Friction stir welding tool and friction stir welding method

Abstract

To provide a tool for friction stir welding which also can be used for friction stir welding of thick steel plate as a tool for friction stir welding which can be used for friction stir welding of a thick plate having a plate thickness of 6 mm or more, and further to provide a friction stir welding method using the tool for friction stir welding.SOLUTION: A tool for friction stir welding is characterized in that the tool has a shoulder part and a probe part provided on a bottom surface of the shoulder part, the probe part having a length of 5.5 mm or more and comprising base material of ceramic containing silicon nitride or sialon as a main phase.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a friction stir welding tool and a friction stir welding method. [Background technology]

[0002] Friction stir welding (FSW) is known as a typical solid-state welding method for metal materials. In friction stir welding, the metal materials to be joined are placed facing each other at the joint, a probe attached to the tip of a rotating tool is inserted into the joint, and the rotating tool is rotated and moved along the joint interface, and the metal materials are made to flow by the frictional heat and the stirring force of the rotating tool, thereby joining the two metal materials. Friction stir welding is characterized by the fact that the maximum temperature reached during joining does not reach the melting point of the joined materials, and the strength loss at the joint is smaller than that of conventional fusion welding, and it has been rapidly put into practical use in recent years.

[0003] However, while friction stir welding has various excellent characteristics, it requires the pressing of a tool stronger than the materials to be welded, and a large stress is applied to the tool, so that the cost and life of the tool become a major problem depending on the materials to be welded. Specifically, when joining thin plates of relatively soft metals such as aluminum and magnesium, the load on the tool is small and there are no particular problems with the tool life and joining conditions, but when joining high-melting-point metals such as steel and titanium, the tool life becomes extremely short. In particular, this becomes a more serious problem when joining thick plates, and in reality, there are almost no tools that can be used industrially, taking into account the joining cost and the range of joining conditions that can obtain a good joint.

[0004] For example, Patent Document 1 (JP Patent Publication No. 2004-522591) proposes a friction stir welding tool equipped with a probe having a shaft and a taper, the probe having a plurality of helically twisted surfaces extending from the base to the tip of the probe, and the diameter of the probe in a longitudinal cross section of the probe continuously decreasing from the base to the tip of the probe.

[0005] The friction stir welding tool described in the above-mentioned patent document is said to be capable of welding thicker workpieces than previously attempted, to reduce the formation of defects, and to maintain the workpieces at an appropriate temperature during joining.

[0006] Furthermore, Patent Document 2 (JP Patent Publication No. 2003-532543) proposes a friction stir welding tool capable of friction stir welding metal matrix composites (MMCs), ferrous alloys, non-ferrous alloys, and superalloys, the friction stir welding tool including a shaft, a shoulder, and a pin, and a highly wear-resistant material disposed on at least a portion of the shoulder and the pin, the shoulder being mechanically fixed to the shaft to prevent rotational motion of the shoulder relative to the shaft, the highly wear-resistant material having a first phase and a second phase, and the highly wear-resistant material being manufactured under ultra-high temperature and ultra-high pressure, and the friction stir welding tool capable of functionally friction stir welding MMCs, ferrous alloys, non-ferrous alloys, and superalloys, and discloses that polycrystalline boron nitride (PCBN) or polycrystalline diamond (PCD) is used as the highly wear-resistant material.

[0007] The friction stir welding tool described in Patent Document 2 above is said to be capable of friction stir welding materials that cannot be joined using conventional friction stir welding methods and tools, i.e., materials including iron alloys such as stainless steel and high melting point superalloys that contain small amounts of iron or no iron. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Special Publication No. 2004-522591 [Patent Document 2] Special Publication No. 2003-532543 Summary of the Invention [Problem to be solved by the invention]

[0009] However, the friction stir welding tool disclosed in the above Patent Document 1 has multiple helical twisted surfaces, and it is difficult to make it into a ceramic tool. In addition, since the tool has a complex, elongated shape, it easily breaks when a large stress is applied to the tool, such as in friction stir welding of steel materials. This is a serious problem, especially when performing friction stir welding on thick plates.

[0010] In addition, since the friction stir welding tool disclosed in Patent Document 2 is manufactured under ultra-high temperature and ultra-high pressure conditions, it is extremely difficult to manufacture a large tool for thick plates. In addition, the manufacturing cost increases significantly with an increase in the tool size, resulting in an expensive tool that is unacceptably expensive for general industrial use.

[0011] In view of the problems in the conventional technology as described above, an object of the present invention is to provide an inexpensive friction stir welding tool that can be used for friction stir welding of thick plates having a thickness of 6 mm or more and that can also be used for friction stir welding of thick steel plates, and further to provide a friction stir welding method using the friction stir welding tool. [Means for solving the problem]

[0012] In order to achieve the above-mentioned object, the inventors conducted extensive research into the shape and material of friction stir welding tools, and discovered that it is extremely effective to use a ceramic having silicon nitride or sialon as a main phase as the base material and to optimize the size and shape of the probe and shoulder portions, thereby arriving at the present invention.

[0013] That is, the present invention provides: Shoulder part, A probe portion provided on a bottom surface of the shoulder portion, The length of the probe portion is 5.5 mm or more, The substrate is made of ceramics having silicon nitride or sialon as the main phase. The present invention provides a friction stir welding tool, comprising:

[0014] The shapes of the shoulder and probe are not particularly limited as long as they do not impair the effects of the present invention, and may be various shapes known in the art. Here, since the friction stir welding tool of the present invention is used for friction stir welding of plate materials having a thickness of 6 mm or more, the length of the probe is 5.5 mm or more. Although it depends on the tool shape and friction stir welding conditions, a certain degree of stirring region is formed below the bottom surface of the probe, so that a defect-free joint can be obtained even with a probe length slightly shorter than the plate thickness.

[0015] The friction stir welding tool of the present invention uses ceramics with silicon nitride or sialon as the main phase as the base material. The main constituent elements of silicon nitride and sialon are Si and N, which are abundant and inexpensive resources, and since no special sintering equipment that applies ultra-high temperature and ultra-high pressure is required, the manufacturing cost per piece can be reduced as the number of pieces manufactured increases. In addition, compared to polycrystalline boron nitride (PCBN) and polycrystalline diamond (PCD), it is easier to process and various shapes can be given to the tool.

[0016] By using ceramics having silicon nitride or sialon as the main phase as the base material, it is possible to realize a tool having strength and durability sufficient to perform friction stir welding on high melting point metals such as steel. In addition, due to the low thermal conductivity, the friction stir welding tool has a high heat storage property, the welding temperature is easily increased, and friction stir welding can be achieved in a relatively short time.

[0017] In the friction stir welding tool of the present invention, it is preferable that the diameter of the probe portion continuously decreases from the bottom surface toward the tip, and that the probe portion is not grooved and / or chamfered. By tapering the probe portion from a thick base to a thin tip, it is possible to suppress breakage from the base of the probe portion and to suppress the formation of defects in the vicinity of the probe tip. It is difficult to generate sufficient plastic flow in the vicinity of the probe tip, but by reducing the probe diameter in this region, the space generated by the passage of the probe becomes smaller, and the formation of defects in this region is suppressed. Furthermore, by tapering the probe portion, material flow can be generated downward in the plate thickness direction, which is effective in forming a good stir portion, especially in friction stir welding of thick plates.

[0018] When the plastic flow generated by the interaction between the tool surface and the workpiece is insufficient, it is necessary to improve the stirring force and promote the plastic flow by groove processing (thread processing) or chamfering the probe part. However, when such processing is performed, the probe part is likely to be damaged due to stress concentration on the processed part. In response to this, the inventors conducted numerous experiments and found that in a friction stir welding tool using a ceramic base material having silicon nitride or sialon as the main phase, the temperature of the workpiece near the tool surface is likely to rise, and even with an unprocessed probe, sufficient plastic flow is generated to form a defect-free stir part, and a good joint can be obtained even with friction stir welding of thick plates.

[0019] In addition, in the friction stir welding tool of the present invention, it is preferable that the diameter of the shoulder portion is 20 mm or less. As the length of the probe portion increases, the diameter of the shoulder portion also needs to be increased, and friction stir welding tools for thick plates are generally large. This is because it is difficult to generate sufficient plastic flow up to the vicinity of the back surface in friction stir welding of thick plates, and a larger shoulder is required to promote plastic flow. However, when ceramics having silicon nitride or sialon as the main phase are used as the base material, sufficient plastic flow occurs on the surface of the probe portion, so even if the diameter of the shoulder portion is 20 mm or less, a good stir portion can be formed even when the plate thickness of the joined material is 6 mm or more. By making the diameter of the shoulder portion 20 mm or less, the tool can be prevented from becoming large, and the price of the tool can be reduced. Since the price of ceramics is greatly affected by the size, the cost reduction effect of miniaturizing the shoulder portion is extremely large.

[0020] In the friction stir welding tool of the present invention, the length of the probe portion is preferably 9.5 mm or more. A more preferable probe length is 11.5 mm or more, and a most preferable probe length is 14.5 mm or more. The friction stir welding tool of the present invention can be used for steel plates with a plate thickness of 12 mm or more, and can also be used for steel plates with a plate thickness of 15 mm or more. Note that the "plate thickness" corresponds to the thickness into which the friction stir welding tool is inserted when overlapping plate materials and friction stir welding is performed.

[0021] Furthermore, in the friction stir welding tool of the present invention, the ceramic preferably contains rare earth and aluminum. By containing rare earth and aluminum, it is possible to maintain wear resistance while maintaining sinterability. More preferably, it is preferable that the ceramic contains 1.5 to 6 wt% aluminum and 1.5 to 10 wt% rare earth.

[0022] The present invention also provides a friction stir welding method, characterized in that plate materials are friction-stir-welded using the friction stir welding tool of the present invention, and a shoulder portion of the friction stir welding tool is brought into contact with the plate material while inserting a probe portion of the friction stir welding tool into the plate material by 5.5 mm or more. As described above, the friction stir welding tool of the present invention is inexpensive even if the probe length is 5.5 mm or more, and the cost of thick plate welding using friction stir welding can be significantly reduced. In addition, since the welding temperature rises quickly and plastic flow of the welded material is likely to occur near the tool surface, a good joint can be obtained in a short welding time. Here, since the friction stir welding tool of the present invention is prone to heat accumulation, it is preferable to use a tool holder that has both heat resistance and strength for holding the tool. Specific materials that can be used include cemented carbide, nickel-based superalloy, cobalt-based superalloy, and various heat-resistant steels.

[0023] In the friction stir welding method of the present invention, the plate material is preferably a steel plate, and more preferably the carbon content of the steel plate is 0.2 mass% or more. The friction stir welding tool of the present invention has excellent heat resistance and high mechanical properties, and can be suitably used for friction stir welding of steel plates. Furthermore, the material flow resistance of steel decreases with an increase in carbon content, and by setting the carbon content to 0.2 mass% or more, the amount of wear on the surface of the friction stir welding tool can be reduced. Here, since it is extremely difficult to melt weld steel with a carbon content of 0.2 mass% or more, the merit of using friction stir welding, which is a solid-phase welding, is great.

[0024] In addition, in the friction stir welding method of the present invention, it is preferable to hold the position of the friction stir welding tool for a certain time after inserting the probe part into the plate material, and move the friction stir welding tool laterally after the reaction force from the plate material applied to the friction stir welding tool has decreased. Usually, in friction stir welding of thick plates, the Z-axis direction (vertical) load is maximum when the friction stir welding tool is inserted. Here, the friction stir welding tool of the present invention has a smaller thermal conductivity than a general friction stir welding tool, and can efficiently heat and soften the workpiece material near the tool surface. As a result, by inserting the friction stir welding tool and holding it at the insertion position, the Z-axis direction load can be significantly reduced. In addition, by moving the friction stir welding tool laterally after reducing the Z-axis direction load, damage to the tool and lifting of the tool can be suppressed.

[0025] Furthermore, in the friction stir welding method of the present invention, it is preferable to arrange a back plate having a lower thermal conductivity than the plate on the back surface of the plate. By arranging a back plate having a lower thermal conductivity than the plate on the back surface of the plate, the temperature in the vicinity of the back surface of the plate during friction stir welding can be increased, and the load in the Z-axis direction of the friction stir welding tool can be significantly reduced. In addition, although the temperature distribution in the plate thickness direction becomes non-uniform as the thickness of the plate increases, the temperature distribution can be made uniform, and the microstructure in the plate thickness direction in the stir portion can be homogenized. Here, a back plate made of silicon nitride can be suitably used. Effect of the Invention

[0026] According to the present invention, there is provided an inexpensive friction stir welding tool that can be used for friction stir welding of thick plates having a thickness of 6 mm or more and that can also be used for friction stir welding of thick steel plates, and further, there is provided a friction stir welding method using the friction stir welding tool. [Brief description of the drawings]

[0027] [Figure 1] 1 is a schematic side view showing an example of a friction stir welding tool of the present invention. [Diagram 2] FIG. 1 is a schematic diagram showing a general friction stir welding situation. [Diagram 3] 1 is a photograph showing the appearance of a silicon nitride tool used in Example 1. [Figure 4] 2 is a cross-sectional macrophotograph of a stir zone of low carbon steel obtained in Example 1. [Diagram 5] 1 is a cross-sectional macrophotograph of a stir portion of medium carbon steel obtained in Example 1. [Figure 6] 1 is a photograph showing the appearance of a silicon nitride tool used in Example 2. [Figure 7] 1 is a cross-sectional macrophotograph of a stir zone of low carbon steel obtained in Example 2. [Figure 8] 1 is a cross-sectional macrophotograph of a stir portion of a medium carbon steel obtained in Example 2. [Figure 9] 1 is a cross-sectional macrophotograph of the stirring section obtained in Example 2 (influence of back plate material). [Figure 10] The microstructure of the stir zone obtained in Example 2 (effect of back plate material) [Figure 11] FIG. 11 is a diagram showing a tool Z-axis load in the second embodiment. [Figure 12] 4 shows the tool wear amount in Example 2 (low carbon steel plate). [Figure 13] 4 shows the tool wear amount in Example 2 (medium carbon steel plate). [Figure 14] 1 is a photograph showing the appearance of a silicon nitride tool used in Example 3. [Figure 15] 1 is a cross-sectional macrophotograph of a stir portion of a medium carbon steel obtained in Example 3. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

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

[0029] (1) Friction stir welding tools FIG. 1 shows a schematic side view of an example of the friction stir welding tool of the present invention. The friction stir welding tool 1 of the present invention has a body 4 having a shoulder 2 and a probe 6 provided on the bottom surface of the body 4, and the length of the probe 6 is 5.5 mm or more. The friction stir welding tool 1 uses a ceramic having silicon nitride or sialon as a main phase as a base material. Here, the shoulder 2 and the probe 6 may be ceramics having the same composition and structure, or may have different compositions and / or structures. The surface of the shoulder 2 and / or the probe 6 may be covered with a hard film formed by PVD, CVD, or the like.

[0030] Here, since the friction stir welding tool 1 is used for friction stir welding of plate materials having a thickness of 6 mm or more, the length of the probe portion 6 is 5.5 mm or more. Although it depends on the tool shape and friction stir welding conditions, a certain degree of stirring region is formed below the bottom surface of the probe portion 6, so a defect-free joint can be obtained even with a probe length slightly shorter than the plate thickness. The length of the probe portion 6 is preferably 9.5 mm or more, more preferably 11.5 mm or more, and most preferably 14.5 mm or more.

[0031] It is also preferable that the diameter of the probe portion 6 continuously decreases from the bottom surface of the shoulder portion 2 toward the tip of the probe portion 6, and that no groove and / or chamfering is performed on the probe portion 6. The taper angle of the probe portion 6 is not particularly limited as long as it does not impair the effects of the present invention, but for example, when the length of the probe portion 6 is 5.75 mm, the diameter at the base can be 7 mm and the diameter at the tip can be 6 mm.

[0032] It is preferable that the surface of the probe portion 6 is not grooved and / or chamfered. By having a smooth surface state without such processing, even if the probe portion 6 has a base material made of ceramics having silicon nitride or sialon as a main phase, damage and wear during friction stir welding can be suppressed. The surface roughness of the probe portion 6 is preferably 0.1 μm to 1.0 μm in terms of Sa. On the other hand, by using a base material made of ceramics having silicon nitride or sialon as a main phase, the materials to be joined can be efficiently heated and stirred, and even if the probe portion 6 has not been grooved and / or chamfered, a sufficient stirring portion can be formed.

[0033] The shape and size of the shoulder portion 2 are not particularly limited as long as they do not impair the effects of the present invention, but the diameter is preferably 20 mm or less. When the substrate is a ceramic having silicon nitride or sialon as the main phase, sufficient plastic flow occurs on the surface of the probe portion 6, so that even if the diameter of the shoulder portion 2 is 20 mm or less, a good stir portion can be formed even if the plate thickness of the workpiece is 6 mm or more. By making the diameter of the shoulder portion 2 20 mm or less, the friction stir welding tool 1 can be prevented from becoming large, and the cost can be reduced. In addition, the width of the surface of the joint can be narrowed.

[0034] It is also preferable that no grooves are formed on the surface of the shoulder portion 2. By making the surface of the shoulder portion 2 smooth, the amount of wear during friction stir welding can be reduced, and deformation of the shoulder portion 2 can be suppressed. On the other hand, by using ceramics having silicon nitride or sialon as the main phase as the base material for the shoulder portion 2, the materials to be joined can be efficiently heated and stirred, and even if no grooves are formed on the surface of the shoulder portion 2, a sufficient stirring portion can be formed.

[0035] The ceramics having silicon nitride or sialon as a main phase, which is the base material of the friction stir welding tool 1, is not particularly limited as long as it does not impair the effects of the present invention, and may be various conventionally known silicon nitride ceramics or sialon ceramics. Here, the ceramics preferably contain rare earth and aluminum. By containing rare earth and aluminum, it is possible to maintain wear resistance while maintaining sinterability. More preferably, it is preferable that the ceramic contains 1.5 to 6 wt% aluminum and 1.5 to 10 wt% rare earth. In addition, sintering aids may be contained, but the amount is preferably small.

[0036] (2)Friction stir welding The friction stir welding method of the present invention is characterized in that plate materials are friction-stir-welded using a friction stir welding tool 1, and the probe portion 6 is inserted into the plate material by 5.5 mm or more while the shoulder portion 2 is brought into contact with the plate material. Figure 2 shows the general state of friction stir welding. The workpiece is placed on a back plate, and the rotating friction stir welding tool 1 is pressed into the workpiece from the surface side to generate plastic flow. The friction stir welding tool 1 is inexpensive even if the probe length is 5.5 mm or more, and the cost of joining thick plates using friction stir welding can be significantly reduced. In addition, the joining temperature rises quickly and plastic flow of the workpiece is likely to occur near the tool surface, so that a good joint can be obtained in a short joining time while suppressing damage and wear during friction stir welding.

[0037] The plate material to be welded is preferably a steel plate, and the carbon content of the steel plate is more preferably 0.2 mass% or more, and most preferably 0.4 mass% or more. As the carbon content of the steel plate increases, the hardness and strength at room temperature increases, but in the welding temperature range of friction stir welding, the material flow stress decreases as the carbon content increases. That is, by using steel with a relatively high carbon content as the welded material, it is possible to suppress damage and wear of the friction stir welding tool 1, and to expand the range of appropriate welding conditions for obtaining a defect-free stir part. The friction stir welding tool 1, which has a ceramic base material with silicon nitride or sialon as the main phase, has mechanical properties that allow friction stir welding to thick steel plates, but is inferior in wear resistance to PCBN tools. However, by selecting a welded material with an appropriate carbon content, it is possible to extend the life of the friction stir welding tool 1.

[0038] When performing friction stir welding, it is preferable to hold the position of the friction stir welding tool 1 for a certain period of time after inserting the probe portion 6 into the plate material, and to laterally move the friction stir welding tool 1 after the reaction force from the plate material applied to the friction stir welding tool 1 has decreased. Here, the main methods of controlling friction stir welding include tool position control, load control, and torque control, but in order to hold the position of the friction stir welding tool 1 for a certain period of time, it is preferable to use position control. In addition, in friction stir welding of thick steel plates, the maximum load in the Z-axis direction applied to the friction stir welding tool 1 may be 10 tons or more in some cases, so it is preferable to use a friction stir welding device with high rigidity that can withstand the reaction force.

[0039] In addition, it is preferable to use a material with a lower thermal conductivity than the materials to be joined for the back surface. By disposing a back plate with a lower thermal conductivity than the materials to be joined on the back surface of the materials to be joined, the temperature near the back surface of the materials to be joined during friction stir welding can be increased, and the load in the Z-axis direction of the friction stir welding tool 1 can be significantly reduced. In addition, although the temperature distribution in the thickness direction becomes non-uniform as the thickness of the plate material increases, the temperature distribution can be made uniform, and the microstructure in the thickness direction in the stir portion can be homogenized. Here, from the viewpoint of thermal conductivity, zirconia, mullite, cermet, alumina, sialon, silicon nitride, etc. can be used as the material for the back plate, but from the viewpoint of strength and durability as a back plate, it is preferable to use silicon nitride or sialon.

[0040] Representative embodiments of the present invention have been described above, but the present invention is not limited to these, and various design modifications are possible, all of which are included in the technical scope of the present invention. EXAMPLES

[0041] Example 1 Using a silicon nitride tool with a probe length of 5.75 mm, friction stir welding was performed on 6 mm thick low and medium carbon steel plates using a start-in plate. The compositions of the low and medium carbon steel plates are shown in Table 1, and a photograph of the appearance of the silicon nitride tool is shown in Figure 3. The composition values ​​are in mass %. The shoulder diameter was 16 mm, the probe diameter at the base (bottom of the shoulder) was 7 mm, and the probe diameter at the tip was 6 mm. No grooves were applied to the bottom of the shoulder or the surface of the probe, resulting in a smooth surface. A backing plate made of tool steel was placed on the back side of the materials to be joined.

[0042] [Table 1]

[0043] The silicon nitride tool is a one-piece molded silicon nitride or sialon using oxides and / or nitrides of rare earth and aluminum as sintering aids, containing 1.5-10 wt% of rare earth and 1.5-6 wt% of aluminum, respectively.

[0044] The friction stir welding conditions for both low carbon steel plate and medium carbon steel plate were a tool rotation speed of 800 rpm and a tool movement speed of 50 mm / min. Friction stir welding was performed with constant tool position control. Cross-sectional macrophotographs of the obtained low carbon steel stir part and medium carbon steel stir part are shown in Figures 4 and 5, respectively. No defects were observed in either stir part, and it was confirmed that friction stir welding of 6 mm thick steel plate is possible using a silicon nitride tool.

[0045] When the tool torque during friction stir welding was measured, it was 40 Nm for the low carbon steel plate and 37 Nm for the medium carbon steel plate, the value being lower for the medium carbon steel plate. Furthermore, these torque values ​​are approximately 10% lower in each case than when friction stir welding was performed under the same welding conditions using a PCBN tool of the same shape as the silicon nitride tool used this time.

[0046] Example 2 Using a silicon nitride tool with a probe length of 9.75 mm, friction stir welding was performed on low carbon steel sheets and medium carbon steel sheets with a thickness of 10 mm using a start-in plate. A photo of the appearance of the silicon nitride tool is shown in FIG. 6. The composition and structure of the silicon nitride tool are the same as those in Example 1, the shoulder diameter is 25 mm, the probe diameter at the base (shoulder bottom surface) is 8.2 mm, and the probe diameter at the tip is 7.25 mm. The shoulder bottom surface and the probe surface are not grooved, and have a smooth surface. In addition, a silicon nitride back plate is installed on the back surface of the workpiece. The composition of the medium carbon steel sheet is the same as that in Example 1, and the composition (mass%) of the low carbon steel sheet is as shown in Table 2.

[0047] [Table 2]

[0048] The friction stir welding conditions for both low carbon steel plate and medium carbon steel plate were a tool rotation speed of 400 rpm and a tool movement speed of 50 mm / min. Friction stir welding was performed with constant tool position control. Cross-sectional macrophotographs of the obtained low carbon steel stir part and medium carbon steel stir part are shown in Figures 7 and 8, respectively. No defects were observed in either stir part, and it was confirmed that friction stir welding of 10 mm thick steel plate is possible using a silicon nitride tool.

[0049] Figure 9 shows cross-sectional macro photographs of the medium carbon stir zone when a silicon nitride backing plate and a tool steel backing plate were used, with the tool rotation speed set to 600 rpm and the tool movement speed set to 25-150 mm / min. When a tool steel backing plate was used, the width of the stir zone near the back surface of the medium carbon steel plate narrowed, and tunnel-shaped defects were formed at a tool movement speed of 150 mm / min. In contrast, when a silicon nitride backing plate was used, the stir zone width increased and the defects disappeared, which is thought to be due to the suppression of heat extraction from the back surface of the medium carbon steel plate, resulting in good plastic flow.

[0050] Figure 10 shows the microstructures of the stir zone obtained using a silicon nitride backing plate and a tool steel backing plate under welding conditions of a tool rotation speed of 600 rpm and a tool travel speed of 25 mm / min (the observation positions are shown in the cross-sectional macrophotographs). In the case of the tool steel backing plate, the microstructure changes along the plate thickness direction, with upper bainite observed on the surface and granular bainite observed near the back surface. On the other hand, in the case of the silicon nitride backing plate, a homogeneous structure is formed in the plate thickness direction, and upper bainite is observed in all regions. When the temperature near the back surface during friction stir welding was measured with a thermocouple, it was about 100°C higher when the silicon nitride backing plate was used than when the tool steel backing plate was used. From these results, it can be seen that by combining a silicon nitride tool and a silicon nitride backing plate, it is possible to form a homogeneous structure in the plate thickness direction even in thick steel plates.

[0051] Figure 11 shows the Z-axis load applied to the tool when using a silicon nitride backing plate and when using a tool steel backing plate under the joining conditions of a tool rotation speed of 600 rpm and a tool movement speed of 25 mm / min. Here, the tool is pressed vertically into the workpieces at the joining position, held in that position for a certain period of time, and then moved horizontally (joining begins). In the case of the tool steel backing plate, the Z-axis load is constant while the tool position is held, but in the case of the silicon nitride backing plate, the Z-axis load drops sharply. In addition, even when the tool is moving horizontally, the silicon nitride backing plate shows a lower Z-axis load. From these results, it can be seen that the tool load during friction stir welding can be effectively reduced by combining a silicon nitride tool and a silicon nitride backing plate.

[0052] Figure 12 shows the amount of tool wear when friction stir welding was performed on low carbon steel plates using a silicon nitride tool and a silicon nitride backing plate under joining conditions of a tool rotation speed of 400-800 rpm and a tool movement speed of 25-75 mm / min, and Figure 13 shows the amount of tool wear when friction stir welding was performed on medium carbon steel plates. Here, friction stir welding was repeated with a friction stir welding distance of about 300 mm per welding, and the tool shape after each welding was measured with a three-dimensional shape measuring device. The average amount of wear on the side of the probe was also calculated from the change in shape obtained.

[0053] Under all conditions, the average wear amount of the tool was about several μm / mm, which shows sufficient wear resistance for practical use as a friction stir welding tool. Here, the wear amount increases with increasing tool rotation speed and decreasing tool movement speed, which is thought to be due to an increase in the welding temperature. In addition, in the case of medium carbon steel, wear is clearly suppressed.

[0054] Example 3 Using a silicon nitride tool with a probe length of 9.6 mm, friction stir welding was performed on low carbon steel sheets and medium carbon steel sheets with a thickness of 10 mm using a start-in plate. A photograph of the appearance of the silicon nitride tool is shown in FIG. 14. The composition and structure of the silicon nitride tool are the same as those in Example 1, the shoulder diameter is 20 mm, the probe diameter at the base (shoulder bottom surface) is 7.5 mm, and the probe diameter at the tip is 6.25 mm. As shown in FIG. 14, the shoulder diameter relative to the probe length is significantly smaller than that of a general friction stir welding tool. The shoulder bottom surface and the probe surface are not grooved, and have a smooth surface. In addition, a silicon nitride back plate is placed on the back surface of the workpiece.

[0055] The material to be joined was medium carbon steel with the composition shown in Table 1, and the tool rotation speed was kept constant at 600 rpm while the tool movement speed was changed in the range of 10 to 100 mm / min (constant tool position control). Cross-sectional macro photographs of the resulting stir zones in the medium carbon steel are shown in Figure 15. No defects were observed in any of the stir zones, and it was confirmed that friction stir welding of 10 mm thick steel plates is possible using a relatively small silicon nitride tool with a shoulder diameter of 20 mm. [Explanation of symbols]

[0056] 1. Friction stir welding tool, 2. Shoulder part, 4... Main body, 6 Probe section.

Claims

1. Shoulder part, A probe portion provided on a bottom surface of the shoulder portion, The length of the probe portion is 5.5 mm or more, The substrate is made of ceramics having silicon nitride or sialon as the main phase, the ceramic comprises rare earth and aluminum; the diameter of the probe portion decreases continuously from the base toward the tip; The probe portion is not grooved and / or chamfered; A tool for friction stir wire welding of thick steel plates.

2. The diameter of the shoulder portion is 20 mm or less.

2. The tool for friction stir line welding of thick steel plates according to claim 1 .

3. The length of the probe portion is 9.5 mm or more; 3. The tool for friction stir line welding of thick steel plates according to claim 1 or 2.

4. Friction stir welding of plate materials is performed using the tool for friction stir wire welding of thick steel plate according to any one of claims 1 to 3, While inserting the probe portion of the thick steel plate friction stir wire welding tool into the plate material by 5.5 mm or more, the shoulder portion is brought into contact with the plate material, The plate material is a steel plate, providing a line bond; A method for friction stir welding of thick steel plates, comprising the steps of:

5. The carbon content of the steel plate is 0.2 mass% or more; 5. The method for friction stir welding of thick steel plates according to claim 4,

6. maintaining a position of the thick steel plate friction stir wire welding tool for a certain period of time after inserting the probe portion into the plate material, and laterally moving the thick steel plate friction stir wire welding tool after the reaction force from the plate material applied to the thick steel plate friction stir wire welding tool is reduced; 6. The method for friction stir welding of thick steel plates according to claim 4 or 5,

7. A back plate having a lower thermal conductivity than the plate is disposed on the back surface of the plate; The thick steel plate friction stir welding method according to any one of claims 4 to 6, characterized in that

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