Titanium You will find the figures mentioned in this article in the German issue of ATZ 2/2005 beginningMATERIALS on page 126. Leichtbau mit Eisenwerkstoffen – Optimierte Gusswerkstoffe für Fahrwerkskomponenten

Lightweight Design Using Iron Perfect Cast Iron Materials for Chassis Components

By Werner Menk

ATZ worldwide 2/2005 Volume 107

A number of metallic materials currently compete for use in chassis components. However, lightweight design is not necessarily synonymous with the use of light alloys. Cast iron too can meet the requirements, provided that suitable materials and design measures are selected. In this article, Georg Fischer Fahrzeugtechnik AG presents its cast iron material Sibodur and shows its advantages over other materials.

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MATERIALS

Iron

1 Introduction

Many associate the term “lightweight design” with “light alloys”, as these words suggest low weight. Consequently, it is a much loved slogan used to denote modernity and progressiveness, based on the considerably lower density of aluminium and magnesium, compared with cast iron. Accordingly, for the past few years, light alloys have been used increasingly in the automotive industry. A study carried out by management consultants Mercer Management Consulting predicts that the proportion of aluminium used will increase further by 2010, from the present 95 to 122 kg. However, modern lightweight design does not necessarily mean the use of light alloys. Cast iron too can meet the requirements called for by the demand for lightweight construction, using suitable materials and designs. Georg Fischer’s (GF Automotive) recent developments in the automotive sector demonstrate this. With the cast iron material Sibodur (trade name SiboDur), the experts at Georg Fischer Fahrzeugtechnik have succeeded in adding ductile cast iron to the range of cast iron materials, thereby offering promising solutions required for high-performance chassis components. Due to the material’s characteristics, components made from Sibodur can easily be adapted to meet the functions required. 2 Materials Used for Passenger Car Chassis

Georg Fischer evaluated materials predominantly used in the production of vehicle chassis, testing 48 models from different manufacturers and classes. At the outset of the evaluation, 48 % of all chassis components were made using steel, 28 % from ductile cast iron and 22 % from aluminium. Within two years, the percentage of steel used in the front suspension was reduced, while that of ductile cast iron increased and that of aluminium remained almost constant. Ductile cast iron is mainly used for steering knuckles, wheel carriers and brake components. Aluminium is predominant in the production of steering gear boxes and has taken a foothold in the manufacture of suspension sub-frames, suspension arms and steering knuckles. During the same period, aluminium castings have been increasingly used in preference to steel for almost all components for rear suspensions. The percentage of ductile cast iron components, however, remained almost constant. These are used mainly for brake and hub carrier components. Nevertheless, there is no question that material density is only one factor. Other

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important material characteristics are, for example, dynamic strength, stiffness, limit of elasticity and modulus of elasticity. As far as these criteria are concerned, ductile cast iron has a distinct advantage over aluminium castings. Because many chassis components are subjected to extremely high stresses, they have to be designed with much higher dimensions when made from aluminium rather than from ductile cast iron. If comparable characteristics are called for, more material is often required when using aluminium, thereby increasing costs and possibly creating space problems during assembly. Therefore, the choice of material used in the manufacture of a component is increasingly difficult to make and encompasses not only purely component characteristics but also production parameters, logistics expenditure and even, later on, service costs. Taking these peripheral requirements into consideration, material density is only one aspect in the selection of a material. 3 Sibodur – a Promising Cast Iron Material

GF Automotive are specialists in both cast iron and aluminium casting, and being experts in both materials they are in a position to offer customers the optimum material solution for each project. Recently, a new, promising material, Sibodur, has become available, which Georg Fischer has developed as a follow-on to conventional ductile cast iron, with the same modulus of elasticity but improved fracture elongation, tensile strength and dynamic strength characteristics. Its name is derived from the addition of silicon and boron and from the word ‘durability’. Sibodur is particularly suitable for chassis components in passenger cars. High dynamic loads make high demands particularly in terms of good dynamic strength and high stiffness because, increasingly, socalled long-arm steering knuckles are being used. A high plastic deformation in relation to strength is also important. The limit of elasticity too should be as high as possible, so that the wheel suspension absorbs as much energy as possible if, for example, the car hits a kerb. The components should visibly distort so that the driver is able to see the damage, Figure 1, but they must not show signs of tearing that go unnoticed, Figure 2, or even break. The new material is based on the idea of improving both strength and fracture elongation, which are, in principle, opposed. The basic material is conventional cast iron with spheroidal graphite, e.g. GJS-400-15. By adding a small amount of silicon and

boron and other elements, the basic strength is increased, although good plastical deformation characteristics are maintained. This means that the dynamic strength of Sibodur 460-17 is increased by 25 % compared with conventional ductile cast iron GJS-400-15 and has a considerably higher, guaranteed proof stress, Figure 3, at 320 against 250 MPa. Therefore, Sibodur can absorb considerably higher static and dynamic loads before a component manufactured from this material undergoes plastic deformation. The higher, guaranteed fracture elongation is not least the result of an improved structural homogeneity, because the structural characteristics of Sibodur components are significantly more uniform, even with different wall thicknesses, than those of components made from GJS-400-15. Strongly fluctuating elongation values of between 15 and 22 % may be expected with the latter material depending on the local structural formation. Due to its homogeneous structural formation, Sibodur 460-17 has far fewer variations. Its fracture elongation value is on average above 20 %, Figure 4. Although Sibodur is somewhat more difficult to cast due to the higher silicon content, which increases the melt viscosity and requires a more elaborate gating and feeder system, Georg Fischer has developed a fail-safe casting process requiring marginally more effort. Overall, it should not involve the customer in additional costs, because Sibodur’s improved material characteristics result in slimmer components and, therefore, a reduction in the material required. 4 Material and Component Characteristics

Sibodur not only shows advantages due to its improved material characteristics, but its characteristics may also be adapted, within certain limits, to meet the demands made on each component. Such a modification is made during the obligatory cleaning process of the component surface after removal from the green sand mould. The surface treatment affects the component characteristics and may increase dynamic strength and reduce plasticity. For instance, during the cleaning process of the cast connecting rods, as much energy as possible is applied to the surface to increase the dynamic strength. An increase in dynamic strength can also be achieved with this effect when using Sibodur, Figure 5. Georg Fischer has analysed and optimised the surface treatment so that the opposite effect may be achieved; with a reduced dynamic strength, component plasticity increases.

ATZ worldwide 2/2005 Volume 107

Titanium

5 High-Tensile Sibodur

Using both variations in surface treatment, Georg Fischer changes the characteristics of the materials Sibodur 440-17 or 460-17. One possibility is a high-tensile material that has the same plasticity as conventional ductile cast iron, but which has a higher proof stress, Table. Form proof stress in particular, which is determined using the spheroidal form test, is very high at 670 MPa. The high-tensile variant of the material Sibodur is predominantly used in the manufacture of steering knuckles and may reduce their mass considerably compared with conventional ductile cast iron, Figure 6. However, compared with an aluminium knuckle, there is still a difference of more than one kilogram. This comparison is based on a knuckle with a bolted-on brake anchor bracket and the latest, thirdgeneration wheel bearing, a flange bearing, Figure 7. Third-generation wheel bearings are impressive due to their clearance tolerances and long working life, although they are more expensive and heavier than previous solutions. Because vehicle manufacturers increasingly demand wheel bearings with life-long durability, second- or third-generation compact bearings in combination with aluminium steering knuckles are increasingly used. Traditional cast-iron steering knuckles with first-generation compact wheel bearings have roughly the same weight as aluminium knuckles with third-generation compact bearings, Figure 8. During the pressing-in process of first-generation bearings, radial forces are not distributed homogenously, and this will lead to non-uniform stresses in the bearing, thereby risking a reduction in its durability. In contrast, the solution using first-generation compact bearings is advantageous because these wheel bearings are very cheap. However, the knuckles must generally be made from ductile cast iron, because aluminium does not have the necessary strength and the force-fit may loosen due to variable thermal expansion. To avoid relatively expensive third-generation compact bearings, Sibodur offers a solution. At present, Georg Fischer, together with well-known bearing manufacturers, is developing solutions that seek to increase precision and, thereby, the working life of first-generation compact bearings by using lower pressing-in forces. Because a steering knuckle made from Sibodur reduces the overall weight of the entire knuckle module, it is even lighter than a comparative module consisting of an alu-

ATZ worldwide 2/2005 Volume 107

minium knuckle with a bolted-on brake anchor bracket and a third-generation compact bearing. 6 Highly Deformable Sibodur

The second material variant consists of Sibodur 440-17 with improved plasticity. With a form proof stress of 530 MPa, it reacts with plastic deformation even at low forces, i.e. earlier than its high-tensile counterpart, but otherwise offers almost identical material characteristics. In comparison with conventional ductile cast iron, this material is suitable for use in all chassis components that are required to have a particularly high energy absorption capability – such as, for example, the transverse control arm – due to its ideal combination of very high plasticity and good physical properties. Transverse control arms must show neither tearing nor breaks when subjected to a defined side impact to the wheel, but can show clear signs of deformation. Georg Fischer already uses transverse control arms made from Sibodur in a chassis produced by a German car manufacturer and used as a platform for different models. So far, only few competitors use transverse control arms produced from cast iron in comparable vehicles. Single or double-layer sheet metal constructions still predominate, and that also applies to the basic engine versions used by the German manufacturer. For higher-performance models with increased axle loads, however, a cast construction using Sibodur is fitted. Because the cast construction should have at least the same characteristics as the sheet metal construction, particularly high requirements have to be met as far as the deformation characteristics of the transverse control arms are concerned. Particularly challenging for Georg Fischer was having to take into account the dynamic processes taking place in the transverse control arm during a crash test, as, so far, mainly static requirements had to be met in respect of cast components. For example, the kerb impact test is defined at an impact speed of 25 km/h and an impact angle of the wheel of 35°. Georg Fischer has designed the transverse control arms made from Sibodur in such a way that they exceed the characteristics of conventional sheet steel control arms and definitely meet all the customer’s requirements. The decision to choose cast components may also have been facilitated by another fact: for other platforms too the manufacturer also trusts cast transverse control arms.

MATERIALS

7 The Future

In addition to the above-mentioned transverse control arms made from Sibodur 44017 for a German car manufacturer, Georg Fischer also supplies other chassis components, such as a steering knuckle and a long-arm steering knuckle – both made from Sibodur 460-17 – to a French and an American company. The new material has been well received by the automotive sector, which is demonstrated by a series of new development and series production projects by well-known car manufacturers. What is notable is the fact that manufacturers who have hitherto mainly used aluminium for chassis components are also interested. It seems that they too come to the conclusion that aluminium is not always the best material to use, but that the choice of material is also influenced by component weight and cost. Therefore, general comparisons between materials are very difficult to make and often only meaningful for a specific component. Because customers make ever-increasing demands regarding the strength of chassis components, it may be expected that Sibodur will meet the requirements of a wide range of applications. This is why intensive research into optimised variants of higher-strength cast iron materials GJS500 and GJS-600, among others, is being carried out. The use of surface treatment may provide two solutions: firstly, an improved GJS-600 with a strength improved by 16 % and somewhat more elongation; secondly, a material in which the present fracture elongation (approximately 3 to 5 %) of GJS-600 is more or less doubled (7 to 10 %). If successful, Sibodur may be of interest in the manufacture of high-strength and highly deformable hubs and suspensions for commercial vehicles. Further use of the material Sibodur could be made based on the cast iron material GJS-500-7, which is used for differential housings, because the Sibodur variant would have a markedly more homogeneous structure in respect of the different wall thicknesses of the housing. Generally, no specific trend is emerging as far as materials used for chassis components are concerned. It is clear, however, that magnesium, because of its tendency to corrode using present technology, will not be in demand for the production of chassis components for the next few years. Nevertheless, aluminium and cast iron components as well as sheet metal and forged parts will still be used as substitutes. With the development of Sibodur, Georg Fischer puts forward convincing reasons for using cast iron. ■

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