Eddy current restraining method and device for eddy current friction stir welding

Abstract

The invention relates to the technical field of friction stir welding, and discloses an eddy current restraining method and device for eddy current friction stir welding. The stirring rod is controlled to press downwards until the end face of the stirring rod makes contact with the to-be-welded workpiece, and then the stirring rod is pressed into the to-be-welded workpiece after the rotating speed is reduced; calculating a torque fluctuation coefficient or a welding temperature fluctuation coefficient in real time, and judging whether the plasticizing eddy current enters a steady-state stage or not; when the plasticized vortex enters a steady state stage, a friction interface is regulated and controlled, and a rigid outer ring constraint boundary is formed; and after the plasticized eddy current under the constraint of the rigid outer ring rotates in situ for a certain time, the stirring rod is driven at a constant welding speed to realize welding. The problem of abrasion of the end face of the stirring tool can be avoided, and a new solution is provided for friction stir welding.

Description

Technical Field

[0001] The invention belongs to the technical field of friction stir welding and relates to an eddy current constraint method and device for eddy current friction stir welding. Background Art

[0002] Friction stir welding (FSW) is a solid-state welding process in which the material remains unmelted, avoiding the cracking and porosity common in fusion welding and ensuring high-quality welds. FSW is suitable for a variety of metals, including aluminum and titanium alloys, and its minimal heat-affected zone helps maintain the mechanical properties of the workpiece. Due to its numerous advantages over other traditional welding methods, it has gained significant popularity in recent years and is widely used in aerospace, marine, and automotive industries.

[0003] During friction stir welding, the stirrer, a core tool, performs the critical tasks of material heating, plastic deformation, and stirring. Low heat input and uniform, stable material flow contribute to welded joints with excellent mechanical properties and microstructure. However, prolonged exposure to high temperatures, pressure, friction, and plastic material flow can significantly challenge the performance of the stirrer, primarily resulting in wear and fracture.

[0004] Chinese invention patent CN110524105A proposes a rotary welding tool and welding method for eddy current friction stir welding. Compared with the conventional friction stir welding method, the eddy current friction stir welding process method uses a stirring rod made of the same material as the base material to replace the stirring rod in the conventional friction stir welding method. The friction between the stirring rod and the workpiece generates heat to soften the material near the friction interface, forming a plasticized eddy current, and finally driving the plasticized eddy current to implement welding. This solves the problem of breaking the stirring rod in the conventional friction stir welding method to a certain extent. However, in order to ensure the welding depth, it is necessary to increase the penetration of the eddy current to the bottom surface of the workpiece. The eddy current friction stir welding process method uses a method in which the support body is in close contact with the surface of the workpiece to constrain the eddy current, which also makes the end face of the support body likely to wear, which is similar to the shoulder wear problem of conventional friction stir welding.

[0005] In conventional friction stir welding methods, shoulder wear is often addressed by designing the material and structure of the stir head, adding a wear-resistant coating, and applying cooling. Chinese invention patent CN108677075B provides a friction stir welding stir head material and a method for preparing the stir head, which utilizes a mixture of titanium carbide powder and zirconium carbide powder to sinter the stir head. Chinese invention patent application CN115647379A proposes a tungsten-rhenium alloy stir head material for friction stir welding and a method for preparing the same, utilizing a tungsten-rhenium precursor prepared from ammonium metatungstate, ammonium rhenate, and oxalic acid, which is then reduced and sintered to produce the stir head. Although designing the stir head material significantly enhances the shoulder's wear resistance, the associated process is complex and costly. Chinese invention patent CN107378167B proposes a method for improving the wear resistance of SiCp / Al composite friction stir welding stir heads, utilizing brazing technology to braze nanodiamond powder onto the end face of the stir head shoulder, forming a nanodiamond wear-resistant layer and improving the wear resistance of the stir head shoulder. However, the related processing is difficult, may also be affected by the mismatch of thermal expansion coefficients, and the cost is high. Chinese invention patent CN110524105B proposes a rotary welding tool and welding method for friction welding, and provides a new internal cooling stirring head structure. The interior of the stirring head is arranged with an internal flow channel to enhance the cooling of the stirring head, which can effectively overcome the problem of increased wear and shortened life of the stirring head due to the heating of the shaft shoulder during use. However, the relevant welding tools need to ensure good sealing, which puts higher requirements on the precision processing of the stirring head and further increases the processing cost. Although the above solutions can improve the wear problem of the shaft shoulder to a certain extent, they are subject to the cost of materials, time and labor, and fail to highlight the advantages. Summary of the Invention

[0006] In view of the shortcomings of the existing technology, the present invention provides an eddy current constraint method and device for eddy current friction stir welding, which can avoid the wear problem of the end face of the stirring tool and provide a new solution for friction stir welding.

[0007] To achieve the above object, the present invention provides the following technical solutions:

[0008] In a first aspect, the present invention provides an eddy current constraint method for eddy current friction stir welding, comprising the following steps:

[0009] Precool the outer ring area of the stirring rod;

[0010] Control the stirring rod to press down until the end of the stirring rod touches the workpiece to be welded, then reduce the speed of the stirring rod and press it into the workpiece to be welded;

[0011] Calculate torque fluctuation coefficient or welding temperature fluctuation coefficient in real time to determine the time required for plasticizing eddy current to enter the steady state stage;

[0012] When the plasticized eddy current enters the steady-state stage, the friction interface is regulated to form a rigid outer ring constraint boundary, and the rotational speed corresponding to the critical slip line velocity V1 is obtained.

[0013] The process parameters obtained in the above process are used as actual welding parameters. After the plasticized eddy current under the constraint of the rigid outer ring rotates in place for a certain period of time, welding is achieved at a constant welding speed.

[0014] This invention creates a gradient temperature field at the friction interface between the stirring rod and the workpiece to be welded by synergistically controlling cooling conditions and rotational speed. This allows the material in the central region of the interface to reach a plasticized flow state, forming eddy currents, while the outer region maintains a rigid state due to low temperature and interfacial slip, thus forming a self-constrained boundary condition. The core principle behind this process is the differential sensitivity of material yield strength to temperature and the differential sensitivity of the friction interface adhesion state to relative linear velocity.

[0015] In combination with the first aspect, further, the pre-cooling of the outer ring area of the stirring rod includes: passing 0-10°C coolant into the water cooling sleeve at a flow rate of 10-40L / min for 30-60 seconds to reduce the temperature of the outer ring area of the stirring rod to 0-20°C.

[0016] In combination with the first aspect, further, the stirring rod is controlled to be pressed down until the end face of the stirring rod contacts the workpiece to be welded, and then the rotation speed of the stirring rod is reduced and the workpiece to be welded is pressed into the workpiece to be welded, including: controlling the stirring rod to move at a rotation speed of 200-500 rpm and a downward pressing speed of 2-10 mm / min until it contacts the workpiece to be welded, switching to a low speed of 0.5-2 mm / min to press the workpiece to be welded; causing friction between the stirring rod and the workpiece to be welded to generate heat, reducing the yield strength of the material, until the shear force on the friction interface is greater than the shear yield strength, and the plasticizing eddy current begins to form.

[0017] In combination with the first aspect, further, the real-time calculation of the torque fluctuation coefficient and the welding temperature fluctuation coefficient to determine whether the plasticizing eddy current has entered the steady-state stage includes: when the eddy current constraint device on which the eddy current constraint method is based includes a torque sensor, the real-time calculation of the torque fluctuation coefficient a1, when a1≤5%, continuing to maintain for t seconds, it can be determined that the plasticizing eddy current has entered the steady-state stage, that is, the interface between the stirring rod and the workpiece is in a completely adhered state; similarly, when the eddy current constraint device on which the eddy current constraint method is based includes a temperature sensor, the real-time calculation of the welding temperature fluctuation coefficient b1, when b1≤5%, continuing to maintain for t seconds, it can be determined that the plasticizing eddy current has entered the steady-state stage.

[0018] In combination with the first aspect, further, t=10.

[0019] In combination with the first aspect, further, when the plasticized eddy current enters the steady-state stage, the friction interface is regulated to form a rigid outer ring constraint boundary, and the rotational speed corresponding to the critical slip line velocity V1 is obtained, including: increasing the rotational speed to 500-2000 rpm in stages, so that the interface linear velocity V of the outer ring area (thickness > 0.2R, R represents the radius of the stirring rod) ≥ the critical slip line velocity V1, inducing the outer ring area of the friction interface to slide completely, and forming a rigid outer ring constraint boundary.

[0020] In combination with the first aspect, further, the torque fluctuation coefficient a1 is obtained by the following formula:

[0021] a1= ×100%;

[0022] Where, is the torque standard deviation, is the average torque value.

[0023] In combination with the first aspect, further, the critical slip line velocity V1 is only related to the material properties and is obtained through experimental calibration. The experimental calibration method is:

[0024] Step a: Increase the speed to the target value Nx in steps of 100-200 rpm, where 500≤Nx≤2000 rpm, and maintain for 10-30 seconds;

[0025] Step b: vertically lifting the stirring rod away from the workpiece to be welded at a speed greater than or equal to 100 mm / s;

[0026] Step c: Observe the fracture morphology of the friction interface: when the interface presents a "texture-like" distribution feature, the friction interface state is "completely adhered"; when the friction interface presents a "discontinuous ring trajectory" distribution feature, the friction interface state is "adhesion-sliding mixed"; when the friction interface area presents a "smooth" distribution feature, the friction interface state is "completely sliding"; when the outer ring area of the friction interface presents a "smooth" distribution feature, it is determined that a rigid outer ring constraint boundary is formed under the target value Nx condition;

[0027] Step d: If the standard is not met, repeat steps a to c, increase the speed to Nx + ΔN, where ΔN = 50-100 rpm, and verify again whether a rigid outer ring constraint boundary is formed until the critical slip line speed V1 is obtained. The corresponding speed is obtained based on the critical slip line speed V1.

[0028] The entire welding process of the present invention is as follows: the stirring rod is controlled to move at a rotation speed of 200-500 rpm and a downward pressing speed of 2-10 mm / min until it contacts the workpiece to be welded, and then switched to a low speed of 0.5-2 mm / min to press into the workpiece to be welded until the time required for the plasticized eddy current to enter the steady state stage is met. Then, the stirring rod speed is adjusted to the speed corresponding to the critical slip line speed V1, and rotated in place for 10 seconds. Then, the stirring rod is driven at a constant welding speed (welding speed range 10-200 mm / min) to achieve welding.

[0029] In the second aspect, the present invention proposes an eddy current restraint device for eddy current stir friction welding, comprising a stirring rod, a torsion sleeve and a water-cooling sleeve; the stirring rod, torsion sleeve and water-cooling sleeve rotate and move synchronously; the torsion sleeve is connected to the main shaft of the stir friction welding machine through a tool handle; the stirring rod is fixedly connected to the torsion sleeve and is arranged inside the torsion sleeve; the water-cooling sleeve is fixedly connected to the torsion sleeve and is arranged outside the torsion sleeve; the water-cooling sleeve circulates cooling water to reduce the temperature of the material in the outer ring area of the stirring rod; the end face of the stirring rod is arranged parallel to the workpiece to be welded.

[0030] In combination with the second aspect, further, the stirring rod includes an outer wall of the stirring rod and an end face of the stirring rod; the torsion sleeve includes an outer wall of the torsion sleeve, an end face of the torsion sleeve and an inner wall of the torsion sleeve; the water-cooling sleeve includes an inner wall of the water-cooling sleeve, an inner end face of the water-cooling sleeve, a lower end face of the water-cooling sleeve and a water-cooling groove; the outer wall of the stirring rod is tightly fitted with the inner wall of the torsion sleeve, the outer wall of the torsion sleeve is tightly fitted with the inner wall of the water-cooling sleeve, the end face of the torsion sleeve is tightly fitted with the inner end face of the water-cooling sleeve, the water-cooling groove is used for circulating cooling water, and the end face of the stirring rod extends out of the end face of the torsion sleeve and the lower end face of the water-cooling sleeve.

[0031] In combination with the second aspect, further, the height of the stirring rod end surface extending from the end surface of the torsion sleeve is 1-5 cm, preferably 2 cm; the height of the stirring rod end surface extending from the lower end surface of the water-cooling sleeve is 1-5 mm, preferably 2 mm.

[0032] In combination with the second aspect, further, the torsion sleeve also includes a torque sensor, which is arranged on the outer side wall of the torsion sleeve and is used to collect the torque of the torsion sleeve; and can measure the torque changes during the welding process in real time.

[0033] In combination with the second aspect, further, the acquisition frequency of the torque sensor is greater than or equal to 1000 Hz, and the measurement accuracy is ≤0.5% FS.

[0034] In conjunction with the second aspect, further, the stirring rod is cylindrical with a diameter of 6-30 mm (preferably 16 mm) and is made of the same material as the workpiece to be welded. It is fixed to the interior of the torsion sleeve and precisely mates with it. The torsion sleeve has a wall thickness of 2-6 mm (preferably 4 mm) and is directly driven by the spindle. The water-cooling sleeve has a wall thickness of 2-6 mm (preferably 4 mm). The water-cooling tank has an L-shaped cross-section with a depth of 20-40 mm and a base width-to-depth ratio of 1:1.5-1:3. The inner wall of the water-cooling tank tightly covers the torsion sleeve and stirring rod.

[0035] In combination with the second aspect, further, the eddy current restraint device of the present invention also includes a temperature sensor, which is used to collect the welding temperature; a first hole and a second hole are vertically opened on the workpiece to be welded, the axis of the first hole is collinear with the axis of the stirring rod, and the second hole is correspondingly arranged directly below the end face of the stirring rod; the first hole and the second hole are both installed with temperature sensors.

[0036] The core principle of the present invention lies in the differences in the sensitivity of material yield strength to temperature and the sensitivity of the friction interface adhesion state to relative linear velocity. The temperature sensitivity of material yield strength refers to the fact that the metal materials involved in friction stir welding generally have a significant temperature dependence of their yield strength. When the temperature rises, the material's yield strength drops sharply and the material softens significantly. In the central region of the vortex, viscous heat dissipation reduces the material strength to an extremely low level, entering a highly plastic state. Under the driving force of the stirring rod, violent plastic flow (forming vortices) occurs, achieving material connection. However, in the outer ring region, excessively high temperatures will cause the outer ring material to soften and its yield strength to be too low. This will lose the rigidity required to restrain the internal vortices, making it impossible to prevent the softened material from overflowing from the stir zone (forming flash) or causing vortex instability. Therefore, the temperature of the outer ring region must be strictly limited to a low range (preferably 0-20°C) through active cooling to maintain its yield strength at a high level sufficient to resist plastic flow and thus maintain the necessary rigidity.

[0037] The linear velocity sensitivity of the adhesion state at the friction interface refers to its high sensitivity to the relative linear velocity of the contact surface between the stir bar and the workpiece. The relative linear velocity is determined by the stir bar's rotational speed and the radial distance from the center of rotation. Therefore, given the stir bar's radius, the stir bar's rotational speed is a key process parameter for the relative linear velocity in the vicinity of the friction interface. In the central region of the stir bar's end face, the relative linear velocity is relatively low. This low relative linear velocity facilitates the maintenance of an adhesion state (materials adhere to each other without macroscopic relative sliding) at the friction interface. This adhesion state, combined with the high-temperature softening effect, is essential for intense plastic deformation (material agitation) and the establishment of plastic flow vortices in the central region. Increasing the stir bar's rotational speed significantly increases the relative linear velocity in the outer annular region. A higher relative linear velocity promotes the transition of the friction interface from an adhesion state to a slip state. This slip state has two key roles: (a) Energy dissipation regulation: The frictional heat generation efficiency in the slip state is usually lower than that in the adhesion state, which helps to suppress the frictional heat generation and temperature rise in the outer ring area (it is easier to control the temperature with external cooling); (b) Shear force limitation and constraint strengthening: Slip means that the shear stress applied to the contact surface material is limited, usually lower than the critical shear strength of the slip interface, which is not enough to cause large-scale plastic flow of the material there. Combined with the aforementioned high yield strength maintained by low temperature, the 'slip-dominated interface state' in the outer ring area further ensures its macroscopic rigidity state, enabling it to form an effective barrier to the plastic flow area in the center. Therefore, the present invention no longer constrains the eddy current by closely contacting the stirring tool with the workpiece surface, and completely avoids the wear problem of the end face of the stirring tool.

[0038] Compared with the prior art, the present invention provides an eddy current constraint method and device for eddy current friction stir welding, which has the following beneficial effects:

[0039] (1) The present invention can solve the problem of shoulder wear during the friction stir welding process.

[0040] (2) The constraint method of the present invention is based on the difference in temperature sensitivity of the material yield strength and the difference in linear velocity sensitivity of the stick-slip state of the friction interface. Through the coordinated control of temperature and rotation speed, a plasticized vortex is formed in the core of the stirring rod, and the outer ring area maintains a rigid state due to the low temperature and interface slip, thereby forming a self-constrained boundary condition.

[0041] (3) The present invention can be used for common metals and alloys such as aluminum, iron, copper, and titanium. For different materials, it is necessary to first explore the process parameter window with good eddy current constraint effect, which also provides a reference standard for quickly selecting process parameters for subsequent welding tests. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] Figure 1is a schematic cross-sectional view of the vortex restraint device in Example 1 of the present invention;

[0043] Figure 2 This is a schematic diagram of the structure of the eddy current restraint device in Example 1 of the present invention. Figure 1 ;

[0044] Figure 3 Schematic diagram of the friction interface of the workpiece in Example 1; wherein, Figure 3 a is a schematic diagram of the friction interface when the rotation speed is 400 rpm and the workpiece material is 6061 aluminum alloy; Figure 3 b is the rotation speed of 600 rpm, and the workpiece material is 6061 aluminum alloy; Figure 3 c is the rotation speed of 800 rpm, and the workpiece material is 6061 aluminum alloy;

[0045] Figure 4 This is a schematic diagram of the structure of the eddy current restraint device in Example 1 of the present invention. Figure 2 , wherein the joint form of the workpiece is butt joint;

[0046] Figure 5 This is a schematic diagram of the structure of the eddy current restraint device in Example 1 of the present invention. Figure 3 , where the joint form of the workpiece is lap joint;

[0047] Figure 6 2 is a schematic structural diagram of the eddy current restraint device in Example 2 of the present invention;

[0048] Figure 7 Schematic diagram of the structure of the eddy current restraint device in Example 3 of the present invention;

[0049] Figure 8 Schematic diagram of the friction interface of the workpiece in Example 3 of the present invention; the rotation speed is 800 rpm, and the workpiece material is 6061 aluminum alloy;

[0050] Figure 9 It is a flow chart of the eddy current constraint method of the present invention.

[0051] The meanings of the reference numerals in the figures are:

[0052] 1. Stirring rod; 1-1. Outer wall of stirring rod; 1-2. End face of stirring rod; 2. Torsion sleeve; 2-1. Outer wall of torsion sleeve; 2-2. End face of torsion sleeve; 2-3. Inner wall of torsion sleeve; 2-4. Torque sensor; 3. Water-cooling sleeve; 3-1. Inner wall of water-cooling sleeve; 3-2. Inner end face of water-cooling sleeve; 3-3. Lower end face of water-cooling sleeve; 3-4. Water-cooling tank; 4. Workpiece; 4-1. Upper surface of workpiece; 4-2. First hole; 4-3. Second hole; 4-4. Temperature sensor; 5. Liquid nitrogen jet cooling device. DETAILED DESCRIPTION

[0053] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0054] Unless otherwise specifically stated, the relative arrangement, numerical expressions and numerical values of the parts and steps set forth in these embodiments do not limit the scope of the present invention. Meanwhile, it should be understood that, for ease of description, the sizes of the various parts shown in the accompanying drawings are not drawn according to actual proportional relationships. Technology, methods and equipment known to those of ordinary skill in the relevant art may not be discussed in detail, but in appropriate cases, the technology, methods and equipment should be considered as a part of the specification. In all examples shown and discussed here, any specific value should be interpreted as being merely exemplary, rather than as a limitation. Therefore, other examples of exemplary embodiments may also include different values. It should be noted that similar numbers and letters represent similar items in the following drawings, and therefore, once an item is defined in an accompanying drawing, it does not need to be further discussed in subsequent drawings.

[0055] In the description of this application, it should be understood that the terms "center", "longitudinal", "lateral", "up", "down", "left", "right", "vertical", "horizontal", "top", "bottom", "inside", "outside", etc., indicating the orientation or position relationship, are based on the orientation or position relationship shown in the accompanying drawings, and are only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as a limitation on the protection content of the present invention.

[0056] Example 1

[0057] like Figures 1 to 5As shown, this embodiment proposes an eddy current restraint device for eddy current stir friction welding, comprising a stirring rod 1, a torsion sleeve 2, a water-cooling sleeve 3, and a workpiece 4. The stirring rod 1 has a diameter of 16 mm (made of the same material as the workpiece) and a non-tapered end face. The stirring rod 1 includes a stirring rod outer wall 1-1 and a stirring rod end face 1-2. The torsion sleeve 2 has a wall thickness of 4 mm and includes a torsion sleeve outer wall 2-1, a torsion sleeve end face 2-2, a torsion sleeve inner wall 2-3, and a torque sensor 2-4, wherein the torque sensor has a sampling frequency of 1000 Hz. The water-cooling sleeve 3 includes a water-cooling sleeve inner wall 3-1, a water-cooling sleeve inner end face 3-2, a water-cooling sleeve lower end face 3-3, and a water-cooling groove 3-4. The depth of the water cooling tank 3-4 is 20-40 mm (preferably 30 mm), the ratio of the bottom width to the depth is 1:1.5-1:3 (preferably 1:2), and the coolant is preferably ethylene glycol coolant with a flow rate of 20 L / min and an inlet temperature of 5±0.5°C. The workpiece 4 includes a workpiece upper surface 4-1.

[0058] The welding machine spindle is connected to the torsion sleeve 2 via a tool handle. The stirring rod 1 is clamped inside the torsion sleeve 2, with the outer wall 1-1 of the stirring rod tightly fitting with the inner wall 2-3 of the torsion sleeve, and the end face 1-2 of the stirring rod extending 2 cm beyond the end face 2-2 of the torsion sleeve. The torsion sleeve 2 is positioned between the stirring rod 1 and the water-cooling sleeve 3, with the outer wall 2-1 of the torsion sleeve tightly fitting with the inner wall 3-1 of the water-cooling sleeve, and the end face 2-2 of the torsion sleeve closely fitting with the inner face 3-2 of the water-cooling sleeve. The torque sensor 2-4 is connected to the outer wall 2-1 of the torsion sleeve. The water-cooling sleeve 3 is located at the outermost part of the device, tightly enveloping the torsion sleeve 2 and the stirring rod 1. The end face 1-2 of the stirring rod extends 2 mm beyond the lower face 3-3 of the water-cooling sleeve. The water-cooling tank 3-4 is filled with circulating cooling water. The workpiece 4 is positioned directly below the stirring rod end face 1-2, with the workpiece upper surface 4-1 parallel to the stirring rod end face 1-2.

[0059] like Figure 9 As shown, this embodiment provides an eddy current constraint method for eddy current friction stir welding, comprising the following steps:

[0060] Step S1 , introducing cooling water into the water-cooling sleeve 3 to pre-cool the outer ring area of the stirring rod 1 .

[0061] Step S2, controlling the stirring rod 1 to press into the workpiece 4 at a relatively low rotation speed and downward pressing speed.

[0062] Step S3, calculating the torque fluctuation coefficient a1 in real time. When the torque fluctuation coefficient a1 is less than or equal to 5% and is maintained for 10 seconds, it is determined that the plasticized eddy current has entered a steady-state stage.

[0063] Step S4: Increase the rotation speed to induce the outer ring area of the friction interface to completely slide, form a rigid outer ring constraint boundary, and obtain the rotation speed corresponding to the critical slip line velocity V1.

[0064] Step S5: Using the process parameters obtained in the above process as actual welding parameters, welding is performed at a constant welding speed after the plasticized eddy current constrained by the rigid outer ring rotates in place for a certain period of time.

[0065] During the test, a 0-10°C coolant was first introduced into the water-cooling sleeve 3 at a flow rate of 10-40 L / min for 30-60 seconds until the temperature of the outer ring area of the stirring rod 1 dropped to 0-20°C. The welding machine spindle then drove the torsion sleeve 2, causing the stirring rod 1, torsion sleeve 2, and water-cooling sleeve 3 to move synchronously at a rotational speed of 200-500 rpm and a downward pressure speed of 2-10 mm / min until the stirring rod 1 contacted the workpiece 4. The pressure was then switched to a low speed of 0.5-2 mm / min until the torque fluctuation coefficient a1 was ≤ 5% and maintained for 10 seconds. The speed was then increased in steps of 100-200 rpm to the target value Nx (500 ≤ Nx ≤ 2000 rpm) and maintained for 10-30 seconds. The stirring rod was then vertically lifted off the workpiece at a speed of ≥100 mm / s. The fracture morphology of the friction interface was observed: when the outer ring area of the friction interface (thickness > 0.2R) exhibited a "smooth" distribution, it was determined that a rigid outer ring constraint boundary could be formed under the Nx condition. If the standard is not met, repeat the above steps and increase the speed to Nx + ΔN (ΔN = 50-100 rpm) and verify again until the critical slip line speed V1 is obtained. The corresponding rotational speed can then be directly derived based on the critical slip line speed V1. During actual welding, the stirring rod is controlled to move at a speed of 200-500 rpm and a downward pressure speed of 2-10 mm / min until it contacts the workpiece to be welded. The speed is then switched to a low speed of 0.5-2 mm / min to press into the workpiece until the plasticizing eddy current enters the steady-state stage. The stirring rod is then adjusted to the speed corresponding to the critical slip line speed V1 and rotated in place for 5 seconds. The welding is then completed by driving the stirring rod at a constant welding speed (welding speed range 10-200 mm / min).

[0066] like Figure 3 As shown in a, Figure 3 Figure a shows the friction interface distribution of Example 1 at a rotational speed of 400 rpm and a workpiece material of 6061 aluminum alloy. The black dashed area exhibits a "textured" distribution characteristic, covering almost the entire friction interface. This indicates that at this rotational speed, the friction interface between the stirring rod end face 1-2 and the workpiece upper surface 4-1 is at a "completely adhered boundary," meaning that the stirring rod is completely softened and the eddy current diameter is approximately equal to the stirring rod diameter, thus preventing the stirring rod from acting as a self-constrained eddy current.

[0067] like Figure 3 As shown in b, Figure 3b shows the distribution state of the friction interface of Example 1 when the rotation speed is 600 rpm and the workpiece material is 6061 aluminum alloy. As shown in the black dotted area, the "texture-like" distribution area shrinks significantly. The annular area between the black dotted line and the white solid line shows a "discontinuous annular trajectory" distribution feature, which indicates that the friction interface at this position is at a "stick-slip mixed boundary". Although the eddy current is subject to a certain constraint in this state, a small amount of material on the friction interface will still flow out in the form of burrs, destroying the eddy current balance, so the stirring rod still cannot play the role of self-constraining the eddy current.

[0068] like Figure 3 As shown in c, Figure 3 Figure c shows the friction interface distribution state of Example 1 when the rotation speed is 800 rpm and the workpiece material is 6061 aluminum alloy. The "textured" distribution area represented by the black dotted area further shrinks, and the "discontinuous annular trajectory" distribution area between the black dotted line and the white solid line also shrinks significantly, while the annular area outside the white solid line shows a "smooth" distribution feature. The "smooth" distribution feature indicates that the friction interface at this position is at a "complete slip boundary" and the eddy current is only located in the core of the stirring rod, that is, the outer ring area of the stirring rod has a good effect of confining the eddy current.

[0069] It should be noted that: Figure 2 The workpiece in is 1, which is mainly used to describe how to obtain parameters with good constraint effect. After obtaining, the parameters can be used for butt welding or lap welding, such as Figure 4 and Figure 5 As shown, Figure 4 There are two workpieces 4 in the middle, and the joint form of the two workpieces 4 is butt joint. Figure 5 There are two workpieces 4 in the middle, and the joint form of the two workpieces 4 is overlap.

[0070] Example 2

[0071] like Figure 6As shown, the difference between Example 2 and Example 1 is that the torsion sleeve 2 no longer integrates the torque sensor 2-4, and is punched in the thickness direction of the workpiece 4, including a first hole 4-2 and a second hole 4-3, wherein the axis of the first hole 4-2 coincides with the axis of the stirring rod, and the diameter of the first hole 4-2 is 0.5-2mm. The axis of the second hole 4-3 corresponds to the 4 / 5 stirring rod radius position, and the diameter is 0.5-2mm. Temperature sensors 4-4 (such as thermocouples) are placed in both holes, and the welding temperature fluctuation coefficient is calculated by the numerical changes of the temperature sensors 4-4 at different positions (the calculation method of the welding temperature fluctuation coefficient is consistent with the calculation method of the torque fluctuation coefficient in Example 1.), the method is the same as Example 1, and it is determined whether the plasticizing eddy current has entered the steady-state stage. The other implementation steps are the same as Example 1. Although the method of Example 2 has inherent interference and is slightly less accurate than Example 1, it is fully capable of solving the problems existing in the prior art.

[0072] The eddy current constraint method for eddy current friction stir welding proposed in this embodiment includes the following steps:

[0073] Precool the outer ring area of the stirring rod;

[0074] Control the stirring rod to press down until the end of the stirring rod touches the workpiece to be welded, then reduce the speed of the stirring rod and press it into the workpiece to be welded;

[0075] The welding temperature fluctuation coefficient b1 is calculated in real time. When b1≤5% and it continues to be maintained for 10 seconds, it is determined that the plasticizing eddy current has entered the steady state stage.

[0076] When the plasticized eddy current enters the steady-state stage, the friction interface is regulated to form a rigid outer ring constraint boundary, and the rotational speed corresponding to the critical slip line velocity V1 is obtained;

[0077] The process parameters obtained in the above process are used as the actual welding parameters. After the plasticized eddy current under the constraint of the rigid outer ring stabilizes again, the stirring rod is driven at a constant welding speed to achieve welding.

[0078] Example 3

[0079] like Figure 7 As shown, Example 3 has made key optimizations based on the built-in cooling scheme of Example 1. By adding a liquid nitrogen injection device 5 near the outer wall 1-1 of the stirring rod and implementing ultra-low temperature (≤-196°C) directional injection, the lower temperature limit of conventional cooling is significantly exceeded (achieving a deep cold state of T≤-20°C in the outer ring area), thereby greatly improving the cooling rate. This enhanced cooling scheme is intended to maximize the high yield strength characteristics of the material at low temperatures, and at the same time, in conjunction with the high rotation speed, ensure a more thorough interface slip state in the outer ring area, thereby constructing a self-constrained boundary in the outer ring with a rigidity and range exceeding that of Example 1. Figure 8As shown in the interface friction state diagram (corresponding to a rotation speed of 800 rpm), compared with the built-in cooling solution of Example 1, the outer ring area of the solution in which the liquid nitrogen injection device 5 is added in this embodiment is further expanded. It can be seen that this embodiment is more advanced than Example 1.

[0080] This embodiment provides an eddy current constraint method for eddy current friction stir welding, comprising the following steps:

[0081] Step S1 , introducing cooling water into the water-cooling sleeve 3 to pre-cool the outer ring area of the stirring rod 1 .

[0082] Step S2, controlling the stirring rod 1 to press into the workpiece 4 at a relatively low rotation speed and downward pressing speed.

[0083] Step S3: adjust the distance between the nozzle of the liquid nitrogen injection device and the outer wall 1-1 of the stirring rod to 20-30 mm, and start the liquid nitrogen injection until the processing is completed, with a flow rate of 5-10 L / min and an injection pressure of 0.1-0.5 MPa.

[0084] Step S4, calculating the torque fluctuation coefficient a1 in real time. When the torque fluctuation coefficient a1 is less than or equal to 5% and is maintained for 10 seconds, it is determined that the plasticized eddy current has entered a steady-state stage.

[0085] Step S5: increasing the rotation speed to induce the outer ring area of the friction interface to completely slide, forming a rigid outer ring constraint boundary, and obtaining the rotation speed corresponding to the critical slip line velocity V1;

[0086] Step S6: Using the process parameters obtained in the above process as actual welding parameters, welding is performed at a constant welding speed after the plasticized eddy current constrained by the rigid outer ring is stabilized again.

[0087] It should be noted that, in this application, relational terms such as first and second, etc. are merely used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply the existence of any such actual relationship or order between these entities or operations. Moreover, the terms "comprises," "comprising," or any other variants thereof are intended to cover non-exclusive inclusion, so that a process, method, article, or device comprising a series of elements includes not only those elements, but also other elements not explicitly listed, or also includes elements inherent to such process, method, article, or device. In the absence of further restrictions, an element defined by the statement "comprising a ..." does not exclude the presence of other identical elements in the process, method, article, or device comprising the element.

[0088] While embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes, modifications, substitutions, and variations may be made to these embodiments without departing from the principles and spirit of the invention, and that the scope of the invention is defined by the appended claims and their equivalents.

Claims

1. An eddy current restraint method for eddy current friction stir welding, characterized in that: The following steps are involved: Precool the outer ring area of the stirring rod; Control the stirring rod to press down until the end of the stirring rod touches the workpiece to be welded, then reduce the speed of the stirring rod and press it into the workpiece to be welded; Welding is performed according to the pre-obtained process parameters. After the plasticized eddy current constrained by the rigid outer ring rotates in place for a certain period of time, welding is performed at a constant welding speed. The pre-obtained process parameters include the time required for the plasticizing eddy current to enter the steady-state stage and the rotation speed corresponding to the critical slip line speed V1; The method for obtaining the process parameters is as follows: Calculate torque fluctuation coefficient or welding temperature fluctuation coefficient in real time to determine the time required for plasticizing eddy current to enter the steady state stage; When the plasticized eddy current enters the steady-state stage, the friction interface is regulated to form a rigid outer ring constraint boundary, and the rotational speed corresponding to the critical slip line velocity V1 is obtained.

2. The eddy current restraint method for eddy current friction stir welding according to claim 1, characterized in that: The pre-cooling of the outer ring area of the stirring rod includes: introducing 0-10°C coolant into the water cooling sleeve at a flow rate of 10-40 L / min for 30-60 seconds to reduce the temperature of the outer ring area of the stirring rod to 0-20°C.

3. The eddy current restraint method for eddy current friction stir welding according to claim 1, characterized in that: The controlling of the stirring rod to press down until the end face of the stirring rod contacts the workpiece to be welded, and then reducing the rotation speed of the stirring rod to press into the workpiece to be welded, includes: controlling the stirring rod to move at a rotation speed of 200-500 rpm and a downward pressing speed of 2-10 mm / min until it contacts the workpiece to be welded, and switching to a low speed of 0.5-2 mm / min to press into the workpiece to be welded.

4. The eddy current restraint method for eddy current friction stir welding according to claim 1, characterized in that: The real-time calculation of the torque fluctuation coefficient or the welding temperature fluctuation coefficient and determination of the time required for the plasticizing eddy current to enter the steady-state stage include: when the eddy current constraint device based on the eddy current constraint method includes a torque sensor, real-time calculation of the torque fluctuation coefficient a1; when a1 ≤ 5%, continuing to maintain for t seconds, it can be determined that the plasticizing eddy current has entered the steady-state stage, that is, the interface between the stirring rod and the workpiece is in a completely adhered state; similarly, when the eddy current constraint device based on the eddy current constraint method includes a temperature sensor, real-time calculation of the welding temperature fluctuation coefficient b1; when b1 ≤ 5%, continuing to maintain for t seconds, it can be determined that the plasticizing eddy current has entered the steady-state stage.

5. The eddy current restraint method for eddy current friction stir welding according to claim 1, characterized in that: When the plasticized eddy current enters the steady-state stage, the friction interface is regulated to form a rigid outer ring constraint boundary, and the speed corresponding to the critical slip line velocity V1 is obtained, including: increasing the speed to 500-2000 rpm in stages, so that the interface line velocity V of the outer ring area ≥ the critical slip line velocity V1, inducing the outer ring area of the friction interface to completely slide, and forming a rigid outer ring constraint boundary.

6. The eddy current restraint method for eddy current friction stir welding according to claim 5, characterized in that: The critical slip line velocity V1 is obtained through experimental calibration, and the experimental calibration method is: Step a: Increase the speed to the target value Nx in steps of 100-200 rpm, where 500≤Nx≤2000 rpm, and maintain for 10-30 seconds; Step b: vertically lifting the stirring rod away from the workpiece to be welded at a speed greater than or equal to 100 mm / s; Step c: Observe the fracture morphology of the friction interface: When the interface exhibits a "texture-like" distribution feature, the friction interface state is "completely adhered"; when the friction interface exhibits a "discontinuous ring trajectory" distribution feature, the friction interface state is "adhesion-slip mixed"; when the friction interface area exhibits a "smooth" distribution feature, the friction interface state is "completely slipping"; when the outer ring area of the friction interface exhibits a "smooth" distribution feature, it is determined that a rigid outer ring constraint boundary has been formed under the target value Nx condition; Step d: If the standard is not met, repeat steps a to c, increase the speed to Nx + ΔN, where ΔN = 50-100 rpm, and verify again whether a rigid outer ring constraint boundary is formed until the critical slip line speed V1 is obtained. The corresponding speed is obtained based on the critical slip line speed V1.

7. An eddy current restraint device for eddy current friction stir welding, characterized in that: It includes a stirring rod, a torsion sleeve and a water-cooling sleeve; the stirring rod, torsion sleeve and water-cooling sleeve rotate and move synchronously; the torsion sleeve is connected to the main shaft of the stir friction welding machine through a tool handle; the stirring rod is fixedly connected to the torsion sleeve and is arranged inside the torsion sleeve; the water-cooling sleeve is fixedly connected to the torsion sleeve and is arranged outside the torsion sleeve; the water-cooling sleeve circulates cooling water to reduce the temperature of the material in the outer ring area of the stirring rod; the end face of the stirring rod is arranged parallel to the workpiece to be welded.

8. The eddy current restraint device for eddy current friction stir welding according to claim 7, characterized in that: The stirring rod includes an outer wall of the stirring rod and an end face of the stirring rod; the torsion sleeve includes an outer wall of the torsion sleeve, an end face of the torsion sleeve and an inner wall of the torsion sleeve; the water-cooling sleeve includes an inner wall of the water-cooling sleeve, an inner end face of the water-cooling sleeve, a lower end face of the water-cooling sleeve and a water-cooling groove; the outer wall of the stirring rod is tightly fitted with the inner wall of the torsion sleeve, the outer wall of the torsion sleeve is tightly fitted with the inner wall of the water-cooling sleeve, the end face of the torsion sleeve is tightly fitted with the inner end face of the water-cooling sleeve, the water-cooling groove is used for circulating cooling water, and the end face of the stirring rod extends out of the end face of the torsion sleeve and the lower end face of the water-cooling sleeve.

9. The eddy current restraint device for eddy current friction stir welding according to claim 8, characterized in that: The torsion sleeve further includes a torque sensor, which is arranged on the outer side wall of the torsion sleeve and is used to collect the torque of the torsion sleeve.

10. The eddy current restraint device for eddy current friction stir welding according to claim 8, characterized in that: It also includes a temperature sensor, which is used to collect welding temperature; a first hole and a second hole are vertically opened on the workpiece to be welded, the axis of the first hole is collinear with the axis of the stirring rod, and the second hole is correspondingly arranged directly below the end face of the stirring rod; both the first hole and the second hole are installed with temperature sensors.

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