Journal of Medical and Biological Engineering, 30(2): 73-78 73

Grindability Evaluation of Dental Cast Ti-20Cr-X Alloys

Hsueh-Chuan Hsu1,2 Shih-Ching Wu1,2 Chang-Hung Pan3

Hsiao-Wei Wang4 Wen-Fu Ho4,*

1Department of Dental Laboratory Technology, Central Taiwan University of Science and Technology, Taichung 406, Taiwan, ROC 2Institute of Biomedical Engineering and Material Science, Central Taiwan University of Science and Technology, Taichung 406, Taiwan, ROC

3Department of Mechanical and Automation Engineering, Da-Yeh University, Changhua 515, Taiwan, ROC 4Department of Materials Science and Engineering, Da-Yeh University, Changhua 515, Taiwan, ROC

Received 10 Aug 2009; Accepted 17 Dec 2009

Abstract

This study aimed to investigate the grindability of ternary Ti-20Cr-X alloy with 20 wt% Cr and 1 wt% alloying

elements X (X = Nb, Mo, Zr or Fe). The grindability was evaluated using an electric dental handpiece using SiC wheels,

with the goal of developing a titanium alloy that exhibited better grindability properties than commercially pure

titanium (c.p. Ti). Because c.p. Ti is considered to be difficult to machine and grind, one method to change these

attributes is to develop and study titanium alloys. Grindability was assessed by measuring the amount of metal volume

removed after 1 min of grinding at each of the four rotational speeds preset on the wheel (500, 750, 1000 or 1200

m/min). Our results indicated that the grinding rate of Ti-20Cr-1Nb was higher than that of c.p. Ti and the other

Ti-20Cr-1X alloys at the speed of 750 and 1000 m/min. Moreover, Ti-20Cr-1Nb exhibited a higher grinding ratio than

did the other Ti-20Cr-1X alloys and c.p. Ti at 1000 m/min, which was about 1.2 times higher than that of c.p. Ti.

Additionally, Ti-20Cr-1Nb had smaller metal chips and no obvious grinding adhesion when ground at the speed of 1000

m/min. Our research suggests that Ti-20Cr-1Nb exhibited the best grindability, specifically at the grinding speed of 750

or 1000 m/min, and can be considered as a good candidate for CAD/CAM application.

Keywords: Dental alloy, Titanium alloy, Structure, Grindability

1. Introduction

Titanium and titanium alloys are materials of choice for

prosthetic devices used by patients with sensitivity to

traditional dental alloys because they possess excellent

corrosion resistance and biocompatibility in the oral

environment [1,2]. For example, pure titanium is used for

dental crowns and bridges, endosseous dental implants and

plates for oral maxillofacial surgery [3-5]. However, when a

higher strength than that provided by pure titanium is needed,

one method is to use titanium alloys, which exhibit

solid-solution hardening, lower fusion temperatures and better

ductility than pure titanium [6]. In fact, many titanium alloys

have been developed for dental use, and their properties have

been extensively studied, mainly in an effort to improve the

strength and castability of titanium [3,6-9].

The recent development of the computer-aided design

and manufacturing (CAD/CAM) method represents a great

advancement over casting technology [10]. CAD/CAM

systems provide an alternative method to produce metal,

* Corresponding author: Wen-Fu Ho

Tel: +886-4-8511888 ext. 4108; Fax: +886-4-8511224

E-mail: fujii@mail.dyu.edu.tw

ceramic, or composite restorations, without the need for any

procedures that require two or more patient appointments.

Although titanium has superior biocompatibility and

corrosion resistance, it exhibits low machinability due to its

intrinsic characteristics of low thermal conductivity, high

chemical reactivity, and low modulus of elasticity [11]. If

pure titanium prostheses are fabricated by CAD/CAM, the

tool life will be short and the processing time long [4,6].

Consequently, there is a need for further development of new

dental materials especially suited for machining. Although

there may be several ways to improve grindability, one

well-documented method is through alloying [12-16]. In fact,

a great deal of effort has been devoted to the study of new

dental materials suited for machining, such as Ti-Cu [12,13],

Ti-Ag [13], Ti-Nb [14], Ti-Hf [15], Ti-Au [16], Ti-Cr [17],

Ti-Cr-Cu [18] Ti-Cr-Fe [19] and Ti-Zr [20].

Among various titanium alloys, Ti-6Al-4V is one of the

most studied titanium alloys and has been used for the

fabrication of denture bases and multiple unit bridges because

of its superior strength compared with pure titanium [2].

Nevertheless, studies have shown that the release of Al and V

ions from the alloy might cause some long-term health

problems [21,22]. Therefore, Al, V-free titanium alloys, such

J. Med. Biol. Eng., Vol. 30. No. 2 2010 74

as Ti-29Nb-13Ta-4.6Zr [23], Ti-13Nb-13Zr [24], Ti-7.5Mo

[25,26] and Ti-Sn [27], have been developed.

In the present study, two types of β stabilizers were added

to commercially pure titanium (c.p. Ti). The β-isomorphous

elements, Nb, Zr and Mo were included, while β-eutectoid

elements, Cr and Fe were chosen [28]. Cr has already been

used for many years as a major constituent in dental casting

alloys [29]. Additionally, Cr is also known to control the

anodic activity of the alloy and increase the tendency of Ti to

passivate [28]. Hsu et al. [30] found that in standard artificial

saliva, the OCP increases with higher Cr content in Ti-Cr

alloys. Also, the Ti-Cr alloys had greater resistance to corrosion

in the fluoride-containing artificial saliva than c.p. Ti and

Ti-6Al-4V. Takemoto et al. [31] reported that the Ti-20Cr alloy

had a greater resistance to corrosion in a fluoride-containing

saline solution than did c.p. Ti. The purpose of the present

study was to evaluate the grindability of four kinds of ternary

cast Ti alloys, including Ti-20Cr-1Nb, Ti-20Cr-1Mo,

Ti-20Cr-1Zr and Ti-20Cr-1Fe, in order to develop a titanium

alloy with better grindability properties than c.p. Ti.

2. Experimental procedures

Alloying elements selected for the study include Cr, Nb, Mo,

Zr and Fe, which were all 99.9% pure. For the sake of simplicity,

throughout the text, “Ti-20Cr-1X” refers to “Ti-20 wt% Cr-1

wt% X”, where X is a given alloying element. The series of Ti

alloys, including Ti-20Cr-1Nb, Ti-20Cr-1Mo, Ti-20Cr-1Zr and

Ti-20Cr-1Fe, were prepared from 99.7% pure titanium, using a

commercial arc-melting vacuum-pressure-type casting system

(Castmatic, Iwatani Corp., Japan). The melting chamber was first

evacuated and purged with argon. An argon pressure of

1.5 kgf/cm2 was maintained during melting. Appropriate amounts

of each metal were melted in a U-shaped copper hearth with a

tungsten electrode. The ingots were re-melted five times prior to

casting to improve chemical homogeneity. Prior to casting, the

ingots were melted once again in an open-based copper hearth

under an argon pressure of 1.5 kgf/cm2. The difference in

pressure between the two chambers allowed the molten alloys to

instantly drop into the graphite mold. The cast alloys were

sectioned using a Buehler Isomet low-speed diamond saw to

obtain specimens for experimental purposes. X-ray diffraction

(XRD) for phase analysis was conducted using a diffractometer

(XRD-6000, Shimadzu, Japan) operated at 30 kV and 30 mA.

Ni-filtered CuKα radiation was used for this study. Phase was

identified by matching each characteristic peak with the JCPDS

files. The microhardness of polished alloys was measured using a

microhardness tester (MVK-E3, Mitutoyo, Japan) at 100 gm for

15 s.

Grindability is defined in terms of the material to be ground,

as the relative ease or difficulty of removing material when

transforming a raw material into a finished product. The same

method used in previous studies by several other researchers

[12-16,32-34] was adopted to evaluate grindability. A silicon

carbide (SiC) wheel (G11, Shofu, Kyoto, Japan) (diameter 13.1

mm, thickness 1.75 mm) on an electric dental handpiece

(Ultimate 500, NSK Nakanishi Inc., Japan) was used to grind the

specimens. Each specimen was placed on the test apparatus, as

referred to in a previous study by Ohkubo et al. [34], so that the

edge of the wheel made contact with the specimen at 90˚. By

applying a force of 100 gf, the specimens were ground at one of

the four rotational speeds of the wheel (500, 750, 1000 or 1200

m/min). The specimen and grinding wheel were housed in a

closed compartment during grinding so that any metal chips

generated could be collected.

The amount of metal removed (mm3) in one minute was

calculated from the density, previously measured using

Archimedes’ principle [34], and the weight loss of the specimen.

The grinding test was performed six times for each kind of metal

at every grinding speed, and a new wheel was applied for every

test. The diameter and weight of each wheel were measured

before and after grinding. The grindability was evaluated by the

grinding rate and grinding ratio. The grinding rate was

determined by the volume of metal removed per minute of

grinding, and the grinding ratio was determined by the volume

ratio of metal removed compared to the wheel material lost

[grinding ratio = (amount of metal removed)/(amount of wheel

lost)]. The wheel volume loss was calculated by measuring the

diameter of each wheel before and after testing. Thus, the

grinding rate represents the ease of metal removal, whereas the

grinding ratio is a measurement of wheel life [15]. At each of the

four grinding speeds, three specimens were used to evaluate the

grindability of each kind of metal. The test was repeated three

times for each specimen at each grinding speed. After testing, the

ground surfaces of the metals were examined using optical

microscopy (BH2, Olympus, Japan). The surface of the chips

resulting from the metal grinding was examined using a scanning

electron microscopy (SEM; S-3000N, Hitachi, Japan). All the

results in this study were analyzed using two-way ANOVA and

Duncan’s multiple comparison test at p < 0.05 level.

3. Results and discussion

3.1 Phase and structure

The phases and crystal structures of the c.p. Ti and

Ti-20Cr-1X alloys are shown in Table 1. The c.p. Ti was

comprised entirely of a hexagonal α' phase. However, the

ternary Ti-20Cr-1X alloys were comprised mainly of the β

phase with a bcc crystal structure. This finding is similar to the

early reports obtained by Ho et al. [35], who reported that the β

phase could be fully retained during quenching on the ternary

Ti-15Mo-1X (X = Nb, Cr, Zr or Fe) alloys. These results are

attributed to the effect of a β stabilizer, especially for Cr, Fe

and Mo, which exhibit a stronger effect on the stabilization of

the β phase. The β phase is retained in metastable form by

quenching the alloy with a sufficient amount of β-stabilizing

elements to depress the martensite finish temperature to below

room temperature [36]. Weiss et al. [37] and Ankem et al. [38]

have shown that β-phase titanium alloys generally can be

processed to higher strength levels and also exhibit better notch

properties and toughness than α + β alloys. Because β-Ti is the

most versatile of the Ti alloys, much research effort has

recently been devoted to its study. These investigations have

Grindability of Titanium Alloys 75

assessed its superior biocompatibility, good mechanical

properties and better processability of β or near-β phase Ti

alloys and their biomedical applications [23,39,40].

Table 1. Phase and crystal structure of c.p. Ti and Ti-20Cr-1X alloys.

Alloy Phase Crystal structure

c.p. Ti α' hexagonal

Ti-20Cr-1Nb β bcc

Ti-20Cr-1Mo β bcc Ti-20Cr-1Zr β bcc

Ti-20Cr-1Fe β bcc

3.2 Grindability

The grinding rates of c.p. Ti and Ti-20Cr-1X alloys at four

different grinding speeds are shown in Fig. 1. With the

exception of Ti-20Cr-1Fe, the grinding rates of the Ti alloys

and c.p. Ti showed a tendency to increase at higher grinding

speed, but decrease at 1200 m/min. For all the alloys, a test

consisting of multiple comparisons was also conducted using

Duncan’s multiple range test. Results show that grinding rates

at 1200 m/min were significantly lower (p < 0.05) than those at

the other three grinding speeds (500, 750 and 1000 m/min). At

the grinding speed of 500 m/min, the grinding rates for all the

Ti-20Cr-1X alloys and c.p. Ti showed no significant difference

(p > 0.05). However, the grinding rates of the Ti-20Cr-1X

alloys were higher than that of c.p. Ti at the speed of 750

m/min, especially for Ti-20Cr-1Nb, although there were no

significant differences among these metals at the 0.05 level. It

is noteworthy that the grinding rate of Ti-20Cr-1Nb was

significantly higher (p < 0.05) than that of c.p. Ti and the other

Ti-20Cr-1X alloys at the speed of 1000 m/min. It is notable that

the ease of grinding these alloys appeared to be dependent on

grinding speed. The grinding rates of all the metals in this study

tended to increase as the grinding speed increased from 500

m/min, which is reasonable, given that the grinding speed

determines the distance the wheel edge travels on the surface of

the specimen per unit of time. However, grinding rates at 1000

m/min were significantly higher than those for the highest

speed of 1200 m/min. This resembles a similar result seen in

another titanium alloy systems [17,19,20,41]. Although further

study is needed, it is possible that the condition of the grinding

wheel deteriorates at the speed of 1200 m/min.

c.p. Ti

Ti-20Cr-1Nb

Ti-20Cr-1Mo

Ti-20Cr-1Zr

Ti-20Cr-1Fe

0

1

2

3

4

500 m/min

750 m/min

1000 m/min

1200 m/min

Grinding rate (mm

3/min)

Figure 1. Grinding rate of c.p. Ti and Ti-20Cr-1X alloys.

The grinding ratios of c.p. Ti and Ti-20Cr-1X alloys are

shown in Fig. 2. The grinding ratios had a similar tendency to

the grinding rates as the grinding speed increased. The ratio of

Ti-20Cr-1Nb was higher than that of c.p. Ti at the speed of 750

m/min. Additionally, Ti-20Cr-1Nb and Ti-20Cr-1Mo exhibited

a higher grinding ratio than did the other Ti-20Cr-1X alloys and

c.p. Ti at 1000 m/min, although there were no significant

differences between Ti-20Cr-1Mo and c.p. Ti at the 0.05 level.

The ratio of Ti-20Cr-1Nb at 1000 m/min was about 1.2 times

higher than that of c.p. Ti. With respect to the grinding ratio, all

the metals had the lowest values at the highest speed, 1200

m/min. Tool life can be predicted based on an evaluation of the

grinding ratio. A higher grinding ratio signifies lower tool wear

for the same volume of metal removed [12]. The ratios varied

widely for all the metals tested, probably because both the

volumes of the metal ground and wheel material lost were very

small.

c.p. Ti

Ti-20Cr-1Nb

Ti-20Cr-1Mo

Ti-20Cr-1Zr

Ti-20Cr-1Fe

0 .0

0.5

1.0

1.5

2.0 500 m/min

750 m/min

1000 m/min

1200 m/min

Grinding ratio

Figure 2. Grinding ratio of c.p. Ti and Ti-20Cr-1X alloys.

c.p. Ti

Ti-20Cr-1Nb

Ti-20Cr-1Mo

Ti-20Cr-1Zr

Ti-20Cr-1Fe

150

200

250

300

350

400

450

Microhardness (HV)

Figure 3. Microhardness of c.p. Ti and Ti-20Cr-1X alloys.

Although the measurements were straightforward, the

interpretation of the mechanism for the measured grindability

values could be quite complex. Such characteristics as

composition, hardness, strength, modulus, ductility and crystal

structure/phase could all affect the grindability of the alloy.

However, the possible mechanisms are not certain at this

moment. Takeyama et al. [42] showed that higher strength or

J. Med. Biol. Eng., Vol. 30. No. 2 2010 76

(a) 500 m/min

(b) 1000 m/min

Figure 4. SEM micrographs of metal chips resulting from grinding at (a) 500 m/min, and (b) 1000 m/min.

(a) 500 m/min

(b) 1000 m/min

Figure 5. Grinding surfaces of c.p. Ti and Ti-20Cr-1X alloys at (a) 500 m/min, and (b) 1000 m/min.

hardness of a material generally make machining of the

material more difficult. However, in this study, all the

Ti-20Cr-1X alloys had much higher hardness values than that

of c.p. Ti (as shown in Fig. 3), but they also had higher grinding

rates, especially at the speed of 750 or 1000 m/min. In early

studies, researchers [43,44] discussed grindability in terms of

the hardness of metals. Nevertheless, in the present study there

appeared to be no correlation between the volume loss and the

hardness of the metals. This result was also in agreement with

that reported by Ohkubo et al. [34], who tested c.p. Ti and

Ti-6Al-4V alloy. In fact, it seems that hardness is not the

principal reason for better grindability. According to Kikuchi et

al. [13], elastic modulus appeared to have no relation to

grindability. Nevertheless, the relatively low elastic modulus of

titanium causes chatter vibration or deformation of the work

piece during machining [45]. Therefore, Kikuchi et al.

suggested that titanium alloys with a higher elastic modulus are

more suitable for machining [13]. In our previous study [46],

the bending moduli of Ti-20Cr-1X alloys, which are in the

range of 134-144 GPa, are considerably higher than that of c.p.

Ti (98 GPa). Thus, considering modulus, these alloys are more

suitable for dental machining. This study showed that the exact

mechanisms which affect the grindability of various alloys are

complex and merit further study.

3.3 Observation of metal chips and ground surfaces

Typical metal chips that result from grinding at 500 m/min

and 1000 m/min are shown in Fig. 4. Although no quantitative

analysis was performed, the size of the metal chips produced at

the grinding speed of 500 m/min generally appeared somewhat

larger than those generated at 1000 m/min, especially for

Ti-20Cr-1Nb and Ti-20Cr-1Mo. However, there were no clear

differences in the appearance of the metal chips of the c.p. Ti

and Ti-20Cr-1X alloys. In general, finer metal chips indicate

that materials are more suitable for grinding than those which

produce larger metal chips [16]. In addition, there were a

number of long chips, particularly for c.p. Ti.

Figure 5 shows optical micrographs of the ground surfaces

of the metals at 500 and 1000 m/min. Grinding marks were

observed for all the metals and speeds. There were no

Grindability of Titanium Alloys 77

pronounced differences in the appearance of the ground

surfaces of c.p. Ti and Ti-20Cr-1X alloys at 500 m/min.

Grinding adhesion was observed at a greater degree for c.p. Ti

and a lesses degree for Ti-20Cr-1Zr and Ti-20Cr-1Fe alloy

ground at 1000 m/min. On the other hand, no obvious grinding

adhesion was found on the surfaces of Ti-20Cr-1Nb and

Ti-20Cr-1Mo at 1000 m/min. In addition, grinding burns could

also be found for c.p. Ti. As can be seen in other research

[13,14,16], the grinding burn typically found on titanium and

titanium alloys results from high-speed grinding. In addition,

appreciable sparking was occasionally observed during the

grinding test, especially at a high grinding speed.

4. Conclusions

On the basis of the results of XRD, the ternary Ti-20Cr-1X

alloys were comprised mainly of the β phase, with a bcc crystal

structure. The grinding rate of Ti-20Cr-1Nb was higher than

that of c.p. Ti and the other Ti-20Cr-1X alloys at the speed of

750 or 1000 m/min. With the exception of Ti-20Cr-1Fe, the

rates of the Ti alloys and c.p. Ti showed a tendency to increase

at higher grinding speed, but decreased at 1200 m/min.

Ti-20Cr-1Nb exhibited a higher grinding ratio than did the

other Ti-20Cr-1X alloys and c.p. Ti at 1000 m/min, which was

about 1.2 times higher than that of c.p. Ti. The size of the metal

chips produced at the grinding speed of 500 m/min generally

appeared somewhat larger than those at 1000 m/min, especially

for Ti-20Cr-1Nb and Ti-20Cr-1Mo. Grinding adhesion was

observed to a greater degree for c.p. Ti and a minor degree for

Ti-20Cr-1Zr and Ti-20Cr-1Fe alloy ground at 1000 m/min. No

obvious grinding adhesion was found on the surfaces of

Ti-20Cr-1Nb and Ti-20Cr-1Mo at 1000 m/min.

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