grindability of cast ti–cu alloys
TRANSCRIPT
Grindability of cast Ti–Cu alloys
Masafumi Kikuchia,*, Yukyo Takadaa, Seigo Kiyosueb, Masanobu Yodac, Margaret Woldud,Zhuo Caid, Osamu Okunoa, Toru Okabed
aDivision of Dental Biomaterials, Department of Oral Rehabilitation and Materials Science, Graduate School of Dentistry, Tohoku University, 4-1
Seiryo-machi, Aoba-ku, Sendai 980-8575, JapanbDepartment of Orthodontics, Fukuoka Dental College, 2-15-1 Tamura Hayara-ku, Fukuoka 814-0193, Japan
cDivision of Fixed Prosthodontics, Department of Oral Rehabilitation and Materials Science, Graduate School of Dentistry, Tohoku University, 4-1
Seiryo-machi, Aoba-ku, Sendai 980-8575, JapandDepartment of Biomaterials Science, Baylor College of Dentistry, Texas A&M University System Health Science Center, 3302 Gaston Ave., Dallas,
TX 75246, USA
Received 16 January 2001; revised 25 March 2002; accepted 11 July 2002
Abstract
Objectives. The purpose of the present study was to evaluate the grindability of a series of cast Ti–Cu alloys in order to develop a titanium
alloy with better grindability than commercially pure titanium (CP Ti), which is considered to be one of the most difficult metals to machine.
Methods. Experimental Ti–Cu alloys (0.5, 1.0, 2.0, 5.0, and 10.0 mass% Cu) were made in an argon-arc melting furnace. Each alloy was
cast into a magnesia mold using a centrifugal casting machine. Cast alloy slabs (3.5 mm £ 8.5 mm £ 30.5 mm), from which the hardened
surface layer (250 mm) was removed, were ground using a SiC abrasive wheel on an electric handpiece at four circumferential speeds (500,
750, 1000, or 1250 m/min) at 0.98 N (100 gf). Grindability was evaluated by measuring the amount of metal volume removed after grinding
for 1 min. Data were compared to those for CP Ti and Ti–6Al–4V.
Results. For all speeds, Ti–10% Cu alloy exhibited the highest grindability. For the Ti–Cu alloys with a Cu content of 2% or less, the
highest grindability corresponded to an intermediate speed. It was observed that the grindability increased with an increase in the Cu
concentration compared to CP Ti, particularly for the 5 or 10% Cu alloys at a circumferential speed of 1000 m/min or above.
Significance. By alloying with copper, the cast titanium exhibited better grindability at high speed. The continuous precipitation of Ti2Cu
among the a-matrix grains made this material less ductile and facilitated more effective grinding because small broken segments more
readily formed.
q 2003 Academy of Dental Materials. Published by Elsevier Science Ltd. All rights reserved
Keywords: Titanium alloys; Grindability; Grinding
1. Introduction
Titanium is an attractive metal for dental applications
because of its combination of excellent biocompatibility,
corrosion resistance and mechanical properties [1,2].
However, there are some characteristics of this metal that
need to be counteracted in order for this metal to become a
viable choice for dental use. Among these characteristics is
the difficulty of grinding or machining titanium [3–6].
Thus, it is important for practical application to develop
titanium metals that are easy to cut and grind.
The ability of a material to be machined has been
termed its ‘machinability.’ [7]. A material has good
machinability if the tool wear is low, the tool life is long,
the cutting forces are low, the chips are well formed and
break into small ringlets instead of long snarls, and the
surface finish is acceptable. Thus, machinability is often
defined based on the machining operational character-
istics. On the other hand, some define machinability in
terms of the material to be machined, as the relative ease
or difficulty of removing material when transforming a
raw material into a finished product. Since the present
experiment did not involve machining but was more
similar to grinding using a wheel in which abrasive
particles are embedded, the word ‘grindability’ will be
0109-5641/03/$ 30.00+0.00 q 2003 Academy of Dental Materials. Published by Elsevier Science Ltd. All rights reserved
PII: S0 10 9 -5 64 1 (0 2) 00 0 80 -5
Dental Materials 19 (2003) 375–381
www.elsevier.com/locate/dental
* Corresponding author. Tel.: þ81-22-717-8317; fax: þ81-22-717-8319.
E-mail address: [email protected] (M. Kikuchi).
used in this article to evaluate experimental alloys by
measuring the amount of alloy ground.
Although there may be several ways to improve
grindability, one method is through alloying. Grindability
is reported to be improved if the microstructure is a two-
phase structure consisting of either a brittle or easily sheared
second phase dispersed throughout a moderately ductile
matrix [8]. There have been many studies performed on
common industrial alloys. The overall machinability of as-
rolled or annealed low-carbon steel improves with micro-
structural changes: increasing the pearlite content and with
increasing ferrite and prior-austenite grain size, which
utilizes the inherent deformation incompatibilities that exist
between the pearlite and the surrounding matrix. Machin-
ability is enhanced by the initiation, growth, and coalesc-
ence of microvoids. One noted method of improving
machinability is to add a small amount of elements, such
as sulfur, lead, and tellurium, to copper alloys and steels,
etc., to increase the brittleness of the alloy with the resultant
[9] machining mentioned above.
For titanium, free-machining pure titanium (DT2F) and
free-machining titanium alloys (DAT5F, DAT52F) have
been developed for industrial use (Daido Steel, Nagoya,
Japan) [10]. The former alloy is made from CP Ti
(equivalent to ASTM grade 2) plus both sulfur and rare
earth metals that produce globular sulfide inclusions,
whereas the latter alloys contain the above alloying elements
in the titanium with aluminum and vanadium. They claimed
a significantly improved tool life when these alloys were cut
compared to cutting CP Ti and Ti–6Al–4V. Taira et al.
performed a comparative study on the machinability of the
wrought form of free-machining titanium (DT2F) vs.
unalloyed CP Ti using dental air turbine burs [11]. The
free-machining titanium was found to possess better
machining efficiency than did CP Ti, leading to longer tool
life. Watanabe et al. investigated the machinability of cast
DT2F and compared it to that of CP Ti and Ti–6Al–4V [12].
(We are using the term machinability instead of grindability
with reference to their study since the carbide bur ‘cut’ the
metals.) Their study revealed that when carbide burs were
used, the machinability of DT2F was much better compared
to that of CP Ti or Ti–6Al–4V. On the contrary, the
grindability with diamond points for DT2F was similar to
these controls. Apparently, granular inclusions dispersed in
the matrix of DT2F were not effective at obtaining efficient
grinding. These studies indicate that alloyed titanium has the
potential to be machined more efficiently than CP Ti.
In another study, the mechanical properties of a series of
cast Ti–Cu alloys [0.5, 1.0, 2.0, 5.0 and 10.0 mass% Cu
(hereafter, ‘mass%’ will be referred to as ‘%’)] were
evaluated in an effort to develop a dental titanium alloy with
a lower fusion temperature for easy casting [13]. By
alloying with copper, the cast titanium became stronger. An
increase in the tensile strength of more than 40% over CP
Ti, as well as an increase in yield strength, were obtained for
the 5 and 10% Cu alloy. The elongation values were
approximately 3% and 1% for the 5 and 10% Cu alloy,
respectively. As for the bulk hardness of these alloys
compared to that of CP Ti (VHN 200), the values for the 5
and 10% Cu alloys were 230 and 280, respectively.
Since the eutectoid concentration of Cu in the Ti–Cu
equilibrium phase diagram is 7% [14,15], titanium alloys
with 5 and 10% Cu are hypo- and hyper-eutectoid alloys,
respectively. As such, we found [13] that the microstructure
of the 5% Cu alloy consists of a basket-weave proeutectoid
a-Ti and the eutectoid constituent (a-Ti þ Ti2Cu) that
formed among the a-Ti. In the 10% Cu alloy, the micro-
structure consists of the multi-grain structure of the lamellar
formed eutectoid constituent that formed from the iso-
thermal decomposition of the remaining phase during
solidification within the proeutectoid Ti2Cu. In these alloys,
strength and hardness were believed to have increased due
to the inclusion of the eutectoid in the ductile a-Ti (5% Cu
alloy) or to a stronger and harder eutectoid structure with an
almost total elimination of the ductile a-Ti (10% Cu alloy).
With the reduced ductility of our experimental alloys
with higher Cu concentrations, it is hypothesized that the
ease of grinding the Ti–Cu alloys, compared to that of a-Ti,
is enhanced because they are less ductile. Another
advantage of alloying copper to titanium is that the fusion
temperature of the titanium (1670 8C) is lowered with an
increase in the amount of copper (approximately 1610 8C
for 5% Cu and 1540 8C for 10% Cu) [14], thus facilitating
the casting process. The addition of copper, which has
greater heat conductivity than titanium, may also contribute
to easier machining.
The purpose of the present study was to evaluate the
grindability of a series of cast Ti–Cu alloys with a goal of
developing a titanium alloy with better machinability than
CP Ti, a metal generally considered to be difficult to machine.
2. Materials and methods
2.1. Alloys examined
Ti–Cu alloys with five concentrations of copper (0.5,
1.0, 2.0, 5.0 and 10.0%) were examined. The alloy
concentrations of 0.5, 1.0 and 2.0% Cu are located within
the a-Ti region at 790 8C (the eutectoid temperature), and
the concentrations of 5.0 and 10.0% Cu correspond to the
hypoeutectoid and the hypereutectoid regions, respectively.
CP Ti (ASTM grade 2, Titanium Industries, Grand Prairie,
TX, USA) and Ti–6Al–4V (Titanium Industries) were used
as controls.
2.2. Preparation of specimens
Each Ti–Cu alloy corresponding to a desired concen-
tration was made by melting sponge titanium (99.8% or
above, Sumitomo Sitix, Amagasaki, Japan) and pieces of
pure Cu [oxygen-free copper (Cu:99.99%, O:0.0005%),
M. Kikuchi et al. / Dental Materials 19 (2003) 375–381376
The Research Institute for Electric and Magnetic Materials,
Sendai, Japan] in an argon-arc melting furnace (TAM-4S,
Tachibana-Riko, Sendai, Japan) into one 30 g button.
Before melting, the furnace was evacuated to 5 £ 1023 Pa
followed by the introduction of high-purity argon gas
(99.9999% or above, Nipponsanso, Kawasaki, Japan) until
the pressure reached 50 kPa. Each alloy button was arc-
melted and cast into a magnesia mold at 200 8C (Selevest
CB, Selec, Osaka, Japan) in a centrifugal casting machine
(Ticast Super R, Selec) using the manufacturer’s suggested
casting conditions [chamber evacuated to 7 £ 1022 Pa
(5 £ 1024 Torr); argon introduced until pressure reaches
27 kPa (200 Torr); electric current: 170–230 A; melting
time: 45–55 s]. To make the castings, wax patterns
(3.5 mm £ 8.5 mm £ 30.5 mm) were each sprued, invested
and cast. A dental X-ray unit (GX 1000, Gendex, Des
Plaines, IL, USA) was used in order to check the soundness
of each cast specimen for any appreciable casting defects
using a procedure similar to that described by Watanabe
et al. [16]. Since in our previous study [13], the depth of the
hardened layer on the cast surface was at most 250 mm, a
250 mm layer was ground from all the surfaces of the
castings prior to the grinding test, producing specimens
measuring 3.0 mm £ 8.0 mm £ 30 mm.
2.3. Grinding test
A silicon carbide (SiC) wheel (703-120, Brasseler USA,
Savannah, GA, USA) (13 mm diameter, 1.5 mm thick) on
an electric dental handpiece (Upower model 501, Brasseler)
was used to grind the specimens. Each specimen was placed
on the test apparatus used in a previous study by Ohkubo
et al. [17] so that the circumferential surface of the wheel
contacted one of the side walls of the rectangular specimen
at 908. Details as to how the applied load was calibrated and
the grinding test was carried out can be found in Ohkubo
et al. [17]. By applying a force of 0.98 N (100 gf), the
specimens were ground at one of the four rotational speeds
of the wheel (500, 750, 1000 or 1250 m/min). The amount
of metal removed (mm3) during one minute was calculated
from density previously measured using Archimedes’
principle [17] and weight loss of the specimen. The
diameter of each wheel was measured before and after
grinding. Grindability was evaluated by volume of metal
removed per minute (grinding rate) and volume ratio of
metal removed compared to wheel material lost (grinding
ratio). The data (n ¼ 4 for each condition) were compared to
the data for CP Ti and Ti–6Al–4V using one-way ANOVA
and Student–Newman–Keuls at a ¼ 0.05.
2.4. SEM observation of chips, metals ground, and wheels
The surfaces of the wheels, the ground surfaces of the
metals after testing and the appearance of the chips resulting
from the metal grinding were examined using a scanning
electron microscope (SEM) (JSM-6300, JEOL, Peabody,
MA, USA) equipped with energy dispersive spectroscopy
(EDS) (Voyager, Noran Instruments, Middleton, WI, USA).
3. Results
3.1. Grinding rate and grinding ratio
The grinding rates are summarized in Fig. 1. At the
lowest rotational speed (500 m/min), the grinding rates of
all of the Ti – Cu alloys and of Ti – 6Al – 4V were
significantly higher ( p , 0.05) compared to CP Ti; note
also that the 10% Cu alloy exhibited the highest grinding
rate. At higher rotational speeds (particularly at 1250 m/
min), the grinding rates of the 5 and 10% Cu alloys were
notably higher ( p , 0.05) than for any other metals at any
of the rotational speeds. However, there was no statistical
difference in the grinding rate among the other Ti–Cu
alloys, CP Ti and Ti–6Al–4V. The grinding rate for the
10% Cu alloy at 1250 m/min was about four times as large
as that for CP Ti. Appreciable sparking accompanied
Fig. 1. Grinding rates of metals tested.
M. Kikuchi et al. / Dental Materials 19 (2003) 375–381 377
grinding as the rotational speed increased, especially with
the 5 and 10% Cu alloys at 1250 m/min.
The grinding ratios are shown in Fig. 2. The grinding
ratio of the 10% Cu alloy at 1000 m/min and above was
notably higher ( p , 0.05) than it was for any other metals
and speeds. The grinding ratio for the 10% Cu alloy at
1250 m/min was almost 20 times as large as that for CP Ti.
3.2. SEM observation of metal chips and wheel surfaces
Fig. 3(a)–(e) show typical SEM micrographs (at two
different magnifications) of metal chips that resulted from
grinding at 1250 m/min for cast CP Ti, Ti–6Al–4V, and the
2, 5, and 10% Cu alloys. Although we did not quantitatively
measure the sizes of the chips, there appears to be a
difference between the size of the chips from CP Ti and
from the 5 and 10% Cu alloys. It looks as though the CP Ti
chips are coarser than any of the other metal chips, while the
Ti–Cu alloy chips are finer.
Fig. 4 shows a magnified view of coagulated particles
found in the 10% Cu alloy chips after grinding at 1250 m/
min. These particles include a mixture of spheres and other
shapes. Examination with SEM/EDS revealed that most of
these spheres mainly consist of Ti, with other spheres
containing Cu and Ti, indicating that partial melting
occurred at a microscopic level from the high-speed
grinding. Similar Ti spheres were also found in the 5% Cu
alloy ground at 1250 m/min. These spheres were not found
in chips from the other metals nor were they seen in either
the 5 or 10% Cu alloys ground at a lower speed (500 m/min).
Fig. 5 shows a typical metal chip adhering to the wheel
after grinding the 10% Cu alloy at 1250 m/min. Some of the
SiC abrasive particles embedded in the matrix of the wheel
showed wear when CP Ti was ground (Fig. 6(a)). Similar
results were found for the Ti–Cu alloys with a lower Cu
Fig. 2. Grinding ratios of metals tested.
Fig. 3. Metal chips resulting from grinding (2 magnifications): (a) CP Ti; (b) Ti–6Al–4V; (c) 2% Cu; (d) 5% Cu; (e) 10% Cu.
M. Kikuchi et al. / Dental Materials 19 (2003) 375–381378
concentration. Note that in Fig. 6(b), fine powdery materials
stuck on some edges of the worn, flat-topped SiC abrasives;
a considerable amount of the powder was attached to the
SiC wheel surfaces used to grind Ti–6Al–4V at 1250 m/
min. Qualitative examination with SEM/EDS showed the
existence of Ti, Al, and V in this powder. Fig. 6(c) reveals a
number of Ti spheres attached to the surface of the wheel;
these Ti particles are similar to those described in Fig. 4.
After grinding Ti–6Al–4V or the 5 and 10% Cu alloys, the
SiC abrasive particles embedded in the matrix of the wheel
seldom wore out, in contrast to the wear occurring from
grinding CP Ti. In many cases, the metal removed during
grinding stuck on the wheel surface. This occurred more
often after grinding the Ti–Cu alloys with higher Cu
concentrations.
4. Discussion
The higher grinding rate of the 5 and 10% Cu alloys
compared to CP Ti is shown by the finer-sized metal chips. The
chips ground from the cast Ti–6Al–4V were similar in size
and appearance to the 1 or 2% Cu alloy chips. The grindability
found for cast Ti–6Al–4V was similar to that of the Ti–Cu
alloys with lower Cu concentrations. Improved grinding ratios
for the 5 and 10% Cu alloys contributed to prolonging the tool
life. It is notable that the ease of grinding these alloys seemed
to be dependent on grinding speed.
Worn-out SiC abrasive particles embedded in the matrix
of the wheel were found when CP Ti and the Ti–Cu alloys
with a lower Cu concentration were ground. What occurred
here is typical ‘galling’ or ‘seizing’, which leads to tool
breakdown when machining titanium [3]. Intense sparking
was observed, and metal spheres were found among the
chips and on the surfaces of the wheels when 5 and 10% Cu
alloys were ground at 1250 m/min. ‘Micro melting’
occurred in these alloys when ground at a high speed
because of the decrease in the fusion temperature as the Cu
concentration increased. The fusion temperature of the 10%
Cu alloy was estimated to be approximately 1540 8C, while
the melting point of Ti is 1670 8C [15].
In the steel industry, it is common to improve machin-
ability by using sulfur [18] or various heat treatments [19] to
decrease ductility. Industrial free-machining aluminum
alloys and free-machining brass generally contain from
one to two percent of low fusion point elements such as Pb,
Bi and Sn, which rarely form a solid solution with the matrix
[20]. These elements are dispersed in the metal as elements
or as a compound; they reduce the ductility of metals and
provide improved machinability by readily forming broken
chips. Industrial free-machining titanium contains both
sulfur and rare earth metals, which produce small globular
sulfide inclusions [10]. Inclusions in the matrix seem to be a
common factor contributing to better grindability.
The improved grindability of the Ti–Cu alloys can be
explained in the same way. Although copper is classified as
a b stabilizing element [21], the b phase is not retained at
room temperature. Even quenched, the b phase is not
retained until the copper concentration reaches 13% because
the eutectoid reaction in this alloy system is very active [21,
22]. It appears that the alloys with a higher Cu concentration
(5 or 10%) could be ground more readily because of their
structure in which the a-Ti/Ti2Cu lamellae formed among
the ductile a-Ti matrix grains [13]. In other words, the
continuous precipitation along grain boundaries of low
melting point Ti2Cu in the Ti–Cu alloy made this material
less ductile and facilitated more effective grinding by
readily forming small broken segments. In contrast, CP Ti
and the alloys with lower Cu concentration (#2%), which
both have mainly ductile a-Ti structures, were more difficult
to grind. The 10% Cu alloy had better grindability, but it had
greater bulk hardness than CP Ti [13]. This signifies that
the hardness values of metals do not necessarily reflect their
ease of grindability [23].
Fig. 5. Ten percent Cu alloy chips attached to SiC wheel (1250 m/min).Fig. 4. Coagulated particles found in the 10% Cu alloy chips (1250 m/min).
M. Kikuchi et al. / Dental Materials 19 (2003) 375–381 379
Titanium alloyed with copper is reported to have
acceptable corrosion resistance [24,25] and biocompat-
ibility [26] for dental use. Considering that the elongation
exhibited by our experimental Ti–Cu alloys with Cu
concentrations of 5% or below was more than 3% [13],
these alloys (except for the 10% Cu alloy) are good
candidates for dental casting alloys. It should also be
noted that the properties of Ti–Cu alloys could be
improved by heat treatment [22,27]. Thus, if needed, the
ductility of an ingot can be controlled by heat treatment
before machining. Better machinability/grindability with
enough elongation may be obtained by adding more than
10% copper if necessary. The majority of the present
dental alloys were developed with dental casting in mind.
However, casting metals is not the only way to fabricate
dental appliances. The CAD/CAM method represents a
great advancement over casting technology. The Ti–Cu
alloys developed here could be good candidates for
machining by computer.
The increase in grindability for the 5 and 10% Cu alloys
may be due to the inclusion of the eutectoid lamellar
aTi/Ti2Cu, which reduces the ductility of the alloy. The
grindability of the Ti–Cu alloys with lower Cu concentrations
(Cu # 2%) was similar to that of CP Ti. At higher grinding
speeds, Ti–6Al–4V had better results than CP Ti. Caution
should be used when grinding a high Cu titanium at a high
rotational speed because of the potential for micro melting to
occur due to the lower fusion temperature of these alloys.
Acknowledgements
This study was partially funded through NIH/NIDCR
grant 11787. The authors thank Mrs Jeanne Santa Cruz for
editing the manuscript.
Fig. 6. Typical SiC wheel surface after grinding (1250 m/min): (a) CP Ti; (b) Ti–6Al–4V; (c) 10% Cu.
M. Kikuchi et al. / Dental Materials 19 (2003) 375–381380
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