1. Early chassis-based construction has now been replaced by body structures of unitary design. The spaceframe concept is increasingly popular, allowing a mix of materials to be used with ease of disassembly and repair.
2. Aluminum design using cast nodes, profiles and sheet has now been proven as a feasible design for volume production although material and vehicle insurance costs remain high.
3. FEM design techniques are now proving invaluable in reducing the timeframe of model development programs. Parameters from a wide range of materials, including high-strength steels, aluminum and polymer variants, can be used to help predict performance in dynamic situations, e.g. a crash. Lower strain-rate programs can help determine forming feasibility.
4. Contemporary design influences can introduce conflicting interests: ease of recyclability is not commensurate with the increased use of plastics used to lighten body structure. There should be no threat to vehicle safety if larger, safer, steel structures are gradually replaced by lighter alternatives.
5. Specialized production techniques offering new forms of materials such as TWBs and hydroformed tube sections are allowing more freedom of design, with opportunities for parts consolidation and weight reduction.
6. TWBs and use of lay-up techniques with composites such as carbon fiber now allow localized strengthening and stiffening of different body zones, thereby shedding superfluous weight.
7. The combination of advanced composites and ultra lightweight honeycomb structures could provide the basis for future alternatively fueled vehicles as demonstrated by current high-performance vehicles.
8. Polymers offer the designer undoubted advantages, extending the range of body shapes and exhibiting good low-speed impact, scuff and dent resistance.
However, the range of materials must be rationalized to allow for simpler specification on drawings/electronic identification systems and ease the task of segregation for dismantlers.
References
1. BRITE EURAM II Low Weight Vehicle Project BE-5652 Contract No. BRE2-CT92- 0264.
2. Brown JC, Robertson AJ, Serpento ST. Motor Vehicle Structures. Butterworth- Heinemann; 2002.
3. Davies GM, Walia S, Austin MD. The Application of Zinc Coated Steel in Future Automotive Body Structures, 5th International Conference on Zinc Coated Steel Sheet, Birmingham; 1997.
4. Garrett K. First Without Chassis? Car Design and Technology May 1992:56e61.
5. Hodkinson R, Fenton J.Lightweight Electric/Hybrid Vehicle Design,SAE International.
Oxford: Butterworth-Heinemann; 2001.
6. Ludke B.Functional Design of a Lightweight Body-in-White. How to determine Body- in-White materials according to structural requirements. VDI Berichte 1543 Sympo- sium, 11 and 12 May 2000. Hamburg.
7. Ludke B. Functional Design of a Lightweight Body-in-White for the new BMW generation.Stahl und Eisen1999;119(5):123e128.
8. Lewandowski J.Audi A8. Bielefeld: Delius Klasing Verlag; 1995.
9. Muraoka H. Development of an All-aluminum Body.Journal of Materials Processing Technology1993;38:655e674.
10. Holt DJ.Saturn: The Vehicle,Automotive Engineering Nov. 1990:34e44.
11. Anon, Aluminum Spaceframe Makes the Lightest Lotus. Materials World Dec.
1995:584.
12. Novak M, Wenzel H. Design Engineering and Production of the Alcoa Spaceframe for Ferrari’s 360 Modena, SAE Paper 1999-01-3174.
13. Anon. The Aston Martin V12 Vanquish,AutoTechnologyAug. 2001;Vol. No. 1:28e29.
14. Bernhardt W.Powertrain 2020: The Business Model Challenge. London: EV Battery Tech 2010; March 26th 2010.
15. Paper No. 750222,SAE 1975.
16. Walia S, et al.The Engineering of a Body Structure with Hydroformed Components.
IBEC Paper 1999-01-31811999.
17. Davies GM, Waddell W. Laser Welding Allows Optimum Door Design.Metal Bulletin MonthlyJuly 1995:40e41.
18. Dieffenbach JR.Not the Delorean Revisited: An Assessment of the Stainless Steel Body- in-White, SAE Paper 1999-01-3239.
19. Dieffenbach JR.Challenging Today’s Stamped Steel Unibody: Assessing Prospects for Steel, Aluminum and Polymer Composites. IBEC ’97 Proceedings, Stuttgart, Germany, 30 Septe2 Oct 1997:113e118.
References
91Materials for consideration and use in automotive body
structures 3
CHAPTER OUTLINE
Objective ... 94 Content ... 94 3.1 Introduction ... 94 3.2 Material candidates and selection criteria... 98 3.2.1 Consistency:a prime requirement ...100 3.3 Steel ... 101
3.3.1 Steel reduction and finishing processes ...103 3.3.1.1 Vacuum degassing ...103 3.3.1.2 Continuous casting ...104 3.3.1.3 Hot- and cold-rolling processes ...104 3.3.1.4 Continuous annealing ...108 3.3.1.5 Skin passing ...110 3.3.2 Surface topography ...111 3.3.3 Effects in processing...117 3.3.4 Higher strength steels ...117 3.3.4.1 Ultra high-strength steels ...118 3.3.4.2 Future developments ...125 3.3.4.3 Stainless steel...125 3.4 Aluminum ... 128 3.4.1 Production process ...128 3.4.2 Alloys for use in body structures...129 3.5 Magnesium ... 132 3.6 Polymers and composites ... 133 3.6.1 Introduction ...133 3.6.2 Thermoplastics ...133 3.6.3 Thermosets ...134 3.6.4 Polymer and composite processing ...135 3.6.4.1 Injection molding ...135 3.6.4.2 Glass-mat thermoplastic compression molding ...135 3.6.5 Advanced composites for competition cars...137 3.7 Repair ... 139 Learning points ... 142 References ... 143
Materials for Automobile Bodies. DOI:10.1016/B978-0-08-096979-4.00003-7
CopyrightÓ2012 Elsevier Ltd. All rights reserved. 93
OBJECTIVE
To review the choice of materials suitable for body manufacture and provide an understanding of salient manufacturing processes, product parameters and associ- ated terminology. This is essential if the limitations of specific materials are to be appreciated, and their advantages and disadvantages weighed up against possible alternatives prior to the design selection stage. The significance of the properties and their impact on the conversion of these base materials into components is given in Chapter 4.
CONTENT
The range of materials that can be realistically considered for body structures is reviewed; the critical need for consistency of properties and their effect on productivity is emphasized, together with the associated advantages of continuous production methods; key stages of both steel and aluminum manufacture are described; the significance of the final skin pass is explained and methods used to vary the texture of work rolls and strip surface are described; the types of high- strength steel (HSS) grades in the yield stress range 180e1200 MPa, and their strengthening mechanisms, are graphically described; finally, an introduction is provided to the relevant polymer types and mode of manufacture.
3.1 INTRODUCTION
The main materials used in body construction are described in the preceding chapter and, as indicated, early choices were governed by the increasing needs of mass production and then during the post-war years by availability, as suppliers struggled to resume production. Obviously, nowadays the choice has broadened considerably as materials technology has responded to the needs of the automotive engineer. This means that a far deeper understanding of materials parameters is required if this enhanced range of properties is to be exploited to maximum advantage. The situa- tion has advanced significantly from the days when ‘mild steel sheet’ was the universal answer to most body parts applications. As apparent from the introduction, any distinction between grades was then generally made on the basis of formability and in the case of the few aluminum specifications by temper (O, annealed;
H, hard; etc.).
The metallurgy of both steel and aluminum alloys have now advanced signifi- cantly, offering a wide choice of mechanical and physical properties together with other attributes. In addition to the greater choice of metallics, it is also necessary to consider plastics, 20 different types of which can be used in the motor vehicle. The traditional requirements of the vehicle body have always been strength, in both static and dynamic terms, and elastic modulus, which governs stiffness/rigidity and 94 CHAPTER 3
Materials for consideration and use
imparts stability of body shape. To these can now be added drawability and work- hardening parameters, which are important with respect to forming and stretching respectively. The latter also has an important effect on energy absorption and impact resistance.
A number of surface parameters are now thought to have considerable tribo- logical (frictional) as well as cosmetic significance. Whereas the main requirement is surface roughness, Ra, a range of ‘deterministic’ (pre-etched) rather than
‘stochastic’ (random, shot-blasted) finishes are now possible. With the advent of computer-aided design (CAD) systems the design engineer need no longer rely on experience from previous models or on a dedicated materials engineer for his materials choice. Instead, he is often confronted with a series of predetermined options, from which he must make his selection. For the correct choice at the engineering stage it is important that the automotive engineer fully understands the parameters presented to him, and their relevance in terms of production and appli- cation, and in controlling metallurgical characteristics. A summary of these key parameters is presented in Table 3.1. It shows some basic properties and their relevance to the automotive engineer. The stage of the strip production process, critical to the development of these parameters, is also identified together with other influencing factors.
The importance of strength, ductility and surface finish will be evident from the preceding text. The ‘r’ value provides a measure of the resistance of the material to thinning in the thickness plane during drawing, via a favorable crystallographic texture. Likewise, during stretching, a high work-hardening ‘n’ value spreads the
Table 3.1 Key Design Parameters and Relevant Processing Details
Parameter Relevance
Influencing Factors and Key Processing Stages
Strength Design Imparted by composition,
deformation and grain size; alloying during smelting and mechanical and thermal treatment
Ductility Forming, collapse
characteristics
Lean composition and optimum heat treatment; careful analysis and extended annealing cycle Drawability index ‘r’
(resistance to thinning)*
Press forming Crystallographic texture requiring optimum rolling and annealing schedules
Work hardenability ‘n’* Stretch forming, energy absorption
Composition and grain size dependent; casting and rolling Surface finish Lubricity during
forming, painted appearance
Imparted by roll finish at temper rolling stage
*Fully defined in Chapter 5
Table 3.2 Extended Choice of Materials and Parameters Used by a Key Car Manufacturer (Courtesy of B. Ludke, formerly of BMW Group)
1 2 3 4 5 6 7 8
Material UK Equivalent E-Module Density E r
E
r Price ffiffiffiffi
pE
ffiffiffiffiE p
r
ffiffiffiffi pE
r Price ffiffiffiffi
3E p
FeP04 St 14 DCO 4 forming grade 210,000 7.85 26,752 22,293 458.3 58.4 48.6 59.4
ZstE 300 P BH HSS bake-hardening 300 MPa YS grade
210,000 7.85 26,752 20,578 458.3 58.4 44.9 59.4
S 420 MC ZstE 420 NbTi
HSS HSLA Grade 420 MPa YS Grade
210,000 7.85 26,752 19,816 458.3 58.4 43.2 59.4
BTR 165 VHF – Stahl
Ultra high-strength steel 1100 MPa YS
210,000 7.85 26,752 19,108 458.3 58.4 41.7 59.4
AlMg5Mn 10%kv Aluminum–magnesium wrought sheet for internal parts
70,000 2.70 25,926 4321 264.6 98.0 16.3 41.2
AlSi1.2Mg0.4 10%
kv, 190C, 0.5hr
Aluminum–silicon skin panel material paint bake-hardened
70,000 2.70 25,926 3704 264.6 98.0 14.0 41.2
AZ 91T6 Magnesium alloy
Heat treated magnesium alloy
45,000 1.75 25,714 4675 212.1 121.2 22.0 35.6
TiAl6V4 F89 Titanium alloy
Titanium alloy for automotive consideration
110,000 4.50 24,444 349 331.7 73.7 1.1 47.9
Kiefer – longitudinal Pinewoodgrain longitudinal
12,000 0.50 24,000 6000 109.5 219.1 54.8 22.9
Kiefer – transverse Grain transverse 12,000 0.50 24,000 6000 109.5 219.1 54.8 22.9 Al2O3 (Keramic,
massiv) ‘spro¨de’
Fused alumina 370,000 3.85 96,104 481 608.3 158.0 0.8 71.8
GFK 55% force parallel to fiber
Glass reinforced plastic –long. fibers
40,000 1.95 20,513 2051 200.0 102.6 10.3 34.2
GFK 55% force normal to fiber
Transverse fibers 12,000 1.95 6154 615 109.5 56.2 5.6 22.9
AFK 55%, TM – Type parallel to fiber
Aramid fiber reinforced epoxy – long
70,000 1.35 51,852 519 264.6 196.0 2.0 41.2
Aramid fiber reinforced epoxy – transverse
6000 1.35 4444 44 77.5 57.4 0.6 18.2
CFK 55% force parallel to fiber
Carbon fiber reinforced epoxy – long
110,000 1.40 78,571 1310 331.7 236.9 3.9 47.9
CFK 55% force normal to fiber
Transverse fibers 8000 1.40 5714 95 89.4 63.9 1.1 20.0
GF-PA-12 (54%) parallel to fibre
Glass reinforced polyester 54% – long
35,400 1.70 20,824 1041 188.1 110.7 5.5 32.8
GF-PA-12 (54%) normal to fiber
54% – transverse 4400 1.70 2588 129 66.3 39.0 2.0 16.4
Glas (massiv)
‘spro¨de’
Fused glass 70,000 2.50 28,000 18,667 264.6 105.8 70.6 41.2
Hinweis:grau hinterlegte Materialien sind Basis fu¨ r Anlage 2 bis 5 jj ¼parallel zur Faser
_j_¼quer zur Faser
Anlage 1:Materialeigenschaften