Copper alloy, semi-finished product and electrical connection element made of copper alloy

Abstract

The invention relates to a copper alloy with the following composition in wt.%: Sn: 3.0 - 6.5%, Ni: 0.30 - 0.70%, P: 0.15 - 0.40%, S: 0.10 - 0.40%, Zn: optionally up to 0.20%, Fe: optionally up to 0.50%, Mn: optionally up to 0.50%, Pb: optionally up to 0.25%, balance copper and unavoidable impurities, wherein the ratio of Ni to P is at least 1.1 and at most 2.8 and wherein the alloy contains nickel phosphides.

Description

[0001] The invention relates to a copper alloy, in particular a wrought copper alloy, a wire- or rod-shaped semi-finished product made of a copper alloy and an electrical connecting element made of a copper alloy.

[0002] In electrical engineering, copper alloys are typically used for components intended to create an electrical connection. Such components are generally referred to as electrical connecting elements. Examples include connectors and electrical terminals. The components can be manufactured by machining from wire- or rod-shaped semi-finished products. The connection of such a component to an electrical line, for example a cable, is usually achieved by a screw connection or a crimp connection. To enable crimping, the material of the component must be easily cold-formable, even in its final state. The electrical connection in such a component is often realized by an external force that leads to elastic deformation of the component or at least part of the component, thus creating a spring effect.Due to the movement of lattice defects, such as dislocations, and alloy atoms, elastic deformation resulting from external stress in the material is transformed over time into plastic deformation. This process, known as relaxation, leads to a decrease in the force maintaining the electrical connection over time. To ensure a secure electrical connection over the long term, the component material must therefore exhibit high resistance to relaxation. Copper alloys for electrical connection elements, especially for connectors, must therefore exhibit not only good cold formability, good machinability, and high electrical conductivity, but also, in particular, high relaxation resistance and high strength.

[0003] Today, connectors are made from the CuSn4Zn4Pb4 alloy. The high lead content of approximately 4 wt.% makes the alloy easy to machine. However, lead is considered a health and environmental concern, so there is a need for a lead-reduced or lead-free alternative to CuSn4Zn4Pb4.

[0004] From patent specification EP 3 225 707 B1 it is known that the lead-containing casting alloy CuSn5Zn5Pb2, which is used to manufacture components for media-carrying gas or water pipes, can be replaced by a copper alloy containing 3.5 to 4.8 wt.% Sn, 1.5 to 3.5 wt.% Zn, 0.25 to 0.65 wt.% S and 0.04 to 0.1 wt.% P. The alloy can optionally contain up to 0.09 wt.% lead and up to 0.4 wt.% nickel. Due to the sulfur content, copper sulfides and zinc sulfides form in the alloy, which improve the machinability of the casting alloy. However, the zinc sulfides have a negative effect on cold formability because they lead to the formation of cracks.Although a reduction in the Zn content improves cold formability, the electrical conductivity and relaxation resistance of the resulting alloy are unsatisfactory, as they do not reach the values of the CuSn4Zn4Pb4 alloy currently used for electrical connectors.

[0005] A lead-free, chipless phosphor bronze rod material is known from patent JP 2019 014946 A.

[0006] The invention is therefore based on the object of specifying a copper alloy, particularly for use in electrical connectors. The alloy must exhibit the following property profile: reduced Pb content, good machinability, high strength, high electrical conductivity, and good relaxation resistance. Furthermore, the alloy must be readily cold-formable in order to produce semi-finished products in small dimensions at low cost and to enable crimping of the finished component made from the alloy. The external dimension of the semi-finished product, for example, its outer diameter, is typically 1.5 to 12 mm.

[0007] The invention is defined by the features of claim 1. The further dependent claims relate to advantageous embodiments and developments of the invention.

[0008] The invention relates to a copper alloy with the following composition in wt.%: Sn: 3,0 - 6,5 %, No: 0,30 - 0,70 %, P: 0,15 - 0,40 %, S: 0,10 - 0,40 %, Zn: optionally up to 0.20%, Fe: optionally up to 0.50%, Mn: optionally up to 0.50%, Pb: optionally up to 0.25%,

[0009] The remainder being copper and unavoidable impurities, the ratio of Ni to P being at least 1.1 and at most 2.8, and the alloy containing nickel phosphides.

[0010] Copper-tin alloys exhibit good spring and sliding properties and excellent fatigue strength. Tin also improves the strength and hardness of the alloy and increases corrosion and wear resistance. As the tin content increases, strength and hardness increase, while electrical conductivity decreases. A favorable combination of these properties results when the tin content is at least 3.0 wt.% and at most 6.5 wt.%. With a tin content of less than 3.0 wt.%, the necessary strength cannot be achieved. With a tin content of more than 6.5 wt.%, the desired conductivity can no longer be achieved. Furthermore, with a tin content above 6.5 wt.%, a brittle delta phase forms and hardness increases, while ductility decreases.

[0011] The solubility of sulfur in copper is low. Sulfur leads to the formation of sulfides with the elements Cu, Zn, Fe, and Mn. The sulfides act as chip breakers and thus improve machinability. Below 0.10 wt.%, the sulfur content in the alloy is too low to improve machinability. If the sulfur content exceeds 0.40 wt.%, the material becomes brittle and cold formability is impaired. Precipitations of the Cu 2 S type only slightly reduce electrical conductivity.

[0012] Nickel, in combination with phosphorus, has a positive effect on electrical conductivity and relaxation resistance. Surprisingly, it has been shown that in copper-tin alloys with a phosphorus content in the range of 0.15 to 0.40 wt.%, the addition of nickel in the range of 0.30 to 0.70 wt.% improves electrical conductivity and relaxation resistance compared to nickel-free alloys. This effect can be attributed to the formation of nickel phosphides. Nickel phosphides can have the compositions Ni2P, Ni5P2, and Ni3P. Surprisingly, it has been shown that the formation of nickel phosphides is favored when the ratio of Ni to P, abbreviated as Ni / P, is at least 1.1 and at most 2.8. In this range, favorable conditions arise for the formation of nickel phosphides alongside competing compounds of other elements with phosphorus.The Ni / P ratio is preferably at least 1.4 and at most 2.5, and more preferably at least 1.6 and at most 2.1. At nickel contents below 0.3 wt.%, insufficient precipitates are formed to improve relaxation resistance.

[0013] The nickel phosphides predominantly form globular precipitates, which are evenly distributed throughout the microstructure. Due to their uniform distribution, they enable uniform forming of the material. The nickel phosphides are very small, so that for a given total amount of nickel phosphides, the number of precipitates per volume is very large. This is why nickel phosphides are particularly effective at fixing lattice defects such as grain boundaries and dislocations in the microstructure, i.e., preventing them from moving. As a result, the nickel phosphides not only contribute to an increase in strength but also significantly improve relaxation resistance. This makes the alloy particularly advantageous, because an increase in strength achieved through cold forming of the material usually leads to a deterioration in relaxation resistance: Cold forming creates dislocations that make the material stronger.When the material is exposed to elevated temperatures during use, dislocation movement occurs (recovery processes), resulting in a reduction in strength. Finely dispersed nickel phosphide precipitates, which pin the dislocations in place, prevent this dislocation movement. This keeps the material strong and ensures relaxation resistance.

[0014] The copper alloy may contain up to 0.20 wt.% zinc. Zinc forms zinc sulfides, which improve machinability but negatively impact cold formability. Therefore, the alloy must not contain more than 0.20 wt.% zinc.

[0015] The copper alloy can contain up to 0.50 wt.% iron. Iron is less soluble in the copper crystal lattice than nickel. It therefore forms precipitates earlier as the melt cools and, in small amounts, can cause grain refinement. Iron sulfides can improve the alloy's machining properties. Iron can react with phosphorus to form iron phosphides, which have a similar effect on relaxation as nickel phosphides. However, these phosphides have a melting point of 1350°C and therefore form from the melt. This can lead to large precipitates that negatively impact cold formability. Therefore, the iron content in the alloy may not exceed 0.50 wt.%, preferably not exceed 0.24 wt.%, and particularly preferably not exceed 0.08 wt.%.

[0016] The copper alloy can contain up to 0.50 wt.% manganese. Manganese, together with sulfur, forms sulfides that promote chip breaking. Manganese thus contributes to improved machinability. If the manganese content exceeds 0.50 wt.%, intermetallic phases can form that negatively affect the properties of the alloy. The manganese content in the alloy is preferably 0.10 wt.% or less.

[0017] The copper alloy can contain up to 0.25 wt.% lead, preferably up to 0.09 wt.% lead. Lead has a positive effect on machinability, but its proportion in the alloy is limited by regulations.

[0018] The remainder of the alloy consists of copper and unavoidable impurities. The impurity content is preferably less than 0.2 wt.%, particularly preferably less than 0.1 wt.%.

[0019] The alloy exhibits excellent properties in terms of cold formability, machinability, strength, electrical conductivity, and relaxation resistance. The low lead content makes it suitable for applications where the lead content is limited by regulatory requirements. The alloy can be machined sufficiently well even with a lead content of less than 0.03 wt.%.

[0020] The alloy's unique property profile is achieved in particular by the specific selection of the alloying elements Ni, P, and S. The proportions of these elements are chosen so that copper sulfides and nickel phosphides are preferentially formed. Copper sulfides improve machinability, while nickel phosphides improve relaxation resistance. The specific selection of the alloy composition makes it possible to produce precisely these two types of precipitates in the correct amounts.

[0021] To produce semi-finished products, the alloy is melted and cast. The cast size can be immediately cold formed without hot forming. In the formed state, the alloy is referred to as a wrought alloy. The alloy can, for example, be cast as cast wire with a diameter of 18 to 25 mm. The cast wire can be cold formed by rolling or drawing. Annealing can then take place in a temperature range between 550 and 700 °C for 2 to 7 hours to recrystallize the microstructure. If the annealing temperature exceeds 750 °C, the electrical conductivity is reduced because above this temperature nickel and phosphorus dissolve again. The sequence of forming and recrystallization annealing can be repeated until the desired dimension of the semi-finished product is achieved. Advantageously, the degree of deformation, defined as the relative reduction in cross-sectional area, is at least 15% in each forming step.In particular, the sequence of forming steps may include at least one step with a degree of forming of at least 60%, particularly preferably at least 70%. The final process in the production of a semi-finished product is usually a forming process, in particular a drawing process.

[0022] Within the scope of one embodiment of the invention, the ratio of P to S, abbreviated as P / S, can be at least 0.70. Nickel can form compounds with both phosphorus and sulfur. In this embodiment, a minimum amount of phosphorus is present relative to the sulfur content, so that the conditions for the formation of nickel phosphides are particularly favorable compared to the formation of nickel sulfides. Therefore, the alloy contains hardly any nickel sulfides. Preferably, the P / S ratio can be at least 0.80.

[0023] In a further embodiment of the invention, the ratio of Ni to Fe, abbreviated as Ni / Fe, can be at least 1.8. Due to the minimum nickel content relative to the iron content selected in this embodiment, nickel phosphides are preferentially formed, with fewer iron-containing precipitates. The Ni / Fe ratio can preferably be at least 2.8, particularly preferably at least 3.0.

[0024] In another particular embodiment of the invention, the tin content can be 4.0 to 5.5 wt.%. If the tin content is selected within this range, it remains predominantly dissolved in the matrix and leads to solid solution strengthening. This results in particularly favorable properties with regard to strength, electrical conductivity, corrosion resistance, and cold formability. Furthermore, elongation and necking exhibit a maximum in this range. The tin content can particularly preferably be 4.5 to 5.0 wt.%.

[0025] In a further advantageous embodiment of the invention, the nickel content can be 0.35 to 0.65 wt.%. Selecting the nickel content within this range results in particularly favorable properties with regard to electrical conductivity, relaxation resistance, and cold formability. The Ni content can particularly preferably be 0.43 to 0.58 wt.%.

[0026] In a further advantageous embodiment of the invention, the phosphorus content can be 0.20 to 0.35 wt.%. Selecting the phosphorus content within this range results in particularly favorable properties with regard to machinability, strength, and relaxation resistance. Sufficient phosphorus is present in the alloy to form both nickel phosphides and copper phosphides. Copper phosphides improve the strength of the alloy, while nickel phosphides improve relaxation resistance and have a more favorable effect on electrical conductivity than copper phosphides. The phosphorus content can particularly preferably be 0.25 to 0.30 wt.%.

[0027] In a further advantageous embodiment of the invention, the sulfur content can be 0.15 to 0.35 wt.%. Selecting sulfur within this range results in particularly favorable properties with regard to machinability and cold formability. The sulfur content is particularly preferably at most 0.30 wt.%.

[0028] The copper alloy described above can be in the form of a wrought copper alloy and in particular in the form of a wire or rod-shaped semi-finished product.

[0029] A further aspect of the invention relates to an electrical connecting element made of a copper alloy described above. In particular, the invention relates to an electrical connecting element in which the electrical connection is achieved by a spring action of the connecting element or by the application of force, for example, by a screw connection and / or a crimp connection. Examples of such connecting elements are plug connectors and electrical terminals. In particular, the electrical connecting element can be produced by machining a wire or rod-shaped semi-finished product made of a copper alloy described above.

[0030] The invention is explained in more detail using exemplary embodiments and comparative examples.

[0031] Table 1 documents the composition (in wt%) of examples of the invention and comparative examples. Samples No. 1 to No. 6 are examples of the invention. Samples No. 7 to No. 10 marked with * are comparative examples, with Sample No. 7 being the lead-containing reference alloy CuSn4Zn4Pb4. The table also contains information on the formability and electrical conductivity of the respective alloy. The "+" sign in the formability column indicates good formability, while the "-" sign indicates poor formability. Table 1: Composition of samples, formability and conductivity Sample No. Sn Ni P S Pb Zn Fe Mn Cu and impurities Ni / P Formability electrical conductivity % by weight % by weight % by weight % by weight % by weight % by weight % by weight % by weight % by weight in S m / mm 2 1 4,51 0,501 0,260 0,197 0,0003 0,010 0,010 0,003 rest 1,92 + 8,54 2 6,24 0,628 0,235 0,120 0,232 0,078 0,076 0,419 rest 2,67 + 8,37 3 4,12 0,569 0,374 0,332 0,025 0,189 0,467 0,048 rest 1,52 + 9,03 4 5,35 0,678 0,284 0,372 0,072 0,134 0,224 0,087 rest 2,39 + 8,41 5 3,37 0,393 0,342 0,287 0,088 0,049 0,043 0,069 rest 1,15 + 8,87 6 4,88 0,339 0,194 0,156 0,115 0,036 0,178 0,022 rest 1,75 + 8,62 7* 3,71 0,10 0,20 0,001 3,48 3,83 0,004 0,001 rest 0,47 + 9,18 8* 4,48 0,01 0,05 0,46 0,00 2,40 0,008 0,003 rest 0,10 - 10,9 9* 3,65 0,01 0,05 <0,002 0,0070 7,67 0,14 0,001 rest 0,29 - 11,2 10* 4,50 0,01 0,28 0,23 0,0003 0,014 0,01 0,003 rest 0,02 + 7,61

[0032] The alloys were melted and cast as wire with a diameter of 20 mm. They were then cold formed in several steps to an outer diameter of 1.5 mm. The degree of deformation, defined as the relative reduction in cross-sectional area, was at least 18% in each forming step. However, deformation degrees of over 70% were also achieved in a single forming step, particularly in the last forming step. Between two forming steps, the samples were annealed at approximately 650 °C. The nickel-containing alloys, samples no. 1 to no. 6, and the nickel-free alloy, sample no. 10, could be formed without problems, as was the reference alloy, sample no. 7. Samples no. 8 (CuSn4Zn2PS) and no. 9 (CuSn4Zn8FeP), could only be formed to a very limited extent, as cracks appeared in the material even at a diameter of 15 mm.

[0033] The electrical conductivity in the annealed condition was determined on samples with an outer diameter of approximately 15 mm. The reference alloy No. 7 has an electrical conductivity of approximately 9.2 S m / mm 2 , which corresponds to 16% IACS. The electrical conductivity of the nickel-containing alloys, samples No. 1 to No. 6, is between 8.4 and 9.0 S m / mm 2 , which corresponds to approximately 15% IACS. The conductivity of these alloys is therefore only slightly below the conductivity of the reference alloy No. 7. The nickel-free alloy, sample No. 10, has a lower electrical conductivity of only 7.6 S m / mm 2 (approximately 13% IACS). Samples No. 8 and No. 9, although they have a relatively high conductivity of approximately 19% IACS, are not considered further due to their poor formability.

[0034] To characterize the relaxation, the decrease in stress Δσ was measured as a function of time at different temperatures using the ring method on 4 mm diameter wires made from samples No. 1 (CuSn5NiSP), No. 7 (CuSn4Zn4Pb4), and No. 10 (CuSn5PS). The test times and temperatures were chosen so that the Larson-Miller parameter covered the range 7 to 11. Details of the measurement procedure can be found in the article "Understanding Stress Relaxation" by M. Bohsmann and S. Gross, in Materials Science and Technology, 2008, Oct., pages 41 to 47. Table 2: Results of stress relaxation tests Sample No. R m relative stress decrease Δσ in % at temperature in °C MPa 50°C 70°C 95°C 120°C 140°C 185°C 230°C 1 796 0,72 2,9 5,18 12,95 20,23 38,86 63,11 7 765 0,66 2,24 7,56 13,46 18,48 38,72 68,84 10 795 0,78 2,09 9,21 19,36 25,47 51,17 76,98 1 843 0,83 3,64 8,35 15,62 23,44 40,52 66,49 7 834 2,53 4,55 8,51 14,77 22,04 38,4 60,72 10 817 0,86 3,53 9,35 19,83 28,83 51,46 74,61

[0035] Table 2 shows the test results. It shows the relative decrease in stress Δσ of the material compared to the initial value at room temperature. The values documented in the top three rows were determined for samples No. 1, No. 7, and No. 10 at a strength condition characterized by a tensile strength of approximately 780 MPa. The values documented in the bottom three rows were determined for samples No. 1, No. 7, and No. 10 at a strength condition characterized by a tensile strength of approximately 830 MPa. The smaller the measured decrease in stress, the better the relaxation resistance of the material.

[0036] The nickel-containing alloy, Sample No. 1, exhibits relaxation resistance for both strength conditions tested approximately at the level of the lead-containing reference alloy, Sample No. 7. In contrast, the nickel-free alloy, Sample No. 10, exhibits a significantly greater decrease in stress than Samples No. 1 and No. 7 for both strength conditions tested at temperatures above 90 °C. The comparison of Samples No. 1 and No. 10 thus demonstrates that the element nickel in the alloy significantly improves relaxation resistance. This is due to nickel phosphides, which are finely distributed in the material's microstructure.

[0037] The investigations show that the described nickel- and phosphorus-containing copper alloy has a property profile that is largely identical to that of the lead-containing alloy CuSn4Zn4Pb4. Thus, it is possible to replace the lead-containing alloy CuSn4Zn4Pb4 with an alloy whose lead content is at a safe level.

Claims

1. Copper alloy with the following composition in % by weight: Sn: from 3.0 to 6.5%, Ni: from 0.30 to 0.70%, P: from 0.15 to 0.40%, S: from 0.10 to 0.40%, Zn: optionally up to 0.20%, Fe: optionally up to 0.50%, Mn: optionally up to 0.50%, Pb: optionally up to 0.25%, the balance being copper and unavoidable impurities, wherein the ratio of the proportion of Ni to proportion of P is at least 1.1 and at most 2.8 and wherein the alloy comprises nickel phosphides.

2. Copper alloy according to claim 1, characterised in that the ratio of the proportion of P to proportion of S is at least 0.70.

3. Copper alloy according to claim 1 or 2, characterised in that the ratio of the proportion of Ni to proportion of Fe is at least 1.8.

4. Copper alloy according to any one of claims 1 to 3, characterized in that the Sn proportion is from 4.0% to 5.5% by weight.

5. Copper alloy according to any one of claims 1 to 4, characterized in that the Ni proportion is from 0.35% to 0.65% by weight.

6. Copper alloy according to any one of claims 1 to 5, characterized in that the P proportion is from 0.20% to 0.35% by weight.

7. Copper alloy according to any one of claims 1 to 6, characterized in that the S proportion is from 0.15% to 0.35% by weight.

8. Semi-finished product in wire or rod form comprising a copper alloy according to any one of the preceding claims.

9. Electrical connecting element comprising a copper alloy according to any one of claims 1 to 7.