The present invention relates to the field of copper alloy manufacturing, and in particular to a softening-resistant copper alloy, a preparation method thereof and applications thereof, belonging to the technical field of novel alloy materials.

Welding is a manufacturing technology that joins metals or other materials by heating, at high-temperature or under high-pressure.

At present, there are mainly three methods for joining materials: fusion welding, pressure welding and braze welding. During the welding process, a workpiece and the solder are molten to form a molten area, and the molten pool is cooled and solidified to form a connection between the materials. During this process, it is usually necessary to apply a pressure. There are a variety of sources of energy for welding, including gas flame, electric arc, laser, electron beams, friction, ultrasonic waves and the like. Before the end of the 19^th century, the only welding process was metal forging already used by the blacksmith for hundreds of years. The earliest modern welding techniques appeared at the end of the 19^th century, first arc welding and oxygen-fuel welding and then resistance welding. In the early 20^th century, as the first and second world wars happened, the demand for cheap and reliable connection methods for military materials was extremely high, so that the development of the welding techniques was also facilitated. With the extensive use of welding robots in industrial applications, researchers are still studying the nature of welding and continuing to develop new welding methods to further improve the welding quality.

Throughout the development of modern welding techniques and equipment, the automation of welding equipment and the improvement of production efficiency are major driving forces for the development of welding techniques. Since copper alloys are good in strength and electrical performance, many consumables in the welding equipment use copper and its alloys, for example, electrode caps in resistance welding, conductive nozzles in braze welding and the like. With the use of modern automatic equipment, particularly welding robots, the requirements on copper alloys used for conductive nozzles, electrode caps and the like, particularly their ability to resist against high-temperature softening, are increasing. During the welding process, due to the need for heating, high temperature or high pressure, the actual copper alloy consumables are often used at a very high temperature, so the requirements on the copper alloys are also increasing. In other fields, there are also examples of using materials in a high-temperature environment. For example, electrified railway contact lines are also to be used for a long period of time at a relatively high temperature. Therefore, it is urgent to develop a copper alloy with better high-temperature softening resistance.

At present, the actually popularized products, such as conductive nozzles for welding equipment, electrode caps and electrified railway contact lines, mostly use conventional copper chromium zirconium alloy (e.g., American Standard C18150) which has been widely applied in the above fields due to its excellent strength and electrical conductivity. However, with the gradual increase of the level of mechanical automation, a strategy of replacing manpower with machines basically comes into use in welding and other industries, in order to improve the production efficiency. This change will present new requirements on the raw material performances of parts, among which high-temperature softening resistance comes first. This is because the wear of the parts will be less if the high-temperature softening resistance is better. Accordingly, the service life of the parts is prolonged and the precision during the welding process is also improved. At present, the conventional copper chromium zirconium alloy (e.g., American Standard C18150) has a high-temperature softening resistance that a hardness loss value is above 15% below 580°C. This already cannot meet the development requirements of the related industries. Therefore, improving the high-temperature softening resistance of materials becomes an urgent need at present. The Chinese patent application CN102534291 A discloses a Cu alloy containing Cr and Zr that offers improved strength and electrical conductivity and is intended for use in applications such as resistance sleeves for welding electrodes, overhead poweline wires and electrical contact materials.

An objective of the present invention is to provide a copper alloy with better high-temperature softening resistance, in order to solve the problem that the high-temperature softening resistance of the existing copper chromium zirconium alloy is to be improved.

To solve the technical problem, the softening-resistant copper alloy, comprises: 0.1%-1.0 wt% Cr, 0.01% -0.2 wt% Zr, 0.01%-0.10 wt% Si, and ≤0.10 wt% Fe, 0.0001-0.10 wt.% Mg, 0.01-2.5 wt.% of at least one of Co, Zn, Mn, Sn and Nb, with their total amount not exceeding 3.5 wt.% and with the remaining of copper and inevitable impurities, characterized in that, the microstructure of the copper alloy comprises: an elemental Cr phase, a Cu₅Zr phase, and a Cr₃Si phase and wherein the elemental Cr phase and the Cr₃Si phase satisfy the following relationship; if the weight of the elemental Cr phase is X and the weight of the Cr₃Si phase is Y, then the ratio X/Y is greater than zero and msaller than twenty (0<X/Y<20). In the copper alloy of the present invention, the high-temperature softening resistance effect of the material is improved by adding a proper amount of Si to form a compound Cr₃Si, and the strength and the high-temperature softening resistance of the material are further improved by strengthening the copper alloy matrix by the elemental Cr phase and the Cu₅Zr phase, using the synergistic effect of the Cr₃Si phase and the elemental Cr phase and by controlling the content of the impurity Fe.

The effects of the alloy elements and the related precipitated phases in the copper will be described below.

The solid solubility of chromium in copper at the normal temperature is very small (less than 0.5%), but the solid solubility of chromium in copper at a high temperature is relatively high (up to 0.65%). Therefore, chromium is able to realize precipitation strengthening and used as a main strengthening element in the copper alloy of the present invention. In the copper alloy, dispersion strengthening phase particles of the elemental Cr can be obtained by heat treatment, so the copper matrix is strengthened. While strengthening the copper matrix, Cr will also form a compound Cr₃Si with Si solid-dissolved in the copper matrix. Researches have indicated that the compound Cr₃Si is a compound phase that is stable at a high temperature and will not be dissolved even at a high temperature of 800 °C, so that the high-temperature softening resistance is very high. The content of chromium in the copper alloy of the present invention is 0.1% to 1.0%. If the content of chromium is less than this range, Cr and Si are difficult to form Cr₃Si or can form a small amount of Cr₃Si so that the desired effect cannot be achieved; however, if the content of chromium is greater than this range, chromium will be largely precipitated to form a strengthening phase, so that the chromium will be largely accumulated at the crystal boundary and the plasticity of the material is damaged.

Zirconium has a certain solubility in the copper alloy. By adding zirconium, the recrystallization temperature of the copper matrix can be increased and the high-temperature softening resistance of the copper alloy can be thus improved. Moreover, zirconium and copper will form an intermediate compound Cu₅Zr, strengthening the copper matrix and also improving the electrical performance of the copper alloy. The content of zirconium in the copper alloy of the present invention is 0.01% to 0.2%. If the content of zirconium is less than this range, the desired effect cannot be achieved; however, if the content of zirconium is greater than this range, although the alloy can be strengthened, the electrical conductivity of the alloy will be greatly reduced and the overall performance of the alloy will be influenced.

Silicon has a certain solid solubility in copper. Silicon can strengthen the copper alloy matrix, but will greatly influence the electrical conductivity of copper and will greatly reduce the electrical conductivity of the copper alloy. However, when there is a proper amount of chromium in the copper alloy, silicon and chromium can form a Cr₃Si phase compound. Since Cr₃Si is a precipitated phase, the electrical conductivity of the material can be greatly improved after Cr₃Si is precipitated, so that the overall performance of the copper alloy is positively influenced. The content of silicon in the copper alloy of the present invention is 0.01% to 0.1%. If the content of silicon is less than this range, the Cr₃Si phase formed in the copper alloy is not enough to achieve the desired effect; however, if the content of silicon is greater than this range, although sufficient Cr₃Si phase can be formed, the precipitation of Cr will be greatly reduced and the overall performance of the alloy will thus be influenced.

In the present invention, Fe is controlled as an impurity element. A small amount of Fe facilitates the improvement of strength, but a too high content of Fe will affect the electrical conductivity. Therefore, in the present invention, the content of Fe is controlled below 0.1 wt.%.

The elemental Cr phase, the Cu₅Zr phase and the Cr₃Si phase in the microstructure of the copper alloy of the present invention have the following effects.

As a primary phase of alloy, the Cr₃Si phase is generated during the liquid state and crystallization process of the alloy, is stable in both structure and performance at a high temperature, and will not be dissolved at 800 °C while still maintaining its original structure. Accordingly, the high-temperature softening resistance of the alloy can be greatly improved. As one of main precipitation strengthening phases in the copper alloy of the present invention, the Cu₅Zr phase is completely dissolved in the copper matrix to form a supersaturated solid solution after solid solution treatment on the alloy, then precipitated out of the copper matrix during the subsequent aging process and dispersedly distributed in the alloy. After the Cu₅Zr phase is precipitated, a pinning effect on the dislocation is achieved, so that the strength and hardness of the copper matrix are improved. Meanwhile, due to the precipitation of the Cu₅Zr phase, the copper matrix becomes pure, the inhibition of electrons is reduced, the electrical resistivity is reduced, and the electrical conductivity is thus greatly improved. Another strengthening phase in the copper alloy of the present invention is the elemental Cr phase. Similarly to the generation principle of the Cu₅Zr phase, the elemental Cr phase is also generated during the heat treatment of the alloy. The elemental Cr phase is completely dissolved in the copper matrix to form a supersaturated solid solution after the solid solution treatment, then precipitated out of the copper matrix during the subsequent aging process and dispersedly distributed in the alloy. As the most important strengthening phase in the alloy of the present invention, the elemental Cr phase plays a crucial role in the improvement of the strength of the alloy.

The three main strengthening phases in the alloy of the present invention exist independently and have a certain dependence. The addition of a suitable proportion of alloy elements to form a rational proportion of phases is very important for the performance of the alloy. The elemental Cr phase, as the main strengthening phase in the alloy, plays a leading role in the strengthening of the alloy; the Cr₃Si phase, as a high-temperature phase, plays a leading role in the high-temperature softening resistance of the alloy; and, the Cu₅Zr phase, as another moderate strengthening phase, can strengthen the alloy and can also increase nucleating particles, refine the elemental Cr phase and the Cr₃Si phase and allow the elemental Cr phase and the Cr₃Si phase to be dispersedly distributed, so that both the strength and the high-temperature softening resistance are further improved.

The elemental Cr phase and the Cr₃Si phase satisfy the following relationship:
if the weight of the elemental Cr phase is X and the weight of the Cr₃Si phase is Y, then 0<X/Y<20.

When the strengthening phases satisfy this ratio, both the high-temperature softening resistance and the strength of the copper alloy will be greatly improved. When the ratio of the strengthening phases is greater than 20, the amount of the Cr₃Si phase in the alloy is very small. As a result, the high-temperature softening resistance of the alloy cannot satisfy the requirements.

The copper alloy further comprises: 0.0001 % - 0.10 wt% Mg. By providing magnesium in this proportion, magnesium can be dissolved in the copper matrix to strengthen the copper alloy, with little influence on the electrical conductivity of the copper alloy; and meanwhile, oxygen in the copper alloy can be effectively eliminated, so that the content of oxygen in the copper alloy is reduced and the quality of the material is improved.

The copper alloy further comprises: 0.01% to 2.5 wt% of any one or more of Co, Zn, Mn, Sn and Nb, and their total amount does not exceed 3.5 wt% of the copper alloy. By adding the above alloy elements in the copper alloy, solid solution strengthening can be realized, the recrystallization temperature of the material is increased, and the softening temperature of the material is further increased. However, the amount of addition of the above alloy elements should not be too large, otherwise the electrical conductivity of the material will be greatly reduced.

Preferably, the softening temperature of the copper alloy is greater than or equal to 580°C. When the softening temperature of the copper alloy is greater than or equal to 580°C, the demands for various welding processes by the material can be greatly increased, and the service life of the welding material is prolonged.

The softening temperature of the copper alloy is determined by tests. Generally, when the material is kept at a certain temperature for 2 hours and then cooled in water, the hardness of the treated material is tested. If the hardness loss of the treated material is within 15%, it is considered that the material is not softened at this temperature; or otherwise, it is considered that the material is softened. The softening temperature of the conventional copper chromium zirconium alloy is about 550°C. If the conventional copper chromium zirconium alloy is kept at 550°C for 2 hours and then cooled in water, the hardness loss of the treated material is about 13% to 15%; and, if the conventional copper chromium zirconium alloy is kept at 580°C, the hardness loss is far greater than 15%.Therefore, the softening temperature of the conventional copper chromium zirconium alloy is 550 °C .However, for the copper alloy of the present invention, under the above experimental conditions, the hardness loss of the material at 550°C is 4% to 8%, and the hardness loss of the material at 550°C does not exceed 10%. Therefore, the softening temperature of the copper alloy of the present invention is greater than or equal to 580°C.

The present invention further discloses a method for preparing copper alloy, the method comprising: alloying and refining-casting into an ingot-ingot sawing, heating and extruding-solid solution heat treatment-stretching and drawing-aging heat treatment-straightening and finalizing;

characterized in that, the casting temperature for the alloying treatment and the covered refining is 1150°C to 1350°C; the temperature for the hot extrusion is 850°C to 950°C; the temperature for the solid solution treatment is 850°C to 1000°C; the cooling medium is water, and the cooling rate is 10°C/min to 150°C/s; the machining rate of the cold stretching and drawing is 20% to 60%; the temperature for the aging heat treatment is 420 °C to 520 °C; and the copper alloy is insulated for 2h to 4h. In the material produced by this production process, the elemental Cr phase, the Cu₅Zr phase and the Cr₃Si phase are rational in size and more dispersive in distribution, so that various performances of the copper alloy of the present invention are improved.

The present invention discloses a method of using the copper alloy, the method comprising using the softening-resistant copper alloy in contact lines and welding materials..

Compared with the prior art, the present invention has the following advantages:

1. In the copper alloy of the present invention, the high-temperature softening resistance effect of the material is improved by adding a proper amount of Si to form a compound Cr₃Si, and the strength and the high-temperature softening resistance of the material are further improved by strengthening the copper alloy matrix by the elemental Cr phase and the Cu₅Zr phase, using the synergistic effect of the Cr₃Si phase and the elemental Cr phase and by controlling the content of the impurity Fe.

2. Since the softening temperature of the copper alloy of the present invention is greater than or equal to 580°C, the requirements on various performances of the copper alloy in the fields of welding and contact lines are better satisfied.

To enable a further understanding of the present invention content of the invention herein, refer to the detailed description of the invention and the accompanying drawings below:

To avoid repetition, the technical parameters involved in the specific implementations will be uniformly described below, and will not be repeated in embodiments.

wt%: weight percentage.

%IACS: used for representing the electrical conductivity of a metal or alloy (reference to the standard annealed pure copper).The electrical conductivity of the standard annealed pure copper is generally defined as 100%IACS, i.e., 5.80F+7(1/Ω·m ) or 58(m/Ω·mm²).The value is the ratio of the resistivity (in volume or mass) specified by the International Annealed Copper Standard to the resistivity of the sample in the same unit multiplied by 100.

HR: Rockwell hardness.

Rem.: remaining amount.

Embodiments 1-20 (embodiments 1-17 and all further embodiments produced using these compositions are not part of the invention)

The finished softening-resistant copper alloy products in Embodiments 21-40 of the present invention were obtained by preparing materials according to the components and their mass percentages of the softening-resistant copper alloy in Embodiments 1-20 in Table 1, then smelting, casting into an ingot, processing and molding, heating to 450°C to 520°C at an average heating rate of 1 °C/min to 30°C/min and holding this temperature for 2h to 4h (Embodiments 21-40 where the finished products were obtained, corresponding to the components and their mass percentages of the softening-resistant copper alloy in Embodiments 1-20, respectively).

The microstructures of the finished softening-resistant copper alloy products in Embodiments 21-40 were analyzed. The results of analysis are shown in Table 2.

In the softening-resistant copper alloys in Embodiments 21-40 of the present invention, microscopic intermediate phases and elementary substances with different properties are formed by various added alloy elements and a particular aging process, and the microscopic phases are dispersedly distributed in the copper matrix, so that various performances of the copper alloys are effectively improved. The related phases and their contents in the softening-resistant copper alloys in Embodiments 21-40 of the present invention are shown in Table 2. The embodiments 21-37 fall outside the scope of the present invention.

Embodiment Second phase Cr(wt%) Cr₃Si (wt%) Cu₅Zr (wt%) Embodiment 21 0.0525 0.0975 0.0495 Embodiment 22 0.1045 0.0975 0.072 Embodiment 23 0.0435 0.1755 0.8505 Embodiment 24 0.119 0.143 0.1215 Embodiment 25 0.154 0.169 0.1305 Embodiment 26 0.224 0.169 0.207 Embodiment 27 0.2375 0.2275 0.216 Embodiment 28 0.251 0.247 0.2655 Embodiment 29 0.1525 0.4225 0.27 Embodiment 30 0.337 0.299 0.324 Embodiment 31 0.486 0.182 0.3555 Embodiment 32 0.3115 0.4355 0.3825 Embodiment 33 0.0312 0.624 0.4095 Embodiment 34 0.469 0.403 0.5175 Embodiment 35 0.609 0.273 0.5715 Embodiment 36 0.7855 0.1235 0.6705 Embodiment 37 0.7195 0.2015 0.6615 Embodiment 38 0.5155 0.5135 0.792 Embodiment 39 0.419 0.533 0.7965 Embodiment 40 0.4365 0.6305 0.873 Comparison embodiment 0.92 - 0.2295

The materials were prepared according to the components and their mass percentages of the softening-resistant copper alloy in Embodiments 1-20 in Table 1, and then treated under the following conditions: the casting temperature for the alloying treatment and the covered refining was 1150°C to 1350°C, the temperature for hot extrusion was 850°C to 950°C, the temperature for solid solution treatment was 850°C to 1000°C, the cooling medium was water, the cooling rate was 10°C/min to 150°C/s, the machining rate of cold drawing was 20% to 60%, the temperature for aging heat treatment was 420°C to 520°C, and the temperature holding time was 2 h to 4 h. Finally, the finished softening-resistant copper alloy bar products in Φ8 in Embodiments 41-60, corresponding to the components and their mass percentages of the softening-resistant copper alloy in Embodiments 1-20, were obtained by finishing.

The tensile strength, hardness, electrical conductivity and softening temperature of the softening-resistant copper alloy bars in Embodiments 58-60 of the present invention were tested by methods specified by the related national and industrial Embodiments 41-57 fall outside the scope of the invention. The test results are shown in Table 3.The room-temperature tensile tests were carried out by an electronic universal mechanical property testing machine according to GB/T228.1-2010 Metal Material Tensile Test Section 1: Test at Room Temperature. The samples were circular cross-section proportional samples having a proportional coefficient of 5.65.The electrical conductivity tests were carried out according to GB/T228.1-2010 Test Methods for Electrical Performance of Electric Wires and Cables Section 2: Metal Material Resistivity Test. As the test instrument, a ZFD microcomputer bridge DC resistance tester was used, and the samples were 1000 mm in length. The electrical conductivity was represented by %IACS. The hardness tests were carried out by a hardometer according to GB/T 230.1-2009 Metal Material: Rockwell Hardness Test.

Table 3: Test results of performances of the softening-resistant copper alloy bars in Embodiments 58-60 of the present invention

In the present invention, the tensile strength is higher than or equal to 470MPa, the Rockwell hardness is above 75, and the electrical conductivity is above 75%IACS.

Embodiments 61-80 The components and their mass percentages of the softening-resistant copper alloys in Embodiments 61-80 are the same as those in Embodiments 41-60. That is, the materials were prepared according to the components and their mass percentages of the softening-resistant copper alloy in Embodiments 1-20 in Table 1, and then treated under the following conditions: the casting temperature for the alloying treatment and the covered refining was 1150°C to 1350°C, the temperature for hot extrusion was 850°C to 950°C, the temperature for solid solution treatment was 850°C to 1000°C, the cooling medium was water, the cooling rate was 10°C/min to 150°C/s, the machining rate of cold drawing was 20% to 60%, the temperature for aging heat treatment was 420 °C to 520 °C, and the temperature holding time was 2h to 4h. Finally, the finished softening-resistant copper alloy bar products in Φ8 were obtained by finishing.

The softening temperature tests were carried out by methods specified by HB5420-89 Copper and Copper Alloys for Resistance Welding Electrodes and Auxiliary Devices. The test temperature was 580°C. The test results are shown in Table 4. Embodiments 61-77 fall outside of the scope of the invention. Embodiment Original hardness (HRB) 580°C Hardness after softening (HRB) Softening rate (%) Embodiment 61 75 70 6.67 Embodiment 62 77 71 7.79 Embodiment 63 75 69 8 Embodiment 64 78 73 6.41 Embodiment 65 78 72 7.69 Embodiment 66 80 75 6.25 Embodiment 67 81 77 4.94 Embodiment 68 80 76 5 Embodiment 69 82 76 7.32 Embodiment 70 81 75 7.41 Embodiment 71 84 79 5.95 Embodiment 72 86 80 6.98 Embodiment 73 82 78 4.88 Embodiment 74 87 81.5 6.32 Embodiment 75 85 80 5.88 Embodiment 76 85 81 4.71 Embodiment 77 87 81 6.90 Embodiment 78 86 80 6.98 Embodiment 79 86 81 5.81 Embodiment 80 88 82 6.82 Comparison embodiment 85 69 18.8

It can be know from the above embodiments that, in accordance with the standard test methods, the hardness loss of the copper alloy of the present invention at 580°C is below 8%, while the hardness loss of the conventional copper chromium zirconium alloy in the comparison embodiment is greater than 18%. It is indicated that the high-temperature softening resistance of the copper alloy of the present invention is greatly improved.

The softening-resistant copper alloy bars in anyone of Embodiments 41-60 are machined into appliances for welding.

The softening-resistant copper alloy bars in anyone of Embodiments 41-60 are machined into contact lines for electrified railways.

In conclusion, the softening-resistant copper alloy of the present invention has high strength, good electrical performance and excellent high-temperature softening resistance, and is particularly applied in industrial fields such as welding appliances and contact lines for electrified railways.