Stress Ratio Effects in Fatigue of Lost Foam Aluminum Alloy 356

David E. Palmer, P.E.BRP – Marine Propulsion Systems Division,

Sturtevant, WI

Introduction• Lost foam casting (LFC) is used to

make outboard engine components (engine block, cylinder head, etc.)

2

Introduction• These components are used in fatigue, generally

with non-zero mean stress

max

min

R

3

Introduction• Fatigue failures typically initiate from porosity

or from the as-cast surface

Fracture surfaceAs-cast surface

Bead structure

Fissure between foam beads

Porosity

4

Problems• Large number of mean stress equations

(Goodman, Soderberg, Walker, etc.) – which one to use?

• Lack of published data on effect of as-cast surface on fatigue of LFCs

• How to account for presence of porosity in LFCs?

5

Motivations1. Provide as-cast mechanical property data for

design engineers

2. Understand factors that influence fatigue of aluminum LFCs in order to find ways to make better castings

3. Gain insight into stress ratio sensitivity of materials

6

Objectives

For LF aluminum alloy 356-T6 and 356-T7 with as-cast and machined surfaces:

1. Evaluate monotonic tensile properties

2. Generate S-N curves (R = –1, R = 0, R > 0)

3. Determine appropriate mean stress correction

4. Evaluate effect of defect size on fatigue life

7

Lost foam casting

• Patterns made from expanded polystyrene (EPS)• Raw bead size 0.25 – 0.50 mm• Impregnated with 5 – 7% hexane (blowing agent)

8

Lost foam casting

Poor pattern

fusion can occur if

beads are above

Tg for insufficient

time during

molding process

9

Lost foam casting

10

• Assembled cluster is coated with refractory slurry

• Coating may penetrate into gaps in foam beads

Lost foam casting

11

• Molten metal is poured directly into the EPS mold

• As metal front advances, EPS degrades, melts, and vaporizes

• LF mold filling is a highly complex process

12

Lost foam casting

Foam

Coating

Sand

Metal

Decomposition layer

Lost foam casting

13

Collapse mode:• Occurs when patterns have density gradients

or poor fusion• Gaps between foam beads provide escape path

for gas, resulting in low local pressures• Metal front advances in “fingers”• This mode results in fold defects as liquid

pyrolysis products are trapped between metal fronts.

Fatigue and mean stress• There are a large number of equations that

relate fatigue with mean stress (R ≠ –1) to an equivalent fully reversed stress (R = 1)

• These include the Goodman, Soderberg, Morrow, Gerber, ASME-Elliptic, Smith-Watson-Topper, Stulen, Topper-Sandor, and Walker equations

14

Fatigue and mean stress

15

Goodman equation

u

m

aeq

1

16

Fatigue and mean stressSoderberg equation

o

m

aeq

1

17

Fatigue and mean stressMorrow equation

f

m

aeq

1

18

Fatigue and mean stressGerber equation

2

1

u

m

aeq

19

Fatigue and mean stressASME-Elliptic equation

2

1

o

m

aeq

20

Fatigue and mean stressSmith-Watson-Topper equation

aeq max

21

Fatigue and mean stressStulen equation

maeq A

• If A = σe / σu , this is equivalent to the Goodman equation; if A = σe / σo , it is equivalent to the Soderberg equation, etc.

• Value of A must be determined from tests at different R ratios

22

Fatigue and mean stressTopper-Sandor equation

maeq

• Power law relationship between σm and σeq

• Value of α must be determined from tests at different R ratios

23

Fatigue and mean stressWalker equation

aeq 1

max

• If γ = 0.5 , this is equivalent to the Smith-Watson-Topper equation

• According to Dowling, γ ≈ 0.45 for aluminum and 0.65 for steels

• Value of γ must be determined from tests at different R ratios

Aluminum alloy 356-T6

24

25

Aluminum alloy 356-T6

Experimental design

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356-T6As-cast

356-T7As-cast

356-T6Machined

356-T7Machined

Tension testing: 5 specimens eachFatigue testing: 15 specimens R = -1 15 specimens R = 0 15 specimens R > 0

SEMporosity

measurements

Sample preparation: machined

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Sample preparation: as-cast

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Pattern fusion testing• Pattern permeability apparatus developed at

University of Alabama-Birmingham (UAB)

• Measures air flow rate when 21 kPa vacuum is applied to surface of foam pattern

• Used to evaluate pattern fusion for as-cast specimens

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Pattern permeability

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Average: 4.3 cm/s

Standard deviation:2.1 cm/s

Tensile testing

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• Performed per ASTM E8• Constant displacement rate (5

mm/min.)• Specimen deflection

measured with extensometer

Stress-strain curves

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356-T6 machined 356-T6 as-cast

Stress-strain curves

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356-T7 machined 356-T7 as-cast

Tensile fracture surfaces

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356-T6 machined 356-T6 as-cast

Tensile fracture surfaces

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356-T7 machined 356-T7 as-cast

Fatigue testing

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• Performed per ASTM E466• Tested in force control• Three different R-ratios (R = -1, R = 0, R > 0)• Six different load levels at each R-ratio• For R > 0 testing, σmax was held at 0.5σy while σmin

was varied to produce R = 0.09, R = 0.26, R = 0.31, R = 0.40, R = 0.44, and R = 0.62 conditions

S-N curves

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356-T6Machined

S-N curves

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356-T6As-cast

S-N curves

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356-T7Machined

S-N curves

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356-T7As-cast

Fatigue fracture surfaces

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356-T6 machined 356-T6 as-cast

Fatigue fracture surfaces

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356-T7 machined 356-T7 as-cast

Weibull analysis

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Weibull analysis

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356-T7As-cast

B50 = 54.8 MPa

B10 = 44.2 MPa

α = 57.1ß = 8.72

Weibull analysis

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Critical pore size

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Average size of critical

pore103 µm

for both as-cast and

machined

Effect of pore size on fatigue

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Difference in fatigue

life between as-cast and machined cannot be attributed to porosity

Folds in as-cast specimens

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Comparison of mean stress equations

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1. Goodman2. Soderberg3. Morrow4. Gerber5. ASME-Elliptic6. Smith-Watson-Topper7. Stulen8. Walker9. Topper-Sandor

Comparison of mean stress equations

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Error = Predicted life – actual life

Actual life

Comparison of mean stress equations

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Condition Surface Goodman Soderberg Morrow

T6Machined 254% 134% 1238%

As-cast 343% 339% 1347%

T7Machined 323% 246% 1040%

As-cast 202% 189% 748%

Comparison of mean stress equations

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Condition Surface GerberASME-Elliptic

SWT

T6Machined 1517% 1840% 18%

As-cast 1947% 2592% 44%

T7Machined 1866% 2362% -23%

As-cast 1091% 1370% -10%

Comparison of mean stress equations

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= Predicted life – actual life

Actual life

Absoluteerror

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Comparison of mean stress equations

Condition Surface StulenTopper-Sandor

Walker

T6Machined 47% 36% 33%

As-cast 44% 38% 40%

T7Machined 40% 30% 34%

As-cast 48% 42% 45%

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Comparison of mean stress equations

356-T7Machined

No mean stress

correction

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Comparison of mean stress equations

356-T7Machined

Goodman correction

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Comparison of mean stress equations

356-T7Machined

ASME-Elliptic

correction

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Comparison of mean stress equations

356-T7Machined

Walker correction

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Mean stress sensitivity parameters

Condition Surface StulenTopper-Sandor

Walker

T6Machined 0.417 0.793 0.530

As-cast 0.454 0.803 0.563

T7Machined 0.341 0.734 0.459

As-cast 0.372 0.749 0.480

Mean stress sensitivity

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Kirby and Beevers (1971):

In air: da/dN = f(ΔK, R) In vacuum: da/dN = f(ΔK) ONLY!

Chalwa et al (2011):

R-ratio effects increase with P(H2O)

Hypothesis:

Greater mean stress sensitivity of 356-T6 compared to 356-T7 is due to greater

oxidation rate on crack surface.

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Mean stress sensitivity

This hypothesis will be tested in future work.

Conclusions

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1. Lost foam 356-T6 and 356-T7 specimens with as-cast surface have significantly lower monotonic and fatigue properties compared to specimens with a machined surface.

Conclusions

63

2. Ranking of mean stress equations:Topper-SandorWalkerStulenSmith-Watson-TopperSoderbergGoodmanMorrowGerberASME-Elliptic

Best

Worst

DO NOT USE

(Tie)

Best if no data for fit

Conclusions

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3. Ranking of effects on fatigue of lost foam aluminum 356:

As-cast surface

Porosity Heat treatment> >

65

Conclusions

4. Lost foam 356-T6 has greater stress ratio sensitivity than lost foam 356-T7.

Future work

• Investigate effect of pattern fusion:

> 10 cm/s (“beady”)

1 – 10 cm/s (present work)

< 0.5 cm/s (smooth)

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Future work

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• Measure crack growth rates (da/dN) for as-cast and machined specimens

Hypothesis: Crack propagation is faster in as-cast specimens due to presence of folds

Future work

68

• Measure polarization resistance of lost foam 356-T6 and 356-T7

Hypothesis: Greater mean stress sensitivity of 356-T6 compared to 356-T7 is due to greater oxidation rate on crack surface

Future work

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• Investigate effect of chills on as-cast properties of lost foam castings

Future work

70

• Investigate other possible means of improving properties of LF castings:

Vibration during solidification

Vacuum-assisted filling

Solidification under pressure

Future work

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• Fully-reversed four-point bending fatigue fixture

Future work

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• Investigate environmental effects on fatigue of LF castings:

Saltwater

Water velocity

Water temperature

Galvanic potential

AcknowledgementsUWM - Dr. Rohatgi, Dr. Venugopalan, Dr. El-Hajjar, Dr.

Church, Betty Warras

BRP - Glover Kerlin, Bill Barth, Jim Bonifield, Ken Chung, Matt Coyne, Todd Craft, Ben Jones, Mark

Noble, Rich Smock, Karl Glinsner, Pete Lucier

IIT - Dr. Sheldon Mostovoy

Virginia Tech - Dr. Norman Dowling

ASU - Dr. Nik Chawla

UAB - Harry Littleton

My family - Thanks for everything!73