23ATZ worldwide 7-8/2003 Volume 105

MATERIALSTitanium

By Hartwig Naploszek,

Jörg-Peter Tepel and

Mark Krause

Aluminium-Strangpressteile

mit geschlitzten Elastomerlagern

in der Radaufhängung

You will find the figures mentioned in this article in the German issue of ATZ 7-8/2003 beginning on page 707.

Extruded AluminiumProfiles with Slit Elastomer Bearings in the Wheel Suspension

There are new prospects for the use of extruded aluminium forstructurally loaded components such as in the wheel suspen-sion or for stabilizing. Connecting links made from chamber-extruded wrought aluminium alloy for instance show superiorproperties when compared to the widely used steel designswith ball joints. The potential of this wrought alloy in combina-tion with high-performance slit elastomer/steel bearings isexplored by the companies Vorwerk autotec and B&T Exact onthe basis of the connecting link module in the PorscheCayenne’s front axle.

24 ATZ worldwide 7-8/2003 Volume 105

MATERIALS Alumimium

1 Introduction

Lightweight design and corrosion resis-tance play a major role in achieving gooddriveability and a high level of robustnessin automotive manufacturing. All the com-ponents of an SUV, including suspensionparts such as connecting links, are meticu-lously examined in order to find ways ofoptimising them.

As part of a vehicle’s suspension, theconnecting link connects the stabilizer (oranti-roll bar) with the wheel suspension.Anti-roll bars serve to reduce the tendencyof the chassis to roll, thus considerably in-creasing safe driving capabilities. As longas the suspension is activated equilaterally,the anti-roll bar will rotate in its supportingbearings without effect. In order to ensureboth the immediate response of the stabi-lizer and an optimum initial spring behav-iour of the primary suspension, all bearingsof the stabilizer module must have a highlevel of radial rigidity as well as low tor-sional spring rates. What is more, the vehi-cle’s tendency to oversteer or understeer isdependent on the proportion of stabilizingof the front and rear axles. The connectinglink is a decisive part of the stabilizingprocess and directly influences the vehicle’sself-steering effect. Moreover, the moduleconsisting of the connecting link and theintegrated bearing elements influences thestabilizer’s noise emission, thus also inter-acting with the sensitive area of drivingcomfort.

This is even more true for the increas-ingly popular Sport Utility Vehicles (SUVs)because in this case the connecting linkswithin the front axle suspension are sub-jected to a much higher level of stress thanmight be expected from their installationposition. This is due to an SUV‘s typicallylong suspension travel, as well as a largewading depth and the permitted high de-gree of articulation of the front and rearaxles. As a consequence, the wheel suspen-sion and stablizer are very much exposed.Since vehicle models such as the PorscheCayenne are fully designed for off-road dri-ving, the front axle design has to allow forhostile operating situations such as intensestone and gravel impact, as well as wetnessand a good deal of mud. Therefore, all ma-terials have to withstand an above-averagerisk of mechanical damage (chipping) plusthe possibility of subsequent corrosion. As aconsequence of the harsh operating situa-tion, a connecting link made of wroughtaluminium alloy in combination with slitelastomer/metal bearings is now beingused. Series production of the module be-gan in Summer 2002.

2 Demands on the ConnectingLink

During driving, the connecting link is sub-jected to tensile loads and compressiveforces. The optimum behaviour of the mod-ule covers a wide range between a high lev-el of radial rigidity, enabling the precisetransmission of forces between the wheelsuspension and the stabilizer, and the tor-sional softness of the bearing. The module’srigidity in the longitudinal axis is providedby the connecting link itself and by the pre-stressed elastomer of the high-performancebearings, in a magnitude appropriate to thematerial. The torsional softness of the bear-ing is a property of the bearing design, withits characteristic slit back.

2.1 The Current State of the Art in the VehicleAt present, connecting links in passengercars typically consist of welded steel de-signs. The bearing is usually a ball joint.While this design does indeed have goodrigidity for tensile and compression stress-es, it is subjected to a particularly high levelof wear and corrosion. A certain amount ofbearing shake that develops over time canbecome a source of noise that is noticeableto the driver. The widespread use of balljoints in connecting links has reasonswhich lie in the history of technology. Butthe introduction of elastomer bearings ofthe slit design type with their inherent pos-itive properties now means that the heav-ier and more costly ball joints can often bereplaced by them.

3 Process-Related MaterialProperties of the ConnectingLink

The connecting link module consists of analuminium connecting link and two high-performance elastomer/metal bearings,Figure 1. The connecting links are made of araw material called EN AW 6082 [AlSi 1MgMn] (F28). This wrought aluminium al-loy of the 6000 series is one of the alloysthat can be hardened, Table 1. Its actual me-

chanical properties are dictated to a largeextent by the treatment during componentmanufacturing. The process of chamber ex-trusion plus quenching and elevated tem-perature age hardening leads to highly de-sirable product properties.

3.1 The Metal Forming ProcessIn its initial state as a soft raw material, thewrought alloy has no guaranteed mechani-cal properties. Its maximum stability re-sults only from the manufacturing process.For hot extrusion, the alloy is heated up toabout 520 °C and the metal, which is nowmalleable, is transported to the extrusiondie under high pressure. Within the die, theinitial billet is divided into several billetsthat flow between the die (exterior con-tour) and the core (interior contour) to be fi-nally re-united in the shape of the extrudedprofile.

3.2 The Heat TreatmentProcessThe high temperature just below the melt-ing point threshold also causes solution an-nealing. During this process, the inter-metallic Mg2Si links are dissolved, and thealloy becomes a homogeneous solution.The extruded profile from the treated mate-rial is abruptly quenched by a water jet im-mediately after leaving the extrusion die.This quenching process takes place soquickly as to ensure that the intermetallicMg2Si links, which are supersaturated atambient temperature, are separated duringcool-down. The result is a granular extrud-ed profile that already has 70 % of the finalgeometry. It has a high tensile strength, ahigh tenacity and a high resistance to inter-metallic corrosion. After quenching, thehardened profile undergoes additional agehardening at a lower temperature (to theT6 state), which causes the formation ofgrains that are oriented differently fromthe aluminium matrix (mixed-crystal for-mation). The manganese content in partic-ular causes a fine grain to form.

Manufacturing a comparable materialstructure using aluminium forging wouldrequire completely different materials and

Legierungs-

bestandteil Si Fe Cu Mn Mg Cr Zn

Anteil in % 0,85 –1,15 0,40 0,10 0,10 0,40 – 0,60 0,05 0,10 – 0,70

Table 1: Alloy ingredients of the material EN AW 6082, mixture in percent of weightof series 6000 aluminium alloys (excerpt)

3 Process-Related Material Properties of the Connecting Link

25ATZ worldwide 7-8/2003 Volume 105

matrix of elastomer/metal bearings. In or-der to fully exploit the potential of this ex-traordinarily long-lasting and robust mate-rial, an essential material property has tobe considered in the bearing design: elas-tomers can withstand shear loads and com-pression stresses but their macromolecularstructure is suceptible to tensile stresses. Ahigh fatigue strength under high loads willtherefore only be achieved if the deforma-tion of the bearing during operation takesplace within the elastomer’s range of resid-ual compressive stresses.

For that purpose, in a SUV, the metallicback of the high-performance bearing isslit. Due to this design, the metal that ad-heres to the elastomer can easily travelwith the comparatively higher shrinkingmovement of the elastomer during thecooling down process that follows vulcani-sation. Without any calibration of the bear-ing, the manufacturing process results in acomponent with a stress-neutral elastomerlayer. Bearings calibrated in the conven-tional way, on the other hand, show unde-sirable tension in the contact area betweenthe metal and the primer caused by defor-mation during calibration. During installa-tion of the bearing into the receptacle of theconnecting link, the slit is closed to form abutt joint, thus giving the bearing an even,predetermined stress. Large loading areasand small relief areas at the front faces ofthe strongly pre-stressed elastomer ac-count for the radial rigidity of the elas-tomer/metal bearing as well as its torsion-al softness.

5 Conclusions

Connecting link modules made fromwrought aluminium alloy and elas-tomer/metal bearings are particularly wellsuited to the manufacturing process, thusresulting in the desired material propertiesof a lightweight, homogenous componentand the high-performance elastomer/met-al bearings. The expertise of two specializedsuppliers has led to the design of a modulethat exhibits the requested fatigue strengthfor SUVs and a particularly long lifetime.The combination of material-related andprocess-related advantages can be exploit-ed in other application areas, such as wheelsuspension modules.

Reference

[1] Kammer, Catrin: Kap. 5.1.6.2., S. 171 ff. In:Aluminium-Taschenbuch. Band 1, Grundlagenund Werkstoffe, 16. Aufl., 3 Bände, Alumini-um-Verlag, Düsseldorf, 2002, ISBN 3-87017-274-6

MATERIALSTitanium

a considerable amount of handling, be-cause it would be difficult to ensure thecritical quenching speed when handling abiscuit. Compared to the extruded profile,forging results rather in a better grain ori-entation (grain texture or grain flow) thana fine grain.

3.3 Expansion to Finish SizeVery precise temperature managementwithin the extrusion process ensures thatthe longitudinal welds have a consistentquality. As an additional element of qualityassurance, the next process step of diepressing and strain hardening to finish sizeincludes an additional degree of stretchforming. For that purpose, the centre-to-centre distance between the two bearingreceptacles is shorter than in the finishedproduct. It is only in the die press that theconnecting link is stretch formed to finishsize and the bearing receptacles are ex-panded. As a result, the reforming processis also a 100 % test of quality and strength.

3.4 Influence of Strain HardeningStrain hardening the connecting link to fin-ish size in the die, which is in fact a form ofsurface pressing (cold pressing), results in acold work hardened component with a pres-sure resistance that is 40 % higher than be-fore, Table 2. The reason for the improvedpressure resistance, Figure 2, is the in-creased area of densification caused by thecold working. This densified structure canbear higher stresses. Research has shownthat cold work hardening can, in principle,even increase durability by up to 100 % [1].

3.5 Robustness and Ductility ofthe Wrought Aluminium AlloyThe high degree of densification that thecold work hardening produces near the sur-face of the connecting link in particular re-duces the influence of stone or gravel im-pact and salt on the component. In this con-text, it is advantageous that the finish sizeof the connecting link is achieved withoutmachining, as the complete surface main-

tains maximum protection against ambi-ent conditions. Consequently, connectinglinks made of wrought aluminium alloy donot require (electrodeposited) coatings as aprotection against corrosion, which there-fore facilitates recycling. Another advan-tage of the aluminium wrought alloy is itsductility. The high degree of elongation atfracture permits a long, predetermined de-formation in the case of a crash, withoutthe risk of rupture.

4 The Role of theElastomer/Metal Bearings

The bearings between the connecting linkand the telescopic leg and between the con-necting link and anti-roll bar have to meetstringent technical requirements. In addi-tion to shocks and impacts of varying fre-quency and intensity as well as torsionalstresses, it is mud and dirt build-up, includ-ing the presence of abrasive particles andcorrosive agents, that are the main factors.In order to ensure a maximum lifetime ofthe connecting link despite these adverseoperating conditions, the bearing has tohave specific properties and a high level ofperformance reserve.

High-performance elastomer bearingsof the slit design type have been developedby Vorwerk autotec since the end of the1980s, Figure 3 and Figure 4. They have typ-ical characteristics such as the highly pre-stressed elastomer in the installed state,which leads to superior performance re-serves. At the same time, the wide rangebetween radial rigidity and torsional soft-ness ensures a direct response of the stabi-lizer and thus a high level of driving com-fort. The slit elastomer/metal bearings fur-ther support the development goal of re-ducing the ancillary spring rates in particu-lar in air-suspended multi-link axles.

4.1 Material Properties of the Elastomer and DesignConclusionsIn contrast to ball joints, movements of thebearing are absorbed within the elastomer

3.4 Influence of Strain Hardening

prior to after unit

strain-hardening strain-hardening

Rm about 340 about 370 N/mm2

Rp0,2 about 320 about 360 N/mm2

A5 about 12 about 7,5 %

Table 2: Strength values of the aluminium wrought alloy for the connecting link prior to and following the surface pressing in the die cast