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

Determination of Crack-Initiating Defects in Cast
Aluminium Alloys by Non-destructive Testing
AVIZO EUGM, 30 May - 1 June, 2012, Bordeaux, France
AVIZO EUGM 2012. 31.5.-1.6.2012, Bordeaux, France

Yakub Tijani and André Heinrietz
1

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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

2

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AVIZO EUGM 2012. 31.5.-1.6.2012, Bordeaux, France Cast aluminium alloys in automobile industry: Motor engine

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

Porsche Cayenne
V8 Motor
Volvo 5 cylinder engine block Ref: www.vdg.de
Source: Müller-Weingarten 3

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Pores in cast aluminium alloy Components

Cast Aluminium alloys: Process Chain in 3 Steps

Liquid Aluminium
+ - Mould Filling
Alloying Elements - Cooling End Products
+ - Solidification
Heating

Die Casting Automotive
Investment Casting Aerospace
Sand Casting Transport
[….] Mechanical Engineering
Energy, Environment, Health
4

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Pores in cast aluminium alloy Components

- Turbulence
- Temperature difference
- Mould Filling
- Change in Solubility
- Cooling
- Solidification (Liquid  Solid)
- Oxidation
[….]
IfG

Material matrix is no longer homogenous
 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
5

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Influence of defects on cast alloys: State of the art

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

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

 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
6

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Influence of defects on cast alloys: Fatigue experiment
170
160 AlSi8Cu3 Ingot
Pü = 50%
150 AlSi8Cu3 with pores
140 Pü = 50%

130
120 Continous
Stress amplitude, MPa

110 casting-ingot
100
90

80 40% reduction in

70 fatigue strength
at N=107
60
σ(N=1e7)
PV (MPa)
Tσ k
(mm³)
50
< 0.5 - 4 54 1:1.34 9.7

Ingot 90 1:1.10 15.6
Ref: Tijani et al., TMS 2012, Orlando FL
40 Int. J. Fatigue (in Review)
3 4 5 6 7 8
10 10 10 10 10 10
Number of cycles to failure [ ] 7

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Influence of defects on cast alloys: Fatigue experiment
10
7
10
6 A i8 u In t
lS C 3 go
P =5 %
ü 0
10
5 A i8 u P < 0 m ³
lS C 3 V .5 m
10
4 P =5 %
ü 0
A i8 u P 0 m ³ - 2m ³
lS C 3 V .5 m m
10
3 P =5 %
ü 0
A i8 u P 2m ³ - 4m ³
lS C 3 V m m
10
2 P =5 %
ü 0
plitude, M a

10
1
P

10
0
90
VPore ~ σN=1E7
80
tress am

70
σ(N=1e7)
PV (MPa)
Tσ k
S

60 (mm³)
< 0.5 59 1:1.16 11.2
0.5 - 2 43 1:1.35 7.2
50
2-4 49 1:1.33 9
Ingot 90 1:1.10 15.6
40 Int. J. Fatigue (in Review)
3 4 5 6 7 8
10 10 10 10 10 10
N m of cy
u ber cles to failu [ ]
res 8

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Influence of defects on cast alloys: Fractography by SEM

SEM fractographic
image of cast AlSi9Cu3
specimen

 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 9

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Influence of defects on cast alloys: Pore shape

Metallography of the fracture surface
10

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Influence of defects on cast alloys: Pore shape

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

1,2

1

0,8

0,6

0,4

0,2

0
1 2 3 4 5 6 7 8 9 10 11 12 13
De fe k t

co n vext y f o rm f akt o r

Crack-initiating pore:
- Biggest pore close to the surface
- Smallest values of Convexity and Shape factor 11

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Development of a parametric model for fatigue life

A C
B
Kt= 2.322 Kt= 2.366 Kt= 2.420

DtS Diameter Pore Volume
Pore (mm) (mm) (mm³)  Different pore sizes
A 0.3 1 0.524
 Different pore distance to surface (DtS)
B 0.45 1.5 1.767
C 0.6 2 4.189  Similar stress concentration factor

Ref: Tijani et al., ECAA 2011, Bremen

Same DtS-Diameter ratio Int. J. Fatigue (in Review)
12

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Development of a parametric model for fatigue life

Effect of pores can be characterized by their stress concentration factors, Kt
K t1  f (distance to surface )
K t 2  f (size) K t  f (distance to surface, size, shape )
K t 3  f (shape)  impact of characteristic material
behavior is excluded

Fatigue notch factor : K f 
Kt
 = notch sensitivity factor


 
 
1 0,71R 0,31 Diameter
Kf  2,74  0,6 R    2,21 R  
 R
1,1
R R   6 Volume 
   
 R   Area 

distance to surface
R  0
pore diameter
Ref: Tijani et al., MS & T 2011, Columbus OH
Int. J. Fatigue (in Review)
13

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Non-destructive Evaluation by Computer Tomography using AVIZO FIRE

 Microstructure discontinuities
can be reconstructed by using
AVIZO Fire on microfocus CT
 Pore parameters can be
determined by 3D image
analysis

 The crack-initiating pore can
be determined from the pore
Pore Volume Area DtS* Shape factor parameters
(mm³) (mm²) (mm) []
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 * DtS = Distance to Surface 14

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Non-destructive Evaluation by Computer Tomography using AVIZO FIRE

Pore Volume Area DtS* Shape factor
(mm³) (mm²) (mm) []

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

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

 Stress concentration factor due to each pore can be determined
15

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Finite Element Models of Cast Pores

EPOS/9125D/1 EPOS/9125D/6 EPOS/9125D/5

EPOS/9125D/4 EPOS/9125D/2 EPOS/9125D/7

16

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FE-Model of Specimen with Pores (AlSi8Cu3)
 Linear elastic calculation
 Nominal stress, σn = 90 MPa
 Max. principal stress σ = 681.5 MPa
 Kt (FEM) = 7.6
 Kt (Par. Model) = 8.0

EPOS/9125D/3

Pore structure from Mikrofocus
CT

Ref: Tijani et al., MS & T 2011, Columbus OH
17

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FE-Model of Specimen with Pores
(AlSi7Mg0.3)
 Linear elastic calculation
 Nominal stress, σn = 90 MPa
 Max. principal stress, σ = 571.7 MPa
 Kt (FEM) = 6.4
 Kt (Par. Model) = 6.9

EPOS/8239A

Pore structure from Mikrofocus
CT

18

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Correlation between FE Models and Parametric Model

8

7
[]
KtKt , FE models
(FEM Berechnung)

6 R2 = 0,9805

5

4

3

2 Tijani et al., TMS 2012, Orlando FL.
2 3 4 5 6 7 8 Tijani et al., MS & T 2011, Columbus OH
KKt , Parametric model
t (Par. Berechnungsmodell) [] Tijani et al., ECAA 2011, Bremen

Good correlation between results of FE Model and computed stress
concentration Kt obtained by using parameters from AVIZO Fire 19

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Validation with industrial SN Curves

1000

Experiment
Pu=50%
Parameter model
Pu=50%
Stress Amplitude, MPa

Fatigue
SN
k strength σk at
Curve
100 N = 107
Exp. 5,3 74 MPa

Model 5,3 78 MPa

Tijani et al., TMS 2012, Orlando FL.
10 Tijani et al., MS & T 2011, Columbus OH
1,00E+03 1,00E+04 1,00E+05 1,00E+06 1,00E+07 1,00E+08
Tijani et al., ECAA 2011, Bremen
No of Cycles to Failure [ ]

Good correlation between experimental and computed fatigue
life results obtained by using parameters from AVIZO Fire 20

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Summary

 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 analysis
can 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

21

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Acknowledgement

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 joint
industrial research and development (IGF) of the Federal Ministry of Economics and Technology (BMWi)

due to a decision of the German Parliament.

Dr.-Ing. Yakub Tijani Dipl.-Ing. André Heinrietz
Fraunhofer Institute for Structural Durability Fraunhofer Institute for Structural Durability
and System Reliability, Darmstadt, Germany. and System Reliability, Darmstadt, Germany.
yakub.tijani@lbf.fraunhofer.de andre.heinrietz@lbf.fraunhofer.de
tijani@szm.tu-darmstadt.de
22

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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