Joining structure of dissimilar metal plates

Abstract

The present invention provides a novel joint structure for joining a titanium plate and a metal plate other than titanium, which can be manufactured easily with fewer structural design constraints and has excellent joint strength. [Solution] The joint structure (1) of the present invention between a titanium plate (2) and a metal plate (3) other than titanium has a point joint portion (4) between the titanium plate (2) and the metal plate (3), At the joint interface between the titanium plate (2) and the metal plate (3) of the point joint (4), one of the metal materials of the titanium plate (2) and the metal plate (3) protrudes from the joint end (4E) into the other metal material. The above-mentioned bonding interface is characterized in that the proportion of intermetallic compound layers with a thickness of 1 μm or less is 50% or more.

Description

TECHNICAL FIELD

[0001] The present invention relates to a joined structure of dissimilar metal plates, specifically to a joined structure of a titanium plate and a metal plate other than titanium.

BACKGROUND ART

[0002] Titanium is excellent in specific strength, corrosion resistance, lightweight properties, workability and the like, and thus is used in a wide range of fields. For example, titanium is used as a coating material for coating the surfaces of fuel tanks for automobiles, motorcycles and the like, muffler components, and building materials such as roof tiles. By coating the surfaces of steel vehicle components and building materials with titanium in this manner, vehicle components and building materials excellent in specific strength, corrosion resistance, lightweight properties and the like can be produced.

[0003] On the other hand, when titanium and an iron-based material (for example, steel, SUS, etc.) are welded by resistance spot welding or the like, an intermetallic compound of titanium and iron may be formed at the joining interface of the welded portion. Since intermetallic compounds are extremely brittle, if such an intermetallic compound is formed at the joining interface, the strength of the joined portion decreases, and it may be difficult to obtain practical strength.

[0004] Therefore, at present, a method of joining titanium and an iron-based material by mechanical means or the like is adopted. In this method, it is necessary to perform predetermined processing on the titanium or the iron-based material before joining, there are structural design restrictions suitable for mechanical joining, and the joining operation of mechanical joining itself is complicated, so a simpler method is required.

[0005] In addition to such mechanical joining, various methods for joining titanium and an iron-based material are known. For example, Patent Document 1 discloses a method of overlapping and joining a titanium plate and a steel plate, in which a low-carbon insert steel material is interposed on the steel plate side and a low-carbon stainless steel mesh insert material is interposed on the titanium plate side, and a copper plate is placed between the titanium plate and an electrode to perform resistance welding. In this Patent Document 1, the joining method involves inserting a low-carbon stainless steel mesh and a low-carbon insert steel material, and performing resistance welding under appropriate welding conditions where the welding current is appropriate for the electrode width or diameter, thereby preventing the formation of brittle carbides at the joint.

[0006] Furthermore, Patent Document 2 discloses a joining structure between steel and a titanium plate with a thickness of 2 mm or less, wherein the joining structure is a lap resistance welding joining structure, and the proportion of the joining interface between the titanium plate and steel at the joining portion where the thickness of the intermetallic compound layer is 10 μm or less is 50% or more. The joining structure described in Patent Document 2 is obtained by forming irregularities on one or both surfaces of the steel and titanium plates before joining, overlapping the steel and titanium plates on the surfaces with the irregularities, and melting the interface by overlap resistance welding. The joining structure described in Patent Document 2 is said to have excellent resistance to embrittlement.

[0007] Furthermore, Patent Document 3 discloses a method for joining metal materials, in which a spot load is applied to the joint portion of a first metal material and a second metal material by a pressurizing means while the joint portion of the two metal materials is overlapped, and if the total thickness of the first and second metal materials at the joint portion is T0 mm before joining and T1 mm after joining, the reduction ratio R = T0 / T1 is 1.8 or more, and by applying a spot load to the joint portion with a pressurizing means, plastic flow occurs at the joint interface, and solid-phase diffusion bonding is achieved at a low temperature, the pressurizing means for applying the spot load is punch-shaped with a circumscribed circle diameter of 3 to 15 mm, the bonding temperature for solid-phase diffusion bonding at a low temperature is in the range of 270 to 450°C, and the thickness of the IMC (intermetallic compound) generated at the joint interface of the joint portion is less than 1 μm. The joining method described in Patent Document 3 involves preheating the joint before applying a spot load, and then applying a spot load to the joint using a pressurizing means. This causes plastic flow to occur at the joint interface from the pressurized area toward the surrounding area, resulting in the appearance of a new surface, which is then joined by solid phase diffusion on this new surface. This joining method described in Patent Document 3 is said to have superior strength and high productivity by suppressing the formation of IMC at the joint. [Prior art documents] [Patent Documents]

[0008] [Patent Document 1] Japanese Patent Application Publication No. 7-266058 [Patent Document 2] Patent No. 4216980 [Patent Document 3] Patent No. 7526404 [Overview of the Initiative] [Problems that the invention aims to solve]

[0009] However, the conventional joining methods described above have room for improvement in terms of simplicity and joint strength. For example, the joining method described in Patent Document 1 requires the insertion of two types of insert materials between the titanium plate and the steel plate, and the placement of a copper plate between the titanium plate and the electrode. This complicates the joining process and also imposes structural design constraints. Furthermore, the need for various insert materials and auxiliary materials such as copper plates raises concerns about increased manufacturing costs.

[0010] In the joining structure described in Patent Document 2, it is necessary to create irregularities on one or both surfaces of the steel and titanium plates before joining, which complicates the joining process. Furthermore, because the interface is melted during joining, there are limitations to suppressing intermetallic compounds, and there is a risk that the desired joint strength may not be achieved.

[0011] In the joining method described in Patent Document 3, the thickness ratio before and after joining is limited as the reduction ratio, which imposes structural design constraints on the structure after joining. Furthermore, in the joining method described in Patent Document 3, it is necessary to preheat the joint before applying a spot load, making the joining process complicated.

[0012] In addition to the joining methods described above, other joining methods such as brazing and friction joining are also possible. However, these joining methods also have structural design constraints, and the joining process becomes complicated, potentially resulting in insufficient joint strength.

[0013] This invention has been made in view of the above circumstances, and aims to provide a novel joint structure for joining a titanium plate and a metal plate other than titanium, which can be manufactured easily with few structural design constraints and has excellent joint strength. [Means for solving the problem]

[0014] The present invention includes the following embodiments.

[0015] (Aspect 1) A joint structure between a titanium plate and a metal plate other than titanium, The above-mentioned joint structure has point joints between the titanium plate and the metal plate, At the joint interface between the titanium plate and the metal plate at the point joint, one of the metal materials, either the titanium plate or the metal plate, protrudes from the joint end into the other metal material. A jointed structure characterized in that, at the above-mentioned joint interface, the proportion of intermetallic compound layers with a thickness of 1 μm or less is 50% or more.

[0016] (Aspect 2) The joint structure according to embodiment 1, characterized in that, at a depth of 0.05 mm from the joint interface of the titanium plate, the maximum hardness within the joint region of the point joint is 15% or more higher than the average hardness of the base material of the titanium plate.

[0017] (Aspect 3) At the above-mentioned joint interface, the metal material of the metal plate protrudes into the metal material of the titanium plate so as to surround the center of the joint of the point joint. The joint structure according to embodiment 1 or 2, characterized in that the amount of metal material overhang of the metal plate is 10% or more of the plate thickness of the base material of the titanium plate.

[0018] (Aspect 4) A joint structure according to any one of the above embodiments 1 to 3, characterized in that a resin is interposed between the titanium plate and the metal plate.

Effects of the Invention

[0019] According to the present invention, it is possible to provide a novel joined structure of a titanium plate and a metal plate other than titanium, which has few restrictions in structural design, can be easily manufactured, and has excellent joining strength.

Brief Description of the Drawings

[0020] [Figure 1] Figure 1 is a cross-sectional view schematically showing a joined structure according to an embodiment of the present invention. [Figure 2] Figure 2 is a cross-sectional view schematically showing a state during joining of the joined structure of the present embodiment.

Modes for Carrying Out the Invention

[0021] Hereinafter, preferred embodiments of the joined structure of the present invention will be described in detail with reference to the drawings.

[0022] In order to achieve the above object, the present inventors have conducted intensive studies focusing on the joining method and joining conditions for dissimilar metal plates. As a result, the present inventors have found that when joining a titanium plate and a metal plate other than titanium, by using an electrode having a specific electrode tip shape and performing solid-state joining under specific joining conditions, the formation of intermetallic compounds is suppressed, the structure of the joining interface is controlled, and joining can be achieved with high joining strength. This method does not require additional steps such as processing before joining, and also does not require auxiliary materials such as insert materials. Therefore, even when joining a titanium plate and a metal plate other than titanium, the process can be easily carried out with few restrictions in structural design, and a joined structure having excellent joining strength can be obtained.

[0023] The present invention has been completed based on such findings, and includes the following aspects.

[0024] Hereinafter, preferred embodiments of the joint structure of the present invention will be described in detail with reference to the drawings. Figure 1 is a schematic cross-sectional view showing a joint structure according to one embodiment of the present invention.

[0025] <Joined structure> As shown in Figure 1, the joining structure 1, which is one embodiment of the present invention, is a joining structure between a titanium plate 2 and a metal plate 3 other than titanium. The joining structure 1 of this embodiment has a point joint portion 4 between the titanium plate 2 and the metal plate 3.

[0026] Furthermore, as shown in Figure 1, the joint structure 1 of this embodiment has a characteristic configuration in which, at the joint interface between the titanium plate 2 and the metal plate 3 of the point joint portion 4, the metal material of the metal plate 3 protrudes from the joint end portion 4E so as to enter into the metal material of the titanium plate 2. In Figure 1, the portion of the metal material of the metal plate 3 that protrudes into the metal material of the titanium plate 2 is shown as the protruding portion 4T.

[0027] In Figure 1, the metal material of the metal plate 3 protrudes into the metal material of the titanium plate 2. However, the joint structure 1 of this embodiment is not limited to this form, and the metal material of the titanium plate 2 may also protrude into the metal material of the metal plate 3.

[0028] Furthermore, the joint structure 1 of this embodiment has a characteristic configuration in which the proportion of the intermetallic compound layer with a thickness of 1 μm or less at the joint interface between the titanium plate 2 and the metal plate 3 of the point joint portion 4 is 50% or more.

[0029] (Effects and Benefits) Generally, the intermetallic compound (IMC) layer formed at the joint interface is very brittle, and as its thickness increases, the overall brittleness of the joint increases. This makes the joint more susceptible to failure under external forces. In addition, a thick IMC layer is prone to stress concentration, which can lead to premature failure of the joint. Furthermore, as the IMC layer thickens, non-uniform structures and defects are more likely to occur within it, which can lead to a decrease in joint strength.

[0030] For the reasons stated above, when the IMC layer is thick, specifically when the IMC layer is 2 μm or thicker, cracks are more likely to occur and propagate in the IMC layer, which may significantly reduce the bonding strength.

[0031] On the other hand, as in the joint structure 1 of this embodiment, if the IMC layer is formed but its thickness is thin, such as 1 μm or less, the joint becomes a more uniform structure, resulting in a better stress distribution, thus reducing brittleness and increasing strength. As a result, crack propagation at the joint interface is suppressed, and when fracture occurs, it is more likely to occur within one of the plate materials. In other words, when the IMC layer is thin, the occurrence and propagation of cracks in the IMC layer are less likely to occur, and excellent joint strength can be achieved.

[0032] Furthermore, in the joint structure 1 of this embodiment, when joining the titanium plate 2 and the metal plate 3, electrodes having a specific electrode tip shape as described later are used, and the structure is manufactured by solid-state joining under specific joining conditions. As a result, the joint structure 1 of this embodiment suppresses the formation of intermetallic compounds, and has a structure in which one of the metal materials, either the titanium plate 2 or the metal plate 3, protrudes from the joint end 4E into the other metal material. When one metal material protrudes into the other metal material in this way, when stress is applied to the joint structure and a crack propagates, the crack tends to propagate along the protruding surface of the metal material (the slope of the protruding portion 4T in Figure 1). At this time, because the direction in which the crack propagates and the direction in which the joint interface extends (the horizontal direction in Figure 1) do not coincide, interfacial fracture is suppressed, and when fracture occurs, the fracture occurs within one of the plate materials. Therefore, even though the joint structure 1 of this embodiment is a joint structure between a titanium plate 2 and a metal plate 3 other than titanium, the formation of intermetallic compounds is suppressed and the structure has such that one of the metal materials protrudes into the other metal material, thus making interfacial fracture less likely and enabling excellent joint strength to be achieved.

[0033] Furthermore, the manufacturing method for the joint structure 1 of this embodiment does not require additional processes such as pre-joining processing, nor does it require auxiliary materials such as insert materials. Therefore, even joint structures made of titanium plates and metal plates other than titanium can be manufactured simply with fewer structural design constraints.

[0034] The components of the joint structure 1 of this embodiment will be described in more detail below.

[0035] (Titanium plate) In this embodiment, the titanium plate is not particularly limited as long as it is a plate material mainly composed of titanium, and for example, a plate material of titanium-based metal material such as pure titanium or titanium alloy can be used.

[0036] The thickness of the titanium plate can be any thickness depending on the structure to which the joint structure 1 of this embodiment is applied. For example, the thickness of the titanium plate may be 0.1 mm or more, 0.2 mm or more, 0.3 mm or more, 0.4 mm or more, 0.5 mm or more, 0.7 mm or more, or 1.0 mm or more. Alternatively, the thickness of the titanium plate may be 5.0 mm or less, 4.5 mm or less, 4.0 mm or less, 3.5 mm or less, or 3.0 mm or less.

[0037] The shape of the titanium plate can be determined according to the shape of the structure to which the joint structure 1 of this embodiment is applied. That is, the shape of the titanium plate may be flat, or it may be molded into a predetermined shape.

[0038] (Metal plates other than titanium) In this embodiment, the metal sheet other than titanium is not particularly limited as long as it is a sheet material mainly composed of a metal other than titanium. For example, sheet materials of metal materials such as iron-based metal materials, aluminum-based metal materials, copper-based metal materials, nickel-based metal materials, zirconium-based metal materials, niobium-based metal materials, tantalum-based metal materials, and hafnium-based metal materials can be used.

[0039] Examples of iron-based metal materials include pure iron, steel, alloy steels such as stainless steel (SUS), and carbon steel. Alloy steels and carbon steels can be any material as long as iron is the main component. Specific examples of stainless steel include martensitic stainless steel, ferritic stainless steel, austenitic stainless steel, austenitic-ferritic duplex stainless steel, and precipitation-hardening stainless steel.

[0040] Examples of aluminum-based metal materials include pure aluminum, aluminum alloys such as the 1000 series, 2000 series, 3000 series, 4000 series, 5000 series, 6000 series, 7000 series, and 8000 series, as well as die-cast materials such as ADC1, ADC3, ADC5, ADC6, ADC10, ADC12, and ADC14.

[0041] Examples of copper-based metal materials include pure copper and copper alloys. Examples of nickel-based metal materials include pure nickel, nickel alloys, and Hastelloy®. Examples of zirconium-based metal materials include pure zirconium and zirconium alloys. Examples of niobium-based metal materials include pure niobium and niobium alloys. Examples of tantalum-based metal materials include pure tantalum and tantalum alloys. Examples of hafnium-based metal materials include pure hafnium and hafnium alloys.

[0042] If a sheet of iron-based metal material other than titanium is used, the surface may be plated with zinc or other plating treatments.

[0043] The thickness of the metal plate other than titanium can be any thickness depending on the structure to which the joint structure 1 of this embodiment is applied. For example, the thickness of the metal plate other than titanium may be 0.1 mm or more, 0.2 mm or more, 0.3 mm or more, 0.4 mm or more, 0.5 mm or more, 0.7 mm or more, 1.0 mm or more, 1.5 mm or more, 2.0 mm or 2.5 mm or more. Alternatively, the thickness of the metal plate other than titanium may be 5.0 mm or less, 4.5 mm or less, 4.0 mm or less, 3.5 mm or less or 3.0 mm or less.

[0044] The titanium plate and the metal plate may have the same thickness or different thicknesses. If the titanium plate and the metal plate have different thicknesses, the ratio of their thicknesses is not particularly limited; for example, a ratio of 3 times or less is acceptable.

[0045] The shape of the metal plates other than titanium can be determined according to the shape of the structure to which the joint structure of this embodiment is applied. That is, the shape of the metal plates may be flat, or they may be molded into a predetermined shape.

[0046] In this embodiment, the joint structure 1 is formed by joining the titanium plate 2 and the non-titanium metal plate 3 by point joints 4, which are formed by a joining method described later.

[0047] (Thickness of the intermetallic compound layer at the bonding interface) The joint structure 1 of this embodiment has a distinctive configuration in which the proportion of intermetallic compound (IMC) layers with a thickness of 1 μm or less at the joint interface between the titanium plate 2 and the metal plate 3 of the point joint portion 4 is 50% or more. As a result, the joint structure 1 of this embodiment is less prone to crack formation and propagation in the IMC layer, and can exhibit excellent joint strength.

[0048] Note that "IMC layer thickness of 1 μm or less" includes cases where the IMC layer thickness is 0 μm, i.e., cases where no IMC layer is formed at the bonding interface of the point joint.

[0049] The ratio at which the thickness of the IMC layer is 1 μm or less at the joining interface of the joined structure can be measured by the following measurement method.

[0050] <Method for measuring the ratio at which the thickness of the IMC layer is 1 μm or less> First, the joined structure to be measured is cut along the plate thickness direction so as to pass through the joining center of the spot joint portion, to expose the cross-section of the joined structure. This cross-section is polished and etched, and then this cross-section is observed as an observation surface using a scanning electron microscope (SEM). At this time, the portion of the metal plate other than titanium (for example, steel) is observed as an etched clear crystal structure. On the other hand, the portion of the titanium plate and the IMC layer do not form an etched structure. The portion of the titanium plate and the IMC layer are distinguished by the difference in luminance.

[0051] Then, from the enlarged image of the observation surface obtained by the SEM, at the joining interface between the titanium plate and the metal plate in the spot joint portion, the total length of the joining interface (that is, the length of the portion where the titanium plate and the metal plate are in contact) and the length of the IMC layer having a thickness of 1 μm or less (including the interface portion where no IMC layer is formed) are measured. The length of the IMC layer measured in this way is divided by the total length of the joining interface, and further multiplied by 100, whereby the ratio (%) at which the thickness of the IMC layer at the joining interface of the joined structure is 1 μm or less is calculated.

[0052] In addition, when the IMC layer having a thickness of 1 μm or less is not continuously formed along the joining interface (that is, when it is formed intermittently in a separated state), the total length thereof is defined as the length of the IMC layer having a thickness of 1 μm or less.

[0053] In the joined structure 1 of the present embodiment, the ratio at which the thickness of the IMC layer at the joining interface of the spot joint portion 4 is 1 μm or less is preferably 60% or more, more preferably 70% or more, further preferably 80% or more, and particularly preferably 100%. The expression that the ratio at which the thickness of the IMC layer is 1 μm or less is 100% means that the thickness of the IMC layer is 1 μm or less over the entire joining interface of the spot joint portion.

[0054] The joint structure 1 of this embodiment is manufactured by solid-phase bonding using electrodes having a specific electrode tip shape as described below, under specific bonding conditions. Here, Figure 2 is a schematic cross-sectional view showing the state of the joint structure of this embodiment during bonding.

[0055] The joint structure 1 of this embodiment is manufactured as follows. First, as shown in Figure 2, a titanium plate 2 and a non-titanium metal plate 3 are stacked to form a plate assembly. Next, the area to be joined in the plate assembly is clamped between a pair of electrodes, consisting of a first electrode 51 and a second electrode 52, which face each other, and current is applied. The resistive heat generated during this current application heats the area to be joined to a degree that does not melt the metal materials, thereby softening the interface between the titanium plate 2 and the metal plate 3.

[0056] Then, in parallel with or after heating the area to be joined, the area to be joined is locally pressurized with a pair of electrodes, applying a large load. As a result, the interface expands at the joining interface, destroying and refining impurity layers such as oxide films, while the metal materials are brought into contact with each other, causing diffusion and thus joining.

[0057] The pair of electrodes used in the above manufacturing method, namely the first electrode 51 and the second electrode 52, have a specific pointed shape, as shown in Figure 2, such that the tip is most convex, causing stress to be concentrated at the center of the joint. By giving the electrode tip such a shape, the current path can be concentrated at the joint interface, making it easier to cause resistive heating at the joint interface, while also applying a large load concentrated at the joint center 4C. By applying a large load concentrated at the joint center 4C in this way, impurities at the joint interface can be broken down and made finer.

[0058] Furthermore, the pair of electrodes are not particularly limited in terms of material or structure other than the electrode tip shape, as long as they have the specific electrode tip shape described above. For example, the pair of electrodes may be copper electrodes such as tungsten copper, chromium copper, or zirconium copper, or tungsten electrodes or electrodes combining tungsten and copper electrodes may be used. By varying the tip shapes of the upper and lower electrodes in relation to the plate thickness of each material, more optimal pressure can be applied, enabling good bonding. In other words, the tip shapes of the upper and lower electrodes may be different from each other.

[0059] Furthermore, in the above manufacturing method, when applying current to the joint of the plate assembly by clamping it with a pair of electrodes, it is necessary to adjust the current application time and current to heat the surface of the titanium plate and the metal plate on the joint interface side to a temperature that does not melt the metal material at the interface. The surface temperature at this time depends on the type and thickness of each metal material in the plate assembly, but for example, it is between 350°C and 950°C.

[0060] Furthermore, in the above manufacturing method, when locally pressurizing the area to be joined with a pair of electrodes, it is necessary to adjust the pressurizing speed and the amount of indentation (maximum load) to apply a load to the joining interface that is suitable for breaking down and refining impurity layers such as oxide films at the joining interface, while simultaneously causing plastic flow of the metal material at the joining interface. The pressure applied when locally pressurizing the area to be joined is a pressure that can generate the aforementioned plastic flow and solid-state join the butt surfaces of the newly formed surfaces that appear due to the plastic flow. This pressure depends on the type and thickness of each metal material in the plate assembly, but is, for example, 110 MPa to 2000 MPa.

[0061] The joint structure obtained by the above manufacturing method is formed as a point joint, where the joint is a spot-like, point-shaped structure, due to localized pressure applied to the area to be joined. At the joint interface between the titanium plate and the metal plate at this point joint, one of the metal materials, either the titanium plate or the metal plate, protrudes from the joint end into the other metal material, and the formation of an IMC layer is suppressed at the joint interface, meaning that the proportion of IMC layers with a thickness of 1 μm or less is 50% or more.

[0062] At the point joint, the plate assembly consisting of a titanium plate and a metal plate is compressed in the thickness direction by a pair of electrodes, forming an indentation. In other words, at the point joint, the thickness of both the titanium plate and the metal plate is reduced.

[0063] The thickness of the titanium plate at the point joint is not particularly limited, but may be, for example, 70% or less, 65% or less, 60% or less, 55% or less, or 50% or less of the thickness of the base material. Alternatively, the thickness of the titanium plate at the point joint may be, for example, 10% or more, 15% or more, 20% or more, 25% or more, or 30% or more of the thickness of the base material.

[0064] The thickness of the metal plate at the point joint is not particularly limited, but may be, for example, 80% or less, 75% or less, 70% or less, or 65% or less of the thickness of the base material. Alternatively, the thickness of the metal plate at the point joint may be, for example, 10% or more, 15% or more, 20% or more, 25% or more, or 30% or more of the thickness of the base material.

[0065] Furthermore, in the joint structure of this embodiment, it is preferable that the maximum hardness within the joint region of the point joint at a depth of 0.05 mm from the joint interface of the titanium plates is 15% or more higher than the average hardness of the base material of the titanium plates.

[0066] Here, the depth position of 0.05 mm from the bonding interface of the titanium plate refers to a position inside the titanium plate that is 0.05 mm away in the thickness direction from the overlapping surface with the metal plate, i.e., the bonding interface. Furthermore, the horizontal direction refers to the virtual axis C that passes through the bonding center 4C of the point bonding portion 4 as shown in Figure 1 and extends in the thickness direction. L This refers to a direction perpendicular to the given direction. Furthermore, the joint region of a point joint refers to the horizontal region between the two joint ends 4E located furthest from the joint center in the joint interface of a point joint, i.e., the part where the titanium plate and the metal plate are in contact, in a cross-section obtained by cutting the joint structure to be measured along the plate thickness direction so as to pass through the joint center of the point joint.

[0067] In a joined structure, if the maximum hardness of the joint interface in the joint region of the titanium plate is 15% or more higher than the average hardness of the base material, when stress is applied to the joined structure, interfacial fracture can be suppressed, and fracture can occur in the softer outer periphery of the joint region or in the base material. In other words, even greater joint strength can be achieved.

[0068] Furthermore, the maximum hardness within the joint area of ​​the point joint at a depth of 0.05 mm from the joint interface of the titanium plate described above may be 20% or more, 25% or more, or 30% or more relative to the average hardness of the base material of the titanium plate.

[0069] In a joined structure, to make the maximum hardness of the joint interface in the joint region of the titanium plate 15% or more higher than the average hardness of the base material, the heating temperature and pressure of the joint interface should be appropriately adjusted in the manufacturing method described above.

[0070] The maximum hardness within the joint area of ​​the point joint and the average hardness of the titanium base material at a depth of 0.05 mm from the joint interface of the titanium plates can be measured as follows.

[0071] <Methods for measuring various hardness levels of titanium plates> First, the joint structure to be measured is cut along the thickness direction, passing through the center of the point joint, to reveal the cross-section of the joint structure. In this cross-section, the Vickers hardness (HV) within the joint area of ​​the point joint at a depth of 0.05 mm from the joint interface of the titanium plate is measured at 0.1 mm intervals, and the maximum value is taken as the maximum hardness (HV) of the above area.

[0072] Furthermore, in the cross-section of the above-mentioned joint structure, the Vickers hardness of the titanium plate base material portion located outside the above-mentioned area, where there are no other joints nearby and no processing has been applied, is measured at the center of the plate thickness for 2 mm (21 points in total), and the average value is taken as the average hardness (HV) of the base material.

[0073] Each of the above Vickers hardness values ​​shall be measured according to the method specified in JIS Z 2244:2009 "Vickers hardness test - Test method".

[0074] Furthermore, as described above, the joint structure of this embodiment has a structure in which, at the joint interface between the titanium plate and the metal plate at the point joint, the metal material of either the titanium plate or the metal plate protrudes so as to penetrate into the other metal material from the joint end. In this embodiment, the amount of protrusion of the metal material of the titanium plate or the metal plate is not particularly limited as long as the amount of protrusion is such that the direction in which the protruding surface (the slope of the protruding portion 4T) extends, i.e., the direction in which the crack propagates, and the direction in which the joint interface extends (i.e., the horizontal direction) are misaligned, thereby suppressing interface fracture. The amount of protrusion may be, for example, 50 μm or more, 60 μm or more, 70 μm or more, 80 μm or more, 90 μm or more, or 100 μm or more, in order to make the direction in which the crack propagates and the direction in which the joint interface extends more likely to be misaligned.

[0075] Furthermore, in order to cause one of the metal materials, either the titanium plate or the metal plate, to protrude 50 μm or more into the other metal material at the bonding interface, the heating temperature and pressure at the bonding interface can be appropriately adjusted in the manufacturing method described above.

[0076] Furthermore, in the joining structure 1 of this embodiment, it is preferable that at the joining interface, the metal material of the metal plate 3 protrudes into the metal material of the titanium plate 2 so as to surround the joining center 4C of the point joining portion 4, and that the amount of protrusion of the metal material of the metal plate 3 is 10% or more of the plate thickness of the base material of the titanium plate 2.

[0077] Here, the structure in which the metal material of metal plate 3 protrudes into the metal material of titanium plate 2 so as to surround the joining center 4C is such that, in a cross-sectional view taken along the thickness direction of the plate passing through the joining center 4C of the point joint 4, as shown in Figure 1, the metal material of metal plate 3 protrudes into the metal material of titanium plate 2 on both sides of the joining center 4C. On the other hand, in a plan view of the point joint 4 seen from above, the metal material of metal plate 3 protrudes into the metal material of titanium plate 2 in an annular shape that surrounds the joining center 4C.

[0078] As described above, at the joining interface, the metal material of the metal plate 3 protrudes into the metal material of the titanium plate 2 so as to surround the joining center 4C, and the protrusion amount of the metal material of the metal plate 3 is 10% or more of the base material thickness of the titanium plate 2. In this case, when stress is applied to the joined structure and cracks propagate, the cracks are more likely to propagate along the protruding surface of the aforementioned metal material, so that interfacial fracture is more reliably suppressed, and even more excellent joining strength can be exhibited.

[0079] Note that, at the joining interface, in order for the metal material of the metal plate to protrude into the metal material of the titanium plate so as to surround the joining center, and for the protrusion amount of the metal material of the metal plate to be 10% or more of the base material thickness of the titanium plate, the heating temperature and pressurizing force at the joining interface may be appropriately adjusted in the aforementioned manufacturing method.

[0080] The protrusion amount of the titanium plate or the metal plate can be measured in the following manner.

[0081] <Method for Measuring Protrusion Amount of Titanium Plate or Metal Plate> In the same manner as the aforementioned <Method for Measuring the Ratio of IMC Layer Thickness of 1 μm or Less>, the joined structure to be measured is cut, the cross section thereof is polished and etched, and then this cross section is observed using an SEM as an observation surface. From the enlarged image of the observation surface obtained by SEM, with a straight line connecting the two joining end portions 4E at the joining interface of the spot joining portion, that is, the two joining end portions 4E located on the farthest side from the joining center in the portion where the titanium plate and the metal plate are in contact with each other, defined as a reference line, the distance (length) between the apex of the protruding portion of the titanium plate or the metal plate and the reference line is measured. Then, the measured value is defined as the protrusion amount (μm) of the titanium plate or the metal plate.

[0082] Furthermore, in the joint structure 1 of this embodiment, a resin may be interposed between the titanium plate 2 and the metal plate 3, as long as it does not hinder the effects of the present invention. When a resin is interposed between the titanium plate 2 and the metal plate 3, the current path during energization can be controlled, the heat generation pattern can be controlled as desired, and a better joint can be achieved. In addition, the resin functions as a sealant at the joint interface, and by isolating it from the external environment during use, corrosion of the point joint can be suppressed. Note that even if the point joint is not completely isolated from the outside by the resin, if it is partially isolated, it will have the effect of improving corrosion resistance.

[0083] In this embodiment, the resin interposed between the titanium plate and the metal plate is not particularly limited as long as it can function as described above, and may be any type of structural adhesive, any type of sealing material, or even a resin plate or sheet.

[0084] Examples of structural adhesives include reinforced epoxy, flexible epoxy, acrylic resin, reinforced acrylic resin, urethane resin, and silicone. In some cases, structural adhesives that can be used as sheets or films may be available. Furthermore, pressure-sensitive structural adhesives such as tapes can also be used.

[0085] Examples of sealants include silicone-based, silylated acrylate-based, modified silicone-based, polysulfide-based, acrylic urethane-based, polyurethane-based, acrylic-based, and butyl rubber-based sealants, as well as oil-based caulking materials and silicone mastic.

[0086] Furthermore, any known resin can be used for the resin plates or sheets. Specific examples of resins for resin plates and sheets include polyester resin, vinyl resin, polyurethane resin, epoxy resin, polyamide resin, polyimide resin, silicon resin, phenolic resin, melamine resin, urea resin, aniline resin, ionomer resin, and polycarbonate resin. The shape of the resin plates or sheets is not particularly limited; any shape that allows for control of the electrical path, such as having irregularities, holes, or a mesh structure, or a shape that can isolate it from the outside world, is acceptable.

[0087] Furthermore, when the resin interposed between the titanium plate and the metal plate is applied by coating, the coating method can be any form as long as it allows for control of the electrical path or provides a shape that can be isolated from the outside world. For example, the resin may be applied uniformly between the materials, applied only to a part near the joint point, applied only directly below the joint point, or applied so that it is not present directly below the joint point but only around it.

[0088] In this embodiment, the joint structure may have another joint, such as one formed by brazing using an arc, laser, or other heating method, placed near or directly above the point joint, as long as it does not hinder the effects of the present invention.

[0089] Furthermore, promoting diffusion at the bonding interface can improve bonding performance. To achieve this, in this embodiment, heat treatment may be applied to the point joint, as long as it does not hinder the effects of the present invention. Any type of heat treatment method may be used. Examples of heat treatment methods include heat treatment performed in a heating furnace (including heat treatment in a separate process such as a painting process), heat treatment utilizing heat from welding / joining near the joint point, and heat treatment using a laser.

[0090] When performing such heat treatment, it is sufficient to control the temperature of the point joint between the titanium and the non-titanium metal plate to be 1000°C or lower, preferably 800°C or lower, so that IMC does not form.

[0091] (Examples of application) As described above, the joint structure of the present invention allows for the simple manufacture of a joint structure between a titanium plate and a metal plate other than titanium, with fewer structural design constraints, and exhibits excellent joint strength. Therefore, the present invention can be suitably used in various structures such as fuel tanks and muffler parts for automobiles and motorcycles, and building materials such as roof tiles, where excellent corrosion resistance and durability are required.

[0092] The joint structures of the present invention are not limited to the embodiments described above, and can be appropriately combined, substituted, or modified without departing from the purpose and spirit of the present invention. In this specification, ordinal numbers such as "1st," "2nd," etc., are used to distinguish the items to which they are assigned, and do not indicate the order, priority, importance, etc. of each item. [Explanation of symbols]

[0093] 1 Jointed structure 2 Titanium plates 3 metal plate 4 point joint 4C Joint center 4E Joint end 4T overhang 51 1st electrode 52 2nd electrode

Claims

1. A joint structure between a titanium plate and a metal plate other than titanium, The aforementioned joint structure has point joints between the titanium plate and the metal plate, At the joint interface between the titanium plate and the metal plate at the point joint, one of the metal materials, either the titanium plate or the metal plate, protrudes from the joint end into the other metal material. A jointed structure characterized in that, at the aforementioned joint interface, the proportion of intermetallic compound layers with a thickness of 1 μm or less is 50% or more.

2. The joint structure according to claim 1, characterized in that, at a depth of 0.05 mm from the joint interface of the titanium plate, the maximum hardness within the joint region of the point joint is 15% or more higher than the average hardness of the base material of the titanium plate.

3. At the aforementioned bonding interface, the metal material of the metal plate protrudes into the metal material of the titanium plate so as to surround the bonding center of the point bonding portion. The joint structure according to claim 1 or 2, characterized in that the amount of metal material overhang of the metal plate is 10% or more of the plate thickness of the base material of the titanium plate.

4. The joining structure according to claim 1 or 2, characterized in that a resin is interposed between the titanium plate and the metal plate.

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