PhD Dissertation Proposal Presentation

Ti-6Al-4V for Functionally Graded
Orthopedic Implant Applications

PhD Dissertation Proposal

Kyle Crosby

University of Connecticut - IMS 4/2/2013

 Motivation
◦ Biomedical implant issues
 Ti-6Al-4V Background
◦ Properties, processing and applications
 Proposed Research Objective
◦ Functionally graded orthopedic implant
 Proposed Methodology
◦ Powder processing and sintering techniques
 Preliminary Results and What’s Left To Do
◦ Characterization of powders and sintered bodies
 Conclusion
◦ Viability as an artificial biological component


 Biomedical implant
◦ Femoral stem, cup and ball socket
 Invasive operation
◦ Long, painfull rehab and recovery time
 Secondary surgery within 5-25 years to repair
or replace implant
◦ Fracture of metallic implants due to stress
concentration sites and contaminates (i.e. pores
and composition gradients)
◦ Implant loosening due to poor mechanical bonding
between metallic implant and native bone


 Metallic components
◦ Stainless steel, pure titanium, titanium alloys, Co-
Cr alloys
 Surface structures
◦ Porous surface network to allow for bone ingrowth
and improved mechanical interlocking
◦ Pores act as stress amplifiers
 Bioactive coatings
◦ Hydroxyapatite ceramic mimics natural bone
because HA is composed of mainly calcium
phosphate


 Ti-6Al-4V vitals
◦ Lightweight
 D = 4.45 g/cm3
◦ High strength
 σYS = 924 MPa
◦ Cheaper than pure Ti
◦ Corrosion resistant
◦ Biocompatible
◦ HCP @ R.T.


 Cast
◦ Sheet, barstock, ingot from a melt
 High cost to maintain melt temperature continuously
 Contamination of gases during melting and from crucible/mold during
pouring

 Forged
◦ Better mechanical properties
◦ Additional processing step = additional cost

 Machining
◦ Difficult due to high hardness

 Powder Metallurgy (PREP)
◦ Tight geometrical tolerances
◦ Sintering process is less expensive than casting with equivalent
mechanical properties


 Grade 5 α/β Alloy
◦ Aerospace
 Engine components
 Fasteners
◦ Marine
 Structural components
◦ Sporting goods
 Golf clubs, bicycles
◦ Jewelry
 Earrings, studs
◦ Biomedical
 Orthodontics
 Screws, pins, staples
 Total joint replacements


 Development of improved hip implant devices Ti-6Al-4V
through functionally graded Ti-6Al-4V + rich core
Hydroxyapatite composite components.
◦ Avoid implant loosening
◦ Avoid bioceramic spallation
◦ Avoid infection and secondary surgery

 Currently, co-sintering of Ti-6Al-4V +HA leads
to formation of oxide and phosphate phases
which have poor mechanical properties and
show adverse bioactivity

 Powder metallurgy and sintering studies are HA rich
conducted to reduce sintering temperature surface
below threshold where undesirable phases
form

 Slurry preparation and co-sintering of Ti+HA
Current Ti Functionally
using solid freeform fabrication technique
hip joint graded Ti/HA
hip joint


 Co-sintering of Ti-6Al-4V + HA components
requires the temperature to be reduced below
1000°C

◦ Thermodynamic alteration
 Diffusion mode shifted via particle size refinement and
increased defect concentration
 bulk lattice limited → grain boundary limited

◦ Kinetic alteration
 Rate of thermal energy application (°C/min)
 Tube furnace, MWS, and SPS densification methods


 Powder processing
◦ Particle size reduction
◦ Crystallite/grain size reduction
◦ Particle morphology changes

 Green body formation
◦ Uniaxial pressure effect
◦ Uniaxial pressing of as rec, 1 hr and 4 hr SPEX powders

 Sintering conditions
◦ Heat generation method, ramp rate, holding time
◦ Crystallite/grain size after sintering
◦ Relative density after sintering


½” stainless steel

¼” stainless steel

Ti-6Al-4V

Stearic acid

*Not to scale


200 µm 250 µm 100 µm

As received PREP 1 hr SPEX milled 4 hr SPEX milled
Avg. Dia. = 110 µm Avg. Dia. = 150 µm Avg. Dia. = 25 µm


10000

9000

8000

Ti 6-4 as rec powder
7000
Relative Intensity (a.u.)

Ti 6-4 SPEX 2wt%/1hr

6000 Ti 6-4 SPEX 3wt%/1hr

5000

4000 Ti 6-4 SPEX 4wt%/2hr

3000

2000
1000
20 30 40 50 60 70 80 90
2 Theta (°)


120

(1011) c 100
Ti 6-4 (002)

Ti 6-4 (101)
(0002)

Crystallite Size (nm)
80

60

40

a2
20

0
0 1 2 3 4 5
Milling Time (hr)

a3

a1


(1011) c 45
Ti 6-4 (002)
40
(0002) Ti 6-4 (101)
35

30

25

20
a2 15

10

5

0
2 3 4 5 6
a3 PCA Concentration (wt%)

a1


As received Ti-6Al-4V PREP powder 4 hr SPEX milled Ti-6Al-4V powder


300 MPa

As received PREP powder 1 hr SPEX milled 4 hr SPEX milled
5 wt% PEG binder no binder no binder
Dtheor = 50% Dtheor = 46% Dtheor = 58%


NO YES
Heavily oxided Light surface
throughout oxidation

Low mechanical Polishes to mirror
strength finish

Useless for load High density
bearing


 Tube furnace sintering (RHS)
◦ Radiant heating of green body, from outside
inward, through furnace atmosphere by electrical
resistance through molybdenum heating elements

 Spark plasma sintering
◦ Electrical resistance heating at contact point
between each powder particle in the green body

 Microwave sintering
◦ Dipole interaction of particle-pores within green
body with microwave radiation


HOT

COLD
V


 Sintering chamber
◦ Inert atmosphere
◦ Microwave transparent crucible (good dielectric)
◦ Particle-pore dipole interaction within green body

COLD

HOT


As received Sintered 2 hr Sintered 2 hr
300 MPa uniaxial @ 1100°C @ 1250°C
Dtheor = 50% Dtheor = 75% Dtheor = >97%


15000
Ti 6-4 as rec tube - 1250C/2hr

14000
Ti 6-4 as rec tube - 1100C/2hr

13000 Ti 6-4 as rec SPS - 1000C/3min

12000 Ti 6-4 SPEX 4/4 tube - 1250C/2hr
Relative Intensity (a.u.)

11000 Ti 6-4 SPEX 4/4 tube - 1100C/2hr

10000 Ti 6-4 SPEX 4/4 SPS - 1000C/3min

9000 Ti 6-4 SPEX 4/4 SPS - 600C/5min

8000 Ti 6-4 SPEX 4/4 MWS - 1250C/30min

Ti 6-4 SPEX 4/4 MWS - 900C/1hr
7000

Ti 6-4 SPEX 4/1 tube - 1250C/2hr
6000
20 30 40 50 60 70 80 90
2 Theta (°) Ti 6-4 SPEX 4/1 tube - 1100C/2hr


250

Ti 6-4 (002)
(1011) c 200 Ti 6-4 (101)
(0002)

150

100

a2
50

0
500 600 700 800 900 1000 1100 1200 1300
Sinteirng Temperature (C)
a3

a1


Ti 6Al-4V as rec. Ti 6Al-4V as SPEX (1hr/4wt%) Ti 6Al-4V as SPEX (4hr/4wt%)
1250°C/2hr → 90% dense 1250°C/2hr → 75% dense 1250°C/2hr → 97% dense

500 µm 500 µm 500 µm

500 µm 500 µm 500 µm
Ti 6Al-4V as rec. Ti 6Al-4V as SPEX Ti 6Al-4V as SPEX (4hr/4wt%)
1100°C/2hr → 78% dense (1hr/4wt%) 1100°C/2hr → 1100°C/2hr → 83% dense
60% dense


Ti-6Al-4V 4 hr SPEX, MWS at 900C Ti-6Al-4V 4 hr SPEX, MWS at 1250C
for 1 hr (95% center, 80% edge) for 30 min (98% center, 81% edge)


3
Ti 6Al-4V as Rec
2.5
Ti 6Al-4V as SPEX 1000°C

2
Displacement (mm)

1.5

1

0.5

0
0 100 200 300 400 500 600 700 800 900 1000
Temperature (°C)

Pressure Max Displacement
Displacement Density
Sample ID Decrease Onset Displacement Plateau Temp
Onset Temp (°C) (g/cm3)
Temp (°C) Temp (°C) (°C)
Ti 6Al-4V as
550 620 800 880 4.31
rec
Ti 6Al-4V as
350 350 600 750 4.27
SPEX 1000


(a) Ti-6Al-4V as (b) Ti-6Al-4V 4 hr SPEX (c) Ti-6Al-4V 4 hr
received, SPS @ 1000C as SPS @ 1000C for 3 SPEX as SPS @ 600C
for 3 min, 99% min, 99% for 5 min, 96%


 SPEX milled powder sinters @ lower temp than as received PREP powder
◦ Smaller particle size = shorter diffusion distance = shorter diffusion time

◦ Increase grain boundary area = rate limiting diffusion mechanism shift DL → Dg.b.
Dg.b. >> DL
 RHS to full theoretical density is possible
◦ Requires strict atmospheric control
◦ Heat penetration lag = longer sintering dwell times

 MWS offers lower temperature, faster sintering than RHS
◦ Pressureless sintering = complex geometry retention is possible
◦ Heat emination lag = porous surface regions

 SPS offers low temperature, rapid sintering
◦ Very high heating rates = rapid diffusion
◦ Uniaxial pressure from conductive die = complex geometry retention is difficult


 Activation energy measurement
◦ Track density as a function of sintering conditions

 Grain size analysis
◦ Monitor grain size as a function of sintering temperature

 Mechanical properties
◦ Compressive and tensile strengths
◦ Rockwell C and microhardness

 Biological properties
◦ Cell attachment (# per unit area)
◦ Cell spreading (lateral area coverage as a function of time in simulated
body fluid)

 Composite Ti-HA co-sintering studies
◦ SPS parameter optimization


2C/min 4C/min 8C/min

2C/min
700 700 700

800 800 800
150

Quench Temp (C)
Relative Density (%)

100 900 900 900

50 1000 1000 1000
0 1100 1100 1100
600 800 1000 1200
1250 1250 1250
Quench Temperature (C)

4C/min 8C/min
150
150

100
100
50
50
0
0
600 800 1000 1200
600 800 1000 1200


4 hr SPEX powder Sintering @ 900C Sintering @ 1250C

Grain size vs. mechanical strength ?
Grain size vs. activation energy barrier?
Grain size vs. cell attachment and spreading?


 Compressive strength
◦ Quasi-static using cross-head speed of 1,10,100
mm/min
◦ High strain rate using Split-Hopkinson Pressure Bar at
300, 900 s-1 frequency

 Tensile strength
◦ Strain until failure
◦ σYS , σUTS , E

 Rockwell C and microhardness
◦ Compare to as received powder, commercially cast or
forged products and between each other


(a) HA as SPS @ 1000C (b) Ti-6Al-4V 4 hr SPEX (c) Ti-6Al-4V 4 hr SPEX
for 3 min, 99% (90 vol%) + HA (10 vol%) (75 vol%) + HA (25 vol%)
as SPS @ 96% as SPS @ 1000C for 3
min, 83%


 Funding
◦ United States National Science Foundation contract CBET-
0930365

 Supervision
◦ Dr Leon Shaw

 Instrumentation
◦ Dr. Claude Estournes (SPS at CIRIMAT in France), Dr. Ashraf Imam
(MWS at NRL in D.C.), Jack Gromek (XRD), Roger Ristau and Lichun
Zhang (TEM/SEM), Bob Bouchard and Matt Bebee (SPEX vial and die
fabrication)

 Support
◦ Monica & Ling (HA synthesis and biostudies) and Girije Marathe
(quartz tube sealing) as well as the rest of my groupmates and
fellow grad students in MSE/IMS


 [1] Hennig, R., Lenosky, T., Trinkle, D., Rudin, S., Wilkins, J. "Classical potential describes martensitic
phase transformations between the alpha, beta, and omega titanium phases," Physical Review B,
78,054121, 2008.
 [2] Kubaschewski O., Wainwright C., and Kirby F.J., “The Heats of Formation in the Vanadium-Titanium-
Aluminium System,” J. Inst. Met., 89, 1960, 139-144.
 [3] Honma BERES 7 Series Driver, honmagolf.com, 2010.
 [4] Solid Grade 5 Titanium, TNG Body Jewlery, 2012.
 [5] Deutsch, A., “Bike review: Merlin works CR 6/4,” Brooklyn Arches Cycling, 2012.
 [6] Titanium dental implants, Naomi Dental, 2011
 [7] Total hip replacement, defectivejoints.com, 2011
 [8] Shaw, L., “Rapid prototyping of functionally graded orthopedic implants via the slurry mixing and
dispensing process,” NSF proposal submitted 2009.
 [9] What’s SPS, “Principles and mechanism of the SPS process,” Fuji Electronic Industrial Co.


?

 Armstrong starting powder
◦ Low apparent density (~5%)
◦ Sponge morphology with high SSA

 Slurry preparation
◦ Stable suspension
◦ Proper pH and viscosity

 Freeform 3D printing
◦ Tip diameter = slurry droplet size
 Lateral and layer thickness resolution

 Software development to allow for composition gradient from core to surface
◦ Must accommodate change in composition within each z-slice

 Mechanical and biological properties of functionally graded component
◦ Ti-HA composites via SPS

 Bioactivity as a function of surface roughness
◦ Cell # and lateral spreading as a function of SiC scratch width

 Surface functionalization
◦ Macropores filled with aerogel particles containing growth hormone or antibiotic


SiC paper

vs.

Ti-6Al-4V


Antibiotic or
pain relieving
aerogel capsules


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