Lost foam casting is a highly versatile metalcasting process that offers significant benefits in terms of design flexibility, energy consumption, and environmental impact. In the present work, the fatigue behavior of lost foam cast aluminum alloy 356, in conditions T6 and T7, was investigated, under both zero and non-zero mean stress conditions, with either as-cast or machined surface finish. Scanning electron microscopy was used to identify and measure the defect from which fatigue fracture initiated. Based on the results, the applicability of nine different fatigue mean stress equations was compared. The widely-used Goodman equation was found to be highly non-conservative, while the Stulen, Topper-Sandor, and Walker equations performed reasonably well. Each of these three equations includes a material-dependent term for stress ratio sensitivity. The stress ratio sensitivity was found to be affected by heat treatment, with the T6 condition having greater sensitivity than the T7 condition. The surface condition (as-cast vs. machined) did not significantly affect the stress ratio sensitivity. The fatigue life of as-cast specimens was found to be approximately 60 – 70% lower than that of machined specimens at the same equivalent stress. This reduction could not be attributed to defect size alone, and may be due to the greater frequency of oxide films near the as-cast surface. Directions for future work, including improved testing methods and some possible methods of improving the properties of lost foam castings, are discussed.
4. Introduction
• Fatigue failures typically initiate from porosity
or from the as-cast surface
Fracture surfaceAs-cast surface
Bead structure
Fissure between
foam beads
Porosity
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5. 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?
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6. Motivations
1. 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
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7. 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
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9. Lost foam casting
• Patterns made from expanded polystyrene (EPS)
• Raw bead size 0.25 – 0.50 mm
• Impregnated with 5 – 7% hexane (blowing agent)
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10. Lost foam casting
Poor pattern
fusion can occur if
beads are above
Tg for insufficient
time during
molding process
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11. Lost foam casting
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• Assembled cluster is
coated with refractory
slurry
• Coating may penetrate
into gaps in foam beads
12. Lost foam casting
12
• 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
14. Lost foam casting
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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.
16. 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
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17. Fatigue and mean stress
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Goodman equation
u
m
a
eq
1
18. 18
Fatigue and mean stress
Soderberg equation
o
m
a
eq
1
19. 19
Fatigue and mean stress
Morrow equation
f
m
a
eq
1
20. 20
Fatigue and mean stress
Gerber equation
2
1
u
m
a
eq
21. 21
Fatigue and mean stress
ASME-Elliptic equation
2
1
o
m
a
eq
23. 23
Fatigue and mean stress
Stulen 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
24. 24
Fatigue and mean stress
Topper-Sandor equation
maeq
• Power law relationship between σm and σeq
• Value of α must be determined from tests at
different R ratios
25. 25
Fatigue and mean stress
Walker 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
33. 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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40. 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
66. 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)
67. 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.
69. 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.
70. Conclusions
70
2. Ranking of mean stress equations:
Topper-Sandor
Walker
Stulen
Smith-Watson-Topper
Soderberg
Goodman
Morrow
Gerber
ASME-Elliptic
Best
Worst
DO NOT
USE
(Tie)
Best if no
data for fit
71. Conclusions
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3. Ranking of effects on fatigue of lost foam
aluminum 356:
As-cast
surface
Porosity
Heat
treatment> >
75. Future work
75
• 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
76. Future work
76
• 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
78. Future work
78
• Investigate other possible means of
improving properties of LF castings:
Vibration during solidification
Vacuum-assisted filling
Solidification under pressure
80. Future work
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• Investigate environmental effects on
fatigue of LF castings:
Saltwater
Water velocity
Water temperature
Galvanic potential
81. Acknowledgements
UWM - 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!
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