JP7121402B2 - Magnesium-lithium alloy joining method and joined body - Google Patents

Description

The present invention relates to a joining method in which at least one of the materials to be joined is a magnesium-lithium alloy and a joined body obtained by the joining method, and more specifically, an efficient joining method using friction stir welding. and a joined body obtained by the joining method.

Magnesium has a high specific strength, and is attracting attention as an alternative material to conventionally used steel materials and aluminum alloys from the viewpoint of reducing the weight of automobiles, trains, aircraft, and the like. On the other hand, since magnesium alloys have an HCP structure, they generally have poor formability at room temperature.

On the other hand, an extremely lightweight Mg-Li (magnesium-lithium) alloy that exhibits excellent room temperature formability by adding the Li (lithium) element has been proposed. It is essential to establish

However, magnesium-lithium alloys have high chemical activity and it is difficult to obtain good joints by fusion welding. For example, in 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.), TIG welding using magnesium - An example of joining a lithium-based alloy is disclosed, but due to the temperature history during welding, grain coarsening and softening of the heat-affected zone are unavoidable.

Further, in Patent Document 1 (Japanese Patent Application Laid-Open No. 2008-254003), a magnesium-lithium alloy plate and an aluminum or aluminum alloy plate are overlapped, and these are overlapped and joined by friction stirring, and then rolled. is disclosed.

In the method for producing the clad material described in Patent Document 1, the upper and lower plate materials are appropriately stirred and diffused by friction stirring, and the joining is performed in a solid state. A strong bond can be achieved without the pretreatment of pickling the contact surface of the aluminum or its alloy plate and polishing it with a metal wire brush to remove the oxide film. It is said that the product cost and manufacturing cost can be kept low compared to roll bonding.

In addition, in the method for producing a clad material described in Patent Document 1, since strong joining is performed by friction stir welding, the joining force can be maintained even if the rolling performed after friction stir welding is performed cold (room temperature). is maintained and does not decrease, it is necessary to heat the clad material at a temperature of about 200 to 300 ° C for a long time after rolling at room temperature to increase the bonding strength and bending workability, as in conventional roll bonding. In this respect, the product cost and manufacturing cost can be kept lower than the conventional roll joining.

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

Japanese Unexamined Patent Application Publication No. 2008-254003

However, with the welding method disclosed in Non-Patent Document 1, the joint strength of the obtained magnesium-lithium alloy is 84% or less of that of the base material, and the ductility is also lower than that of the base material. In addition, fusion welding of magnesium-lithium based alloys requires the use of large amounts of argon to completely exclude oxygen from the atmosphere.

Further, in the clad material manufacturing method disclosed in Patent Document 1, a tool for friction stir welding is inserted from the upper aluminum alloy plate side, and the upper aluminum alloy plate and the lower magnesium- It joins lithium-based alloy plates, and almost no material flow occurs on the magnesium-lithium alloy plate side due to the action of the tool. be. That is, it is difficult to say that a method for obtaining good joints of magnesium-lithium alloys by friction stir welding has been established.

In view of the problems in the prior art as described above, an object of the present invention is to provide a simple and efficient friction stir welding method when at least one of the materials to be welded is made of a magnesium-lithium alloy, comprising: An object of the present invention is to provide a joining method capable of imparting strength and plastic workability higher than those of a base material, and a joined body obtained by the joining method. In addition, the present invention provides a modification method capable of imparting strength and plastic workability equal to or greater than those of the base material to an arbitrary region of a magnesium-lithium alloy, and a metal structure obtained by the modification method. is also intended.

In order to achieve the above object, the present inventors have made intensive research on the materials of friction stir welding tools, etc. As a result, it has been found that the adhesion of the material to be welded (magnesium-lithium alloy) to the tool surface can be suppressed. The inventors have found that this is extremely effective, and arrived at the present invention.

That is, the present invention
A method for friction stir welding one material to be welded and the other material to be welded, comprising:
At least one of the materials to be joined is a magnesium-lithium alloy,
Using any one of a cemented carbide tool, a cermet tool, a ceramic tool, an intermetallic compound tool and a ceramic coated tool as a friction stir welding tool,
inserting the friction stir welding tool into the one workpiece side;
A friction stir welding method for metal materials characterized by:

Aluminum alloys and magnesium alloys can be easily friction-stirred with steel tools, and there are no particular problems with tool life. Hot work tool steel: SKD61, etc.) tools have been used.

However, when the present inventor attempted to join a magnesium-lithium alloy by a conventionally known friction stir welding method using a tool steel tool, the softened magnesium-lithium alloy adhered to the tool surface, It was difficult to form a good stirrer.

On the other hand, in the friction stir welding method for metal materials of the present invention, any one of cemented carbide tools, cermet tools, ceramic tools, intermetallic compound tools and ceramic coated tools is used as the friction stir welding tool. Adhesion of the magnesium-lithium alloy, which is the material to be joined, can be effectively reduced by using the above. Here, by making the tool surface made of inorganic non-metal, it is possible to reduce the affinity with the magnesium-lithium alloy (cemented carbide and cermet have a metallic bonding phase, but the inorganic non-metallic phase is the main component). In addition, as a result, the temperature of the friction stir welding can be lowered, and a stir zone having a finer and more homogeneous structure and excellent strength and plastic workability can be formed.

Further, in the method of joining metal materials of the present invention, (1) the ends of the metal plates are butted together to form a joining portion, and the rotating tool is rotated and moved along the longitudinal direction of the processed portion to join the metal plates together. (2) spot welding in which the ends of metal plates are butted against each other to form a joint portion, and a rotating tool is rotated at the joint portion without moving and joined by rotating; (3) metal plates are overlapped at the joint portion spot welding, in which the metal plates are joined together by inserting a rotating tool into the joint and rotating the rotary tool without moving it at that spot; Including the four aspects (1) to (4) of joining metal plates by inserting a tool and rotating and moving the rotary tool along the longitudinal direction of the joint, and combinations thereof, A friction stir welding tool is inserted into a magnesium-lithium alloy. In the metal material joining method of the present invention, adhesion of magnesium-lithium alloy to the tool surface is suppressed, and the tool life is extended by inserting the tool into a magnesium-lithium alloy with low plastic deformation resistance. can also be improved.

One of the features of the metal material joining method of the present invention is that the friction stir welding tool is inserted into one of the materials to be joined (magnesium-lithium alloy side). More specifically, when one material to be welded is a magnesium-lithium alloy and the other material to be welded is not a magnesium-lithium alloy, in butt welding, the center of the friction stir welding tool (probe The center of the part) is placed on one of the welded materials side from the butt surface, and in lap welding, one welded material is placed on the upper side, and a friction stir welding tool is inserted from the one welded material side. .

General friction stir welding conditions such as tool rotation speed, movement speed, insertion amount and applied load may be appropriately set from the viewpoint of defect formation in the stir zone, welding efficiency, and the like. Moreover, the shape of the tool is not particularly limited as long as the effects of the present invention are not impaired, and various shapes of conventionally known friction stir welding tools can be used.

Further, in the method for joining metal materials according to the present invention, it is preferable that the other material to be joined is a magnesium-lithium alloy. By using a magnesium-lithium alloy as both the materials to be joined, the importance of the friction stir action on the magnesium-lithium alloy increases, so that the effects of the metal material joining method of the present invention can be obtained more remarkably. can be done.

In the method for joining metal materials according to the present invention, it is preferable that the surface of the friction stir welding tool is not grooved. In friction stir welding of aluminum alloys and magnesium alloys, tool wear and tool breakage are unlikely to occur, so groove processing such as threading is generally performed on the bottom surface of the shoulder portion and the side surface of the probe portion in order to increase the stirring effect of the tool. target. On the other hand, the adhesion of the magnesium-lithium-based alloy to the tool surface can be suppressed by not performing such groove processing on the tool. On the other hand, since the magnesium-lithium alloy has excellent plastic workability, it is possible to sufficiently flow the material even with a tool that does not have grooving, and to form a stir zone. The friction stir welding tool that is not grooved means a tool that has a planar shoulder bottom surface and probe side surface.

In the method for joining metal materials according to the present invention, it is preferable that the peripheral speed of the outermost periphery of the shoulder portion of the friction stir welding tool is 23.6 mm/s to 78.5 mm/s. This peripheral speed range is an uncommonly low speed range for friction stir welding of magnesium alloys, but sufficient friction stir action can be obtained when a magnesium-lithium alloy with room temperature workability is used as the material to be welded. can. Here, by setting the peripheral speed of the outermost periphery of the shoulder portion to 23.6 mm/s or more, it is possible to suppress the formation of defects due to insufficient stirring, and by setting it to 78.5 mm/s or less, it is possible to suppress the formation of defects on the tool surface. It is possible to suppress the formation of defects due to the adhesion of magnesium-lithium alloys, the surface oxidation of the stirring part, and the formation of a liquid phase.

Further, in the metal material joining method of the present invention, it is preferable that the peripheral speed of the outermost periphery of the probe portion of the friction stir welding tool is 9.5 mm/s to 31.4 mm/s. This peripheral speed range is an uncommonly low speed range for friction stir welding of magnesium alloys, but sufficient friction stir action can be obtained when a magnesium-lithium alloy with room temperature workability is used as the material to be welded. can. Here, by setting the peripheral speed of the outermost periphery of the probe part to 9.5 mm/s or more, it is possible to suppress the formation of defects due to insufficient stirring, and by setting it to 31.4 mm/s or less, it is possible to suppress the formation of defects on the tool surface. Formation of defects due to adhesion of the magnesium-lithium alloy can be suppressed.

Moreover, in the method for joining metal materials according to the present invention, it is preferable to use butt joining. In butt welding, the formation of a stir zone is more important than in lap joining. In the method of joining metal materials according to the present invention, a good stir zone can be formed even when a magnesium-lithium alloy is used as the material to be joined. Since it can be formed, efficient bonding can be achieved even if it is a butt bonding.

Friction stir welding for the purpose of joining and the friction stir process for the purpose of modification are basically techniques using the same principle, and the above-described method for joining metal materials of the present invention can be used to modify metal materials. It can also be used as a method. Specifically, considering the desired shape, size, position, etc. of the modified portion, friction stir may be performed using a tool having an appropriate shape and size.

In addition, the present invention
A joined body in which one member to be joined and the other member to be joined are joined via a stirrer,
At least one of the materials to be joined is a magnesium-lithium alloy,
The stirring part contains α-phase recrystallized grains of the magnesium-lithium alloy,
The crystal orientation of the recrystallized grains is randomized;
Also provided is a conjugate characterized by:

In the joined body of the present invention, since the stir zone is formed by friction stir welding at a relatively low temperature, the stir zone contains recrystallized grains of the α phase having the HCP structure and the β phase having the BCC structure. The recrystallized grains of the α phase are clearly refined compared to the base material.

In addition, the crystal orientation of the α-phase is randomized in the stirring part. Since the α-phase has an HCP structure, a strong texture is generally formed after undergoing a working process in which simple shear stress is applied, such as extrusion or rolling. On the other hand, the crystal orientation of the α-phase can be randomized by friction stirring under appropriate conditions using the bonding method of the present invention, resulting in a decrease in mechanical properties due to a strong texture and anisotropy. can be suppressed. Here, randomization means that a strong basal 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 are displayed only in the same system color. I wish I didn't. More specifically, in the pole figure, the Texture Intensity is preferably 1-10, more preferably 1-5.

In addition, in the joined body of the present invention, the structure of the stirred portion is finer than that of the base material, so that the joined body has higher hardness than that of the base material. In addition, since there is no heat-affected zone due to temperature rise during welding on the outer edge of the stirring part, there is no region where the hardness is lower than that of the base material.

Further, in the joined body of the present invention, it is preferable that the magnesium-lithium alloy region of the joined body has a minimum hardness of 50 HV or more, and the joint efficiency for the one joined material is 100%. is more preferred. By performing friction stir welding under appropriate conditions, a joined body having the above mechanical properties can be obtained.

Further, in the joined body of the present invention, the tensile strength of the stirring part is preferably 1.1 times or more, more preferably 1.3 times or more, the base material tensile strength of the one workpiece. , 1.5 times or more. Regarding the stir zone formed by general friction stir welding, the tensile strength of the stir zone may be slightly higher than the tensile strength of the base material under welding conditions where the heat input is relatively small. However, in the joined body of the present invention, the stir zone is formed under low heat input conditions, which is uncommon for friction stir welding of magnesium alloys, and the stir zone is nanostructured (the average crystal grain size of the α phase is less than 1 μm). By doing so, the tensile strength of the stirring part can be made 1.1 times or more the tensile strength of one of the members to be welded. In addition, the stirring part having the nanostructure exhibits excellent superplasticity, and is therefore preferable from the viewpoint of workability.

Furthermore, in the bonded body of the present invention, the other material to be bonded is preferably a magnesium-lithium alloy, more preferably a butt bonded body. By using a magnesium-lithium alloy as the materials to be joined together and forming a butt joint, the weight of the joined body can be extremely efficiently reduced.

The joined body of the present invention can be suitably manufactured by the method of joining metal materials of the present invention described above.

Furthermore, the present invention provides
A metal structure including a modified portion of a magnesium-lithium alloy,
The modified portion includes recrystallized grains of the α phase of the magnesium-lithium alloy,
The crystal orientation of the recrystallized grains is randomized;
Also provided is a metal structure characterized by:
Since the metal structure of the present invention has a modified portion containing randomized recrystallized grains of the α-phase, it has isotropically excellent mechanical properties and plastic workability.

In the metal structure of the present invention, it is preferable that the recrystallized grains have an average grain size of less than 1 μm. By setting the average grain size of the recrystallized grains to less than 1 μm, the modified portion can have higher hardness and strength, and can be imparted with excellent superplasticity.

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

According to the present invention, there is provided a simple and efficient friction stir welding method in the case where at least one of the materials to be welded is a magnesium-lithium alloy, in which the strength and plastic workability are imparted to the stir zone greater than or equal to those of the base material. It is possible to provide a bonding method capable of achieving the above bonding, and a bonded body obtained by the bonding method. Further, according to the present invention, a modification method capable of imparting strength and plastic workability equal to or greater than those of the base material to any region of a magnesium-lithium alloy, and a metal structure obtained by the modification method are provided. You can also

BRIEF DESCRIPTION OF THE DRAWINGS It is a schematic diagram which shows one aspect|mode of the joining method of the metal material of this invention. FIG. 3 is a schematic cross-sectional view of the vicinity of the joint in the joined body of the present invention; 1 is a cross-sectional photograph of a joint obtained in an example. 4 is a hardness distribution of a joint section obtained in an example. 1 shows tensile properties of joints obtained in Examples. 4 is a general view photograph of a stirrer test piece after evaluation of plastic deformation characteristics. 1 is a SEM photograph of the center of a stirring part obtained in a base material and an example. 2 is an orientation map image of the α phase at the center of the stir zone obtained in the base material and the example.

Hereinafter, representative embodiments of the method for joining metal materials 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 thereto. In the following description, the same or corresponding parts are denoted by the same reference numerals, and redundant description may be omitted. Also, since the drawings are for the purpose of conceptually explaining the present invention, the dimensions and ratios of the depicted components may differ from the actual ones.

(1) Method for Joining Metal Materials In the method for joining metal materials according to the present invention, friction stir welding is used. Friction stir welding is called FSW (Friction Stir Welding), in which the ends of two metal materials to be welded are butted against each other, and a projection (probe) provided at the tip of a rotary tool is pressed. It is a method of joining two metal members by inserting between both ends and rotating and moving a rotating tool along the longitudinal direction of these ends.

As described above, the method of joining metal materials according to the present invention comprises: (1) joining ends of metal plates together to form a joining portion; (2) spot welding in which the ends of metal plates are butted against each other to form a joint, and a rotating tool is rotated at the joint without moving it to join them; (3) metal plates are joined at the joint spot welding in which the metal plates are joined together by inserting a rotating tool into the joint and rotating the rotary tool without moving it at the spot, (4) overlapping the metal plates at the joint, and Insert a rotating tool into the joint and move the rotating tool along the longitudinal direction of the joint to join the metal plates together (1) to (4) including four aspects and combinations thereof However, hereinafter, as a representative mode, "(1) the ends of the metal plates are butted together to form a joint, and the rotating tool is rotated and moved along the longitudinal direction of the processed portion to join the metal plates together. "Joining" will be described in detail.

FIG. 1 is a schematic diagram showing one aspect of the method for joining metal materials according to the present invention. The material to be welded 2 (one material to be welded) and the material to be welded 2′ (the other material to be welded) are butted against each other, the rotated tool 4 is inserted into the desired welding area, and moved along the welding line. A junction 6 can be obtained by Here, when both the material to be bonded 2 and the material to be bonded 2' are made of a magnesium-lithium alloy, the tool 4 is basically inserted so that the center of the projecting portion (probe portion) 8 of the tool 4 is aligned with the butt line. . Further, when the material to be bonded 2 is a magnesium-lithium alloy and the material to be bonded 2' is another metal material, the metal material bonding method of the present invention uses a protrusion (probe) 8 that constitutes the tool 4. And most of the body portion (shoulder portion) 10 comes into contact with the workpiece 2 side.

As the tool 4, any one of a cemented carbide tool, a cermet tool, a ceramic tool, an intermetallic compound tool, and a ceramic coated tool can be used to obtain a magnesium-lithium-based Alloy adhesion can be effectively reduced. Here, by making the surface of the tool 4 inorganic and non-metallic, it is possible to reduce the affinity with the magnesium-lithium alloy. Moreover, as a result, the temperature of the friction stir welding can be lowered, and the stir portion 12 having a finer and more homogeneous structure and excellent in strength and plastic workability can be formed.

Here, the composition and structure of cemented carbide, cermet, ceramics and intermetallic compounds used as tool materials are not particularly limited as long as the effects of the present invention are not impaired, and conventionally known various compositions and structures can be used. Although possible, it is preferable to reduce the metallic bonding phase for cemented carbides and cermets. As ceramics, for example, silicon carbide, silicon nitride, sialon, boron nitride, zirconia, alumina, titanium diboride, etc. can be used. intermetallic compounds of the system can be used.

In the case of ceramic coating, the main body of the tool 4 may be made of metal, for example, hot work tool steel (SKD61). The composition, structure, thickness, etc. of the ceramic coating are not particularly limited as long as the effects of the present invention are not impaired, and various known compositions, structures, and thicknesses can be used. Various hard coatings known in the art can be used.

In addition, the above-mentioned tool material and ceramic coating are preferably selected from the viewpoint of wettability with the magnesium-lithium alloy, and the contact angle between the magnesium droplet and these materials is preferably 90 ° or more. . The torque applied to the tool 4 can be reduced by selecting a tool material and a ceramic coating that provide a contact angle of 90° or more.

FIG. 1 shows the case of using a tool 4 having a columnar projection (probe) 8 on the bottom surface of a columnar main body (shoulder) 10. The shape of the tool 4 is There are no particular limitations as long as the effects of the present invention are not impaired, and various conventionally known friction stir welding tool shapes can be used. Further, it is preferable that the bottom surface of the main body portion (shoulder portion) 10 and the side surface of the protruding portion (probe portion) 8 contacting the workpieces 2 and 2' do not have groove processing such as screw processing. Adhesion of the magnesium-lithium alloy to the surface of the tool 4 can be suppressed by not performing the groove processing on the tool. On the other hand, since the magnesium-lithium alloy has excellent plastic workability, it is possible to sufficiently form the stirring portion 12 even with a tool that does not have groove processing.

The magnesium-lithium alloy used as at least one of the materials to be joined in the metal material joining method of the present invention is a broadly used alloy containing magnesium as a main component and lithium added to impart plastic workability at room temperature. includes. For example, the alloy includes those to which aluminum, zinc, manganese, yttrium, lanthanides, zirconium, silver, silicon, calcium, etc. are added in order to improve strength and heat resistance.

Here, lithium is preferably contained within the range of 5 to 15% by weight. If the lithium content is less than 5% by weight, the plastic workability at room temperature is not significantly improved, and if the lithium content exceeds 15% by weight, intergranular cracks (surface cracks) may occur. Also, since lithium is expensive, the cost is high.

Also, the other member to be joined 2' is preferably a magnesium-lithium alloy. By using a magnesium-lithium alloy for both the materials 2 and 2' to be joined, the importance of the friction stir action on the magnesium-lithium alloy increases, so that the effect of the metal material joining method of the present invention becomes more pronounced. can get to

General friction stir welding conditions such as rotation speed, movement speed, insertion amount and applied load of the tool 4 may be appropriately set from the viewpoint of defect formation in the stirrer 12 and welding efficiency.

Here, it is preferable that the peripheral speed of the outermost circumference of the body portion (shoulder portion) 10 of the tool 4 is 23.6 mm/s to 78.5 mm/s. This peripheral speed range is an uncommonly low speed range in friction stir welding of magnesium alloys, but when magnesium-lithium alloys having room temperature workability are used as the materials 2, 2' to be welded, sufficient friction stir action can be achieved. Obtainable. Here, by setting the peripheral speed of the outermost periphery of the main body portion (shoulder portion) 10 to 23.6 mm/s or more, it is possible to suppress the formation of defects due to insufficient stirring, and it is set to 78.5 mm/s or less. can suppress the formation of defects due to the adhesion of the magnesium-lithium alloy to the surface of the tool 4, the oxidation of the surface of the stirring part 12, and the generation of liquid phase. When the diameter of the main body (shoulder portion) 10 of the tool 4 is 15 mm, the peripheral speed of the outermost periphery is approximately 23.6 mm/s when the rotation speed is 30 rpm, and approximately 78.5 mm when the rotation speed is 100 rpm. /s.

Further, it is preferable that the peripheral speed of the outermost periphery of the protruding portion (probe portion) 8 of the tool 4 is 9.5 mm/s to 31.4 mm/s. This peripheral speed range is an uncommonly low speed range in friction stir welding of magnesium alloys, but when magnesium-lithium alloys having room temperature workability are used as the materials 2, 2' to be welded, sufficient friction stir action can be achieved. Obtainable. Here, by setting the peripheral speed of the outermost periphery of the protrusion (probe portion) 8 to 9.5 mm/s or more, it is possible to suppress the formation of defects due to insufficient stirring, and it is set to 31.4 mm/s or less. , it is possible to suppress the formation of defects due to the adhesion of the magnesium-lithium alloy to the surface of the tool 4 . When the diameter of the protruding portion (probe portion) 8 of the tool 4 is 6 mm, the peripheral speed of the outermost circumference is approximately 9.5 mm/s when the rotational speed is 30 rpm, and approximately 31.4 mm when the rotational speed is 100 rpm. /s.

In the butt joint, the formation of the stir portion 12 is more important than in the lap joint. Since it is possible to form a good stirring portion 12 even at the highest temperature, the bonding can be efficiently achieved by using the bonding method of the present invention.

(2) Joined Body FIG. 2 shows a schematic cross-sectional view of the vicinity of the joined portion in the joined body of the present invention. Note that FIG. 2 shows a butt joint as a representative aspect of the joint in the joined body of the present invention.

A joined body 20 of the present invention is a joined body in which one member to be joined 2 and the other member to be joined 2' are joined together via a stirrer 12, and at least one member to be joined 2 is magnesium-lithium It is characterized by containing recrystallization of the α-phase in the region of the magnesium-lithium alloy in the stirring part 12 . The shape and size of the members 2 and 2' to be joined are not particularly limited as long as they do not impair the effects of the present invention, and may be joined by the joining method of the present invention.

A magnesium-lithium alloy has an α phase with an HCP structure and a β phase with a BCC structure. On the other hand, in the stirring part 12 as well, although the α-phase is distributed in a streaky macroscopic manner, the α-phase is recrystallized and is in a state in which equiaxed α-phases are aggregated.

The magnesium-lithium alloy used as at least one member 2 to be joined includes a wide range of alloys containing magnesium as a main component and lithium added to impart plastic workability at room temperature. For example, the alloy includes those to which aluminum, zinc, manganese, yttrium, lanthanides, zirconium, silver, silicon, calcium, etc. are added in order to improve strength and heat resistance.

Here, lithium is preferably contained within the range of 5 to 15% by weight. If the lithium content is less than 5% by weight, the plastic workability at room temperature is not significantly improved, and if the lithium content exceeds 15% by weight, intergranular cracks (surface cracks) may occur. Also, since lithium is expensive, the cost is high.

In the joined body 20, since the structure of the stirring portion 12 is finer than that of the base material, the joined body 20 has higher hardness than that of the base material. Further, since there is no heat-affected zone along the outer edge of the stirring part 12 due to temperature rise during welding, there is no region where the hardness is lower than that of the base material. The average grain size of recrystallized grains of the α-phase is preferably less than 5 μm, more preferably less than 3 μm, and most preferably less than 1 μm. By refining the recrystallized grains of the α-phase, not only can the hardness and strength of the stirring portion 12 be increased, but also good plastic deformability can be imparted.

In addition, it is preferable that the minimum hardness in the region of one of the joined materials 2 (magnesium-lithium alloy) of the joined body 20 is 50 HV or more. ) is 100%.

In addition, the tensile strength of the stirring part 12 is preferably 1.1 times or more, more preferably 1.3 times or more, and 1.5 times or more the tensile strength of one member 2 to be welded. is most preferred. Regarding the stir zone formed by general friction stir welding, the tensile strength of the stir zone may be slightly higher than the tensile strength of the base material under welding conditions where the heat input is relatively small. However, in the joined body 20, the stir portion 12 is formed under a low heat input condition that is uncommon for friction stir welding of magnesium alloys, and the stir portion 12 is nanostructured (the average crystal grain size of the α phase is less than 1 μm). Thus, the tensile strength of the stirring part 12 can be made 1.1 times or more the tensile strength of the one workpiece 2 . In addition, since the stirring part 12 having the nanostructure exhibits excellent superplasticity, it is also preferable from the viewpoint of workability.

In addition, in the joined body 20 , it is preferable that the crystal orientation of the α-phase is randomized in the recrystallized region of the stirring portion 12 . By friction stirring under appropriate conditions using the above-described bonding method of the present invention, the crystal orientation of the α phase can be randomized, and the deterioration and anisotropy of mechanical properties due to strong texture can be suppressed. can do.

Furthermore, in the joined body 20, the other joined material 2 is preferably a magnesium-lithium alloy, more preferably a butt joined body. By using a magnesium-lithium alloy as both the members 2 and 2' to be joined and forming a butt joined body, the weight of the joined body 20 can be reduced extremely efficiently.

Although representative embodiments of the present invention have been described above, the present invention is not limited to these, and various design changes are possible, and all such design changes are included in the technical scope of the present invention. be Note that 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≫
LZ91 magnesium alloy (9 wt % Li-1 wt % Zn-Mg Bal.) plates of 200 mm in length, 65 mm in width and 3 mm in thickness were butted against each other in the arrangement shown in FIG. 1, and a joined body was obtained by friction stir welding. 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 was used as a tool for friction stir welding. Friction stir welding was performed with tool position control under the conditions of No shielding gas was used during the friction stir welding, and groove processing was not performed on the bottom of the shoulder portion and the side surface of the probe portion of the tool.

≪Comparative example≫
Friction stir welding was performed in the same manner as in Example 1, except that the tool was made of hot work tool steel (SKD61).

[Appropriate bonding conditions]
In order to confirm suitable bonding conditions for obtaining a good bonded body, the surface and cross section of the bonded portion (stirred portion) were observed with an optical microscope. × if defects are formed in the stir zone, △ if no defects are formed but a large amount of burrs and surface roughness are observed, and if no defects are formed and the surface condition of the stir zone is smooth The results of Examples are shown in Table 1. In addition, since defects were formed under all conditions in the comparative example (evaluation of x under all conditions), the results are not shown.

As an example of the cross-sectional observation results of the joint, FIG. 3 shows a cross-sectional photograph of the joint obtained in the example (tool rotation speed: 200 rpm, tool movement speed: 100 mm/min). Formation of defects was not observed, and a good stirring part was obtained.

In the example, small defects were formed in the stirrer at the slowest rotation speed of 25 rpm, but no defects were formed under other conditions, and the surface of the stirrer was in good condition. . From the results, it can be seen that good joints can be obtained under a wide range of joining conditions with the joining method of the present invention. It should be noted that when the rotational speed is higher than 150 rpm, the metallic luster of the surface of the stirrer is lost, and the reason for this is thought to be oxidation or the formation of a liquid phase due to the rise in the bonding temperature. By using argon gas as the shielding gas, it was possible to obtain a good stirrer surface with a metallic luster. On the other hand, in the comparative example, the material to be welded adhered to the tool surface under all welding conditions, and a groove-like defect was formed in the stir zone.

[Adhesion of material to be welded to tool surface]
With regard to the welding conditions of a tool rotation speed of 100 rpm and a tool moving speed of 100 mm/min, the tool surface condition after friction stir welding was visually observed. In addition, as tools that can be used in the present invention, silicon nitride tools, silicon carbide tools, sialon tools, cermet tools, and tools with a TiAl coating formed on the surface of the tool steel main body have the same friction. Stir welding was performed, and the surface condition of the tool after the welding was visually observed.

In the tool steel tool used as a comparative example, the material to be welded adhered to the side surface of the probe portion and the bottom surface of the shoulder portion, and it was difficult to grasp the original shape of the tool. On the other hand, in the other tools, although there were portions where coloring due to the material to be welded was observed on the side surface of the probe portion and the bottom surface of the shoulder portion, no significant adhesion was confirmed.

[Hardness measurement]
In the comparative example, groove-like defects were formed in the stir zone under all joining conditions, so the Vickers hardness measurement was performed on the cross section of the joined zone obtained in the example. The Vickers hardness measurement was performed under the conditions of a load of 0.1 kgf and a load application time of 15 seconds.

FIG. 4 shows the hardness distribution (horizontal direction of the joint at the plate thickness center) of the cross section of the joint obtained at a tool moving speed of 100 mm/min and a tool rotating speed of 100 to 300 rpm. The hardness of all the stirring parts is higher than that of the base material, and the increase in hardness of the stirring parts is particularly noticeable at lower tool rotation speeds. In addition, no softened region is observed on the outer edge of the stirrer.

[Tensile test]
In the comparative example, groove-like defects were formed in the stir zone under all welding conditions, so the tensile properties of the joints obtained in the examples were evaluated. For the tensile test, a tensile test piece having a parallel portion (the center of the parallel portion is the abutting surface) of length 50 mm, width 12 mm, and thickness 2 mm was used, and the plate width direction of the joint was taken as the tensile axis. The tensile speed was set to 1 mm/min.

FIG. 5 shows the tensile properties of joints obtained at a tool moving speed of 100 mm/min and a tool rotating speed of 30 to 100 rpm. For comparison, the tensile test results of the base material are shown. In addition, in order to evaluate the tensile properties of the stirring part, a small tensile sample having a parallel part of 4 mm in length × 2 mm in width × 2 mm in thickness was extracted from the stirring part obtained at a tool moving speed of 100/min and a tool rotating speed of 30 rpm. A test piece was prepared, and the results of evaluation at a tensile speed of 0.08 mm/min with the joining direction as the tensile axis are also shown.

The joints obtained in the examples have tensile properties equivalent to those of the base material, and the joint efficiency is 100%. Moreover, the tensile properties of the stirring part are significantly higher than those of the base material, and the tensile strength is about twice that of the base material.

[Evaluation of plastic deformation characteristics]
A small tensile test piece having a parallel portion with a length of 4 mm, a width of 2 mm, and a thickness of 2 mm was prepared from the base material and the stirring part obtained in the example at a tool moving speed of 100 / min and a tool rotating speed of 30 rpm. , 200° C., and a strain rate of 3×10 ⁻⁴ , the amount of tensile deformation was evaluated.

The amounts of deformation of the base material and the stirrer until breakage were approximately 150% and approximately 1100%, respectively, confirming that the stirrer was endowed with extremely remarkable superplastic deformability. FIG. 6 shows an overview photograph of the stirrer test piece after evaluation. The 4 mm long parallel portion is approximately 44 mm when broken.

[Microstructure Observation]
In order to confirm the grain size and shape of the crystal grains in the stirred portion, SEM observation and EBSD measurement of the cross section of the joint portion were performed. For SEM observation and EBSD measurement, FE-SEM (JSM-7001FA manufactured by JEOL Ltd.) and OIM data Collection ver 5.31 manufactured by TSL were used.

FIGS. 7 and 8 show SEM photographs of the center of the stir zone and orientation map images of the α-phase obtained with the base material and the example (tool moving speed of 100 mm/min, tool rotating speed of 100 to 300 rpm). The base material is composed of α phase and β phase, and α phase is distributed in streaks. On the other hand, in the stirred part obtained in the example, the α-phase was divided to form equiaxed recrystallized grains. In addition, the α-phase of the base material has a strong texture, while the α-phase of the stir zone is randomly oriented. In addition, the crystal grain size of the stirring part is made finer than that of the base material, and this is more remarkable under the condition that the tool rotation speed is low. The average crystal grain size of the α phase is less than 5 μm at 300 rpm, less than 3 μm at 200 rpm, and less than 1 μm at 100 rpm.

2, 2' ... materials to be joined,
4... tools,
6 ... junction,
8 ... Protruding portion (probe portion),
10 ... body portion (shoulder portion),
12... Stirrer,
20 ... Joined body.

Claims (14)

A joined body in which one member to be joined and the other member to be joined are joined via a stirrer,
At least one of the materials to be joined is a magnesium-lithium alloy,
The stirring part contains α-phase recrystallized grains of the magnesium-lithium alloy,
The crystal orientation of the recrystallized grains is randomized;
A zygote characterized by There is no region softened more than the base material on the outer edge of the stirring part;
The joined body according to claim 1, characterized by: The minimum hardness in the region of the magnesium-lithium alloy is 50 HV or more,
The joined body according to claim 1 or 2, characterized by: The tensile strength of the stirring part is 1.1 times or more the tensile strength of the one workpiece,
The joined body according to any one of claims 1 to 3, characterized by: the other material to be joined is a magnesium-lithium alloy;
The conjugate according to any one of claims 1 to 4, characterized by: being a butt zygote;
The conjugate according to any one of claims 1 to 5, characterized by: A metal structure including a modified portion of a magnesium-lithium alloy,
The modified portion includes recrystallized grains of the α phase of the magnesium-lithium alloy,
The crystal orientation of the recrystallized grains is randomized;
A metal structure characterized by: The average grain size of the recrystallized grains is less than 1 μm,
The metal structure according to claim 7, characterized by: A method for friction stir welding one material to be welded and the other material to be welded, comprising:
At least one of the materials to be joined is a magnesium-lithium alloy,
Using any one of a cemented carbide tool, a cermet tool, a ceramic tool, an intermetallic compound tool and a ceramic coated tool as a friction stir welding tool,
inserting the friction stir welding tool into the one workpiece side;
A friction stir welding method for metal materials, characterized by: the other material to be joined is a magnesium-lithium alloy;
The friction stir welding method for metal materials according to claim 9, characterized in that: groove processing is not performed on the surface of the friction stir welding tool;
The friction stir welding method for metal materials according to claim 9 or 10, characterized in that: setting the peripheral speed of the outermost circumference of the shoulder portion of the friction stir welding tool to 23.6 mm/s to 78.5 mm/s;
The friction stir welding method for metal materials according to any one of claims 9 to 11, characterized in that: setting the peripheral speed of the outermost periphery of the probe portion of the friction stir welding tool to 9.5 mm / s to 31.4 mm / s;
The friction stir welding method for metal materials according to any one of claims 9 to 12, characterized in that: being a butt joint;
The friction stir welding method for metal materials according to any one of claims 9 to 13, characterized by:

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