Hindawi Publishing Corporation
Journal of Metallurgy
Volume 2009, Article ID 853092, 4 pages
doi:10.1155/2009/853092

Research Article
Machinable Cu-40%Zn Composites Containing Graphite
Particles by Powder Metallurgy Process
Katsuyoshi Kondoh,1 Hisashi Imai,1 Yoshiharu Kosaka,2 Akimichi Kojima,2and Junko Umeda1
1 Joining and Welding Research Institute, Osaka University, 11-1 Mihogaoka, Ibaragi, Osaka 567-0047, Japan
2 Development and engineering division, SAN-ETSU METAL Co. Ltd, 1892 Ota, Tonami,Toyama 939-1315, Japan
Correspondence should be addressed to Katsuyoshi Kondoh, kondoh@jwri.osaka-u.ac.jp
Received 29 December 2008; Accepted 11 March 2009
Recommended by Ludo Froyen
Advanced Cu-40mass%Zn alloys with a high tensile strength and excellent machinability have been developed via a powder
metallurgy (P/M) process. They are environmentally benign because fine graphite particles are included instead of lead (Pb)
to improve their machinability. The eﬀect of the content and particle size of the graphite on the mechanical properties andmachinability are investigated in this paper. When applying a conventional P/M process consisting of the cold compaction andhot extrusion to consolidate elementally premixed mixture of Cu-40mass%Zn and graphite powder, the addition of 1 mass%graphite particles with 5 ∼ 10 μm in diameter is suitable to obtain a high tensile strength and good machinability at the sametime.
Copyright © 2009 Katsuyoshi Kondoh et al. This is an open access article distributed under the Creative Commons Attribution
License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly
cited.

1. Introduction
Cu-40mass%Zn (brass) alloys have a good balance ofstrength and ductility at room temperature, nonmagnetic
properties, and excellent malleability and machinability,
which are strongly required in fabricating industrial components. They also show a superior corrosion resistance inneutral and acidified NaCl solution [1]. It is well known
that lead (Pb) is one of the hazardous materials and should
be reduced in the use of products. It is, however, necessary
to add 1∼4 mass% of Pb for the improvement of themachinability of Cu-40mass%Zn alloys used for the watersupply parts and electrical products. From a viewpoint of
the environmentally benign issues, Pb-free brass alloys witha good machinability should be developed. It is also wellknown that graphite particles are environmentally benign
and not expensive materials. The addition of them to themetals can reduce and stabilize the friction coeﬃcient under
dry sliding condition. Previous works reported the use of
the graphite particles instead of Pb as the additives into

brass alloys were investigated in the conventional castingprocess [2–5]. It was diﬃcult to uniformly disperse themin the matrix of a cast brass billet because the large
diﬀerence of the specific gravity between the graphite andbrass alloy causes the floatation of the graphite particles
in the molten brass alloy and their segregation in the
cast billet [5]. In this study, P/M process is employed toprepare brass alloys uniformly dispersed graphite particles
because there is a very limited eﬀect of the large diﬀerenceof the specific gravity on the segregation of the graphite
particles in the elemental mixture powder. The eﬀect ofthe particle size and content of the added graphite particles
on the mechanical properties and machinability of P/M
extruded Cu-40mass%Zn-graphite composites is investigated.

2. Experimental Procedures
Water-atomized Cu-40mass%Zn alloy powders without Pb,Bi, and Cd, having a mean particle size of 245 μm, were10 μm(c) P/M (5 μm-Gr; 1.0%)

10 μm

10 μm

(b) P/M billet (Gr; 0%)

10 μm(a) Cast billet

10 μm

Journal of Metallurgy

10 μm

2Graphite
20 μm(d) P/M (15 μm-Gr; 0.8%)(e) P/M (15 μm-Gr; 1.0%)(f) P/M (15 μm-Gr; 1.2%)

Figure 1: Optical microstructures of hot extruded Cu-40%Zn alloys by using cast billet (a) and P/M billets with various contents of graphite
particles (b)–(f). (unit; mass%).

used as the raw materials. It was useful to prepare metallic
powder with refined microstructures, which would improvethe mechanical property [6]. Graphite powders with a mean
particle size of 5 μm, 15 μm, and 50 μm were employed as
the additives. The maximum content of graphite particles
in the elemental mixture of brass and graphite powder
was 1.2 mass%. After ball-milling the mixture for 3.6 ks,
each one was compacted at room temperature by the2000 kN hydraulic press machine. The green compact washeated at 1033 K for 2.4 ks in nitrogen gas atmosphere andimmediately hot extruded as rod materials with 10 mm indiameter. The short heating time of 2.4 ks is applied in
hot extrusion because Zn vaporization easily occurs during
heating, and the composition at the surface of the green
compact changes. Optical microstructure observation andtensile tests of each extruded brass alloy were carried out.
Concerning the machinability evaluation, the comparison of
the machining time to drill a 4.5 mm diameter hole with
5 mm depth in each material by applying 9.8 N load wasevaluated. After 10 drilling tests, the mean machining time
is used as a machinability parameter of each brass alloy inthis study.

3. Results and Discussion
Figure 1 shows optical microstructure of P/M extruded
brass alloys with various contents of graphite particles,

compared with the extruded Cu-40mass%Zn cast alloy. Even
when containing 1.2 mass% of graphite particles, they areuniformly dispersed in the matrix, and no segregation ofthem is observed. The mean grain size of P/M materials is
8∼10 μm, which is finer than that of the extruded brass castalloy with a mean grain size of 18 μm. This is because therefined microstructures by the rapid solidification during
the atomization are maintained in the extruded materials.
Figure 2 indicates a dependence of the tensile test resultson the content of the graphite particles. In the case of
0 mass% graphite content, P/M brass alloy shows a superiortensile strength to extruded brass cast alloy due to its finegrains as mentioned in Figure 1. There is no remarkabledecrease of tensile strength when adding 1.2 mass% graphite.
However, its elongation gradually decreases with increasing
the graphite content because a poor ductility of graphite
particulates causes the decrease of the elongation of thecomposite. Figure 3 shows a fractured surface of the tensile
test specimen containing 0.5 mass% graphite particles with
5 μm. Small and uniform dimple patterns are observedat the brass matrix. It means a good bonding between
the primary brass alloy powders by hot extrusion andresults in high tensile strength. However, a poor bonding
between the graphite particle and primary brass powder also
causes the decrease of the ductility of the P/M materials.
Figure 4 shows a dependence of the machining time in
the machinability test by drilling holes on the graphite
particle content of P/M brass alloys. The performance of theconventional brass alloy containing Pb is shown as a broken

Journal of Metallurgy

3550

Tensile strength, σ (MPa)Graphite particle
500

450

400

00.51Graphite content (mass%)

1.5

SEI 5.0 kV ×5000 1 μm WD 8.7 mm

Figure 3: SEM observation on fractured surface of tensile test
specimen of P/M extrude brass alloy with 0.5 mass% graphite sizeof 5 μm in diameter.

P/M (50 μm-Gr)
P/M (5 μm-Gr)

Cast billet
P/M (Gr; 0%)
P/M (15 μm-Gr)

None(a)Poor machinability

4040
Machining time, t (s)Elongation to failure, ε (%)

5030

20

103020
Reference (Cu-Zn-Pb)
100Applied load; 9.8 N
00.51Graphite content (mass%)

1.5

P/M (50 μm-Gr)
P/M (5 μm-Gr)

Cast billet
P/M (Gr; 0%)
P/M (15 μm-Gr)(b)

Figure 2: Tensile property dependence on graphite particle contentof P/M extruded Cu-40%Zn alloys; (a) tensile strength and (b)elongation to failure.

000.51Graphite content (mass%)
Cast billet
P/M (Gr; 0%)
P/M (15 μm-Gr)

1.5

P/M (50 μm-Gr)
P/M (5 μm-Gr)

Figure 4: Dependence of the machining time in drilling hole teston graphite particle content of P/M brass alloys, compared with Cu40mass%Zn-3mass%Pb alloy.

4. Conclusion
line. In the case of 0.1 mass% graphite, it was impossible
to drill a hole in the specimen demonstrating a very poor
machinability. The machining time proportionally decreases
with increase in the graphite content. When containing
1.2 mass% graphite particles, the machining time is almost
same as that of the brass with Pb alloy. Accordingly, theaddition of graphite particles is eﬀective to improve themachinability of P/M brass alloys. However, their elongationis smaller than that of a conventional brass alloy containingPb.

A powder metallurgy process is available to prepare
extruded Cu-40mass%Zn brass alloys uniformly dispersed
fine graphite particles. The increase of the graphite particle
content causes the decrease of a ductility of the wrought alloy
while the tensile strength is higher than the conventionalbrass alloy prepared via ingot metallurgy process. The
machinability strongly depends on the graphite content,
and about 1 mass% content is enough to serve the samemachinability to P/M brass alloy as the conventional brassalloy containing lead.

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Journal of Metallurgy