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Cast Titanium Alloys for Dental Applications Marie Koike and Toru Okabe

Baylor College of Dentistry Texas A&M Health Science Center

Dallas, TX, USA ABSTRACT

In view of the proven, excellent biocompatibility of titanium, this metal has been considered as a potential metal for biomedical applications. A research group at Baylor College of Dentistry has formulated experimental titanium alloys to evaluate various characteristics in search of good candidate alloys for dental prosthetic use. By adapting applicable technologies from the titanium casting industry, the evolution of casting the casting machines and investment materials specifically made for dental casting has greatly advanced during last 20 years. It appears that dental titanium casting has now almost reached the stage where its practical applications should seriously be assessed. This presentation reports our on-going research project in which we have tested the casting performance, mechanical properties, grindability, wear resistance, electrochemical behavior and biocompatibility. The alloys we tested were biocompatible experimental titanium alloys with a low fusion temperature and also some industrial titanium alloys as well. INTRODUCTION Beta (β) titanium alloys offer an effective combination of high strength and low modulus, two characteristics needed for biomedical titanium alloys. These alloys may be used after heat treatment to improve their mechanical properties. Examples of elements used to make β alloys are Nb, Ta, Mo, and Zr.1 However, metals with these elements are not easy to melt and cast because they have a higher melting point than titanium, and they are more expensive. We have made alloys using low-melting, simple metals such as Al, Si and Sn and transition metals like Fe, Co, Cr, Cu, Hf, Mn, and Zr because many of these alloys are inexpensive, biocompatible and are easy to cast into fixed and removable dental prostheses. Studies of such titanium alloys for dental use

are not new. Since trials on casting titanium crowns were done by Waterstrat at the U.S. National Institute of Standards and Technology), research groups at Kyoto University, Tokyo Medical and Dental University and Northwestern University followed with studies of titanium casting. They examined the mechanical properties of titanium alloys for medical and dental applications. As time went on, all the technology of casting titanium advanced considerably. Using all the currently available materials and equipment, we began our research in evaluating cast titanium not only for their strength properties but also for other characteristics required for dental applications. This paper summarizes our progress and findings at characterizing various properties of cast titanium alloys. SCREENING TITANIUM ALLOYS for DENTAL APPLIANCES

Since most dental appliances must withstand forces in service during mastication, dental alloys must meet minimum mechanical properties requirements. ISO Standard 22694 for metallic materials for fixed and removable appliances classifies dental alloys into 5 types with minimum mechanical properties requirements: 80 MPa (Type 1) to 500 MPa (Type 5) for yield strength; from 18% (Type 1) to 2% (Type 5) for elongation, and 150 GPa (only Type 5) for modulus of elasticity. On the other hand, the yield strength, tensile strength, elongation, and elastic modulus of as-cast commercial dental casting alloys currently available on the U.S. market range from 490-1060 MPa, 220-1145 MPa, 1.5-35%, and 86-203 GPa, respectively.2 The yield strength and tensile strength in particular are much higher than those specified in the ISO standard. Thus, we set our own standard for candidate alloys and proceeded with research on titanium alloy development. We also set screening criteria for corrosion and cytotoxicity, mold

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filling capacity (castability), grindability and wear resistance. MECHANICAL PROPERTIES

The tensile test was performed for commercial and experimental alloys which were cast with a MgO-based investment (Selevest CB, Selec, Japan) using a casting machine (Ticast Super R, Selec). The yield strengths and elongation ranged from 200 (pure Ti) to 930 MPa and 1 to 15% (pure Ti), respectively. The Ti-Al-Fe and Ti-Al-Cu alloys exhibited a good combination of yield strength and elongation. MOLD FILLING CAPACIITY (CASTABILITY)

We found the best way for screening how well an alloy fills a mold was to use a wedge-shaped cavity with 15° and 30° angles.3 The mold filing capacity is a measure of the extent to which the metals fills the mold, which is determined from the gap (μm) between the tip of the casting and the theoretical acute tip of the triangle in a photograph of the cross section of each casting. There were no significant differences for all the cast titanium tested in the 30° wedge angle. However, the mold filling of the 1% and 4% Cu alloys was significantly different in the 15° wedge angle (p<0.05). Their poor liquid fluidity was responsible for the results. GRINDABILITY

The grindability and machinability of titanium is generally considered to be poor compared to many common metals and alloys.4 However, we found that a much better grindability than is typical for pure titanium can be attained by carefully selecting the alloying elements. Grindability of the cast alloys was examined by grinding titanium specimens with a SiC wheel at the rotational speeds of 500-1,250 m/min (circumferential speed) at 100 gf.5 Some particular differences in grindability were found between types of metals. There were some trends in terms of the microstructures and existing phases of the alloys. Chan et al. derived an expression of grindability in terms of the tensile strength and plastic strain at fracture or tensile ductility.6

The model predictions indicated that the grindability of the titanium alloys increases with grinding speed but increases with decreasing fracture toughness or tensile ductility. Evidently the grindability is favorable when the metals have low ductility and a multiphase structure (α+β) and/or eutectoid structure.7 Thus, highly ductile metals, such as pure titanium and some β alloys (e.g. single-phase Ti-15V-3Cr-3Sn-3Al and the Ti-Cr alloys) do not have good grindability. WEAR BEHABIOR

Titanium is noted for its poor tribological characteristics.4 Our investigation found that wear is similar to grindability in that the wear resistance of titanium is improved by alloying. A two-body wear testing apparatus that simulates chewing action was used to measure the amount of wear of cast titanium teeth.8 These ductile alloys had inferior wear resistance. On the other hand, the metals with two-phase α+β microstructure had better wear resistance. Alloying Cu was effective at improving the wear resistance of the experimental alloys. These results indicate that the less ductile α+β alloys and the alloys having a eutectoid microstructure showed improved wear. We think that any microstructural modifications restricting plastic deformation can improve the wear characteristics. The wear behavior was analyzed using the elastic-plastic fracture of individual alloys in response to the relevant contact stress field.9 Model predictions suggested that the wear of titanium alloys increased with greater hardness but increases with reduced fracture toughness or tensile ductility. CORROSION BEHAVIOR and BIOCOMPATIBILITY

It is well known that pure titanium has extremely low toxicity and is well tolerated by both bone and tissue.10 The effect of alloying is an important consideration for the corrosion characteristics and cytotoxicity of titanium. It is possible that adding alloying elements may compromise the outstanding corrosion behavior and cytotoxicity. We examined many of our cast experimental alloys using a dynamic

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polarization method in artificial saliva to determine open circuit potential (OCP) value, polarization or corrosion resistance, corrosion current density and passivation current density.11 No unusual activity was observed in the anodic polarization with the normal intraoral potential (-58 to 212 mV).12 The cytotoxicity of selected alloys was evaluated by the MTT method using CP Ti, Ti-6Al-4V and Teflon® as controls, according to ISO 10993-5. The cytotoxicity of some selected titanium alloys expressed as values normalized to the Teflon® control ranged from 82% to 96%.13 FRACTURE TOUGHNESS and FATIGUE

Our studies of the fracture toughness and fatigue resistance of our cast experimental alloys are just getting underway. The fracture toughness of alloys depends on their alloy composition and microstructure. High fracture toughness and tensile ductility produce poor grindability, so the alloy composition and microstructure must be carefully chosen to produce a combination of good grindability, high strength, fracture toughness, wear resistance and tensile ductility. On the other hand, the surfaces of small dental titanium devices are generally polished and smooth. The fatigue life will mostly be spent in crack initiation, and due to this small size, the fatigue cracks will easily reach critical size. The number of fatigue cycles to failure was considered to be the fatigue life. We have just begun investigating the fracture toughness and fatigue behavior of cast titanium alloys. CONCLUSION

Some characteristics of inexpensive titanium alloys have been evaluated with the goal of developing alloys to be used to cast dental prostheses. These alloys show promise: they are easy to cast and they have good corrosion resistance and biocompatibility. In the future, studies must be done on other properties needing improvement, such as casting efficiency and accuracy, bonding ability to porcelain, and machinability for CAD/CAM applications. The potential use of these alloys to fabricate fixed bridges and removable partial dentures including

superstructures for dental implants looks encouraging. ACKNOWLEDGEMENTS

The authors appreciate the efforts of the dedicated research collaborators who have contributed to the above findings. They also appreciate the editorial assistance by Mrs. Darla Benson. This study was partially funded by the U.S. National Institutes of Health/National Institute of Dental and Craniofacial Research grant DE11787. REFERENCES 1. Niinomi M, Akahori T, Takeuchi T, Katsura

S, Fukui H, Toda H (2005). Mechanical properties and cytotoxicity of new beta type titanium alloys with low melting points for dental applications. Mater Sci and Eng C 25:417-425.

2. O'Brien WJ (2002). Dental Materials and Their Selection, 3rd ed. Chicago: Quintessence Pub. Co., Inc.

3. Shimizu H, Habu T, Takada Y, Watanabe K, Okuno O, Okabe T (2002). Mold filling of titanium alloys in two different wedge-shaped molds. Biomat 23:2275-81.

4. Donachie MJ (2000). Titanium: A Technical Guide. Materials Park, OH: ASM International.

5. Koike M, Itoh M, Okuno O, Kimura K, Takeda O, Okabe TH, Okabe T (2005). Evaluation of Ti-Cr-Cu alloys for dental applications. J Mater Eng Perf 14:778-783.

6. Chan KS, Koike M, Okabe T (2006). Grindability of Ti alloys. Metall Mater Trans 87A:1323-1331.

7. Okabe T, Kikuchi M, Ohkubo C, Koike M, Okuno O, Oda Y (2004). Improving grindability and wear resistance of titanium alloys. In: FH Froes, M Ashraf Imam, D Fray, eds., Cost-affordable titanium symposium dedicated to Professor Harvey Flower, TMS.

8. Ohkubo C, Shimura I, Aoki T, Hanatani S, Hosoi T, Okabe (2002). In vitro wear assessment of titanium alloy teeth. J Prosthod 11:263-269.

9. Chan KW, Koike M, Okabe T (2007). Modeling wear of cast Ti alloys. Acta Biomaterialia, 3: 383-389.

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10. Steinemann SG (1980). Corrosion of surgical implants - in vivo and in vitro tests. Winter GD, Leroy JL, deGroot K, eds. NY: John Wiley & Sons, pp. 1-34.

11. Koike M, Cai Z, Fujii H, Brezner M, Okabe T (2003). Corrosion behavior of cast titanium with reduced surface reaction layer made by a face-coating method. Biomat 24:4541-4549.

12. Ewers GJ, Greener EH (1985). The electrochemical activity of the oral cavity - a new approach. J Oral Rehabil 12:469-476.

13. Koike M, Lockwood PC, Wataha JC, Okabe T (2007). Initial cytotoxicity of novel titanium alloys. J Biomed Mater 83B: 327-331, 2007.

CONTACT Marie Koike, D.D.S, Ph.D. Assistant Professor Dept. of Biomaterials Science Baylor College of Dentistry Texas A&M University System Health Science Center 3302 Gaston Ave. Dallas, TX 75246 e-mail: mkoike@bcd.tamhsc.edu PH: 214-370-7005 FAX: 214-370-7001

Cast Titanium Alloys forCast Titanium Alloys for Dental Applicationspp

Mari Koike and Toru Okabe

Baylor College of DentistryBaylor College of DentistryTexas A&M Health Science Center

Dallas, TX, USA

1

IntroductionIntroduction• Titanium – a potential metal for biomedicalp

applications

• Formulated binary and ternary titanium alloyswith inexpensive and biocompatible elements toevaluate their potential as a dental casting alloyevaluate their potential as a dental casting alloy

• Tested casting performance electrochemical• Tested casting performance, electrochemicalbehavior, biocompatibility, grindability, wearresistance, capacity for porcelain veneering

2

Alloy DevelopmentAlloy Development• Traditional dental alloys:Traditional dental alloys:

– noble and precious alloys (Au, Pd, Ag)– non-precious base metal alloys (Co-Cr, Ni-Cr alloys)p y ( y )

• Biomedical alloys must have:– high corrosion resistance– biocompatibility– a minimal release of ions

• The importance of titanium and titanium alloys in biomedical applications continues to increase

3

ISO Criteria vs. Properties RangesISO Criteria vs. Properties RangesISO (22694) Criteria vs. Properties Ranges

of Commercial Dental Casting Alloysof Commercial Dental Casting Alloys

ISO Criteria Commercial Alloys

YS80 MPa (Type 1)

– 500 MPa (Type 5)490-1060 MPa

TS – 220-1145 MPa

18% (Type 1), 10% (Type 2), El

( yp ) ( yp )5% (Type 3), 2% (Types

4/5)1.5 – 35%

4

E 150 MPa (only Type 5) 86-203 MPa

Minimum Criteria for Screening Alloys

Yield Strength 500 MPaInitial screening

gTensile Strength 580 MPaElongation 2%

Yield Strength 750 MPaScreening based on application

Fixed Bridgese d S e g 50 a

Tensile Strength 800 MPaElongation 5%

Removable Partial

Yield Strength 750 MPaTensile Strength 800 MPa

5DenturesElongation 2%

Alloys TestedCommercial

AlloysExperimental

Alloys

Commercial Dental Ally y Alloys

Pure Ti

Commercially Pure (CP Ti)Ti-6Al-4V (α + β)

Ti-CuTi64-Cu

Ney-Oro B-2(Au-Ag-Cu-Pd)( β)

Ti-6Al-4V ELI (α + β)Ti-6Al-7Nb (α + β)Super TIX800® (Fe-O-N)

Ti-CrTi-Cr-CuTi-Hf

( g )Vitallium(Co-Cr-Mo)Talladium VSuper TIX800 (Fe O N)

(α + β)Super TIX51AF® (Al-Fe) (α + β)

Ti HfTi-SiTi-Si-CuTi-Al

Talladium V(Ni-Cr-Ti)

(α + β)TIMETAL 62S® (α + β)TIMETAL 21SRx® (β)Ti-13Zr-13Nb (13-13) (β)

Ti AlTi-Al-CuTi-MnTi-Mn-Cu

6

Ti-13Zr-13Nb (13-13) (β)Ti-15V-3Cr-3Sn-3Al (15-3) (β)

Ti-Mn-Cu

14

Yield Strengths Elongation10005Al2Fe 64Cu

12 Pure Ti900 64 Cr

CrCu5AlCu5Al1Fe

10

n (%

)

Hf700

800

h (M

Pa)

67

MnMoNb

6

8

onga

tion

CPTiSiAl

5Al5Cu600

700

Stre

ngth

1FeSiCu

67

AlMnCu

15-3

4

6

Elo

645Al1Fe

C

Cr

CrCu500Yiel

d S

13-13

CPTi

CuHf

Si15 3

21Fe Cu SiCu

6764CuMnCu400

CPTi

Pure Ti

70

ExperimentalTi alloys

CommercialTi alloys

67 Mn300Experimental

Ti alloysCommercial

Ti alloys

Pure Ti

CastabilityCastability

• Used a wedge-shaped mold (15° andUsed a wedge shaped mold (15 and 30°)

• Castability/mold filling is judged by the gap• Castability/mold filling is judged by the gap between the tip of the casting and the theoretical acute tip of the triangletheoretical acute tip of the triangle

8Castability: μm

Evaluation of Castability600

700

m

30° 15°

y

400

500

ity, μ

m

300

400

asta

bil

100

200

C

0

100

Au Hf Hf P N 67 64 AF Cu Cr Au P Hf Hf N 67 64 AF Cu CrAu40

Hf10H

fCP LN 67 64 AF5C

uNiC

r Au CP40

Hf10H

fLN 67 64 AF5CuNiC

r

Influencing FactorsInfluencing Factors• Mold filling determined by:Mold filling determined by:

– flowability (limited by heat transfer)– fillability (limited by surface tension)

• Fluidity influenced by:– metal variables (surface tension, kinematic ( ,

viscosity, fusion temperature vs. mold temperature, liquidus-solidus temperatures)

– mold-metal variables– equipment variables

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Kinematic viscosityKinematic viscosity = viscosity/density

• Ti: 0.012 cm2/secA 0 003• Au: 0.003

• Cu: 0.005• Water: 0.010

Viscosity seldom a limitingViscosity seldom a limiting factor in mold filling

11

s 3 8

4 .0

4 .2

T i-H f

T i-M o

T i-N bViscosity

osity

, mPa

.3 .4

3 .6

3 .8

T i-C r

T i-M o

T i-S nT i Z r

Visc

2 .8

3 .0

3 .2

T i-A lT i-C o

T i-C u

T i-F e T i-M n

T i-Z r

T i-S i

M o le F ra c tio n0 .0 0 .1 0 .2 0 .3 0 .4 0 .5

m 2 2

2 .4T i-M o

Surface Tension

ensi

on, N

/m

1 .8

2 .0

2 .2

T i-A l

T i-N b

Surface Tension

Surf

ace

Te

1 .4

1 .6

T i-C r

T i-H f

T i-M n

T i-S n

T i-Z rT i-S i

12M o le F ra c tio n

0 .0 0 .1 0 .2 0 .3 0 .4 0 .51 .2

T i-C o T i-C uT i-F e

17001670 Phase

e°C 1500

L1670 Phase

Diagram f Ti C

erat

ure

1300

1100

of Ti-CuTe

mpe 1100

900

(βTi)

T 900

700 (αTi)790ºC

2Cu

500100 4020 30

Ti2

αTi+Ti2Cu

13

Cu (%)

Liquidus line

Temperaturegradient

erat

ure

erat

ure

Tem

pe

Tem

pe

Ti Cu(%)

Solidus line

Distance from mold surface

Molten metal

Cu(%)

Mold

sta ce o o d su ace

Solidified layer

Molten metalMold

14Mushy solid-liquid

y

GrindabilityGrindability

• Abrasive wheels and burs are used inAbrasive wheels and burs are used in dentistry to grind, cut and polish castings.G i d bilit d hi bilit f• Grindability and machinability of pure titanium is known to be poor because of its

tiproperties.• Improved grindability is possible through p g y p g

alloying.

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Grindability ExperimentsGrindability Experiments

• Volume loss (mm3) after 1 minute• Volume loss (mm3) after 1 minute of grinding–Using an SiC wheel–500 - 1 250 m/min–500 - 1,250 m/min

(circumferential speed)100 gf–100 gf

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Grindability RangesGrindability Ranges• Commercial and experimental alloys:Commercial and experimental alloys:

0.66 to 1.63 mm3 (500 m/s)0 59 to 5 73 mm3 (1 250 m/s)0.59 to 5.73 mm (1,250 m/s)

• CP Ti is the lowestC i l Ti 5Al 1F ll• Commercial Ti-5Al-1Fe alloys

– excellent• Experimental Ti-5Al+Cu alloys

– excellent17

Grindability of Metals Tested

6

7

3 ) 5Al5Cu

4

5

y (m

m3

64

5Al1Fe5Cu

3

4

abili

ty

6

CPTi1Fe Si

SiCu

5Al

1

2

Grin

d

P Ti

CPTi

CrCrCu

5Al

0

1

ExperimentalCommercial

Pure Ti

18

pTi alloysTi alloys

Grindability ResultsGrindability Results• Model predictions indicated that the p

grindability of titanium alloys:– increases with grinding speed– increases with decreasing fracture toughness

or tensile ductility

G i d bilit i f bl h t l• Grindability is favorable when metals have:

low ductility– low ductility– multiphase structure (α+β) and/or eutectoid

structure

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Wear BehaviorWear Behavior

• Titanium is noted for its poor tribologicalTitanium is noted for its poor tribological characteristics.

• Wear resistance is important for the longevity of restored occlusion.

• Alloying can improve wear resistance.

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Wear Testing at TsurumiWear Testing at Tsurumi

• Wear investigation of cast maxillary• Wear investigation of cast maxillary and mandibular teeth: – two-body wear-testing equipment

simulated chewing action– applied load of 49 N (5 kgf)– 60 cycles/min60 cyc es/– assessed as volume loss after 50,000

cyclescycles

Wear Resistance ResultsWear Resistance Results

Remarkable findings:Remarkable findings:• β alloys (15-3, Timetal 21SRx, etc.) – poor

β ll (Ti 6Al 4V Ti 6Al 7Nb• α+β alloys (Ti-6Al-4V, Ti-6Al-7Nb, Ti-5Al-Fe, Ti-Cr-Cu) – excellent

• Cu – very beneficial• Stop plastic deformation!Stop plastic deformation!

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4 15-3Volume Loss of Metals Tested

3)

MoNb

15 3

3

s (m

m3 )

13Cr3Cu

2

me

loss CPTi(3) 3Cu13-3

5CuCPTi(2)

1Volu

m

64

1Fe 13Cr5Cu5Cu

64-4Cu64-1Cu

5Al5Cu

( )

0

Au67

5Al1Fe

64 4Cu

13Cr7Cu 5Al

23

0Experimental

Ti alloysCommercial

Ti alloysCommercialdental alloy

Corrosion Behavior• Pure titanium has low toxicity and is well-

tolerated by bone and tissue

• Concerns that alloying might jeopardize corrosion behavior and cytotoxicityy y

• Alloys were tested using dynamic polarization method in artificial saliva

• Evaluated open circuit potential (OCP) value, polarization or corrosion resistance, corrosion pcurrent density, passivation current density

• Electrochemical behavior of alloys was similar to

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ypure titanium

Cytotoxicityy y• Tested cytotoxicity of commercial and

experimental alloysexperimental alloys– Polished specimens were placed in contact

with Balb/c 3T3 fibroblasts for 72 hrs.– Toxicity was evaluated by MTT method– Controls were CP Ti, Ti-6Al-4V, Teflon®

– Cytotoxicity results: 82-96% (values normalized to Teflon® control)

• Cytotoxicity of experimental alloys was similar to other commonly used alloys and

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y yCP Ti

SummarySummary• Characteristics of dental titanium alloys:Characteristics of dental titanium alloys:

– easy to cast– have good corrosion resistance andhave good corrosion resistance and

biocompatibility

• Future studies should improve:• Future studies should improve:– casting efficiency and accuracy

bonding with porcelain– bonding with porcelain– fracture toughness and fatigue

machinability for CAM/CAD applications26

– machinability for CAM/CAD applications

Summary (cont.)Summary (cont.)

• Cast Ti alloys met the properties• Cast Ti alloys met the properties criteria set

• At the completion of our studies, we should be able to identify alloys fory y– Inlays and crowns– Fixed bridgesed b dges– Removable partial dentures– Implant superstructuresImplant superstructures

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AcknowledgmentsAcknowledgments

Funded partially by National Institutes ofFunded partially by National Institutes of Health/National Institute of Dental and Craniofacial Research grant DE11787Craniofacial Research grant DE11787.

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