Friction element joining method

Abstract

The purpose of the present invention is to provide a friction element joining method. The present invention is a friction element joining method for joining a plate group by pressing an element having a tapered portion at the tip thereof into the plate group in which two or more metal plates are stacked while rotating the element, the friction element joining method comprising: a first step of removing an oxide film on the upper surface of a lower plate disposed on the lowermost layer of the plate group; and a second step of pressing the component into the lower plate to form the joint, in the first step, oxide removal is performed by controlling the immersion ratio of the component in the lower plate to be 0.5-1.3, the rotation speed to be 4000-8000 rpm, and the pressure to be 4000-8000 N, and in the second step, the component is pressed into the lower plate to form the joint, and in the first step, the component is pressed into the lower plate to form the joint by controlling the immersion ratio to be 0.5-1.3, the rotation speed to be 4000-8000 rpm. The increase in the rotational speed of the second step with respect to the rotational speed of the first step increases the rotational speed of the second step so as to satisfy a prescribed conditional expression in accordance with the immersion ratio.

Description

Technical Field

[0001] The present invention relates to a friction element joining method for joining a stack of at least two metal plates by joining friction elements. Background Art

[0002] In recent years' automotive industry, for the purpose of reducing environmental load based on vehicle body lightweighting while taking into account safety, the adoption of high-strength steel plates known as ultra-high-strength steel has been promoted, and it is expected to develop steel plates with higher strength and better workability in the future. However, with the increase in strength and workability of steel plates, a problem has arisen in fusion joining such as resistance spot welding due to embrittlement of the solidification structure, resulting in a decrease in joint strength. For this reason, joining technologies that can join two or more steel plates, including high-strength steel plates, in a non-fusion manner have attracted attention.

[0003] As a non-fusion joining technology, for example, the application of FEW (Friction Element Welding), which involves joining by pressing a rotating element into a stack of plates at high speed, has been studied as described in Reference 1. As the element used in this friction element joining (FEW), for example, the structures disclosed in Patent Document 1 and Patent Document 2 have been proposed.

[0004] [Reference 1]

[0005] Jamie D. Skovron, Brandt J. Ruszkiewicz, and Laine Mears, “INVESTIGATION OF THE CLEANING AND WELDING STEPS FROM THE FRICTION ELEMENT WELDING PROCESS”, (ASME 2017 12th International Manufacturing Science and Engineering Conference collocated with the JSME / ASME 2017 6th International Conference on Materials and Processing, June 4 - 8, 2017 Los Angeles, California, USA)

[0006] A component is disclosed in Patent Document 1. The front end of the mandrel portion of the component (equivalent to the "connection element" in this Patent Document 1) is a substantially flat end face, and a pin-shaped centering portion projects from the center of the flat end face. By configuring the component in such a manner, each connection process is performed by the method of aiming at the connection portion using the pin-shaped centering portion. Thereby, the applied frictional energy can be concentrated at the respective positions of the components to be inserted.

[0007] A joining structure is disclosed in Patent Document 2, in which a plurality of overlapping steel components are joined using a connecting member having a shaft portion. In this joining structure, when the maximum diameter at the boundary between the second steel component disposed on the uppermost side and the shaft portion is designated as Dmax2, and the minimum diameter of the shaft portion at a position farther from the second steel component than the measurement position of Dmax2 is designated as Dmin, Dmax2 is 1.20 times or more of Dmin.

[0008] Patent Document 1: Japanese Patent Application Laid-Open No. 2013-527804

[0009] Patent Document 2: Japanese Patent No. 6795124

[0010] In Patent Document 1, only the shape of the pin at the front end of the mandrel portion of the component is disclosed. However, in actual joining using the component, it is necessary to set the shape of the component corresponding to the joining conditions, but this is not disclosed in Patent Document 1.

[0011] In addition, if an oxide remains on the upper surface of the lower plate that forms a joint with the component, there is a problem that the oxide mixes into the joint portion and a sound joint portion cannot be formed, resulting in a reduction in joint strength. For these reasons, when joining using the component, it is required to remove the oxide (specifically, the oxide film) on the upper surface of the lower plate that forms a joint with the component. However, in the component described in Patent Document 1, the contact area between the pin-shaped centering portion of the component and the steel plate becomes narrow. Therefore, there is a problem that the time required to remove the oxide on the steel plate surface (here, the upper surface of the lower plate) due to the contact between the centering portion and the steel plate becomes long.

[0012] In Patent Document 2, the diameter of the connecting member (i.e., the component diameter) and the joint portion diameter are described, but the relationship between the joining conditions and the shape of the connecting member is not clear. Therefore, there is a problem that sufficient joint strength cannot be obtained by the obtained connecting member. Summary of the Invention

[0013] The present invention has been completed to solve the above problems, and an object thereof is to provide a method for joining friction elements, which can shorten the joining process time regardless of the plate group and the composition of the materials to be joined constituting the plate group, and improve the joint property of the obtained joint member.

[0014] In the present invention, dedicated research has been conducted to solve the above problems in the friction element joining method. The friction element joining method is a joining method in which an element having a tapered portion at its front end is rotated at high speed and pressed into a stack of two or more metal plates (steel plates) to join the stack of plates.

[0015] In addition to the above problems, in the case of a friction element joining method using an element having a conventional front-end shape, although the upper and lower plates are joined by the element, it takes time until the front end of the element reaches a specified depth of the lower plate. As a result, there is also a problem that the heat input to the material to be joined becomes too large and the joining strength of the obtained joined part is reduced.

[0016] Therefore, the present inventors conducted the following experiments: a joining method designed to have two or more stages as the friction element joining process, using metal plates including ultra-high strength steel and various elements having different front-end shapes, and varying the pressing force and rotational speed in each process. As a result, it was found that in solving the above problems, by specifying the optimum relationship between the front-end shape of the element and the rotational speed in each process, it is possible to achieve high-speed joining of the joining process and joining of the joint under high strength.

[0017] That is, the process from when the element penetrates the upper plate of the stack of plates until the front end of the high-speed rotating element abuts against the upper surface of the lower plate of the stack of plates, and the element is pressed into the upper surface of the lower plate to form a joined part is designed as a process of at least two stages. Specifically, the process of abutting the front end of the element against the upper surface of the lower plate of the stack of plates to remove the oxide film existing on the upper surface of the lower plate, and pressing the element into the lower plate to a specified position is defined as the first process, and the process of further pressing the element into the lower plate to form a joined part is defined as the second process. At this time, the increase (ΔR, described later) of the rotational speed of the second process with respect to the rotational speed of the first process is specified based on the penetration ratio (L / t, described later) calculated from the distance from the pressing start position to the specified position in the first process and the conical height of the front end of the element. As a result, the joining time of the entire joining process is shortened. In addition, it was clarified that it is effective in suppressing the thermal influence on the stack of plates and the element, and thus in suppressing the reduction of the joint strength.

[0018] The present invention has been completed based on such an insight, and the main points are as follows.

[0019] [1] A friction element joining method in which an element having a tapered portion at its front end is rotated and pressed into a stack of two or more metal plates to join the stack of plates, comprising:

[0020] a first process of removing the oxide film on the upper surface of the lower plate disposed at the lowermost layer of the stack of plates; and

[0021] Second process: Press the above-mentioned component into the above-mentioned lower plate to form a joint portion.

[0022] In the above-mentioned first process,

[0023] When setting the pressing force in the first process as P1 (N), the rotational speed in the first process as R1 (rpm), the height of the above-mentioned tapered portion as t (mm), setting the cylinder position at the moment when the front end of the component contacts the upper surface of the above-mentioned lower plate as 0 and the cylinder displacement at the end of the first process as L (mm), and the immersion ratio obtained by dividing the above-mentioned cylinder displacement by the height of the above-mentioned tapered portion as L / t,

[0024] Control the removal of oxides on the upper surface of the above-mentioned lower plate within the range of 0.5 ≤ L / t < 1.3, 4000 ≤ R1 ≤ 8000, and 4000 ≤ P1 ≤ 8000.

[0025] Next, in the above-mentioned second process,

[0026] When setting the rotational speed in the second process as R2 (rpm) and the increment of the rotational speed in the second process relative to the rotational speed in the first process as △R,

[0027] When the above-mentioned immersion ratio L / t at the end of the first process is 0.5 or more and less than 0.8, increase the rotational speed in the second process in such a way that the above-mentioned increment satisfies Equation (1).

[0028] When the above-mentioned immersion ratio L / t at the end of the first process is 0.8 or more and less than 1.0, increase the rotational speed in the second process in such a way that the above-mentioned increment satisfies Equation (2).

[0029] When the above-mentioned immersion ratio L / t at the end of the first process is 1.0 or more and less than 1.3, increase the rotational speed in the second process in such a way that the above-mentioned increment satisfies Equation (3).

[0030] △R ≥ 4×(2 - L)×((12000 - R1) / 100)…(1)

[0031] △R ≥ 2×(3.2 - L)×((12000 - R1) / 100)…(2)

[0032] △R ≥ 0.4×(12 - L)×((12000 - R1) / 100)…(3)

[0033] [2] According to the friction element joint method described in [1], wherein,

[0034] The above-mentioned metal plate is a steel plate, and the tensile strength of this steel plate is 590 MPa or more.

[0035] [3]The friction element joining method according to [1] or [2], wherein

[0036] The height of the conical portion of the above-mentioned element exceeds 0 mm and is 1.0 mm or less, and the elevation angle of the conical portion is 10 to 45°.

[0037] The present invention provides a friction element joining method for improving joinability. According to the joining method of the present invention, the friction element can be joined to the metal plate based on the optimal relationship between the front-end shape of the element and the rotational speed in each process. As a result, it is possible to achieve high-speedization (i.e., shortening of the time) of the joining process and joining under high strength. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] Figure 1 It is a schematic diagram showing an example of a state in which a plate group composed of two metal plates is joined by the friction element joining method of the present invention.

[0039] Figure 2 It is a schematic diagram for explaining the friction element joining method of the present invention.

[0040] Figure 3 It is a cross-sectional view showing an example of the element shape used in the friction element joining method of the present invention.

[0041] Figure 4 It is a schematic diagram showing an example of the joining state using an existing element. DETAILED DESCRIPTION OF THE INVENTION

[0042] Hereinafter, the present invention will be described. In addition, the present invention is not limited to this embodiment.

[0043] First, with reference to Figures 1 to 3 , a friction element joining method as one embodiment of the present invention will be described. Figure 1 It is a cross-sectional view showing an example of a state in which a plate group composed of two metal plates is joined by the friction element joining method of the present invention. Figure 2 It is a diagram for explaining the cylinder displacement amount and the like in each process in the present invention. Figure 3 It is a cross-sectional view showing an example of the element shape used in the friction element joining method of the present invention.

[0044] In the friction element joining method of the present invention (hereinafter, there are also cases where it is referred to as "joining method"), while rotating an element having a conical portion at the front end, it is pressed into a plate group in which two or more metal plates are overlapped to join the plate group (refer to Figure 1 ). This joining is a friction joining using frictional heat generation. The above-mentioned "plate group in which two or more metal plates are overlapped" means a plate group in which two or more metal plates are laminated.

[0045] In addition, as Figure 3 shown, the element 1 used in the present invention includes a mandrel portion 2 and a collar portion 3, and has a tapered portion 4 at the front end of the element (also referred to as "the front end of the mandrel portion"). The tapered portion 4 is formed into a shape with a height of the tapered portion (taper height: t) and an elevation angle of the tapered portion (taper angle: θ) to be described later.

[0046] In addition, in the following description, the case where two metal plates are overlapped to join the friction elements is described, but the present invention can also be applied to the case where three or more metal plates are overlapped to join, and the same effect can be obtained.

[0047] In the joining method of the present invention, the process from penetrating the upper plate to pressing the element onto the upper surface of the lower plate to form a joint portion is constituted by at least two-stage processing. The process of the first stage (i.e., the first process) is a process of removing the oxide film on the upper surface of the lower plate, and the process of the second stage (i.e., the second process) is a process of pressing the element onto the lower plate to form a joint portion, which is constituted by the above two-stage processes. Specifically, the second process includes: a process of generating plastic deformation caused by frictional heat generation on the contact surface between the front end of the element (i.e., the lower surface of the element) and the lower plate, and a process of performing joining based on crimping.

[0048] In addition, as an example, the case designed as "two-stage processing" is described, but the above process can also be divided into three or more stages of processing.

[0049] Here, the "upper plate" in the present invention refers to the uppermost metal plate arranged at the uppermost side among the metal plates (materials to be joined) constituting the plate group, and the "lower plate" refers to the lowermost metal plate arranged at the lowermost side among the metal plates (materials to be joined) constituting the plate group. The upper plate and the lower plate can be, for example, plate-shaped components.

[0050] In the following description, as an embodiment, the method of frictionally joining metal plates using the joining method of the present invention having the above first process and second process is described in detail. Here, as Figure 1 and Figure 2 shown, a plate group in which two metal plates (in this example, an upper plate and a lower plate) are overlapped is used.

[0051] Although the illustration is omitted, first, a plate group composed of an upper plate and a lower plate of a plate-like member is disposed on a support table of a bonding device so as to face an element mounted on the bonding device. A cylinder of the bonding device abuts against the upper surface side of the element. The element is pressed from above by this cylinder and pressed into the lower plate. Next, the rotation speed of the element is adjusted by a control unit of the bonding device, and the element rotates and contacts the upper plate of the plate-like member, and then a pressing force adjusted by the above control unit is applied to press the element into the upper plate. As a result, the upper plate is plasticized due to frictional heat generation, the core shaft portion of the element enters the upper plate, and then the element penetrates the upper plate. This process is called the upper plate entry process. Moreover, the lower surface of the element contacts the upper surface of the lower plate.

[0052] Next, the first process of the present invention, that is, the oxide film removal process on the upper surface of the lower plate, is started by pressing the element into the lower plate while rotating at high speed.

[0053] 〔First Process〕

[0054] In the first process, the oxide film existing on the upper surface of the lower plate is discharged by bringing the front end of the element (that is, the lower surface of the element) into contact with the upper surface of the lower plate.

[0055] By removing the oxide film, a fresh surface between the element and the upper surface of the lower plate is exposed, and a sound solid-phase bonding portion can be formed. In order to obtain such an effect, as described above, it is important to adjust (control) so that the front end shape of the element, the pressing force, and the rotation speed in the first process are in an optimal relationship. Therefore, in the present invention, the bonding conditions for the first process are specified as follows.

[0056] Specifically, in the first process, when the pressing force in the first process is set to P1 (N), the rotation speed in the first process is set to R1 (rpm), the height of the tapered portion of the element is set to t (mm), the cylinder position at the moment when the front end of the element contacts the upper surface of the lower plate is set to 0, and the cylinder displacement amount at the end of the first process is set to L (mm), and the immersion ratio obtained by dividing this cylinder displacement amount by the height of the tapered portion is set to L / t, the oxide on the upper surface of the lower plate is removed using an element that rotates while the immersion ratio (L / t) is controlled within the range of 0.5 ≤ L / t < 1.3, the rotation speed in the first process is controlled within the range of 4000 ≤ R1 ≤ 8000, and the pressing force in the first process is controlled within the range of 4000 ≤ P1 ≤ 8000.

[0057] Here, use Figure 2 The above cylinder displacement amount will be described.

[0058] As Figure 2As shown in (a) therein, the moment when the front end of the element contacts the upper surface of the lower plate is the start time of the first process. Based on the cylinder position at the start time of this first process (hereinafter, also referred to as the "cylinder initial position"), this reference value is set to "0".

[0059] After the start of the first process, the element rotates while being pressed by the cylinder from above the element with a pressing force P1 and a rotational speed R1 that are controlled to specified values within the above numerical range, and is pressed into the lower plate. As Figure 2 shown in (b) therein, when the cylinder is displaced to a preset length, the first process ends. The displacement amount from the cylinder initial position at the end time of this first process is the above-mentioned "cylinder displacement amount (L)".

[0060] When the immersion ratio (L / t) is less than 0.5, the contact surface between the lower part of the element and the upper surface of the lower plate is insufficient, and the removal of the oxide film is insufficient. On the other hand, when the immersion ratio (L / t) is 1.3 or more, there is a concern that the device load will increase because the element enters the lower plate deeply. Therefore, the immersion ratio is set to 0.5 or more and less than 1.3.

[0061] If the rotational speed is less than 4000 rpm, there is a concern that plastic deformation will be hindered due to insufficient heat generation. On the other hand, if the rotational speed exceeds 8000 rpm, there is a concern that the element will deform due to excessive heat generation. Therefore, the rotational speed in the first process is set to 4000 ≤ R1 ≤ 8000.

[0062] In addition, when the pressing force (P1) in the first process is less than 4000 N, there is a concern that the entry into the upper plate cannot be promoted. On the other hand, when the pressing force (P1) in the first process exceeds 8000 N, there is a concern that the device load will increase because the element enters in a state of insufficient heating. Therefore, the pressing force in the first process is set to 4000 ≤ P1 ≤ 8000.

[0063] Furthermore, in the present invention, for the end of the first process, the control unit determines that the removal of the oxide on the upper surface of the lower plate is completed when the cylinder displacement amount reaches L due to the element being pressed in the plate thickness direction (refer to Figure 2 ).

[0064] Next, the subsequent second process is performed.

[0065] 〔Second Process〕

[0066] In the second process, a joint portion is formed by friction joining treatment.

[0067] First, under the pressing force adjusted by the above control unit, while rotating the element at high speed, the element is pressed into the lower plate (refer to Figure 2in (c)). Thus, the friction joining process starts. In the friction joining process, the front end of the element that enters the lower plate plastically deforms the lower plate and the element due to frictional heat generation. This process is called the plastic deformation process. The front end of the mandrel part of the element is connected to the lower plate by friction element joining, and the material of the upper plate extruded due to plastic deformation contacts the upper part of the element and is pressed by the collar part.

[0068] In the subsequent crimping process, mechanical joining of the element to two or more metal plates constituting the plate group is achieved (see Figure 1 ). The above control unit finally applies a pressing force to the element in a state where the rotation of the element has stopped, causing the material of the metal plate extruded due to plastic deformation to be crimped to the element, thereby completing the friction joining process.

[0069] As described above, in the second process, it is important to define the increment (ΔR, described later) of the rotational speed of the second process with respect to the rotational speed of the first process according to the immersion ratio of the element. Therefore, in the present invention, the joining conditions of the second process are defined as follows.

[0070] Specifically, in the second process, when the rotational speed of the second process is set to R2 (rpm) and the increment of the rotational speed of the second process with respect to the rotational speed of the first process is set to ΔR, according to the cylinder displacement amount at the end of the first process,

[0071] When the immersion ratio (L / t) is 0.5 or more and less than 0.8, the rotational speed (R2) in the second process is increased in such a way that the increment (ΔR) satisfies Equation (1).

[0072] When the immersion ratio (L / t) is 0.8 or more and less than 1.0, the rotational speed (R2) in the second process is increased in such a way that the increment (ΔR) satisfies Equation (2).

[0073] When the immersion ratio (L / t) is 1.0 or more and less than 1.3, the rotational speed (R2) in the second process is increased in such a way that the increment (ΔR) satisfies Equation (3).

[0074] ΔR ≥ 4×(2 - L)×((12000 - R1) / 100)…(1)

[0075] ΔR ≥ 2×(3.2 - L)×((12000 - R1) / 100)…(2)

[0076] ΔR ≥ 0.4×(12 - L)×((12000 - R1) / 100)…(3)

[0077] As specified in the conditional expression of the second process, by appropriately varying the rotational speed increment (ΔR) from the first process to the second process according to the immersion ratio (L / t) in the first process, the frictional heat generated by rotation can be effectively utilized to promote the plasticization of the lower plate. As a result, the formation of the joint can be promoted.

[0078] That is, when the immersion ratio (L / t) is 0.5 or more and less than 0.8, by increasing the rotational speed (R2) of the second process in such a way that the increment (ΔR) satisfies Equation (1), the plastic deformation effect caused by frictional heat can be obtained. As a result, the joining is promoted. Preferably, the increment (ΔR) is 200 or more, that is, Equation (1) is set as ΔR ≥ 200. The upper limit value of the increment (ΔR) in the case where the immersion ratio is within this numerical range is not particularly specified. Since sound joining cannot be achieved due to excessive frictional heat, it is preferably that the upper limit of the increment (ΔR) is 600 or less, that is, Equation (1) is set as 600 ≥ ΔR.

[0079] In addition, when the immersion ratio (L / t) is 0.8 or more and less than 1.0, compared with the case of 0.5 ≤ L / t < 0.8 described above, the element is immersed and advanced. Therefore, the rotational speed (R2) of the second process is increased more slowly than Equation (1) above in such a way that the increment (ΔR) satisfies Equation (2). When the immersion ratio is 0.8 or more and less than 1.0, if it is less than the right side value of Equation (2) (that is, the value calculated by "2×(3.2 - L)×((12000 - R1) / 100)"), the plastic deformation effect caused by frictional heat cannot be obtained and sound joining cannot be achieved. Preferably, the right side value of Equation (2) is 150 or more, that is, Equation (2) is set as ΔR ≥ 150. The upper limit value of the increment (ΔR) in the case where the immersion ratio is within this numerical range is not particularly specified. Since sound joining cannot be achieved due to excessive frictional heat, it is preferably that the upper limit of the increment (ΔR) is 500 or less, that is, Equation (2) is set as 500 ≥ ΔR.

[0080] In addition, when the immersion ratio (L / t) is 1.0 or more and less than 1.3, the immersion of the component is advanced to a certain extent. If the rotation speed (R2) of the second process is excessively increased, the thermal influence becomes larger. Therefore, the rotation speed is slightly increased compared to the above formulas (1) and (2). When the immersion ratio is 1.0 or more and less than 1.3, and when it is less than the right side value of formula (3) (i.e., the value calculated by "0.4×(12 - L)×((12000 - R1) / 100)"), the plastic deformation effect caused by frictional heat generation cannot be obtained, and sound joining cannot be achieved. It is preferable that the right side value of formula (3) is 100 or more, that is, formula (3) is set as ΔR≥100. The upper limit value of the increment (ΔR) when the immersion ratio is within this numerical range is not particularly specified. Since sound joining cannot be achieved due to excessive frictional heat generation, it is preferable that the upper limit of the increment (ΔR) is 400 or less, that is, formula (3) is set as 400≥ΔR.

[0081] In addition, as Figure 2 shown in (c) of, if the cylinder in the second process is displaced to a preset length (S), the second process ends. The rotation speed of the second process determines the increment (ΔR) based on the immersion ratio (L / t) of the cylinder displacement amount (L) at the end of the first process, and performs the above-mentioned various processes at this rotation speed (constant).

[0082] According to the joining method of the present invention, a sound joining state between the metal plate constituting the plate group and the component can be obtained.

[0083] As described above, by changing the increment (ΔR) of the rotation speed of the second process according to the shape of the front end of the component and the immersion ratio (L / t), it is possible to ensure shortening of the joining process and joint strength.

[0084] In addition, in the joining method of the present invention, when using a metal plate provided with a through hole as a bottom hole on the upper plate, the above-mentioned upper plate entry process is omitted. That is, after the plate group is set on the support table of the joining device in such a manner that the overlapping metal plates (i.e., the plate group) face the component mounted on the joining device, the above-mentioned first process and subsequent processes are performed.

[0085] In the present invention, it is possible to apply metal plates including ultra-high strength steel to the upper plate 7 and the lower plate 8. In the present invention, "ultra-high strength steel" refers to a steel plate having a tensile strength (TS) of 590 MPa or more. However, when using ultra-high strength steel for the upper plate, it is necessary to previously provide the above-mentioned through hole.

[0086] Next, use Figure 3 to illustrate an embodiment of the shape of the component used in the present invention. Figure 3 is a longitudinal sectional view taken along the length direction cut in a manner passing through the center of the component 1.

[0087] Element 1 is a tool for joining friction elements of a plate group by pressing it into a plate group in which two or more metal plates are overlapped while rotating Element 1 by its side (see Figure 1 ).

[0088] The mandrel portion 2 is provided at the axis of Element 1, and the portion other than the front end is formed in a cylindrical shape. The diameter of this cylindrical shape is the diameter (D) (mm) of the element. The front end of the mandrel portion (i.e., the front end of the element) is inclined so that the front end portion protrudes, and is formed in a conical shape. This conical portion becomes the tapered portion 4. As shown in the sectional view of Figure 3 , the angle formed by the straight line perpendicular to the axis at the front end of the element in the tapered portion 4 and the conical side surface (i.e., the conical surface) is defined as the elevation angle (θ) (°).

[0089] In the present invention, in order to promote the discharge of oxides, it is preferable to set the elevation angle (θ) to 10 to 45°. If the elevation angle (θ) is less than 10°, there is a concern that the effect of discharging oxides cannot be obtained. On the other hand, if the elevation angle (θ) exceeds 45°, there is a concern that the contact area between the lower part of the element and the upper surface of the lower plate is reduced. The elevation angle is preferably 20° or more and preferably 30° or less.

[0090] In addition, as shown in the sectional view of Figure 3 , the tapered portion 4 sets the length from the front end position (t1) of the element to the intersection point (t2) of the diameter of the bottom surface of the cone and the axis as the height (t) (mm) of the tapered portion. In the present invention, in order to suppress the core jitter of the element when the upper surface of the lower plate is pressed in, it is preferable to set the height (t) of the tapered portion to more than 0 mm and 1.0 mm or less.

[0091] If the height (t) of the tapered portion is 0 mm, the front end of the element becomes a flat surface, and the discharge of oxides occurs. That is, the effect of the present invention cannot be obtained. On the other hand, if the height (t) of the tapered portion exceeds 1.0 mm, the front end becomes an acute angle, and in order to obtain a bonding surface of the same degree as the element diameter, the element needs to be pressed deeper. The height of the tapered portion is preferably 0.1 mm or more.

[0092] The total length S (mm) of the mandrel portion 2 is a length equal to or less than the total value of the total plate thickness of the metal plates constituting the plate group 6. The total length S of the mandrel portion 2 only needs to be adjusted to a length that can join the front end of the mandrel portion 2 to the lower plate 8.

[0093] The collar portion 3 is provided on the upper part of the mandrel portion 2, that is, the head of Element 1. As shown in Figure 3 , the shape of the collar portion 3 can be formed into a shape such as a wafer or a torus, for example. The collar portion 3 only needs to be a shape that can suppress the material of the upper plate 7 extruded due to plastic flow.

[0094] Next, useFigure 1 and Figure 4 explain the joined state of the obtained joined joint.

[0095] Figure 1 An example of the state in which a plate group composed of two metal plates is joined using the element 1 of the present invention is shown. In Figure 4 the existing example, an example of the state in which a plate group composed of two metal plates is joined using the existing element 10 is shown. Figure 1 and Figure 4 both use the above-described joining method of the present invention as the joining method. In addition, Figure 1 and Figure 4 are cross-sectional views in the plate thickness direction cut in a manner passing through the center of the element in the obtained joined joint.

[0096] As Figure 4 the existing example shows, when joining is performed using the existing element 10 by the joining method of the present invention, the plate group 6 composed of two metal plates ( Figure 4 the upper plate 7 and the lower plate 8 in Figure 4 ) becomes a state joined by the element 10. However, as Figure 4 shows, although the upper plate 7 and the lower plate 8 are joined by the element 10, there is a gap between the material of the upper plate 7 plastically deformed and extruded during the joining process and the collar portion 3 provided at the upper part of the mandrel.

[0097] The present inventors believe that this gap is caused by the oxide film removal process on the lower plate surface of the friction element joining method. If the cross-section of the joined portion of the joined joint where the discharge of the oxide film existing on the upper surface of the lower plate is insufficient in the oxide film removal process is observed, oxides caused by the oxide film on the upper surface of the lower plate remain at the joining interface between the element 10 and the lower plate 8. As a result, the joined state is not sound. This is caused by insufficient time for the oxide film removal process and insufficient discharge of the oxide film.

[0098] Here, an element having a conical shape at the front end of the mandrel portion of the element is used as the above-mentioned "existing element". However, the regulation of the tapered portion as in the present invention is not performed.

[0099] In contrast, as Figure 1 shows, when joining is performed using the element 1 having the above-described tapered portion 4 (refer to Figure 3 ) and by the joining method of the present invention, the two metal plates ( Figure 1The plate group 6 composed of the upper plate 7 and the lower plate 8) in it is in a state where it is joined to the element 1. Specifically, for the plate group 6 composed of two plate-like members (i.e., the upper plate 7 and the lower plate 8), the front end of the mandrel 2 is connected to the lower plate 8 by friction element joining, and the material of the upper plate 7 that is plastically deformed and extruded during the joining process is pressed by the collar portion 3 provided on the upper part of the mandrel. Thus, each metal plate 7, 8 and the element 1 are in a joined state.

[0100] As Figure 1 shown, when observing the cross-section of the joined portion in the above-mentioned element 1, it was confirmed that it was in a sound joined state. As shown in the embodiments described later, according to the present invention, there are no voids as in the example shown in Figure 4 . Thus, it can also be known that the residual oxides can be reduced.

[0101] As described above, in the present invention, when joining the material to be joined using an element having the height (t) of the above-mentioned tapered portion, the change amount (ΔR) of the rotational speed (R2) of the joining process (i.e., the second process) for performing the friction joining process and the rotational speed (R1) of the process (i.e., the first process) for removing the oxide on the upper surface of the lower plate, which is the previous process, is set to satisfy the conditional expressions of Formula (1) to Formula (3) according to the immersion ratio (L / t) for joining. Thus, the heat input to the material to be joined can be suppressed, and the front end of the element can be pressed into a specified depth. Therefore, according to the present invention, compared with the existing element joining method, the reduction in the joining strength of the obtained joining joint can be suppressed. In addition, the shortening of the joining process can also be achieved.

[0102] Embodiment

[0103] Hereinafter, in order to further understand the present invention, embodiments will be used for description. In addition, the present embodiments do not limit the present invention, and as long as they satisfy the gist of the present invention, they are all included in the technical scope of the present invention.

[0104] As test specimens, the metal plates shown in the upper plate and the lower plate of Table 1 were used, and they were overlapped to form a plate group. Here, the upper plate used was the upper plate with a pre-hole processed with a diameter of Φ7 mm. In the present embodiment, only the above-mentioned first process and second process were performed using this plate group, and the evaluation described later was carried out.

[0105] In the first process and the second process, an element with the front end shape shown in Table 2 was used, and it was controlled to be in the pressing force, rotational speed, and cylinder displacement amount (L) shown in Table 2 for carrying out. In addition, for the shape of the element, the element diameter (D) was set to 4.55 mm, and the elevation angle (θ) and height (t) of the tapered portion were set to the values shown in Table 2 (refer to Figure 3 ).

[0106] Moreover, after the second process was completed, the following evaluation was carried out.

[0107] 〔Evaluation of fracture morphology after tensile test〕

[0108] The cross tensile test specified in JIS Z 3137 was carried out, and the fracture morphology after fracture was investigated. The fracture morphology was evaluated by observing the cross section of the area where the components contacted with SEM (Scanning Electron Microscope). Here, when a ductile cross section with an area equal to or larger than the component diameter (D) was obtained, it was evaluated with the symbol "〇 (qualified)". On the other hand, when the ductile cross section had an area smaller than the component diameter (D), it was evaluated with the symbol "× (unqualified)". In addition, a high joint strength can be obtained through the area where the ductile cross section is equal to or larger than the component diameter.

[0109] 〔Evaluation of shortening of time〕

[0110] The evaluation of shortening of time from the start of the first process to the end of the second process of the present invention was carried out as follows.

[0111] In this embodiment, the time required from the start of the first process to the completion of the second process was set as "time required to complete the process (required time) (s)". In addition, in this embodiment, the upper limit value of this required time was set to 5 s. The above "start of the first process" refers to the moment when the oxide film removal process on the upper surface of the lower plate described above starts, and the above "completion of the second process" refers to the moment when the friction joining process described above is completed.

[0112] Here, when the required time was 0 s or more and less than 2 s, it was evaluated with the symbol "A", when the required time was 2 s or more and less than 3 s, it was evaluated with the symbol "B", and when the required time was 3 s or more, it was evaluated with the symbol "C". In addition, when the first and second processes were not completed within the above upper limit time, it was evaluated with the symbol "F". The evaluations "A" and "B" were qualified, and "A" represented the best. The evaluations "C" and "F" were unqualified, and "F" represented the worst. The obtained evaluation results are shown in Table 2.

[0113] In addition, the examples (No. 7, 14) with the evaluation of "-" in the "shortening of time" column of Table 2 were comparative examples that were outside the range of the above conditional formula due to the penetration ratio (L / t) of 1.3 and were not completed within 5 s, and thus were outside the scope of evaluation. In addition, No. 26 and 27 were also comparative examples that were outside the scope of evaluation because the penetration ratio (L / t) was not within the above numerical range.

[0114] [Table 1]

[0115]

[0116] [Table 2]

[0117]

[0118] As shown in Table 2, in the examples of the present invention, a ductile cross-section having an area equal to or larger than the element diameter can be obtained in any case. By having a ductile cross-section with an area equal to or larger than the element diameter, high joint strength can be obtained.

[0119] Description of Reference Numerals

[0120] 1, 10... elements; 2... mandrel portion; 3... ferrule portion; 4... tapered portion; 6... plate group; 7... upper plate; 8... lower plate; 9... interface of joint portion between element and lower plate.

Claims

1. A method for joining a friction element, in which an element having a tapered portion at its front end is rotated and pressed into a stack of two or more metal plates to join the stack of plates. The method for joining a friction element is characterized by comprising: a first step of removing an oxide film on the upper surface of a lower plate disposed at the lowermost layer of the stack of plates; and a second step of pressing the element into the lower plate to form a joint portion. In the first step, when the pressing force in the first step is P1 (N), the rotational speed in the first step is R1 (rpm), the height of the tapered portion is t (mm), the cylinder position at the moment when the front end of the element contacts the upper surface of the lower plate is set to 0 and the cylinder displacement at the end of the first step is L (mm), and the penetration ratio obtained by dividing the cylinder displacement by the height of the tapered portion is L / t, the removal of the oxide on the upper surface of the lower plate is controlled within the range of 0.5 ≤ L / t < 1.3, 4000 ≤ R1 ≤ 8000, and 4000 ≤ P1 ≤ 8000. Next, in the second step, when the rotational speed in the second step is R2 (rpm) and the increase in the rotational speed in the second step relative to the rotational speed in the first step is ΔR, when the penetration ratio L / t at the end of the first step is 0.5 or more and less than 0.8, the rotational speed in the second step is increased in such a manner that the increase satisfies formula (1). when the penetration ratio L / t at the end of the first step is 0.8 or more and less than 1.0, the rotational speed in the second step is increased in such a manner that the increase satisfies formula (2). when the penetration ratio L / t at the end of the first step is 1.0 or more and less than 1.3, the rotational speed in the second step is increased in such a manner that the increase satisfies formula (3). ΔR ≥ 4 × (2 - L) × ((12000 - R1) / 100)…(1) ΔR ≥ 2 × (3.2 - L) × ((12000 - R1) / 100)…(2) ΔR ≥ 0.4 × (12 - L) × ((12000 - R1) / 100)…(3).

2. The method for joining a friction element according to claim 1, characterized in that the metal plate is a steel plate having a tensile strength of 590 MPa or more.

3. The method for joining a friction element according to claim 1 or 2, characterized in that the height of the tapered portion of the element exceeds 0 mm and is 1.0 mm or less, and the elevation angle of the tapered portion is 10 to 45°.

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