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.

1. Stress Ratio Effects in
Fatigue of Lost Foam
Aluminum Alloy 356
David E. Palmer, P.E.
BRP – Marine Propulsion Systems Division,
Sturtevant, WI

2. Introduction
• Lost foam casting (LFC) is used to
make outboard engine components
(engine block, cylinder head, etc.)
2

3. Introduction
• These components are used in fatigue, generally
with non-zero mean stress
max
min


R
3

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
4

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?
5

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
6

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
7

8. Lost foam
casting
8

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)
9

10. Lost foam casting
Poor pattern
fusion can occur if
beads are above
Tg for insufficient
time during
molding process
10

11. Lost foam casting
11
• 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

13. 13
Lost foam casting
Foam
Coating
Sand
Metal
Decomposition layer

14. Lost foam casting
14
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.

15. 15
Fatigue and
mean stress

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
16

17. Fatigue and mean stress
17
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





22. 22
Fatigue and mean stress
Smith-Watson-Topper equation
aeq  max

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

26. 26
Aluminum alloy
356

27. Aluminum alloy 356-T6
27

28. 28
Aluminum alloy 356-T6

29. 29
Experimental

30. Experimental design
30
356-T6
As-cast
356-T7
As-cast
356-T6
Machined
356-T7
Machined
Tension testing: 5 specimens each
Fatigue testing: 15 specimens R = -1
15 specimens R = 0
15 specimens R > 0
SEM
porosity
measurements

31. Sample preparation: machined
31

32. Sample preparation: as-cast
32

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
33

34. Pattern permeability
34
Average:
4.3 cm/s
Standard
deviation:
2.1 cm/s

35. Tensile testing
35
• Performed per ASTM E8
• Constant displacement rate (5
mm/min.)
• Specimen deflection
measured with extensometer

36. Stress-strain curves
36
356-T6 machined 356-T6 as-cast

37. Stress-strain curves
37
356-T7 machined 356-T7 as-cast

38. Tensile fracture surfaces
38
356-T6 machined 356-T6 as-cast

39. Tensile fracture surfaces
39
356-T7 machined 356-T7 as-cast

40. Fatigue testing
40
• 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

41. S-N curves
41
356-T6
Machined

42. S-N curves
42
356-T6
As-cast

43. S-N curves
43
356-T7
Machined

44. S-N curves
44
356-T7
As-cast

45. Fatigue fracture surfaces
45
356-T6 machined 356-T6 as-cast

46. Fatigue fracture surfaces
46
356-T7 machined 356-T7 as-cast

47. Weibull analysis
47

48. Weibull analysis
48
356-T7
As-cast
B50 = 54.8 MPa
B10 = 44.2 MPa
α = 57.1
ß = 8.72

49. Weibull analysis
49

50. Critical pore size
50
Average
area of
critical pore
0.111 mm²
(machined);
0.120 mm²
(as-cast)

51. Effect of pore size on fatigue
51
Results for
T6 and T7
lie along the
same line

52. Effect of pore size on fatigue
52
Difference
in fatigue
life between
as-cast and
machined
cannot be
attributed
to porosity

53. Folds in as-cast specimens
53

54. 54
Comparison of
mean stress
equations

55. Comparison of mean stress
equations
55
1. Goodman
2. Soderberg
3. Morrow
4. Gerber
5. ASME-Elliptic
6. Smith-Watson-Topper
7. Stulen
8. Walker
9. Topper-Sandor

56. Comparison of mean stress
equations
56
Error =
Predicted life – actual life
Actual life

57. Comparison of mean stress
equations
57
Condition Surface Goodman Soderberg Morrow
T6
Machined 254% 134% 1238%
As-cast 343% 339% 1347%
T7
Machined 323% 246% 1040%
As-cast 202% 189% 748%

58. Comparison of mean stress
equations
58
Condition Surface Gerber
ASME-
Elliptic
SWT
T6
Machined 1517% 1840% 18%
As-cast 1947% 2592% 44%
T7
Machined 1866% 2362% -23%
As-cast 1091% 1370% -10%

59. Comparison of mean stress
equations
59
=
Predicted life – actual life
Actual life
Absolute
error

60. 60
Comparison of mean stress
equations
Condition Surface Stulen
Topper-
Sandor
Walker
T6
Machined 47% 36% 33%
As-cast 44% 38% 40%
T7
Machined 40% 30% 34%
As-cast 48% 42% 45%

61. 61
Comparison of mean stress
equations
356-T7
Machined
No mean
stress
correction

62. 62
Comparison of mean stress
equations
356-T7
Machined
Goodman
correction

63. 63
Comparison of mean stress
equations
356-T7
Machined
ASME-
Elliptic
correction

64. 64
Comparison of mean stress
equations
356-T7
Machined
Walker
correction

65. 65
Mean stress sensitivity
parameters
Condition Surface Stulen
Topper-
Sandor
Walker
T6
Machined 0.417 0.793 0.530
As-cast 0.454 0.803 0.563
T7
Machined 0.341 0.734 0.459
As-cast 0.372 0.749 0.480

66. Mean stress sensitivity
66
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.
67
Mean stress sensitivity
This hypothesis will be tested in
future work.

68. 68
Conclusions

69. Conclusions
69
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
71
3. Ranking of effects on fatigue of lost foam
aluminum 356:
As-cast
surface
Porosity
Heat
treatment> >

72. 72
Conclusions
4. Lost foam 356-T6 has greater stress ratio
sensitivity than lost foam 356-T7.

73. 73
Future work

74. Future work
• Investigate effect of pattern fusion:
> 10 cm/s (“beady”)
1 – 10 cm/s (present work)
< 0.5 cm/s (smooth)
74

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

77. Future work
77
• Investigate effect
of chills on as-cast
properties of lost
foam castings

78. Future work
78
• Investigate other possible means of
improving properties of LF castings:
Vibration during solidification
Vacuum-assisted filling
Solidification under pressure

79. Future work
79
• Fully-reversed
four-point
bending fatigue
fixture

80. Future work
80
• 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!
81