irons. Due to the strict laws imposed on the regulation of Co2 emissions, there is a need for the development of new concepts or methodologies to reduce the overall weight of the vehicle and thereby reducing the pollution. Through the studies conducted in Europe, it is observed that the amount of Co2 per kilometer can be significantly reduced by reducing the weight of the car. Hence reduction of the weight of the vehicle plays an important role in reducing the fuel consumption and the emissions. The weight reduction can be achieved mainly in two ways; first is by altering the materials used in the vehicle; second is by the structural optimization of the Body in White (BIW). When choosing a new light material several aspects like cost, material properties, impact on the product design and the production process etc., have to be taken into consideration. Most importantly the several performance parameters like the dynamics of vehicle, operational strength, crashworthiness etc., should be satisfactory. The evaluation of such parameters can be done with the help of a testing a physical prototype of the final design of the vehicle. If the performance is not as expected then there is a need to improve the structure or change the material used and repeat the process of testing through an updated physical prototype. This continues until we get a model with desired performance and considerable weight reduction. It is clearly a time consuming as well as costly process and hence the numerical methods like Finite Element Methods (FEM) are used to generate virtual prototypes and test in the computer without the need of a physical prototype. Using FEM in combination with the optimization techniques like Multi-Disciplinary Optimization, the structures can be optimized for the weight and functional targets like static and dynamic performances, during the initial stages of design itself (Gaetano). In this way the time to design a new product with optimized material usage can be reduced along with the simultaneous consideration of the multiple disciplines like dynamics of driving, structural statics and crashworthiness etc.

Aluminum

Aluminum has been widely used for manufacturing different sub-systems of a car like, chassis, Body in White (BIW), powertrain etc.

Powertrain

The powertrain mainly consists of engine blocks, intake manifolds, cylinder heads, transmission housings etc. Aluminum is commonly used in all these components of the powertrain since long time but its use in the engine block to reduce the engine weight is recently started. Since engine will be running at high temperatures it is required that the material possess properties like good thermal conductivity, fatigue strength, vibrational resistance etc., at elevated temperatures. Also for the efficient combustion of the fuel with air, the engine components should have a very good resistance against leaks. Since Al has the ability to meet all the above mentioned requirements, it is being efficiently employed in the manufacturing of the powertrain components.

Chassis

Formerly, the chassis of the car was manufactured out of steel which leads to higher unsprung mass of the vehicle. The vehicle’s vibrational behavior directly depends on its unsprung mass, lower the unsprung mass better will be ride comfort and the safety due to better tire-road contact. Therefore auto makers have started to use Al for the chassis components for the weight reduction. The typical chassis components like wheels, suspension, braking system, fuel system etc., are produced out of casted Al. In the casting process the rate at which solidification of the microstructure occurs is very important and needs to be effectively controlled in order to produce defect free castings. Moreover the porosity and the formation of oxide films can be eliminated by avoiding the turbulence during the metal pouring. This is achieved with the help of computer assisted heat and fluid flows and proper designing of the location and geometries of the foundry components like sprue pin, risers and ingates. In this way the Al castings with very good fatigue and impact resistances can be obtained. In chassis Al 5000 series alloys are primarily used due to their good forming capability, weldability, and good after forming strength, and the supreme corrosion resistance even without the need of any coatings. However when the components are located close to heat sources like engine or exhaust systems, to resist the thermal loads the Mg content in the alloy should be less than 3%, which otherwise would lead to intergranular corrosion (IGC). The figure below shows the components in chassis made out of Al 5000 series. Apart from these, wheels are also manufactured from casted Al, especially in luxury cars both to reduce the weight and to have a superior appearance of the customized design. In case of the heavy machinery like trucks and construction equipment, where the magnitude of loads are high, the wheels are produced from forged Al to meet the requirements of higher mechanical strength. On the whole the application of Al in chassis components not only reduces the fuel consumption but also most importantly enhances the driving comfort and safety of the vehicle.

Front end

An efficient design of a front end helps in improving the driving dynamics as well as the ability of the vehicle to take up the crash loads. BMW has been able to successfully employ Al in manufacturing the structure of front end and achieving a weight reduction of 30% when compared to steel structure. From the figure below it can be observed that the front end modules are manufactured from different techniques like extrusion, casting, and sheet stamping, with each having their own contribution to meet the requirements of strength, stiffness and resistance to corrosion. The formability of the structure can further be improved by increasing the Mg content in the Al alloy.

Body In White (BIW)

BIW which was formerly manufactured out of steel is the heaviest part of the vehicle and contributes to about one third of the whole weight of the car. Hence the auto makers found a great weight reduction potential of the whole car through building of BIW with Al alloys. BIW mainly comprises the sole structure and the hanging parts like Door, Hood, Fender etc. attached to the structure through bolts. Both the 5000 and 6000 series Al alloys in the form of thin sheets are used for the manufacturing of the BIW, and the selection of the material is done based on whether the component is subjected to thermal loads or not and also whether it is a part of the inner or outer structure. Heat treatable Al 6000 alloys with low Mg content are used for the BIW outer panels and the hanging parts in order to make use of the heat during the paint baking process for strengthening of the material. In addition, a very good surface finish and corrosion resistance can be imparted which is very essential for the outer panels as they are directly exposed to outside world. Whereas for the BIW structure and inner panels, strength and formability are main parameters and since they are not exposed to heat or thermal loads, using Al alloys with high Mg content is preferable. Therefore Al 5000 series alloys with Mg content greater than 5% by mass have been developed and successfully employed in BIW structures.

BIW of a car can be designed and fabricated mainly in two ways: first is a structure which is primarily made from stamping process of different panels of the body, which are welded together by means of spot welding. The other method is a BIW structure where different components are made from different manufacturing processes like metal casting, extrusion and of course conventional stamping. For employing the standalone stamping process, the Al sheets used should be feasible enough to be able to form into required complex shapes at reasonable cost. The section below presents the different models in the Industry employing different methodologies to manufacture a BIW.

Application of Al in BIW of several car models

Audi which is one of the leading automotive manufacturer in the world, has employed the Space Frame Concept for building the BIW of A8, which weighs about 277 kg. It has the parts mainly manufactured using extrusion process and also few components are produced from the casting and sheet metal stamping processes. Different parts are joined together by using techniques like Riveting, Metal Inert Gas welding, Laser welding, and adhesives. Through the application of Al alloys, Audi was able to successfully reduce the weight of BIW by 40% and found the Space Frame Concept really efficient. Eventually they are producing very high volumes of cars every year using the same concept and with even lighter BIWs.

However, the Jaguar in their XJ Model 2002, are still employing the conventional ‘‘Stamped Sheet Monocoque’’ for building the BIW. It can be observed that in contrary to the Space Frame Concept used by Audi, the number of parts produced from Sheet metal stamping are very high than that of the parts manufactured from casting and extrusion. Also the primary procedures employed for joining are adhesives and rivets which leads to higher weight of the BIW as compared to BIW of Space Frame Concept.

Comparison of Casting and Extrusion

Extrusions

Aluminum extrusion is a widely employed manufacturing process in the automotive domain, especially to produce parts with complex designs. The weight reduction of the components can be achieved simultaneously meeting the required functionalities and hence extrusion process is often employed in series production. Currently the Al extrusions are used in Space Frame Concept model of BIW and also in complex components like bumpers (fig), crash absorbing elements, air bags etc. Due to high strength requirements for extrusions, heat treatable Al 6000 and Al 7000 series alloys are employed. The strength and formability of the Al 6000 series alloys can be improved through hardening over the period of time. Research is still going on to develop new alloys which have enhanced extrudability, mechanical strength, and tolerance levels to be able to operate in toughest conditions without failure.

Castings

A major section of the automobile parts are manufactured using casting process, which include engine components like block, cylinder head, and other sub systems like suspension, chassis, brakes etc. Due to best casting ability, strength, and endurance, cast iron was widely used, however in order to reduce the weight Al castings are recently employed. Extensive research lead to the development of high performance Al alloys possess significantly improved mechanical properties to meet the real world requirements. Also the development of computer aided casting process and advanced casting methods enables the application of Al casting to produce complex parts in an automobile. Newly developed AlSiMgMn alloys are used to manufacture complex structures like a pillar with many rib sections, which connects the B-pillar instrument panel and front- end structure. For high-pressure die-castings (HPDC), it is required that the development of alloy and material models happen in parallel, in order to observe how the casted part behaves under the load. Also we can compare the force at which fracture occurs and the crack propagation rate for the simulation as well as for the real experiment.

Magnesium in a Car

Magnesium has all the potentialities to reduce the weight of the car as it is one third times lighter than Al and three quarters times lighter than steel or cast iron. In addition the components made out of Mg can be easily blended to required shapes and sizes with high surface finish due to very good manufacturability and machinability. Moreover Mg alloys when in pure form have high corrosion resistance and when casted without pores leads to high impact strength. On the other hand, it has lower ultimate tensile strength, lower fatigue and creep strengths, which can be compensated using a distinctive or application specific design with additional features like ribs, beads, and supports. Also Mg exhibits poor performance at high temperatures and hence cannot be employed to withstand thermal loads. Solidification time is a very key aspect of casting process and Mg has a low solidification time compared to steel or Al and hence the productivity is higher.

Even though the cost of Mg as a raw material is almost twice than that of the Al, but due to its low density, the actual cost of the component is nearly same as that of the component having the equal volume made out of Al. When good manufacturability of Mg is taken in to consideration, the final cost of the part produced from Mg may be even less than that manufactured out of conventional steel or Al. However due to its poor mechanical properties and poor performance at high temperatures its application in the automotive domain is limited. The figure below depicts the current and future application of Mg as a raw material for automotive components.