Friction joining method and joining structure for galvanized steel sheets

Abstract

To provide a friction joining method for galvanized sheet iron, which can effectively suppress mixing of zinc in a joint part and provide the joint part covered with a galvanized layer, and a joint structure formed by this joining method.SOLUTION: A friction joining method comprises: a first step of abutting one member 2 against another member 4 to form a joint interface; a second step of repeatedly sliding the members 2 and 4 on the same track and discharging flashes from the joint interface; and a third step of stopping the sliding to form a joint surface 12. At least one of the members 2 and 4 is galvanized sheet iron, and mixing of a zinc component in the joint surface is suppressed through flash discharging.SELECTED DRAWING: Figure 5

Description

[Technical field]

[0001] The present invention relates to a friction joining method for solid-state joining of galvanized steel sheets and a joined structure obtained by the friction joining method. [Background technology]

[0002] Steel materials are used in a variety of industrial products because they have excellent strength, ductility, and low cost. However, they have the disadvantage of being susceptible to corrosion, so in corrosive environments, hot-dip galvanized steel sheets, which have a zinc coating on the surface, are used.

[0003] However, when welding galvanized steel sheets, zinc, which has a lower boiling point than the melting point of steel, turns into zinc vapor, which can make it difficult to achieve a good weld. Specifically, the zinc vapor can cause blowholes in the bead, or the arc formation can become unstable, resulting in increased spattering.

[0004] Furthermore, during fusion welding, the zinc coating on the surface evaporates, changing the surface condition of the zinc-coated steel sheet, which deteriorates the corrosion resistance. In addition, there is also the problem that the mechanical properties and reliability of the joint are reduced due to the inclusion of zinc in the weld.

[0005] In response to this, for example, Patent Document 1 (JP 2016-179483 A) discloses a method for welding a galvanized steel sheet, the method including: a step of preparing a galvanized steel sheet; a step of forming an arc between the galvanized steel sheet and a welding wire while shielding with a shielding gas to heat and melt the galvanized steel sheet and the welding wire to form a molten pool; and a step of solidifying the molten pool, wherein the shielding gas is a mixed gas containing carbon dioxide and argon, with the balance being unavoidable impurities, the welding wire has a diameter of 0.9 mm or less, and the carbon dioxide contained in the shielding gas is 4% by volume or more and 10% by volume or less, or the welding wire has a diameter of more than 0.9 mm and 1.0 mm or less, and the carbon dioxide contained in the shielding gas is 4% by volume or more and 6% by volume or less.

[0006] The method for welding galvanized steel sheets described in Patent Document 1 above claims that it is possible to provide a method for welding galvanized steel sheets that can achieve a good welded state by optimizing the ratio of carbon dioxide contained in the shielding gas, which has a large effect on the welded state of the galvanized steel sheets, and the diameter of the welding wire.

[0007] Furthermore, Patent Document 2 (JP 2015-167981 A) discloses a method for welding galvanized steel sheet by MAG welding, characterized in that a low-viscosity solid wire that reduces the viscosity of the molten pool is used as the welding wire, and a three-type mixed gas consisting of 13 to 18 volume % oxygen gas, 5 to 15 volume % carbon dioxide gas, and the remainder argon gas is used as the shielding gas.

[0008] In the welding method of zinc-plated steel sheet described in the above Patent Document 2, instead of the high-viscosity wire that has been generally used when MAG welding zinc-plated steel sheet, a low-viscosity solid wire that can reduce the viscosity of the molten pool is used, making it easier for zinc vapor generated during welding to escape from the molten pool.It is said that by using a ternary system of argon gas (Ar), carbon dioxide gas (CO2), and oxygen gas (O2) as the optimal shielding gas composition when MAG welding using the low-viscosity solid wire, and mixing these in a predetermined ratio, it is possible to reproducibly obtain the effect of suppressing the occurrence of blowholes and pits in the weld without reducing the mechanical strength of the weld metal. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] JP 2016-179483 A [Patent Document 2] JP 2015-167981 A Summary of the Invention [Problem to be solved by the invention]

[0010] However, in the welding methods of zinc-coated steel sheets disclosed in the above Patent Documents 1 and 2, the zinc-coated layer of the weld is completely vaporized, so that it is not possible to obtain a weld covered with a good zinc-coated layer. In addition, since the vaporized zinc inevitably gets mixed into the weld, it is extremely difficult to suppress the deterioration of the mechanical properties of the joint caused by the mixing.

[0011] In view of the problems in the conventional techniques as described above, an object of the present invention is to provide a method for joining galvanized steel sheets, which effectively suppresses the intrusion of zinc into the joint and enables the production of a joint covered with a zinc plating layer, and a joint structure obtained by the joining method. [Means for solving the problem]

[0012] In order to achieve the above-mentioned object, the present inventors have conducted extensive research into methods for joining galvanized steel sheets, and as a result, have discovered that applying friction joining methods such as linear friction joining and friction welding to the joining of galvanized steel sheets and preventing the intrusion of zinc into the joint by expelling burrs from the joint interface is extremely effective, and have arrived at the present invention.

[0013] That is, the present invention provides: a first step of contacting one member with another member to form a joining interface; a second step of repeatedly sliding the one member and the other member on the same trajectory while applying pressure substantially perpendicular to the joined interface, thereby removing burrs from the joined interface; and a third step of stopping the sliding and forming a joining surface, At least one of the one member and the other member is a zinc-plated steel plate, By removing the burrs, the inclusion of zinc plating components in the joining surface is suppressed. The present invention provides a friction joining method comprising the steps of: In the friction joining method of the present invention, the principles of linear friction joining and friction pressure welding, which expel burrs from the welded interface, can be used, but linear friction joining will be described in detail below as a representative example.

[0014] Figure 1 shows a schematic diagram of the friction welding method (linear friction welding). Linear friction welding is a solid-state welding method in which the main heat source is frictional heat generated when the materials to be welded are rubbed together in a linear motion. The oxide film that formed at the welded interface is removed by expelling the softened material as burrs from the welded interface as a result of heating, and a weld can be obtained by bringing the newly formed surfaces together.

[0015] Here, burrs are first discharged from the joined interface in a direction approximately perpendicular to the sliding direction, then discharged in a direction approximately parallel thereto, and finally discharged from the entire circumference of the joined interface. That is, even if evaporation or melting of the zinc plating occurs during joining, the incorporation of the zinc plating components into the joint can be effectively suppressed. In particular, in the case of linear friction joining of zinc-plated steel sheets, linear sliding in a direction perpendicular to the sheet thickness allows burrs to be quickly discharged from the long sides that occupy most of the joint surface, and extremely effective suppression of incorporation of zinc plating components into the joint.

[0016] In addition, the inventors of the present invention observed linear friction joints of galvanized steel sheets in detail and found that the galvanized layer formed on the surface of the steel sheet deforms and / or moves following the moderately softened burrs, so that the linear friction joint is covered with the galvanized layer up to the base of the burrs. In other words, the use of linear friction welding not only prevents zinc from being mixed into the joint, but also allows the surface of the joint to be sufficiently covered with the galvanized layer.

[0017] The type, size and shape of the galvanized steel sheet to which the friction joining method of the present invention is applied are not particularly limited as long as the effects of the present invention are not impaired, and various conventionally known galvanized steel sheets can be used.

[0018] In the friction joining method of the present invention, it is preferable to set the pressure to be equal to or higher than the yield stress of the galvanized steel sheet at the desired joining temperature, and set the joining temperature to be equal to or lower than the boiling point of zinc. In linear friction joining and friction welding, the joining temperature can be accurately determined by the joining pressure, and by setting the joining temperature to be equal to or lower than the boiling point of zinc, it is possible to suppress changes in the galvanized layer formed on the surface of the steel sheet. Here, in the present invention, "joining temperature" means "the desired maximum temperature reached at the joined interface in the second step".

[0019] In the friction joining method of the present invention, it is preferable that the joining temperature is equal to or lower than the melting point of the zinc plating. By setting the joining temperature equal to or lower than the melting point of the zinc plating, changes in the zinc plating layer formed on the surface of the steel sheet can be more effectively suppressed.

[0020] In the friction joining method of the present invention, it is preferable that the joining temperature is equal to or lower than the A1 point of the galvanized steel sheet. By setting the joining temperature to equal to or lower than the A1 point of the galvanized steel sheet, not only is the joining temperature surely equal to or lower than the boiling point of zinc, but softening and embrittlement of the steel sheet can be suppressed. In steel, brittle martensite is formed by phase transformation, which may make joining difficult or may cause the joint to become embrittled. In contrast, by setting the joining temperature to equal to or lower than the A1 point, no phase transformation occurs, and therefore the formation of brittle martensite can be completely suppressed. In addition, the lowering of the joining temperature can suppress softening in the heat-affected zone.

[0021] Here, when the applied pressure of linear friction welding (friction welding) is increased, the frictional heat increases, but since the softened material is continuously discharged as burrs, the "welding temperature" is determined by the pressure applied to the softened material (the force that discharges the burrs). In other words, when the applied pressure is set high, the welded materials with higher strength (higher yield strength) can be discharged as burrs. Here, "higher yield strength" means "lower temperature", so the "welding temperature" decreases with an increase in applied pressure. Since the relationship between yield strength and temperature is approximately constant depending on the material, the welding temperature can be controlled extremely accurately compared to the case of using frictional heat.

[0022] Furthermore, in the friction joining method of the present invention, it is preferable that the tensile strength of the galvanized steel sheet is 340 MPa or more. The friction joining method of the present invention is a solid-phase joining method, and can obtain a joint having high strength and excellent reliability even with steel sheets having high tensile strength. In addition, since the joining temperature is set to a low value to effectively suppress the inclusion of zinc plating, it is possible to express good joint characteristics in which softening of the heat-affected zone is suppressed even with high-tensile steel sheets.

[0023] The present invention also provides a joint structure having a friction joint in which one member and another member are integrated through a friction joint interface, at least one of the one member and the other member is a zinc-plated steel sheet, and no zinc plating components are mixed into the friction joint. The shape and size of the zinc-plated steel sheet are not particularly limited, and widely include plate materials, square members (rectangular columns), U-shaped members, pipe materials, etc.

[0024] Here, "zinc plating components are not mixed into the linear friction joint" can be confirmed, for example, by performing elemental analysis using SEM-EDS on the cross section of the joint. The method of SEM-EDS measurement is not particularly limited, and may be performed using various conventionally known devices and measurement conditions. More specifically, elemental mapping may be obtained for the cross section of the joint to confirm whether elements originating from the zinc plating layer present on the surface of the joint are mixed into the inside of the joint. The allowable content of zinc plating components in the joint depends on the steel material used as the base material, but it is sufficient as long as it does not affect the strength of the steel material, and for example, the average content in the joint is preferably less than 1.0 mass%, and the maximum value in the joint is also preferably less than 1.0 mass%. The joint efficiency of the friction joint is preferably 90% or more, more preferably 95% or more, and most preferably 100%.

[0025] In the joint structure of the present invention, it is preferable that a burr is formed on the outer edge of the friction joint interface, and the surface of the friction joint is covered with a zinc plating layer up to the root of the burr. By covering the surface of the friction joint with a zinc plating layer up to the root of the burr, a joint with excellent corrosion resistance can be realized. Effect of the Invention

[0026] According to the present invention, it is possible to provide a method for joining zinc-plated steel sheets, which can effectively suppress the intrusion of zinc into the joint and obtain a joint covered with a zinc plating layer, and a joint structure obtained by the joining method. [Brief description of the drawings]

[0027] [Figure 1] FIG. 1 is a schematic diagram showing one embodiment (linear friction joining) of the friction joining method of the present invention. [Diagram 2] FIG. 2 is a schematic diagram showing a joining step of the friction joining method (linear friction joining) of the present invention. [Diagram 3] 1 is a graph showing the flow stress (yield stress) of carbon steel at each temperature. [Figure 4] 1 is a graph showing the tensile strength of various metals at each temperature. [Diagram 5] 1 is a schematic cross-sectional view showing an example of a joint structure of the present invention. [Figure 6] 1 is a graph showing the temperature dependence of strength of a hot-dip galvanized steel sheet used in an example. [Figure 7]1 shows an external view and a cross-sectional view of a joint obtained in an example (200 MPa). [Figure 8] 1 shows an SEM photograph and element mapping of a cross section of a hot-dip galvanized steel sheet used in an example. [Figure 9] 1 shows an SEM photograph and element mapping of a cross section of a joint obtained in an example (200 MPa). [Figure 10] FIG. 2 is a schematic diagram showing the shape and dimensions of a tensile test specimen used in the examples. [Figure 11] FIG. 2 is a stress-strain diagram showing the tensile properties of the joint obtained in the example. [Figure 12] 1 is a cross-sectional photograph of a joint in a steel plate having a thickness of 1.2 mm obtained in an example. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0028] Representative embodiments of the friction welding method and welded structure of the present invention will be described in detail below with reference to the drawings, but the present invention is not limited to these. In the following description, the same or equivalent parts are given the same reference numerals, and duplicated descriptions may be omitted. In addition, since the drawings are intended to conceptually explain the present invention, the dimensions of each component shown and their ratios may differ from the actual ones.

[0029] (1)Friction welding method 2 is a schematic diagram showing the joining steps of the friction joining method of the present invention when linear friction joining is used. The friction joining method of the present invention includes a first step of bringing one member 2 into contact with the other member 4 to form a joining interface 6, a second step of repeatedly sliding the one member 2 and the other member 4 on the same trajectory while applying pressure substantially perpendicular to the joining interface 6, and discharging burrs 8 from the joining interface substantially parallel and perpendicular to the sliding direction, and a third step of stopping the sliding to form a joining surface. Each step will be described in detail below.

[0030] (1-1) First step The first step is a step of bringing one member 2 into contact with the other member 4 to form a bonded interface 6. The one member 2 and / or the other member 4 are moved to a location where it is desired to form a bond, and the surfaces to be bonded are brought into contact with each other to form the bonded interface 6.

[0031] At least one of the first member 2 and the second member 4 is a galvanized steel sheet. The type, size and shape of the galvanized steel sheet are not particularly limited as long as they do not impair the effects of the present invention, and various conventionally known galvanized steel sheets can be used. Examples of galvanized steel sheets include hot-dip galvanized steel sheets (GI), alloyed hot-dip galvanized steel sheets (GA), electrogalvanized steel sheets (EG) and two-layer alloyed hot-dip galvanized steel sheets (GAE). The same method can also be applied to galvanized steel sheets of different compositions, such as highly corrosion-resistant hot-dip zinc, aluminum, magnesium alloy-plated steel sheets (ZAM (registered trademark), Super Dima (registered trademark): highly weather-resistant plated steel sheets), zinc-aluminum alloy-plated steel sheets, zinc-nickel alloy-plated steel sheets, and zinc-magnesium-plated steel sheets. In addition, in each galvanized steel sheet, the coating weight (plating thickness) is not particularly limited as long as it does not impair the effects of the present invention, and can be various conventionally known values.

[0032] The mechanical properties of the galvanized steel sheet used as the joined material are not particularly limited as long as they do not impair the effects of the present invention, but it is preferable that the tensile strength is 340 MPa or more. Even when a steel sheet with high tensile strength is used, a joint having high strength and excellent reliability can be obtained. In addition, since the joining temperature is set to a low value in order to effectively suppress the inclusion of zinc plating, softening of the heat-affected zone is suppressed even in the case of high-tensile steel sheets, and good joint characteristics can be expressed. The tensile strength of the galvanized steel sheet is more preferably 780 MPa or more, and most preferably 980 MPa or more.

[0033] (1-2) Second process The second step is a step in which one component 2 and the other component 4 are repeatedly slid on the same trajectory while applying pressure P approximately perpendicular to the joined interface 6, and burrs 8 are expelled from the joined interface 6 approximately parallel to and approximately perpendicular to the sliding direction.

[0034] The method of repeatedly sliding one member 2 and the other member 4 on the same trajectory is not particularly limited as long as it does not impair the effects of the present invention, and both members may be vibrated together, or one may be fixed and the other may be vibrated.

[0035] Here, the pressure P during linear friction joining can be set to be equal to or higher than the yield stress and equal to or lower than the tensile strength of one member and / or the other member at the desired joining temperature, thereby controlling the joining temperature. In the friction joining method of the present invention, the pressure P can be set to be equal to or higher than the yield stress and equal to or lower than the tensile strength of the hot-dip galvanized steel sheet at the desired joining temperature, thereby determining the joining temperature based on the hot-dip galvanized steel sheet. By setting the pressure P to be equal to or higher than the yield stress of the hot-dip galvanized steel sheet, the discharge of burrs 8 from the joined interface 6 begins, and by increasing the pressure P until the tensile strength is reached, the discharge of burrs 8 is accelerated. As with the yield stress, the tensile strength at a specific temperature is also approximately constant depending on the joined materials, so that the joining temperature corresponding to the set pressure P can be realized. In addition, this allows joining without deforming the base material even when joining thin plates. For example, the plate thickness is preferably 2.0 mm or less.

[0036] As a specific example, the flow stress (yield stress) of carbon steel at each temperature is shown in Figure 3, and the tensile strength of various metals at each temperature is shown in Figure 4. Note that Figure 3 is a graph published in "Tetsu to Hagane, No. 11, 1981, p. 140," and Figure 4 is a graph published in "Tetsu to Hagane, No. 6, 1986, p. 55." As shown in these figures, the tensile strength and yield stress at a specific temperature are approximately constant depending on the material. In other words, by creating a database of such data for the materials to be joined, joining at any temperature can be carried out efficiently and easily.

[0037] When the joining pressure P is set high, the joined materials (hot-dip galvanized steel sheets) with higher yield strength and tensile strength can be discharged as burrs, and the joining temperature can be reduced. In addition, as shown in Figures 3 and 4, the tensile strength and yield stress at a specific temperature are approximately constant depending on the material, so by setting the joining pressure P based on the temperature dependency of the strength of the hot-dip galvanized steel sheets, the joining temperature of the hot-dip galvanized steel sheets can be controlled very accurately.

[0038] In linear friction welding, it is necessary to set welding parameters other than the pressure P (frequency and amplitude for vibrating the workpieces, welding time, and gap, etc.), but these values ​​are not limited as long as they do not impair the effects of the present invention, and may be set appropriately depending on the material, shape, size, etc. of the workpieces. Here, by increasing the amplitude and frequency for sliding the workpieces, the heating rate and the cooling rate after welding increase, but the maximum temperature (welding temperature) remains unchanged.

[0039] The joining temperature is preferably set to the boiling point of zinc (907°C) or lower, and more preferably to the melting point of the zinc plating (or to the melting point of the alloy plating if alloyed). In linear friction joining, the joining temperature can be accurately determined by the joining pressure P, and by setting the joining temperature to the boiling point of zinc or lower, changes to the zinc plating layer formed on the surface of the steel sheet can be suppressed. Furthermore, by setting the joining temperature to the melting point of the zinc plating or lower, changes to the zinc plating layer can be more reliably suppressed.

[0040] In addition, in the friction joining method of the present invention, it is preferable that the joining temperature is equal to or lower than the A1 point of the galvanized steel sheet. By setting the joining temperature to equal to or lower than the A1 point of the galvanized steel sheet, not only is the joining temperature surely equal to or lower than the boiling point of zinc, but softening and embrittlement of the steel sheet can be suppressed. In steel, brittle martensite is formed by phase transformation, which may make joining difficult or may cause the joint to become embrittled. In contrast, by setting the joining temperature to equal to or lower than the A1 point, no phase transformation occurs, and therefore the formation of brittle martensite can be completely suppressed. In addition, the reduction in the joining temperature can suppress softening in the heat-affected zone.

[0041] (1-3) Third step The third step is a step of stopping the sliding in the second step to form a joining surface. In the friction joining method of the present invention, a good joint can be obtained by stopping the sliding after the burrs 8 are discharged from the entire surface of the joining interface 6. In addition, by discharging the burrs 8 from the entire surface of the joining interface 6, it is possible to suppress the incorporation of zinc plating components into the joint. The pressure P applied to the joined materials in the second step may be maintained as it is, or may be increased in order to discharge the burrs 8 and to make the new surface abut more firmly. In the joining process, if the joining area increases, the pressure P decreases, and there are cases where the joining temperature increases unintentionally, but this phenomenon can be suppressed by increasing the pressure P.

[0042] Here, the timing for stopping the sliding is not limited as long as the burrs 8 are discharged from the entire surface of the joined interface 6, but by observing the joined interface 6 from a direction approximately perpendicular to the sliding direction and stopping the sliding at the moment the burrs 8 are discharged approximately parallel to the sliding direction, it is possible to form a good joint while minimizing the amount of burrs 8 discharged (while minimizing consumption of the joined materials). Note that "a direction approximately perpendicular to the sliding direction" and "a direction approximately parallel to the sliding direction" are both directions approximately perpendicular to the applied pressure.

[0043] (2) Joined structure 5 is a schematic cross-sectional view showing an example of a joined structure of the present invention. The joined structure 10 is formed by linear friction joining one member 2 and another member 4, and at least one of the one member 2 and the other member 4 is a hot-dip galvanized steel sheet.

[0044] One member 2 and the other member 4 are metallurgically joined via a linear friction joint 12, and the linear friction joint 12 is characterized in that the components of the zinc plating layer 14 formed on the surface of the hot-dip galvanized steel sheet are not mixed in. "Zinc plating components are not mixed in the linear friction joint" can be confirmed by elemental analysis using SEM-EDS on the cross section of the joint, but since the quantitative value of zinc will be subject to errors due to the influence of peaks caused by iron, for example, it can be judged by obtaining elemental mapping of the entire cross section of the joint and checking whether or not there is a clear location of zinc inside the joint.

[0045] In the welded structure 10, it is preferable that a burr 8 is formed on the outer edge of the linear friction weld interface (welded interface 6), and the surface of the linear friction weld 12 is covered with a zinc plating layer 14 up to the base of the burr 8. By covering the surface of the linear friction weld 12 with a zinc plating layer 14 up to the base of the burr 8, a weld with excellent corrosion resistance can be realized.

[0046] Although the representative embodiments of the present invention have been described above, the present invention is not limited to these, and various design modifications are possible, and all of these design modifications are included in the technical scope of the present invention. For example, when the bonding temperature exceeds the boiling point of zinc, a treatment is performed to attach the zinc vapor to the bonded body surface. EXAMPLES

[0047] The materials to be joined were hot-dip galvanized steel sheets (JIS-SGHC: 0.05%C-0.01%Si-0.15%Mn-0.17%P-0.04%S), and the end faces of the hot-dip galvanized steel sheets were butted together to perform linear friction welding. The size of the hot-dip galvanized steel sheets was 2mm x 50mm x 63mm, and the butted end faces were 2mm x 50mm.

[0048] In addition, in order to determine the joining pressure for linear friction welding, a high-temperature tensile test was used to investigate the temperature dependence of the strength of hot-dip galvanized steel sheets. The tensile strength was measured at 500°C, 600°C, 700°C, and 800°C, and the results are shown in Figure 6. From the results in Figure 6, joining pressures of 50 MPa, 100 MPa, and 200 MPa were set as the joining pressures at which the joining temperature during linear friction welding would be below the boiling point of zinc (907°C). The predicted joining temperatures at each joining pressure are 50 MPa: approximately 800°C, 100 MPa: approximately 700°C, and 200 MPa: approximately 560°C.

[0049] The linear friction joining conditions other than the joining pressure were constant: frequency: 50 Hz, amplitude: 2 mm, and distance: 2.5 mm. When the temperature of the hot-dip galvanized steel sheet surface at a position 1 mm from the joining interface when each joining pressure was used was measured with a K-type thermocouple, the results were 50 MPa: approximately 605°C, 100 MPa: approximately 566°C, and 200 MPa: approximately 330°C. The temperature decreased as the joining pressure increased, which corresponds to the results in Figure 6. Note that the temperature measurement position was 1 mm away from the joining interface, so the measured values ​​were lower than the values ​​predicted from Figure 6.

[0050] In all joints obtained at each joining pressure, burrs were observed to have been expelled from the entire periphery of the welded interface. Furthermore, in cross-sectional observation of all joints, no joint defects such as unjoined areas or cracks were found. As a representative result, Fig. 7 shows the appearance and cross-sectional photographs of a joint obtained at 200 MPa. In the appearance photograph, it can be seen that the surface condition of the hot-dip galvanized steel sheet (state of the zinc plating layer) has hardly changed even in the vicinity of the burrs.

[0051] Next, SEM-EDS analysis of the cross section of all the joints obtained at each joining pressure was performed. In addition, to confirm the initial state of the hot-dip galvanized layer, SEM-EDS analysis of the cross section of the hot-dip galvanized steel sheet before joining was also performed. The SEM used was a JSM-7001FA manufactured by JEOL Ltd.

[0052] Figure 8 shows an SEM photograph and element mapping results of the hot-dip galvanized steel sheet before joining. It can be seen that a zinc plating layer with a thickness of about 4 μm has been formed on the surface of the steel sheet. As a representative joint result, Figure 9 shows an SEM photograph and element mapping results of a joint obtained at 200 MPa. The zinc (Zn) mapping results show that no zinc has been mixed into the joint. In addition, it can be confirmed that the surface of the joint is covered with a zinc plating layer up to the base of the burr (area surrounded by a dotted line in the figure). Note that, in the joints obtained at 50 MPa and 100 MPa, no zinc was found to be mixed into the joint, as in the joint obtained at 100 MPa, and the surface of the joint is covered with a zinc plating layer up to the base of the burr.

[0053] To evaluate the mechanical properties of the joints, tensile tests were conducted on the joints obtained at each joining pressure and on the hot-dip galvanized steel sheets before joining. The test pieces shown in Figure 10 were prepared so that the joint interface was located at the center of the parallel part, and the tensile axis was perpendicular to the joint interface. The tensile strength of the joints was measured using a tensile testing machine (SHIMADZU Autograph AGS-X 10kN) at a crosshead speed of 0.06mm / min. The stress-strain curves obtained at each joining pressure are shown in Figure 11.

[0054] The joints obtained with all joining pressures showed the same tensile strength as the hot-dip galvanized steel sheets before joining, and the base material fractured. The elongation value of the joints was slightly smaller, but this is thought to be due to the increase in hardness caused by the refinement of the structure of the linear friction joint.

[0055] Linear friction welding was performed at a joining pressure of 50 MPa in the same manner as for the 2.0 mm plate thickness, except that the thickness of the hot-dip galvanized steel plate used as the welded material was 1.2 mm. A cross-sectional photograph of the welded part of the obtained joint is shown in Figure 12. Even when the plate thickness was 1.2 mm, good removal of burrs from the welded interface was confirmed, and it can be seen that a similar joint was formed to the case of the 2.0 mm plate thickness.

[0056] From the above results, it can be seen that the use of linear friction joining suppresses the intrusion of zinc plating layer components into the joint, and produces a joint in which the surface of the joint is covered with a zinc plating layer, thereby imparting tensile properties to the joint equivalent to those of the base material. [Explanation of symbols]

[0057] 2...One member, 4...the other member, 6... bonded interface, 8. Bali, 10...joint structure, 12...linear friction joint, 14. Zinc-plated layer.

Claims

1. a first step of contacting one member with another member to form a joining interface; a second step of repeatedly sliding the one member and the other member on the same trajectory while applying pressure substantially perpendicular to the joined interface, thereby removing burrs from the joined interface; and a third step of stopping the sliding and forming a joining surface, At least one of the one member and the other member is a zinc-plated steel plate, By removing the burrs, the inclusion of zinc plating components in the joining surface is suppressed. A friction joining method comprising the steps of:

2. The pressure is set to be equal to or greater than the yield stress of the galvanized steel sheet at a desired joining temperature; The bonding temperature is equal to or lower than the boiling point of zinc; The friction joining method according to claim 1,

3. The joining temperature is set to a melting point of zinc plating or lower; The friction joining method according to claim 2,

4. The joining temperature is A of the galvanized steel sheet. １ points or less, The friction joining method according to claim 2,

5. The tensile strength of the zinc-plated steel sheet is 340 MPa or more; The friction joining method according to any one of claims 1 to 4, characterized in that

6. A friction welded portion is formed by joining one member and another member together via a friction welded interface, At least one of the one member and the other member is a galvanized steel plate, The zinc plating components are not mixed into the friction joint. A joint structure comprising:

7. A burr is formed on the outer edge of the friction welded interface, the surface of the friction joint is covered with a zinc plating layer up to the base of the burr; The bonded structure according to claim 6 .

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