Mechanical Properties of Copper Sulfide Dispersed Lead Free Bronze

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Mechanical Properties of Copper Sulfide-Dispersed Lead-Free Bronze

*1

Hidekazu Sueyoshi

1

, Yuki Yamano

1;*2

, Kensuke Inoue

1;*2

,

Yoshikazu Maeda

2

and Kosaku Yamada

3

1

Graduate School of Science and Engineering, Kagoshima University, Kagoshima 890-0065, Japan

2Faculty of Engineering, Kagoshima University, Kagoshima 890-0065, Japan 3Department of Management, Kyushu Tabuchi Co. Ltd., Kirishima 899-4462, Japan

Microstructure, tensile properties and Vickers hardness of the copper sulfide (Cu2S)-dispersed bronze produced by adding MoS2to molten bronze consisting of Cu and Sn were examined. The amount of Cu2S and the number of casting defects increase with the amount of added MoS2. The Vickers hardness of Cu2S-dispersed bronze increases with the amount of added MoS2. This is because the solid-solution hardening owing to Mo and the hardening due to the dispersion of Cu2S are superior to the softening due to casting defects. The tensile strength of Cu2S-dispersed bronze decreases with increasing amount of added MoS2. This is because the softening due to casting defects outweights the solid-solution hardening due to Mo and the strengthening due to the dispersion of Cu2S. The fracture elongation of Cu2S-dispersed bronze decreases with the amount of added MoS2. This is because the number of casting defects increases and Cu2S acts as a nucleus of voids. To produce Pb-free bronze having excellent machinability and mechanical properties equivalent to those of CAC406 containing Pb and CAC902 containing Bi, adequate control of the amount and size of Cu2S and the number of casting defects is of great importance. [doi:10.2320/matertrans.D-MRA2008841]

(Received October 24, 2008; Accepted January 8, 2009; Published February 25, 2009)

Keywords: lead-free bronze, copper sulfide, microstructure, mechanical property

1. Introduction

Bronze (JIS: CAC406) containing several mass% Pb has been widely used for water faucets and pipes for fresh water supply because of its good machinability. However, Pb is harmful to humans. To limit the amount of permitted Pb in drinking water supplies, the leaching standard value of Pb was severely revised to 0.01 mg/L in Japan.

Recently, Pb-free bronze (JIS: CAC902) containing Bi as an alternative of Pb has widely been used.1)However, Bi is a minor metal and has some disadvantages, such as the exhaustion of resources2)and its being harmful to humans.3) From the viewpoint of life cycle assessment (LCA), the development of a new Pb-free bronze containing an alter-native free-machining additive is desirable.

The authors have reported that (1) copper-sulfide (Cu2 S)-dispersed bronze is obtained by adding molybdenum sulfide (MoS2) to molten bronze, (2) the dispersion of Cu2S reduces the coefficient of friction on the rake surface and increases the shear angle, resulting in a reduction in cutting force, (3) Cu2S-dispersed bronze has a good chip disposability because Cu2S acts as a chip breaker, (4) Cu2S has no influence on the finished surface roughness, and (5) Cu2S-dispersed bronze has an excellent machinability equivalent to those of CAC406 and CAC902.4) In order to use Cu

2S-dispersed bronze for water faucets, the estimation of its mechanical properties is of importance. Maruyama et al.5)reported that sulfide-dispersed Pb-free copper alloy castings can be produced by S activity control with Ni or Fe addition to molten copper alloy. However, in high S content (such as 0.81 mass%) the tensile strength and the elongation were below the standardized values of CAC406.

In the present study, tensile and hardness tests were carried out using various types of Cu2S-dispersed bronze, and the

effects of microstructure on mechanical properties were examined.

2. Experimental Procedure

Cu and Sn were melted using a high-frequency induction furnace with a carbon crucible under nitrogen gas atmo-sphere. MoS2 was added to the molten bronze at 1463 K, followed by agitation and holding for 1.8 ks. Then, the molten bronze was poured into a shell mold. The obtained ingot was 20 mm in plain part diameter and 25 mm in chuck part diameter. Four types of bronze (BLMS, BSMS1, BSMS1.5 and BSMS2) were produced by adding large-size MoS2(mean grain size: 15mm) of 2.0 mass% and small-size MoS2 (mean grain size: 1.2mm) of 1.0, 1.5 and 2.0 mass%, respectively. As reference materials, the bronze without free-machining additives (BCFZ), the bronze containing Pb (CAC406) and the bronze containing Bi (CAC902) were also produced. Chemical compositions of these specimens are listed in Table 1. The amount of Cu2S was obtained by assuming that all S formed Cu2S.

The microstructure of the bronze specimens was charac-terized by an electron-probe microanalyzer (EPMA). A tensile test was carried out using a rod specimen (JIS No. 4: 14 mm in diameter and 60 mm in gauge length). After the tensile test, the fracture surface was examined using EPMA. Vickers hardness was measured at the cross section of the tensile specimen.

3. Results and Discussion

Figure 1 shows back-scattered electron (BSE) images of the specimens. In BLMS and BSMS, black parts are casting defects (shrinkage cavity and gas defect). The size of casting defects in BSMS2 was smaller than that in BLMS. However, the number of casting defects in BSMS2 was larger than that in BLMS. The number of casting defects in BSMS1.5 was smaller than that in BSMS2. The number

*1This Paper was Originally Published in Japanese in J. JRICu47(2008)

147–152.

*2Graduate Student, Kagoshima University

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of casting defects in BSMS1 was smaller than that in BSMS1.5. Thus, the size of casting defects became small as the size of added MoS2 was small, and the number of casting defects decreased with decreasing amount of added MoS2. In BLMS and BSMS2, a uniform distribution of gray phases was observed.

Figure 2 shows EPMA analysis for BSMS2. In the gray phases in the SE image, large amounts of Cu and S and a small amount of Mo were detected, while no Sn was detected. By the point analysis (quantitative analysis) of the gray phase, it was found that the ratio of Cu to S is Cu:S¼

2 : 1at%. This indicates that the gray phase is Cu2S. The formation of Cu2S was also confirmed by thermodynamic analysis, in which the standard free energy for Cu2S (calculated by Outokumpu Research HSC chemistry 5) became negative at the addition temperature of MoS2 (1463 K). As shown in Fig. 1(a), the size of Cu2S in BLMS was the same as that of added MoS2(15mm), while its size in BSMS2 was larger than that of added MoS2 (1.2mm).

In Fig. 1(c), the white phase is Pb. A uniform distribution of Pb having a grain size of severalmmwas observed. The size of casting defects was small. The number of casting Table 1 Chemical compositions of the specimens (mass%).

Specimen Cu Sn Pb Bi Zn Fe Ni P Al Si Sb Mo S Cu2S

BLMS 92.90 6.12 — — — — — 0.031 — — — 0.334 0.838 4.16

BSMS1 92.50 6.37 — — — — — 0.028 — — — 0.256 0.538 2.67

BSMS1.5 92.80 6.34 — — — — — 0.036 — — — 0.278 0.771 3.83

BSMS2 92.80 6.13 — — — — — 0.024 — — — 0.383 0.860 4.27

BCFZ 94.50 5.48 — — — — — 0.035 — — — — — —

CAC406 83.59 4.65 5.98 — 5.25 0.049 0.203 0.011 0.001 0.001 0.054 — — —

CAC902 85.62 4.02 — 2.83 7.24 0.016 0.039 0.018 0.001 0.001 0.173 — — —

(a) (b) (c) (d)

50µm 50µm 50µm 50µm

Fig. 1 BSE images of the specimens. (a) BLMS, (b) BSMS2, (c) CAC406 and (d) CAC902.

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(d)

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defects was smaller than that in BSMS2. In Fig. 1(d), the white phase is Bi. A uniform distribution of Bi was observed. The size of casting defects was small. The number of casting defects was smaller than that in BSMS2.

As shown in Fig. 2(e), Mo was detected in both Cu2S and the bronze matrix. However, the content of Mo shown in Table 1 was smaller than that predicted from the amount of added MoS2. Figure 3 shows the X-ray diffraction profile of the slag skimmed off from the surface of molten bronze after the addition of MoS2. It is found from Fig. 3 that the slag mainly consists of Mo oxide (MoO2).

Figure 4 shows standard free energies for Cu, Sn, S and Mo oxides. Although each oxide had a negative value at the addition temperature of MoS2 (1463 K), MoO2 had a maximum negative value. This suggests that MoO2 is preferentially formed in the molten bronze. It is considered from these results that a portion of Mo produced by the decomposition of MoS2 or the reaction between S in MoS2 and Cu becomes MoO2 and then, it is skimmed off from the surface of molten bronze as a slag, while the remaining Mo dissolves into the bronze matrix during solidification. This suggests that the bronze matrix is hardened by the solid-solution hardening due to Mo.

Figure 5 shows the relationship between Vickers hardness (load: 9.8 N) and the amount of Cu2S. The hardness increased with the amount of Cu2S or the amount of added MoS2.

Hardness tests were also carried out under different loads. In BLMS, Hv90.0 under a load of 980 mN was obtained at a casting defects-free matrix. On the other hand, a relatively large Cu2S hadHv134 under a load of 98 mN.Hv95.8, which was higher than the matrix, was obtained in the Cu2S-matrix mixed region without casting defects under a load of 980 mN. Thus, the hardness of Cu2S was higher than that of the matrix. Although Cu2S has an orthorhombic structure at room temperature, it transforms into a hexagonal structure, which has a high deformability, at temperatures higher than 377 K.6) Because Cu2S in the bronze is transformed into a soft hexagonal structure by the heat produced during machining, the machinability of the bronze is improved. On the other hand, Cu2S has a high hardness at room temperature because of an orthorhombic structure, resulting in an increase in the hardness of the bronze. The hardness shown in Fig. 5 was measured under a load of 9.8 N. The indentation had a diagonal length of about 150mm. As a result, a large amount of Cu2S and many casting defects were present in the indentation. It is considered that the hardness change shown in Fig. 5 is affected by microstructural factors and the solid-solution hardening due to Mo. The absence of hardness difference between CAC406 and BSMS1 may be attributed to the cancellation of the solid-solution hardening due to Mo, the hardening due to dispersed Cu2S, and the softening due to the increase in casting defects. In the case of Cu2S-dispersed bronze, the hardness change with the amount of Cu2S is related to both the hardening due to dispersed Cu2S and the softening due to the increase in casting defects. Thus, the hardness depends on both Cu2S and casting defects. In BSMS2, the dispersion of Cu2S contributes significantly to the hardening compared with the softening due to casting defects, resulting in a high hardness. In CAC406 and CAC902, the softening caused by Pb and Bi is significant compared with the hardening due to few casting defects and the addition of Zn, resulting in a hardness lower than that of BCFZ.

Figure 6 shows the relationship between tensile strength and the amount of Cu2S. Contrary to the hardness change with the amount of Cu2S (Fig. 5), tensile strength decreased with an increase in the amount of Cu2S or the amount of MoS2. This suggests that the softening due to casting defects has a significant influence compared with the solid-solution hardening due to Mo and the strengthening due to dispersed Cu2S.

Intencity(a.u.)

(deg)

θ

MoO2

10 20 30 40 50 60 70 80

Fig. 3 X-ray diffraction profile of the slag.

Temperature,T/K

400 –300 –250 –200 –150 –100 –50 0

600 800 1000 1200 1400 1600

Standard free energy,

G ° /kJ · mol -1 CuO

Cu2O SO2

SO

SnO2

SnO

MoO2

Fig. 4 Standard free energies for Cu, Sn, S and Mo oxides.

Vickers hardness,HV

Amount of Cu2S,m(mass%)

C C

Cu2S size

Small CAC406 Large CAC902 90 80 70 60 50 40 30

0 1 2 3 4 5

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Figure 7 shows the relationship between fracture elonga-tion and the amount of Cu2S. The fracture elongation decreased with an increase in the amount of Cu2S. The reason for this is discussed later.

Figure 8 shows the relationship between tensile strength and fracture elongation. The fracture elongation decreased with decreasing tensile strength. The standard of Japan Water Works Association (JWWA) is showed by dashed lines in Fig. 8. As shown in Fig. 8, Cu2S-dispersed bronzes satisfied this standard.

Figure 9 shows the SE image of the fracture surface of BCFZ. As shown in the SE image, the fracture surface consisted of a number of large and deep dimples. An inclusion was observed in a dimple. It is considered that voids are formed at not only the casting defects but also the inclusion/matrix interface, and then, the coalescence of adjacent voids occurs with the growth of voids, followed by fracture. This explains why BCFZ had a large fracture elongation.

Figure 10 shows EPMA analysis for the fracture surface of BLMS. As shown in the SE image, the fracture surface consisted of dimples smaller and shallower than those of BCFZ (Fig. 9). An inclusion was observed in a dimple. As shown in Fig. 10(b), the content of S was high in the inclusion. It was found by point analysis (quantitative analysis) that the inclusion is Cu2S. This suggests that Cu2S acts as a nucleus of voids. As a large amount of MoS2 is added to molten bronze, many casting defects and a large

amount of Cu2S are induced in the bronze matrix. As a result, the number of voids produced during tensile test increases because Cu2S acts as a nucleus of voids. The increase in the number of voids makes it easy to coalesce adjacent voids. As shown in Fig. 7, the fracture elongations of BCMS2 and BLMS containing large amounts of Cu2S were low. In the case of BSMS2 and BLMS having the same amount of Cu2S, the fracture elongation of BSMS2, which had more voids and Cu2S, was lower than that of BLMS. These results are strongly related to the nucleation-growth-coalescence behav-ior of voids, as mentioned above.

Figure 11 shows EPMA analysis for the fracture surface of CAC406. The fracture surface consisted of many dimples. Although the size of Pb was several mmbefore the tensile test, Pb on the fracture surface was enlarged as shown in Fig. 11(b). This suggests that Pb also deforms with the matrix deformation due to the nucleation-growth of voids in the tensile test because Pb is very soft.

Figure 12 shows EPMA analysis for the fracture surface of CAC902. The fracture surface consisted of many dimples. Cu2S size

Small Large

Tensile strength,

B

/MPa

Amount of Cu2S,m(mass%)

300

250

200

150

100

0 1 2 3 4 5

Fig. 6 Relationship between tensile strength and the amount of Cu2S.

Amount of Cu2S,m(mass%)

Fracture elongation,

(%)

Cu2S size

Small Large 45

40

35

30

25

20

15

10

0 1 2 3 4 5

Fig. 7 Relationship between fracture elongation and the amount of Cu2S.

Tensile strength,

B

/MPa

Fracture elongation, (%)

300

200

100

JWWA

BSMS1

BSMS1.5

BSMS2 BLMS BCFZ

CAC406 CAC902 0

0 10 20 30 40 50

Fig. 8 Relationship between tensile strength and fracture elongation.

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(b)

50

µ

m

Fig. 10 EPMA analysis for the fracture surface of BLMS. (a) SE and (b) S.

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50

µ

m

Fig. 11 EPMA analysis for the fracture surface of CAC406. (a) SE and (b) Pb.

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50

µ

m

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As shown in Fig. 12(b), the content of Bi was high in the inclusion in a dimple. This suggests that Bi acts as a nucleus of voids. However, the degree of enlargement of Bi was smaller than that of Pb. This is because Bi is harder than Pb.

Thus, the mechanical properties of Cu2S-dispersed bronze depend on the matrix, casting defects and Cu2S. To develop the Cu2S-dispersed bronze having good machinability and mechanical properties equivalent to those of CAC406 and CAC902, the adequate control of these structural factors is of great importance.

4. Conclusions

The Cu2S-dispersed bronze was prepared by adding MoS2 to molten bronze consisting of Cu and Sn, and its micro-structure and mechanical properties were examined. The obtained results are as follows:

(1) The amounts of Cu2S and pores increase with increas-ing amount of added MoS2.

(2) The hardness increases with increasing amount of added MoS2. This is because the solid-solution hard-ening due to Mo and the hardhard-ening due to dispersed Cu2S contribute significantly to the increase in hardness compared with the softening due to casting defects. (3) The tensile strength decreases with an increase in the

amount of added MoS2. This is because the softening due to the increase in casting defects is large compared with the solid-solution hardening due to Mo and the strengthening due to dispersed Cu2S.

(4) The fracture elongation decreases with an increase in the amount of MoS2. This is because many voids produced from casting defects and Cu2S, which acts as a nucleus of voids, enhance the coalescence of adjacent voids.

(5) To develop the Cu2S-dispersed bronze having good machinability and mechanical properties equivalent to those of CAC406 containing Pb and CAC902 contain-ing Bi, the adequate control of the number and size of casting defects and the amount of Cu2S is of great importance.

Acknowledgments

This work was supported by a Grant-in-Aid for Scientific Research from the Japan Society for the Promotion of Science (No. 19510101) and a 2006 Grant from the Japan Research Institute for Advanced Copper-Base Materials and Technologies.

REFERENCES

1) T. Kobayashi and T. Maruyama: Materia Japan43(2004) 647–650. 2) I. Yamauchi:New Materials Data Book18(2002) pp. 455–460. 3) G. Lagier: Therapie35(1980) 315–317.

4) H. Sueyoshi, K. Inoue, Y. Yamano, Y. Maeda and K. Yamada: J. JRICu 47(2008) 142–146.

5) T. Maruyama, H. Wakai, T. Kobayashi and H. Abe: AFS Trans.116 (2008) 299–307.

Figure

Fig. 2EPMA analysis for BSMS2. (a) SE, (b) Cu, (c) Sn, (d) S and (e) Mo.
Fig. 2EPMA analysis for BSMS2. (a) SE, (b) Cu, (c) Sn, (d) S and (e) Mo. p.2
Fig. 1BSE images of the specimens. (a) BLMS, (b) BSMS2, (c) CAC406 and (d) CAC902.
Fig. 1BSE images of the specimens. (a) BLMS, (b) BSMS2, (c) CAC406 and (d) CAC902. p.2
Fig. 5Relationship between Vickers hardness and the amount of Cu2S.
Fig. 5Relationship between Vickers hardness and the amount of Cu2S. p.3
Fig. 3X-ray diffraction profile of the slag.
Fig. 3X-ray diffraction profile of the slag. p.3
Fig. 4Standard free energies for Cu, Sn, S and Mo oxides.
Fig. 4Standard free energies for Cu, Sn, S and Mo oxides. p.3
Fig. 10EPMA analysis for the fracture surface of BLMS. (a) SE and (b) S.
Fig. 10EPMA analysis for the fracture surface of BLMS. (a) SE and (b) S. p.5
Fig. 11EPMA analysis for the fracture surface of CAC406. (a) SE and (b) Pb.
Fig. 11EPMA analysis for the fracture surface of CAC406. (a) SE and (b) Pb. p.5
Fig. 12EPMA analysis for the fracture surface of CAC902. (a) SE and (b) Bi.
Fig. 12EPMA analysis for the fracture surface of CAC902. (a) SE and (b) Bi. p.5

References