Method for joining dissimilar materials, and dissimilar-material joined body

Abstract

This method for joining dissimilar materials comprises a joining step for solid-phase joining a first material to be joined and a second material to be joined. The joining step includes: a first step in which a first pressure is applied to the first material to be joined and the second material to be joined, and the interface between the first material to be joined and the second material to be joined is heated to a prescribed temperature; and a second step in which, after the first step, a second pressure is applied to deform the first material to be joined and the second material to be joined. The first pressure corresponds to the yield strength of the first material to be joined at a prescribed temperature and is lower than the yield strength of the second material to be joined at the prescribed temperature. The second pressure is 80-110% of the yield strength of the first material to be joined at room temperature. There is no intersection between the temperature-yield strength curve of the first material to be joined and the temperature-yield strength curve of the second material to be joined.

Description

Dissimilar material joining method and dissimilar material joined body

[0001] The present disclosure relates to a method for joining dissimilar materials and a joined body of dissimilar materials.

[0002] In the past, research and development has been actively conducted into joining materials containing different metals (dissimilar material joining) from the viewpoint of reducing the weight of structures accompanying the electrification of automobiles and the development of the aerospace industry. By implementing the dissimilar material joining, it becomes possible to manufacture a joined body that is lighter than a joined body made of steel plates. For example, Patent Document 1 listed below discloses a friction joining device that frictionally joins one member to another member by repeatedly moving the two members relative to each other along the same trajectory while they are in contact with each other.

[0003] JP 2015-164738 A

[0004] When dissimilar materials are joined by solid-state welding using the above-mentioned friction welding apparatus or the like, it is desirable to join the materials at a temperature at which the strengths of the two materials to be joined are the same, from the viewpoint of obtaining good joint efficiency. However, when the materials to be joined are a steel and an aluminum alloy, for example, there is no temperature at which the strengths of the two materials to be joined are the same. In this case, even if solid-state welding such as friction welding is simply performed, oxides are likely to be contained in the joint and unjoined areas are likely to be formed, making it difficult to obtain good joint efficiency.

[0005] On the other hand, even if there is a temperature at which the strength of the welded materials is the same, that temperature may be too low. If solid-state welding such as friction welding is simply performed on such a combination of welded materials, a metallurgical weld interface due to the recrystallization mechanism will not be formed, and good joint efficiency will not be achieved.

[0006] An object of one aspect of the present disclosure is to provide a dissimilar material joined body that exhibits good joint efficiency even when there is no temperature at which the strengths of the joined materials are the same, and even when the temperature at which the strengths of the joined materials are the same is too low, and to provide a dissimilar material joining method capable of producing the same.

[0007] A dissimilar material joining method according to one aspect of the present disclosure includes a preparation step of butting together a first material to be joined that includes a first metal and a second material to be joined that includes a second metal different from the first metal, and a joining step of solid-state joining the first material to be joined and the second material to be joined, the joining step including a first step of applying a first pressure to the first material to be joined and the second material to be joined in a butting direction of the first material to be joined and heating an interface between the first material to be joined and the second material to be joined to a predetermined temperature, and a second step of applying a second pressure higher than the first pressure in a butting direction to the first material to be joined and the second material to be joined after the first step. and a second step of applying a pressure to the second material to be joined to deform the first material and the second material to be joined, wherein the first pressure corresponds to the yield strength of the first material to be joined at a predetermined temperature and is lower than the yield strength of the second material to be joined at the predetermined temperature, the second pressure is 80% or more and 110% or less of the yield strength of the first material to be joined at room temperature and is equal to or greater than the yield strength of the second material to be joined at the predetermined temperature, and there is no intersection between the temperature-yield strength curve of the first material to be joined and the temperature-yield strength curve of the second material to be joined, or the temperature of the intersection is lower than the predetermined temperature.

[0008] A dissimilar material joined body according to one aspect of the present disclosure comprises a first material to be joined and a second material to be joined that overlap each other in a predetermined direction, and a joining portion that integrates the first material to be joined and the second material to be joined, wherein the joining portion has a joining region located between the first material to be joined and the second material to be joined in the predetermined direction, and a burr portion located outside the joining region when viewed from the predetermined direction, and the burr portion has a first portion having a first metal contained in the first material to be joined and a second portion having a second metal contained in the second material to be joined, and when there is no intersection between the temperature-yield strength curve of the first material to be joined and the temperature-yield strength curve of the second material to be joined, or when the temperature of the intersection is lower than 150°C, and when the first maximum tensile strength of the first material to be joined is lower than the second maximum tensile strength of the second material to be joined in the predetermined direction, the maximum tensile strength of the joining portion is 90% or more of the first maximum tensile strength.

[0009] According to one aspect of the present disclosure, it is possible to provide a dissimilar material joined body that exhibits good joint efficiency even when there is no temperature at which the strengths of the joined materials are the same, and even when the temperature at which the strengths of the joined materials are the same is too low, and a dissimilar material joining method that can produce the same.

[0010] FIG. 1 is a cross-sectional schematic diagram showing a joined body according to an embodiment. FIG. 2 is a diagram showing the temperature dependence of the material strength of the first member and the second member according to an embodiment. FIGS. 3A and 3B are cross-sectional schematic diagrams for explaining a dissimilar material joining method according to an embodiment. FIG. 4 is a diagram showing changes in pressure applied to the first member and the second member in the dissimilar material joining method. FIG. 5 is a diagram showing the temperature dependence of the material strength of the first member and the second member according to a modified example. FIG. 6 is a diagram showing the temperature dependence of the material strength of the first and second members to be joined in Experimental Example 1. FIG. 7A is an enlarged photograph of a main portion of the dissimilar material joined body formed in Experimental Example 1, and FIG. 7B is an enlarged photograph of a main portion of the first and second members to be joined after the dissimilar material joining method of Experimental Example 6. FIG. 8 is a diagram showing measurement results of the maximum tensile strength of joints in Experimental Examples 1 to 6. Fig. 9(a) is an enlarged photograph of a main portion of a joined body of dissimilar materials formed in Experimental Example 7, Fig. 9(b) is an enlarged photograph of a main portion of a joined body of dissimilar materials formed in Experimental Example 8, Fig. 9(c) is an enlarged photograph of a main portion of a joined body of dissimilar materials formed in Experimental Example 9, and Fig. 9(d) is an enlarged photograph of a main portion of a joined body of dissimilar materials formed in Experimental Example 10. Fig. 10(a) is a cross-sectional photograph showing a central portion of a joint formed in the joined body of dissimilar materials of Experimental Example 8, Fig. 10(b) is a cross-sectional photograph showing an end portion of a joint formed in the joined body of dissimilar materials of Experimental Example 8, Fig. 10(c) is a cross-sectional photograph showing a central portion of a joint formed in the joined body of dissimilar materials of Experimental Example 10, and Fig. 10(d) is a cross-sectional photograph showing an end portion of a joint formed in the joined body of dissimilar materials of Experimental Example 10. Fig. 11(a) shows a microstructure photograph of the center of the second workpiece near the weld interface of the joint formed in the joined body of dissimilar metals of Experimental Example 10, and Fig. 11(b) shows a microstructure photograph of the end of the second workpiece near the weld interface of the joint formed in the joined body of dissimilar metals of Experimental Example 10. Fig. 12 is a diagram showing the measurement results of the maximum tensile strength of the joints of Experimental Examples 7 to 10. Fig. 13 is a graph showing the distribution of Vickers hardness of Experimental Examples 8 to 10.

[0011] Hereinafter, embodiments of the present disclosure will be described in detail with reference to the accompanying drawings. In the following description, identical elements or elements having the same functions will be designated by the same reference numerals, and redundant explanations will be omitted. In this specification, the term "same" and similar words are not limited to "completely identical." Furthermore, since the drawings are intended to conceptually explain the embodiments, the dimensions and ratios of the depicted components may differ from the actual dimensions.

[0012] (1) Overview of the method for joining dissimilar materials In the method for joining dissimilar materials according to this embodiment, workpieces containing different metals are joined by friction welding. As a specific example, the method for joining dissimilar materials according to this embodiment includes a preparation step of butting together a first workpiece containing a first metal and a second workpiece containing a second metal different from the first metal, and a joining step of solid-state joining the first workpiece and the second workpiece, in which the joining step includes a first step of applying a first pressure to the first workpiece and the second workpiece along the butt direction of the first workpiece and the second workpiece, and heating the interface between the first workpiece and the second workpiece to a predetermined temperature, and a second step of applying a second pressure, which is higher than the first pressure, to the first workpiece along the butt direction of the first workpiece. and a second step of applying a pressure to the composite material and the second material to be joined, thereby deforming the first material and the second material to be joined, wherein the first pressure corresponds to the yield strength of the first material to be joined at a predetermined temperature and is lower than the yield strength of the second material to be joined at the predetermined temperature, the second pressure is 80% or more and 110% or less of the yield strength of the first material to be joined at room temperature and is equal to or greater than the yield strength of the second material to be joined at the predetermined temperature, and there is no intersection between the temperature-yield strength curve of the first material to be joined and the temperature-yield strength curve of the second material to be joined, or the temperature of the intersection is lower than the predetermined temperature. The dissimilar material joined body according to this embodiment, which is manufactured by the above-described dissimilar material joining method, comprises a first material to be joined and a second material to be joined that overlap each other in a predetermined direction, and a joining portion that integrates the first material to be joined and the second material to be joined, wherein the joining portion has a joining region located between the first material to be joined and the second material to be joined in the predetermined direction, and a burr portion located outside the joining region when viewed from the predetermined direction, and the burr portion has a first portion having a first metal contained in the first material to be joined and a second portion having a second metal contained in the second material to be joined, and when there is no intersection between the temperature-yield strength curve of the first material to be joined and the temperature-yield strength curve of the second material to be joined, or when the temperature of the intersection is lower than 150°C, and when the first maximum tensile strength of the first material to be joined is lower than the second maximum tensile strength of the second material to be joined in the predetermined direction, the maximum tensile strength of the joining portion is 90% or more of the first maximum tensile strength.

[0013] (2) Details of the Method for Joining Dissimilar Materials Next, details of the method for joining dissimilar materials according to this embodiment will be described. First, a joined body manufactured by the method for joining dissimilar materials according to this embodiment will be described with reference to FIG.

[0014] FIG. 1 is a cross-sectional schematic diagram showing a joined body according to this embodiment. As shown in FIG. 1, the joined body 1 is a structure including a first member 2, a second member 3, and a joint 4 that joins the first member 2 and the second member 3. The joined body 1 is used, for example, as a part of an automobile, transportation equipment, or the like. In this embodiment, the joined body 1 is a rod-shaped member having a quadrangular prism shape, but is not limited thereto. For example, the joined body 1 may be a rod-shaped member having a cylindrical or polygonal prism shape, or a plate-shaped member.

[0015] The first member 2 and the second member 3 are each a workpiece to be joined by a joint 4. Therefore, in the joined body 1, the first member 2 (first workpiece) and the second member 3 (second workpiece) are integrated with each other. In the joined body 1, the first member 2 and the second member 3 are butt-joined along the direction D1 shown in FIG. 1 and overlap each other in the direction D1. As will be described later, in this embodiment, the first member 2 and the second member 3 are joined by linear friction welding. The shapes, sizes, etc. of the first member 2 and the second member 3 are not particularly limited and may be bulk materials such as plates, sheets, bars, and square timber. The shapes of the first member 2 and the second member 3 may be the same or different from each other. In this embodiment, the shapes of the first member 2 and the second member 3 are each columnar. The sizes of the first member 2 and the second member 3 may be the same or different from each other. For example, the cross-sectional area of ​​the first member 2 perpendicular to the direction D1 may be the same as or different from the cross-sectional area of ​​the second member 3 perpendicular to the direction D1.

[0016] Each of the first member 2 and the second member 3 may have room temperature strength sufficient to withstand pressure (details of which will be described later) applied during the manufacturing of the bonded structure 1. In this case, deformation at unintended locations during the manufacturing of the bonded structure 1 can be prevented.

[0017] The first member 2 and the second member 3 each have a metal phase that can be metallurgically joined together. In this embodiment, the first member 2 and the second member 3 include different materials. That is, the joined body 1 is a joined body of dissimilar materials. The metal included in the first member 2 and the metal included in the second member 3 are, for example, aluminum, iron, magnesium, copper, titanium, nickel, zinc, cobalt, niobium, zirconium, silver, etc. The first member 2 and the second member 3 may each include an alloy containing the above metal (e.g., aluminum alloy, steel such as carbon steel, nickel-chromium steel, Invar (alloy), titanium alloy, stainless steel, copper alloy, magnesium alloy, etc.). In one example, the hardness of the first member 2 is lower than the hardness of the second member 3, and the melting point of the metal included in the first member 2 (first metal) is lower than the melting point of the metal included in the second member 3 (second metal). In one example, the first member 2 (first material to be joined) is an aluminum alloy material containing aluminum (first metal), and the second member 3 (second material to be joined) is a steel material containing iron (second metal).

[0018] 2 is a diagram showing the temperature dependence of the material strength of the first member 2 and the second member 3 according to this embodiment. In FIG. 2, the vertical axis represents yield strength, the horizontal axis represents temperature, graph 11 represents the temperature-yield strength curve of the first member 2, and graph 12 represents the temperature-yield strength curve of the second member 3. As shown in FIG. 2, in this embodiment, the yield strength (yield stress) of the first member 2 is generally lower than the yield strength of the second member 3, and there is no intersection between graphs 11 and 12. Note that plot 13 in graph 11 represents the yield strength of the first member 2 at room temperature.

[0019] The joint 4 is a portion of the joined body 1 that integrates the first member 2 and the second member 3. In this embodiment, when the maximum tensile strength (first maximum tensile strength) of the first member 2 is lower than the maximum tensile strength (second maximum tensile strength) of the second member 3 in the direction D1, the maximum tensile strength of the joint 4 is 90% or more of the first maximum tensile strength. The joint 4 has a joint region 5 located between the first member 2 and the second member 3 in the direction D1, and a flash portion 6 located outside the joint region 5 when viewed from the direction D1.

[0020] The joining region 5 corresponds to a portion that contributes to joining the first member 2 and the second member 3 and includes an interface B (joined interface) between the first member 2 and the second member 3. The thickness of the intermetallic compound layer in the joining region 5 is, for example, 1 μm or less. In this case, the adverse effects of the intermetallic compound layer on the joining strength can be effectively reduced. The burr portion 6 is a portion where a burr generated when the first member 2 and the second member 3 are joined has been cooled. The burr portion 6 has a first portion 6 a containing metal contained in the first member 2 and a second portion 6 b containing metal contained in the second member 3. The first portion 6 a is formed from a softened portion of the first member 2 that is generated when the first member 2 and the second member 3 are joined. The second portion 6 b is formed from a softened portion of the second member 3 that is generated when the first member 2 and the second member 3 are joined. In FIG. 1 , the shape and size of the first portion 6 a are similar to those of the second portion 6 b, but are not limited thereto. From the viewpoint of material strength, the first portion 6 a may be larger than the second portion 6 b. The first portion 6 a may contain the metal contained in the second member 3, and the second portion 6 b may contain the metal contained in the first member 2.

[0021] (Dissimilar Material Joining Method) Hereinafter, an example of a dissimilar material joining method according to this embodiment will be described in detail with reference to FIGS. 3A, 3B, and 4. FIGS. 3A and 3B are cross-sectional schematic views for explaining the dissimilar material joining method according to this embodiment. FIG. 4 is a diagram showing changes in pressure applied to the first member 2 and the second member 3 in the dissimilar material joining method. FIG. 4 shows graphs 11 and 12 and plots 13 to 15 similar to those in FIG. 2. Plot 14 indicates a first pressure P1 applied to the first member 2 and the second member 3 at a temperature T1 (described later). Plot 15 indicates a second pressure P2 applied to the first member 2 and the second member 3 at a temperature T1 (described later). In FIG. 4, plot 14 overlaps with graph 11, and plot 15 overlaps with graph 12, but this is not a limitation. Plot 14 may be located above graph 11. Plot 15 may be located below graph 12 or above graph 12.

[0022] First, as shown in Fig. 3(a), the first member 2 is butted against the second member 3 (preparation step). In this preparation step, one end 2a of the first member 2 is butted against one end 3a of the second member 3. This forms an interface B between the first member 2 and the second member 3. This completes the preparation for carrying out the dissimilar material joining method.

[0023] Next, the first member 2 and the second member 3 are solid-state welded (joining process). In this embodiment, the joining process is performed within, for example, several seconds or one second. Therefore, the first and second processes described below can be performed continuously and quickly. In the joining process, as shown in FIG. 3B, a first pressure P1 is applied to the first member 2 and the second member 3 along a direction D1, and the interface B between the first member 2 and the second member 3 is heated to a temperature T1 (a predetermined temperature) (first process). In this embodiment, the first process deforms (plastically deforms) the peripheral portion of the interface B in the first member 2, generating a burr 8 near the interface B. In the first process, a softened region of the first member 2 is formed near the interface B, and at least a portion of the softened region is pushed out from the interface B, generating the burr 8. In the first process, the second process described below may be performed before the deformation of the first member 2. Note that the second member 3 is not deformed in the first process. Furthermore, the first member 2 does not necessarily have to be deformed in the first step.

[0024] The temperature T1 and the first pressure P1 in the first step are set, for example, in accordance with the temperature dependence of the yield strength of the first member 2 and the second member 3 (the temperature-yield strength curve of the first member 2, the temperature-yield strength curve of the second member 3), the yield stress of the first member 2 at room temperature, the temperature dependence of the tensile strength of at least one of the first member 2 and the second member 3, and the temperature dependence of the flow stress of at least one of the first member 2 and the second member 3. That is, the temperature T1 and the first pressure P1 are set appropriately depending on the material of the first member 2 and the material of the second member 3, for example. In one example, the first pressure P1 is set in accordance with the temperature-yield strength curve of the first member 2. In this embodiment, the first pressure P1 corresponds to the yield strength of the first member 2 at the temperature T1. In addition, the first pressure P1 is lower than the yield strength of the second member 3 at the temperature T1. Therefore, as described above, the first member 2 may deform in the first step, while the second member 3 does not. In one example, the first pressure P1 is constant, for example, between 100 MPa and 200 MPa. Note that when the first pressure P1 corresponds to the yield strength of the first member 2 at temperature T1, the first pressure P1 does not have to coincide with the yield strength of the first member 2 at temperature T1. For example, the first pressure P1 may be an adjusted value using the yield strength of the first member 2 at temperature T1 as a reference value. The adjusted value is a value obtained by adjusting the reference value in consideration of the influences of the temperature dependence of the tensile strength of at least one of the first member 2 and the second member 3, the temperature dependence of the flow stress of at least one of the first member 2 and the second member 3, and the like.

[0025] The heating method for the first step to temperature T1 varies depending on the type of solid-state welding. For example, if the solid-state welding is friction welding, linear friction welding, or the like, the friction between the first member 2 and the second member 3 corresponds to the heating method. On the other hand, if the solid-state welding is a voltage welding method or the like, the application of electricity to the first member 2 and the second member 3 corresponds to the heating method. In this embodiment, the interface B is heated to temperature T1 by linear friction between the first member 2 and the second member 3 along direction D2 perpendicular to direction D1. Temperature T1 is determined by, for example, the temperature-yield strength curve of the first member 2, the temperature-yield strength curve of the second member 3, the first pressure P1, the temperature dependence of the tensile strength of at least one of the first member 2 and the second member 3 (e.g., the temperature-tensile strength curve of the first member 2 and / or the second member 3), the temperature dependence of the flow stress of at least one of the first member 2 and the second member 3 (e.g., the temperature-flow stress curve of the first member 2 and / or the second member 3), and the like. In this embodiment, the predetermined temperature is determined by the temperature-yield strength curve of the first member 2 or the temperature-yield strength curve of the second member 3, and the first pressure P1. In one example, the temperature T1 is set so that the yield strength of the first member 2 at the temperature T1 corresponds to the first pressure P1. As a result, when the interface B is heated to the temperature T1 in the first step, the softened region of the first member 2 is formed as described above. Note that the temperature T1 may be set after the first pressure P1 is determined, or the first pressure P1 may be set after the temperature T1 is determined. For example, when the first pressure P1 is constant in the first step, deformation of the first member 2 occurs when the interface B is heated to the temperature T1. Alternatively, when the temperature T1 is constant in the first step, deformation of the first member 2 occurs when the pressure applied to the first member 2 and the second member 3 reaches the first pressure P1.

[0026] The yield strength of the second member 3 at temperature T1 is, for example, 80% to 110% of the yield strength of the first member 2 at room temperature. In this case, deformation of unintended locations of the first member 2 can be suppressed during the joining process. From the viewpoint of preventing deformation of unintended locations of the first member 2 during the joining process, the yield strength of the second member 3 at temperature T1 may be lower than the yield strength of the first member 2 at room temperature. Alternatively, from the viewpoint of preventing deformation, if the yield strength of the second member 3 at temperature T1 is equal to or greater than the yield strength of the first member 2 at room temperature, the entire periphery of the first member 2 may be fixed with a jig or the like. In these cases, the shape of the first member 2 can be well maintained in the second process described later. Furthermore, from the viewpoint of forming a metallurgical bonded interface by a recrystallization mechanism during the joining process, the temperature T1 depends on the materials of the first member 2 and the second member 3, but in one example, the temperature T1 is 150°C or higher, 180°C or higher, or 200°C or higher. The conditions for linear friction between the first member 2 and the second member 3 are not particularly limited, but for example, the frequency of the relative movement of the first member 2 and the second member 3 along the direction D2 is greater than or equal to 50 Hz and less than or equal to 100 Hz.

[0027] After the first step, a second pressure P2 higher than the first pressure P1 is applied to the first member 2 and the second member 3 along the direction D1, deforming (plastically deforming) the peripheral portions of the interface B in the first member 2 and the second member 3 (step 2). In step 2, a softened region of the first member 2 and a softened region of the second member 3 are formed near the interface B, and at least a portion of each softened region is extruded from the interface B. As a result, the newly formed surfaces of the first member 2 and the second member 3 come into contact with each other in step 2, forming a favorable metallurgical bonded interface through a recrystallization mechanism. In addition, burrs are expelled from the interface B. As a result, impurities such as oxides present at the interface B are expelled along with the burrs. As a result, a bonded portion 4 that firmly bonds the first member 2 and the second member 3 is formed in step 2, and the bonded body 1 is produced.

[0028] The second step starts when a predetermined condition is satisfied after the start of the first step. That is, when a predetermined condition is satisfied after the start of the first step, the first step ends and the second step starts. The predetermined condition may be the time elapsed since the start of the first step, the temperature of the interface B, or the amount of dimensional change (amount of allowance consumption) of the first member 2. If deformation of the first member 2 occurs in the first step, the second step may be started when the amount of dimensional change of the first member 2 exceeds a predetermined threshold.

[0029] In the second step, the temperature of the interface B may be maintained at temperature T1 or may be higher than temperature T1. For example, in the second step, linear friction between the first member 2 and the second member 3 is also performed, so that the temperature of the interface B may be maintained at temperature T1 or may be higher than temperature T1.

[0030] The second pressure P2 is applied to the first member 2 and the second member 3 to deform them. From the viewpoint of deforming the second member 3 in addition to the first member 2 in the second step, the second pressure P2 is 80% or more or 85% or more of the yield strength of the second member 3 at temperature T1. From the viewpoint of reliably deforming the second member 3 in the joining step, the second pressure P2 may be equal to or greater than the yield strength of the second member 3 at temperature T1. From the viewpoint of suppressing deformation of unintended locations of the first member 2 in the joining step, the second pressure P2 is 80% or more and 110% or less of the yield strength of the first member 2 at room temperature. From the viewpoint of preventing deformation of unintended locations of the first member 2 in the joining step, the second pressure P2 may be lower than the yield strength of the first member 2 at room temperature. Alternatively, when the second pressure P2 is equal to or greater than the yield strength of the first member 2 at room temperature, the entire periphery of the first member 2 may be fixed by a jig or the like. This makes it possible to prevent unintended deformation of the first member 2 in the second step.

[0031] The effects of a dissimilar-material joined body manufactured by the dissimilar-material joining method according to the present embodiment described above will be described in comparison with conventional solid-state welding. For example, when two workpieces made of different materials do not have an intersection point on the temperature-yield strength curve, attempting to join the two workpieces using conventional solid-state welding can result in a problem of insufficient joint strength. From the perspective of achieving sufficient joint strength, solid-state welding can be performed at a temperature and pressure that minimizes the difference in yield strength between the two workpieces. However, when such solid-state welding is performed, deformation or other problems can occur at unintended locations in one of the workpieces (the workpiece with the lower yield strength). Or, a weld interface may not even be formed. On the other hand, when solid-state welding is performed at a temperature and pressure that takes into account the workpiece with the lower yield strength, the workpiece with the higher yield strength does not deform. As a result, oxides and other impurities remain in the welded joint, preventing the production of a dissimilar-material joined body with good joint efficiency. Therefore, it has been conventionally believed that solid-state joining of joined materials, which do not have an intersection point on the temperature-yield strength curve, cannot produce a dissimilar material joint having good joint efficiency and shape.

[0032] In contrast, in the dissimilar material joining method according to this embodiment, the first step is performed in the joining process, in which a first pressure P1 is applied to the first member 2 and the second member 3 along a direction D1, and the interface B is heated to a temperature T1. The first pressure P1 corresponds to the yield strength of the first member 2 at temperature T1 and is lower than the yield strength of the second member 3 at temperature T1. Therefore, only the first member 2 is deformable in the first step. Subsequently, after the first step, the second step is performed in which a second pressure P2 is applied to the first member 2 and the second member 3 along a direction D1, thereby deforming the first member 2 and the second member 3. In this way, by performing the joining process taking into account the yield strengths of both the first member 2 and the second member 3, a joined body 1 with good joint efficiency can be obtained even in solid-state joining of joined materials where no intersection point exists in the temperature-yield strength curve. In addition, since the second pressure P2 is 80% or more and 110% or less of the yield strength of the first member 2 at room temperature, deformation of unintended locations of the first member 2 can be suppressed during the joining process.

[0033] In one example, the first and second steps may involve linear friction between the first member 2 and the second member 3. In this case, the joining step can be performed without requiring complicated equipment, etc. In addition, the temperature of the interface B can be easily adjusted in the first and second steps.

[0034] In one example, the second pressure P2 may be equal to or greater than the yield strength of the second member 3 at the temperature T1. In this case, the portion of the second member 3 near the interface B can be reliably deformed in the second step.

[0035] In one example, the second pressure P2 may be lower than the yield strength at room temperature of the first member 2. In this case, deformation of unintended locations of the first member 2 can be prevented in the second step.

[0036] In one example, the first step may deform the first member 2. Alternatively, in the first step and the second step, a softened region of the first member 2 is formed in the vicinity of the interface B, and at least a portion of the softened region is extruded from the interface B, and in the second step, a softened region of the second member 3 is formed in the vicinity of the interface B, and at least a portion of the softened region is extruded from the interface B. In these cases, the first step is not simply a heating step at the interface B, but also includes a deforming step of the first member 2.

[0037] In one example, the first metal contained in the first member 2 may be aluminum, and the second metal contained in the second member 3 may be iron. Alternatively, the first member 2 may be an aluminum alloy material, and the second member 3 may be a steel material. In these cases, a joined body 1 useful for structures such as automobiles can be obtained.

[0038] The dissimilar material joining method and dissimilar material joined body according to the present disclosure are as described in the following [1] to

[10] , and have been described in detail based on the above embodiments. [1] A method for manufacturing a welding machine, comprising: a preparation step of butting together a first material to be welded, the first material including a first metal, and a second material to be welded, the second material including a second metal different from the first metal; and a joining step of solid-state joining the first material to be welded and the second material to be welded, the joining step comprising: a first step of applying a first pressure to the first material to be welded and the second material to be welded in a butt direction of the first material to be welded and the second material to be welded, and heating an interface between the first material to be welded and the second material to be welded to a predetermined temperature; and a second step of applying a second pressure to the first material to be welded and the second material to be welded, the second pressure being higher than the first pressure in the butt direction of the first material to be welded, to deform the first material to be welded and the second material to be welded, the first pressure being equal to or greater than the yield strength of the first material to be welded at the predetermined temperature, and lower than the yield strength of the second material to be welded at the predetermined temperature. A method for joining dissimilar materials, wherein the second pressure is 80% or more and 110% or less of the yield strength of the first material to be joined at room temperature and is equal to or greater than the yield strength of the second material to be joined at the predetermined temperature, and there is no intersection between the temperature-yield strength curve of the first material to be joined and the temperature-yield strength curve of the second material to be joined, or the temperature at the intersection is lower than the predetermined temperature. [2] The method for joining dissimilar materials according to [1], wherein linear friction is performed on the first material to be joined and the second material to be joined in the first step and the second step. [3] The method for joining dissimilar materials according to [1] or [2], wherein the second pressure is lower than the yield strength of the first material to be joined at room temperature. [4] The method for joining dissimilar materials according to any of [1] to [3], wherein the intersection is 150°C or less. [5] The method for joining dissimilar materials according to any of [1] to [4], wherein the first material to be joined is deformed in the first step.[6] The method for joining dissimilar materials according to any one of [1] to [5], wherein in the first step and the second step, a softened region of the first workpiece material is formed in the vicinity of the interface, and at least a part of the softened region is extruded from the interface, in at least one of the first step and the second step, a softened region of the second workpiece material is formed in the vicinity of the interface, and in the second step, at least a part of the softened region is extruded from the interface. [7] The method for joining dissimilar materials according to any one of [1] to [6], wherein the first metal is aluminum and the second metal is iron. [8] The method for joining dissimilar materials according to [7], wherein the first workpiece material is an aluminum alloy material and the second workpiece material is steel. [9] A dissimilar material joint comprising: a first material to be joined and a second material to be joined that overlap each other in a predetermined direction; and a joint that integrates the first material to be joined and the second material to be joined, wherein the joint has a joint region located between the first material to be joined and the second material to be joined in the predetermined direction, and a flash portion located outside the joint region as viewed from the predetermined direction, and the flash portion has a first portion having a first metal contained in the first material to be joined and a second portion having a second metal contained in the second material to be joined, wherein there is no intersection between a temperature-yield strength curve of the first material to be joined and a temperature-yield strength curve of the second material to be joined, or the temperature of the intersection is lower than 150°C, and when a first maximum tensile strength of the first material to be joined is lower than a second maximum tensile strength of the second material to be joined in the predetermined direction, the maximum tensile strength of the joint is 90% or more of the first maximum tensile strength.

[10] The dissimilar metal welded body according to [9], wherein the first material to be welded is an aluminum alloy material, and the second material to be welded is a steel material.

[0039] However, one aspect of the present disclosure is not limited to the above embodiment and the above [1] to

[10] . One aspect of the present disclosure can be further modified within the scope of the gist thereof.

[0040] FIG. 5 is a diagram showing the temperature dependence of the material strength of the first member and the second member according to the modified example. In FIG. 5, graph 21 shows the temperature-yield strength curve of the first member according to the modified example, graph 22 shows the temperature-yield strength curve of the second member according to the modified example, plot 23 shows the intersection of graphs 21 and 22, plot 24 is located on graph 21, and plot 25 is located on graph 22. In this modified example, the temperature T2 of plot 23 is 150°C or less, and the temperatures T3 of plots 24 and 25 are higher than temperature T2, for example, 200°C or higher. The pressure P4 of plot 24 is lower than the pressure P3 of plot 23. The pressure P5 of plot 25 is higher than the pressure P4 of plot 24 and is lower than the yield strength of the first member at room temperature, for example.

[0041] When the first and second members according to this modification are solid-state welded at the temperature of plot 23, the temperature is too low to form a metallurgical bonded interface due to the recrystallization mechanism in the dissimilar metal joined body. In contrast, in this modification, as in the above embodiment, pressure P4 is applied to the first and second members to heat the interface between the first and second members to temperature T3, and then pressure P5 is applied to the first and second members. That is, the first and second members according to this modification are solid-state welded under conditions that shift the temperature from plot 24 to plot 25. In this case, a metallurgical bonded interface due to the recrystallization mechanism can be formed between the first and second members, resulting in a dissimilar metal joined body with good joint efficiency. In addition, because pressure P5 in plot 25 is lower than the yield strength of the first member at room temperature, unintended deformation of the first member can be prevented during the solid-state welding.

[0042] The present disclosure will be explained in more detail by the following experimental examples, but the present disclosure is not limited to these examples.

[0043] (Experimental Example 1) A rod-shaped member (A7075) made of aluminum alloy (model number: A7075-T6) was prepared as the first material to be joined, and a rod-shaped member (S45C) made of medium carbon steel (model number: S45C) was prepared as the second material to be joined. The first material to be joined had a length of 71 mm (extension length: 10 mm), a width of 20 mm, and a thickness of 5 mm. The second material to be joined had a length of 65 mm (extension length: 4 mm), a width of 20 mm, and a thickness of 5 mm. Figure 6 shows the temperature dependence of the material strength of the first and second materials to be joined in Experimental Example 1. In Figure 6, graph 31 is the temperature-yield strength curve for the first material to be joined, and graph 32 is the temperature-yield strength curve for the second material to be joined. There is no intersection between graphs 31 and 32. The tensile shear strength of the first material to be joined at room temperature was 574 MPa, and the tensile shear strength of the second material to be joined at room temperature was 742 MPa. Each tensile shear strength was measured using an autograph testing machine (Shimadzu Corporation, SHIMADZU Autograph AG-10TB) under conditions of a gripping distance of 80 mm and a crosshead speed of 1 mm / min.

[0044] Next, the first and second workpieces were placed in a linear friction welding test device (LFD15, manufactured by ACB), and the first and second workpieces were butted together. A thermocouple was welded to the second workpiece at a position 0.5 mm away from the interface between the first and second workpieces.

[0045] Next, the first and second workpieces were linearly friction welded together under the conditions of a frequency of 50 Hz, an amplitude of 2 mm, a contact distance of 7 mm, and a pressure of 50 MPa. A dissimilar-material welded body of the first and second workpieces was formed as shown in FIG. 7A. The pressure was applied to the first and second workpieces along the direction D1 shown in FIG. 7A. The first and second workpieces were also moved relative to each other along the direction D2 shown in FIG. 7A. In FIG. 7A, burrs (Al burrs) formed from a portion of the first workpiece were observed around the weld interface between the first and second workpieces, but no burrs formed from a portion of the second workpiece were observed.

[0046] (Experimental Example 2) Linear friction welding was carried out under the same conditions as in Experimental Example 1, except that the pressure was set to 100 MPa. In this way, a dissimilar material welded body was produced.

[0047] (Experimental Example 3) Linear friction welding was carried out under the same conditions as in Experimental Example 1, except that the pressure was set to 200 MPa. In this way, a dissimilar material welded body was produced.

[0048] (Experimental Example 4) Linear friction welding was carried out under the same conditions as in Experimental Example 1, except that the pressure was set to 300 MPa. In this way, a dissimilar material welded body was produced.

[0049] (Experimental Example 5) Linear friction welding was performed under the same conditions as in Experimental Example 1, except that the pressure was set to 400 MPa. In Experimental Example 5, a welded interface between the first and second workpieces was not formed, and therefore a dissimilar metal welded body was not produced.

[0050] (Experimental Example 6) Linear friction welding was performed under the same conditions as in Experimental Example 1, except that the pressure was set to 500 MPa. In Experimental Example 6, as shown in (b) of Fig. 7, a welded interface between the first and second workpieces was not formed. Therefore, a dissimilar material welded body was not produced.

[0051] FIG. 8 shows the measurement results of the maximum tensile strength of the joints of Experimental Examples 1 to 6. In FIG. 8, the vertical axis represents the maximum tensile strength of the joint, and the horizontal axis represents the pressure (applied pressure) applied to the first and second workpieces. Here, the maximum tensile strength of the joint corresponds to the tensile shear strength of the dissimilar material joint, measured using a method similar to that used to measure the tensile shear strength of the first and second workpieces. Therefore, in Experimental Examples 5 and 6, dissimilar material joints were not produced, and therefore the maximum tensile strength of the joint was not obtained. As shown in FIG. 8, the maximum tensile strength of Experimental Example 2 was the highest, approximately 250 MPa. The maximum tensile strength of the joint of Experimental Example 1 was similar to that of the joint of Experimental Example 2. The maximum tensile strength of the joints of Experimental Examples 3 and 4 was significantly lower than that of the joints of Experimental Examples 1 and 2. The maximum joint efficiency in Experimental Examples 1 to 6 was approximately 43%. The joint efficiency is a value obtained by dividing the maximum tensile strength of the joint by the tensile shear strength of the first material to be joined.

[0052] (Experimental Example 7) As in Experimental Example 1, the first and second workpieces were butted together. Next, linear friction was performed between the first and second workpieces under the following conditions: frequency: 50 Hz, amplitude: 2 mm, and pressure (first pressure): 50 MPa. When the contact allowance of the first workpiece reached 1 mm, the pressure (second pressure) applied between the first and second workpieces along direction D1 was changed to 300 MPa. A dissimilar-material bonded body of the first and second workpieces was formed, as shown in FIG. 9A. The temperature measured by a thermocouple during the change from the first pressure to the second pressure was approximately 400°C. This suggests that the maximum temperature at the bonded interface in dissimilar-material bonding between the first and second workpieces is also approximately 400°C.

[0053] (Experimental Example 8) Under the same conditions as in Experimental Example 1, linear friction was performed between the first and second workpieces. When the contact allowance of the first workpiece reached 1 mm, the pressure (second pressure) applied to the first and second workpieces along direction D1 was changed to 400 MPa. A dissimilar-material joint was then formed between the first and second workpieces, as shown in FIG. 9B. As shown in FIG. 9B, deformation was observed in a portion of the second workpiece.

[0054] Experimental Example 9: Under the same conditions as Experimental Example 1, linear friction was performed between the first and second workpieces. When the contact allowance of the first workpiece reached 1 mm, the pressure (second pressure) applied to the first and second workpieces along direction D1 was changed to 500 MPa. A dissimilar-material joint was then formed between the first and second workpieces, as shown in FIG. 9C. As shown in FIG. 9C, a burr (Fe burr) was observed to have formed from a portion of the second workpiece.

[0055] Experimental Example 10: Under the same conditions as Experimental Example 1, linear friction was performed between the first and second workpieces. When the contact allowance of the first workpiece reached 1 mm, the pressure (second pressure) applied to the first and second workpieces along direction D1 was changed to 600 MPa. A dissimilar-material joint was then formed between the first and second workpieces, as shown in FIG. 9(d). As shown in FIG. 9(d), a burr (Fe burr) was observed to have formed from a portion of the second workpiece.

[0056] Fig. 10(a) is a cross-sectional photograph showing the center of a joint formed in a joined body of dissimilar metals of Experimental Example 8. Fig. 10(b) is a cross-sectional photograph showing an end of a joint formed in a joined body of dissimilar metals of Experimental Example 8. As shown in Fig. 10(a) and (b) , in the joint formed in the joined body of dissimilar metals of Experimental Example 8, no defects such as an unbonded region were observed in the center, but an unbonded region of about 500 µm was observed at the end. It is presumed that this unbonded region was formed because the interface of the second joined material was not expanded sufficiently, and therefore oxides and the like were not sufficiently expelled from the joined interface.

[0057] FIG. 10(c) is a cross-sectional photograph showing the center of the joint formed in the joined body of dissimilar materials of Experimental Example 10. FIG. 10(d) is a cross-sectional photograph showing the end of the joint formed in the joined body of dissimilar materials of Experimental Example 10. As shown in FIGS. 10(c) and 10(d), no defects such as unbonded regions were observed in either the center or end of the joint formed in the joined body of dissimilar materials of Experimental Example 10. Therefore, it is presumed that the interface of the second joined material was sufficiently enlarged in Experimental Example 10. It is presumed that this interface enlargement was achieved because the second pressure exceeded the material strength of the second joined material at the maximum joining temperature, resulting in good deformation of the second joined material. Although not shown, it is presumed that the interface of the second joined material was also sufficiently enlarged in Experimental Example 9.

[0058] FIG. 11(a) shows a microstructure photograph of the center of the second workpiece near the weld interface of the joint formed in the joined body of dissimilar metals of Experimental Example 10. FIG. 11(b) shows a microstructure photograph of the end of the second workpiece near the weld interface of the joint formed in the joined body of dissimilar metals of Experimental Example 10. As shown in FIGS. 11(a) and 11(b), in Experimental Example 10, the pearlite structure was segmented and spheroidized cementite was observed in both the center and end of the joint. In addition, refinement of crystal grains was also observed near the weld interface. As described above, the maximum joining temperature in dissimilar metal joining between the first workpiece and the second workpiece is approximately 400°C, which does not reach the transformation temperature of the steel material. Therefore, it is believed that no change in the structure occurred, and only segmentation of the pearlite structure was observed.

[0059] Figure 12 shows the measurement results of the maximum tensile strength of the joints of Experimental Examples 7 to 10. In Figure 12, the vertical axis represents the maximum tensile strength of the joint, and the horizontal axis represents the second pressure applied to the first and second workpieces. As shown in Figure 12, the maximum tensile strength of the joint of Experimental Example 10 was the highest, at 535 MPa. The maximum tensile strength of the joint of Experimental Example 9 was 531 MPa, similar to that of Experimental Example 10. The maximum tensile strengths of the joints of Experimental Examples 7 and 8 were 267 MPa and 291 MPa, respectively, which were significantly lower than the maximum tensile strengths of the joints of Experimental Examples 9 and 10, but higher than the maximum tensile strengths of the joints of Experimental Examples 1 to 6. As mentioned above, these differences are presumed to be due to whether or not the second workpiece was deformed satisfactorily. The maximum joint efficiency of Experimental Examples 7 to 10 was approximately 93%, significantly higher than the maximum joint efficiency of Experimental Examples 1 to 6.

[0060] (Hardness Evaluation) The mechanical properties of the joints of Experimental Examples 8 to 10 were evaluated as follows. FIG. 13 is a graph showing the distribution of Vickers hardness for Experimental Examples 8 to 10. In FIG. 13, the horizontal axis represents the distance from the joint center of the joint along direction D1, and the vertical axis represents Vickers hardness. In FIG. 13, the left side of "0" on the horizontal axis represents the Vickers hardness of the first joined material, and the right side of "0" on the horizontal axis represents the Vickers hardness of the second joined material. As shown in FIG. 13, the hardness of the second joined material at the joint center in Experimental Examples 9 and 10 is higher than the hardness of the second joined material at the joint center in Experimental Example 8. This is thought to be due to an increase in hardness during the joining process. However, as described above, since no structural changes occurred in the second joined material, no significant increase in hardness was observed in any of Experimental Examples 8 to 10. On the other hand, no increase in hardness was observed in the first joined material in any of Experimental Examples 8 to 10. This is presumably because burrs were formed from a part of the first workpiece before the second pressure was applied.

[0061] DESCRIPTION OF SYMBOLS 1... Joined body, 2... First member (first member to be joined), 3... Second member (second member to be joined), 4... Joining part, 5... Joining area, 6... Burr part, 6a... First part, 6b... Second part, B... Interface, P1... First pressure, P2... Second pressure, T1... Temperature (predetermined temperature).

Claims

1. A welding method comprising: a preparation step of butting together a first material to be welded, the first material including a first metal, and a second material to be welded, the second material including a second metal different from the first metal; and a joining step of solid-state welding the first material to the second material to be welded, the joining step comprising: a first step of applying a first pressure to the first material to be welded and the second material to be welded in a direction in which the first material to be welded and the second material to be welded together, and heating the interface between the first material to be welded and the second material to a predetermined temperature; and a second step of applying a second pressure to the first material to be welded and the second material to be welded in a direction in which the first material to be welded and the second material to be welded, the second pressure being higher than the first pressure and being applied in the direction in which the first material to be welded is welded together, the second pressure being equivalent to the yield strength of the first material to be welded at the predetermined temperature and lower than the yield strength of the second material to be welded at the predetermined temperature. a temperature-yield strength curve of the first material to be joined and a temperature-yield strength curve of the second material to be joined, the second pressure being 80% or more and 110% or less of the yield strength of the first material to be joined at room temperature and equal to or greater than the yield strength of the second material to be joined at the predetermined temperature; and there is no intersection between the temperature-yield strength curve of the first material to be joined and the temperature-yield strength curve of the second material to be joined, or the temperature of the intersection is lower than the predetermined temperature.

2. The method for joining dissimilar materials according to claim 1, wherein in the first step and the second step, the first workpiece and the second workpiece are subjected to linear friction.

3. The method for joining dissimilar materials according to claim 1 or 2, wherein the second pressure is lower than the yield strength of the first material to be joined at room temperature.

4. The method for joining dissimilar materials according to claim 1 or 2, wherein the intersection is at a temperature of 150°C or less.

5. The method for joining dissimilar materials according to claim 1 or 2, wherein the first step involves deforming the first workpiece.

6. A method for joining dissimilar materials as set forth in claim 1 or 2, wherein in the first step and the second step, a softened region of the first material to be joined is formed in the vicinity of the interface, and at least a part of the softened region is extruded from the interface; in at least one of the first step and the second step, a softened region of the second material to be joined is formed in the vicinity of the interface; and in the second step, at least a part of the softened region is extruded from the interface.

7. The method for joining dissimilar materials according to claim 1 or 2, wherein the first metal is aluminum and the second metal is iron.

8. The method for joining dissimilar materials according to claim 7, wherein the first material to be joined is an aluminum alloy material, and the second material to be joined is a steel material.

9. A dissimilar material joint comprising: a first material to be joined and a second material to be joined that overlap each other in a predetermined direction; and a joint portion that integrates the first material to be joined and the second material to be joined, wherein the joint portion has a joint area located between the first material to be joined and the second material to be joined in the predetermined direction, and a burr portion located outside the joint area when viewed from the predetermined direction, wherein the burr portion has a first portion having a first metal contained in the first material to be joined and a second portion having a second metal contained in the second material to be joined, wherein there is no intersection between the temperature-yield strength curve of the first material to be joined and the temperature-yield strength curve of the second material to be joined, or the temperature of the intersection is lower than 150°C, and when the first maximum tensile strength of the first material to be joined is lower than the second maximum tensile strength of the second material to be joined in the predetermined direction, the maximum tensile strength of the joint portion is 90% or more of the first maximum tensile strength.

10. A dissimilar material joint according to claim 9, wherein the first material to be joined is an aluminum alloy material, and the second material to be joined is a steel material.

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