Development aluminum bodies for fuel efficient vehicles

Development aluminum bodies for fuel efficient vehicles Masaaki Saito et al Honda R&D Co Ltd, Tochigi R&D Center, 4630 Shimota Kanezausa, Haga-machi, Haga-gun, Tochigi 371-3393, Japan

As the c u r t a i n of the 21st c e n t u r y rises, e n v i r o n m e n t related c o n c e r n s will be in the spotlight, a n d it has b e c o m e an u r g e n t task for the autom o t i v e i n d u s t r y to m e e t the d e m a n d for high fuel efficiency as a solution to the p r o b l e m of global warming. In tracking this task, Honda research and d e v e l o p m e n t (R&D) e x a m i n e d ways to double the fuel efficiency of its Civic automobile, w h i c h is the representative of higher performance of the company. As a result of these efforts, w e decided to p u r s u e three courses of action simultaneously: i m p r o v e m e n t of the e f f i c i e n c y of the e n g i n e itself, an energy r e g e n e r a t i o n t e c h n o l o g y application and the w e i g h t r e d u c t i o n of the v e h i c l e body, s h o w n in Fig. 1. In terms of making a significant reduction in the weight of the vehicle, we realized that this w o u l d n o t be

possible without replacing steel with a l u m i n u m for the body in white. This paper reports the development of the a l u m i n u m body for Honda Insight which achieves an ultrahigh fuel efficiency.

Development objectives In d e v e l o p i n g the n e w a l u m i n u m body, we set the following objectives in order not only to achieve an ultrahigh fuel e c o n o m y but also to keep cost within a feasible level: (1) 50% reduction in the weight of body in white (compared with the 3-door Honda Civic). (2) High rigidity of the body. (3) Collision safety performance at the top level in its category. (4) Reductions in tooling investments and the n u m b e r of parts used.

200% * Power regeneration • Idling stop * Improvement Ln combustion emciency • Lean-burn • NOx=.ldsorption-type catalytic converter • Friction reduction • Weight reduction • Aerodyn#mms • Low rolllhg resistarii~e tires

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Integrated Motor Asist system

luminum body 100%

Issues associated with

development of aluminum body Honda had already launched onto the market the NSX in 1990, a car with an all-aluminum body. In the NSX, the structure featured only a few changes from the same m o n o c o q u e as a conventional body with steel.As a result, a body which was 40% lighter than a steel body was obtained, shown in Fig. 2. Compared with steel sheets, however, aluminum sheets are subject to some major manufacturing restrictions, they have inferior press-molding characteristics, and difficulty in welding. As a result, use of aluminum wotOd increase the production time, the n u m b e r of parts used and the n u m b e r of welding points, and it would also require large investments in dies.The problem, therefore, would be higher costs stemming from small-lot production. The a l u m i n u m bodies developed in recent years have started to feature alum i n u m space frame structure.The alum i n u m space frame structure uses extrusion molded a l u m i n u m w h i c h enables the characteristics a l u m i n u m to be utilized more freely than the m o n o c o q u e structure a n d e n a b l e s r e d u c t i o n of the n u m b e r of large pressed parts and higher rigidity. This could be an answer to the problems of an aluminum body structure. Nevertheless, the results of our studies revealed that the space frame structure requires fastening parts and other fittings for m o u n t i n g the outer panels and various functional parts, and that these would make it impossible to achieve our cost target.

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We therefore set a b o u t d e v e l o p i n g an a l u m i n u m hybrid body structure w h i c h w o u l d c o m b i n e the merits of both the m o n o c o q u e and space frame structures w i t h m a x i m u m util i z a t i o n of the c h a r a c t e r i s t i c s of aluminum. By optimally c o m b i n i n g the three molding methods of press molding, extrusion molding and casting, a balance in the rigidity and collision safety performance with accomplishments in cost savings and productivity was achieved s h o w n in Fig. 3. Our general approach was to: (1) Increase the application of extruded materials that can be readily molded even w h e n complex crosssectional shapes are used, a n d achieve a more adequate strength and rigidity with the m i n i m u m weight.

Rear hatch frame Trunk lid anel ~ ~ ~ ; ~

fender ~=~ / ///~J~"x'x Sidesill ' ~ ;ront feDn°;erskin • -~

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(2) Employ cast parts to improve the coupling rigidity and to reduce both the n u m b e r of parts and the

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(3)Reduce

pressed

parts

and cut

COSTS.

;ontribution of

Table 1 lists the major characteristics of the materials a n d p r o d u c t i o n m e t h o d while Fig. 4 shows the cross sections of the e x t r u d e d materials.

parts forming type

t-Tr

The m a i n frames s h o u l d b e basically straight and the c u r v e d areas should have smooth shapes featuring minimal changes in curvature w h e r e v e r this was possible. These frames w e r e c o n n e c t e d t o g e t h e r w i t h high rigidity a n d w i t h sufficient w e l d i n g l e n g t h e i t h e r directly or through f u n c t i o n - i n t e n s i v e cast members. The resulting c o n s t r u c t i o n is o n e w h i c h receives the s u s p e n s i o n i n p u t or e n e r g y g e n e r a t e d d u r i n g collisions m o r e efficiently a n d w h i c h e n s u r e s that the load is d i s t r i b u t e d o v e r t h e e n t i r e frame, s h o w n in Fig. 5. Fig. 6 shows a c o m p a r i s o n b e t w e e n a pressed structure and structure con-

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L .... ~

. . . . Front tfillar lower frame

Front side-frame~ J

sisting of cast parts for the rear outrigger as an example of a cast memb e r that c o n n e c t s t h e s e frames

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together. Compared with the aluminum mono-

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Forming

Material

Application

Merits

Issue

Sheet stamping

A50520-O

Inner panel

A5182P-O SG112P-T4 (Bake hardness type)

Outer panel

-High elongation .Thin thickness =Large-sized part aMass production ,High strength

• Low yield • High material cost • High die cost (comparied with extrusion)

A6063S-T5

Frame

*Closed cross section *Lower material cost than sheet ,Lower die cost than stamping

• Lower elongation than sheet

A356

Joint

• High Strength • Complicated shape in one process

• Lower elongation than sheet • Machining cost

Extrusion

Gravity Casting Thixo

m

Extruded parts Die-casting Darts

As a result, a lightweight a l u m i n u m body having a higher rigidity than a body made of steel has b e c o m e possible, s h o w n in Fig. 9.

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Conventionalsteel body

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c o q u e body (NSX), the a l u m i n u m hybrid body has 15% fewer parts and 24% fewer welding points, s h o w n in Fig. 7. Furthermore, compared with the 3-door Civic with its steel body, the body in white unit weights 47% less, s h o w n in Fig. 8.

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Body rigidity and collision safety performance

Aluminum has a Young's modulus which is onethird of that of steel. For this reason, we managed A,.,,~)~ y ""~ to fashion lightweight
For the collision energy absorption m e m b e r s of side frame front and of the rear frame, we used polygonal-ribstructured cross sections w h i c h make the most of the special features i n h e r e n t to extruded aluminum. In view of the correlation b e t w e e n the average stress and the thickness-to-width ratio of rectangularly shaped sheets, we decided on a ratio of approximately 5% for the cross-section dimensions of the sheets: this makes it possible for the sheets to be compressed and collapsed stably and for the energy generated during a collision to be absorbed efficiently, s h o w n in Fig. 10 [ 1]. To use the side frame front members as an example, this construction increases the energy absorption by 50% and reduces its weight by 37% compared with a steel body frame for a virtually identical fully assembled vehicle weight, shown in Figs. 11 and 12. To minimize head-on collision damage, the side frame was divided into two parts, and the front and rear parts were connected using cast parts. The front and rear parts were extruded aluminum materials with different sections formed in such a way that the front part will be compressed and collapsed stably and the rear part will b e n d and alleviate any penetration and impact on the body, shown in Fig. 13. To m i n i m i z e rear-end collision damage, the same

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Number of welding spots

Number of parts

r% 50/,

100%

Steel body Aluminumhybridbody (Civic3-door) (INSIGHT)

a p p r o a c h was a d o p t e d in designing the frame cross sections w i t h the result that the e n e r g y a b s o r p t i o n rate is n o w i m p r o v e d a n d the weight is r e d u c e d significantly.

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+1 3%

O The structural improvements outlined above help to ensure that the collision safety p e r f o r m a n c e n o w meets Honda's in-company standards, w h i c h include the head-on 40% offset impact test at 64 k m p h and the head-on fulllap collision test at 55 kmph. The speeds in both of these tests are higher than the speeds stipulated by the regulations in Japan, the United States and Europe.

Steel body (Civic 3door)

Aluminum Steel body hybrid body (Civic 3door) (INSIGHT)

Materials and production technology

large-sized a l u m i n u m cast parts are developed.

In order to develop the a l u m i n u m hybrid body, n e w b e n d i n g m e m b e r s made of extruded a l u m i n u m and n e w

Table 2 s h o w s a comparison b e t w e e n the b e n d i n g m e t h o d s . Stretch b e n d i n g m e t h o d s are generally used to b e n d e x t r u d e d materials. U n d e r this m e t h o d , however, bending dies are required over the entire length of the m e m b e r s , and these dies are costly.We w a n t e d to e m p l o y the p u s h - t h r o u g h b e n d i n g m e t h o d , b e c a u s e it does n o t involve the use

Nithstiffeningrib Thickness-width

ratio 5%

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withoutstiffeningrib

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Bending

Torsional

Aluminum hybrid body (INSIGHT)

of b e n d i n g dies. To solve the machining p r e c i s i o n p r o b l e m w i t h this method, so as to maintain shape precision, the three dimensional b e n d i n g system hardware and its control software were improved, and also the proof stress level of the a l u m i n u m was controlled to ±10 MPa in its raw material state. As a result, we were able to reduce the variations in b e n d i n g form to one-third of what they were before a n d t h e r e b y attained the target precision.

Thickness-widthratio 7%

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Stroke

Materials

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Conventional Steel frame

Stroke

33

Method

Draw-bend

Push through a

Stretch-bend movable die

the w o r l d ' s highest fuel efficiency for a fully a s s e m b l e d v e h i c l e w a s achieved.

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S t:i ution Bending limit

Good

Excellent

Acceptable

Residual Strain

Good

Excellent

Acceptable

Accuracy

Good

Acceptable

Good

37%

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Aluminum extrusion Conventional frame Steel frame

As m e n t i o n e d before, w e p l a c e d largesized aluminum cast joints at t h e four c o r n e r s o f t h e c a b i n in o r d e r to reduce the n u m b e r o f parts and e n h a n c e the connecting rigidity. We used thixotropic forming for the first time in the world with the skeletal m e m b e r s of a car body, such as the rear outrigger.

a l u m i n u m materials and cast parts additional to the aluminum body technology w h i c h d e b u t e d in the NSX, was a d o p t e d for the Insight. As a result, t h e w e i g h t r e d u c t i o n s and f u r t h e r i m p r o v e m e n t s in rigidity, high c o l l i s i o n safety a n d c o s t savings, w h i c h w e r e o u r original d e m a n d i n g objectives, w e r e obtained, and also

Side frame front

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References [1] Satoh, T., Takada, K., Collapse and e n e r g y a b s o r p t i o n o f thinwalled frames w i t h polygonal sections, 98$3-W-19, 15th Int. Technical ConE on Enhanced Safety of Vehicles, pp. 518-524, M e l b o u r n e , Australia (1996).

Engine/Transmission mount bracket (casting) Front floor frame

Side frame rear

(extrusion)

Using this method, which involves the extrusion molding of A356 aluminum billets in a semi-melted state, thin-waUed casting was achieved, and both the thickness and weight were reduced by more than 20% compared with conventional methods. Fig. 6 shows a comparison with conventional structures while Fig. 14 is a photo of the product made by thixotropic forming.

7. Conclusions

In a d d i t i o n to t h e universal a u t o m o tive issues o f safety, t h e d e m a n d for w e i g h t r e d u c t i o n s in a u t o m o b i l e s has b e e n i n c r e a s i n g in r e c e n t years because of global environmental issues, a n d p r o g r e s s is b e i n g m a d e , n o t only in i m p r o v i n g c o n s t r u c t i o n over t h e i r c o n v e n t i o n a l c o u n t e r p a r t s c o n s i s t i n g mainly of steel parts, b u t also in efforts to e n g i n e e r w e i g h t reductions which include changing o v e r to o t h e r materials s u c h as aluminum. We h o p e that t h e t e c h n o l o g y w h i c h w e have d e v e l o p e d will h e l p s t i m u l a t e e f f o r t s to r e d u c e t h e w e i g h t of a u t o m o b i l e s .

Front lower ann bracket

(casting)

7

5911 x 21111 x 2 5 0 | i i i n

5

A hybrid aluminum b o d y structure, employing extrusion-molded

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