Achieving Fully Equiaxed Microstructure in Additive Manufacturing of Ti-6Al-4V

Feasibility of Attaining Fully Equiaxed
Microstructure
through Process Variable Control
for Additive Manufacturing of Ti-6Al-4V
Sarah Kuntz, B.S. Mechanical Engineering, WSU
Advised by Dr. Nathan Klingbeil
Wright State University
Dayton, Ohio 45434
Supported by the National Science Foundation
Grant No. CMMI-1131266 & CMMI-1335196

Acknowledgements
• Thesis Committee
− Dr. Nathan Klingbeil
− Dr. Joy Gockel
− Dr. Raghu Srinivasan
• Additive Manufacturing Research Group (AMRG)
− Dr. Greg Loughnane
− Luke Sheridan
− Nate Levkulich
− Laura Gliebe
− Jason Beckman
2

Outline
• Background & Literature Review
• Thesis Contributions
• Modeling Approach
• Results & Discussion
• Summary & Future Work
3

What is Additive Manufacturing?
• 3-D Printing
• An alternative manufacturing technique
− Less weight
− Less wasted material
− Fewer pieces  increased strength / longer life
− * Easy to customize
• A challenge
− Thermal history determines microstructure
− Microstructure determines material properties
4
LENSTM Powder Fed Process
(Hofmeister, 1999)

Consistent & Desirable Microstructure
Mixed β grains
Gockel 2014, NASA Langley, EBF3
(modified Sciaky)
4
• Phase diagram
− β grains (form at solidification)
− α grains (from at β transus)
• Morphologies of interest: β grains
− Fully columnar
− Fully equiaxed
− Mixed  not desirable
• Have achieved
− Fully columnar & mixed
Goal:
Determine process variables
for fully equiaxed microstructure

Process Variable Control
6Inspired by Beuth et. al. 2013
• Process variables of interest
− Beam power
− Velocity
− Preheat temperature
• Commercial Processes Considered
− LENS
− Sciaky
− EOS
− Arcam
• How do process variables relate to
microstructure?

Relating Process Variables to Thermal
Conditions
• 3-D Rosenthal Solution (1946)
− Solves 3D Heat Transfer Equation
− Assumptions:
o Temperature independent material properties (c, ρ, k)
o Constant point heat source (Q)
o Constant, linear velocity (V) only in the x-direction
o Solid & semi-infinite substrate
− * Previous research suggests this is a good approximation (Bontha, 2003;
Davis, etc.)
− Solution is an equation for temperature as a function of distance from the heat
source
7
(Rosenthal, 1946; Bontha, 2006)
𝑇 − 𝑇0 =
𝛼𝑄
2𝜋𝑘
𝑒−λ𝑉x0
𝑒−λ𝑉𝑟
𝑟
, 𝑟 = 𝑥0
2
+ 𝑦0
2
+ 𝑧0
2

8
(Kuntz’s Summary of Bontha 2006)
- Working with dimensionless quantities
 Will make it easier to switch material
systems, size scale, etc.
- Relationship between P-V and thermal
conditions
 Need relationship between thermal
conditions and microstructure
𝑆𝑅 =
1
𝛻𝑇
𝜕𝑇
𝜕𝑡
Solidification Rate
Dimensionless Rosenthal Solution

Relating Thermal Conditions to
Solidification Microstructure
• Hunt’s Criterion Boundary Curves
− Originally for welding
− Divide thermal process space into microstructural regions…
− By plotting microstructure morphology boundaries in terms
of thermal conditions!
9
Original Curves (Hunt, 1984)
G-R Map in Ti64, Casting Samples (Kobryn, 2003)
𝑮 𝑹 < 𝟎. 𝟔𝟏𝟕𝑵 𝑶
𝟏
𝟑
𝟏 − ∆𝑻 𝑵
𝟑 𝑹𝑪 𝒐
𝑨
−𝟑
𝟐 𝑹𝑪 𝒐
𝑨
𝟏
𝟐
Equiaxed Boundary
𝑮 𝑹 > 𝟎. 𝟔𝟏𝟕 𝟏𝟎𝟎𝑵 𝑶
𝟏
𝟑 𝟏 − ∆𝑻 𝑵
𝟑 𝑹𝑪 𝒐
𝑨
−𝟑
𝟐 𝑹𝑪 𝒐
𝑨
𝟏
𝟐
Columnar Boundary

Relating Process Variables to Microstructure
• Changing process variables  various thermal conditions 3-D Rosenthal Solution
• Specific thermal conditions  solidification microstructure Hunt’s Criterion Curves
10Gockel 2014
Bontha 2006
*Power & Velocity  Microstructure:
Select a specific location for thermal conditions

Specific Location for Thermal Conditions
11
1
2
• Meet the melt pool
− Heat source & direction of motion
− Melt pool boundary / liquidus isotherm
− Trailing edge / solidification front
• Points of Interest
1: Top Surface
 Top of trailing edge
 Visible in-situ to co-axial cameras
 Have achieved equiaxed grains
2: Deepest Point
 Bottom of melt pool, aka “melt pool depth”
& “deepest point”
 Have not achieved equiaxed grains

Importance of the Deepest Point…
when adding a new layer
Why the look at the bottom of the melt pool?
−Because Additive Manufacturing adds layers!
− The top of the melt pool gets
re-melted…
− But the bottom won’t be “overwritten”
when the next layer is added!
− If we have equiaxed at the top when
we deposit the next layer…
12
Added Material Deposition
Use this Point!

If we have equiaxed at the top when we deposit the next layer…
− Equiaxed at the top will be absorbed into long columnar grains
UNLESS…
− Equiaxed grains at the bottom get in the way
− Also, if the bottom is equiaxed, the whole melt pool
will likely be equiaxed…
* Photos on next slide
13
Case 1: Equiaxed at top
Columnar at bottom
Case 2: Equiaxed at top
Equiaxed at bottom
Case 1: Columnar at bottom
Case 2: Equiaxed at bottom
when considering grain growth

Loughnane 2015
450 W, Thin Wall, 1 Bead, 4 sec pause
Gliebe 2015
14
• Grain propagation
− Equiaxed grains necessary to prevent columnar expansion
• Bontha’s work (2006)
− If deepest point is equiaxed, entire melt pool is equiaxed
Bloomin’
columnar grains!
Bontha 2006
when finding thermal conditions

Recap of Background & Literature Review
• Answered the questions:
− What is Additive Manufacturing?
− What are Process Variables?
− What is Fully Equiaxed Microstructure?
• Introduced the approach:
− Process Variables are related to Thermal Conditions (Rosenthal Solution)
− Thermal Conditions are related to Microstructure (Hunt’s Curves)
− To relate Process Variables directly to Microstructure, need to pick a location
− This location should be the Deepest Point in the melt pool!
15

Contributions
Analytic Model for thermal conditions at melt pool depth
Process Variable impact on Thermal Conditions
at melt pool surface & depth
Evaluation of Four Commercial Processes
Range of Process Variables for equiaxed grain growth
at melt pool depth
Examines impact of 4) on Melt Pool Dimensions
16
1
2
3
4
5

• 3-D Rosenthal provides equation for
temperature  Location of melt pool
• Take the derivative to find
− Thermal gradient, G
− Cooling rate
 Solidification rate, R:
• Problem:
− At any instant in time:
 Cooling rate equation equals zero
Thermal Conditions at Deepest Point
17
Moving Point Source Solution
Why is cooling
rate zero?

• Physical meaning:
− At a given instant in time, bottom
is transition between heating & cooling
• Artifact of the mathematics:
− Dimensionless cooling rate equals
x-component of thermal gradient
− At deepest point, x-component of
thermal gradient equals zero
* See next slide
18
Cooling Rate Equation Equals Zero at Depth

Artifact of the Mathematics…
At an instant in time, cooling rate might be zero…
− But cooling isn’t instantaneous!
19

T = TL
T = TS
Non-Instantaneous Cooling Rate
• Approximate derivative as finite difference
• Commonly done in FEA
* Can now find cooling rate
20

Contributions
at melt pool depth
21
1
2
3
4
5

22
10
-4
10
-3
10
-2
10
-1
10
0
10
1
10
2
10
3
10
0
10
1
10
2
10
3
10
4
10
5
10
6
Impact of Changing Power & Velocity
on Thermal Trends at Surface of Melt Pool
Solidification Rate (cm/s)
ThermalGradient(K/cm)
Columnar Grains
Equiaxed Grains
Mixed Morphology
10
-4
10
-3
10
-2
10
-1
10
0
10
1
10
2
10
3
10
0
10
1
10
2
10
3
10
4
10
5
10
6
on Thermal Trends at 99% of Melt Pool Depth
Columnar Grains
Equiaxed Grains
Mixed Morphology
G = 104.75
*R
50 W
100 W
500 W
1000 W
10000 W
50000 W
75000 W
100000 W
0.05 mm/s
0.5 mm/s
5 mm/s
10 mm/s
50 mm/s
100 mm/s
500 mm/s
1000 mm/s
At Melt Pool Surface At Melt Pool Depth
Changing Power and Velocity

23
10
-4
10
-3
10
-2
10
-1
10
0
10
1
10
2
10
3
10
0
10
1
10
2
10
3
10
4
10
5
10
6
Columnar Grains
Equiaxed Grains
Mixed Morphology
10
-4
10
-3
10
-2
10
-1
10
0
10
1
10
2
10
3
10
0
10
1
10
2
10
3
10
4
10
5
10
6
Columnar Grains
Equiaxed Grains
Mixed Morphology
G = 104.75
*R
50 W
100 W
500 W
1000 W
10000 W
50000 W
75000 W
100000 W
0.05 mm/s
0.5 mm/s
5 mm/s
10 mm/s
50 mm/s
100 mm/s
500 mm/s
1000 mm/s
At Surface At Depth
Comparison with Prior Work
Bontha 2006
* General behavior is consistent!
However…

• Powers: 50 W to 100 kW
• Velocities: 0.05 mm/s to 1 m/s
• Preheat: none
• Prediction: Not Fully Equiaxed
*Remember, one of the four
commercial processes had a preheat
temperature…
24
10
-4
10
-3
10
-2
10
-1
10
0
10
1
10
2
10
3
10
0
10
1
10
2
10
3
10
4
10
5
10
6
Columnar Grains
Equiaxed Grains
Mixed Morphology
G = 104.75
*R
50 W
100 W
500 W
1000 W
10000 W
50000 W
75000 W
100000 W
0.05 mm/s
0.5 mm/s
5 mm/s
10 mm/s
50 mm/s
100 mm/s
500 mm/s
1000 mm/s
At Melt Pool Depth
Impact of Power and Velocity

10
-4
10
-3
10
-2
10
-1
10
0
10
1
10
2
10
3
10
0
10
1
10
2
10
3
10
4
10
5
10
6
Impact of Changing Pre-Heat & Velocity
Columnar Grains
Equiaxed Grains
Mixed Morphology
25o
C
100
o
C
500o
C
850o
C
1000o
C
1500o
C
0.05 mm/s
0.5 mm/s
5 mm/s
10 mm/s
50 mm/s
100 mm/s
500 mm/s
1000 mm/s
10
-4
10
-3
10
-2
10
-1
10
0
10
1
10
2
10
3
10
0
10
1
10
2
10
3
10
4
10
5
10
6
Impact of Changing Pre-Heat & Velocity
Columnar Grains
Equiaxed Grains
Mixed Morphology
25
At Melt Pool Surface At Melt Pool Depth _
Changing Preheat and Velocity

Contributions
at melt pool depth
26
1
2
3
4
5

10
-4
10
-3
10
-2
10
-1
10
0
10
1
10
2
10
3
10
0
10
1
10
2
10
3
10
4
10
5
10
6
Arcam
(750o
C pre-heat)
Arcam range
(no pre-heat)
EOS
Sciaky
LENS
Morphology Prediction:
Four Commercial Processes
Columnar Grains
Equiaxed Grains
Mixed Morphology
Results for Four Commercial Processes
• Prediction: Not Equiaxed
• LENS, Sciaky & EOS
− No preheat
• Arcam
− Preheat is too small
* Sciaky gets closest
− Very high powers
− Low velocities
27
Sciaky: 1 – 40 kW, 0.04 – 42 mm/s
LENS: 100 – 500 W, 0.04 – 42 mm/s
EOS: 50 – 500 W, 42 – 1060 mm/s
Arcam: 50 – 2000 W, 42 – 1060 mm/s
Arcam: 50 – 2000 W, 42 – 1060 mm/s
Closest to boundary

-3.5 -3 -2.5 -2 -1.5 -1 -0.5 0 0.5
-1
-0.9
-0.8
-0.7
-0.6
-0.5
-0.4
-0.3
-0.2
-0.1
0
x-position (cm)
z-position(cm)
Melt Pool Contours for Representative Cases
Arcam, 750o
C
Arcam, 25o
C
LENS
EOS
Sciaky
Representative Melt Pool for Each Process
28
Representative Points
10 kW, 4.23 mm/s (Sciaky)
500 W, 4.23 mm/s (LENS)
100 W, 635 mm/s (EOS)
1000 W, 635 mm/s (Arcam)
1000 W, 635 mm/s (Arcam)-0.6 -0.5 -0.4 -0.3 -0.2 -0.1 0 0.1
-0.14
-0.12
-0.1
-0.08
-0.06
-0.04
-0.02
0
x-position (cm)
z-position(cm)
Melt Pool Contours for Representative Cases
Arcam, 750o
C
Arcam, 25
o
C
LENS
EOS
Melt Pool Contours: Zoomed View
* Prediction:
Since Sciaky gets closest, fully
equiaxed melt pool will be large
(cm-size scale)

Contributions
at melt pool depth
29
1
2
3
4
5

10
-4
10
-3
10
-2
10
-1
10
0
10
1
10
2
10
3
10
0
10
1
10
2
10
3
10
4
10
5
10
6
Columnar Grains
Equiaxed Grains
Mixed Morphology
50 W
100 W
500 W
1000 W
10000 W
50000 W
75000 W
100000 W
0.05 mm/s
0.5 mm/s
5 mm/s
10 mm/s
50 mm/s
100 mm/s
500 mm/s
1000 mm/s
10
-2
10
-1
10
2
10
3
Process Variables for Equiaxed
The Process
• Consider a range of
Powers & Velocities
at various preheat
temperatures:
• 750oC, 850oC, 1000oC
1100oC, 1200oC, 1300oC
• Consider more P-V
combinations at and around
1300oC…
30

• A high preheat temperature is
necessary… but how high?
− Depends on power & velocity
• For a ~75% Melt Temp. preheat:
− Power: 50 – 100 kW
− Velocity: 4 – 8 mm/s
− Power: 50 W – 100 kW
− Velocity: 0.2 mm/s – 4 m/s
31
Process Variables for Equiaxed
0
10
1
10
2
1550 C
1500 C
1475 C
1450 C
1425 C
1400 C
1350 C
1300 C
1250 C
50 W
94% Tm
50 kW
76% Tm
0.2 mm/s
4 m/s

Contributions
at melt pool depth
32
1
2
3
4
5

33
Melt Pool Dimensions: Sciaky-Size?
* Prediction: Since Sciaky gets closest, fully equiaxed melt pool will
be large (cm-size scale)  FALSE

− Power: 50 – 100 kW
− Trailing Edge Length: 35 – 71 cm
− Power: 50 W – 100 kW
− Trailing Edge Length: 1.5 mm to
2.80 m
• Trailing Edge length depends
on absorbed power & preheat
34
Melt Pool Length for Equiaxed
0
10
1
10
2
1550 C
1500 C
1475 C
1450 C
1425 C
1400 C
1350 C
1300 C
1250 C
Constant Velocity
Constant Power
Increasing Velocity
2.8 m
35-70 cm
1.5 mm

− Power: 50 – 100 kW
− Melt Pool Depth: 4.2 – 5.0 cm
− Power: 50 W – 100 kW
− Melt Pool Depth: 0.8 mm to 14 cm
• Melt pool depth depends on
power, velocity & preheat
35
Melt Pool Depth for Equiaxed
0
10
1
10
2
1550 C
1500 C
1475 C
1450 C
1425 C
1400 C
1350 C
1300 C
1250 C
Constant Velocity Constant Power
0.8 mm
4.2-5 cm
14 cm

• If all equiaxed melt pools were huge…
− Equiaxed would not be attainable for small scale applications
− Achieving equiaxed would essentially require melting the substrate
(i.e. making a casting)
• Melt pools with preheats between 75-95% of Melt Temp.
Can be large… but don’t have to be!
• Near melt temperature preheats
− Do not correspond to melting the entire substrate
− Allow for equiaxed grain growth at melt pool depth
for a wide range of powers and velocities
36
Melt Pool Size for Equiaxed

Contributions
at melt pool depth
37
1
2
3
4
5

Summary
• Microstructure:
− Want either fully columnar or fully equiaxed
− Have not yet obtained fully equiaxed
• Modeling:
− Bontha’s analytic model accurately describes thermal trends
− A time-dependent element is added to cooling rate to describe melt pool depth
• Impact of Process Variables:
− Thermal conditions respond differently at melt pool surface & depth
− Equiaxed is not feasible at depth without an added preheat
• Evaluation of Commercial Processes:
− None are expected to produce fully equiaxed microstructure
38

• Fully equiaxed microstructure is attainable through process
variable control
• A substrate preheat of at least 75% of the melt temperature is
required for fully equiaxed microstructure
• Melt pools created using near-melt-temperature preheats are not
necessarily large (centimeter scale)
• No commercially existing processes are capable of producing
fully equiaxed microstructure because none have near-melt-
temperature preheats
39
Conclusions

• Finite Element Modeling
− Take into account latent heat effects & temperature dependence of
properties
− Fine-tune numeric predictions
− Impact of added material, more complex geometries, etc.
• Feasibility of Implementing near-melt-temperature preheats
(1250oC+)
− Can this be done? If so, how?
− Are other methods of obtaining fully equiaxed less difficult?
• Explore relationship between thermal conditions at surface and
depth of melt pool
40
Future Work

Dimensionless Relationships:
Melt Pool Surface & Depth
42
-8 -6 -4 -2 0 2 4 6 8
-0.05
0
0.05
0.1
0.15
0.2
0.25
0.3
Ln(Tmbar)
SolidificationRateRatio(SR
depth
/SR
surf
)
Relationship Between Solidifcation Rate Ratio &
Dimensionless Temperature
Rosenthal Values
Initial Curve Fit
Curve Fit Equation
r2 is 0.9953
-8 -6 -4 -2 0 2 4 6 8
0
5
10
15
20
25
30
35
40
45
50
Ln(Tmbar)
ThermalGradientRatio(Gdepth
/Gsurf
)
Relationship Between Thermal Gradient Ratio &
Dimensionless Temperature
Rosenthal Values
Exponential Curve Fit
Curve Fit Equation
r2 is 0.9981

10
-3
10
-2
10
-1
10
0
10
1
10
0
10
1
10
2
10
3
10
4
10
5
Fully Columnar
Fully Equiaxed
Mixed
Solidification Map, G vs. R: Hunt's Criterion Curves
Solidification Rate, R (cm/s)
ThermalGradient,G(K/cm)
Columnar Check Points
Equiaxed Check Points
Hunt's Columnar Criterion
Hunt's Equiaxed Criterion
Hunt’s Curve Re-creation for Ti64
43
G-R Map in Ti64 (Kobryn, Brown, 2003, Bontha, Gockel)
Kuntz (2015)

Model Verification (Const. CR, Ti64)
Recreation of High Power Plot Previously Published Plot
44
(Bontha, 2006)

Average Solidification Cooling Rate
• Based on FEA cooling rate extraction
45
-0.04 -0.035 -0.03 -0.025 -0.02 -0.015 -0.01 -0.005 0
-0.035
-0.03
-0.025
-0.02
-0.015
-0.01
-0.005
0
x-position (cm)
z-position(cm)
Depth vs Solidification Location: P=325W, v=8.47mm/s
Liquidus Isotherm
Solididus Isotherm
Liquidus 10 pts
Solidus 10 pts
Liquidus Depth
Solidus at Liquidus Depth
10
-3
10
-2
10
-1
10
0
10
1
10
2
10
3
10
0
10
1
10
2
10
3
10
4
10
5
10
6
Solidification Rate, R (cm/s)ThermalGradient,G(K/cm)
Average Cooling Rate Verification: P=325W, v=8.47mm/s, ND=0.1512
Columnar Boundary
Equiaxed Boundary
Instantaneous Liquidus: All Depth Points
Instantaneous Liquidus: 10 pts
Approximated: 10 pts
Instantaneous Liquidus at Depth
Solidus at Liquidus Depth
Approximated at Liquidus Depth

Achieving Fully Equiaxed Microstructure in Additive Manufacturing of Ti-6Al-4V

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