A method for friction stir welding with nearly no thinning and control of geometric dimensions at the bottom of a thin-walled box

Abstract

The present invention provides a method for friction stir welding with nearly no thinning and control of geometric dimensions for a thin-walled box bottom, comprising the following steps: assembling a thin-walled box bottom with a diameter of 5 m; performing milling on the thin-walled box bottom with a diameter of 5 m by using an assembly precision control method; and welding the thin-walled box bottom with a diameter of 5 m by using a friction stir welding defect control method with nearly no thinning. The beneficial effects of the present invention are as follows: realizing the friction stir welding production with nearly no thinning for a thin-walled aluminum-lithium alloy box bottom with a diameter of 5 m, effectively controlling the generation of welding defects and welding deformation, achieving qualified welding of the product at one time, filling the technical blank of the engineering application technology of friction stir welding for a thin-walled aluminum-lithium alloy box bottom with a diameter of 5 m, providing a solid technical support for the successful development of the new generation of launch vehicles in China, and having important practical significance.

Description

Technical Field

[0001] The present invention belongs to the technical field of rocket equipment, and particularly relates to a friction stir welding and geometric dimension control method for a thin-walled tank bottom with nearly no thinning. Background Art

[0002] In the prior art, the new generation of launch vehicles in China adopt a 5m diameter common-bottom storage tank structure, and most of the materials are 2219 aluminum alloy and 2195 aluminum-lithium alloy. Among them, the tank bottom of 2195 aluminum-lithium alloy is a spherical crown tank bottom. Each tank bottom contains 8 longitudinal seams, 1 transition and circular ring seam, 1 central top cover and circular ring seam, and several flange ring seams. The total length of the welds is 37.5m - 43.5m, and the thickness of the welding area is only 3mm at the thinnest. For the 2195 aluminum-lithium alloy material, a fusion welding process cannot obtain a weld joint with excellent performance, and only a friction stir welding process can be used for welding.

[0003] Disadvantages of the prior art: The material of the 5m diameter thin-walled tank bottom structure is 2195 aluminum-lithium alloy. The weldability of aluminum-lithium alloy is special, and its fusion welding weldability is poor, which is mainly reflected in the following four aspects: high sensitivity to welding porosity, high sensitivity to welding oxidation, high sensitivity to welding hot cracks, and low mechanical properties of the welded joint. Therefore, a fusion welding process cannot be used for welding production.

[0004] When using a friction stir welding process for production, during the welding process, affected by the forging pressure of the shoulder of the stirring head, the plastic metal extruded on the front of the weld will form flash and be extruded out of the weld area, resulting in a certain degree of material loss in the weld area and causing thinning on the front of the weld. At the same time, to prevent the stirring pin from piercing the backing plate too long, in actual welding, the length of the stirring pin will be less than the thickness of the actual product welding area by 0.15 - 0.25mm. Therefore, a shallow weak bonding defect will be formed on the back of the weld. After welding, the back of the weld needs to be ground to remove the weak bonding defect, resulting in a certain degree of thinning on the back of the weld. For medium-thickness or large-thickness products, meeting the weld thinning within the standard range has little impact on the mechanical properties of the weld and the overall quality of the product. However, for 3mm thin plate products, the weld thinning has a greater impact on the performance of the weld joint and the overall quality of the product. The thinning amount will not only reduce the tensile forging force of the weld, but also have an adverse impact on the geometric dimensions of the thin-walled product, and greatly reduce the repair welding margin for weld defects. Summary of the Invention

[0005] In view of this, the present invention aims to propose a friction stir welding and geometric dimension control method for a thin-walled tank bottom with nearly no thinning, so as to solve problems such as the assembly accuracy and geometric dimension control of the thin-walled tank bottom, and the control of friction stir welding defects with nearly no thinning of the thin-walled aluminum-lithium alloy tank bottom, and greatly improve the welding quality of the product.

[0006] To achieve the above object, the technical solution of the present invention is realized as follows:

[0007] A method for friction stir welding with nearly no thinning and control of geometric dimensions of a thin-walled box bottom, comprising the following steps:

[0008] S1. Assemble the thin-walled box bottom with a diameter of 5 m;

[0009] S2. Perform profile milling on the thin-walled box bottom with a diameter of 5 m by using an assembly accuracy control method;

[0010] S3. Weld the thin-walled box bottom with a diameter of 5 m by using a defect control method for friction stir welding with nearly no thinning;

[0011] The assembly accuracy control method in step S2 includes an assembly accuracy control method for the longitudinal seams of the segments and an assembly accuracy control method for the circumferential seams of the box bottom. The assembly accuracy control method for the circumferential seams of the box bottom includes an in-situ profile milling method for the circumferential seams between the circular ring and the transition ring and a profile milling method for the circumferential seams between the circular ring and the top cover;

[0012] The defect control method for friction stir welding with nearly no thinning in step S3 includes a defect control method for the longitudinal seams of the segments and a defect control method for the circumferential seams of the box bottom.

[0013] Further, the assembly accuracy control method for the longitudinal seams of the segments includes the following steps:

[0014] A1. Milling the allowance of the longitudinal seams of the segments, installing the segments on the longitudinal seam die according to the butting line, adjusting the assembly positions of the segments in the axial and circumferential directions, supporting the large ends of the segments with a leveling mechanism, and fixing the middle parts of the segments with a clamping belt;

[0015] A2. Install a pressing strip between the welding area and the pressing mechanism. The thickness of the pressing strip is 4 - 6 mm, and the distance between the edge of the pressing strip and the weld joint is controlled within 15 ± 2 mm;

[0016] A3. Operate the pneumatic pressing mechanism to press both sides of the segments. According to the profile of the segment die, program the milling procedure for the segments and then mill the allowances on both sides of the segments. First, perform rough milling, leaving a margin of 1 - 2 mm, and then perform finish milling, with the flatness of the processed edge being less than or equal to 0.2 mm;

[0017] A4. Milling the other 7 segments in sequence, and leaving a margin of 2 - 3 mm when milling the last segment. Perform profile milling according to the dimensions after welding the 7 segments;

[0018] A5. Lower the segment die after milling and check the butting condition of the longitudinal seams.

[0019] Further, the in-situ profile milling method for the circumferential seams between the circular ring and the transition ring includes the following steps:

[0020] B1. After pushing out the welding backing plate and the wedge-shaped compensating block on the equipment die, measure the roundness of the whole circle of the welding backing plate and confirm that the roundness is within ±0.2 mm;

[0021] B2. Hoist the transition ring onto the annular seam membrane tire, so that the bottom of the large end of the transition ring contacts the leveling device at the bottom of the annular seam tooling membrane tire, and adjust the transition ring to a horizontal level by adjusting the leveling device;

[0022] B3. Hoist the transition ring pressing tooling to the annular seam work surface, so that the pneumatic pressing plate presses the outer plane of the transition ring;

[0023] B4, support the inner support plate and wedge-shaped compensation block of the transition ring;

[0024] B5. Lift the annular welded membrane by 50mm, hoist the ring onto the annular welded membrane, and adjust the level of the ring according to the distance from the theoretical mark on the large end of the ring to the surface of the welding platform;

[0025] B6. Hoist the ring pressing mechanism onto the annular seam film tire and lock it to make the pneumatic pressing plate press the large end of the ring, measure the gap between the inner surface of the ring and the mold tire, and use mechanical pressing to make manual local adjustments to the gap to control the gap within 0.2mm;

[0026] B7. Call out the five-axis program in the coordinate system, make the spindle axis position coincide with the center position of the X-axis of the annular seam film tire, adjust the spindle to the horizontal plane angle of 31°, install a milling cutter with a diameter of 20mm, and perform rough milling and fine milling of the large end of the ring according to the down-milling principle, with a rough milling amount of 10-15mm and a fine milling amount of 1-3mm;

[0027] B8. When the milling is to 1mm from the theoretical scale line at the large end of the ring, lower the annular seam membrane, check the transition ring and the ring, and check the amount of misalignment. If the amount of misalignment is greater than the set value, mill the excess at the large end of the ring again until the misalignment is less than 0.2mm after the transition ring and the ring are butted together, and the butt clearance is less than 0.2mm.

[0028] Furthermore, the method for milling the annular seam between the circular ring and the top cover comprises the following steps:

[0029] C1. Measure the roundness of the top cover annular seam or annular annular seam welding pad and confirm that the roundness is within ±0.2mm;

[0030] C2. Lower the welding pad to leave space for the milling cutter at the small end of the ring. The lowering height should not be less than 20mm.

[0031] C3. Call out the five-axis program in the coordinate system, make the spindle axis position coincide with the center position of the X-axis of the annular seam film tire, adjust the spindle to the horizontal plane angle of 82°, install a milling cutter with a diameter of 20mm, and perform rough milling and fine milling of the small end of the ring according to the down-milling principle, with a rough milling amount of 10-15mm and a fine milling amount of 1-3mm;

[0032] C4. When the milling distance is 1mm from the theoretical scale line of the small end of the ring, lift the welding pad and check the top cover and the ring. If the top cover cannot be inserted into the ring, mill the small end of the ring again with a feed amount of 0.2 to 0.3mm each time until the top cover is inserted into the ring and the butt clearance is less than 0.2mm.

[0033] Furthermore, the melon slice longitudinal seam defect control method comprises the following steps:

[0034] D1, adopt 5-section positioning welding, the 5-section positioning welding is in a fixed assembly state before the near-no thinning welding, the length of each positioning weld of the 5-section positioning welding is about 200-250mm, and a positioning stirring head with a shaft shoulder diameter of 10mm and a stirring needle length of 1-1.5mm is used;

[0035] The distribution positions are: one at each end of the melon slice longitudinal seam, and three evenly distributed in the middle of the longitudinal seam. The welding sequence is: positioning the lower end of the longitudinal seam, positioning the middle section of the longitudinal seam, positioning the upper end of the longitudinal seam, positioning the 2 / 5 section from the lower end of the longitudinal seam, and positioning the 4 / 5 section from the lower end of the longitudinal seam.

[0036] D2. After the 5-stage tack welding is completed, full tack welding is performed, using the same stirring head as the 5-stage tack welding to fix the assembly state;

[0037] D3. Perform 5-section tack welding and full tack welding on the remaining 7 longitudinal seams in turn to fix the gap state of each longitudinal seam;

[0038] D4, pre-weld the 8 melon slice longitudinal seams, the Archimedean spiral is used as the structural curve of the shoulder, the needle length of the stirring head is 0.2-0.25mm less than the thickness of the welding area, and the shoulder diameter is 1-2mm less than the formal welding stirring head;

[0039] D5. Formal welding of 8 melon slice longitudinal seams was carried out, using a nearly non-thinning stirring head, the needle length of the stirring head was 0.25-0.3mm less than the thickness of the welding area, and the shaft shoulder was 1-2mm larger than the pre-welding stirring head;

[0040] D6. Clean the front of the weld, conduct phased array in-situ testing, and check the internal quality of the weld;

[0041] D7. Remove the ring from the rack, clean the back of the weld, perform phased array testing and X-ray fluoroscopy, and measure the longitudinal seam thickness.

[0042] Furthermore, the main axis inclination angle of the cross-symmetrical welding method is 2° to 2.5°.

[0043] Furthermore, the box bottom annular seam defect control method comprises the following steps:

[0044] E1. Adopt the diagonal multi - point segment positioning method, use a positioning stirring head with a shoulder diameter of 10 mm and a stirring pin length of 1 - 1.5 mm. The circumferential seam between the ring and the transition ring is evenly segment - positioned at 16 places within the entire circumference, and the length of each positioning weld is 300 - 400 mm. The circumferential seam between the ring and the top cover is evenly segment - positioned at 8 places within the entire circumference, and the length of each positioning weld is 200 - 300 mm;

[0045] E2. Carry out circumferential positioning welding. Also use a positioning stirring head with a shoulder diameter of 10 mm and a stirring pin length of 1 - 1.5 mm. The welding angle is 358°, and 2° is reserved without welding;

[0046] E3. Carry out pre - welding. Use the same stirring head as that for the pre - welding of the longitudinal seam of the segment. The length of the stirring pin of the stirring head is 0.2 - 0.25 mm less than the thickness of the welding area, and the shoulder diameter is 1 - 2 mm less than that of the formal welding stirring head;

[0047] E4. Carry out formal welding. The formal welding uses a retractable stirring head. During the end stage of welding, the stirring pin is retracted uniformly. The main shaft inclination angle of the retractable stirring head is 2° - 2.5°, and the welding angle is 360°. For formal welding +5°, and for repeated welding +7° retracted welding. The length of the retractable stirring pin is 0.25 - 0.3 mm less than the thickness of the welding area, and the shoulder is 1 - 2 mm larger than the pre - welding stirring head;

[0048] E5. Clean the front of the weld, carry out phased array in - place detection, and check the internal quality of the weld;

[0049] E6. After the welding of the bottom of the box is completed, take it off the shelf, clean the back of the weld, carry out phased array detection and X - ray fluoroscopy, measure the weld thickness, carry out hydraulic and helium mass spectrometry leak detection on the bottom of the box, and conduct assessment and evaluation on the overall quality of the bottom of the box.

[0050] Compared with the prior art, the method for friction stir welding with nearly no thinning and geometric dimension control of a thin - walled box bottom of the present invention has the following advantages:

[0051] The method for friction stir welding with nearly no thinning and geometric dimension control of a thin - walled box bottom of the present invention realizes the friction stir welding production with nearly no thinning of the 5 - m - diameter thin - walled aluminum - lithium alloy box bottom, effectively controls the generation of welding defects and welding deformation, realizes the qualified welding of the product at one time, fills the technical gap in the engineering application of friction stir welding for 5 - m - diameter thin - walled aluminum - lithium alloy box bottoms, and provides solid technical support for the successful development of the new generation of launch vehicles in our country, having important practical significance. BRIEF DESCRIPTION OF THE DRAWINGS

[0052] The drawings constituting a part of the present invention are used to provide a further understanding of the present invention. The schematic embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute an improper limitation to the present invention. In the drawings:

[0053] Figure 1 Schematic diagram of the first spindle milling and cutting position and posture according to an embodiment of the present invention;

[0054] Figure 2 Schematic diagram of the second spindle milling and cutting position and posture according to an embodiment of the present invention;

[0055] Figure 3 Schematic diagram of the third spindle milling and cutting position and posture according to an embodiment of the present invention;

[0056] Figure 4 Schematic diagram of the five-segment positioning welding according to an embodiment of the present invention;

[0057] Figure 5 Schematic diagram of the cross-symmetric welding sequence according to an embodiment of the present invention;

[0058] Figure 6 Schematic diagram of the conventional near-zero thinning stir welding head welding according to an embodiment of the present invention;

[0059] Figure 7 Schematic diagram of the first multi-segment positioning sequence according to an embodiment of the present invention;

[0060] Figure 8 Schematic diagram of the second multi-segment positioning sequence according to an embodiment of the present invention;

[0061] Figure 9 Schematic diagram of the retractable near-zero thinning friction stir welding according to an embodiment of the present invention. Detailed implementation manners

[0062] It should be noted that, without conflict, the embodiments in the present invention and the features in the embodiments can be combined with each other.

[0063] In the description of the present invention, it should be understood that the orientation or positional relationship indicated by the terms "center", "longitudinal", "transverse", "upper", "lower", "front", "rear", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", etc. are based on the orientation or positional relationship shown in the 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 thus should not be construed as a limitation to the present invention. In addition, the terms "first", "second", etc. are only used for descriptive purposes and cannot be understood as indicating or implying relative importance or implicitly indicating the number of the indicated technical features. Thus, the features defined with "first", "second", etc. can explicitly or implicitly include one or more of such features. In the description of the present invention, unless otherwise specified, the meaning of "a plurality" is two or more.

[0064] In the description of the present invention, it should be noted that unless otherwise clearly specified and defined, the terms "installation", "connection", and "coupling" should be understood in a broad sense. For example, it may be a fixed connection, a detachable connection, or an integral connection; it may be a mechanical connection or an electrical connection; it may be a direct connection or an indirect connection through an intermediate medium, and it may be the communication inside two components. For those of ordinary skill in the art, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0065] The present invention will be described in detail below with reference to the drawings and in conjunction with embodiments.

[0066] As Figures 1 to 9 shown, a method for friction stir welding and geometric dimension control of a thin-walled tank bottom with nearly no thinning includes the following steps:

[0067] S1. Assemble the 5m diameter thin-walled tank bottom;

[0068] S2. Use an assembly accuracy control method to perform milling on the 5m diameter thin-walled tank bottom;

[0069] S3. Use a defect control method for friction stir welding with nearly no thinning to weld the 5m diameter thin-walled tank bottom;

[0070] In the assembly accuracy control method in step S2, it includes an assembly accuracy control method for the longitudinal seams of the segment and an assembly accuracy control method for the circumferential seams of the tank bottom. The assembly accuracy control method for the circumferential seams of the tank bottom includes an in-situ milling method for the circumferential seams between the circular ring and the transition ring and a milling method for the circumferential seams between the circular ring and the top cover;

[0071] In the defect control method for friction stir welding with nearly no thinning in step S3, it includes a defect control method for the longitudinal seams of the segment and a defect control method for the circumferential seams of the tank bottom.

[0072] Advantages of the present invention: It realizes the friction stir welding production with nearly no thinning of the 5m diameter thin-walled aluminum-lithium alloy tank bottom, effectively controls the generation of welding defects and welding deformation, achieves qualified welding of the product at one time, fills the technical gap in the engineering application of friction stir welding for the 5m diameter thin-walled aluminum-lithium alloy tank bottom, provides a solid technical support for the successful development of the new generation of launch vehicles in our country, and has important practical significance.

[0073] The present invention has developed an engineering application technology for friction stir welding with nearly no thinning of the thin-walled tank bottom, solved problems such as the control of assembly accuracy and geometric dimensions of the thin-walled tank bottom, and the control of defects in friction stir welding with nearly no thinning of the thin-walled aluminum-lithium alloy tank bottom, and significantly improved the welding quality of the product.

[0074] The friction stir welding of the 5m diameter thin-walled tank bottom is carried out on the 5m tank bottom friction stir welding system. The weld types include the longitudinal seams of the segmental plates and the circumferential seams of the tank bottom. The assembly accuracy of the tank bottom components will have a huge impact on the weld quality and the overall geometric dimensions of the ultra-thin aluminum-lithium alloy tank bottom. Before welding production of the 5m diameter ultra-thin tank bottom, it is necessary to study the assembly accuracy control technology of the ultra-thin tank bottom, so as to realize the control of the assembly quality such as the assembly gap, butt joint misalignment and fitting degree of the tank bottom welds, and realize the control of geometric dimensions through fine perimeter matching and equipment accuracy, improve the welding quality of the tank bottom, and ensure that the 5m diameter ultra-thin tank bottom is qualified when it comes off the production line.

[0075] 1. Assembly Accuracy Control Technology for 5m Diameter Thin-Walled Tank Bottom

[0076] For the longitudinal seams of the segmental plates, assemble them according to the following method. First, measure the accuracy of the equipment die mold. The fitting accuracy of the milling and welding trajectories of the longitudinal seams of the segmental plates of the equipment with the theoretical profile should reach 0.1mm. The in-situ milling and the method of machining the last segmental plate with the remaining allowance are used to control the assembly gap and the perimeter dimension. Before assembly welding, mill the remaining allowance of the longitudinal seams of the segmental plates, install the segmental plates on the longitudinal seam die mold according to the butting line, adjust the assembly positions of the segmental plates in the axial and circumferential directions, support the large end of the segmental plate with a leveling mechanism, and fix the middle of the segmental plate with a tape. To ensure that the weld to be welded is closely fitted with the rigid support backing plate within the full contact range, install a thick strip with stronger rigidity between the welding area and the pressing mechanism. The thickness of the strip is 4 - 6mm, and the distance between the edge of the strip and the weld butt joint is controlled within 15 ± 2mm, so that the pressing force of the pressing mechanism is transmitted to the weld area more closely and effectively, thus significantly increasing the pressing force effect received by the weld area, and then improving the situation of non-fitting with the backing plate and the misalignment phenomenon in the weld area.

[0077] Operate the pneumatic pressing mechanism to press both sides of the segmental plate. According to the profile of the segmental plate die mold, program the milling procedure for the segmental plate and then mill the remaining allowance on both sides of the segmental plate. To ensure the flatness of the edge, first perform rough milling, leaving a 1 - 2mm allowance, and then perform finish milling. The flatness of the edge can reach within 0.2mm. Use this milling method to mill 7 segmental plates. To ensure the perimeter of the ring after welding, leave a 2 - 3mm allowance when milling the last segmental plate, and perform machining according to the actual dimensions after welding of the 7 segmental plates to effectively ensure the assembly accuracy of the last segmental plate. Achieve precise control of the butt joint gap of the longitudinal seams of the segmental plates and the perimeter of the ring.

[0078] After milling is completed, lower the segmental plate die mold and check the butting situation of the longitudinal seam.

[0079] For the circumferential seams between the ring and the transition ring and the circumferential seams between the ring and the top cover, first measure the end face runout and radial runout of the turntable tabletop of the circumferential seams of the tank bottom, which need to be controlled within 0.2mm. Then, the in-situ machining method is also used to control the assembly gap and perimeter matching.

[0080] The in-situ milling method for the annular gap between the circular ring and the transition ring is:

[0081] (1) After the welding pad and wedge-shaped compensation block on the equipment mold are extended, measure the roundness of the entire welding pad to confirm that the roundness is within ±0.2mm;

[0082] (2) hoist the transition ring onto the annular seam membrane tire so that the bottom of the large end of the transition ring contacts the leveling device at the bottom of the annular seam tooling membrane tire, and adjust the transition ring to a horizontal level by adjusting the leveling device;

[0083] (3) Lift the transition ring pressing tooling to the annular seam work surface so that the pneumatic pressing plate presses the outer plane of the transition ring;

[0084] (4) Support the inner support plate and wedge-shaped compensation block of the transition ring;

[0085] (5) Raise the annular welded membrane by 50 mm, hoist the ring onto the annular welded membrane, and adjust the level of the ring according to the distance from the theoretical mark on the large end of the ring to the surface of the welding platform;

[0086] (6) Hoist the ring clamping mechanism onto the annular seam film tire and lock it to make the pneumatic pressure plate clamp the large end of the ring, measure the gap between the inner surface of the ring and the mold tire, and use mechanical clamping to manually adjust the gap to the tire in the part with a large gap, so as to control the gap within 0.2 mm;

[0087] (7) Call out the five-axis program in the coordinate system, make the spindle axis position coincide with the center position of the X-axis of the annular seam film tire, adjust the spindle to a 31° angle with the horizontal plane, install a milling cutter with a diameter of 20 mm, and perform rough milling and fine milling of the large end of the ring according to the down-milling principle, with a rough milling amount of 10 to 15 mm and a fine milling amount of 1 to 3 mm;

[0088] (8) When the milling is 1mm away from the theoretical mark line of the large end of the ring, lower the ring seam film, check the transition ring and the ring, and check the amount of misalignment. If the amount of misalignment is large (the set value can be given), mill the large end of the ring again until the misalignment of the transition ring and the ring is less than 0.2mm after docking, and the docking gap is less than 0.2mm. Figures 1 - 2 shown.

[0089] The milling method of the ring and the top cover ring seam is:

[0090] (1) Measure the roundness of the top cover / circular ring seam welding pad and confirm that the roundness is within ±0.2mm;

[0091] (2) Lower the welding pad to leave space for the milling cutter at the small end of the ring. The lowering height should not be less than 20 mm.

[0092] (3) Recall the five-axis program in the coordinate system to align the spindle axis with the X-axis center position of the circumferential seam film tire. Adjust the angle between the spindle and the horizontal plane to 82°. Install a milling cutter with a diameter of 20 mm and perform rough milling and finish milling on the small end of the ring according to the principle of down milling. The rough milling amount is 10 - 15 mm, and the finish milling amount is 1 - 3 mm.

[0093] (4) When milling to 1 mm away from the theoretical marking line at the small end of the ring, raise the welding backing plate and check the fit between the top cover and the ring. If the top cover cannot be inserted into the ring, mill the remaining amount at the small end of the ring again with a feed rate of 0.2 - 0.3 mm each time until the top cover can be inserted into the ring and the butt joint gap is less than 0.2 mm; as Figure 3 shown.

[0094] 2. Defect control technology for friction stir welding of thin-walled box bottom with near-zero thinning

[0095] To achieve the goal of qualified welding in one pass and avoid the occurrence of repair welding as much as possible, a defect control method of multi-point segment positioning welding + full positioning welding + pre-welding + formal welding is proposed.

[0096] For the longitudinal seams of the segmental petals, the defect control method is as follows:

[0097] (1) Adopt 5-segment positioning welding. The 5-segment positioning can fix the assembly state before near-zero thinning welding, prevent the change of the assembly state of the product under the action of welding force during near-zero thinning welding, and cause welding defects. The pin length of the stirring head for segment positioning welding is 1 - 1.5 mm, and the shoulder diameter is 10 mm. The length of each positioning weld for the 5-segment positioning welding is about 200 - 250 mm. Use a positioning stirring head with a shoulder diameter of 10 mm and a stirring pin length of 1 - 1.5 mm. The distribution positions are: one at each end of the longitudinal seam of the segmental petal, and three evenly distributed in the middle position of the longitudinal seam. The welding sequence is: segment positioning at the lower end of the longitudinal seam - segment positioning in the middle position of the longitudinal seam - segment positioning at the upper end of the longitudinal seam - segment positioning at the 2 / 5 position from the lower end of the longitudinal seam - segment positioning at the 4 / 5 position from the lower end of the longitudinal seam, as Figure 4 shown in the following serial number sequence;

[0098] (2) After the segment positioning welding is completed, perform full positioning welding, that is, continuously position weld the entire weld seam, using the same stirring head as the segment positioning welding to further fix the assembly state;

[0099] (3) Perform segment positioning and full positioning welding on 8 longitudinal seams in sequence to fix the gap state of each longitudinal seam, prevent a large gap from occurring due to the accumulation of welding shrinkage in the last weld seam during formal welding, and ensure the welding quality and the post-weld perimeter of the last weld seam;

[0100] (4) Pre-welding is carried out. Similar to the structure of the near-zero-thinning stirring head, the shoulder also adopts an Archimedean spiral as the structural curve. The length of the stirring head needle is 0.2 - 0.25 mm less than the actual thickness of the welding area, but the shoulder diameter is 1 - 2 mm less than that of the formal welding stirring head. During the welding process, the weld can be closely attached to the back rigid support under the action of the upsetting force of the main shaft, eliminating the non-conforming phenomenon between the two, making the weld between the stirring head and the backing plate form a complete closed extrusion die, and achieving penetration to a certain extent. Affected by the assembly state, welding defects inside the weld may occur during this process.

[0101] At the same time, to control the circular ring position size, when pre-welding the 8 longitudinal seams of the petal segments, the cross-symmetric welding method is adopted, and the welding sequence is as follows Figure 5 As shown, the inclination angle of the main shaft is 2° - 2.5°.

[0102] (5) Formal welding is carried out. A conventional near-zero-thinning stirring head is used. The length of the stirring head needle is 0.25 - 0.3 mm less than the actual thickness of the welding area, and the shoulder is 1 - 2 mm larger than the pre-welding stirring head, which can stir the flash generated during pre-welding into the weld again, reducing the loss of base metal. During the formal welding stage, the weld has achieved close fitting between the rigid support backing plate, and the assembly state is better than that during pre-welding. Therefore, the internal weld defects generated during pre-welding can be eliminated during the formal stirring welding stage, obtaining a complete and near-defect-free longitudinal seam, achieving a qualified one-time welding of the petal longitudinal seam, as Figure 6 shown.

[0103] (6) Clean the front of the weld and conduct phased array in-situ detection to check the internal quality of the weld;

[0104] (7) Remove the circular ring from the shelf, clean the back of the weld, conduct phased array detection and X-ray fluoroscopy, and measure the thickness of the longitudinal seam.

[0105] For the bottom ring seam of the tank, the defect control method is as follows:

[0106] (1) Adopt the diagonal multi-point segment positioning method. Use a positioning stirring head with a shoulder diameter of 10 mm and a stirring needle length of 1 - 1.5 mm. The circular ring and the transition ring seam are evenly positioned at 16 points within the entire circle, and the length of each positioning weld is 300 - 400 mm. The circular ring and the top cover ring seam are evenly positioned at 8 points within the entire circle, and the length of each positioning weld is 200 - 300 mm. The positioning sequence is as follows Figures 7 - 8 shown;

[0107] (2) Carry out full-circle positioning welding. Also use a positioning stirring head with a shoulder diameter of 10 mm and a stirring needle length of 1 - 1.5 mm. The welding angle is 358°, and 2° is reserved without welding for seam alignment during pre-welding;

[0108] (3) Perform pre-welding. Use the same stirring head as that for the pre-welding of the longitudinal seam of the segment. The length of the stirring head needle is 0.2 - 0.25 mm less than the actual thickness of the welding area, but the diameter of the shoulder is 1 - 2 mm less than that of the stirring head for the formal welding. During the welding process, the weld can be closely attached to the rigid support on the back under the action of the upsetting force of the main shaft, eliminating the non-conforming phenomenon between the two, so that the weld between the stirring head and the backing plate forms a complete closed extrusion die, and penetration is achieved to a certain extent. Affected by the assembly state, welding defects inside the weld may be generated during this process;

[0109] (4) Perform formal welding. Use a near-thicknessless stirring head with a retractable stirring pin for the formal welding. During the end stage of welding, the stirring pin is retracted uniformly to eliminate the keyhole defect. The inclination angle of the main shaft is 2° - 2.5°, and the welding angle is 360° formal welding + 5° repeated welding + 7° retraction welding. The length of the retractable stirring head needle is 0.25 - 0.3 mm less than the actual thickness of the welding area, and the shoulder is 1 - 2 mm larger than the stirring head for the pre-welding, which can stir the flash generated during the pre-welding into the weld again, reducing the loss of base metal. During the retractable friction stir welding stage, the weld has been closely attached to the rigid support backing plate, and the assembly state is better than that during the pre-welding. Therefore, this welding stage can eliminate the welding defects inside the weld generated during the pre-welding process, obtain a complete and near-defect-free weld, and achieve a qualified one-time welding of the bottom ring weld. The state of the retractable near-thicknessless friction stir welding is as follows Figure 9 as shown.

[0110] (5) Clean the front of the weld and perform phased array in-situ detection to check the internal quality of the weld.

[0111] (6) After the bottom welding is completed, remove it from the shelf, clean the back of the weld, perform phased array detection and X-ray fluoroscopy, measure the weld thickness, perform hydraulic and helium mass spectrometry leak detection on the bottom, and evaluate the overall quality of the bottom.

[0112] The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A method for friction stir welding with nearly no thinning and control of geometric dimensions of a thin-walled box bottom, characterized in that: It includes the following steps: S1. Assemble the thin-walled box bottom with a diameter of 5m; S2. Perform milling on the 5m-diameter thin-walled box bottom by using an assembly accuracy control method; S3. Weld the 5m-diameter thin-walled box bottom by using a method for controlling defects in friction stir welding with nearly no thinning; The assembly accuracy control method in step S2 includes a method for controlling the assembly accuracy of the longitudinal seams of the segmental plates and a method for controlling the assembly accuracy of the circumferential seams of the box bottom. The method for controlling the assembly accuracy of the circumferential seams of the box bottom includes an in-situ milling method for the circumferential seam between the circular ring and the transition ring and a milling method for the circumferential seam between the circular ring and the top cover; The method for controlling defects in friction stir welding with nearly no thinning in step S3 includes a method for controlling defects in the longitudinal seams of the segmental plates and a method for controlling defects in the circumferential seams of the box bottom; The method for controlling the assembly accuracy of the longitudinal seams of the segmental plates includes the following steps: A1. Milling the allowance of the longitudinal seams of the segmental plates, installing the segmental plates on the longitudinal seam die according to the butting line, adjusting the assembly positions of the segmental plates in the axial and circumferential directions, supporting the large ends of the segmental plates with a leveling mechanism, and fixing the middle parts of the segmental plates with a clamping belt; A2. Install a pressing strip between the welding area and the pressing mechanism, the thickness of the pressing strip is 4 - 6mm, and the distance between the edge of the pressing strip and the butt joint of the weld is controlled within 15 ± 2mm; A3. Operate the pneumatic pressing mechanism to press both sides of the segmental plates. After programming the milling program for the segmental plates according to the die surface of the segmental plates, mill the allowances on both sides of the segmental plates. First, perform rough milling, leaving a margin of 1 - 2mm, and then perform finish milling, with the flatness of the processed edge less than or equal to 0.2mm; A4. Milling the remaining 7 segmental plates in sequence, and when milling the last segmental plate, leave a margin of 2 - 3mm, and perform milling according to the dimensions after welding the 7 segmental plates; A5. After milling is completed, lower the segmental plate die and check the butting situation of the longitudinal seam; The method for controlling defects in the longitudinal seams of the segmental plates includes the following steps: D1. Adopt a five-segment positioning welding. The five-segment positioning welding fixes the assembly state before nearly no thinning welding. The length of each positioning weld in the five-segment positioning welding is 200 - 250mm, and a positioning stirring head with a shoulder diameter of 10mm and a stirring pin length of 1 - 1.5mm is used; The distribution positions are: one at each end of the longitudinal seams of the segmental plates, and three evenly distributed at the middle position of the longitudinal seams. The welding sequence is: positioning of the lower segment of the longitudinal seam, positioning of the middle position segment of the longitudinal seam, positioning of the upper segment of the longitudinal seam, positioning of the segment at the 2 / 5 position from the lower end of the longitudinal seam, positioning of the segment at the 4 / 5 position from the lower end of the longitudinal seam; D2. After the five-segment positioning welding is completed, perform full-positioning welding, using the same stirring head as the five-segment positioning welding to fix the assembly state; D3. Perform five-segment positioning welding and full-positioning welding on the remaining 7 longitudinal seams in sequence to fix the gap state of each longitudinal seam; D4. Perform pre-welding on the 8 longitudinal seams of the segmental plates. The shoulder adopts an Archimedean spiral as the structural curve, the length of the stirring pin of the stirring head is 0.2 - 0.25mm less than the thickness of the welding area, and the shoulder diameter is 1 - 2mm less than that of the formal welding stirring head; D5. Perform formal welding on the 8 longitudinal seams of the segmental plates, using a nearly no thinning stirring head. The length of the stirring pin of the stirring head is 0.25 - 0.3mm less than the thickness of the welding area, and the shoulder is 1 - 2mm larger than the pre-welding stirring head; D6. Clean the front of the weld seam, conduct in-situ phased array detection, and inspect the internal quality of the weld seam; D7. Lower the ring, clean the back of the weld seam, conduct phased array detection and X-ray fluoroscopy, and measure the thickness of the longitudinal seam; The method for controlling the defects of the bottom ring weld seam of the box includes the following steps: E1. Adopt the diagonal multi-point segment positioning method, use a positioning stirring head with a shoulder diameter of 10 mm and a stirring needle length of 1 - 1.5 mm. The circumferential seam between the ring and the transition ring is evenly segmented and positioned at 16 places within the whole circle, and the length of each positioning weld seam is 300 - 400 mm. The circumferential seam between the ring and the top cover is evenly segmented and positioned at 8 places within the whole circle, and the length of each positioning weld seam is 200 - 300 mm; E2. Conduct circumferential positioning welding. Similarly, use a positioning stirring head with a shoulder diameter of 10 mm and a stirring needle length of 1 - 1.5 mm, and the welding angle is 358°, leaving 2° unwelded; E3. Conduct pre-welding, use the same stirring head as that for the pre-welding of the longitudinal seam of the segment. The length of the stirring needle of the stirring head is 0.2 - 0.25 mm less than the thickness of the welding area, and the shoulder diameter is 1 - 2 mm less than that of the formal welding stirring head; E4. Conduct formal welding. The formal welding uses a retractable stirring head. During the end stage of welding, the stirring needle is retracted evenly. The spindle inclination angle of the retractable stirring head is 2° - 2.5°, and the welding angle is 360°. For formal welding +5°, and for repeated welding +7° retraction welding. The length of the retractable stirring head needle is 0.25 - 0.3 mm less than the thickness of the welding area, and the shoulder is 1 - 2 mm larger than the pre-welding stirring head; E5. Clean the front of the weld seam, conduct in-situ phased array detection, and inspect the internal quality of the weld seam; E6. After the bottom of the box is welded, lower it to clean the back of the weld seam and conduct phased array detection and X-ray fluoroscopy, measure the thickness of the weld seam, conduct hydraulic and helium mass spectrometry leak detection on the bottom of the box, and conduct assessment and evaluation on the overall quality of the bottom of the box.

2. According to the method for friction stir welding with nearly no thinning and control of geometric dimensions of a thin-walled box bottom described in claim 1, it is characterized in that: The method for in-situ matching milling of the circumferential seam between the ring and the transition ring includes the following steps: B1. After the welding backing plate and the wedge-shaped compensation block on the equipment die are propped out, measure the roundness of the whole circle of the welding backing plate, and confirm that the roundness is within ±0.2 mm; B2. Lift the transition ring to the circumferential seam die, make the bottom of the large end of the transition ring contact the leveling device at the lower part of the circumferential seam tooling die, and adjust the transition ring to be horizontal by adjusting the leveling device; B3. Lift the transition ring pressing tooling to the circumferential seam workbench surface, and make the pneumatic pressing plate press the outer plane of the transition ring; B4. Prop out the inner support backing plate and the wedge-shaped compensation block of the transition ring; B5. Lift the circumferential seam welding die by 50 mm, lift the ring to the circumferential seam die, and adjust the level of the ring according to the distance from the theoretical scribed line of the large end of the ring to the surface of the welding platform; B6. Lift the ring pressing mechanism to the circumferential seam die and lock it. Make the pneumatic pressing plate press the large end of the ring, measure the gap between the inner profile surface of the ring and the die, and for the gap part, use the mechanical pressing method for manual local adjustment to control the gap against the die within 0.2 mm; B7. Call out the five-axis program in the coordinate system, make the spindle axis position coincide with the center position of the X-axis of the annular seam film tire, adjust the spindle to a 31° angle with the horizontal plane, install a milling cutter with a diameter of 20mm, and perform rough milling and fine milling of the large end of the ring according to the down-milling principle, with a rough milling amount of 10~15mm and a fine milling amount of 1~3mm; B8. When the milling is to 1mm from the theoretical scale line at the large end of the ring, lower the annular seam membrane, check the transition ring and the ring, and check the amount of misalignment. If the amount of misalignment is greater than the set value, mill the excess at the large end of the ring again until the misalignment is less than 0.2mm after the transition ring and the ring are butted together, and the butt clearance is less than 0.2mm.

3. According to claim 1, a method for friction stir welding and shape and position size control of a thin-wall box bottom with near-zero thinning, Features: The method for milling the annular gap between the circular ring and the top cover comprises the following steps: C1. Measure the roundness of the top cover annular seam or annular annular seam welding pad and confirm that the roundness is within ±0.2mm; C2. Lower the welding pad to leave space for the milling cutter at the small end of the ring. The lowering height should not be less than 20mm. C3. Call out the five-axis program in the coordinate system, make the spindle axis position coincide with the center position of the X-axis of the annular seam film tire, adjust the spindle to the horizontal plane angle of 82°, install a milling cutter with a diameter of 20mm, and perform rough milling and fine milling of the small end of the ring according to the down-milling principle, with a rough milling amount of 10~15mm and a fine milling amount of 1~3mm; C4. When the milling distance is 1mm from the theoretical scale line of the small end of the ring, lift the welding pad and check the top cover and the ring. If the top cover cannot be inserted into the ring, mill the small end of the ring again with a feed amount of 0.2~0.3mm each time until the top cover is inserted into the ring and the butt clearance is less than 0.2mm.

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