Fatigue Behavior of Al-Si-Cu-Mg Casting Alloys Behavior of Al-Si-Cu Aluminum...• Application of...

3/03/03 TMS 2003, San Diego

Fatigue Behavior of Al-Si-Cu-Mg Casting Alloys

Qigui Wang and Peggy JonesAdvanced Materials

GM - Powertrain


Background

• Application of Al-Si-Cu-Mg Casting Alloys– Complex engine components because of reasonable

castability, good high temperature mechanical properties, and low cost compared with other primary alloys

– Concerns of fatigue resistance

• Objectives– To study the influence of Sr, grain refinement with

“TiBloy”, and low pressure filling on fatigue;– To determine weak links that controls fatigue, and– To develop microstructure tolerant design (MTD)

method based on statistics and fracture mechanics.


hTest castings of cylinder heads were lost foam cast using both gravity pouring and low pressure fill.

hT7 heat treatment including solution treatment for 12 hr at 493°C, Quenching into agitated warm water (70°C), and artificial aging for 8 hr at 249°C.

hBoth tensile and fatigue specimens were prepared and tested at room temperature at WMT&R per ASTM E466.

hAlloy (319)

Experimental

Chemistry

(wt%) SPECIFIC GRAVITY

(g/cc)

Si Cu Fe Mn Mg Ti Sr Mn:Fe Ratio

Base 2.68 6.86 3.26 0.17 0.059 0.31 0.091 0.0005 0.42 Base +TiB2 2.67 6.96 3.27 0.21 0.086 0.31 0.100 0.0008 0.55

Sr 2.67 7.01 3.20 0.19 0.068 0.29 0.091 0.0151 0.49 Sr+TiB2 2.68 7.03 3.30 0.20 0.079 0.30 0.101 0.0194 0.54 LP Base 6.66 3.26 0.20 0.048 0.31 0.102 0.0002 0.24 2.68


Experimental

• Metallography– Porosity: vol%, maximum Feret diameter, area– Dendrite cell size (DCS): a circle grid (5 circles) used

Max. Size

Max. Depth

• Fractography– Fatigue crack origin;– Initial crack (pore/oxide)

size; and – Weak links controlling

crack propagation


Microstructure

319 Base

319 Sr

319 TiB2

319Sr TiB2

One interconnected pore

LP 319 Base


Microstructure (Grain Structure)

319 Base

319 Sr

319 TiB2

319Sr TiB2


Microstructure Characterization

Porosity vs. Alloys

Alloys and casting method

-2.00

0.00

2.00

4.00

6.00

8.00

Are

a pe

rcen

t of p

oros

ity o

n as

-po

lishe

d p

lane

s

319-T7 Alloys

Mean value

Max/Min values

+/- 95% Confidence interval of mean

Av. % Porosity Stand. Dev. # samples

Gravity 319

Gravity 319+TiB2

Gravity 319Sr

Low pressure 319

Gravity 319Sr+TiB2

0.060.12 44

2.040.83 33

1.460.70 27

1.980.78 32

1.840.74 34

0.06

4.07

2.04

0.70

2.83

1.46

0.59

4.32

1.98

0.66

4.41

1.84

0.570.63

0.00

areapon.grf

Dendrite Cell Size (DCS) vs. Alloys


0

40

80

120

160

Den

drite

cel

l siz

e, D

CS

(µm

)

319-T7 Alloys

Mean value

Max/Min values



Gravity 319

Gravity 319+TiB2

Gravity 319Sr

Low pressure 319

Gravity 319Sr+TiB2

0.060.12 44

2.040.83 33

1.460.70 27

1.980.78 32

1.840.74 34

90

76

59

48

79

60

48

96

66

54

83

65

55

72

51

DCS&319.grf


Maximum Pore Size vs. Alloys

Microstructure Characterization


-400

0

400

800

1200

1600

Ma

xim

um p

ore

siz

e o

nas

-pol

ishe

d p

lan

es (

µm

)

319-T7 Alloys

Mean value

Max/Min values



Gravity 319

Gravity 319+TiB2

Gravity 319Sr

Low pressure 319

Gravity 319Sr+TiB2

0.060.12 44

2.040.83 33

1.460.70 27

1.980.78 32

1.840.74 34

440

772

445

164

765

368

226

632

391

189

721

415

183167

20

mic&pore.grf

-200 0 200 400 600 800 1000Maximum pore size

on as-polished plane, amax (µm)

0.0

0.2

0.4

0.6

0.8

1.0

Cum

ulat

ive

dist

ribu

tion

func

tion,

F (a

max

)

A319-T7 alloys

LP A319

A319

A319 + TiB2

A319+Sr

A319Sr+TiB2

w-evs-f.grf


Fatigue Crack Initiation and Propagation

SEM fractograph BEI fractograph

Initiation Site at Shrink

Slow Crack Propagation Region

Overload

Slow Crack Propagation Boundary

Shrinkage porosity initiated fatigue crack in a HCF specimen (LP 319 Base)which failed after 1,666,452 cycles at 60MPa



SEM fractograph

Shrinkage porosity initiated fatigue crack in a LCF specimen (319 Base) which failed after 6,869 cycles at 145.5MPa

BEI fractograph

Origin at Shrink

Secondary Origin at Gas Pore




Shrinkage porosity initiated fatigue crack in a LCF specimen (319 TiB2) which failed after 3,156 cycles at 144MPa

Origin at ShrinkSecondary Origin at Shrink

Secondary Origin at Shrink




Shrinkage porosity initiated fatigue crack in a LCF specimen (319 Sr) which failed after 3,609 cycles at 138MPa

Cu-rich intermetallics

Origin at Shrink




Shrinkage porosity initiated fatigue crack in a LCF specimen (319Sr+TiB2) which failed after 7,369 cycles at 150MPa

Origin at Gas Pore

Secondary Origin at ShrinkGas Pore


Fatigue Strength vs. Micros and Tensile (LCF)

No clear relationship between fatigue properties and micros

No strong relationship between fatigue and

tensile ductility for T7


0

200

400

600

800

1000

Max

imu

m p

ore

size

on

as-p

olis

hed

pla

nes

(µm

)


Gravity 319

Gravity 319+TiB2

Gravity 319Sr

Low pressure 319

Gravity 319Sr+TiB2

0.060.12 44

2.040.83 33

1.460.70 27

1.980.78 32

1.840.74 34

fatmic.grf

0

2

4

6

8

10

12

%A

rea

of

po

ros

ity (

as-p

olis

hed

pla

nes

)

110

120

130

140

150

Fatig

ue s

tren

gth

(LC

F) a

t RT

(MP

a)

319 - T7 alloysR = -1, LCF, f=5Hz

Fatigue strength at RT

Pore size (as-polished)

Area% of porosity (as-polished)

Alloys and castin method

100

110

120

130

140

150

160

Fat

igu

e st

reng

th (L

CF

) at R

T, σ

w (M

Pa)

150

200

250

300

Ten

sile

str

eng

th (M

Pa)

fat-ten.grf

Low pressure Gravity Gravity Gravity Gravity 319 319 319+TiB2 319Sr 319Sr+TiB2

Av. % Porosity 0.06 2.04 1.46 1.98 1.84 Stand. Dev. 0.12 0.83 0.70 0.78 0.74 #samples 44 33 27 32 34

Pla

stic

str

ain

to fr

actu

re, ε

p (%

)

319 alloys, R = -1 (LCF, f = 5Hz)(T7 - 12h@915°F & 8h@480°F)


Ultimate strength at RT

Yiels strength at RT

Plastic strain to fracture

2.5

2.0

1.5

1.0

0.5

0.0


Comparison of Pore Sizes

5 6 7 8 9 10 11

Ln ( amax )

-4.0

-3.0

-2.0

-1.0

0.0

1.0

2.0

3.0

4.0ln

ln (1

/(1-f

(am

ax))

)400 81002980

319-T7 alloysPores initiated fatigue cracks

alloys b amax, 0

LP base 3.3 1756

Base 3.1 1085

TiB2 4.0 1220

Sr 2.7 2790

Sr + TiB2 4.0 2237

f(a m

ax)

0.632

0.934

0.999

0.049

0.127

0.308

Maximum sizes of poresinitiated cracks, amax (µm)

150 Wpcrack1.grf

0.018

1100

f(amax ) = 1 - exp ( )-bamax

amax,0

22000

140MPa 130MPa144MPa

132MPa136MPa


Weak Links and Fatigue Properties

Good correlation between fatigue life and maximum pore size

Strong correlation between fatigue strength and area% of porosity and intermetallics on

fracture surface


0

10

20

30

40

50

60

Are

a fr

actio

n of

por

e an

d in

term

etal

lics

on fr

actu

re s

urfa

ce (%

)


Gravity 319

Gravity 319+TiB2

Gravity 319Sr

Low pressure 319

Gravity319Sr+TiB2

0.060.12 44

2.040.83 33

1.460.70 27

1.980.78 32

1.840.74 34

fpinterm.grf

100

110

120

130

140

150

Fatig

ue s

tren

gth

at R

T (M

Pa)

Error bar stands for one (1) Std Dev.

319 - T7 alloys, R = -1 (LCF, f = 5 Hz)


Total %area of pore and intermetallics

%Area of porosity

%Area of intermetallics

0 2000 4000 6000 8000Maximum pore size, amax

(µm)

1E+2

1E+3

1E+4

1E+5

1E+6

1E+7

1E+8

Cyc

les

to fr

actu

re

sn-m

ax.g

rf

319-T7 alloysR = -1

LP base

Base

TiB2

Sr

Sr+TiB 2

Base


Microstructure Tolerant Design (MTD)

For applicationsN

f> 107 cycles

Initial effective stress intensity factor range

m

i

pieff a

NCK

1

,

−

⋅=∆

Nf / a max (cycle/m)

1

10

Eff

ectiv

e in

itial

str

ess

inte

nsity

fac

tor

Kef

f, i (

MP

a m

1/2 )

105 106 107 108

sn-dK2.grf

104 1010 1011

319-T7, R = -1500µm < amax < 6000 µm

LP Base

Base

TiB2

Sr

Sr+TiB2

109

-1

6.67

2

3

4

5

6

8

0 1000 2000 3000 4000 5000 6000Maximum pore size, amax

(µm)

0

50

100

150

200

Str

ess

ampl

itud

e, σ

a (M

Pa)

Predict eventual fatiguefailure from defect

Predict no fatiguefailure from defect

sa319.g

rf

319-T7 alloysR = -1

LP base

Base

TiB2

Sr

Sr+TiB 2


For applicationsN

f< 107 cycles

Long crack modelm

aip

Ca

N

⋅=

σ1


Nf and Nf * amax (cycles x µm)

100

Str

ess

ampl

itude

, σa

(MP

a)

108 1010 1012

SNAmax.grf

106

σa= 786*(Nf* amax )-0.11

1014

Nf* amax dataNf data319 alloys, T7

R = -1, LCF & HCF500µm < amax< 6000µm

LP Base

Base

TiB 2

Sr

Sr + TiB 2

1016102 10440

50

80

60

200

R-squared = 0.956129

Calculated propagation life, Np (Cycles)

Cyc

les

to fa

ilure

, Nf

102

103

104

105

106

107

102 103 104 105 106 107

lcnp-max.grf

319 - T7 alloysGravity and low pressure

R = -1, LCF and HCF

LP Base

Base

TiB2

Sr

Sr + TiB2


For applicationsN

f< 107 cycles

Short crack models

⋅

⋅=

i

fn

a

ysp a

aCN ln

σ

σ

⋅

⋅

⋅=i

fn

a

ysp a

aCN ln

max

'

σε

σ


ln(af /amax ) * Np-1 (cycle-1)

1.0

σ a / σ y

s

10-8 10-210-7 10-6 10-5 10-4 10-3

319 - T7 alloysR = -1, LCF and HCF

LP base, HCF

Base, LCF

Base + TiBloy, LCF

Sr, LCF

Sr + TiBloy, LCF

snys-max.grf

n = 7.9C5= 2.45 x 10 -3

R2 = 0.925178

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

ln(af /a max ) * Np-1 (cycle-1)

ε max

* (σ

a / σ

ys )

10-8 10-210-7 10 -6 10-5 10-4 10-3

319 - T7 alloysR = -1, LCF and HCF

LP base, HCF

Base, LCF

Base + TiBloy, LCF

Sr, LCF

Sr + TiBloy, LCF

snduc-mx.grf

n = 4.1C6= 6.4 x 107

R2 = 0.928111

0.0003

0.0005

0.003

0.002

0.0040.005

0.0001

0.001

(M.J. Caton et al., Met. Trans. 1999)


Comparison of Microporosity as Observed Metallurgically and in a SEM

4.0 5.0 6.0 7.0 8.0ln (amax)

0.0

0.2

0.4

0.6

0.8

1.0

Cu

mul

ativ

e d

ensi

ty

Max pore size (amax)

319 Base alloy, T7

as-polished plane

fracture surface

crack initiators

148.5 403.5 1096.6 2981319cumu.grf

Gas Porosity

3.0 4.0 5.0 6.0 7.0 8.0 9.0ln (amax)

0.0

0.2

0.4

0.6

0.8

1.0

Cu

mul

ativ

e d

ensi

ty

Max pore size, amax (µm)

319Sr alloy (T7)

as-polished plane

fracture surface

crack initiators

148.5 403.5 1096.6 2981319cuSr.grf 54.6 8103

Shrinkage Porosity


Comparison of Microporosity as Observed Metallurgically and in a SEM

(J.M. Boileau, 2000)

Gas Porosity

Shrinkage Porosity


Extreme Value Statistics (EVS)

Alloys Maximum defect size (µm)

As-polished

(mean ± 3σ)

Fracture surface

(mean ± 3σ)

EVS estimation

(mean ± 3σ)

LP Base 167 ± 316 1573 ± 1645 1594 ± 560

Base 445 ± 477 968 ± 1210 2646 ± 1072

TiB2 368 ± 402 1104 ± 944 2231 ± 1034

Sr 391 ± 400 2483 ± 3342 2246 ± 1030

SR + TiB2 415 ± 467 2027 ± 1816 2562 ± 1136

0 1000 2000 3000 4000Maximum size of pores

on as-polished planes, amax (µm)

-2

0

2

4

6

8

10

12

14

16

18

20

-ln

(-ln

(F(a

max

)))

T

(re

turn

per

iod)

0 1000 2000 3000 4000

Maximum pore size on as-polished planes (µm)

2.2x104

1.6x105

36.79

98.185

99.9999997

99.966

99.999

99.999998.9x106

1.2x106

6.6x107

A319-T7 alloys Extreme-Value Statistics of defects

alloys λ δ

LP Base 119 84.6

Base 370.7 130.6

TiB2 305.3 110.5

Sr 329.2 110.0

Sr+TiB2 343 127.3

F(a) = exp (-exp (- ))a - λδ

w-evsmc.grf

Cum

ulat

ive

dis

trib

uti

on f

unc

tion

, F(a

max

) (%

)

−−−=

δλx

xF expexp)(

x – pore size, λ, δ - EVS scale parameters

0VV

T =

46

0

107.3100

107.3×=

×==

VV

T

74 107.31000107.3 ×=××=bT

1000*TTb =(Murakami, et al, 1994, 97)


Application of Fracture Mechanics and EVS

Number of cycles to failure, Nf (cycles)

0

50

100

150

200

250S

tres

s am

plitu

de, σ

a (M

Pa)

103 104 105 106

sn&cal.grf

102 107 108

319-T7, R = -1500µm < amax < 6000 µm

LP Base

LP TiB2

Base

TiB2

Sr

Sr + TiB2

Long crack model with EVS

Short crack model with EVS


Summary and Conclusions

• Porosity plays the most important role in determining fatigue resistance. Compared with porosity, eutectic structure and intermetallic phases play a minor role in crack initiation. However, they can influence crack propagation rates late in life.

• Strontium modification of eutectic Si leads tomacrosegregation of Cu-rich and Fe-rich intermetallicphases, and increases microshrinkage porosity.

• Compared with conventional gravity pouring, low pressure filling appears beneficial. It significantly reduced the volume fraction of porosity and increased tensile and fatigue strengths.


Summary and Conclusions

• The effect of grain refinement using “TiBloy” additions on microstructure and mechanical properties is marginal. It showed no significant benefit in unmodified (Sr-free) alloys. In Sr-modified alloys, TiBloy additions slightly reduce the volume fraction and size of porosity as well as the degree of intermetallic phase segregation, leading to a slight improvement in fatigue performance compared to Sr-modified material. The gains do not completely restore the strength of Sr-modified material to that of the base alloy.

• Fatigue cracks initially propagate mainly through the dendrites, leading to fewer eutectic (intermetallic) particles in the fatigue crack propagation region compared with tensile overload area.


Summary and Conclusions• For the studied alloys with lives of ~106 – 107 cycles, the

effective threshold stress intensity factor (∆Keff, th) is about 1MPa√m. For stress and defect combinations exceeding a stress intensity factor of 1MPa√m, the fatigue crack would initiate from the largest pores located at the free surface of the materials.

• Fatigue life can then be predicted using both long crack (LEFM – linear elastic fracture mechanics) and short crack (CTOD – crack-tip opening displacement) models together with inherent material characteristics.

• The largest defect (pore) size in a cast component can be estimated using extreme-value statistics (EVS) applied tometallographic measurements of pore size. Maximum pore size prediction by EVS agrees quite well with measurements of the initiation pore sizes from the fracture surface.

Fatigue Behavior of Al-Si-Cu-Mg Casting Alloys Behavior of Al-Si-Cu Aluminum...• Application of...

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