2020 Volume 61 Issue 8 Pages 1684-1688
C6932 is a lead-free 75.5Cu–3Si–Zn alloy (hereinafter, “C6932”) which is excellent in machinability, strength, corrosion resistance, hot workability, and castability. C6932 is widely used for water-related products and in automobile and electricity fields, and over 40,000 tons of this alloy is distributed world-wide under the brand name of ECOBRASS®.
In the United States, a law to restrict lead in drinking water was put into force across the nation in 2014. Lead is going to be restricted in Europe also in the automobile field as well as electronic and electrical field. Considering this trend, more free-cutting copper alloys are likely to be free of lead. Concurrently, in the automobile field, for instance, there is a higher demand for improved fuel efficiency by reducing component weight and improved reliability by using components of higher strength.
Based on these backgrounds, we have developed a lead-free, high-strength free-cutting copper alloy by optimizing the composition of a Cu–Si–Zn alloy and controlling its metallographic structure. The developed alloy shows significantly higher strength compared to C6932 without sacrificing its machinability and ductility. The alloy we have developed is a lead-free, high strength free-cutting copper alloy with tensile strength of 650 MPa and an elongation of 43%, which can be realized by annealing a 76.3Cu–3.2Si–0.2Sn–Zn alloy and controlling the grain size to 8 µm.
This Paper was Originally Published in Japanese in J. Japan Inst. Copper 58 (2019) 40–44. Captions of Figures and Tables were slightly modified.
Fig. 7 Microstructure of the developed alloy (a) C6932, (b) The developed alloy, (c) Enlarged imaged of (b), (d) SEM image of developed alloy.
C6932 is a lead-free 75.5Cu–3Si–Zn alloy (hereinafter, “C6932”) which is excellent in machinability, strength, corrosion resistance, hot workability, and castability. C6932 is widely used for water-related products and in automobile and electricity fields, and over 40,000 tons of this alloy is distributed world-wide under the brand name of ECOBRASS®.1)
In the United States, a law to restrict lead contained in drinking water-related devices was put into force across the nation in 2014. Lead is going to be restricted in Europe also in the automobile field as well as electronic and electrical field by End-of Life Vehicles Directive and Restriction of Hazardous Substances Directive in 2021. Considering this trend, more free-cutting copper alloys are likely to be free of lead.
Lead-containing, free-cutting brass has been conventionally used in various industries due to its excellent machinability and good hot workability. Lately, however, in the field of automobile components, for instance, there is a higher demand for better fuel efficiency through reduced component weight and improved reliability by increasing the strength of components. Because of that, depending on the intended use, conventional lead-containing brass is no longer able to meet customer requirements. In future, demand for stronger free-cutting brass is expected to increase in addition to lead-free, free-cutting brass.
In this background, we have developed a lead-free, high-strength copper alloy having a strength much higher than that of C6932 without impairing machinability or ductility by optimizing the composition of a Cu–Si–Zn alloy and controlling its microstructure.
We have investigated influence of heat treatment (annealing) using hot-extruded materials of C6932 and C3771 produced with mass production facility. In this investigation, only the C6932 material was annealed for 2 hours at 550°C. For comparison, a SUS316 material of ϕ35 mm available on the market was used. Chemical compositions of these test materials are listed in Table 1.
In a laboratory, 21 kinds of alloy were made of Cu, Si, and Sn with their contents varied between 75.0–77.5 mass%, 2.9–3.5 mass%, and 0–0.2 mass%, respectively then cast to a diameter of 95 mm. Subsequently, the cast material was hot-extruded to a diameter of 24 mm (extrusion ratio: 19) using a 500-ton extruder in the laboratory with the extrusion conditions adjusted such that the grain size became approximately 15 µm. The extruded material was then annealed for 2 hours at 550°C to see the influence of annealing. A 76.2Cu–3.25Si–Zn alloy, which is one of the aforementioned alloys, was extruded on 3 different extruding conditions adjusted such that the grain size became 11–25 µm then annealed for 2 hours at 550°C.
A 76.3Cu–3.2Si–0.2Sn–Zn alloy was cast with mass production facility then hot-extruded to a diameter of 25 mm with a 3000-ton direct extruder on 4 different extruding conditions (A to D) adjusted such that the grain size became 8–16 µm. Afterwards, the resultant bars were annealed for 2 hours at 550°C in the laboratory.
Tensile testing, observation of microstructure, and measurement of grain size were performed on each specimen. Incidentally, in the measurement of grain size, the grain sizes of α phase and κ phase were deemed uniform, and for that reason, the comparison method of JIS H0501 was used. With respect to the test materials made C6932 and 76.3Cu–3.2Si–0.2Sn–Zn alloy prepared using mass production facility, observation by an electron microscope and surface analysis by an EPMA were performed. In addition, machining test was performed using a lathe to evaluate the appearance of the chips.
Figure 1(a) shows an electron microscopic image of the C6932 material extruded with mass production facility. Figure 1(b) shows an electron microscopic image of the same C6932 material after being annealed. These pictures show that the simply extruded C6932 material has three phases consisting of α, κ, and γ phases,2) but after annealing, γ phase disappears and the microstructure changes to be a binary-phase structure consisting of α and κ phases. κ phase has higher Si concentration than α phase by approximately 2% (approximately 2% lower than γ phase) and is hard, but has a moderate degree of ductility.2) Further, some precipitated phases looking like lines were observed inside α phases at the locations circled in Fig. 1(b), which are estimated to have a plate-like shape in a three-dimensional image. The result of surface analysis by an EPMA ascertained that these linear precipitated phases are κ phases. The reason is that γ phase is the hardest phase to disappear by annealing, meanwhile linear κ phases are precipitated in α phases, resulting α phases to be divided into refined crystal grains. These microstructure change leads to improvements in strength and ductility.
Microstructure of C6932 (a) Simply extruded material, (b) Further annealed material.
Figures 2 and 3 show the results of tensile testing performed on specimens prepared in a laboratory containing 75.0–77.5 mass% Cu and 21 different amounts of Si within a range from 2.9 mass% to 3.5 mass%. The grain sizes of the alloys were adjusted to approximately 15 µm. Figure 2 shows that regardless of whether annealing was conducted, tensile strength increased directly in proportion to the increase in Si amount. Further, by annealing, tensile strength improved by about 20 MPa and elongation improved by 10%, compared with the same material before annealing. With respect to C6932, which contains 3.0 mass% Si, it was ascertained that when Si content is increased to about 3.2 mass%, tensile strength improves by about 25 MPa with very little deterioration in ductility. This suggests that a material prepared by annealing a 76.3Cu–3.2Si–Zn alloy has tensile strength which is approximately 45 MPa higher than C6932 (a 75.5Cu–3.0Si–Zn alloy) due to an increase in the amount of Si and the effect of annealing.
Si content-tensile strength relation in extruded or annealed Cu–Zn–Si materials.
Si content-elongation relation in extruded or annealed Cu–Zn–Si materials.
Figure 4 shows the relations between tensile strength and grain size, and 0.2% proof strength and grain size of a 76.2Cu–3.25Si–Zn material whose grain sizes were varied into three. Both tensile strength and 0.2% proof strength were found to be directly proportionate to d−1/2, satisfying the Hall-Petch relation.3,4) The coefficient of the Hall-Petch relation (inclination in the Hall-Petch plot) is about 23 for both tensile strength and 0.2% proof strength. For reference, the coefficient for the yield strength of a 64Cu–36Zn alloy is 125) indicating that these 76.2Cu–3.25Si–Zn alloys’ coefficients are much larger. This suggests that by reducing grain size, tensile strength and 0.2% proof strength significantly improve in 76.2Cu–3.25Si–Zn alloys.
Relations between tensile strength/0.2% proof stress and grain size in annealed 76.2Cu–3.25Si–Zn material.
Table 2 shows the mechanical properties of 76.2Cu–3.2Si–0.2Sn–Zn alloys manufactured on four different extrusion conditions A–D using mass production facility. This table shows that the 76.2Cu–3.2Si–0.2Sn–Zn alloy extruded on Condition A has approximately the same grain size as the extruded C6932 material because Condition A is equivalent to the condition used for preparing the extruded C6932 material. However, it exhibited 45 MPa higher tensile strength than the extruded C6932 material. Si content of 76.2Cu–3.2Si–0.2Sn–Zn alloy is 0.2 mass% larger than that of C6932 whose Si content is 3.0 mass%. Also, unlike the production condition of C6932, Condition A includes annealing. Therefore, it is considered that the improvement in tensile strength by 45 MPa is a result of 25 MPa improvement due to a slight increase in Si content and additional 20 MPa improvement due to annealing, as indicated by the laboratory specimen shown in Fig. 2.
Further, the grain size of the specimen made in accordance with Condition A is 16 µm. However, it gradually became smaller as the extrusion condition was changed to Condition B, C, and to D. When Condition D was applied, the tensile strength further improved by 55 MPa due to the effect of grain refinement. Hereinafter, the 76.2Cu–3.2Si–0.2Sn–Zn alloy manufactured in accordance with Condition D and further annealed for 2 hours at 550°C is referred to as the “developed alloy”.
Compared with C6932, the developed alloy was found to have high tensile strength of 650 MPa, which is 100 MPa higher than that of C6932 and high elongation of 43%, which is an improvement of 8%. When compared with free-cutting brass alloy C3771, the tensile strength of the developed alloy was 1.63 times higher, and the 0.2% proof strength was about twice as high. When compared with a rod made of SUS 316 also, the developed alloy turned out to have a tensile strength over 80 MPa higher.
Figure 5 shows the stress-strain curve of the developed alloy. Generally speaking, even though tensile strength of a brass material can easily be improved by work hardening, as the tensile strength increases, the elongation declines. However, as shown in Fig. 5, the tensile strength of the developed alloy is higher than C6932 and SUS316 across the entire range of strain, and as the grains become finer, the alloy becomes stronger with no deterioration in ductility observed. In addition, the developed alloy has a ductility equivalent to that of free-cutting brasses.
Stress-strain curve of the developed alloy.
Figure 6 shows the Hall-Petch relations of the test material (made of 76.2Cu–3.2Si–0.2Sn–Zn alloy) manufactured with mass production facility and the laboratory specimen (made of 76.2Cu–3.25Si–Zn alloy). This figure shows that their coefficients (inclination in the Hall-Petch plot) are approximately the same, and the effect of grain refinement is significant in the test material manufactured with mass production facility, too. That the test material produced with mass production facility generally has a higher tensile strength than the laboratory specimen is considered to be induced by the difference in extrusion ratio.
Tensile strength-grain size relation in laboratory specimen (76.2Cu–3.25Si–Zn) and production-line specimen (76.2Cu–3.2Si–0.2Sn–Zn).
Table 3 shows the κ phase ratios of annealed C6932 alloy and the developed alloy.
Figure 7 shows the microstructures of these alloys. The κ phase ratio of the developed alloy is about 10% higher than that of annealed C6932. The Cu–Si pseudobinary phase diagram of a Cu–Si–Zn alloy (cross-section where Zn content is 20 wt%)6) shown in Fig. 9 suggests that the higher the Si content, the higher the κ phase ratio becomes. That is, it is considered that the tensile strength increases in proportion to the increase in Si content because the ratio of κ phase, which is a strong phase, increases if Si content increases. Figure 7 shows that due to annealing, γ phase, which is a hard phase, is not present either in C6932 or the developed alloy. Whereas C6932 has crystal grains of 16 µm, the developed alloy has finer crystal grains whose size is 8 µm. It also has a larger number of linear phases precipitated inside α phases as shown by Fig. 7 at the circled locations. It is considered that the tensile strength improved significantly without deterioration in ductility due to the effects of disappearance of γ phase, appearance of the linear phases precipitated inside α phases, and grain refinement.
Microstructure of the developed alloy (a) C6932, (b) The developed alloy, (c) Enlarged imaged of (b), (d) SEM image of developed alloy.
Microstructure of the developed alloy (a) SEM image of the developed alloy, (b) Results of surface analysis.
Pseudobinary phase diagram for Cu–Si–Zn alloys (Cross section where Zn = 20 wt%).
The results of surface analysis by an EPMA shown in Fig. 8 have revealed that the linear phases precipitated in the developed alloy are κ phases, and the thickness of these linear κ phases is approximately 0.3 µm. Although precipitation directions are not uniform, many of such κ phases have both of their ends on a grain boundary of α phase. It is presumed that α phases have been further refined due to their division by these linear κ phases, and linear κ phases have also strengthened α phases themselves. These effects are presumed to allow the coefficient in the Hall-Petch relation (inclination in the Hall-Petch plot) to increase and significantly improve the tensile strength of the developed alloy.
3.5 Results of machining testsFigure 10 shows the appearances of chips produced by machining C6932 and the developed alloy with a lathe to a depth of 1.0 mm at a machining speed of ϕ14 × 900 rpm (40 m/min) and a feed rate of 0.11 mm/rev. Both alloys produced chips of approximately the same shape having good brittleness. Despite disappearance of γ phases, which have excellent brittleness, due to annealing, the developed alloy had the same level of machinability as C6932. It is considered that the increase in κ phase ratio promoted by increased Si content, and the linear κ phases precipitated inside α phases play an important role in such machinability improvement.
Chip shape (a) C6932, (b) Developed alloy.
The developed alloy is patented in Japan under patent number 6448167.
