Determination of crack-initiating defects in cast aluminium alloys by Non-destructive Testing

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Determination of Crack-Initiating Defects in Cast Aluminium Alloys by Non-destructive Testing

AVIZO EUGM, 30 May - 1 June, 2012, Bordeaux, France

Yakub Tijani and André Heinrietz

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Outline

Pores in cast aluminium components Influence of defects on cast aluminium Development of a parametric model for fatigue life Non-destructive evaluation by computer tomography using AVIZO

FIRE Finite element models of cast pores Correlation between FE models, experiments and parametric model Summary

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Cast aluminium alloys in automobile industry: Motor engine

Porsche Cayenne

V8 Motor Volvo 5 cylinder engine block

Source: Müller-Weingarten

Good strength-weight ratio Castability of complex geometries Cheap to produce

Ref: www.vdg.de

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Cast Aluminium alloys: Process Chain in 3 Steps

Automotive

Aerospace

Transport

Mechanical Engineering

Energy, Environment, Health

Die Casting

Investment Casting

Sand Casting

[….]

- Mould Filling

- Cooling

- SolidificationEnd Products

Liquid Aluminium

+

Alloying Elements

+

Heating

Pores in cast aluminium alloy Components

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Pores and/or Oxides may be present

Unwanted notch effect in the material leads to crack initiation

Reduction in fatigue life

Processes to reduce pores lead to increase in cost

- Turbulence

- Temperature difference

- Change in Solubility

(Liquid Solid)

- Oxidation

[….]

- Mould Filling

- Cooling

- Solidification

Material matrix is no longer homogenous

IfG

Pores in cast aluminium alloy Components

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Defects in the components interior increase the notch effectin the loaded section

Influence of porosity on fatigue strength is scientifically proven, but not quantitatively predictable based on measured variables

Influence of defects on cast alloys: State of the art

Poor explicit correlation between fatigue strength and pore size/morphology Difficult to experimentally measure the effect of mechanical stress on pores The radiographic quality assurance is carried-out by comparison with reference

catalog Cast products will be unnecessarily scrapped in the event of pores The cast simulation cannot quantitatively predict porosity

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103 104 105 106 107 10840

50

60

70

80

90

100

110120130140150160170

Number of cycles to failure [ ]

Stre

ss a

mpl

itude

, MP

a

AlSi8Cu3 IngotPü = 50%AlSi8Cu3 with poresPü = 50%

40% reduction in fatigue strengthat N=107

Continouscasting-ingot

PV (mm³)

σ(N=1e7)

(MPa)Tσ k

< 0.5 - 4 54 1:1.34 9.7

Ingot 90 1:1.10 15.6

Influence of defects on cast alloys: Fatigue experiment

Ref: Tijani et al., TMS 2012, Orlando FL

Int. J. Fatigue (in Review)

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103 104 105 106 107 10840

50

60

70

80

90

100

110120130140150160170

Number of cycles to failures [ ]

Stre

ss a

mpl

itude

, MPa

AlSi8Cu3 IngotPü = 50%AlSi8Cu3 PV < 0.5 mm³Pü = 50%AlSi8Cu3 PV 0.5 mm³ - 2 mm³Pü = 50%AlSi8Cu3 PV 2 mm³ - 4 mm³Pü = 50%

PV (mm³)

σ(N=1e7)

(MPa)Tσ k

< 0.5 59 1:1.16 11.2

0.5 - 2 43 1:1.35 7.2

2 - 4 49 1:1.33 9

Ingot 90 1:1.10 15.6

VPore ~ σN=1E7



Influence of defects on cast alloys: Fatigue experiment

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SEM fractographicimage of cast AlSi9Cu3specimen

Crack initiated by big pore close to the surface

Image can be analyzed to determine pore parameters

Relationship between pore parameters and fatigue life is only qualitative

The approach is a destructive testing method

Influence of defects on cast alloys: Fractography by SEM

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Metallography of the fracture surface

Influence of defects on cast alloys: Pore shape

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0

0,2

0,4

0,6

0,8

1

1,2

1 2 3 4 5 6 7 8 9 10 11 12 13

Defekt

convexty formfaktor

1 23

45

6 78 9

10 1112

13

Crack-initiating pore:- Biggest pore close to the surface- Smallest values of Convexity and Shape factor

Influence of defects on cast alloys: Pore shape

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Different pore sizes

Different pore distance to surface (DtS)

Similar stress concentration factor

A B C

Kt= 2.322 Kt= 2.366 Kt= 2.420

Development of a parametric model for fatigue life

Pore

DtS

(mm)

Diameter (mm)

Pore Volume (mm³)

A 0.3 1 0.524

B 0.45 1.5 1.767

C 0.6 2 4.189

Same DtS-Diameter ratioRef: Tijani et al., ECAA 2011, Bremen


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)shape(3 fKt

)size(2 fKt

)surface todistance(1 fKt

Effect of pores can be characterized by their stress concentration factors, Kt

t

fKK :factornotch Fatigue

)shape size, surface, todistance(fKt

impact of characteristic material behavior is excluded

= notch sensitivity factor

AreaVolume

DiameterRRR

RR

RRK f 621,231,0

1,171,06,074,21

0 diameter pore

surface todistanceR

Development of a parametric model for fatigue life

Ref: Tijani et al., MS & T 2011, Columbus OH


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Non-destructive Evaluation by Computer Tomography using AVIZO FIRE

Pore Volume(mm³)

Area (mm²)

DtS* (mm)

Shape factor[ ]

1 0.13 1.43 3.53 0.8

2 0.45 4.42 3.36 0.5

3 0.09 1.35 3.56 0.6

4 1.56 10.92 3.12 0.5

Microstructure discontinuitiescan be reconstructed by usingAVIZO Fire on microfocus CT

Pore parameters can bedetermined by 3D image

analysis

The crack-initiating pore canbe determined from the poreparameters

* DtS = Distance to Surface



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Pore Volume(mm³)

Area (mm²)

DtS* (mm)

Shape factor[ ]

1 0.15 2.47 3.53 0.4

2 0.21 3.81 3.43 0.3

3 0.68 8.04 2.74 0.3

4 0.19 2.94 2.95 0.4

5 0.60 7.77 2.33 0.3

6 0.14 1.96 3.42 0.6

7 0.12 1.92 1.98 0.5

* DtS = Distance to Surface

A database of pore parameters can be prepared using AVIZO Fire on microfocus CT

Stress concentration factor due to each pore can be determined

Non-destructive Evaluation by Computer Tomography using AVIZO FIRE

Tijani et al., Int. J. Fatigue (in Review)

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EPOS/9125D/1

EPOS/9125D/2EPOS/9125D/4

EPOS/9125D/5EPOS/9125D/6

EPOS/9125D/7

Finite Element Models of Cast Pores

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EPOS/9125D/3

Linear elastic calculation Nominal stress, σn = 90 MPa Max. principal stress σ = 681.5 MPa Kt (FEM) = 7.6 Kt (Par. Model) = 8.0

FE-Model of Specimen with Pores (AlSi8Cu3)

Ref: Tijani et al., MS & T 2011, Columbus OH


Pore structure from Mikrofocus CT

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EPOS/8239A

Pore structure from Mikrofocus CT

Linear elastic calculation Nominal stress, σn = 90 MPa Max. principal stress, σ = 571.7 MPa Kt (FEM) = 6.4 Kt (Par. Model) = 6.9

FE-Model of Specimen with Pores(AlSi7Mg0.3)



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Correlation between FE Models and Parametric Model

Good correlation between results of FE Model and computed stress concentration Kt obtained by using parameters from AVIZO Fire

R2 = 0,9805

2

3

4

5

6

7

8

2 3 4 5 6 7 8Kt (Par. Berechnungsmodell)

Kt (

FEM

Ber

echn

ung)

Kt , Parametric model [ ]

Kt, F

E m

odel

s[ ]


Tijani et al., TMS 2012, Orlando FL.

Tijani et al., MS & T 2011, Columbus OH

Tijani et al., ECAA 2011, Bremen

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Validation with industrial SN Curves

Good correlation between experimental and computed fatigue life results obtained by using parameters from AVIZO Fire

SN Curve k

Fatiguestrength σk at

N = 107

Exp. 5,3 74 MPaModel 5,3 78 MPa

10

100

1000

1,00E+03 1,00E+04 1,00E+05 1,00E+06 1,00E+07 1,00E+08

Stre

ss A

mpl

itude

, MPa

No of Cycles to Failure [ ]

Experiment Pu=50%Parameter modelPu=50%


Tijani et al., TMS 2012, Orlando FL.

Tijani et al., MS & T 2011, Columbus OH

Tijani et al., ECAA 2011, Bremen

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The presence of defects reduces fatigue strength

By using Avizo Fire, the crack-initiating pores can be determined in a microstructure

Fatigue life of cast aluminum alloy components based on non-destructive analysiscan be calculated

The obtained parameters provide reliable predictions over the range of conditions found in industrial castings, where pores are among the major driving forces for reduction of structural durability

This approach will reduce production costs and is expected to improve control mechanisms within the scope of quality assurance

Summary

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The presented work is a part of the research project IGF 295 ZN of the Research Association Casting Technology (FVG), Sohn-straße 70, 40237 Düsseldorf, Germany. It was sponsored by the German Federation of Industrial Research Associations (AiF) under the program for the promotion of jointindustrial research and development (IGF) of the Federal Ministry of Economics and Technology (BMWi) due to a decision of the German Parliament.

Acknowledgement

Dipl.-Ing. André HeinrietzFraunhofer Institute for Structural Durabilityand System Reliability, Darmstadt, Germany. [email protected]

Dr.-Ing. Yakub TijaniFraunhofer Institute for Structural Durabilityand System Reliability, Darmstadt, Germany. [email protected]@szm.tu-darmstadt.de

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Thank You For Your Attention

Source: Honsel via www.vdg.de

Determination of crack-initiating defects in cast aluminium alloys by Non-destructive Testing

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Transcript of Determination of crack-initiating defects in cast aluminium alloys by Non-destructive Testing