Formability and Failure ofFormability and Failure ofAutomotive Sheet MaterialAutomotive Sheet Material

AA 5754AA 5754

Materials Science and Engineering

701 Seminar

February 16, 2005

Presented by Diana Zdravecky

Supervised by Dr.Wilkinson and Dr.Jain

OutlineOutline

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• Material properties

• Processing methods

• Formability of the material

• Microstructural comparison

• Failure mechanisms

Project OverviewProject Overview

• GM sponsored project inconjunction with Novelis (formerlyAlcan Inc.)

• Potential use of Aluminum sheetmaterial for automotive applications

• Research activities carried out atMcMaster for approximately threeyears

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Driving ForceDriving Force

• Government regulationsfor the reduction inexhaust emissions

• 10% reduction in vehicleweight can improve thefuel efficiency by 5.5%

• Replacing structuralframe components (innerdoor panels)

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http://shermanparts.com

Materials of InterestMaterials of Interest

• AA 5754-O (5xxx series)

- Work hardenable -Solution hardened alloy

- Non-heat treatable -Main alloying element Mg

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Chemical composition of alloy (wt%) thickness: 1mmMaterial Type Al Mg Mn Si Fe Cr Zn

CC 96.09 3.42 0.23 0.05 0.21 - -DC 96.3 3.11 0.25 0.06 0.21 0.04 0.02

Comparison with Mild SteelComparison with Mild Steel

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Property Aluminum Alloys – 5xxx Mild SteelDensity (Mg/m3) 2.8 7.8

Young’s Modulus (GPa) 69-79 200Yield Strength (MPa) 90 - 120 187

% Elongation 26 42R-value 0.7 - 0.9 1.9

Weldability Yes YesCorrosion Resistance Exceptional Galvanizing necessary

Cost ($/kg) 3.0 0.7

Casting MethodsCasting Methods

• Direct Chill (DC)

- conventional process

- semi-continuous process

- production of Al extrusionbillets, forging ingots androlling slabs

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www.wagstaff.com/

Casting MethodsCasting Methods

• Twin-Belt ContinuousCasting (CC)

- set up in 1971

- produces thinner cross-section ingots (strip,sheet and foil products)

- significant cost reductions

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LiquidMetal

ContinuousCast

(19mm)

Hot Roll– 5mm

ColdRoll

BatchAnneal

www.hazelett.com

Formability of Al SheetFormability of Al Sheet

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Composition

Processing

Microstructure andTexture

- ductility- strain localization- failure- strength level- work hardening

Modelling

ObjectivesObjectives

• Compare the formability of continuouscast (CC) material to direct chill (DC)material

- evaluate the forming and fracture limits over arange of strain paths

- determine the damage mechanisms and identifythe parameters which influence fracture

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Forming and Fracture LimitForming and Fracture LimitDiagram (FLD)Diagram (FLD)

Matls 701 - Feb 16, 2005 11Minor strain

Major strain

Forming Limit

Fracture limit

Development of FLDDevelopment of FLD

• Use of Dome testerto obtainexperimental results

• Reliable andreproducible results

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Development of FLDDevelopment of FLD

• Variation of strain paths

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Development of FLDDevelopment of FLD

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• Failure of specimens

Calculating FLD’sCalculating FLD’s

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MajorStrain

MinorStrain

Major Strain = (major axis length – Do ) x 100

DoMinor Strain = (minor axis length – Do ) x 100

Do

FLD of AA 5754FLD of AA 5754

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DC fracturelimitCC fracturelimit

DC and CCforminglimit

Forming Limit ObjectiveForming Limit Objective

• Forming limit of CC and DC materialvary minimally

• Fracture limit varies significantlybetween two material

- CC 5754 showing considerably lower strainsto failure

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Fracture LimitsFracture Limits

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• Dependent on variations in materialinhomogeneities

• Microstructural parameters are influencinglocalize damage

• Influenced by stress (or strain) state, strainrate and temperature

Microstructural ObjectiveMicrostructural Objective

• To investigate how microstructure isinfluencing the fracture limit and damageprocess

- look at region between necking limit andfracture limit

• Range of strain paths “pure modes”

- simple tension

- plane strain

- biaxial tension

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Failure of AA 5754Failure of AA 5754

• Characteristic modes of failure

1) Ductile fracture involving nucleation,growth and coalescence of microvoids

- debonding and cracking of second phaseparticles

- coalescence by impingement

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Failure of AA 5754Failure of AA 5754

2) Localized shear fracture after strainlocalization in the form of a macroscopicshear band

- catastrophic failure due to rapid accumulationof damage

- coalescence by instability

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Microstructural AspectsMicrostructural Aspects

• Development of second phase particlesin casting and rolling

- intermetallic particles – Fe based (1 µm)

- due to low solubility of Fe in Al

- situated in grain boundaries and throughoutmatrix

- distribution of second phase particles vary inDC and CC material

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Intermetallic ParticlesIntermetallic Particles

- Particles form a interdendritic network duringsolidification

- Network breaks up during subsequent rollingoperations

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CC 0.21%Fe DC 0.21%Fe

Rollingdirection

Thicknessdirection

Fracture MechanismsFracture Mechanisms

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FractureMechanisms

Mechanics- Critical stress tocause decohesion- Influenced strongly byhydrostatic stresses

MicrostructuralAspects

- role of secondphase particles- properties of matrixand inclusions

Mechanics of DuctileMechanics of DuctileFractureFracture

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• Dependent upon geometry of specimen

- change of stress state change offailure mechanism

• Triaxial stress states developed after theonset of necking

- related to hydrostatic stresses

- responsible for void nucleation and growth

σmean = 1/3 (σ11 + σ22 + σ33 )

Mechanics of DuctileMechanics of DuctileFractureFracture

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• Simple Tension

σmean = 1/3(σ11 + σ22 + σ33 )

σmean = 1/3(σ11 + 0 +0 )

σmean = σ/3

Mechanics of DuctileMechanics of DuctileFractureFracture

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• Plane Strain

σmean = 1/3(σ11 + σ22 +0 )

In plane strain εwidth= 0 , σ22 = 1/2σ11

σmean = 1/3(σ11 + 1/2σ11 +0 )

σmean = σ/2

Mechanics of DuctileMechanics of DuctileFractureFracture

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• Biaxial Tension

σmean = 1/3(σ11 + σ22 +0 )

In biaxial tension, σ22 = σ11 = σ

σmean = 1/3(σ + σ +0 )

σmean = 2 /3 σ

Damage FormationDamage Formation

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• Is there evidence of uniform damagedevelopment?

- related to ductile fracture mechanism

• If not localized shear fracture

- Involves two simultaneous processes

(i) Localization of plastic flow into macroscopic bands

(ii) Nucleation and growth of voids

FractographyFractography

• Microstructural techniques to determinemode of failure

• Quantify analysis of plasticity and damageunder different stress states

- Optical metallography

- SEM (polished and fracture surfaces)

- Void density measurements

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Future Group WorkFuture Group Work

• Quantitative linkage between continuummodels and the actual microstructure

• Solve boundary value conditions to predictfracture limits in dome test

• Future development of realistic micro-mechanical damage models that simulatereal parts

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Questions?Questions?

Thank you for your time!

Project Goals of Research GroupProject Goals of Research Group

• To understand the role of microstructureon the formability of strip cast Al alloys

- effect of changing of strain path on forminglimits

- development of constitutive laws for damageand ductility

- develop a microstructurally based finite elementmodel for the prediction of formability

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Formability of Al SheetFormability of Al Sheet

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Ductility:controlled by stressstate, temperature,

inclusion level

Interaction ofmaterial

parameterswhich influence

formability

Anisotropy: controlledby texture, thermal-mechanical history,

elongated inclusions,second phase particles

Strength Level:controlled by

composition grainsize, precipitates,solutes in solution

Surface Properties:controlled by oxide

films lubricants,surface plasticdeformation

Work Hardening:controlled by

temperature, solutes,strain gradients around

particles, degree ofdeformation

Localized NeckingLocalized Necking

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Fracture Strain in CC & DC 5754Thickness: 1 mm

-15

-10

-5

0

5

10

15

20

25

30

35

40

45

50

55

60

65

-15 -10 -5 0 5 10 15 20 25 30 35 40 45 50 55 60 65

εTD / %

ε RD

/%

Tension, CC:Fe:0.08, RDTension, CC,Fe:0.21, RDTension, CC,Fe:0.08, TDTension, CC,Fe:0.21, TDTension, DC, RD

Tension, DC, TD

Plane Strain, CC,Fe:0.08, RDPlane Strain, CC,Fe:0.08, TDPlane strain, CC,Fe:0.21, TDPlane Strain, DCRDPlane Strain, DCTDBulge, CC,Fe:0.08Bulge, CC,Fe:0.21Bulge, DC

CC,Fe:0.21 DC, Fe:

limit strain

CC,Fe 0.08