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Light Weight Materials in Automotive Industry

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Introduction

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 (Cole) . 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 (Hirsch). 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 (Hirsch). Hence reduction of the weight of the vehicle plays an important role in reducing the fuel consumption and the emissions (Hirsch). 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 (Henriksson).

Most importantly the several performance parameters like the dynamics of vehicle, operational strength, crashworthiness etc., should be satisfactory (Cole). 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 (Moroncini). 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. (Duddeck). In this paper firstly, the weight reduction of a car though the usage of lightweight materials like Al, Mg, composites etc., will be discussed and later the application of FEM based Multi-Disciplinary Optimization technique to minimize the weight, keeping the functional targets in check will be detailed.

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Light Weight Materials in Automotive Industry:

1. 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 (Cole).

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 behaviour 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 (Hirsch). The typical chassis components like wheels, suspension, braking system, fuel system etc., are produced out of casted Al (Cole). 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 (Cole).

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) (Hirsch). The figure below shows the components in chassis made out of Al 5000 series (Hirsch). 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 (Cole). 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 (Cole). 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 (Hirsch).

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 (Hirsch).

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 (Hirsch). BIW mainly comprises the sole structure and the hanging parts like Door, Hood, Fender etc attached to the structure through bolts (Cole). 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 (Cole). 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 (Hirsch).

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 (Hirsch).

Manufacturing processes:

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, 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 (Hirsch).

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 (Hirsch).

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 (Cole).

Composites:

Composites as the name suggests is a combination of multiple materials to achieve enhanced properties in order to suit wide range of applications. The composites are basically of two types

: natural composites; engineered (metal matrix) composites. One of the widely employed natural composites is the hypereutectic silicon in Aluminum matrix, where large amount of silicon content is introduced to achieve higher strength and resistance to wear. The entire process of producing this composite is naturally controlled though the parameters like composition of the alloy, nucleation and the solidification environment. On the other hand in the metal matrix composites the nonmetallic particles and/or the fibers are added in required quantities to the metal, through a well-controlled engineering process. The main advantage of such composites is that they have a very good performance characteristics at elevated temperatures, which is not the case with sole Mg alloys. As already mentioned cost plays a key role in employing a material and research is going on to bring down the cost of the composites. Now the metal matrix composites are available at a reasonable cost so that they can be employed where high strength and hardness are required (Cole).

Advanced Multi-Material ‘‘MM’’ Design Concepts

Each and every material has its own pros and cons and is suitable for certain application under specific environment of operation. In order to weigh the advantages of Aluminum in car manufacturing against the different materials like enhanced steels, magnesium, and composites, the auto makers in the Europe have designed a multi-material concept. The main goal of this multi- material concept model is to find a best material to suit a specific requirement, taking in to account the different aspects like the raw material, fabrication, and recycling costs, performance, compliance with the joining methods, environmental and operational safety. A European Union funded project namely “Super Light Car (SLC)” employed this multi material concept to build a body in white of VW Golf V car and evaluated different materials used based on the above mentioned aspects including the life cycle analysis (LCA). After several iterations to finalize the concept, a prototype was built using upgraded Aluminum sheets and best forming technologies and tested. The results are quite promising as the weight of BIW can be reduced by 34% along with the cost savings and satisfactory crash resistance using joining methods favorable to series production. Considering the several aspects involved with the material replacement, Aluminum was chosen for several parts of SLC.

Impact on Design process due to material replacement:

As already mentioned the BIW of a car made out of steel accounts for nearly half of the total weight and hence when the steel is replaced by a light weight material like Aluminum, the total weight of the car can be effectively reduced. There are basically two types of projects involved with material replacement namely; a mere substitution project or a completely new product development (NPD) project. In case of a substitution project, materials of a very few components in the vehicle are changed and hence a less degree of freedom is available. Whereas with the New Development Project the materials of almost all the components are freely chosen during the design phase itself. Till now when replacing the material of a component only the factors like material properties, cost, manufacturing processes are considered but it is equally important to take into account the impact on the design process, especially in case of substitution projects.

Any product development project comprises the activities like: gathering the needs, formulating the problem statement, create an abstract of the tasks to be performed, analyze the feasibility and finally start implementing.

Breadth-first vs. depth-first analysis

There are several possibilities or concepts available to solve a particular problem but care should be taken while choosing one among them taking into consideration different aspects like quality, cost and time. This can be viewed as an algorithm to divide the problem in to several levels of hierarchy and find a best solution. Based on the way a solution to the problem is obtained, there are two different approaches employed namely breadth-first analysis, and depth-first analysis. In breadth- first analysis, as the name suggests, all the concepts available to solve a problem at a certain level are evaluated first, before going to the next level. In contrast to this, in depth- first analysis, all the concepts at different hierarchical levels are independently explored and developed in detail and then the best path to find the solution is determined.

A case study was conducted in order to investigate the difficulties faced during the design and production phases of a product due to the changing of the material. The case study mainly deals with a material substitution project where, aluminum is going to replace the steel in the body side of a Volvo XC90 MY2015. Since it is a substitution project, the boundary conditions are tight in order to not disturb the adjacent parts. In addition, different aspects like geometry, material and the fabrication process should be taken into consideration.

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