Friction stir welding partitioned gradient cooling device and gradient cooling method

Abstract

The invention relates to a friction stir welding partition gradient cooling device which comprises a cooling tank arranged in a supporting tank on a welding machine platform, and a filler strip divides the cooling tank into a first cooling tank and a second cooling tank which are located on the two sides of a to-be-welded seam. The first cooling groove and the second cooling groove filled with cooling liquid are used for achieving gradient cooling in the thickness direction of a to-be-welded seam and gradient cooling in the length direction of a pair of to-be-welded plates when the stirring head conducts friction stir welding between the two welding edges. Meanwhile, the invention further provides a cooling method of the cooling device. The non-uniform thermal cycle effect of materials in different areas of a welding seam in the friction stir welding process is effectively improved, and high-quality uniform friction stir welding can be completed.

Description

Technical Field

[0001] The invention relates to the field of friction stir welding machines, and in particular to a cooling method and a device for friction stir welding. Background Art

[0002] Friction stir welding is a solid-phase connection technology. During the welding process, the high-speed rotating stirring head and the plate to be welded are mechanically stirred and rubbed to form friction heat, which softens the material plastically and makes the material reach a plastic flow state. At the same time, it is squeezed by the stirring head, thus forming a reliable joint in the solid state. However, the upper material of the weld, which is mechanically stirred and rubbed by the larger stirring head shoulder, has a higher thermal coupling input, while the bottom material of the weld, which is relatively smaller in size, experiences a lower thermal coupling input. There is a large difference in the degree of plastic softening of the weld material along the plate thickness direction. The plasticized metal experiences abnormal flow under the action of different driving forces, and even forms a "turbulent" phenomenon. At the same time, during the welding process, the heat dissipation cooling rate of the material in different regions of the weld along the plate thickness direction is different, the heat conduction from the high temperature zone to the lower low temperature zone, and the uneven thermal cycle caused by different welding temperatures and heat dissipation lead to large differences in the microstructure and properties of the weld, and even abnormal tensile fracture occurs in the weld core zone at the bottom. As for the friction stir welding of long welds in thick aluminum alloy plates, the entire welded parts are in a cold state at the initial stage. As the friction stir welding proceeds, heat is transferred from the high-temperature zone to the unwelded low-temperature zone, and the temperature of the pad continues to increase, causing its heat dissipation effect on the bottom of the long weld to continue to decrease. When welding to the subsequent welded materials that have undergone preheating, the thermal cycle is uneven due to different welding temperatures and heat dissipation, resulting in different initial welding states of the long welds along the length direction, and large differences in thermal cycles. As a result, there are large differences in the microstructure and properties of the long welds along the length direction. Summary of the invention

[0003] The purpose of the present invention is to avoid the shortcomings of the prior art and provide a stir friction welding partitioned gradient cooling device and gradient cooling method that overcomes the uneven thermal cycle effect of materials in different areas of the weld during welding, where the high-temperature residence time of the weld will not be increased due to the early heat conduction heating of the preceding weld, and the mechanical properties of the joint along the length direction and the plate thickness direction are homogenized, thereby achieving high-quality uniform welding.

[0004] To achieve the above object, the technical solution adopted by the present invention is: a friction stir welding zone gradient cooling device, comprising a support groove installed on a welding machine platform, a pair of plates to be welded are fixed in the support groove, and two opposite welding edges of a pair of plates to be welded are spliced ​​to form a weld; A cooling groove is provided in the supporting groove, and a pad is provided in the cooling groove corresponding to the weld to be welded, and is used to form an integral friction stir welding matrix in combination with two welding edges; At the same time, the pad strip divides the cooling groove into a first cooling groove and a second cooling groove located on both sides of the weld to be welded. The first cooling groove and the second cooling groove filled with coolant are used to achieve gradient cooling in the thickness direction of the weld to be welded and gradient cooling in the length direction of a pair of plates to be welded when the stirring head stirs friction welding between the two welding edges. It also includes a coolant circulation system connected to both ends of the first cooling groove and the second cooling groove respectively. The coolant circulation system is used to dynamically adjust the coolant filling flow rate near the starting position of the stir friction welding and the coolant outflow flow rate near the end of the welding end, so that the temperature of the outflowing coolant is within the range of 30 to 60°C, further ensuring gradient cooling along the thickness direction of the weld and gradient cooling in the length direction of a certain plate to be welded.

[0005] Furthermore, the height of the first cooling groove and the second cooling groove are both 8-30 mm, and the width is both 20-100 mm; the height of the cushion strip is 8-30 mm, and the width is 10-200 mm; The plate to be welded has a thickness of 3 to 50 mm and a width of 80 to 300 mm. It is used to ensure the gradient cooling along the thickness direction of the weld and the gradient cooling effect along the length direction of the plate to be welded.

[0006] Furthermore, the diameter of the stirring head is 10-60 mm, and the width of the pad is 10-200 mm, so as to ensure that the weld is not collapsed and the first cooling groove and the second cooling groove can effectively cool the plates on both sides of the weld.

[0007] Furthermore, it also includes a clamping tool for fixing a pair of plates to be welded in the support groove, and the clamping tool includes a clamping block and an adjusting wedge block; A clamping block, the clamping blocks are arranged in pairs on both sides of a pair of plates to be welded, one end of the clamping block is fixed on the supporting groove, the other end of the clamping block is arranged on the pair of plates to be welded, and a clamping nut passes through a through hole on the clamping block and is fixed to the bottom of the supporting groove, so as to clamp the end of the clamping block on the pair of plates to be welded; Adjusting wedges, the adjusting wedges are arranged in pairs on both sides of a pair of plates to be welded, and the wedge-shaped joints of the adjusting wedges are clamped between the clamping nut and the pair of plates to be welded, the inclined surface of the wedge-shaped joint is in contact with the clamping nut, and the supporting surface of the wedge-shaped joint is in contact with the pair of plates to be welded. The adjusting wedges are used to clamp the pair of plates to be welded to ensure the concentricity between the pair of plates to be welded and the stirring head.

[0008] Furthermore, the pad is made of iron, copper, stainless steel or stone; the pair of plates to be welded are 2XXX series or 7XXX series aluminum alloy rolled plates; and the support groove is made of die steel.

[0009] Furthermore, it also includes a thermocouple and a temperature measurement system, one end of the thermocouple is electrically connected to a pair of plates to be welded, and the other end of the thermocouple is electrically connected to the temperature measurement system, which is used to detect the temperature in the thickness direction of the weld under different cooling conditions, that is, to measure the gradient cooling effect along the thickness direction of the weld; at the same time, the thermal cycle and heat dissipation of different areas in the length direction of the pair of plates to be welded are detected, that is, to measure the gradient cooling effect along the length direction of the pair of plates to be welded; The thermocouple is electrically connected to a connection hole set on a pair of plates to be welded. There are multiple connection holes, which are set in pairs on the plates to be welded along the length direction of the pair of plates to be welded and the thickness direction of the weld. The setting direction of the connection holes is perpendicular to the weld, and the distance between any two connection holes along the length direction of the pair of plates to be welded is 100 to 300 mm. The connection holes are evenly arranged along the thickness direction of the weld.

[0010] Further, the coolant circulation system includes a filling tank for storing coolant and a pipeline between the first cooling tank and the second cooling tank and the filling tank, and the filling tank fills the coolant into the first cooling tank and the second cooling tank through a pump and a pipeline; It also includes an outflow pipeline, the first cooling tank and the second cooling tank are connected to a waste liquid pool through the outflow pipeline, the waste liquid pool collects the cooling liquid flowing out of the outflow pipeline, and is used to allow excess cooling liquid to enter the waste liquid pool through the outflow pipeline; The coolant temperature in the outflow pipeline is measured according to the temperature gauge, and the filling amount and outflow amount of the coolant are dynamically regulated by the filling tank, the pump and the outflow pipeline.

[0011] Furthermore, the pair of plates to be welded are a pair of rectangular welding plates, the welds to be welded are the long sides of the rectangular welding plates, and the size of the rectangular welding plates is 300-1500×80-500×3-50mm.

[0012] The present invention also provides a gradient cooling method of the friction stir welding zone gradient cooling device as described above, comprising the following steps: Step 1: Mechanically grind a pair of plates to be welded, and then clean the thermocouple temperature measurement connection holes and the weld seams on the plates to be welded with anhydrous ethanol for later use; Step 2: Install the thermocouple, and select the stirring head and the pad according to the thickness of the pair of plates to be welded; fix the support groove on the welding machine platform, and at the same time, fix the pair of plates to be welded on the support groove with the clamping block and the adjusting wedge block; adjust the centering between the level of the pair of plates to be welded and the stirring head, and move the stirring head linearly to determine the friction welding starting position, end position, and welding length; Step 3: The coolant circulation system fills the coolant into the cooling groove through the pipeline, and waits for the first cooling groove and the second cooling groove to be filled with coolant; Step 4: The stirring head performs friction stir welding at a rotation speed of 200 to 1000 rpm, a welding speed of 60 to 200 mm / min, and a welding inclination angle of 1.5° to 3.5°; In the penetration stage, the stirring head is penetrated with a progressive feed speed of 1 to 10 mm / min. The penetration is completed when the stirring head shoulder contacts the two opposite welding edges of a pair of plates to be welded, and then stir friction welding is performed after maintaining for 1 to 40 seconds, so that the non-plastic softened metal materials on the two welding edges and the stirring head form a stable flow of the plastic softened metal in the extrusion mold; During friction stir welding, according to the flash and forming morphology of the surface of the plate to be welded, the downward pressure of the stirring head is adjusted to 0.02-0.5mm; Step 5: After welding is completed, when the test temperature of the thermocouple drops below 100°C, turn off the temperature measurement system and disassemble the welded plate.

[0013] Furthermore, in step 3, the coolant is filled into the first cooling tank and the second cooling tank and overflows until the upper surface of the pair of plates to be welded is immersed, and the excess coolant flows into the waste liquid pool through the outflow pipeline; Before step 5, the method also includes collecting temperature curves of thermocouple connection points at different positions through a temperature measurement system to study the heat dissipation regulation in the gradient cooling of the first cooling tank and the second cooling tank on both sides of the weld corresponding to the length and thickness directions of the plate to be welded; wherein heat dissipation refers to the convection heat exchange between the weld and the air, the heat conduction heat dissipation between the weld and the cold pad, the heat conduction heat dissipation between the welding area at the bottom of the weld and the coolant in the cooling tank, and the heat conduction heat dissipation state between the upper surface of the plate to be welded and the stirring head and the first cooling tank and the second cooling tank are filled and overflowed until the upper surface of the plate to be welded is immersed in the coolant.

[0014] The beneficial effects of the present invention are as follows: the present invention provides a reliable homogenized partitioned gradient cooling method suitable for stir friction welding, which effectively improves the uneven thermal cycling effect of materials in different areas of the weld during stir friction welding. Finally, experimental verification proves the feasibility and reliability of the method provided by the present invention, and is capable of completing high-quality uniform stir friction welding.

[0015] Specifically, the cooling method of the present invention can effectively regulate the heat dissipation of different areas of the weld as needed, improve the large gradient thermal cycle of the weld along the plate thickness direction and the high temperature residence time of different areas, so that the heat input of the subsequent part to be welded during welding is not increased due to the early heat conduction heating of the previous weld, and avoid the different initial states of welding caused by temperature differences along the length direction, thereby improving the organizational structure of the weak area of ​​the weld, homogenizing the weld organization, reducing the difference in mechanical properties of the joint along the length direction and the plate thickness direction, and improving its mechanical properties.

[0016] Compared with direct cooling of the iron pad, the peak temperature of the friction stir welding joint with local coolant immersion and partitioned gradient cooling at a rotation speed of 300rpm and a welding speed of 60mm / min is lower, the high temperature residence time is shortened, the temperature gradient is reduced, the minimum values ​​of the overall yield strength, tensile strength and elongation after fracture of the joint are increased by 21.96%, 11.41% and 12.29% respectively, and the minimum values ​​of the corresponding mechanical properties of the three-equal-division and layered slices are increased by 17.57%, 13.35% and 6.37% respectively. The uniformity of the mechanical properties along the length direction of the welding plate and the thickness direction of the weld is significantly improved.

[0017] By optimizing the welding process parameters of rotation speed of 400rpm and welding speed of 120mm / min, the peak temperature of stir friction welding with local coolant immersion and partitioned gradient cooling was further reduced, the high temperature residence time was further shortened, the temperature gradient was further reduced, the uniformity was significantly improved, the minimum values ​​of the corresponding mechanical properties of the overall joint were increased by 28.08%, 19.74% and 40.41% respectively, and the minimum values ​​of the corresponding mechanical properties of the three-equal-division and layered slices were increased by 21.72%, 16.58% and 41.40% respectively. The uniformity of mechanical properties along the length direction of the welding plate and the thickness direction of the weld was further significantly improved, and abnormal fracture of the weld core zone due to incomplete yield did not occur. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 This is a schematic diagram of the friction stir welding structure of the present invention; Numbers in the figure: 1, support groove; 2, welding machine platform, 3, pad, 4, a pair of plates to be welded, 41, weld seam to be welded, 5, stirring head, 6, cooling groove, 61, first cooling groove, 62, second cooling groove, 7, filling box, 71, pump, 8, pipeline; 9, outflow pipeline; 10, waste liquid tank; 101, thermometer, 11, clamping block; 12, clamping nut; 13, adjusting wedge; 14, thermocouple; 15, temperature measurement system; Figure 2 It is a structural schematic diagram of the present invention; Figure 3 It is a schematic diagram of the cross-section cooling principle of the present invention; Figure 4 It is a schematic diagram of the friction stir welding structure of the existing steel backing plate with overall cooling; Figure 5 The temperature results of long welds of friction stir welding of high-strength aluminum alloy thick plates prepared in specific examples 1, 2 and 3 of the present invention; in, Figure 5 (a) The steel plate is cooled as a whole; Figure 5 (b) Add coolant to the lower surface for local gradient cooling; Figure 5 (c) Local cooling of the lower surface by immersion of coolant; Figure 6 The tensile performance results of the long welds of high-strength aluminum alloy thick plates prepared by friction stir welding in specific examples 1, 2 and 3 of the present invention along the length direction; in, Figure 6 (a) The steel plate is cooled as a whole; Figure 6 (b) Local cooling of the lower surface by adding coolant; Figure 6 (c) Local cooling of the lower surface by immersion in coolant; Figure 7 The tensile performance results of long weld layered slices of high-strength aluminum alloy thick plate friction stir welding prepared in specific examples 1, 2 and 3 of the present invention along the length direction; in, Figure 7 (a) The steel plate is cooled as a whole; Figure 7 (b) Local cooling of the lower surface by adding coolant; Figure 7 (c) Local cooling of the lower surface by immersion in coolant; Figure 8 The macroscopic morphology results of the tensile fracture of the long welds of the friction stir welding of high-strength aluminum alloy thick plates prepared in specific examples 1, 2 and 3 of the present invention along the length direction; in, Figure 8 (a) The steel plate is cooled as a whole; Figure 8 (b) Add coolant to the lower surface for local gradient cooling; Figure 8 (c) Local cooling of the lower surface by immersion in coolant; Fig. 9 The macroscopic morphology results of tensile fracture along the length direction of long welds of friction stir welding of high-strength aluminum alloy thick plates prepared for specific examples 1, 2 and 3 of the present invention; in, Fig. 9 (a) The steel plate is cooled as a whole; Fig. 9 (b) Add coolant to the lower surface for local gradient cooling; Fig. 9 (c) The lower surface is locally immersed in coolant for gradient cooling. DETAILED DESCRIPTION

[0019] The principles and features of the present invention are described below in conjunction with the accompanying drawings. The examples given are only used to explain the present invention and are not used to limit the scope of the present invention.

[0020] During the friction stir welding process, the material in the area where the stirring head shoulder acts experiences the highest peak temperature thermal cycle, and the material at the bottom of the weld affected by the relatively small stirring needle experiences a lower friction thermal cycle. However, the high temperature area of ​​the stirring head shoulder conducts heat to the lower low temperature area, and the material at the bottom of the weld experiences a long period of high temperature thermal cycle, which softens, especially the heat affected zone. In addition, for the friction stir welding of long welds of aluminum alloy, the entire plate to be welded is in a cold state at the initial stage. As the friction stir welding proceeds, the heat is conducted from the high temperature area to the unwelded low temperature area, and the temperature of the pad continues to increase, which continuously reduces the heat dissipation effect on the bottom of the long weld. When welding to the subsequent material to be welded that has undergone preheating, the thermal cycle caused by different welding temperatures and heat dissipation is uneven, resulting in different initial welding states of the long weld along the length direction, and large differences in thermal cycles.

[0021] The present invention adopts the following main methods to embody the purpose of the present invention: 1. During the friction stir welding process, the coolant is added by using the zoned gradient cooling system to adjust the heat dissipation during the welding process from the back of the weld according to the demand, improve the thermal cycle of the weak area of ​​the weld, and then improve the local organizational structure, thereby improving the local mechanical properties; Second, by using a zoned gradient cooling device and method to add coolant from the back of the weld until the entire weld is completely immersed, it helps to reduce the peak temperature of each area of ​​the weld along the plate thickness direction and shorten the high temperature residence time, homogenize the temperature gradient from high to low, and improve the mechanical properties of the joint; 3. The partitioned gradient cooling device and method can reduce the heat conduction temperature rise from the previous high-temperature weld to the subsequent cold weld, reduce the initial temperature difference of the plates to be welded along the length direction, and the high-temperature residence time of the subsequent welds is not increased due to the early heat conduction heating of the previous welds, thereby homogenizing the mechanical properties of the joint along the length direction, thereby improving the overall performance of the joint.

[0022] In order to achieve the above object, the present invention provides the following specific implementation methods: Example 1: Figure 1 , Figure 2 As shown, a friction stir welding zone gradient cooling device comprises a support groove 1 mounted on a welding machine platform 2, a pair of plates to be welded 4 are fixed in the support groove 1, and two opposite welding edges of a pair of plates to be welded 4 are spliced ​​to form a weld seam 41; The pair of plates to be welded 4 is a pair of rectangular parallelepiped welding plates, the weld seams 41 are the long sides of the rectangular parallelepiped welding plates, and the size of the rectangular parallelepiped welding plates is 300-1500×80-500×3-50 mm.

[0023] A cooling groove 6 is provided in the supporting groove 1, and a pad strip 3 is provided in the cooling groove 6 corresponding to the weld seam 41 to be welded, and is used to form an integral friction stir welding matrix in combination with two welding edges; At the same time, the cushion strip 3 divides the cooling groove 6 into a first cooling groove 61 and a second cooling groove 62 located on both sides of the weld 41. The first cooling groove 61 and the second cooling groove 62 filled with coolant are used for realizing gradient cooling in the thickness direction of the weld 41 and gradient cooling in the length direction of the plate 4 to be welded when the stirring head 5 stirs friction welding between the two welding edges. The height of the first cooling groove 61 and the second cooling groove 62 are both 8 to 30 mm, and the width is both 20 to 100 mm; The height of the cushion strip 3 is 8 to 30 mm, and the width is 10 to 200 mm; The thickness of the plate 4 to be welded is 3 to 50 mm, and the width is 80 to 300 mm. It is used to ensure the gradient cooling along the thickness direction of the weld 41 and the gradient cooling effect along the length direction of the plate 4 to be welded.

[0024] The diameter of the stirring head 5 is 10-60 mm, and the width of the cushion strip 3 is 10-200 mm, so as to ensure that the weld 41 is not collapsed and the first cooling groove 61 and the second cooling groove 62 can effectively cool the plates on both sides of the weld 41 .

[0025] It also includes a coolant circulation system respectively connected to the two ends of the first cooling groove 61 and the second cooling groove 62. The coolant circulation system is used to dynamically adjust the coolant filling flow rate near the starting position of the stir friction welding and the coolant outflow flow rate near the end of the welding end, so that the outflowing coolant temperature is within the range of 30 to 60°C, further ensuring gradient cooling along the thickness direction of the weld seam 41 and gradient cooling in the length direction of a certain plate 4 to be welded.

[0026] The coolant circulation system includes a filling tank 7 for storing coolant and a pipeline 8 between the first cooling tank 61 and the second cooling tank 62 and the filling tank 7. The filling tank 7 fills the coolant into the first cooling tank 61 and the second cooling tank 62 through a pump 71 and the pipeline 8. It also includes an outflow pipeline 9, and the first cooling groove 61 and the second cooling groove 62 are connected to a waste liquid tank 10 through the outflow pipeline 9. The waste liquid tank 10 collects the coolant flowing out of the outflow pipeline 9, and is used to allow excess coolant to enter the waste liquid tank 10 through the outflow pipeline 9; the coolant temperature in the outflow pipeline 9 is measured according to the thermometer 101, and the filling amount and outflow amount of the coolant are dynamically regulated by the filling box 7, the pump 71 and the outflow pipeline 9.

[0027] It also includes a clamping tool for fixing a pair of plates to be welded 4 in the support groove 1, and the clamping tool includes a clamping block 11 and an adjusting wedge block 13; A clamping block 11 is arranged in pairs on both sides of a pair of plates 4 to be welded, one end of the clamping block 11 is fixed on the supporting groove 1, and the other end of the clamping block 11 is arranged on a pair of plates 4 to be welded, and a clamping nut 12 passes through a through hole on the clamping block 11 and is fixed to the bottom of the supporting groove 1, so as to clamp the end of the clamping block 11 on the pair of plates 4 to be welded; Adjust the wedge 13, which is arranged in pairs on both sides of a pair of plates to be welded 4, and the wedge joint of the adjusting wedge 13 is clamped between the clamping nut 12 and the pair of plates to be welded 4, the inclined surface of the wedge joint is in contact with the clamping nut 12, and the supporting surface of the wedge joint is in contact with the pair of plates to be welded 4. The adjusting wedge 13 is used to press the pair of plates to be welded 4 to ensure the concentricity between the pair of plates to be welded 4 and the stirring head 5.

[0028] The backing strip 3 is made of iron, copper, stainless steel or stone; the pair of plates to be welded 4 is a 2XXX series or 7XXX series aluminum alloy rolled plate; and the support groove 1 is made of die steel.

[0029] Embodiment 2 is the same as Embodiment 1, except that it further comprises a thermocouple 14 and a temperature measuring system 15, one end of the thermocouple 14 is electrically connected to a pair of plates to be welded 4, and the other end of the thermocouple 14 is electrically connected to the temperature measuring system 15, and is used for detecting the temperature in the thickness direction of the weld 41 under different cooling conditions, that is, measuring the cooling effect along the thickness direction of the weld 41; at the same time, detecting the thermal cycle and heat dissipation in different areas in the length direction of a pair of plates to be welded 4, that is, measuring the gradient cooling effect along the length direction of a pair of plates to be welded 4; The thermocouple 14 is electrically connected to a connecting hole 42 set on a pair of plates to be welded 4. There are multiple connecting holes 42, which are multiple connecting holes set in pairs on the plates to be welded along the length direction of the pair of plates to be welded 4 and the thickness direction of the weld 41. The setting direction of the connecting holes 42 is perpendicular to the weld 41, and the distance between any two connecting holes 42 along the length direction of the pair of plates to be welded 4 is 100 to 300 mm. The connecting holes 42 are evenly arranged along the thickness direction of the weld 41.

[0030] Embodiment 3: A gradient cooling method of the friction stir welding zone gradient cooling device as in Embodiment 1, comprising the following steps: Step 1: mechanically polish a pair of plates to be welded 4, and then clean the thermocouple temperature measurement connection holes 42 and the weld seams 41 on the pair of plates to be welded 4 with anhydrous ethanol for later use; Step 2: Install the thermocouple 14, and select the stirring head 5 and the pad 3 according to the thickness of the pair of plates to be welded 4; fix the support groove 1 on the welding machine platform 2, and at the same time, fix the pair of plates to be welded 4 on the support groove 1 with the clamping block 11 and the adjusting wedge block 13; adjust the centering between the level of the pair of plates to be welded 4 and the stirring head 5, and move the stirring head 5 linearly to determine the friction welding starting position, end position, and welding length; Step 3: The coolant circulation system adds the coolant to the cooling tank 6 through the pipeline 8, and waits until the first cooling tank 61 and the second cooling tank 62 are filled with the coolant; Step 4: The stirring head 5 performs friction stir welding at a rotation speed of 200 to 1000 rpm, a welding speed of 60 to 200 mm / min, and a welding inclination angle of 1.5° to 3.5°; The stirring head 5 is inserted in a progressive manner at a feed rate of 1 to 10 mm / min. The insertion is completed when the shaft shoulder of the stirring head 5 contacts the two opposite welding edges of the plate 4 to be welded. Then, the stirring friction welding is performed after maintaining the state for 1 to 40 seconds, so that the non-plastic softened metal materials on the two welding edges and the stirring head 5 form a stable flow of the plastic softened metal in the extrusion die. During friction stir welding, according to the flash and forming morphology of the surface of the plate 4 to be welded, the downward pressure of the stirring head 5 is adjusted to 0.02-0.5 mm; Step 5: After welding is completed, when the test temperature of the thermocouple 14 drops below 100° C., the temperature measurement system 15 is turned off and the welded plate is disassembled.

[0031] Embodiment 4: A gradient cooling method of the friction stir welding zone gradient cooling device as in Embodiment 1, having the same steps as in Embodiment 3, except that: In step 3, the coolant is filled into the first cooling tank 61 and the second cooling tank 62 and then overflows until the upper surface of the pair of plates to be welded 4 is submerged, and the excess coolant flows into the waste liquid tank 10 through the outflow pipe 9; Before step 5, the method also includes collecting temperature curves of the connection points of the thermocouples 14 at different positions by the temperature measuring system 15, so as to study the heat dissipation control in the gradient cooling of the first cooling groove 61 and the second cooling groove 62 on both sides of the weld seam 41 corresponding to the length and thickness direction of the welded plate; The heat dissipation refers to the convection heat exchange between the weld 41 and the air, the heat conduction heat dissipation between the weld and the cold pad 3, the heat conduction heat dissipation between the welding area at the bottom of the weld 41 and the coolant in the cooling tank 6, and the heat conduction heat dissipation between the upper surface of the plate to be welded 4 and the stirring head 5 and the first cooling tank 61 and the second cooling tank 62 which are filled and overflow until the upper surface of the plate to be welded is immersed in the coolant.

[0032] In order to verify the effect of the present invention, the present invention provides the following specific examples, in which friction stir welding is performed on 7050-T7451 aluminum alloy with a thickness of 12.7 mm, the same stirring head is selected, the welding process parameters are optimized, and three types of cooling are completed respectively: 1. The existing steel pad is cooled as a whole, that is, there is no cooling groove 6 structure, and the steel pad 3 is directly arranged in the supporting groove 1 corresponding to the weld; 2. Add coolant to the lower surface of the weld plate in the heat-affected zone at the bottom of the weld for zone gradient cooling, that is, fill the first cooling groove 61 and the second cooling groove 62 with coolant to achieve cooling; 3. Completely immerse the plate to be welded in zoned gradient cooling, that is, fill the first cooling tank 61 and the second cooling tank 62 and then overflow until the upper surface of the plate to be welded 4 is immersed to achieve cooling.

[0033] During the test welding process, the temperature curves of characteristic points at different positions and the tensile properties after welding are taken to illustrate the technical effects of the present invention. Now, the present invention is described in detail in conjunction with specific examples and drawings: Specific example 1: (1) A 12.7 mm thick 7050-T7451 aluminum alloy rolled plate was cut into 900×100×12.7 mm blocks to be welded, and holes with a diameter of 2.4 mm for placing thermocouples were machined at different positions along the length of the welded plate (30 mm, 210 mm, 370 mm, 570 mm, 730 mm, and 870 mm from the starting end, respectively) for standby use; (2) Mechanically polish the parts to be welded, use an anhydrous ethanol cleaning machine to place the holes for thermocouple temperature measurement and the parts to be welded for later use; (3) placing a thermocouple for testing the temperature curve during friction stir welding, fixing it in the hole and connecting it to the temperature measurement system; (4) At the middle groove of the water-cooling tank body, a steel pad with the same size as the groove is mechanically fixed on the support groove 1, that is, the cooling groove 6 structure is omitted, and the steel pad 3 is directly arranged in the support groove 1 corresponding to the weld; (5) Mechanically fix the support tank 1 with the steel pad to the friction stir welding machine platform, and adjust the horizontal and the centering between the stirring head; (6) Fix the plate to be welded on the support groove 1 with a pressing plate, and use a triangular wedge to push it sideways, move the stirring head linearly, adjust the centering, and obtain the three-dimensional coordinates of the welding starting position, the ending position X, Y and Z, and the required welding length; (7) Set the welding process parameters: rotation speed 300 rpm, welding speed 120 mm / min, stirring head counterclockwise rotation, welding angle 2.5°; (8) According to step (6), the three-dimensional coordinates and welding length are obtained to set the welding program, and stir friction welding is performed. The stirring head is inserted into the mold by a progressive feeding method to complete the insertion of the stirring head. After the insertion is completed, it is maintained for 10 seconds to allow the plastic softened metal in the extrusion mold composed of the non-plastic softened metal material and the stirring head to flow stably; (9) The stirring head moves horizontally at the set welding speed. During welding, the downward pressure is manually adjusted according to the flash and forming morphology of the surface, and the temperature curves of the characteristic points at different positions are collected. At the same time, the partitioned gradient cooling method is not used to regulate the heat dissipation during the welding process. The heat dissipation is mainly convective heat exchange with the air and heat conduction heat dissipation between the cold pad and the welding plate. (10) After welding is completed, when the test temperature of the thermocouple drops below 100°C, turn off the temperature measurement system and remove the welding test plate.

[0034] After testing and characterization, it was found that under the conditions of rotation speed of 300rpm and welding speed of 60mm / min, the overall cooling friction stir weld of 7050-T7451 aluminum alloy thick plate iron pad was well formed without obvious macro defects, and the joint strength and toughness were high. The temperature curves of characteristic points at different positions along the length direction of the weld were as follows: Figure 5 As shown in (a), the tensile properties of the joint as a whole and the three-equally divided slices along the thickness direction are as follows: Figure 6 (a) and Figure 7 (a) It can be seen that the average values ​​of yield strength, tensile strength and elongation of the joint along the length direction are 272±17.79MPa, 407±16.97MPa and 7.00±0.53%, respectively, the maximum values ​​are 312MPa, 444MPa and 7.82%, respectively, and the minimum values ​​are 253MPa, 390MPa and 5.37%, respectively. Figure 8 As shown in (a), the tensile failure mainly occurs in the heat affected zone of the weld due to 45° shear ductile fracture, and some fractures occur in the heat affected zone near the weld core.

[0035] The average yield strength, tensile strength and elongation of the upper part of the joint along the thickness direction are 278±17.17MPa, 392±7.30MPa and 4.66±0.58%, the maximum values ​​are 324MPa, 403MPa and 6.08%, and the minimum values ​​are 260MPa, 378MPa and 3.97%. The average yield strength, tensile strength and elongation of the middle part are 286±26.28MPa, 402±25.07MPa and 5.22±1.03%, the maximum values ​​are 342MPa, 467MPa and 8.74%, and the minimum values ​​are 266MPa, 381MPa and 3.62%. The average values ​​of yield strength, tensile strength and elongation after fracture of the bottom slice are 266±26.14MPa, 391±26.44MPa and 5.46±0.68%, respectively, the maximum values ​​are 343MPa, 461MPa and 6.52%, respectively, and the minimum values ​​are 249MPa, 366MPa and 3.14%, respectively. The minimum values ​​of yield strength, tensile strength and elongation after fracture of the three-equally divided slices of the friction stir welding joint with integral cooling of the iron pad are 249MPa, 366MPa and 3.14%, respectively. Fig. 9 As shown in (a), the tensile failure mainly occurs in the heat affected zone of the weld due to 45° shear ductile fracture, and some fractures occur in the heat affected zone near the weld core.

[0036] Specific example 2: (1) A 12.7 mm thick 7050-T7451 aluminum alloy rolled plate was cut into 900×100×12.7 mm blocks to be welded, and holes with a diameter of 2.4 mm for placing thermocouples were machined at different positions along the length of the welded plate (30 mm, 210 mm, 370 mm, 570 mm, 730 mm, and 870 mm from the starting end, respectively) for standby use; (2) Mechanically polish the parts to be welded, use an anhydrous ethanol cleaning machine to place the holes for thermocouple temperature measurement and the parts to be welded for later use; (3) placing a thermocouple for testing the temperature curve during friction stir welding, fixing it in the hole and connecting it to the temperature measurement system; (4) According to the thickness of the plate to be welded and the size of the selected stirring head, a suitable steel pad is selected, and the selected steel pad is mechanically fixed in the middle groove of the water cooling tank; (5) Mechanically fix the support tank 1 with steel pads and symmetrically distributed cooling grooves on both sides to the platform of the friction stir welding machine, and adjust the horizontal and the centering between the stirring heads; (6) Fix the plate to be welded on the water-cooling tank with a pressing plate, and use a triangular wedge to push it sideways, move it linearly, adjust the centering, and obtain the X, Y and Z three-dimensional coordinates of the welding starting position and ending position, as well as the required welding length; (7) Use the coolant circulation system to add coolant from the injection hole. When the first cooling groove and the second cooling groove on both sides between the groove body and the cushion strip are filled with coolant, manually control the flow rate until the coolant does not overflow the bottom cooling groove, and the coolant flows out from the outflow hole and enters the waste liquid pool; (8) Set the welding process parameters: rotation speed 300 rpm, welding speed 120 mm / min, stirring head counterclockwise rotation, welding angle 2.5°; (9) According to step (6), the three-dimensional coordinates and welding length are obtained to set the welding program, and stir friction welding is performed. The stirring head is inserted into the stage by using a progressive feed to complete the insertion of the stirring head. After the insertion is completed, it is maintained for 10 seconds to allow the plastic softened metal in the extrusion die composed of the non-plastic softened metal material and the stirring head to flow stably; (10) The stirring head moves horizontally at the set welding speed. During welding, the downward pressure is manually adjusted according to the flash and forming morphology of the surface, and the temperature curves of the characteristic points at different positions are collected. At the same time, the zone gradient cooling method is used to regulate the heat dissipation during the welding process. The heat dissipation mainly includes convection heat exchange with the air, heat conduction heat dissipation between the cold steel pads, and heat conduction heat dissipation between the heat affected zone at the bottom of the weld and the coolant in the cooling tank; (11) After welding is completed, when the test temperature of the thermocouple drops below 100°C, turn off the temperature measurement system and remove the welding test plate.

[0037] After testing and characterization, it was found that under the conditions of rotation speed of 300rpm and welding speed of 60mm / min, the friction stir weld of 7050-T7451 aluminum alloy thick plate was well formed with local cooling liquid and gradient cooling without obvious macro defects, and the joint had high strength and toughness. The temperature curves of characteristic points at different positions along the length direction of the weld were as follows: Figure 5 As shown in (b), the tensile properties of the joint as a whole and the three-equally divided slices along the thickness direction along the length direction are as follows: Figure 6 (b) and Figure 7 As shown in (b), it can be seen that compared with the characteristic points at the same position of the friction stir weld with overall cooling of the steel pad in Specific Example 1, the peak temperature of the weld with gradient cooling of the local coolant on the lower surface is reduced, the high temperature residence time is shortened, and the temperature gradient in different areas is reduced.

[0038] The average values ​​of yield strength, tensile strength and elongation of the joint along the length direction are 325±8.80MPa, 458±6.74MPa and 6.85±0.35%, respectively, the maximum values ​​are 336MPa, 466MPa and 7.57%, respectively, and the minimum values ​​are 308MPa, 435MPa and 6.03%, respectively. Figure 5As shown in (b), the tensile failure mainly occurs in the 45° shear ductile fracture in the heat affected zone of the weld, and some fractures occur in the heat affected zone near the weld core. Compared with the direct cooling of the iron plate, the overall yield strength, tensile strength and minimum elongation after fracture of the friction stir welding joint with local coolant on the lower surface increased by 21.96%, 11.41% and 12.29% respectively. Figure 8 As shown in (b), tensile failure mainly occurs in the heat-affected zone of the weld due to 45° shear ductile fracture.

[0039] The average yield strength, tensile strength and elongation of the upper part of the joint along the thickness direction are 320±10.77MPa, 438±11.82MPa and 5.66±0.62%, the maximum values ​​are 342MPa, 457MPa and 6.47%, and the minimum values ​​are 293MPa, 415MPa and 3.85%. The average yield strength, tensile strength and elongation of the middle section are 335±7.07MPa, 454±9.85MPa and 5.31±0.55%, the maximum values ​​are 343MPa, 468MPa and 5.98%, and the minimum values ​​are 320MPa, 432MPa and 3.34%. The average values ​​of yield strength, tensile strength and elongation after fracture of the bottom slice are 347±13.43MPa, 461±8.22MPa and 5.15±0.23%, respectively, the maximum values ​​are 368MPa, 471MPa and 5.59%, and the minimum values ​​are 320MPa, 443MPa and 4.56%, respectively. The minimum values ​​of yield strength, tensile strength and elongation after fracture of the three-divided slices of the friction stir welding joint with gradient cooling of the lower surface with local coolant are 293MPa, 415MPa and 3.34%, respectively, which are 17.57%, 13.35% and 6.37% higher than the minimum values ​​of 249MPa, 366MPa and 3.14% of the three-divided slices of the friction stir welding joint with overall cooling of the iron pad. Fig. 9 As shown in (b), tensile failure mainly occurs in the heat-affected zone of the weld due to 45° shear ductile fracture.

[0040] At the same time, under the conditions of a rotation speed of 300rpm and a welding speed of 120mm / min, the uniformity of the mechanical properties of the friction stir welding joint along the length direction and the plate thickness direction of the 7050-T7451 aluminum alloy thick plate with local coolant gradient cooling on the lower surface is significantly improved compared with specific example 1.

[0041] Specific example 3: (1) A 12.7 mm thick 7050-T7451 aluminum alloy rolled plate was cut into 900×100×12.7 mm blocks to be welded, and holes with a diameter of 2.4 mm for placing thermocouples were machined at different positions along the length of the welded plate (30 mm, 210 mm, 370 mm, 570 mm, 730 mm, and 870 mm from the starting end, respectively) for standby use; (2) Mechanically polish the parts to be welded, use an anhydrous ethanol cleaning machine to place the holes for thermocouple temperature measurement and the parts to be welded for later use; (3) placing a thermocouple for testing the temperature curve during friction stir welding, fixing it in the hole and connecting it to the temperature measurement system; (4) According to the thickness of the plate to be welded and the size of the selected stirring head, a suitable steel pad is selected, and the selected steel pad is mechanically fixed in the middle groove of the water cooling tank; (5) Mechanically fix the tank body with steel pads and symmetrically distributed cooling grooves on both sides on the platform of the friction stir welding machine, and adjust the horizontal and the centering between the stirring heads; (6) Fix the plate to be welded on the water-cooling tank with a pressing plate, and use a triangular wedge to push the side, move the stirring head linearly, adjust the centering, and obtain the X, Y and Z three-dimensional coordinates of the welding starting position and ending position, as well as the required welding length; (7) Use the coolant circulation system to add coolant from the injection hole, and wait until the cooling grooves symmetrically distributed on both sides between the groove body and the pad are filled with coolant, overflowing until the upper surface of the plate to be welded is just immersed, and the coolant flows out from the outflow hole and enters the waste liquid pool; (8) Set the welding process parameters: rotation speed 300 rpm, welding speed 120 mm / min, stirring head counterclockwise rotation, welding angle 2.5°; (9) According to step (6), the three-dimensional coordinates and welding length are obtained to set the welding program, and stir friction welding is performed. The stirring head is inserted into the stage by using a progressive feed to complete the insertion of the stirring head. After the insertion is completed, it is maintained for 10 seconds to allow the plastic softened metal in the extrusion die composed of the non-plastic softened metal material and the stirring head to flow stably; (10) The stirring head moves horizontally at the set welding speed. During welding, the downward pressure is manually adjusted according to the flash and forming morphology of the surface, and the temperature curves of the characteristic points at different positions are collected. At the same time, the partitioned gradient cooling method is used to regulate the heat dissipation during the welding process. The heat dissipation mainly includes convection heat exchange with the air, heat conduction heat dissipation between the cold steel pad, heat conduction heat dissipation between the heat affected zone at the bottom of the weld and the coolant in the cooling tank, and heat conduction heat dissipation between the upper surface of the weld and the stirring head and the coolant; (11) After welding is completed, when the test temperature of the thermocouple drops below 100°C, turn off the temperature measurement system and remove the welding test plate.

[0042] After testing and characterization, it was found that the friction stir weld of 7050-T7451 aluminum alloy thick plate was well formed with no obvious macro defects and high joint toughness under the conditions of rotation speed of 400rpm and welding speed of 120mm / min. The temperature curves of characteristic points at different positions along the length direction of the weld were as follows: Figure 5 As shown in (c), the tensile properties of the joint as a whole and the three-equally divided slices along the thickness direction along the length direction are as follows: Figure 6 (c) and Figure 7 As shown in (c), it can be seen that compared with the characteristic points at the same position of the steel pad overall cooling friction stir weld in specific example 1, the peak temperature of the weld with local gradient cooling by immersion of coolant on the lower surface is further reduced, the high temperature residence time is shortened, and the temperature gradient in different areas is further reduced and more uniform.

[0043] The average values ​​of yield strength, tensile strength and elongation of the joint along the length direction are 336±7.01MPa, 472±2.56MPa and 8.50±0.51%, respectively, the maximum values ​​are 348MPa, 476MPa and 9.41%, respectively, and the minimum values ​​are 323MPa, 468MPa and 7.54%, respectively. Figure 5 As shown in (b), all tensile failures occurred in the heat affected zone of the weld at 45° shear toughness fracture. Compared with the joints with direct cooling of the iron plate at a rotation speed of 300 rpm and a welding speed of 60 mm / min, the overall yield strength, tensile strength and minimum elongation after fracture of the friction stir welding joints with a rotation speed of 400 rpm and a welding speed of 120 mm / min and local cooling of the lower surface with coolant immersion and gradient cooling increased by 28.08%, 19.74% and 40.41% respectively. Figure 8 As shown in (c), the tensile failure mainly occurs in the heat-affected zone of the weld, which is a 45° shear ductile fracture, and no abnormal fracture occurs in the weld core zone where the weld has not fully yielded.

[0044] The average yield strength, tensile strength and elongation of the upper part of the joint along the thickness direction are 337±15.19MPa, 457±14.63MPa and 6.38±0.77%, the maximum values ​​are 361MPa, 476MPa and 7.95%, and the minimum values ​​are 304MPa, 427MPa and 4.56%. The average yield strength, tensile strength and elongation of the middle part are 354±9.10MPa, 477±9.63MPa and 6.41±0.53%, the maximum values ​​are 374MPa, 487MPa and 6.97%, and the minimum values ​​are 329MPa, 442MPa and 4.44%. The average values ​​of yield strength, tensile strength and elongation after fracture of the bottom slice are 349±13.26MPa, 478±12.01MPa and 6.54±0.35%, respectively, the maximum values ​​are 363MPa, 489MPa and 7.27%, respectively, and the minimum values ​​are 303MPa, 434MPa and 5.94%, respectively. The minimum values ​​of yield strength, tensile strength and elongation after fracture of the three-divided slices of the friction stir welding joint with gradient cooling by local cooling liquid immersion on the lower surface are 329MPa, 442MPa and 5.94%, respectively, which are 21.72%, 16.85% and 41.40% higher than the minimum values ​​of 303MPa, 427MPa and 4.44% of the three-divided slices of the friction stir welding joint with overall cooling of the iron pad. Fig. 9 As shown in (c), the tensile failure mainly occurs in the heat-affected zone of the weld, which is a 45° shear ductile fracture, and no abnormal fracture occurs in the weld core zone where the weld has not fully yielded.

[0045] At the same time, under the conditions of a rotation speed of 400rpm and a welding speed of 120mm / min, the uniformity of the mechanical properties of the 7050-T7451 aluminum alloy thick plate stir friction welded joint along the length direction and the plate thickness direction was further significantly improved compared with specific examples 1 and 2.

[0046] The above description is only a preferred specific example of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims

1. A zoned gradient cooling device for friction stir welding, characterized in that: It comprises a support groove (1) installed on a welding machine platform (2), a pair of plates to be welded (4) are fixed in the support groove (1), and two opposite welding edges of the plates to be welded (4) are spliced ​​to form a weld seam (41); A cooling groove (6) is provided in the support groove (1), and a cushion strip (3) is cushioned in the cooling groove (6) corresponding to the weld seam (41) to be welded, and is used to combine with two welding edges to form an integral friction stir welding matrix; At the same time, the cushion strip (3) divides the cooling groove (6) into a first cooling groove (61) and a second cooling groove (62) located on both sides of the weld (41). The first cooling groove (61) and the second cooling groove (62) filled with cooling liquid are used for realizing gradient cooling in the thickness direction of the weld (41) and gradient cooling in the length direction of a plate (4) to be welded when the stirring head (5) stirs friction welding between two welding edges. It also includes a coolant circulation system connected to both ends of the first cooling groove (61) and the second cooling groove (62), respectively. The coolant circulation system is used to dynamically adjust the coolant filling flow rate near the start position of the friction stir welding and the coolant outflow flow rate near the end position of the welding, so that the outflowing coolant temperature is within the range of 30 to 60°C, further ensuring gradient cooling along the thickness direction of the weld seam (41) and gradient cooling along the length direction of the plate (4) to be welded.

2. The friction stir welding zone gradient cooling device according to claim 1, characterized in that: The first cooling groove (61) and the second cooling groove (62) both have a height of 8 to 30 mm and a width of 20 to 100 mm; The height of the cushion strip (3) is 8 to 30 mm, and the width is 10 to 200 mm; The plate to be welded (4) has a thickness of 3 to 50 mm and a width of 80 to 300 mm. It is used to ensure the gradient cooling effect along the thickness direction of the weld (41) to be welded and the gradient cooling effect along the length direction of the plate (4) to be welded.

3. The friction stir welding zone gradient cooling device according to claim 1, characterized in that: The diameter of the stirring head (5) is 10 to 60 mm, and the width of the cushion strip (3) is 10 to 200 mm, so as to ensure that the weld seam (41) is not crushed, and the first cooling groove (61) and the second cooling groove (62) can effectively cool the plates to be welded on both sides of the weld seam (41).

4. The friction stir welding zone gradient cooling device according to claim 1, characterized in that: It also includes a clamping tool for fixing a pair of plates to be welded (4) in the support groove (1), the clamping tool including a pressing block (11) and an adjusting wedge block (13); A clamping block (11), the clamping blocks (11) are arranged in pairs on both sides of a pair of plates to be welded (4), one end of the clamping block (11) is fixed on the supporting groove (1), and the other end of the clamping block (11) is arranged on a pair of plates to be welded (4), and a clamping nut (12) passes through a through hole on the clamping block (11) and is fixed to the bottom of the supporting groove (1), so as to clamp the end of the clamping block (11) on the pair of plates to be welded (4); An adjusting wedge (13) is provided in pairs on both sides of a pair of plates to be welded (4), and a wedge-shaped joint of the adjusting wedge (13) is clamped between a clamping nut (12) and a pair of plates to be welded (4), an inclined surface of the wedge-shaped joint is in contact with the clamping nut (12), and a supporting surface of the wedge-shaped joint is in contact with a pair of plates to be welded (4), and the adjusting wedge (13) is used to clamp the pair of plates to be welded (4) to ensure the centering between the pair of plates to be welded (4) and the stirring head (5).

5. The friction stir welding zone gradient cooling device according to claim 1, characterized in that: The pad strip (3) is made of iron, copper, stainless steel or stone; the pair of plates to be welded (4) is a 2XXX series or 7XXX series aluminum alloy rolled plate; and the support groove (1) is made of die steel.

6. The friction stir welding zone gradient cooling device according to claim 1, characterized in that: It also includes a thermocouple (14) and a temperature measurement system (15), one end of the thermocouple (14) is electrically connected to a pair of plates to be welded (4), and the other end of the thermocouple (14) is electrically connected to the temperature measurement system (15), and is used to detect the temperature in the thickness direction of the weld (41) under different cooling conditions, that is, to measure the gradient cooling effect along the thickness direction of the weld (41); at the same time, it detects the thermal cycle and heat dissipation of different regions in the length direction of the pair of plates to be welded (4), that is, to measure the gradient cooling effect along the length direction of the pair of plates to be welded (4); The thermocouple (14) is electrically connected to a connection hole (42) provided on a pair of plates to be welded (4). There are a plurality of connection holes (42), which are arranged in pairs on the plates to be welded along the length direction of the plates to be welded (4) and the thickness direction of the weld seam (41). The setting direction of the connection holes (42) is perpendicular to the weld seam (41), and the distance between any two connection holes (42) along the length direction of the plates to be welded (4) is 100 to 300 mm. The connection holes (42) are evenly arranged along the thickness direction of the weld seam (41).

7. The friction stir welding zone gradient cooling device according to claim 1, characterized in that: The coolant circulation system comprises a filling box (7) for storing coolant and a pipeline (8) between the first cooling tank (61) and the second cooling tank (62) and the filling box (7); the filling box (7) fills the coolant into the first cooling tank (61) and the second cooling tank (62) through a pump (71) and the pipeline (8); It also includes an outflow pipeline (9), wherein the first cooling tank (61) and the second cooling tank (62) are connected to a waste liquid pool (10) via the outflow pipeline (9), and the waste liquid pool (10) collects the cooling liquid flowing out of the outflow pipeline (9) so as to allow excess cooling liquid to enter the waste liquid pool (10) through the outflow pipeline (9); The temperature of the coolant in the outflow pipeline (9) is measured by the temperature meter (101), and the filling amount and outflow amount of the coolant are dynamically regulated by the filling box (7), the pump (71) and the outflow pipeline (9).

8. The friction stir welding zone gradient cooling device according to any one of claims 1 to 7, characterized in that: The pair of plates to be welded (4) are a pair of rectangular parallelepiped welding plates, the weld seams (41) to be welded are the long sides of the rectangular parallelepiped welding plates, and the size of the rectangular parallelepiped welding plates is 300-1500×80-500×3-50mm.

9. A gradient cooling method for the friction stir welding zone gradient cooling device as claimed in claims 1-8, characterized in that: The following steps are involved: Step 1: mechanically grinding a pair of plates to be welded (4), and then cleaning the thermocouple temperature measurement connection holes (42) and the weld seams (41) on the pair of plates to be welded (4) with anhydrous ethanol for later use; Step 2: Install a thermocouple (14), and select a stirring head (5) and a pad (3) according to the thickness of a pair of plates to be welded (4); fix the support groove (1) on the welding machine platform (2), and at the same time, fix the pair of plates to be welded (4) on the support groove (1) with a clamping block (11) and an adjustment wedge block (13); adjust the alignment between the level of the pair of plates to be welded (4) and the stirring head (5), and linearly move the stirring head (5) to determine the friction welding starting position, end position, and welding length; Step 3: The coolant circulation system adds the coolant into the cooling tank (6) through the pipeline (8), and waits until the first cooling tank (61) and the second cooling tank (62) are filled with the coolant; Step 4: The stirring head (5) performs friction stir welding at a rotation speed of 200 to 1000 rpm, a welding speed of 60 to 200 mm / min, and a welding inclination angle of 1.5° to 3.5°; The stirring head (5) is inserted in a progressive manner at a feed rate of 1 to 10 mm / min during the insertion stage. The insertion is completed when the stirring head (5) contacts two opposite welding edges of a pair of plates to be welded (4). The stirring head (5) is then kept in contact for 1 to 40 seconds before friction stir welding is performed, so that the non-plastic softened metal material on the two welding edges and the stirring head (5) form a stable flow of the plastic softened metal in the extrusion die. During friction stir welding, the downward pressure of the stirring head (5) is adjusted to 0.02 to 0.5 mm according to the flash and forming morphology of the surface of the pair of plates to be welded (4). Step 5: After welding is completed, when the test temperature of the thermocouple (14) drops below 100° C., the temperature measurement system (15) is turned off and the welded plate is disassembled.

10. The gradient cooling method according to claim 9, characterized in that: In step 3, the coolant is filled into the first cooling tank (61) and the second cooling tank (62) and then overflows until the upper surface of the plate to be welded (4) is submerged, and the excess coolant flows into the waste liquid pool (10) through the outflow pipeline (9); Before step 5, the method further includes collecting temperature curves of connection points of thermocouples (14) at different positions by means of a temperature measurement system (15) to study the heat dissipation control in the gradient cooling of the first cooling groove (61) and the second cooling groove (62) on both sides of the weld seam (41) corresponding to the length and thickness direction of the welded plate; The heat dissipation refers to the convection heat exchange between the weld (41) and the air, the heat conduction heat dissipation between the weld (41) and the cold pad (3), the heat conduction heat dissipation between the welding area at the bottom of the weld (41) and the coolant in the cooling tank (6), and the heat conduction heat dissipation between the upper surface of the plate to be welded (4) and the stirring head (5) and the first cooling tank (61) and the second cooling tank (62) which are filled and overflow until the upper surface of the plate to be welded (4) is immersed in the coolant.

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