Low Carbon Vehicle Technology Project (LCVTP)

WS 7 Lightweight Structures - Technology Review

November 17th 2011

Geraint Williams

WMG

Manufacturing UHSS (boron steel)

Developing joining solutions for composites

Rahul Bhattacharya, Dave Cooper, Mike Cromarty, Richard Dashwood, Darren J. Hughes, Dave Mossop, Mark Pharaoh, John Pillier, Neil Reynolds,

Darren Stewardson, Geraint Williams, Terry Wheeldon

Associate Partner: Alpha Adhesives – David Shaw, Darryl Taylor

Presentation overview

A – Processing boron steel 1 – Applications 2 – Manufacturing considerations 3 – Facility development 4– Initial results

B – The joining challenge for thermoplastic composites 1 – The options available 2 – A fitting solution? 3 – Matching to existing processes 4 – Optimisation and results

Demonstrator components (next talk)

UHSS (boron steel)

Significant opportunities in using UHSS > Light-weighting via down-gauging

> Improved crash performance

HOWEVER : Challenges to forming > Limited RT formability (low elongation, high forces)

> High spring-back

Hot forming – die quenching > Eliminates spring-back > High ductility > Fully hardened (cooling rate

dependent)

Hot-forming route

‘Direct process’ 1 –Austenitisation of blank @ 850-950 C 2 – Rapid transfer 3 – Forming-cooling operation

Austenitization in

furnace

As-received

Ferrite-Pearlite

Forming & quenching

Fully hardened

part

Typical applications

Typically A/B-pillar, roof, side rails....

Ford Fiesta (2011) A-pill a r

Volvo C70

ROPS – reinforced A-pillar

Challenges: tailored microstructure

Best crash performance achieved with different properties

- B-pillar Can be achieved by:

tailor welded blanks Differential tool cooling

- Tailored microstructure

Fully hardened microstructure

Controlled cooling

Challenges

Differential tool cooling Heat flow – final microstructure

–thermal control mechanisms

Interplay between forming strain and microstructure

Controlling oxide – tool wear –joining implications

Addressing information needs of partners for designing with UHSS

Facility development at WMG

Main aims: Develop a facility capable of forming UHSS To create a facility to fabricate demonstrator hardened parts in boron steel for

input to LCVTP test programme

Hot forming requirements

FURNACE - rated to ~ 1000 C

PRESS - high forming speed

TOOL - integral cooling

INSTRUMENTATION- thermal tracking

MATERIALOGRAPHY - microscopy, hardness

Facility

Furnace

Blanksto1m 2

To1200 C Up-and-over door

Facility

Press and tool

500 tonne press Final forming sp Water-cooled to LCVTP top-hat d accounted for

Facility

Facility

Part transfer critical

Spring-back

First parts – short top-hats tool time~30s

Degree of spring-back obtained using

GOM scanner

Virtually zero spring-back observed in hot-formed part

Hardness and microstructure

Hardness (macro-Vickers)

As-received stock - 253 VHN 30

Average test-part 1 - 396 VHN 30

Average test-part 2 - 393 VHN30

Microstructure : to come...

Progress-to-date

Press facility commissioned - Industry relevant forming speed (100 mms -1 over 125 mm via accumulators)

Controllable tool temperature via channels

Initial parts are hardened – thermal instrumentation needed

Spring-back effectively zero

Full length parts into test programme next week

Joining solutions for composites

Principal aims > Evaluate the state-of-the-art for joining processes > Investigate appropriate (production-relevant) joining methods for the

candidate materials selected in WS7

> Provide timely input for demonstrator (top-hat/seat) testing

Candidate material – recap!

PA – GF composite (aligned glass fibre reinforcement – thermoplastic matrix)

Joining is critical

Jaguar X350 (XJ) monocoque

284 stampings

35 extrusions

15 castings

Majority rivets + adhesive

Joining TPC’s?

SPR for composites?

SPR – Self Pierce Riveting

Cold-forming process

Widely used in AA bodies

" No pre-drilled hole

Y Valid for a range of materials

" Readily automated

( Excellent repeatability

x Two-sided access required

x Recycling issues

x Weight x Aesthetics

Joining proposal & questions

Proposal: Combine the advantages of adhesives and mechanical (SPR) fixtures

High fail strength (adhesive) Improved energy absorption (mechanical)

Good peel strength (mechanical) Environmental performance (adhesive) Immediate fixing – no jigging (mechanical)

? Can we use the e-coat process to cure a 1-k adhesive?

simpler tooling/automation mechanical fixture holds form

? Is SPR relevant to TPC’s?

? Can we combine it with a (1-k) adhesive without modifying existing processes? (paint line)

> Current status

Alpha Adhesives signed as ‘associate’ (adhesive supply and advice)

Initial tests performed to establish appropriate adhesive

Lap-shear coupon testing (JLR standard geometry) + ageing

DSC – cure kinetics

DMTA – adhesive strength development and thermal resistance

CT–joint integrity

μCT analysis

Initial results show applicability to hybrid and similar joints

X-ray μCT in WMG can show integrity of joint (NDT)

- same concept as medical scanner

3000 images over 180 degs

1s per2D image

Relies on X-ray contrast between phases

Reconstruction takes 5 mins

‘Fly through’ analysis possible

15 μm voxel

SPR joint – suitable for composites?

μCT data

Fibres into plane Fibres horizontal

Fibre layup: 90,0,0,90,90,90,0,0,90

μCT data

Suitable flare – could be optimised

Fibre layup: 90,0,0,90,90,90,0,0,90

Cure of 1-k via paint line?

Thermal simulation of paint line profile (thermocouple data) using DMTA

Work with Alpha Adhesives to

optimise properties of 1-k

(epoxy) adhesive

> Initial adhesive – cures (but

only just)

> Alpha re-formulated

adhesive for LCVTP

> Lower temp cure – OK

(reduced shelf-life)

Composite surface finish – peel ply or

not?

finish Peel ply cloth

Surface

Composite surface finish – peel ply or

not?

Is the peel ply useful?

> Aesthetically better

> May give better bond – literature disagrees

Investigated by lap-shear testing of:

1k epoxy and methyl-methacrylate

P60 abraded surface Aligned glass – PA composite (P60 prep)

Aligned glass – PA composite (peel ply)

Lap-shear tests – peel ply worth it?

Tests using 1-k epoxy and MM

>Approx two-fold increase in

UTS using peel ply

Joint efficiency (η) = Joint UTS/Parent UTS

1-k epoxy

Peel ply

Non- Peel ply

Lap-shear tests – peel ply worth it?

Tests using 1-k epoxy and MM Joint efficiency (η) = Joint UTS/Parent UTS

Substrate failure with peel ply

Substrate

failure

Lap-shear tests – peel ply worth it?

Tests using 1-k epoxy and MM Joint efficiency (η) = Joint UTS/Parent UTS

BUT

> Results not replicated

using 2-k or MM

Recall DMTA....

- initial viscosity drop

- flow into ‘pores’

- strong interlock on cure

MM

Peel ply

Non- Peel ply

Lap-shear tests – peel ply worth it?

Tests using 1-k epoxy and MM Joint efficiency (η) = Joint UTS/Parent UTS

Summary

Highlights:

> Identified mechanical (SPR) and adhesive as a route to usage in automotive structure

Adhesive and SPR systems already in use – minor changes to incorporate composites

> Optimisation of 1-k adhesive achieved with Alpha Adhesives – cure within 15 minute bake

> Proven usefulness of peel-ply finish

> Excellent joint in hybrid structures. Further work needed to optimise similar joints. >

System used for demonstrator longitudinal parts (NEXT TALK....)

Thanks for listening!