Method for controlling welding hot cracking with bilateral symmetrical thermal strain

Abstract

This invention discloses a method for controlling welding hot cracks with bilateral symmetrical thermal strain assistance. The method includes preparation before welding and welding. In the welding step, the temperature generated by the two induction coils is stabilized at 302±5℃ before welding begins. During welding, the welding heat source and the two induction coils move synchronously and uniformly from back to front, so that the thermal strain generated by the two induction coils continuously heats the workpieces to be welded on both sides of the weld within the brittle temperature range behind the molten pool until welding is completed. In this way, the thermal strain generated by the two induction coils continuously heats the corresponding workpieces to be welded on both sides of the weld within the brittle temperature range behind the molten pool. The thermal expansion of the workpieces generates a transverse compressive strain pointing towards the weld center in the weld metal within the brittle temperature range (BTR zone) behind the molten pool, thus counteracting the crack-inducing tensile strain generated in the brittle temperature range during welding, thereby effectively controlling the generation of hot cracks.

Description

Technical Field

[0001] This invention relates to the field of welding technology, and more specifically to a method for controlling welding hot cracks with bilateral symmetrical thermal strain assistance. Background Technology

[0002] Welding hot cracking is a common problem in the welding process. The formation of welding hot cracking is the result of the combined effect of metallurgical and mechanical factors. During the solidification process of the weld, the presence of the low-melting-point eutectic liquid phase at the grain boundary is the metallurgical factor for the formation of hot cracking, while the weld metal in the Brittle Temperature Range (BTR) will be subjected to a certain tensile strain, which is the mechanical factor for the formation of welding hot cracking.

[0003] Especially for high-strength aluminum alloys, which have lower density and higher specific strength compared to conventional aluminum alloys, and due to their higher thermal conductivity, coefficient of thermal expansion, and lower critical instability stress in thin-plate structures, they are prone to significant welding deformation, hot cracking, and softening of weld joints during welding. The high susceptibility to hot cracking exhibited by high-strength aluminum alloys during welding poses a significant safety hazard to their structural reliability: welding hot cracks can easily lead to macroscopic fracture of the joint after welding or during service, and can also trigger other early failure problems, such as fatigue failure during long-term service, ultimately severely shortening the service life of the structure. Therefore, controlling welding hot cracking in high-strength aluminum alloys remains a crucial scientific problem that urgently needs to be solved in this field.

[0004] To prevent welding hot cracking, one can generally address it from the perspectives of metallurgical and mechanical factors. From a metallurgical perspective, the main approach is to adjust the alloy composition and crystal structure of the weld by using welding wire to improve the crack resistance of the weld metal. However, metallurgical methods may sacrifice the performance of the welded joint due to the addition of dissimilar welding wires, which limits the practical application of this method. Therefore, in recent years, research focus has gradually shifted to mechanical control strategies. Currently, most methods employ energy field assistance to achieve active control of hot cracks. Specific methods include welding hammering, pre-set transverse compressive stress, welding rotary extrusion, or welding electromagnetic impact, etc. Among these, the pre-set transverse compressive stress method suffers from attenuation of the fixed contact mechanical force due to displacement during the transverse contraction of the weldment, deviating from the expected extrusion effect. Welding hammering and welding rotary extrusion cannot directly generate transverse compressive stress pointing towards the weld center; they can only play a stretching role, which may even cause outward diverging tensile stress, thus promoting crack formation. Correspondingly, the aforementioned welding electromagnetic impact method, such as Chinese patent publication number CN1943969A, directly places an electromagnetic coil above the weld. Its working principle is: the electromagnetic coil is directly above the weld, and the electromagnetic field... The pulsed current generates a downward pulsed electromagnetic force on the weld behind the molten pool, causing the weld metal to plastically extend and thus releasing tensile stress. However, the force is downward, unlike the two induction coils in this embodiment which directly generate a transverse compressive force pointing towards the center of the weld. Therefore, this type of welding method is prone to producing the opposite effect in controlling the transverse tensile strain, which is most sensitive to hot cracking. That is, it has the same problem as the welding hammer method and the welding rotary extrusion method. Moreover, for this type of welding method (i.e., the welding electromagnetic impact method), the induced magnetic field generated by the electromagnetic coil and the compressive stress generated by the current in the induced magnetic field are only applicable to the conditions where welding current flows through the positive and negative poles in the arc welding method. However, laser welding converts mechanical energy of a certain power into heat energy. No current flows through the workpiece, and the magnetic field only affects the current in the field. It has no mechanical effect on laser welding and is therefore not suitable for laser welding.

[0005] In view of this, this application has conducted in-depth research on this basis, resulting in this case. Summary of the Invention

[0006] The purpose of this invention is to provide a method and apparatus for controlling welding hot cracks with bilateral symmetrical thermal strain. This method enables non-contact welding with the workpiece, avoiding damage to its fatigue performance. At the same time, the compressive strain generated by the external heating load balances the tensile strain during the welding process, offsetting the crack-inducing tensile strain in the BTR zone, so that the transient tensile strain in the BTR zone is less than the minimum ductility of the metal, resulting in good control of welding hot cracks.

[0007] To achieve the above objectives, the solution of the present invention is: a method for controlling welding hot cracking with bilateral symmetrical thermal strain assistance, comprising the following steps: Step 1, Preparations before welding: Step 1-1: Determine the brittle temperature range, the butt joint gap between the two workpieces to be welded, and the misalignment of the workpieces; Step 1-2: Install a welding heat source above the weld between the two workpieces to be welded. The welding heat source is directly facing the weld. Symmetrically arranged induction coils are set on both sides of the weld. The two induction coils are located above the corresponding workpieces to be welded. Both induction coils are rectangular. In this case, with the welding direction from back to front, the welding heat source is located in front of the induction coil, and the center line connecting the two induction coils corresponds to the starting position of the brittle temperature range. The adjustment distance between the center points of the two induction coils and the center line of the weld is adjusted, and the total distance between the center points of the two induction coils is the optimal extrusion stress transmission distance. Step 2, Welding: Welding begins after the temperatures generated by the two induction coils stabilize at 302±5℃. During the welding process, the welding heat source and the two induction coils move synchronously and uniformly from back to front, so that the thermal strain generated by the two induction coils continuously heats the workpieces to be welded on both sides of the weld seam within the brittle temperature range behind the molten pool until welding is completed.

[0008] In step 1-1, the mating gap is ≤0.1mm and the workpiece misalignment is ≤0.2mm.

[0009] After step 1-1, tack welds are performed on both ends of the weld, forming tack weld points at each end of the weld.

[0010] The length of the two positioning weld points is 2-3mm, the welding heat source is a laser beam, and the laser power of the welding heat source is 90% of the laser power during the actual welding.

[0011] The adjustment distance between the center point of each of the two induction coils and the center line of the weld is 4-7mm.

[0012] After steps 1-2, parameters are preset. The welding heat source is a laser beam, the laser power of the welding heat source is 1100-1800W, the welding speed of the welding heat source is 12-16mm / s, and the defocusing amount of the welding heat source is -0.5-0mm.

[0013] The heating power of the heating device matching the induction coil is 400 - 700 W, the output frequency of the heating device is 50 - 80 kHz. Among them, the peak temperature of the heating area of the induction coil is 280 - 320 °C, and the thermal strain following distance of the induction coil is 4 - 8 mm.

[0014] The welding process of step 2 is as follows: Step 2 - 1: First, preheat in advance and start welding when the temperatures generated by the two induction coils are both stable at 302 ± 5 °C. The welding heat source uses a linearly increasing power mode for starting welding, so as to increase from 0 W to the set power within the set time; Step 2 - 2: The welding heat source and the two induction coils respectively move synchronously and uniformly from back to front along the length direction of the weld. During the movement, keep the relative positions of the welding heat source and the two induction coils unchanged. The thermal strains generated by the two induction coils continuously heat the corresponding workpieces to be welded on both sides of the weld within the brittle temperature range behind the molten pool; Step 2 - 3: After the weld is completed, the welding heat source uses a linearly decreasing power mode for ending, so as to decrease from the set power to 0 W within the set time. At the same time, the two induction coils and the welding heat source move to leave the workpieces to be welded respectively and then are turned off.

[0015] In step 2 - 2, during the heating process of the workpiece to be welded, ensure that the fluctuation range of the laser power of the welding heat source and the temperature generated by the thermal strain is within ±2%.

[0016] The formula for the transverse true extrusion stress generated at the weld after the workpiece to be welded expands due to heat is: , where is the elastic modulus of the material, is the transverse true extrusion strain of the workpiece to be welded due to heat expansion, is the linear expansion coefficient of the high - strength aluminum alloy, is the length of the weld before being extruded, is the width of the weld before being extruded, is the temperature rise change of the workpiece to be welded in the part heated by the induction coil; Among them, the temperature rise change of the workpiece to be welded is , where is the heat conversion efficiency, The value range of is 0 - 1; is the heat generated by the resistance, is the specific heat capacity, is the cross - sectional radius of the resistance wire of the induction coil, is the length of the resistance wire of the induction coil, The width of the resistance wire in the induction coil. This represents the number of resistance wires in the induction coil. This represents the magnitude of the current flowing through the resistor.

[0017] After adopting the above method, the present invention has the following beneficial effects: the thermal strain generated by two induction coils continuously heats the two workpieces to be welded within the brittle temperature range. The thermal expansion of the workpieces to be welded generates a transverse compressive strain pointing towards the center of the weld on the weld metal behind the molten pool in the brittle temperature range (BTR zone). This counteracts the crack-inducing tensile strain generated in the brittle temperature range during welding, thereby controlling the generation of hot cracks from the mechanical source. In other words, it actively counteracts the crack-inducing strain and improves the control effect of welding hot cracks.

[0018] Furthermore, by using two induction coils to heat the workpieces to be welded separately, that is, to heat both sides of the weld, the two induction coils do not come into contact with the workpieces to be welded during the heating process, so that no indentations or damage are left on the surface of the workpieces to be welded, effectively avoiding damage to their fatigue performance. Attached Figure Description

[0019] Figure 1 This is a schematic diagram illustrating the principle of welding hot cracking in this invention.

[0020] Figure 2 The mechanical conditions that cause hot cracks during normal welding.

[0021] Figure 3 This is a visual representation of a weld seam welded using conventional methods.

[0022] Figure 4 This is a view of the weld seam produced using the method of the present invention.

[0023] Figure 5 This is a schematic diagram of the structure of the device of the present invention.

[0024] Figure 6 This is a structural schematic diagram of the device of the present invention from another angle.

[0025] In the picture: 1a-Welding platform; 2a-Clamping fixture; 1-Welding heat source; 2-Induction coil; 22-Heating device; 3-Weld seam. Detailed Implementation

[0026] To further explain the technical solution of the present invention, the present invention will be described in detail below through specific embodiments.

[0027] Example 1 A method for controlling welding hot cracking with bilateral symmetrical thermal strain assistance, such as Figure 1As shown, it welds the weld 3 formed between two workpieces to be welded. In this embodiment, for ease of description, a rectangular test plate with a thickness of 1-3mm for both workpieces to be welded is used as an example. The workpieces to be welded are rectangular test plates cut from high-strength aluminum alloy plates using laser cutting. The workpieces to be welded are conventional high-strength aluminum alloys. It should be noted that the above specifications and materials of the workpieces to be welded are illustrative examples. This embodiment is not limited to the above thickness, material, and shape. That is, it is applicable to all workpieces to be welded used in conventional welding, that is, materials with high thermal cracking sensitivity. The shape is not limited, but a thin plate with a thickness of 1-3mm is preferred.

[0028] The method for controlling welding hot cracks with bilateral symmetrical thermal strain in this embodiment includes the following steps.

[0029] Step 1, preparations before welding, specifically: Step 1-1: Determine the length of the brittle temperature range, the butt joint gap between the two workpieces to be welded, and the misalignment of the workpieces.

[0030] Furthermore, the above-mentioned butt joint gap is ≤0.1mm, and the workpiece misalignment is ≤0.2mm; where the butt joint gap refers to the gap distance between the butt joint end faces of the two workpieces to be welded, and the workpiece misalignment refers to the height difference formed between the surfaces of the two workpieces to be welded due to misalignment along the thickness direction of the workpieces to be welded.

[0031] Furthermore, after step 1-1, the above-mentioned weld 3 and the workpiece to be welded are pre-treated. The specific treatment process is as follows: First, a stainless steel wire brush is used to uniformly grind the mating surface and side area of ​​the two workpieces to be welded along the front-back direction of the weld seam 3. The mating surface refers to the side of the two workpieces to be welded facing each other. The side area is the area of ​​the workpieces to be welded from the side of the weld seam 3 to the area extending 4-8mm away from the weld seam 3. In this embodiment, the side area is 5mm as an example. The two workpieces to be welded are ground until a uniform silvery-white metallic luster is exposed to completely remove the Al2O3 oxide film on the surface and improve the laser absorption rate.

[0032] Next, using a lint-free cloth soaked in pure acetone, wipe the side area of ​​weld seam 3 repeatedly three times to thoroughly remove oil, moisture, and fingerprints. It should be noted that the pure acetone can be replaced with other commonly used cleaning agents.

[0033] Step 1-2: After determining and pre-processing the two workpieces to be welded according to the above parameters, perform tack welding on both ends of weld 3 so that tack welding points are formed at both ends of weld 3 (i.e., the front end and the rear end) to ensure that the workpieces to be welded do not shift during the welding process.

[0034] Specifically, in this embodiment, the length of both positioning weld points is 2-3mm. The distance between the positioning weld point at the front end and the front end of the weld 3 is 10mm. Correspondingly, the distance between the positioning weld point at the rear end and the rear end of the weld 3 is 10mm. The distance can be set according to the actual welding situation and is not limited to the above values.

[0035] Furthermore, the welding heat source 1 described below is used for tack welding. The welding heat source 1 can be a conventional welding gun such as a laser welding gun. In this embodiment, a laser beam is used as the welding heat source 1 as an example. This laser beam is the beam emitted by an existing conventional laser welding gun. The welding heat source 1 (i.e., the laser beam) is equipped with a laser. The laser power of the welding heat source 1 is 90% of the laser power during the actual welding. For example, if the power of the welding heat source 1 during the actual welding is 1600W, then the laser power during laser tack welding is 1440W. This ensures that the workpiece to be welded does not shift during the welding process.

[0036] It should be noted that the laser positioning welding described above uses welding heat source 1 for positioning welding, which is a well-known existing technology, so it will not be described in detail here.

[0037] Steps 1-3: Install a welding heat source 1 above the two weld seams 3, with the welding heat source 1 facing the weld seam 3. Symmetrically arranged induction coils 2 are set on both sides of the weld seam 3. The two induction coils 2 are located above the corresponding workpieces to be welded. The two induction coils are rectangular to make the heating area rectangular and the heating temperature more uniform.

[0038] In this configuration, the welding heat source 1 is located in front of the two induction coils 2, and the center line connecting the two induction coils 2 corresponds to the starting position of the brittle temperature range (BTR region). Then, the adjustment distance between the center points of the two induction coils 2 and the center line of the weld 3 is adjusted (i.e.,...). h / 2), so that the total distance between the two induction coils 2 is 2 h The total distance, which is twice the adjustment distance, is the optimal distance for transferring compressive stress.

[0039] Furthermore, in this embodiment, the adjustment distance between the center points of the two induction coils 2 and the center line of the weld 3 is 4-7mm, preferably 5mm, with a total distance of 10mm, which is the optimal distance for transmitting compressive stress. The height between the two induction coils 2 and the workpiece to be welded is 3-5mm. It should be noted that this height can be set according to actual conditions and is not limited to the above-mentioned height.

[0040] Furthermore, the line connecting the centers of the two induction coils 2 is located 6 mm directly behind the center of the welding heat source 1. It should be noted that during the welding process described below, the thermal strain generated by the two induction coils 2 causes localized expansion of the workpieces to be welded on both sides of the weld 3. This expansion of the workpieces generates a transverse compressive strain directed towards the weld centerline, caused by a temperature gradient, on the weld metal behind the molten pool in the brittle temperature range. The aforementioned 6 mm is the length (D) of the weld 3 behind the molten pool in the brittle temperature range; where the molten pool refers to the center position of the welding heat source 1. This determines the relative positions of the induction coils 2 and the welding heat source 1.

[0041] Furthermore, both induction coils 2 mentioned above are commercially available rectangular induction coils. In this embodiment, the length of both induction coils 2 is 50 mm, the width is 10 mm, both induction coils 2 are wound with φ=0.5 mm high-temperature resistant enameled wire, and both induction coils have 5 turns. It should be noted that the dimensions of the two induction coils 2 can be adjusted according to actual conditions and are not limited to the above dimensions.

[0042] In this embodiment, the center line between the two induction coils 2 refers to the connecting line between the center points of the two induction coils 2; the center line of the weld 3 is the middle line of the weld 3 arranged along its length.

[0043] Furthermore, after determining the relative positions of the welding heat source 1 and the two induction coils, the two workpieces to be welded are rigidly clamped. Specifically, the two pre-treated workpieces are fixedly installed. This installation method adopts conventional methods in the welding field. For example, the two workpieces to be welded are installed on the welding platform 1a described below, and then conventional pneumatic clamps or evenly distributed multi-point pressure plates are used to press the two workpieces to be welded, ensuring that the butt joint gap and assembly gap between the two workpieces to be welded are strictly controlled within the range of 0.05mm to 0.10mm to prevent the workpieces to be welded from becoming unstable and deformed during the welding process.

[0044] Furthermore, after steps 1-3, parameter presets are performed, specifically as follows: In welding heat source 1, the focal length of the laser spot is 240 mm, the wavelength of the fiber laser is 1064 nm, the laser spot diameter is approximately 1 mm, the laser power is 1100-1800 W, the welding speed of welding heat source 1 is 12-16 mm / s, and the defocusing amount of welding heat source 1 is -0.5-0 mm. Here, the laser power refers to the welding power range used in this welding process.

[0045] Two induction coils 2 are equipped with heating devices. The two induction coils 2 and the heating devices together form a double-sided symmetrical thermal strain heating device. In the double-sided symmetrical thermal strain heating device, the heating power is 400-700W and the output frequency of the heating device is 50-80kHz. The peak temperature of the heating area of ​​the induction coil 2 is 280-320℃ and the thermal strain following distance is 4-8mm. The thermal strain following distance refers to the relative distance between the induction coil 2 and the welding heat source 1 in the welding direction, which is used to match the position of the BTR zone.

[0046] It should be noted that the peak temperature of the bilateral symmetrical thermal strain should not exceed 320℃ to prevent the high-strength aluminum alloy workpiece to be welded from over-aging and softening, losing its ability to transfer transverse extrusion stress, which would instead exacerbate the generation of hot cracks.

[0047] Step 2, Welding (i.e., welding and application of symmetrical thermal strain load on both sides): After the temperature generated by the two induction coils 2 stabilizes at 302±5℃, the laser is turned on to start welding. During the welding process, the welding heat source 1 and the two induction coils 2 move synchronously and uniformly from back to front, so that the thermal strain generated by the two induction coils 2 continuously heats the corresponding workpieces to be welded on both sides of the weld in the brittle temperature range behind the molten pool until the welding is completed.

[0048] It is worth mentioning that, in this embodiment, the thermal strain generated by the two induction coils 2 is applied to the corresponding local workpieces to be welded on both sides of the weld 3. After the two workpieces to be welded expand, they generate a transverse compressive strain pointing towards the center line of the weld 3 caused by the temperature gradient behind the molten pool, in order to offset the tensile stress of the welding process and thus control the generation of hot cracks. In other words, the induction coils 2 heat the workpieces to be welded. Since the workpieces to be welded do not have a brittle temperature range, but the metal in the weld 3 behind the molten pool has a brittle temperature range, the temperature of the metal in the weld 3 is within this range, and the ductility of the metal in the weld 3 is very small. As long as there is a little tensile stress, it will be torn apart, and hot cracks will be generated. Therefore, the thermal strain generates an additional transverse compressive strain pointing towards the center line of the weld caused by the temperature gradient to offset the tensile stress of the welding process and thus control the generation of hot cracks.

[0049] To elaborate, the specific welding process in step 2 is as follows.

[0050] Step 2-1: First, turn on the two induction coils 2 30 seconds in advance. When the temperature generated by the two induction coils 2 is stable at 302±5℃, start welding. Then, the welding heat source 1 adopts a linear power increase mode to start welding, so as to increase from 0W to the set power within a set time (0.3s in this embodiment) to prevent the welded workpiece from burning through at the weld start point. In this embodiment, the set power is 1600W as an example. The specific set power value is set according to the actual situation.

[0051] Furthermore, to ensure that the thermal strain generated by the two induction coils 2 continuously heats the corresponding workpieces to be welded on both sides of the weld within the brittle temperature range behind the molten pool, the welding heat source 1 moves for a set time before the two induction coils 2 begin to move forward, at which point all three move synchronously and uniformly. For example, when the length of the weld 3 in the brittle temperature range behind the molten pool is 6mm, the welding heat source 1 first moves at a speed of 12mm / s for 0.5s, and then the induction coils 2 move synchronously at a speed of 12mm / s.

[0052] Step 2-2: The welding heat source 1 and the two induction coils 2 move synchronously and uniformly from back to front along the length of the weld 3. During the movement, the relative positions of the welding heat source and the two induction coils remain unchanged. The thermal strain generated by the two induction coils continuously heats the corresponding workpieces to be welded on both sides of the weld in the brittle temperature range behind the molten pool. At the same time, the laser power of the welding heat source 1 and the temperature generated by the thermal strain are monitored in real time to ensure that the fluctuation range of the laser power and the temperature generated by the thermal strain are within ±2%.

[0053] It should be noted that the temperature at which thermal strain occurs here is maintained at 302±5℃ as mentioned above.

[0054] Steps 2-3: After welding is completed, the welding heat source 1 adopts a linear power reduction mode to finish off the process, reducing the power from 1600W to 0W within 0.3s. At the same time, the two induction coils 2 and the welding heat source 1 continue to move forward until they are outside the front end of the workpiece to be welded before shutting off. In this embodiment, the distance between the front side of the induction coil 2 and the front side of the welding heat source 1 is 50mm as an example. This distance can be set according to the actual situation and is not limited to the above value. That is, the welding heat source 1 is shut off after it leaves the front end of the workpiece to be welded, and the two induction coils 2 continue to move 50mm before shutting off.

[0055] Step 3: After the workpiece has cooled to room temperature naturally, inspect the appearance of the weld. The surface of weld 3 should be smooth and continuous, with no macroscopic hot cracks visible to the naked eye.

[0056] It is worth mentioning that in this embodiment, conventional welding is compared with welding in the manner described above. Conventional welding here refers to welding without the assistance of two induction coils 2, while other parameters are the same. Therefore, it can be seen that... Figure 3 As shown, under conventional welding conditions, the weld crack is clearly visible and its length is relatively long, measuring approximately 57 mm. In comparison, as... Figure 4 As shown, when the output power of the two induction coils 2 is 560 W and the temperature is about 302 ℃, the cracks on the weld 3 have completely disappeared, demonstrating the good control effect of the double-sided symmetrical thermal strain on the thermal cracks under this power.

[0057] This embodiment presents a method for controlling welding hot cracks using a dual-sided symmetrical thermal strain-assisted approach. During the welding process, two induction coils 2 continuously heat the corresponding workpieces on both sides of the weld within the brittle temperature range behind the molten pool, generating thermal strain. The thermal expansion of the workpieces causes a transverse compressive strain pointing towards the weld center in the weld metal within the brittle temperature range (BTR zone) behind the molten pool. This counteracts the crack-inducing tensile strain generated in the brittle temperature range during welding, controlling the generation of hot cracks from a mechanical perspective. This method offers flexible parameter adjustment, adaptability to workpieces of different materials and sizes, strong adaptability, and stable and reliable control effects, making it suitable for widespread application in actual industrial welding production.

[0058] To elaborate further, Figure 2 The figure shows the mechanical conditions that cause hot cracking during welding, with the horizontal axis representing the... It represents the strain produced under tensile stress; Indicates the brittle temperature range The plasticity of the inner weld metal varies with temperature. , A function representing the change in plasticity of weld metal with temperature; Indicates the liquidus temperature; f(T) The function representing strain as a function of temperature, where, = f(T) In this embodiment, This is represented by curves I, II, and III. Among them, when a liquid thin film appears, there exists a minimum plasticity reserve between the grains (…). (See curve II).

[0059] If the tensile strain varies with temperature according to curve I, at the solidus temperature... Near the point where the plasticity of weld 3 is at its minimum within the brittle temperature range, only tensile strain is generated. ,Depend on Figure 4 It can be seen that the metal still has a tensile strain margin within the BTR range at this time. for: ,in, This represents the numerical value of the tensile strain.

[0060] because ,so Therefore, thermal cracking will not occur under these conditions.

[0061] If the strain is as shown in curve III, then the strain generated by the tensile stress is... This exceeds the minimum plasticity reserve that weld metal possesses within the brittle temperature range. ),Right now At this point, the liquid film between the crystals will be separated, forming hot cracks.

[0062] Therefore, it can be concluded that during the welding process, if... Figures 1-2 As shown, in a direction perpendicular to the center section of weld 3, a bilateral symmetrical thermal strain is applied to both sides of the weld. The local workpieces to be welded on both sides of the weld expand due to heat, which in turn generates a transverse compressive strain pointing towards the center line of the weld caused by the temperature gradient on the weld behind the molten pool in the BTR interval. This counteracts the transverse tensile strain experienced by the metal during solidification and shrinkage, making the difference between the two less than the minimum plasticity value of the metal in the BTR interval. This can reduce and control the generation of welding hot cracks. The bilateral symmetrical thermal strain is continuously applied throughout the welding process until the welding is completed.

[0063] Furthermore, if additional transverse compressive strain is applied, the strain will decrease. The transverse compressive strain generated after the application of bilateral symmetrical thermal strain is the driving force causing curve III to shift to the left. The tensile strain margin that the metal could originally withstand within the BTR range was... When a bilaterally symmetrical thermal strain is applied, the transverse compressive strain it generates on the weld metal is: At this point, the expression for the tensile strain margin that the metal can withstand within the BTR interval becomes: Therefore, it can be known that the transverse compressive strain The production of [something] is beneficial to increase This change deprives the weld metal of the mechanical conditions necessary for the generation of welding hot cracks.

[0064] In this context, under constrained conditions, any deformation of the workpiece to be welded will generate elastic strain. Applying a bilateral symmetrical thermal strain causes the workpiece to expand due to heat, resulting in displacement changes of particles within the workpiece and inducing stress. Therefore, to accurately control the magnitude of the lateral compressive strain generated by the two induction coils 2, it is obtained in the following manner.

[0065] Establish the relationship between the transverse compressive strain generated by the thermal expansion of the workpiece under bilateral symmetrical thermal strain and the change in temperature.

[0066] Let the radius of the resistance wire of the induction coil be... The length of the resistance wire in the induction coil is The width of the resistance wire in the induction coil is The number of resistance wires in the induction coil is Then the circumference of each induction coil is: The cross-sectional area of ​​the resistance wire is: .

[0067] Assume the resistivity of the resistance wire is Then the resistance value of the induction coil is: .

[0068] Let the magnitude of the current flowing through the resistor be... According to Joule's law, the heat generated by the resistance is: .

[0069] Let the heat absorbed by the workpiece to be welded be... The relationship between temperature and heat is: In the formula, For specific heat capacity, For quality, The initial temperature, The temperature of the workpiece to be welded after being heated. This refers to the heat conversion efficiency (including all energy losses, including heat dissipation). The value range is 0-1.

[0070] Among them, the temperature rise change of the workpiece to be welded in the part heated by the induction coil. for: .

[0071] Absorb heat The temperature of the workpiece to be welded is: .

[0072] As the workpiece expands towards the weld due to heat, the weld is compressed. Let the length of the weld before compression be... Width is The area of ​​the weld before it is subjected to compression is: The thermal expansion area is: , In the formula, The coefficient of linear expansion of high-strength aluminum alloy. This represents the lateral compression area.

[0073] Thus, the nominal strain of the transverse expansion of the workpiece after thermal expansion is: In the formula, Let be the nominal strain. Since the nominal strain cannot truly reflect the effect of the changing base length on the strain, while the true strain, i.e., the logarithmic strain, can truly reflect the cumulative process of deformation and has superposition properties, the transverse strain of the workpiece under thermal expansion can be expressed as the true strain. In the formula, The area of ​​the weld after being compressed, The initial area of ​​the weld, For the weld area, This represents the change in weld area; therefore, the actual transverse compressive strain of the workpiece under thermal expansion is: The formula for the actual transverse compressive stress generated at the weld after the workpiece expands due to heat is: In the formula, This is the elastic modulus of the material.

[0074] Thus, based on the above formula, the corresponding parameters in this embodiment can be obtained. For example, the transverse compressive stress formula can be used to obtain the transverse compressive strain required in this embodiment.

[0075] It is worth mentioning that this embodiment of a method for controlling welding hot cracks with bilateral symmetrical thermal strain assistance differs from conventional welding methods that form a preheating zone around the workpiece to be welded for welding, such as Chinese Patent Publication No. CN107877016A. The working principle of this type of welding method is as follows: an induction coil is placed in front of the stirring head to preheat the area in front of the stirring head. This type of welding method is a solid-state welding process that relies on the frictional heat and mechanical stirring between the stirring head and the workpiece to be welded to achieve connection. However, at room temperature, materials such as steel have extremely high yield strengths, and the stirring head cannot effectively agitate the base material, leading to severe wear of the stirring head. Even if it causes severe damage or breakage, the sole purpose of preheating here is to significantly reduce the material's yield strength by increasing the temperature, allowing the stirring head to be smoothly inserted and agitated to achieve a reliable solid-state bond. Therefore, preheating in friction stir welding is not directly related to the control of welding hot cracks. In other words, the purpose of this type of welding method is to reduce material flow stress and stirring head wear, thereby lowering the material's yield strength, not to control welding hot cracks. It does not target the BTR zone behind the molten pool, nor does it generate transverse tensile strain pointing towards the weld center. This type of welding method is an auxiliary heat source used to soften the material and does not involve generating specific mechanical effects on the BTR zone.

[0076] Compared to conventional welding methods that control hot cracking by setting preheating coils and postheating coils at the front and rear of the welding torch, such as Chinese Patent Publication No. CN102380688A, the method in this embodiment works by reducing the weld cooling rate and strain rate in the BTR region through weld preheating and slow cooling after welding, while improving the weld crystallization structure and mitigating the tendency for hot cracking from a mechanical auxiliary perspective. However, it cannot completely prevent or eliminate the generation of hot cracks, and the method does not counteract the crack-inducing tensile strain in the BTR region from a mechanical root cause.

[0077] Example 2 In this embodiment, a device for controlling welding hot cracks with bilateral symmetrical thermal strain is provided. It is an improvement on the conventional welding hot cracking device and is adapted to the method in Embodiment 1.

[0078] like Figures 5-6 As shown, the device includes a welding heat source 1 and two induction coils 2. For ease of description, the reference direction of the device is the normal operating state, and the welding direction is from back to front. The left and right directions are the horizontal direction perpendicular to the front and back. That is, the two sides of the weld 3 are the left and right sides.

[0079] The above-mentioned welding heat source 1 is a laser beam already available on the market. The welding heat source 1 is located directly above the weld seam 3, and the laser port of the welding heat source 1 is facing the weld seam 3; both induction coils 2 are rectangular in shape. The two induction coils 2 are respectively arranged on the left and right sides of the weld seam 3, and the two induction coils 2 are respectively arranged in one-to-one correspondence with the two workpieces to be welded. The two induction coils 2 are respectively located above the corresponding workpieces to be welded. Among them, the two induction coils 2 are symmetrically arranged along the center line of the weld seam 3. Preferably, the two induction coils 2 are arranged parallel to the center line of the weld seam 3, that is, the left and right sides of each induction coil are parallel to the center line of the weld seam 3; among them, the center connection line between the two induction coils 2 is located directly behind the welding heat source 1, and the center connection line between the two induction coils 2 corresponds to the starting position of the brittle temperature zone (i.e., the BTR zone) of the workpiece to be welded; in this example, the welding heat source 1 and the two induction coils 2 can move synchronously and uniformly along the welding direction.

[0080] Specifically, the device further includes a welding platform 1a. Above the welding platform 1a, there is a frame (not shown in the figure). The above-mentioned welding heat source 1 is installed on the frame in a manner that can move back and forth. The two induction coils 2 are respectively installed on the frame in a manner that can move back and forth. Among them, the welding heat source 1 and the two induction coils 2 respectively achieve back-and-forth movement through moving devices. Each moving device is respectively installed on the upper side of the welding platform 1a, and each moving device is controlled by an operating machine to achieve synchronous movement. In this embodiment, each moving device is a conventional moving device in the art, and the operating machine is an automatic control device. The moving device and the operating machine here are both well-known technologies in the art, so no further description will be given.

[0081] Furthermore, the above-mentioned two induction coils 2 are jointly equipped with a heating device 22. The heating device 22 is used to control the corresponding parameters of the two induction coils 2, such as heating power, heating temperature, and heating time, etc. In this embodiment, the heating device 22 supporting the induction coil is a well-known technology, so no further description will be given. Among them, for the convenience of description, the two induction coils 2 and the heating device in this embodiment jointly form a bilateral symmetric thermal strain heating device. In addition, the two induction coils 2 can also be respectively equipped with a heating device 22.

[0082] Furthermore, the device of this embodiment further includes a fixture 2a. The fixture 2a is installed on the upper surface of the welding platform 1a. The installation structure of the fixture 2a is a conventional existing structure, so no further description will be given; the fixture 2a is used to fix and position the two workpieces to be welded to ensure that the two workpieces to be welded do not shift during the welding process. In this embodiment, the above-mentioned fixture 2a selects the fixture used in a conventional welding hot crack device, and no further description will be given here.

[0083] In this embodiment, the output frequency of the heating device 22 is 50-80kHz. After a high-frequency alternating current is applied, an alternating magnetic field of the same frequency is generated around the induction coil 2. When the alternating magnetic field passes through the conductive aluminum alloy workpiece to be welded, according to Faraday's law of electromagnetic induction, a closed eddy current is induced inside the workpiece. The eddy current generates Joule heat (Q=I) under the action of the workpiece's own resistance. 2 Rt) enables localized rapid heating of the side areas on both sides of weld 3. The heating area is a uniform rectangular strip distribution consistent with the shape of induction coil 2 (i.e., thermal strain), with a heating rate ≥50℃ / s and a temperature control accuracy of ±5℃.

[0084] This embodiment discloses a device for controlling welding hot cracks with bilateral symmetrical thermal strain assistance. Neither of the two induction coils 2 covers the centerline of the weld 3; they are only aligned with the corresponding workpieces to be welded on either side of the weld 3, allowing the workpieces to expand upon heating. During welding, the welding heat source 1 and the two induction coils 2 move synchronously from back to front along the length of the weld 3. The thermal field generated by the two induction coils can precisely act on the workpieces to be welded on both sides of the weld within the brittle temperature range, causing the workpieces to expand upon heating within the brittle temperature range. This generates lateral (i.e., left-right) compressive strain pointing towards the center of the weld 3, thereby counteracting the crack-inducing tensile strain generated within the brittle temperature range during welding. This eliminates the critical conditions for hot cracking from a mechanical perspective, achieving the goal of controlling hot cracking. Furthermore, the induction coils do not need to directly contact the surface of the workpieces to be welded throughout the process, preventing damage such as indentations and scratches, and avoiding notch effects that affect the fatigue performance of the joint. The device also has a simple overall structure, with easily adjustable parameters for each component, facilitating practical welding production applications.

[0085] Furthermore, both induction coils are rectangular in design. Compared to the circular induction coils commonly used in the art, the rectangular heating area results in more uniform heating temperature and is more suitable for the method in this embodiment.

[0086] The above description is only a preferred embodiment of this invention. Any equivalent changes and modifications made within the scope of the claims of this invention shall fall within the scope of the claims of this invention.

Claims

1. A method for controlling welding hot cracking with bilateral symmetrical thermal strain assistance, characterized in that, It includes the following steps: Step 1. Preparation before welding: Step 1-1. Determine the brittle temperature range, the butt joint gap between the two workpieces to be welded, and the misalignment of the workpieces; Step 1-2. Install a welding heat source above the weld between the two workpieces to be welded. The welding heat source is directly opposite to the weld. Symmetrically arranged induction coils are respectively provided on both sides of the weld. The two induction coils are respectively located above the corresponding workpieces to be welded, and both of the two induction coils are rectangular; Among them, taking the welding direction as from back to front, the welding heat source is located in the front of the induction coils. The central connection line between the two induction coils correspondingly lies at the starting position of the brittle temperature range. Adjust the adjustment distance between the center points of the two induction coils and the center line of the weld respectively. The total distance between the center points of the two induction coils is the optimal extrusion stress transfer distance; Step 2. Welding: Start welding after the temperatures respectively generated by the two induction coils are stable at 302±5°C. During the welding process, the welding heat source and the two induction coils move synchronously and uniformly from back to front, so that the thermal strains respectively generated by the two induction coils continuously heat the workpieces to be welded on both sides of the weld in the brittle temperature range behind the molten pool until the welding is completed.

2. The method for controlling welding hot cracking with bilateral symmetrical thermal strain according to claim 1, characterized in that: In Step 1-1, the butt joint gap ≤0.1mm, and the misalignment of the workpieces ≤0.2mm.

3. The method for controlling welding hot cracking with bilateral symmetrical thermal strain according to claim 2, characterized in that: After Step 1-1, tack welding is carried out at both ends of the weld, and positioning weld spots are respectively formed at both ends of the weld.

4. The method for controlling welding hot cracking with bilateral symmetrical thermal strain according to claim 3, characterized in that: The length of the two positioning weld spots is 2-3mm. The welding heat source is a laser beam, and the laser power of the welding heat source is 90% of the laser power during formal welding.

5. The method for controlling welding hot cracking with bilateral symmetrical thermal strain according to any one of claims 1-4, characterized in that: The adjustment distance between the center points of the two induction coils and the center line of the weld respectively is 4-7mm.

6. The method for controlling welding hot cracking with bilateral symmetrical thermal strain according to claim 5, characterized in that: After Step 1-2, parameter presetting is carried out. The welding heat source is a laser beam, the laser power of the welding heat source is 1100-1800W, the welding speed of the welding heat source is 12-16mm / s, and the defocus amount of the welding heat source is -0.5-0mm.

7. The method for controlling welding hot cracking with bilateral symmetrical thermal strain according to claim 6, characterized in that: The heating power of the heating device supporting the induction coil is 400-700W, and the output frequency of the heating device is 50-80kHz. Among them, the peak temperature of the heating area of the induction coil is 280-320°C, and the thermal strain following distance of the induction coil is 4-8mm.

8. The method for controlling welding hot cracking with bilateral symmetrical thermal strain according to claim 5, characterized in that: The welding process of Step 2 is as follows: Step 2-1. First, preheat in advance. Start welding when the temperatures respectively generated by the two induction coils are stable at 302±5°C. The welding heat source adopts a power linear gradual increase mode for starting welding to increase from 0W to the set power within the set time; Step 2-2. The welding heat source and the two induction coils respectively move synchronously and uniformly from back to front along the length direction of the weld. During the movement, keep the relative positions of the welding heat source and the two induction coils unchanged. The thermal strains generated by the two induction coils continuously heat the corresponding workpieces to be welded on both sides of the weld in the brittle temperature range behind the molten pool; Steps 2-3: After the weld is completed, the welding heat source adopts a linear power reduction mode to finish, so as to reduce the power from the set power to 0W within a set time. At the same time, the two induction coils and the welding heat source move away from the workpiece to be welded and then turn off.

9. The method for controlling welding hot cracking with bilateral symmetrical thermal strain according to claim 8, characterized in that: In step 2-2, during the heating process of the workpiece to be welded, the fluctuation range of the laser power of the welding heat source and the temperature generated by thermal strain is ensured to be within ±2%.

10. The method for controlling welding hot cracking with bilateral symmetrical thermal strain according to claim 8, characterized in that: The formula for the transverse compressive stress generated at the weld joint after the workpiece expands due to heat is: In the formula, The elastic modulus of the material, This represents the actual transverse compressive strain of the workpiece under thermal expansion. The coefficient of linear expansion of high-strength aluminum alloy. The length of the weld before it is subjected to compression. The width of the weld before it is subjected to compression. This represents the temperature rise change of the workpiece to be welded in the part heated by the induction coil. Among them, the temperature rise change of the workpiece to be welded is In the formula, For heat conversion efficiency, The value range is 0-1; The heat generated by the resistance For specific heat capacity, For quality, Let be the radius of the resistance wire cross-section of the induction coil. The length of the resistance wire in the induction coil is given. The width of the resistance wire in the induction coil. This represents the number of resistance wires in the induction coil. This represents the magnitude of the current flowing through the resistor.

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