aluminium in innovative light-weight car design

7/28/2019 Aluminium in Innovative Light-weight Car Design

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Hydro Deutschland GmbHSpeaker: Prof. Dr.-Ing. Jürgen Hirsch

Lightweight Vehicle Structure Conference 101

Aluminium in innovative light-weightcar design

Hydro Deutschland GmbH

Prof. Dr.-Ing. Jürgen HirschHydro Deutschland GmbH, R&D, Bonn / Germany

Speaker

Daniele BassanCentre Richerche Fiat S.C.p.A, Turin / Italy

Chris LahayeCorus Technology B.V., Ijmuiden / Netherlands

Dr.-Ing. Martin GoedeVolkswagen AG, Group research, Wolfsburg / Germany





Illustration 1 shows a selection of Aluminium parts used in modern light-weight cars:

Illustration 1: Aluminium products for advanced automotive applications

The average total aluminium content per car for European cars was 132kg in 2005 /9/. It

has been analysed systematically as:

• Power-train (Engine, Fuel system, Liquid lines):

69 kg (25 components analysed) in engine block & cylinder head, transmission housingsand radiators.

• Chassis & Suspension (cradle, axle):37 kg (17 components analysed) in wheels, suspension arms and steering systems.

• Car Body (Body-In-White "BIW", hoods, doors, wings, bumpers and interiors):26 kg (20 components analysed) in bonnets & doors, front structure and bumper

beams.

This shows that for the body the most potential exists. Seen as one component the BIW isthe heaviest part of a conventional car with a share between 25 and 30% of the completecar's weight, depending mainly on options installed, engine size, and integrated safety fea-tures.





For the new concept of "Multi-material designs" (for high volume cars) is an alternativeto the all-aluminium designs of BIW's mentioned above. It consists of applications of alu-minium together with high and ultra-high strength steels, magnesium and plastics or com-posites, where applicable. The principle idea is to use the "best" material for theappropriate functions. The additional goal is to achieve an overall cost efficient light-weight

design. here state-of-the-art in this area is BMW 5 - E60 (Illustration 4) which uses 20% asdeep drawing steels, 42% as higher strength steels, 20% as highest strength steels and18% Aluminum alloys. The front-end substructure consists of 16,4 kg steel and 29,4 kg in86 Aluminum parts (as stamped sheet, extrusions, high-pressure die castings, and hydrofor-med tubes).

Illustration 4: State-of-the-art Body in White multimaterial concept

3 Wrought aluminium alloys for automotive applica-tions

In the multi-material SLC design the contribution of the aluminium concern new alloys aswell as alternative production methods for aluminium parts. Aluminium sheet is predomi-nantly used for BIW panels and closures. Despite the existing "all aluminium vehicles" like

Audi A8 and Jaguar XJ, aluminium in mass produced vehicles needs to reduce developmenttime and other additional costs in new production methods and/or new alloys.

The main Aluminium alloy classes for automotive sheet application are the non-heat treat-able Al-Mg (EN 5xxx series) and the heat treatable Al-Mg-Si (EN 6xxx series) alloy system,some especially tailored by variations in chemical composition and processing, e.g. Al-Mgalloys optimized for strength and corrosion resistance for use in chassis or Al-Mg-Si alloysapplied for autobody sheets have been improved for formability, surface appearance and

age hardening response. The specific properties and principal differences are illustrated inIllustration 5. The effects of varying alloy additions and process parameters as described in

/4/ are well developed and controlled for enhanced performance and efficient manufactur-ing.




Lightweight Vehicle Structure Conference106

3.1 Age hardening Al-Mg-Si alloys

6xxx series alloys contain magnesium and silicon. Current 6xxx alloys used for autobodysheets are AA6016 (Europe) and AA6111 (Amerika) and, more recently, AA6181A wasadded for recycling aspects. In the US, AA6111 is often used for outer panels in gauges of

0.9-1.0 mm which combines high strength with good formability. In Europe, EN-6016 ispreferred and applied in gauges of around 1-1.2 mm. It shows a superior formability andfiliform corrosion resistance and allows flat hems even on parts with local pre-deformation.

However, the bake-hardened strength of 6016 is significantly lower than that of AA-6111 /6/.

Illustration 5: EN-AW 5xxx and 6xxx alloys competing for car body sheets

In recent years alloy and processing modifications have been introduced to meet the in-creased requirements /5/. Higher strength alloys may allow outer panel thickness reductionswith no loss of dent resistance, provided stiffness requirements are met. As paint-bake tem-peratures decrease, there is increasing demand for a significantly higher age hardening re-sponse. However, for some parts formability remains the major difficulty. Therefore specialalloy modifications with either improved formability or strength have recently been devel-

oped by European Aluminium sheet manufacturers and agreed upon as standards by theautomotive industry.





3.2 Non heat-treatable Al-Mg-Mn alloys

Al-Mg-Mn alloys show an optimum combination of formability and strength achieved bythe mechanism of solid solution and deformation hardening due to their specific high strainhardening. Further improvement in properties required for specific applications (e.g. surface

appearance, corrosion resistance, thermal stability) have been achieved by small additionsof other alloy elements and/or modified processing routes /4,7-9/, e.g. stretcher strain free("SSF") sheet, avoiding Lüders-lines /10/

Non heat-treatable Al-Mg-Mn alloys are applied in Europe for automobile parts in largerquantities as hot and cold rolled sheet and hydroformed tubes, due to their good formabil-

ity which can always be regained during complex forming operations by inter-annealingwhere quenching is needed for age hardening. In chassis parts or wheel applications thebenefit is twofold since the weight reduction in the unsprung mass of moving parts addi-tionally enhances driving comfort and reduces noise levels.

A well established high Mg containing alloy, AlMg5Mn (AA5182), is used for high strengthand complex stampings. For 5xxx alloys containing > 3% Mg the precipitation of ß -Mg5Al8 particles at grain boundaries can result in susceptibility to intergranular corrosioncracking ("ICC") by long term exposure at > 80°C. For these conditions special high Mg al-loys have been developed with a good compromise for sufficient strength and ICC resist-

ance.For all other cases special high Mg alloys (>6%Mg) have been introduced which show highstrength and strain hardening, thus also enhancing formability. Al-Mg-Mn alloy sheet hasalso been successfully applied or is currently being tested in many parts for structural sup-

port, pedal boxes, heat reflectors, lever arms e.t.c...Al-Mn EN-AW 3xxx alloys are applied for heat-exchangers which is another success story of

Aluminium sheet and extrusion applications that started in Europe many years ago. It is anincreasing market with intensive R&D, established for advanced light-weight technology forradiators and air conditioning systems in cars (and elsewhere) world wide.

3.3 New aluminium alloys for automotive applications

Several new product developments were introduced in the SLC project to meet specific de-mands of the BIW that cannot be met by the present aluminium alloys. E.g. a high Mg 5xxx

alloy specially dedicated to warm forming /12/. New 6xxx alloys and 7xxx alloys for structuralapplications were introduced /11/, such as 'Crash alloy' is used for the crash members inthe front structure of the SLC model or a 'Roof alloy' with special attention when placedon steel structure. Here a 6xxx alloy (6013 type) has been introduced /11/ with fast paintbake response to withstand the thermally induced plastic deformation.The final SLC structure has a magnesium roof where the finished roof is mounted after theBIW has passed through the paint line. High strength 7xxx alloys applied in aerospace havebeen tested in the SLC study to determine their potential for weight saving replacing steelin the Golf V B-pillar for a side impact simulation. The alloy selected was a 7081 type alloy

with yield strength around 600 MPa in an artificially aged temper. The result of the impactsimulation shows that a 3.5mm thick 7081 matches the performance of 2mm thick boronsteel. More importantly the aluminium part is 2.4kg lighter, so achieving a weight saving ofaround 40% /11/.





Other aspects investigated in detail in the SLC project were:

• Heat Forming is a new technique for making complex aluminium tubular shapes usinginternal gas pressure to form hollow bodies or tubes within a warm environment /12/.It provides a competitive alternative to hydro- or superplastic- (SPF-) forming.

• Tailor Welded Blanks (TWB) is a mature product for steel automotive applications easyto apply to aluminium. There is only one example of aluminium TWBs in series produc-tion; the backplate of the front wheel house of the Lamborghini Gallardo /11. The SLC

project proved that aluminium TWBs can be applied for demanding deep drawn partsat higher volumes.

Figure 6 shows the geometry of the TWB and a final result of a pressed part of door inner.The aluminium Tailor Weld Blanks have been successfully stamped to produce inner doorpanels, using a two-step operation to obtain 140 mm deep drawing depth without breaksin laser weld seam. Geometrical accuracy of stamped panel was checked and found to be

acceptable for production use, showing that the process is technical feasible.

Illustration 6: Door inner produced from TWB

Laser brazing steel-aluminium

The next logical step after steel TWBs and aluminium TWBs is a combination steel-alumini-

um TWB. This however is more complicated because of the joining of steel to aluminium.Conventional fusion welding is gives poor quality joints due to the formation of brittle Fe-Al intermetallics. Besides the well-known technologies such as mechanical fastening andadhesive bonding, a recently developed technology called laser brazing shows good poten-tial for joining steel to aluminium /11/





4 Aluminium in the final SLC body concept

The final SLC-body concept (Illustration 9) chosen shows an optimum between weight re-duction of 95 kg (34%), i.e. a weight saving of 41% vs. reference (from 65 kg to 110 kg)and a additional part costs of 5 /kg. It has a Mg roof and a steel floor frame (i.e. lighter on

top than underneath) and torsion ring of the side structure in form-hardened high strengthsteel combined with an aluminium sheets frame (Illustration 7).

For the inner B-pillar TWB steel sheets are used with an external Aluminium skin. Aluminium

is used as sheet panels and as extrusion in front rail and for bumper and crash elements andin the rear underbody rail and wheelhouse structure as HPDC (high pressure die cast). Com-

pared to the good formable (conventional) steel grades Aluminium is less formable, so themanufacturability of such parts is not obvious. The procedure is to start with a Simulationand by mutual agreement with design and engineering departments, to adapt the designof a part so it can be produced (simulated) while it fulfils all requirements. This interactivedevelopment procedure can take some time, but in case of the SLC demonstrator it is suf-ficient to show the feasibility, not necessarily to develop a failure free simulation.

Illustration 7: Final SLC-body multi material concept

5 Aluminium sheet forming simulationAs an example here the side panel is shown based on present steel design. It is a very diffi-cult part for Aluminium because of steep deep drawing around sharp corners. Easiest - butunrealistic - Solutions for SLC are to split the panel into more sections or to drasticallychange the design by creating corners with larger radii. Here the Simulation strategy is toinvestigate different options to control material flow to lower the strain in critical areas, us-ing AutoForm incremental 4.1. Input material data are taken from the SLC SP2 database

generated, tool geometry is generated by the program based in the CAD files of the parts.Illustration 8 shows the initial Simulation. It is obvious that a straightforward pressing of the

part will not lead to an OK part. Indicated at the problem areas are the options to improvethe material flow.





Illustration 8: Initial simulation of SLC side panel in aluminium

To improve the formability of the side panel draw beads are included in the centre sectionsand the radius is adjusted to 10mm in the doors. Increasing the radius and the draw beads

are clearly an improvement. It is obvious that the new Simulation is safer (more green area).Some critical areas disappeared and some have become less critical. This can also be ob-served when comparing the FLD plots. The red area is clearly shifted from critical deep draw-

ing on the left side to the middle section. One option to go forward is to further optimisethe deep drawing process with simulation in order to reduce the red zones. However this

middle zone of an FLC is typically an area that is also strongly influenced by friction and typ-ically friction is one of the parameters that can easily be controlled in prototyping.

Illustration 9: Final simulation of SLC side panel in aluminium





Therefore this is the point where simulation shows that the side panel is feasible in Alumin-ium. The proposed layout of the deep drawing process behind the simulation from Illustra-tion 9 is taken as the starting point for the prototype toolmakers. The part being producednow is perfect. Thus simulations are used as a last - but necessary - step to describe condi-tions of the deep drawing process for the manufacturability of Aluminium parts. Aluminium

makes an important contribution to this multi-material concept due to the favourable com-bination of low density and low production costs, with Aluminium solutions easily applica-ble for high volume production.

6 Extrusions

Another wide field of Aluminium solutions and applications is opened by making use of thewell established technology of Aluminium extrusions. Here quite complex shapes of profilescan be achieved allowing innovative light weight design with integrated functions. In Eu-rope complete new and flexible car concepts (e.g. the aluminum space frame, Illustration3a) and complex sub-structures (e.g. in chassis parts, bumpers, crash elements, air bags,

e.t.c.) have been developed using Aluminium extrusions. Their high potential for complexdesign and functional integration is most suitable for cost-effective mass production.

Medium strength AA6xxx and high strength AA7xxx age hardening alloys are used, sincethe required quenching occurs during the extrusion process. Formability and final strengthis controlled by subsequent heating for age hardening. Extrusions are applied for bumperbeams and crash elements/boxes (Illustration 10), also in the SLC car.

Illustration 10: Aluminiumm extrusions in bumper beams and crash boxes [Hydro]





7 Castings

The highest volume of aluminium components in cars are castings, such as engine blocks,cylinder heads and special chassis parts. The substitution of cast iron engine blocks contin-ues. Even diesel engines, which continue to gain a substantial increase in market share in

Europe now, are being cast in Aluminium where, due to the high requirements on strengthand durability, cast iron has generally been used. However progress in Aluminium alloy de-velopment (Al-Si-Cu-Mg-Fe-type) and new casting techniques came up with improved ma-terial properties and functional integration that enables Aluminium to meet therequirements. Aluminium castings are also gaining acceptance in the construction of space

frame, axle parts and structural components. Complex parts are produced by special castingmethods that ensure optimal mechanical properties and allow enhanced functional integra-tion /5/. For high pressure die cast "HPDC" new AlSiMgMn alloys have been developed with

enhanced strength and ductility combination. In the SLC project structural parts in the

wheel house architecture have been designed using advanced Al die cast with an integratedstriker plate.

8 Summary and Conclusions

Due to its low weight, good formability and corrosion resistance, aluminum is the materialof choice for many automotive applications such as chassis, autobody and many structuralcomponents /13/. Aluminum alloys tailored by suitable variations in chemical compositionand processing best fit many requirements, like the non-heat treatable Al-Mg alloys used in

chassis optimized for superb resistance against intercrystalline corrosion and concurrenthigh strength or the heat treatable AlMgSi alloys for extrusions and autobody sheet modi-

fied for improved age hardening response during the automotive paint bake cycle.

With a sound knowledge about the specific material properties and effects excellent lightweight solutions for automotive applications have been successfully applied by the Europe-

an automobile industries. Intensive R&D and continuous collaboration of material suppliersand application engineers provided optimum solutions for sometimes contradicting aspectsof the specific requirements, e.g. for the specific material selection and optimum combina-

tions of strength and formability.

Material specific processing routes and individual solutions have been developed in closecooperation with OEM partners and suppliers. Applying the full knowledge about the phys-

ical processes involved and the microstructure / properties correlation a tuning of propertiesis possible in order to produce optimum and stable products required for the high demandsin automobile applications.

The examples given for the successful prove the major breakthrough in automotive appli-cations for Aluminium that has been achieved during recent years by developing innovativelight weight and cost efficient solutions. With the reference of the SLC project results it is

expected that in the near future the use of Aluminium with specifically improved propertieswill grow in many automobile applications meeting the increased economical and ecologi-cal demands.





Due to the positive experience gained in the project and from former successful applicationsits volume fraction used in cars of all classes and all sizes will grow significantly.

The SLC concept shows clearly that aluminium can be used for the car body structure andthat there can be a weight advantage of at least 30% without losing performance. For most

parts the present grades used for exterior panels can be applied. In some cases where veryhigh strengths are demanded, 7xxx series alloys can be used to maintain this significantweight advantage. For large volume aluminium solutions are most cost effective. Castingswill be applied for areas where strong part integration is feasible. Extrusions can be easily

applied as straight profiles, but also forming of an extruded profile is a competitive processfor high volumes, e.g. as bumper beams as used in the SLC prototype car.

Aluminium is the ideal light-weighting material as it allows a weight saving of up to 50%over competing materials in most applications without compromising safety.

9 Acknowledgement

The research activity presented in this paper has been performed within the European fund-

ed project SLC (Sustainable Production Technologies of Emission Reduced Light weight Carconcepts) Proposal/Contract no.: 516465 /8/ in the EU 6th Framework Programme which isgratefully acknowledged. The authors also thank the SLC consortium and the European

Aluminium Association EAA for their support. The support of Dr.Wieser, Mr.Brünger, Dr.Ju-pp, Dr.Brinkman is gratefully acknowledged.

10 Literature / references

[1] "Automobil-Produktion" Juni 2001, S.136

[2] "Aluminium materials technology for automobile construction", ed. by W.J.BartzMech.Eng.Publ. London (1993) p.1

[3] "Aluminiumwerkstofftechnik für den Automobilbau" Kontakt & Studium Werkstoffe,Band 375, TAE, Expert Verlag, Ehningen (1992)

[4] J.Hirsch, ICAA5 (4) Materials Science Forum Volume, 242 Transtec Publications, Swit-

zerland, (1997) S.33-50[5] J.Hirsch: "Automotive Trends in Aluminium - The European Perspective", ICAA9, edt.

J.F.Nie, A.J.Morton, B.C.Muddle, Mater.Forum 28, Inst. of Mat. Eng. AustralasiaLtd.ISBN 1876855 223, Vol.1 p. 15-23

[6] A.K.Gupta, G.B.Burger, P.W.Jeffrey, D.J.Lloyd; ICAA4, Proc. 4th Int. Conf. on Al alloys,Atlanta/GA USA (1994) ed. by T.H.Sanders, E.A.Starke, Vol.3 p.177

[7] E.Brünger, O.Engler, J.Hirsch: "Al-Mg-Si Sheet for Autobody Application" in "VirtualFabrication of Aluminium Products" chapter I-6, Wiley-VCH Verlag, Weinheim 2006(ISBN: 3-527-31363-X), pp. 51-61

aluminium in innovative light-weight car design

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