Electrical contact spring

Abstract

To provide an electrical contact spring that can achieve both spring reaction force and stress relaxation resistance by a simple method.SOLUTION: An electrical contact spring 1 is made of a metallic material. When the boundary between the elastic and plastic regions in the true stress-true strain curve of the metallic material is approximated by the Voce equation (Equation 1), the S / Y ratio is between 1.2 and 2.5. σ=S-(S-Y) exp-cε (Equation 1). (In the equation, S is the maximum true stress σ of the metallic material, Y is the elastic limit stress, and c is a constant related to the logarithmic plastic strain ε.)SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a spring for an electrical contact. [Background technology]

[0002] When connecting high-voltage wires used as electrical wiring in a vehicle to a mating connector (e.g., a motor, an inverter, or a high-voltage battery), the connection must be structured to maintain good electrical contact even under vibration and heat. For this reason, high-voltage terminals with a spring structure that maintains contact by the spring's reaction force are used at such connections. However, when such high-voltage terminals are used in high-temperature environments, the spring's reaction force may decrease due to stress relaxation. Therefore, Patent Document 1 discloses a copper alloy material that suppresses the decrease in spring reaction force due to stress relaxation.

[0003] Patent Document 1 discloses a copper alloy material for electric and electronic parts having high strength, high electrical conductivity, and excellent stress relaxation resistance. Specifically, the copper alloy material contains 0.10 to 0.50 mass% of Cr, 0.005 to 0.50 mass% of Ti, and 0.005 to 0.20 mass% of Si, with O being restricted to 150 ppm or less and H being restricted to 5 ppm or less, with the balance being Cu and unavoidable impurities. The copper alloy has a metal structure in which the average grain size in the rolling direction and the average grain size in the thickness direction are 15 μm or less as determined by cross-sectional SEM observation, and the compounds containing Cr, Si, and other elements have grain sizes of 5 μm or less and 500 μm or less. 2 The present invention discloses a copper alloy material for electric and electronic parts having 30 or less copper alloy particles. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2016-20543 Summary of the Invention [Problem to be solved by the invention]

[0005] Conventionally, metal materials for electrical contacts have been designed to improve spring reaction force and stress relaxation resistance by adjusting the alloy composition and internal structure, as in Patent Document 1. However, adjusting the alloy composition and internal structure requires optimization from numerous options, which requires numerous experiments. Therefore, a simple method for achieving both spring reaction force and stress relaxation resistance is desired.

[0006] The present invention has been made in view of the problems inherent in the prior art, and an object of the present invention is to provide a spring for an electrical contact that can achieve both a spring reaction force and stress relaxation resistance by a simple method. [Means for solving the problem]

[0007] A spring for electrical contacts according to an embodiment of the present invention is a spring for electrical contacts made of a metal material, and when the vicinity of the boundary between the elastic region and the plastic region in the true stress-true strain curve of the metal material is approximated by the Voce equation expressed by Equation 1, S / Y is 1.2 or more and 2.5 or less.

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[0008] According to the present invention, it is possible to provide a spring for an electrical contact that can improve the spring reaction force and stress relaxation resistance by a simple method. [Brief explanation of the drawings]

[0009] [Figure 1] 1 is a perspective view showing an example of a spring for electrical contacts according to an embodiment of the present invention; [Figure 2] 1 is a graph showing a true stress σ-true strain ε curve of beryllium copper, as well as an approximate curve according to Voce's law and an approximate curve according to Hooke's law. [Figure 3]This graph shows the state in which, when calculating the elastic limit stress Y, the true stress-true strain curve of a metal material is linearly approximated in the range where the maximum stress is 0 to 30% of the actual measured true stress-true strain curve, and then a tangent line A to the apparent elastic modulus is drawn. [Figure 4] 4 is a graph showing the results of determining the relationship between true strain and the amount of deviation between tangent line A and the true stress-true strain curve from FIG. 3. [Figure 5] 1 is a graph showing the measured values ​​of the true stress-true strain curve of beryllium copper. [Figure 6A] FIG. 10 is a diagram showing the results of a structural analysis. [Figure 6B] 10 is a graph showing an example of the relationship between the indentation depth and the spring load of a long plate material, obtained by structural analysis. [Figure 7] FIG. 10 is a diagram showing the results of structural analysis to determine the region where no plastic strain occurs when the longitudinal center of the convex portion of a long plate material is pressed. [Figure 8] FIG. 1 is a diagram showing true stress-true strain curves of several types of metal materials, and the results of structural analysis when S / Y=1.03 and 3.8. [Figure 9] FIG. 1 is a diagram showing true stress-true strain curves of several types of metal materials, as well as the spring load F and volume fraction V, which is an index of stress relaxation resistance V, when the S / Y ratio is 1 to 3.4. [Figure 10] FIG. 10 is a diagram illustrating how to determine the creep speed. [Figure 11] 1 is a table showing the relationship between the S value and the Y value for various metal materials. [Figure 12] 1 is a graph showing a principal component plot in which principal component scores obtained as a result of principal component analysis are plotted. [Figure 13A] 10 is a graph showing the principal component scores of the sum of the spring load F and the volume ratio V. [Figure 13B] 13B is a graph showing the principal component scores of the H / t ratio corresponding to the sum of the spring load F and the volume ratio V in FIG. 13A. DETAILED DESCRIPTION OF THE INVENTION

[0010] The spring for electrical contacts according to the present embodiment will be described in detail below with reference to the drawings. Note that the dimensional proportions in the drawings are exaggerated for the sake of explanation and may differ from the actual proportions.

[0011] 1, the spring for electrical contacts 1 of this embodiment can be a leaf spring made of a long plate material 2. In the spring for electrical contacts 1, the long plate material 2 has a convex portion 4 in the center in the longitudinal direction, which is more convex than both end portions 3 of the long plate material 2. Therefore, when the spring for electrical contacts 1 is viewed from the side, the long plate material 2 has a curved shape like a bow, and the convex portion 4 is generally arc-shaped.

[0012] The spring 1 for electrical contacts is used as a part of a spring component provided inside a high-voltage terminal, for example, to come into contact with a mating terminal inserted into the high-voltage terminal. In the spring component, the spring 1 for electrical contacts can be used as a doubly supported spring.

[0013] The spring 1 for electrical contacts is a spring made of a metal material, and is characterized in that when the vicinity of the boundary between the elastic region and the plastic region in the true stress-true strain curve of the metal material is approximated by the Voce equation expressed by the following Equation 2, S / Y is 1.2 or more and 2.5 or less.

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[0014] More specifically, the true stress-true strain curve of a metallic material can be obtained by measuring the elastic-plastic deformation characteristics of the metallic material using a uniaxial tensile test. Specifically, the elastic-plastic deformation characteristics of the metallic material are measured in accordance with Japanese Industrial Standards JIS Z2201 (Tensile test specimen for metallic materials) and JIS Z2241 (Metallic material tensile test method). As a result, the actual measured value of the true stress-true strain curve of the metallic material used in the electrical contact spring 1 can be obtained. Note that true strain is also called logarithmic plastic strain. Figure 2 shows the actual measured value of the true stress σ-true strain ε curve of beryllium copper.

[0015] Next, an approximation calculation is performed using Voce's law on the measured values ​​of the obtained true stress σ-true strain ε curve. For this approximation calculation, for example, the optimize function curve_fit in the numerical analysis library scipy of the programming language Python can be used. This approximation calculation can obtain an approximation curve using Voce's law, as shown in Figure 2. Then, from this approximation curve, the maximum value S of true stress σ and the constant c related to logarithmic plastic strain ε of the metallic material can be obtained.

[0016] The elastic limit stress Y in Equation 2 can be calculated as follows. First, as shown in Figure 3, a true stress-true strain curve in the range of maximum stress 0 to 30% is linearly approximated to the measured true stress-true strain curve of the metal material, and then a tangent line A to the apparent elastic modulus is drawn. Next, the deviation between the tangent line A and the true stress-true strain curve is calculated in terms of true strain. The true stress at which the deviation of the obtained true strain is 0.02% is defined as the elastic limit stress Y.

[0017] Specifically, the true stress-true strain curve for beryllium copper shown in Figure 3 is linearly approximated for the true stress-true strain curve in the range of maximum stress of 0 to 30%, and then a tangent line A to the apparent elastic modulus is drawn. Next, as shown in Figure 4, the relationship between the tangent line A and the deviation of the true stress-true strain curve is plotted as a graph, and the true strain at which the deviation of the tangent line A from the true stress-true strain curve is 0.02% is determined. From Figure 4, the true strain at which the deviation of the tangent line A from the true stress-true strain curve is 0.02% is determined to be 0.7%.

[0018] Figure 5 shows the measured true stress-true strain curve for beryllium copper, and from Figure 5, it can be seen that when the true strain is 0.7% (0.007), the true stress is 1003 MPa. This 1003 MPa is the elastic limit stress Y.

[0019] Here, the region where the true stress is equal to or less than the elastic limit stress Y is the elastic region where the metal material undergoes elastic deformation, and can be approximated by Hooke's law in Equation 3.

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[0020] In contrast, the region where the true stress exceeds the elastic limit stress Y transitions from the elastic-plastic boundary region of the metallic material to the plastic region, and can therefore be approximated by the Voce law shown in Equation 2. Note that stress-strain curve models such as the Voce law are generally intended for the plastic region after the offset yield strength (0.2% yield strength, etc.), but in the present invention, they are used to represent the behavior of the elastic-plastic boundary.

[0021] Structural analysis is then performed based on the S-value and Y-value of the metal material thus obtained. For the structural analysis, AFDEX (forging analysis software), version V21R03, manufactured by JSOL Corporation, can be used. The shape of the long plate material 2 used for the structural analysis is the shape shown in Figure 1. Specifically, a rectangular parallelepiped plate material is used, which is curved into an arch shape. The longitudinal length L is set to 14.02 mm, the height H from the bottom surface of each end 3 of the long plate material 2 to the top surface of the convex portion 4 is set to 1.0 mm, the width W is set to 0.65 mm, and the plate thickness t is set to 0.30 mm. The length L, height H, width W, and plate thickness t of the long plate material 2 are values ​​obtained by optimizing the shape of the electrical contact spring through structural analysis using the software described above.

[0022] This type of structural analysis is used to obtain the spring load F and stress relaxation resistance V, which are the component performance of the electrical contact spring. As shown in Figure 6A, the spring load F is calculated as the load that the indent receives when it presses against the longitudinal center B of the convex portion 4 of the long plate material 2. Figure 6B shows an example of the simulation results for the indent pressing depth (mm) and the spring load (N) of the long plate material 2. As shown in Figure 6B, the value of point C where the spring load is maximum relative to the indent pressing depth is taken as the spring load F.

[0023] As shown in Figure 7, when the center B of the longitudinal direction of the convex portion 4 of the long plate material 2 is pressed, the region where no plastic strain occurs is determined, and the volume fraction of this region D where no plastic strain occurs is calculated to determine the stress relaxation resistance V. The volume fraction of the region D where no plastic strain occurs is the ratio of the volume of the region D where no plastic strain occurs to the total volume of the long plate material 2. Such plastic strain is a cause of stress relaxation in the spring, and the larger the volume fraction of the region D where no plastic strain occurs, the less likely stress relaxation occurs, resulting in better stress relaxation resistance V.

[0024] Then, using multiple types of metals as the metallic material for the electrical contact spring 1, true stress-true strain curves were measured, and the S and Y values ​​were calculated. Furthermore, the calculated S and Y values ​​were used to obtain the spring load F and stress relaxation resistance V through structural analysis. Figure 8 shows the true stress-true strain curves obtained in this manner for multiple types of metallic materials, as well as the results of structural analysis when S / Y = 1.03 and 3.8. Figure 8 reveals that when S / Y is low, such as when S / Y = 1.03, when the longitudinal center B of the convex portion 4 of the long plate material 2 is pressed, the deformation of the long plate material 2 does not propagate throughout the long plate material 2, resulting in large localized plastic strain. Therefore, when S / Y is low, the stress relaxation resistance V decreases. On the other hand, when S / Y is high, such as S / Y=3.8, the plastic deformation spreads as the longitudinal center B of the convex portion 4 of the long plate material 2 is pressed, and small plastic strains are uniformly distributed over a wide area. Therefore, the creep rate, which will be described later, is 1×10 -5 If the region below / s is defined as the unstrained region, the volume fraction of region D where no plastic strain occurs is higher at higher S / Y ratios, and an improvement in stress relaxation resistance V is expected.

[0025] FIG. 9 shows the true stress-true strain curves of several types of metal materials obtained as described above, as well as the spring load F and the volume fraction V, which is an index of stress relaxation resistance V, when the S / Y ratio is 1 to 3.4. Note that the volume fraction V is the value obtained when the creep rate, which will be described later, is 1×10 -5 / s(1e -5 / s). As shown in Figure 9, as the S / Y ratio increases, the spring load F gradually decreases, while the volume ratio V gradually increases.

[0026] 9, normalization is performed by setting the maximum value of the spring load F to 1 and the minimum value of the spring load F to 0. Similarly, normalization is performed by setting the maximum value of the volume fraction V to 1 and the minimum value of the volume fraction V to 0. The normalized spring load F and volume fraction V are then added together to determine the performance balance. As a result, as shown in FIG. 9, when S / Y is 1.2 or more and 2.5 or less, the balance between the spring load F and the volume fraction, which is an index of the stress relaxation resistance V, is excellent, and both excellent spring load and stress relaxation resistance can be achieved.

[0027] In this specification, the creep rate at volume fraction V is 1×10 -5 The region below 1 / s is defined as the unstrained region, and this creep rate can be calculated as follows: As shown in Figure 10, we first assumed that creep occurs solely through dislocation motion within crystal grains, and based on the relationship between dislocation motion and macroscopic crystal deformation, we defined the creep rate as shown in Equation 4 below.

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[0028] And the dislocation density ρ [m -2 ] and the average velocity of dislocation ν [m / s] can be calculated as follows:

[0029] The dislocation density ρ [m -2 ] can be calculated from Equation 5 using the relational expression relating to dislocation motion and macroscopic crystal deformation, similar to Equation 4.

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[0030] Although x is a value that differs depending on the material, in this specification, we assume a state in which dislocations exist at a high density (cold-worked material), which is typical of copper alloys, and set the mean free path (travel distance) of dislocations to 1e -8 Using this equation, ρ was calculated at any logarithmic plastic strain ε on the true stress σ-logarithmic plastic strain ε curve.

[0031] The average velocity of dislocation motion ν [m / s] in Equation 4 is expressed as follows using the average movement distance of dislocations and their occurrence frequency p:

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[0032] Using these formulas 6 to 8, the average velocity of motion ν of dislocations at any true stress σ on the true stress σ-logarithmic plastic strain ε curve can be calculated.

[0033] As described above, the spring for electrical contacts 1 according to this embodiment is a spring for electrical contacts made of a metal material, and when the vicinity of the boundary between the elastic region and the plastic region in the true stress-true strain curve of the metal material is approximated by the Voce equation expressed by Equation 2, S / Y is 1.2 or more and 2.5 or less. When S / Y is 1.2 or more and 2.5 or less, the balance between the volume fraction, which is an index of the spring load F and the stress relaxation resistance V, is excellent, so that it is possible to obtain a spring for electrical contacts 1 that has both excellent spring load and stress relaxation resistance.

[0034] The following methods can be used to control the S / Y ratio of the metal material for the electrical contact spring 1 to 1.2 or more and 2.5 or less. It is generally known that the mechanical response of a metal material varies greatly depending on the microstructure, which is the microscopic structure inside the material. The S / Y ratio specified in this invention is a numerical representation of the slight plastic deformation that occurs during the transition period from the elastic region to the plastic region of the mechanical response. Therefore, the S / Y ratio is controlled by the generation of dislocations and the structural factors related to their movement. In the case of copper alloys, the main structural factors that affect the properties are crystal defects (dislocations and grain boundaries), precipitated phases, and solute atoms, and by controlling these, a material with a desired S / Y ratio can be obtained.

[0035] FIG. 11 shows the relationship between the S value and the Y value for various metal materials. As shown in FIG. 11, metal materials for the electrical contact spring 1 that satisfy S / Y = 1.2 to 2.5 include, for example, copper-beryllium alloys, copper-titanium alloys, copper-nickel-silicon alloys, copper-chromium alloys, copper-magnesium alloys, and austenitic stainless steel (SUS). Examples of copper-beryllium alloys include C1720-HT. Examples of copper-titanium alloys include C19900-EH. Examples of copper-nickel-silicon alloys include C70350-TM06, C70252-SH, and C64790-ST. Examples of copper-chromium alloys include C18070-H, R550, and C18080-TR08. Examples of copper-magnesium alloys include C10850-SH. Examples of austenitic stainless steel (SUS) include SUS301 and SUS304. However, the alloy is not limited to these, and as mentioned above, materials with a desired S / Y ratio can be obtained by controlling crystal defects (dislocations and grain boundaries), precipitated phases, and solute atoms.

[0036] As described above, the spring for electrical contacts 1 of this embodiment is made of a long plate material 2, and can be shaped such that the central portion in the longitudinal direction of the long plate material 2 has a convex portion 4 that is more convex than both end portions 3 of the long plate material 2. Here, the height H (H / t) from both end portions 3 of the long plate material 2 to the convex portion 4 relative to the thickness t of the long plate material 2 is preferably 1.8 or more and 3.7 or less. The height H is the height from the lower surface of both end portions 3 of the long plate material 2 to the upper surface of the convex portion 4. By having the height H within this range, it is possible to achieve both excellent spring load and stress relaxation resistance.

[0037] The reason why the height H (H / t) from both ends 3 of the long plate material to the convex portion 4, relative to the thickness t of the long plate material 2, is preferably 1.8 to 3.7, is determined by principal component analysis. Principal component analysis (PCA) is a typical dimensionality reduction technique that reduces dimensionality by projecting data onto a hyperplane (a plane defined by n-1 dimensions in n-dimensional space) in a multidimensional data space. The data projected onto the coordinate axes that make up the hyperplane are called principal components, and PCA can analyze the correlation between data from factor loadings, which are a correlation coefficient matrix between these principal components and the original data.

[0038] In this specification, PCA is used to extract shape features that contribute to the balance of the performance (spring load and stress relaxation resistance) of the electrical contact spring 1. Specifically, a 39-row, 9-column dataset was created containing shape factors (i.e., the ratios of length L, height H, width W, and thickness t, i.e., L / t ratio, L / W ratio, L / H ratio, H / t ratio, H / W ratio, and W / t ratio), as well as material factors (i.e., S / Y ratio, spring load F, and stress relaxation resistance index V). Principal component analysis was then performed using this dataset. Table 1 shows specific values ​​for the dataset of the L / t ratio, L / W ratio, L / H ratio, H / t ratio, H / W ratio, W / t ratio, S / Y ratio, spring load F, and stress relaxation resistance V. In Table 1, a circle indicates an index that satisfies S / Y = 1.2 to 2.5, and a double circle indicates an index that satisfies S / Y = 1.2 to 2.5 and H / t = 1.8 to 3.7.

[0039] [Table 1]

[0040] FIG. 12 shows a principal component plot of the principal component scores obtained from the results of the principal component analysis. From the results of FIG. 12, the H / t ratio is extracted as a design factor that affects the sum of the spring load F and the volume fraction V, which is the performance balance of the electrical contact spring. FIG. 13A shows the principal component score of the sum of the spring load F and the volume fraction V, and FIG. 13B shows the H / t ratio corresponding to the sum of the spring load F and the volume fraction V in FIG. 13A. From FIG. 13A, it can be seen that a principal component score in the range of -0.6 to 0.2 for PC3 indicates that the performance balance of the electrical contact spring is good. From FIG. 13B, it can be seen that the H / t ratio at which the performance balance of the electrical contact spring is good is 1.8 to 3.7.

[0041] Thus, the results of the principal component analysis show that in a spring 1 for electrical contacts made of a metal material that satisfies S / Y = 1.2 to 2.5, when H / t is 1.8 or more and 3.7 or less, excellent spring load and stress relaxation resistance can both be achieved.

[0042] Although the present embodiment has been described above, the present embodiment is not limited to this, and various modifications are possible within the scope of the gist of the present embodiment. [Explanation of symbols]

[0043] 1 Electrical contact spring 2 Long board material 3 Both ends 4 Convex part

Claims

1. A spring for electrical contacts made of a metal material, A spring for electrical contacts, wherein when the vicinity of the boundary between the elastic region and the plastic region in the true stress-true strain curve of the metal material is approximated by the Voce equation expressed by Equation 1, S / Y is 1.2 or more and 2.5 or less. [Equation 1] (In the formula, S is the maximum value of the true stress σ of the metal material, Y is the elastic limit stress, and c is a constant related to the logarithmic plastic strain ε.)

2. The electrical contact spring is made of a long plate material, The long plate material has a convex portion at a central portion in a longitudinal direction, the convex portion being more convex than both end portions of the long plate material, 2. The spring for electrical contacts according to claim 1, wherein a height H (H / t) from each end of said long plate material to said convex portion relative to a thickness t of said long plate material is 1.8 or more and 3.7 or less.

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