Electrical contact spring

Abstract

An electrical contact spring (1) includes a metal material, and when the vicinity of the boundary between an elastic region and a plastic region in a true stress-true strain curve of the metal material is approximated using the Voce equation represented by Equation 1 [sigma] = S-(S-Y) exp-c [epsilon], where S is the maximum value of the true stress [sigma] of the metal material, and c is the maximum value of the true stress [sigma] of the metal material. Y is the elastic limit stress of the metal material, and c is a constant related to the logarithmic plastic strain [epsilon] of the metal material.

Description

TECHNICAL FIELD

[0001] The present application relates to an electric contact spring. BACKGROUND

[0002] When a high-voltage wire used as a vehicle wire is connected to a counterpart connector body (for example, a motor, an inverter, and a high-voltage battery), it is necessary to have a structure that can well maintain the electric contact state of the connection portion even under vibration and high temperature. Therefore, a high-voltage terminal having a spring structure and maintaining contact by the reaction force of the spring is used for the connection portion. However, when such a high-voltage terminal is used in a high-temperature environment, the reaction force of the spring can decrease due to a stress relaxation phenomenon. JP 2016-20543 A discloses a copper alloy material that controls the decrease in the reaction force of the spring due to the stress relaxation phenomenon.

[0003] JP 2016-20543 A discloses a copper alloy material for electrical and electronic components, which has high strength, high electrical conductivity, and excellent stress relaxation resistance. Specifically disclosed is a copper alloy material for electrical and electronic components, which: 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; is limited to 150 ppm or less of O and 5 ppm or less of H; contains Cu and inevitable impurities as residues; has a metal structure with an average grain size of 15 μm or less in the rolling direction and an average grain size of 10 μm or less in the plate thickness direction, as viewed from the cross-sectional SEM; has a hardness of 120 HV or more, as measured by a Vickers hardness tester; and has a spring reaction force of 0.5 N or more, as measured by a spring reaction force tester. 2 has 30 or less compounds containing Cr, Si, or other elements, and a particle size of 5 μm or less. SUMMARY

[0004] Conventionally, for a metal material for electric contact, as described in JP 2016-20543 A, the spring reaction force and the stress relaxation resistance are improved by adjusting the alloy composition and the internal structure. However, the adjustment of the alloy composition and the internal structure requires optimization from an infinite number of options, and many experiments are required. Therefore, a simple method is required to achieve both the spring reaction force and the stress relaxation resistance.

[0005] The present application was made in view of the problems in the related art. An object of the present application is to provide an electric contact spring that can achieve both the spring reaction force and the stress relaxation resistance using a simple method.

[0006] An electrical contact spring according to an embodiment of the present invention comprises: a metallic material, wherein, when the boundary between the elastic and plastic regions in the true stress-strain curve of the metallic material is approximated using the Voce equation expressed in Equation 1, the ratio S / Y is 1.2 or more and 2.5 or less.

[0007] [Equation 1]

[0008] σ=S-(SY)exp -cε

[0009] Where S is the maximum value of the true stress σ of the metallic material, Y is the elastic limit stress of the metallic material, and c is a constant related to the logarithmic plastic strain ε of the metallic material.

[0010] According to the present invention, an electrical contact spring can be provided that can achieve improved spring reaction force and stress relief using a simple method. Attached Figure Description

[0011] Figure 1 This is a perspective view showing an example of an electrical contact spring according to an embodiment.

[0012] Figure 2 This is a graph showing the true stress σ-true strain ε curve of beryllium copper, as well as the approximate curves according to Voce's law and Hooke's law.

[0013] Figure 3 This is a graph showing the state of the actual stress-strain curve relative to the measured value of the actual stress-strain curve of the metallic material when the elastic limit stress Y is obtained, with the actual stress-strain curve in the range of 0% to 30% of the maximum stress linearly approximated, and then the state of the tangent A of the apparent elastic modulus is plotted.

[0014] Figure 4 It shows the basis Figure 3 A graph showing the relationship between the actual strain and the deviation between the tangent A and the actual stress-actual strain curve.

[0015] Figure 5 This is a graph showing the measured values ​​of the true stress-true strain curve of beryllium copper.

[0016] Figure 6A This is a diagram showing the results of the structural analysis.

[0017] Figure 6B This is a graph illustrating an example of the relationship between the indentation depth of the indenter and the spring load of the long plate, based on structural analysis.

[0018] Figure 7is a graph showing results of determining regions in which no plastic strain occurs when a center portion in a length direction of a convex portion of a long plate is pressed, using structural analysis.

[0019] Figure 8 is a graph showing true stress-true strain curves of several metal materials and results of structural analysis of S / Y = 1.03 and S / Y = 3.8.

[0020] Figure 9 is a graph showing true stress-true strain curves of several metal materials, spring load F when S / Y ratio is 1 to 3.4, and volume percentage V as an index of stress relaxation resistance V.

[0021] Figure 10 is a graph showing how to determine a creep speed.

[0022] Figure 11 is a table showing a relationship between S values and Y values of various metal materials.

[0023] Figure 12 is a graph showing principal component scores in which principal component scores obtained by principal component analysis are plotted.

[0024] Figure 13A is a graph showing principal component scores of a value obtained by adding spring load F and volume percentage V.

[0025] Figure 13B is a graph showing principal component scores of an H / t ratio corresponding to an added value of spring load F and volume percentage V in Figure 13A . DETAILED DESCRIPTION

[0026] With reference to the drawings, a description will be given of an electrical contact spring according to the present embodiment. Note that the dimensional ratios in the drawings are exaggerated for the sake of description and are sometimes different from actual ratios.

[0027] As shown in Figure 1 , the electrical contact spring 1 according to the present embodiment can be a leaf spring including a long plate 2. In the electrical contact spring 1, the long plate 2 includes a convex portion 4 at a central portion thereof in a length direction, the convex portion 4 being more convex than both end portions 3 of the long plate 2. Thus, when the electrical contact spring 1 is viewed from a side, the long plate 2 is in an arcuately curved shape, and the convex portion 4 is in a substantially arc shape.

[0028] For example, the electrical contact spring 1 contacts a counterpart terminal inserted into a high-voltage terminal and functions as a part of a spring member provided inside the high-voltage terminal. In the spring member, the electrical contact spring 1 can function as a double support spring.

[0029] The electrical contact spring 1 is a spring made of a metal material in which S / Y is 1.2 or more and 2.5 or less when a Voce equation expressed in the following Equation 1 is used to approximate a vicinity of a boundary between an elastic region and a plastic region in a true stress-true strain curve of the metal material.

[0030] [Equation 1]

[0031] σ = S - (S - Y) exp -cε

[0032] Note that, in Equation 1, S is a maximum value of a true stress σ of the metal material, Y is an elastic limit stress, and c is a constant related to a logarithmic plastic strain ε.

[0033] For detailed explanation, the true stress-true strain curve of the metal material can be obtained by measuring the elasto-plastic deformability of the metal material using a uniaxial tensile test method. Specifically, the elasto-plastic deformability of the metal material is measured in accordance with Japanese Industrial Standards JIS Z2201 (Test piece for tensile testing of metallic materials) and JIS Z2241 (Metallic materials - Tensile testing - Method of test at room temperature). Thus, a measured value of the true stress-true strain curve of the metal material for the electrical contact spring 1 can be obtained. Note that the true strain is also referred to as the logarithmic plastic strain. Figure 2 A measured value of a true stress σ-true strain ε curve of beryllium copper is shown.

[0034] Next, the measured value obtained using the Voce law is approximated. For example, for the approximation, an optimization function curve_fit in a numerical analysis library scipy of a programming language Python can be used. By using this approximation, an approximate curve based on the Voce law as shown in Figure 2 can be obtained. From this approximate curve, a maximum value S of the true stress σ of the metal material and a constant c related to the logarithmic plastic strain ε of the metal material can be obtained.

[0035] The elastic limit stress Y in Equation 1 can be obtained as follows. First, as shown in Figure 3 , a linear approximation of the true stress-true strain curve is made in a range of 0% to 30% of a maximum stress with respect to the measured value of the true stress-true strain curve of the metal material, and then a tangent line A of an apparent elastic modulus is drawn. Next, a deviation amount between the tangent line A and the true stress-true strain curve is obtained with respect to the true strain. Then, a true stress at which the obtained deviation amount of the true strain is 0.02% is used as the elastic limit stress Y.

[0036] Specifically, as shown in Figure 3As shown, relative to the measured values ​​of the true stress-true strain curve of beryllium copper, a linear approximation is made for the true stress-true strain curve in the range of 0% to 30% of the maximum stress, and then the tangent line A of the apparent elastic modulus is plotted. Next, as... Figure 4 As shown, after plotting the relationship between the deviation of tangent A and the true stress-true strain curve, the true strain with a deviation of 0.02% between tangent A and the true stress-true strain curve is obtained. According to... Figure 4 The deviation between tangent A and the true stress-strain curve is 0.02%, and the true strain is 0.7%.

[0037] Figure 5 The measured values ​​of the true stress-true strain curve for beryllium copper are shown. According to... Figure 5 The actual stress at a strain of 0.7% (0.007) is 1003 MPa. This 1003 MPa is used as the elastic limit stress Y.

[0038] Here, the region where the actual stress is less than or equal to the elastic limit stress Y is the elastic region of elastic deformation of the metallic material, and can be approximated using Hooke's law in Equation 3.

[0039] [Equation 3]

[0040] σ=kε

[0041] In Equation 3, σ is the true stress, k is Young's modulus, and ε is the true strain.

[0042] In contrast, the region where the actual stress exceeds the elastic limit stress Y is the plastic region of the elastoplastic boundary region of the metallic material and can be approximated using Voce's law as shown in Equation 1. Note that stress-strain curve models, such as Voce's law, are typically used for plastic regions after offsetting the yield stress (such as 0.2% of the yield stress), but in this invention, it is used to represent the behavior of the elastoplastic boundary.

[0043] Then, structural analysis is performed based on the S and Y values ​​of the obtained metallic material. Structural analysis can be performed using AFDEX (forging analysis software) version 21R03 manufactured by JSOLCORPORATION. Note that the shape of the long plate 2 used for structural analysis is as follows... Figure 1As shown. Specifically, a long plate 2, obtained by bending a cuboid plate into an arc shape, is used. The length L in the longitudinal direction is set to 14.02 mm, the height H from the lower surface of the two ends 3 of the long plate 2 to the upper surface of the protrusion 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. Note that the length L, height H, width W, and plate thickness t of the long plate 2 are values ​​obtained by optimizing the shape of the electrical contact spring through structural analysis using the software described above.

[0044] Through structural analysis, the spring load F and stress relief performance V of the component acting as an electrical contact spring were obtained. As for the spring load F, ... Figure 6A As shown, when the pressure head is pressed against the center B of the protrusion 4 in the length direction of the long plate 2, the load applied to the pressure head is calculated. Figure 6B An example of simulation results for the indentation depth (mm) of the indenter and the spring load (N) of the long plate 2 is shown. Figure 6B As shown, the spring load F is the value of the point C where the spring load is at its maximum indentation depth relative to the indenter.

[0045] As a stress-relieving property, V, such as Figure 7 As shown, when the center B of the protrusion 4 of the long plate 2 is pressed along its length, the region where no plastic strain occurs is identified, and the volume percentage of the region D where no plastic strain occurs is calculated. Note that the volume percentage of the region D where no plastic strain occurs refers to the percentage of the volume of the region D where no plastic strain occurs relative to the total volume of the long plate 2. Plastic strain causes stress relief in the spring, and the larger the volume percentage of the region D where no plastic strain occurs, the less stress relief occurs, and the better the stress relief resistance V.

[0046] Then, using various metals as the metallic materials for the electrical contact spring 1, the true stress-strain curves were measured, and the S and Y values ​​were further calculated. Based on the calculated S and Y values, the spring load F and stress mitigation resistance V were obtained through structural analysis. Figure 8 The actual stress-strain curves for various metallic materials obtained as a result are shown, along with the results of structural analyses for S / Y = 1.03 and S / Y = 3.8. From... Figure 8 It is evident that when S / Y is low, such as S / Y = 1.03, and when the central B along the length of the protrusion 4 of the long plate 2 is pressed, the deformation of the long plate 2 does not propagate throughout the entire long plate 2, and thus concentrates a large local plastic strain. Therefore, when S / Y is low, the stress relief V decreases. In contrast, it is clear that when S / Y is high, such as S / Y = 3.8, the plastic deformation spreads as the central B along the length of the protrusion 4 of the long plate 2 is pressed, and small plastic strains are uniformly distributed over a wide area. Therefore, if the creep rate described below is 1 × 10⁻⁶, the deformation will be significantly reduced.-5 The region of 1e-13 / s or less is defined as an unstrained region, and thus the volume percentage of the region D in which no plastic strain occurs is higher at a high S / Y ratio, and the stress relaxation resistance V is expected to be improved.

[0047] Figure 9 The true stress-true strain curves of the various metal materials obtained as described above, the spring load F when the S / Y ratio is 1 to 3.4, and the volume percentage V as an index of the stress relaxation resistance V are shown. Note that the volume percentage V is the value when the creep rate is 1x10 -5 / s (1e -5 / s) or less, which will be described below. As Figure 9 shown, the spring load F gradually decreases and the volume percentage V gradually increases as the S / Y ratio increases.

[0048] In Figure 9 , normalization is performed in a case where the maximum value of the spring load F is 1 and the minimum value of the spring load F is 0. Similarly, normalization is performed in a case where the maximum value of the volume percentage V is 1 and the minimum value of the volume percentage V is 0. Then, a performance balance is obtained by adding the normalized spring load F and the volume percentage V. Thus, as Figure 9 shown, when the S / Y is 1.2 or more and 2.5 or less, the balance between the spring load F and the volume percentage as an index of the stress relaxation resistance V is very good, and both excellent spring load and excellent stress relaxation resistance can be achieved.

[0049] Note that in the present specification, the creep rate at the volume percentage V is 1x10 -5 / s or less is defined as an unstrained region. The creep rate can be obtained in the following manner. As Figure 10 shown, assuming that the creep phenomenon proceeds only through dislocation movement in a crystal grain, based on the relationship between the dislocation movement and macroscopic crystal deformation, the creep rate is defined as the following Equation 4.

[0050] [Equation 4]

[0051]

[0052] where p [m -2 ], b [m], and are the strain rate, the dislocation density, the Burgers vector (constant, 2.56e -10 m in the case of copper), and the average movement speed of dislocations, respectively.

[0053] The dislocation density p [m -2 ] and the average movement speed v [m / s] of dislocations in Equation 4 can be calculated as follows.

[0054] The dislocation density p [m"1] in Equation 4 can be calculated from Equation 5 using the relationship between dislocation motion and macroscopic crystal deformation. -2 ]as in Equation 4 can be calculated from Equation 5 using the relationship between dislocation motion and macroscopic crystal deformation.

[0055] [Equation 5]

[0056]

[0057] where ε, b and x are the logarithmic true strain, Burgers vector (constant) and average free path (moving distance) of dislocations, respectively.

[0058] The value of x differs depending on the material, but in the present specification, it is assumed that the average free path (moving distance) of dislocations is le"10m under the condition that dislocations exist at a high density (cold worked material), which is common in copper alloys. Using this equation, p at any logarithmic plastic strain ε on the true stress σ-logarithmic plastic strain ε curve is calculated. -8

[0059] The average motion velocity v [m / s] of dislocations in Equation 4 is expressed by the average moving distance of dislocations and the occurrence frequency p as follows.

[0060] [Equation 6]

[0061]

[0062] where the temperature and load stress as the creep condition are related to the occurrence frequency p. The occurrence frequency p is expressed by the following Equation 7.

[0063] [Equation 7]

[0064]

[0065] where L, v d , k, T and G * are the distance between obstacles (precipitates or solid solution atoms) in dislocation motion, the thermal frequency (le 13 ) of copper, the Boltzmann constant, the temperature and the thermal activation energy of dislocation motion, respectively. Here, Equation 8 is used for the thermal activation energy G * of dislocation motion.

[0066] [Equation 8]

[0067]

[0068] where τ m , d * and τ * are the critical resolved shear stress, the obstacle size and the load stress, respectively. ​

[0069] Equations 6 to 8 can be used to calculate the average movement speed v of dislocations at any real stress s on a real stress s-log plastic strain e curve.

[0070] As described above, the electrical contact spring 1 according to the present embodiment is made of a metal material in which S / Y is 1.2 to 2.5 when the Voce equation represented in Equation 1 is used to approximate the vicinity of the boundary between the elastic region and the plastic region in the real stress-real strain curve of the metal material. When S / Y is 1.2 to 2.5, the balance between the spring load F and the volume percentage as an index of stress relaxation resistance V is very good, and thus an electrical contact spring 1 that realizes excellent spring load and excellent stress relaxation resistance can be obtained.

[0071] In the metal material used for the electrical contact spring 1, the S / Y ratio can be controlled to be 1.2 or more and 2.5 or less using the following method. It is well known that the mechanical response of a metal material greatly changes depending on the structure that is a microstructure inside the material. The S / Y ratio specified in the present invention quantifies the slight plastic deformation during the transition from the elastic region to the plastic region in the mechanical response. Therefore, the S / Y ratio is controlled by the generation of dislocations and structural factors related to the movement thereof. In the case of a copper alloy, the main structural factors related to the properties include crystal defects (dislocations and grain boundaries), precipitated phases, and solid solution atoms. By controlling these factors, a material having a predetermined S / Y ratio can be obtained.

[0072] Figure 11 The relationship between the S value and the Y value of various metal materials is shown. As shown in Figure 11 the metal material used for the electrical contact spring 1 that satisfies S / Y = 1.2 to 2.5 includes a copper beryllium alloy, a copper titanium alloy, a copper nickel silicon alloy, a copper chromium alloy, a copper magnesium alloy, and an austenitic stainless steel (SUS). An example of the copper beryllium alloy is C1720-HT. An example of the copper titanium alloy is C19900-EH. Examples of the copper nickel silicon alloy include C70350-TM06, C70252-SH, and C64790-ST. Examples of the copper chromium alloy include C18070-H, C18070-R550, and C18080-TR08. An example of the copper magnesium alloy is C10850-SH. Examples of the austenitic stainless steel (SUS) include SUS301 and SUS304. However, the metal material is not limited to these alloys, and as described above, by controlling the crystal defects (dislocations and grain boundaries), the precipitated phases, and the solid solution atoms, a material having a predetermined S / Y ratio can be obtained.

[0073] As described above, the electric contact spring 1 according to the present embodiment includes the long plate 2 which can have a shape having the convex portion 4 at the central portion in the length direction of the long plate 2, the convex portion 4 being more convex than the both end portions 3 of the long plate 2. Here, the height H from the both end portions 3 of the long plate 2 to the convex portion 4 with respect to the thickness t of the long plate 2 (H / t) is preferably 1.8 or more and 3.7 or less. Note that the height H is from the lower surface of the both end portions 3 of the long plate 2 to the upper surface of the convex portion 4. When the height H is within this range, both excellent spring load and excellent stress relaxation resistance can be achieved.

[0074] The reason why the height H from the both end portions 3 of the long plate 2 to the convex portion 4 with respect to the thickness t of the long plate 2 (H / t) is preferably 1.8 or more and 3.7 or less is determined by performing principal component analysis. Principal component analysis (PCA) is a typical method of dimensionality reduction. It is a method of performing dimensionality reduction by projecting data onto a hyperplane (a plane defined by n-1 dimensions in an n-dimensional space) in a multidimensional data space. Data projected onto the coordinate axes constituting the hyperplane is called a principal component. In PCA, the correlation between data can be analyzed by factor loading, which is a correlation factor matrix between principal components and original data.

[0075] In the present specification, PCA is used to extract shape features which contribute to the balance of the performance (spring load and stress relaxation resistance) of the electric contact spring 1. Specifically, shape factors of L / t ratio, L / W ratio, L / H ratio, H / t ratio, H / W ratio, and W / t ratio of length L, height H, width W, and plate thickness t; a material factor of S / Y ratio; spring load F; and stress relaxation resistance V are set as a data set of 39 rows and 9 columns. Principal component analysis is performed using the data set. Table 1 lists specific values of the data set of 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 index V. Note that in Table 1, “B” is a parameter satisfying S / Y = 1.2 to 2.5, and “A” is a parameter satisfying S / Y = 1.2 to 2.5 and H / t = 1.8 to 3.7.

[0076] [Table 1]

[0077]

[0078] Figure 12 A principal component plot in which principal component scores obtained by principal component analysis are plotted is shown. According to the results in Figure 12 , the H / t ratio is extracted as a design factor which affects a value obtained by adding spring load F and volume percentage V, which is the performance balance of the electric contact spring. Figure 13A is a plot of principal component scores of a value obtained by adding spring load F and volume percentage V.Figure 13B The corresponding H / t ratio of the value obtained by adding the spring load F and the volume percentage V in Figure 13A Figure 13A As is apparent from Table 1 in Figure 13B As is apparent from Table 1 in

[0079] Thus, from the results of the principal component analysis, it is seen that the electrical contact spring 1 including the metal material satisfying S / Y = 1.2 to 2.5 can achieve both an excellent spring load and an excellent stress relaxation resistance when H / t is 1.8 or more and 3.7 or less.

[0080] While specific embodiments have been described in detail, these particular embodiments are shown and described by way of illustration only, and are not intended to limit the scope of the application. Indeed, the novel embodiments described herein can be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the embodiments described herein can be made without departing from the spirit of the application. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the application.​

Claims

1. An electrical contact spring, comprising: Metallic materials, among which, When using the Voce equation shown in Equation 1 to approximate the area near the boundary between the elastic and plastic regions in the true stress-strain curve of a metallic material, S / Y is greater than 1.2 and less than 2.

5. [Equation 1] σ=S-(S-Y)exp -cε Where S is the maximum value of the true stress σ of the metallic material, Y is the elastic limit stress of the metallic material, and c is a constant related to the logarithmic plastic strain ε of the metallic material.

2. The electrical contact spring according to claim 1, wherein, The electrical contact spring includes a long plate material having a protrusion at its central portion along its length, the protrusion being more prominent than both ends of the long plate material. The height from both ends of the long plate material to the protrusion is 1.8 or more and 3.7 or less relative to the thickness of the long plate material.

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