Competitive Growth in Spiral Grain Selector
during Investment Casting of Single-Crystal
Gas Turbine Blades
Hongbiao Dong, Manish Javahar, Huijuan Dai
Department of Engineering, University of Leicester
Neil D’Souza Precision Casting Facility, Rolls-Royce Plc.
PARSONS 2011, 5-8 September 2011, Portsmouth, UK
Parsons 2011, 5-8 September 2011, Portsmouth UK 2
Outlines
Introduction
Numerical Modelling - Heat Transfer Analysis
and Microstructure Modelling
Experiments - Casting & EBSD Analyses
What we found and what we propose
Parsons 2011, 5-8 September 2011, Portsmouth UK 3
Grain Selection in Practice
Ceramic Mould
Molten
Metal
Water
Cooled Chill
Radiation
Cooling
Radiation
Heating
Columnar
Starter Block
V
Spiral Grain
Selector
Turbine
Blade
Spiral Grain
Selector
Starter
Block
R.C. Reed, ‘The Superalloys Fundamental and Applications’, Cambridge University Press, 2006
Parsons 2011, 5-8 September 2011, Portsmouth UK 4
Prior Study
Spiral Designs
In practice: There is no 'standard practice' for grain selection;
the design of the selector rely upon trial-and-error based optimisation
In Practice
Angled Spiral (Helix) Restrictor
Parsons 2011, 5-8 September 2011, Portsmouth UK 5
Motivation of Our Study
What are the physical processes occurring in the grain
selector?
What are the optimum dimensions and geometry of the
grain selector?
Can numerical modelling be used to answer the above
questions?
Can we move away from a purely empirical choice of
grain selector to one which is designed and optimised?
Simulation
Heat Transfer
&
Microstructure
Parsons 2011, 5-8 September 2011, Portsmouth UK
Parsons 2011, 5-8 September 2011, Portsmouth UK 7
Thermal Analysis (VeriCast Model)
Investment Furnace Turbine Blade Grain Selector
VeriCast Blade
8
0 50 100 150 200 250 300 350 4001250
1300
1350
1400
1450
1500
Tem
pera
ture
Time (s)
30mm
22.5mm
15mm
7.5mm
0mm
Thermal Analysis
P. Carter, D.C. Cox, C.A. Gandin, R.C. Reed,
Mater. Sci. Eng. A280, 2000, 233-246
(˚C
)
ExperimentSimulation
Parsons 2011, 5-8 September 2011, Portsmouth UK
Parsons 2011, 5-8 September 2011, Portsmouth UK 9
Grain Structure Evolution
Mixed Colours
G <001>
Parsons 2011, 5-8 September 2011, Portsmouth UK 10
Grain Structure Evolution
Stochastic Study
Linear Colours
G
Grain
Deviation
<001>
Parsons 2011, 5-8 September 2011, Portsmouth UK 11
the Role of Spiral?
Starter
Block
Spiral
10
3738
29
43
G
Grain
Deviation
2
5
Distance from the chill (mm)
0
5
10
15
20
25
30
1 2 3 4 5 6 7
Avera
ge G
rain
Devia
tio
n
2 5 10 29 37 38 432 5 10 29 37 38 43
Starter Block Spiral
<001>
Parsons 2011, 5-8 September 2011, Portsmouth UK 12
Quantitative Description of Spiral Size and Geometry
T
Case 1 2 3 4 5 6 7
d T (mm) 2 3 4 5 6 7 8
d S (mm) 12 13 14 15 16 17 18
d C (mm)
d B (mm)
L P (mm)
20
10.5
10
(dC= dS – T)
dS
θ
Case 8 9 10
d T (mm)
d S (mm) 13 15 17
d B (mm)
L P (mm)
5
20
10.5
Case 11 12 13 14 15 16 17 18
d T (mm)
d S (mm)
θ 20 25 28 29 35 40 50 70
d B (mm)
L P (mm) 7.5 9.3 10.5 11.2 14 17 24 55
5
15
20
q
dB
dS
T
LP
q = LP/2(dS-T)
Parsons 2011, 5-8 September 2011, Portsmouth UK 13
bc
d
e
f(d1) (e1) (f1)(b1) (c1)
(d2) (e2) (f2)(b2) (c2)(a)
A
B
C
A
D
Grain Structure Evolution
Pole Figures Along Sample <001>
Grain Selection in Spiral Selector
Parsons 2011, 5-8 September 2011, Portsmouth UK 14
Criteria for Determining Spiral Efficiency
SX Height
SX Orientation
%100
15 30 40 550 15 30 40 550
27.8˚
Degrees
Parsons 2011, 5-8 September 2011, Portsmouth UK 15
2 3 4 5 6 7 80
10
20
30
40
50
SX
Ori
enta
tio
n
dT (mm)
2 3 4 5 6 7 80
2
4
6
8
10
12
14
16
18
20
22
SX
Hei
gh
t (m
m)
dT (mm)
No apparent correlation
between SX orientation and
spiral thickness
SX height INCREASES
with larger spiral thickness
Effect of Spiral Thickness (T)
T
Parsons 2011, 5-8 September 2011, Portsmouth UK 16
13 14 15 16 175
10
15
20
25
30
35
40
45
50
SX
Ori
enta
tion
dS (mm)
13 14 15 16 1713
14
15
16
17
18
19
20
21
SX
Heig
ht
(mm
)
dS (mm)
SX height DECREASES
with larger spiral diameter
No apparent correlation
between SX orientation and
spiral diameter
Effect of Spiral Diameter (dS)
LARGER
Spiral Diameter
dS
Parsons 2011, 5-8 September 2011, Portsmouth UK 17
20 30 40 50 60 70
1012141618202224262830323436384042
SX
Hei
gh
t (m
m)
Take-off Angle
20 30 40 50 60 705
10
15
20
25
30
35
40
45
SX
Ori
enta
tio
ns
Take-off Angle
Effect of Spiral Take-off Angle θ
SX height INCREASES
with larger take-off angle
No apparent correlation
between SX orientation
and take-off angle
SMALLER
Take-off angle
Experiments
Casting
&
Grain Structure
Parsons 2011, 5-8 September 2011, Portsmouth UK
Parsons 2011, 5-8 September 2011, Portsmouth UK 19
Case 13
Experiments
Case 17 Case 18 Cylinder bar
Location where SX
structure occurs
(28˚) (50˚) (70˚)
Required SX Height
INCREASES
with larger take-off angle
Parsons 2011, 5-8 September 2011, Portsmouth UK 20
0 10 20 30 40 50 60 70 800
10
20
30
40
50
Take-off Angle θ (° )
SX
Heig
ht
(mm
)Simulations
Experiments
Efficiency of Grain Selection in Spiral
Spiral becomes more efficient with a smaller take-off angle
Parsons 2011, 5-8 September 2011, Portsmouth UK 21
22°
EBSD Grain Structure Map
Inverse Pole Figures along sample <001>
b
c
d
e
(a) Case 17
(d) (e)(b) (c)
Grain Orientation
Geometrical Blocking
Mechanism ?
Parsons 2011, 5-8 September 2011, Portsmouth UK 22
Mechanism - Geometrical Blocking?
Grain selection depends on the geometry of the spiral
R.C. Reed, ‘The Superalloys Fundamental and Applications’,
Cambridge University Press, 2006, p133
Parsons 2011, 5-8 September 2011, Portsmouth UK 23
Mechanism - Geometrical Blocking?
Shorten LP Increase q
Grain A & B survive Grain A & B survive
Only grain A survives
Grain selection depends on the geometry of the spiral
Parsons 2011, 5-8 September 2011, Portsmouth UK 24
What We Found
The Role of Cylindrical Base - Majority of the selection of grain orientations occurs in the cylindrical base and only a couple of grains with well aligned orientations can grow into the spiral.
The Role of Spiral – randomly select one grain via Geometrical Blocking Mechanism with no optimisation on grain orientations
An quantitative description of the effect of the spiral on grain selection. The spiral becomes more efficient with:
SMALLER Spiral Thickness; LARGER Spiral Diameter; SMALLER
Take-off Angle.
What We Propose – 2D Selector?
dS
tanq =LP / 2 (dS-dW)
Take-off Angle (q)
3D 2D
dW
LP
dW
dS
LP
Fully automated casting / wax-
assembly production line?
Parsons 2011, 5-8 September 2011, Portsmouth UK 26
Acknowledgements
• modelling work:
Dr. Jean-Christophe Gebelin, Prof. Roger Reed, University of
Birmingham and Dr. Paul Brown, Rolls-Royce.
• casting experiments
• Dr. Yizhou Zhou, Prof. Nick Green, University of Birmingham
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