TWI683715B - Joint structure, metal structure and friction stir joining method for metal materials - Google Patents

Abstract

The invention provides a simple and efficient friction stir joining method for use when at least one of the materials being joined is a magnesium-lithium alloy, wherein the method imparts strength and plastic workability to the stirred part equaling or exceeding that of the base metal, and also provides a joint structure obtained using this joining method. Further, the invention also provides a modification method which can impart strength and plastic workability equaling or exceeding that of the base metal to an arbitrary region of the magnesium-lithium alloy, and a metal structure obtained using this modification method. The friction stir joining method for metal materials of the invention is a method for joining one material to be joined to another material to be joined, wherein at least one of the materials to be joined is a magnesium-lithium alloy, the tool for friction stir joining employs any one of a cemented carbide tool, cermet tool, ceramic tool, intermetallic compound tool, or ceramic-coated tool, and this tool for friction stir joining is inserted on the side of one of the materials to be joined.

Description

Friction stir welding method of joint body, metal structure body and metal material

The present invention relates to a joining method when at least one material to be joined is a magnesium-lithium-based alloy and a joined body obtained by the joining method, and more specifically, to an efficient joining method using friction stir welding And a joined body obtained by this joining method.

Magnesium has a high specific strength. Considering the light weight of automobiles, trains, and aircraft, it has attracted attention as an alternative to iron-steel materials and aluminum alloys used in the past. On the other hand, since magnesium alloys have an HCP structure, they generally lack formability at room temperature.

In response to this, it has been proposed to add an extremely lightweight Mg-Li (magnesium-lithium) alloy that exhibits excellent room temperature formability by adding Li (lithium) elements, but in order to make effective use of it widely, it is necessary to establish a bonding technique.

However, the magnesium-lithium alloy has high chemical activity, and it is difficult to obtain a good joint by fusion welding. For example, Non-Patent Document 1 ("Microstructure and mechanical properties of Mg-Li alloy after TIG "Welding", Transactions of Nonferrous Metals Society of China, 21.3 (2011), 477-481.) discloses an example of using TIG welding to join magnesium-lithium alloys, but due to the temperature history at the time of welding, crystallization of the welded part cannot be avoided Grain coarsening and softening of the heat affected zone.

In addition, Patent Document 1 (Japanese Unexamined Patent Publication No. 2008-254003) discloses a method for manufacturing a clad material, which is characterized by overlapping a magnesium-lithium alloy plate with an aluminum plate or its alloy plate and using Friction stir to make them overlap and join, and then calender.

In the manufacturing method of the coating material described in the above Patent Document 1, the upper and lower plates are moderately stirred and diffused by friction stirring and the bonding is performed in the solid phase state, so even if the magnesium-lithium alloy plate is not performed as in the conventional rolling bonding The surface in contact with the aluminum plate or its alloy plate is pre-acid-washed and polished with a metal wire brush to remove the oxide film and other pretreatments, and it can also be firmly bonded. In this respect, it is compared with the conventional rolling bonding , Can reduce product costs and manufacturing costs.

In addition, in the method of manufacturing a coating material described in Patent Document 1, the friction stir welding is used for strong bonding, so the rolling performed after the friction stir welding can be maintained even in a cold room (room temperature) The bonding force is not reduced. It is not necessary to perform heat treatment at a temperature of about 200 to 300°C for a long time after the rolling at room temperature as in the conventional rolling bonding to improve the bonding force and bending workability. In other words, compared with the conventional rolling bonding, the product cost and manufacturing cost can also be reduced.

[Prior Technical Literature]

[Non-patent literature]

[Non-Patent Document 1] "Microstructure and mechanical properties of Mg-Li alloy after TIG welding", Transactions of Nonferrous Metals Society of China, 21.3(2011), 477-481.

[Patent Literature]

[Patent Document 1] Japanese Unexamined Patent Publication No. 2008-254003

However, in the welding method disclosed in the above-mentioned non-patent document 1, the joint strength of the obtained magnesium-lithium alloy is 84% or less of the base material, and the extensibility is also reduced compared to the base material. In addition, in the fusion welding of magnesium-lithium alloys, in order to completely remove oxygen in the environment, a large amount of argon gas must be used.

In addition, in the method of manufacturing a coating material disclosed in Patent Document 1, a tool for friction stir welding is inserted from the aluminum alloy plate side disposed on the upper side, and the aluminum alloy plate on the upper side and the magnesium-lithium-based alloy on the lower side are inserted. When the plates are joined, material flow due to the action of the tool hardly occurs on the magnesium-lithium alloy plate side. Considering the "friction stirring technique", it is equivalent to friction stir welding of aluminum alloys. That is, it is difficult to say that a method for obtaining a good joint of a magnesium-lithium alloy by friction stir welding has been established.

In view of the above-mentioned problems in the prior art, an object of the present invention is to provide a joining method that is a simple and efficient friction stir welding method in which at least one material to be joined is a magnesium-lithium alloy, and can The strength and plastic workability of the base material are given to the stirring part; and the joined body obtained by this joining method is provided. In addition, the present invention also aims to provide a modification method capable of imparting strength and plastic workability higher than that of a base material to any region of a magnesium-lithium-based alloy, and a metal structure obtained by the modification method.

In order to achieve the above object, the inventors of the present invention have made intensive studies on the materials of friction stir welding tools, and found that it is extremely effective to suppress the adhesion of the material to be joined (magnesium-lithium alloy) on the tool surface, and completed the present invention. .

That is, the present invention provides a friction stir welding method of metal materials, which is a method of friction stir welding a joined material and another joined material; characterized in that at least the one joined material is a magnesium-lithium alloy, The tool for friction stir welding uses any one of a cemented carbide tool, a cermet tool, a ceramic tool, an intermetallic compound tool, and a ceramic coated tool, and inserts the friction stir welding tool into the tool One is the material side.

Aluminum alloys and magnesium alloys can be easily friction-stirred with steel tools, and there is no particular problem with tool life. Therefore, even in consideration of tool prices, conventional friction stir welding systems use steel (for example, Hot working tool steel: SKD61, etc.) tools.

However, the inventors of the present application tried the joining of magnesium-lithium-based alloys using a conventionally known friction stir welding method using tools made of tool steel. As a result, the softened magnesium-lithium-based alloy adhered to the surface of the tool, making it difficult to form a good stirring portion.

In this regard, in the friction stir welding method of the metal material of the present invention, by using any one of cemented carbide tools, cermet tools, ceramic tools, intermetallic compound tools, and ceramic coated tools as friction stir welding With a tool, the adhesion of the magnesium-lithium alloy that is the material to be joined can be effectively reduced. Here, by making the tool surface made of inorganic non-metal, the affinity with magnesium-lithium-based alloys can be reduced (although cemented carbide and cermet have a metal-bonded phase, but the main component is an inorganic non-metal phase). In addition, as a result, the temperature of friction stir welding can be lowered, and it is possible to form a stirring part having a finer and homogeneous structure with excellent strength and plastic workability.

In addition, the metal material joining method of the present invention includes the following four aspects (1) to (4) and their combinations: (1) The ends of the metal plates are butted against each other to form a joint, and the rotary tool is rotated Rotate along the longitudinal direction of the processed portion while moving to join the metal plates to each other; (2) The ends of the metal plates are butted against each other to form a joint, and the joint is rotated without moving the rotating tool at the joint Spot welding; (3) The metal plates are overlapped at the joint, and the rotary tool is inserted into the joint, and the rotary tool is rotated at this location to join the metal plates. Point bonding; (4) overlapping the metal plates at the joint, inserting the rotary tool into the joint, and moving the rotary tool along the longitudinal direction of the joint to move the joint to join the metal plates; friction The tool for stir welding is inserted into the magnesium-lithium alloy. In the method of joining metal materials of the present invention, the magnesium-lithium alloy is The adhesion of the tool surface is suppressed, and by inserting the tool into a magnesium-lithium alloy with low plastic deformation resistance, the tool life can also be improved.

One of the characteristics of the method for joining metal materials of the present invention is that a tool for friction stir welding is inserted on a side of a material to be joined (magnesium-lithium alloy side). More specifically, when one material to be joined is a magnesium-lithium-based alloy and the other material to be joined is other than a magnesium-lithium-based alloy, the center of the tool for friction stir welding (the center of the probe portion) is set at The side to be joined is closer to the material to be joined than the butting surface. In the overlapping joining, a material to be joined is arranged on the upper side, and a tool for friction stir welding is inserted from the side to be joined.

In addition, the general friction stir welding conditions such as the rotation speed, moving speed, insertion amount, and applied load of the tool may be appropriately set in consideration of the formation of defects in the stirring section, the joining efficiency, and the like. In addition, as long as the effect of the present invention is not impaired, the shape of the tool is not particularly limited, and various shapes of conventionally known tools for friction stir welding can be used.

Moreover, in the method of joining metal materials of the present invention, the other material to be joined is preferably a magnesium-lithium alloy. Since the materials to be joined are all magnesium-lithium-based alloys, the importance of the friction stir action of the magnesium-lithium-based alloys increases, so that the effects of the metal material joining method of the present invention can be obtained more remarkably.

In addition, in the method for joining metal materials of the present invention, it is preferable that the surface of the tool for friction stir welding not be grooved. In friction stir welding of aluminum alloy and magnesium alloy, tool wear and tool breakage are not likely to occur. Therefore, in order to enhance the stirring effect caused by the tool, groove processing such as thread processing is generally applied to the bottom surface of the shoulder and the side of the probe. On the contrary, by not applying these grooves to the tool, the magnesium-lithium system can be suppressed Gold is attached to the tool surface. On the other hand, the magnesium-lithium-based alloy has excellent plastic workability, so even a tool without groove processing can sufficiently generate material flow and form a stirring part. In addition, the tool for friction stir welding without groove processing means a tool having a shoulder bottom surface and a probe side surface in a planar state.

In addition, in the method of joining metal materials of the present invention, the peripheral speed of the outermost periphery of the shoulder portion of the tool for friction stir welding is preferably set to 23.6 mm/s to 78.5 mm/s. This range of peripheral speeds is a low-speed range beyond the common sense in friction stir welding of magnesium alloys. However, when a magnesium-lithium alloy having room temperature processability is used as the material to be joined, a sufficient friction stirring effect can be obtained. Here, by setting the peripheral speed of the outermost periphery of the shoulder to 23.6 mm/s or more, the formation of defects due to insufficient stirring can be suppressed, and by setting it to 78.5 mm/s or less, the adhesion of magnesium-lithium-based alloys can be suppressed Defects formed on the surface of the tool, surface oxidation of the stirring part and the formation of liquid phase.

Furthermore, in the method of joining metal materials of the present invention, the peripheral speed of the outermost periphery of the probe portion of the tool for friction stir welding is preferably set to 9.5 mm/s to 31.4 mm/s. This range of peripheral speeds is a low-speed range beyond the common sense in friction stir welding of magnesium alloys. However, when a magnesium-lithium alloy having room temperature processability is used as the material to be joined, a sufficient friction stirring effect can be obtained. Here, by setting the peripheral speed of the outermost periphery of the probe part to 9.5 mm/s or more, the formation of defects due to insufficient stirring can be suppressed, and by setting it to 31.4 mm/s or less, the adhesion of magnesium-lithium-based alloys can be suppressed Defects caused on the tool surface.

In addition, in the method of joining metal materials of the present invention, butt joint is preferable. Compared with overlap welding, the formation of the stirring part in butt welding becomes important. In the method of joining metal materials of the present invention, even if magnesium is used When the lithium-based alloy is used as the material to be joined, a good stirring portion can be formed, so even in butt joining, the joining can be efficiently achieved.

In addition, the friction stir welding for the purpose of joining and the friction stir processing for the purpose of modification are basically the same technology, and the above-mentioned method of joining metal materials of the present invention can also be used as a method of modifying metal materials. Specifically, considering the shape, size, and position of the desired modified portion, friction stir may be performed using a tool having an appropriate shape and size.

Moreover, the present invention also provides a joined body formed by joining one joined material and another joined material by a stirring part; characterized in that at least the one joined material is a magnesium-lithium alloy, and the stirring part contains the In the recrystallized grains of the α phase of the magnesium-lithium-based alloy, the crystal orientation of the recrystallized grains is randomized.

The joined body of the present invention uses a relatively low temperature friction stir welding to form the agitated portion, so the agitated portion contains the recrystallized crystal grains of the α phase with the HCP structure and the β phase with the BCC structure, compared with the base material , The recrystallized grains of the α phase are significantly refined.

In the stirring section, the crystal orientation of the α phase is randomized. The α-phase has an HCP structure, so generally, a strong texture is formed after a processing process that applies simple shear stress through extrusion, calendering, or the like. In this regard, by using the joining method of the present invention and performing friction stirring under appropriate conditions, the alpha phase can be crystallized The orientation becomes random, and the reduction in mechanical properties and anisotropy due to strong texture can be suppressed. Here, randomization means that the strong bottom texture of the α phase is not formed. For example, in the orientation map of the EBSD measurement, the recrystallized grains of the α phase may not be represented by the same system color. . More specifically, in the pole figure, the texture intensity (Texture Intensity) is preferably 1-10, more preferably 1-5.

In addition, in the joined body of the present invention, the structure of the agitating portion is finer than that of the base material, so it has a higher hardness than the base material. In addition, there is no heat-affected portion accompanying the temperature increase during joining at the outer edge of the stirring portion, so there is no area having a lower hardness than the base material.

Moreover, in the joined body of the present invention, the minimum hardness in the region of the magnesium-lithium alloy of the joined body is preferably 50 HV or more, and the joint efficiency of the material to be joined is more preferably 100%. By performing friction stir welding under appropriate conditions, a joined body having such mechanical properties can be obtained.

In addition, in the joined body of the present invention, the tensile strength of the agitating portion is preferably 1.1 times or more, more preferably 1.3 times or more, and most preferably 1.5 times or more the tensile strength of the base material of the material to be joined. Regarding the stirring part formed by general friction stir welding, under the joining condition where the heat input is relatively small, sometimes the tensile strength of the stirring part is slightly higher than that of the base material. However, in the joined body of the present invention, since the friction stir welding of the magnesium alloy forms an agitating portion for low heat input conditions beyond common sense, and organizes the nanostructure of the agitating portion (the average crystal grain size of the α phase is less than 1μm), the tensile strength of the stirring part can be 1.1 times or more of the tensile strength of a material to be joined. Moreover, since the stirring part having a nanostructure exhibits excellent superplastic performance, it is also good from the viewpoint of workability.

In addition, in the bonded body of the present invention, the other material to be bonded is preferably a magnesium-lithium-based alloy, and is more preferably a butt bonded body. Since all the materials to be joined are magnesium-lithium alloys and are made into butt joints, the weight of the joints can be extremely efficiently reduced.

In addition, the joined body of the present invention can be suitably produced by the above-mentioned joining method of the metal material of the present invention.

In addition, the present invention also provides a metal structure containing a modified portion of a magnesium-lithium-based alloy; characterized in that the modified portion contains the recrystallized grains of the α-phase of the magnesium-lithium-based alloy, and the The crystal orientation is randomized.

The metal structure of the present invention has a modified portion containing recrystallized crystal grains of randomized α phase, and therefore has excellent isotropic mechanical properties and plastic workability.

In the metal structure of the present invention, the average particle size of the recrystallized crystal grains is preferably less than 1 μm. By setting the average particle size of the recrystallized grains to less than 1 μm, the modified portion can be made to have higher hardness and higher strength, and excellent superplastic properties can be imparted.

In addition, the metal structure of the present invention can be suitably manufactured by the same method of modifying the metal material as the metal material joining method of the present invention described above.

According to the present invention, a joining method can be provided, which is a simple and efficient friction stir joining method when at least one material to be joined is a magnesium-lithium alloy, and can impart strength and plastic workability of the base material or more to the stirring portion; and A bonded body obtained by this bonding method is provided. Furthermore, according to the present invention, it is also possible to provide a modification method capable of imparting strength and plasticity to the base material to any region of the magnesium-lithium alloy, and a metal structure obtained by the modification method.

2, 2’‧‧‧ to be joined

4‧‧‧Tool

6‧‧‧Joint

8‧‧‧Protrusion (probe part)

10‧‧‧Body (shoulder)

12‧‧‧Mixer

20‧‧‧Conjugate

FIG. 1 is a schematic diagram showing one aspect of the method for joining metal materials of the present invention.

Fig. 2 is a schematic cross-sectional view of the vicinity of the joint portion in the joint body of the present invention.

[FIG. 3] It is a cross-sectional photograph of the joint obtained in the examples.

[Fig. 4] It is the hardness distribution of the cross section of the joint obtained in the examples.

[Fig. 5] The tensile properties of the joints obtained in the examples.

[Fig. 6] An overview photograph of a test piece of a stirring part after evaluation of plastic deformation characteristics.

[Fig. 7] SEM photographs of the base material and the center of the stirring part obtained in Examples.

[Fig. 8] A crystal orientation distribution image of the base material and the α phase at the center of the stirring section obtained in Examples.

Hereinafter, the representative embodiment of the metal material joining method and the joined body of the present invention will be described in detail with reference to the drawings, but the present invention is not limited to these. In addition, in the following instructions, the When parts are given the same symbol, duplicate descriptions are sometimes omitted. In addition, the drawings are used to conceptually explain the present invention, so the dimensions and ratios of the components shown may sometimes differ from the actual ones.

(1) How to join metal materials

In the method of joining metal materials of the present invention, friction stir welding is used. Friction stir welding is called FSW (Friction Stir Welding), which connects the ends of the two materials to be joined made of metal materials, and inserts the protrusions (probes) provided on the front end of the rotary tool into both. Between the ends of the two, by rotating the rotary tool along the longitudinal edges of these ends while moving, a method of joining two metal members.

The joining method of metal materials in the present invention, as described above, includes the following four aspects (1) to (4) and their combinations: (1) The ends of the metal plates are butted against each other to form a joined portion, so that The rotary tool moves along the longitudinal direction of the processing portion while rotating to join the metal plates to each other; (2) The ends of the metal plates are butted against each other to form a joint portion where the rotary tool is not moved at the joint portion Rotate to join the joints; (3) Overlap the metal plates at the joint, insert the rotary tool into the joint, and rotate the rotary tool without moving the part to join the joints of the metal plates (4) The metal plates are overlapped at the joint, the rotary tool is inserted into the joint, and the rotary tool is rotated along the longitudinal direction of the joint to move and join the metal plates to each other; the following represents In terms of behavior, "(1) joining the ends of the metal plates to form a joint, and moving the rotary tool along the longitudinal direction of the processing portion while rotating to join the metal plates" is detailed Instructions.

FIG. 1 is a schematic diagram showing one aspect of the method for joining metal materials of the present invention. Butt the joined material 2 (one joined material) and the joined material 2'(another joined material), and insert the rotating tool 4 into the desired joining area to move it along the joined line. Obtain the joint 6. Here, when both the material-to-be-joined 2 and the material-to-be-joined 2'are magnesium-lithium-based alloys, the mating wire is inserted so that the center of the protruding portion (probe portion) 8 of the tool 4 substantially coincides. In addition, when the material to be joined 2 is a magnesium-lithium-based alloy and the material to be joined 2'is another metal material, in the method of joining metal materials of the present invention, the protruding portion (probe portion) 8 and the main body portion (shoulder) constituting the tool 4 Most of the part 10 is in contact with the side of the material to be joined 2.

In addition, by using any of cemented carbide tools, cermet tools, ceramic tools, intermetallic tools, and ceramic coated tools as the tool 4, the magnesium-lithium alloy that is to be joined can be effectively reduced Attachment. Here, since the surface of the tool 4 is made of inorganic non-metal, the affinity with the magnesium-lithium-based alloy can be reduced. In addition, as a result, the temperature of friction stir welding can be lowered, and the agitating portion 12 having a finer and homogeneous structure with excellent strength and plastic workability can be formed.

Here, the composition and structure of cemented carbide, cermet, ceramic and intermetallic compound used as the material of the tool are not particularly limited as long as the effect of the present invention is not impaired, but may be various compositions and structures known in the past, but In the case of cemented carbide and cermets, the metal bonding phase should be reduced. In addition, for ceramics, for example, silicon carbide, silicon nitride, silicon aluminum oxynitride (sialon), boron nitride, zirconium oxide, aluminum oxide, titanium diboride, etc., for intermetallic compounds, for example Intermetallic compounds of Ti-Al series and Ni-Al series can be used.

In addition, in the case of ceramic coating, the body of the tool 4 may be made of metal, for example, may be made of hot working tool steel (SKD61). The composition, structure, and film thickness of the ceramic coating are not particularly limited as long as the effects of the present invention are not impaired, and can be any conventionally known composition, structure, and film thickness. For example, various hard materials used as cutting tools can be used Envelope.

In addition, the tool material and the ceramic coating described above should be selected in consideration of the wettability with the magnesium-lithium-based alloy, and the contact angle of the magnesium droplet with these materials should be 90° or more. By selecting the tool material and ceramic coating with a contact angle of 90° or more, the torque applied to the tool 4 can be reduced.

In addition, FIG. 1 shows a case where the tool 4 has a cylindrical protrusion (probe portion) 8 on the bottom surface of the cylindrical body portion (shoulder portion) 10, but as long as the effect of the present invention is not impaired, the shape of the tool 4 There is no particular limitation, and various shapes of conventionally known tools for friction stir welding can be used. In addition, it is preferable that the bottom surface of the body portion (shoulder portion) 10 and the side surface of the protruding portion (probe portion) 8 that are in contact with the materials to be joined 2, 2'have no groove processing such as screw processing. By not applying the groove processing to the tool, the adhesion of the magnesium-lithium-based alloy to the surface of the tool 4 can be suppressed. On the other hand, the magnesium-lithium-based alloy has excellent plastic workability, so that even a tool without groove processing can sufficiently form the stirring portion 12.

In the method for joining metal materials of the present invention, a magnesium-lithium-based alloy that becomes at least one material to be joined widely includes an alloy containing magnesium as a main component and added with lithium in order to impart plastic workability at room temperature. Examples include those in which aluminum, zinc, manganese, yttrium, lanthanides, zirconium, silver, silicon, calcium, etc. are added to the alloy in order to improve strength and heat resistance.

Here, the content of lithium is preferably in the range of 5 to 15% by weight. If the lithium content is less than 5% by weight, the plastic workability at room temperature will not be improved. Conversely, if the lithium content is greater than 15% by weight, it may sometimes cause grain boundary cracks (surface cracks), and Since lithium is expensive, the cost becomes high.

Also, the other material to be joined 2'is preferably a magnesium-lithium-based alloy. Since both of the materials to be joined 2 and 2'are magnesium-lithium-based alloys, the importance of the friction stir action of the magnesium-lithium-based alloy increases, so that the effect of the metal material joining method of the present invention can be obtained more remarkably.

In addition, the general friction stir welding conditions such as the rotation speed, the moving speed, the insertion amount, and the applied load of the tool 4 may be appropriately set in consideration of the formation of defects and the joining efficiency in the stirring section 12.

Here, the peripheral speed of the outermost periphery of the body portion (shoulder portion) 10 of the tool 4 is preferably set to 23.6 mm/s to 78.5 mm/s. This range of peripheral speeds is a low-speed range beyond the common sense in friction stir welding of magnesium alloys. However, when a magnesium-lithium alloy having room temperature processability is used as the materials to be joined 2, 2', a sufficient friction stirring effect can be obtained. Here, by setting the peripheral speed of the outermost periphery of the body portion (shoulder portion) 10 to 23.6 mm/s or more, the formation of defects due to insufficient stirring can be suppressed, and by setting it to 78.5 mm/s or less, it can be suppressed The formation of defects due to the adhesion of the magnesium-lithium-based alloy to the surface of the tool 4, the surface oxidation of the agitating portion 12, and the generation of the liquid phase. In addition, when the diameter of the body portion (shoulder portion) 10 of the tool 4 is 15 mm, by setting the rotation speed to 30 rpm, the peripheral speed at the outermost periphery is about 23.6 mm/s, and by setting the rotation speed to 100 rpm , So that the peripheral speed of the outermost periphery is about 78.5 mm/s.

In addition, the peripheral speed of the outermost periphery of the protruding portion (probe portion) 8 of the tool 4 is preferably set to 9.5 mm/s to 31.4 mm/s. This range of peripheral speeds is a low-speed range beyond the common sense in friction stir welding of magnesium alloys. However, when a magnesium-lithium alloy having room temperature processability is used as the materials to be joined 2, 2', a sufficient friction stirring effect can be obtained. Here, by setting the peripheral speed of the outermost periphery of the protruding portion (probe portion) 8 to 9.5 mm/s or more, the formation of defects due to insufficient stirring can be suppressed, and by setting it to 31.4 mm/s or less, it can be suppressed Defects caused by the magnesium-lithium-based alloy adhering to the surface of the tool 4 are formed. In addition, when the diameter of the protruding portion (probe portion) 8 of the tool 4 is 6 mm, by setting the rotation speed to 30 rpm, the peripheral speed at the outermost periphery is about 9.5 mm/s, and by setting the rotation speed to 100 rpm , So that the peripheral speed of the outermost periphery is about 31.4 mm/s.

In addition, the formation of the agitating portion 12 in butt joining becomes more important than in the case of overlapping joining. In the method of joining metal materials of the present invention, even when a magnesium-lithium alloy is used as the materials to be joined 2, 2', it can be formed Since the stirring part 12 is good, by using the joining method of the present invention, joining can be achieved efficiently.

(2) Joint body

FIG. 2 shows a schematic cross-sectional view of the vicinity of the joint in the joint body of the present invention. In addition, as a representative aspect of the joint portion in the joint body of the present invention, the butt joint portion is shown in FIG. 2.

The joined body 20 of the present invention is a joined body formed by joining a material to be joined 2 and another material to be joined 2 ′ by a stirring part 12, characterized in that at least one material to be joined 2 is a magnesium-lithium-based alloy and is stirred The region 12 of the magnesium-lithium-based alloy contains recrystallization of the α phase. In addition, as long as the effects of the present invention are not impaired, the shapes and sizes of the materials to be joined 2, 2'are not particularly limited as long as they can be joined by the joining method of the present invention.

The magnesium-lithium alloy has the α phase of the HCP structure and the β phase of the BCC structure. However, in the extruded material and the rolled material, the α phase is elongated and distributed in a strip shape. In contrast, although the α phase is macroscopically distributed in the stirring section 12, the α phase is recrystallized, and the equiaxed α phase is aggregated.

The magnesium-lithium-based alloy that becomes at least one material to be joined 2 widely includes an alloy containing magnesium as a main component and added with lithium in order to impart plastic workability to room temperature. Examples include those in which aluminum, zinc, manganese, yttrium, lanthanides, zirconium, silver, silicon, calcium, etc. are added to the alloy in order to improve strength and heat resistance.

Here, the content of lithium is preferably in the range of 5 to 15% by weight. If the lithium content is less than 5% by weight, the plastic workability at room temperature will not be improved. Conversely, if the lithium content is greater than 15% by weight, it may sometimes cause grain boundary cracks (surface cracks), and Since lithium is expensive, the cost becomes high.

In the joined body 20, the structure of the agitating portion 12 is finer than that of the base material, so it has a higher hardness than the base material. In addition, there is no heat-affected portion accompanying the temperature rise during joining at the outer edge of the stirring portion 12, so there is no area having a lower hardness than the base material. In addition, the average particle size of the recrystallized crystal grains of the α phase should preferably be less than 5 μm, more preferably less than 3 μm, and most preferably less than 1 μm. By refining the α-phase recrystallized crystal grains, the hardness and strength of the agitating portion 12 can be improved. Not only that, but also good plastic deformation ability can be imparted.

In addition, the minimum hardness in the region of one of the joined bodies 20 to be joined 2 (magnesium-lithium alloy) is preferably 50 HV or more, and the joint efficiency for one of the joined bodies 20 to be joined 2 (magnesium-lithium alloy) is 100% Better.

In addition, the tensile strength of the stirring part 12 is preferably 1.1 times or more of the tensile strength of the material to be joined 2, more preferably 1.3 times or more, and most preferably 1.5 times or more. Regarding the stirring part formed by general friction stir welding, under the joining condition where the heat input is relatively small, sometimes the tensile strength of the stirring part is slightly higher than that of the base material. However, in the joined body 20, the stirring part 12 is formed by low heat input conditions beyond the common sense in the case of friction stir welding of magnesium alloys, and the stirring part 12 is organized into nanometers (the average crystal grain size of the α phase is less than 1 μm), the tensile strength of the stirring portion 12 can be 1.1 times or more of the tensile strength of the material 2 to be joined. Moreover, since the stirring part 12 having a nanostructure exhibits excellent superplastic performance, it is also good from the viewpoint of workability.

In addition, in the bonded body 20, in the recrystallization region of the stirring portion 12, the crystal orientation of the α phase is preferably randomized. By using the above-mentioned joining method of the present invention and performing friction stirring under appropriate conditions, the crystal orientation of the α phase can be made random, and the reduction in mechanical properties and anisotropy due to strong texture can be suppressed.

Furthermore, in the bonded body 20, the other material to be bonded 2'is preferably a magnesium-lithium-based alloy, and is more preferably a butt bonded body. Since both of the materials to be joined 2 and 2'are magnesium-lithium-based alloys and they are made into a butt joint body, the weight of the joint body 20 can be extremely efficiently reduced.

The representative embodiments of the present invention have been described above, but the present invention is not limited to these, and various design changes can be made, and these design changes are all included in the technical scope of the present invention. In addition, the modified portion in the metal structure of the present invention has the same characteristics as the stirring portion in the joined body of the present invention.

[Example]

≪Examples≫

The LZ91 magnesium alloy (9wt%Li-1wt%Zn-Mg Bal.) plates with a length of 200mm, a width of 65mm, and a thickness of 3mm were butted against each other in the configuration shown in FIG. 1, and the joint was obtained by friction stir welding. The tool for friction stir welding is a cemented carbide tool with a shoulder diameter of 15 mm, a probe diameter of 6 mm, and a probe length of 2.8 mm. The tool rotation speed is 25 to 300 rpm, the tool movement speed is 20 to 100 mm/min, and the advancing angle is 3°. , Implement friction stir welding with tool position control. In addition, shielding gas was not used for friction stir welding, and the bottom of the shoulder and the side of the probe of the tool were not grooved.

≪Comparative example≫

Except that the material of the tool was changed to hot-working tool steel (SKD61), the same procedure as in Example 1 was performed, and friction stir welding was performed.

[Appropriate joining conditions]

In order to confirm appropriate bonding conditions for obtaining a good bonded body, the surface and cross section of the bonded portion (agitated portion) were observed with an optical microscope. The case where a defect is formed in the agitating portion is ×. Although no defect is formed, a large amount of burrs and a rough surface are observed is △. The case where no defect is formed and the surface state of the agitating portion is also smooth is ○. Table 1. In addition, in the comparative example, defects were formed under all conditions (all conditions are evaluated by ×), and therefore were not shown.

As an example of the cross-sectional observation result of the joint, a cross-sectional photograph of the joint obtained in the examples (tool rotation speed 200 rpm, tool movement speed 100 mm/min) is shown in FIG. 3. No defect formation was observed, and a good stirring part was obtained.

In the examples, when the rotation speed is the slowest at 25 rpm, small defects are formed in the agitating portion, but no defect formation is observed under other conditions, and the surface of the agitating portion is also in good condition. From this result, it can be seen that the joining method of the present invention can obtain good joints under a wide range of joining conditions. In addition, if the rotation speed is higher than 150 rpm, the surface of the agitating portion loses metallic luster. It is considered that this is due to the formation of oxidation and liquid phase accompanying the increase in bonding temperature. In addition, by using argon gas as a shielding gas, it is possible to obtain a surface of the stirring portion that maintains a good metallic luster. On the other hand, in the comparative example, in all joining conditions, the material to be joined adhered to the surface of the tool, and a groove-shaped defect was formed in the stirring portion.

[Adhesion of the material to be joined to the surface of the tool]

Regarding the joining conditions of the tool rotating speed of 100 rpm and the tool moving speed of 100 mm/min, visually observe the state of the tool surface after friction stir welding. Also, regarding tools that can be used in the present invention, silicon nitride tools, silicon carbide tools, silicon aluminum oxynitride tools, cermet tools, and tools The tool formed by forming a TiAl film on the surface of the steel body part was also subjected to the same friction stir welding, and the surface state of the tool after the welding was visually observed.

For the tool steel tool used as a comparative example, the material to be joined is attached to the side surface of the probe portion and the bottom surface of the shoulder portion, which makes it difficult to grasp the status of the tool prototype. In contrast to tools other than steel tools, although there is a part where the coloring of the material to be joined is observed on the side surface of the probe portion and the bottom surface of the shoulder, no significant adhesion is confirmed.

[Hardness measurement]

In the comparative example, groove-shaped defects were formed in the agitating portion at all the joining conditions. Therefore, Vickers hardness was measured on the cross section of the joining portion obtained in the example. In addition, the Vickers hardness measurement is performed under the conditions of load: 0.1 kgf and load load time: 15 s.

Fig. 4 shows the hardness distribution of the joint section obtained at the tool moving speed of 100 mm/min and the tool rotating speed of 100 to 300 rpm (in the horizontal direction of the joint at the center of the plate thickness). All the hardness of the mixing part is higher than that of the base material, especially at lower tool rotation speeds, the hardness of the mixing part increases significantly. In addition, no softened area was observed at the outer edge of the stirring part.

[Stretching test]

In the comparative example, groove-shaped defects were formed in all the joining conditions in the stirring part, so the joints obtained in the examples were evaluated for tensile properties. Used in the tensile test with a length of 50mm × width 12mm × thickness The tensile test piece of the parallel part of 2 mm (taking the butting surface as the center of the parallel part), and the width direction of the joint plate as the tensile axis. In addition, the stretching speed was set to 1 mm/min.

Figure 5 shows the tensile properties of the joints obtained at a tool movement speed of 100 mm/min and a tool rotation speed of 30 to 100 rpm. In addition, the comparison shows the tensile test results of the base material. In addition, in order to evaluate the tensile properties of the stirring section, it was also shown that a small tensile test piece having a parallel section with a length of 4 mm×width 2 mm×thickness 2 mm was produced in the stirring section obtained at a tool moving speed of 100/min and a tool rotating speed of 30 rpm , With the joining direction as the stretching axis, and the result of evaluation at a stretching speed of 0.08 mm/min.

The joints obtained in the examples have the same tensile properties as the base material, and the joint efficiency is 100%. In addition, the tensile properties of the agitating portion are significantly higher than that of the base material, and the tensile strength is about twice that of the base material.

[Evaluation of plastic deformation characteristics]

From the base material and the examples, a small tensile test piece having a parallel portion of length 4 mm×width 2 mm×thickness 2 mm was prepared in the stirring part obtained at a tool moving speed of 100/min and a tool rotating speed of 30 rpm. The amount of tensile deformation was evaluated under a strain rate of 3×10 ^-4 .

The amount of deformation until the base material and the stirring part were broken was about 150% and about 1100%, respectively, and it was confirmed that the stirring part was given extremely remarkable superplastic deformation ability. An overview photograph of the test piece of the stirring section after the evaluation is shown in FIG. 6. The parallel part of 4 mm in length becomes approximately 44 mm when broken.

[Microstructure observation]

In order to confirm the particle size and shape of the crystal grains in the stirring portion, SEM observation and EBSD measurement of the cross section of the joint portion were carried out. In addition, FE-SEM (JSM-7001FA manufactured by JEOL Ltd.) and OIM data Collection ver5.31 manufactured by TSL were used for SEM observation and EBSD measurement.

The SEM photographs of the center of the stirring part and the crystal phase distribution images of the α phase obtained from the base material and the examples (at a tool moving speed of 100 mm/min and a tool rotating speed of 100 to 300 rpm) are shown in FIGS. 7 and 8, respectively. The base material system is composed of α phase and β phase, and the α phase is distributed in strips. On the other hand, it can be seen that the α phase in the stirring section obtained in the example is blocked and becomes equiaxed recrystallized crystal grains. Furthermore, although the α-phase of the base material has a strong texture, the α-phase system of the stirring section is randomly aligned. In addition, the crystal grain size of the agitating portion is finer than that of the base material, which is more pronounced when the tool rotation speed is low. In addition, the average crystal grain size of the α phase is less than 5 μm when the tool rotation speed is 300 rpm, less than 3 μm when the tool rotation speed is 200 rpm, and less than 1 μm when the tool rotation speed is 100 rpm.

Claims (13)

A joined body formed by joining a joined material and another joined material by a stirring portion; characterized in that at least the one joined material is a magnesium-lithium-based alloy, and the stirring portion contains α of the magnesium-lithium-based alloy With respect to the recrystallized crystal grains, the crystal orientation of the recrystallized crystal grains is randomized, and the average particle diameter of the recrystallized crystal grains is less than 5 μm. As in the joint body according to item 1 of the patent application scope, there is no area softened from the base material outside the agitating portion. For example, in the joint body of claim 1 or 2, the minimum hardness in the region of the magnesium-lithium alloy is 50 HV or more. For example, in the joint body of claim 1 or 2, the tensile strength of the stirring part is 1.1 times or more of the tensile strength of the material to be joined. As in the joint body of claim 1 or 2, the other material to be joined is a magnesium-lithium alloy. If the joint body of the patent application item 1 or 2 is a butt joint body. A metal structure containing a modified portion of a magnesium-lithium-based alloy; characterized in that the modified portion contains recrystallized grains of the α-phase of the magnesium-lithium-based alloy, and the crystal orientation of the recrystallized grains is randomized The average particle size of the recrystallized grains is less than 5μm. For example, in the metal structure of claim 7, the average particle size of the recrystallized grains is less than 1 μm. A friction stir welding method of metal materials is a method of manufacturing a joined body as described in any one of patent application items 1 to 6, which is to perform friction stir welding of one material to be joined and another material to be joined; : At least one of the materials to be joined is a magnesium-lithium-based alloy, and tools for friction stir welding use tools made of cemented carbide tools, cermet tools, ceramic tools, tools made of intermetallic compounds, and ceramic coated tools Either way, insert the tool for friction stir welding to the side of the material to be joined; the peripheral speed of the outermost periphery of the shoulder portion of the tool for friction stir welding is set to 23.6 mm/s to 78.5 mm/s. For example, the friction stir welding method of the metal material according to item 9 of the patent scope, wherein the other material to be joined is a magnesium-lithium alloy. For example, the friction stir welding method of the metal material according to item 9 or 10 of the patent application scope, wherein the surface of the friction stir welding tool is not grooved. For example, the friction stir welding method for metal materials according to item 9 or 10 of the patent application range, wherein the peripheral speed of the outermost periphery of the probe part of the friction stir welding tool is set to 9.5 mm/s to 31.4 mm/s. For example, the friction stir welding method of metal materials in the 9th or 10th of patent application is butt welding.

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