A Variable-Polarity Plasma Arc Welding Method for Thick Aluminum Alloy Based on Laser-Assisted Perforation

Abstract

The present invention relates to a variable polarity plasma arc welding method for thick aluminum alloy plates based on laser-assisted piercing. On the basis of variable polarity plasma welding of aluminum alloy, the laser heat source and the plasma arc heat source are coaxially placed on the upper and lower sides of the plate to be welded. At the starting point of welding, the plasma arc first melts the aluminum alloy plate at a fixed point. At the same time, under the action of the arc force, the liquid surface gradually sinks to form a blind hole. When the depth of the blind hole reaches the penetration limit of the plasma arc, the laser is turned on. Using the high energy density of the laser, the metal is melted from the back of the workpiece, and the blind hole is "pierced" under the action of the evaporation reaction force to form a penetrated "keyhole", guiding the arc plasma to flow out of the keyhole. After the piercing is formed, the plasma arc forms a stable flow pattern inside the keyhole, and the liquid metal surrounds the plasma arc. The laser is turned off, and the quasi-rigid characteristics of the plasma arc are used to maintain the stability of the keyhole wall-attached molten pool, realizing high-quality piercing welding of thick aluminum alloy plates.

Description

Technical Field

[0001] The present invention belongs to the field of forming manufacturing methods, and particularly relates to a variable polarity plasma arc welding method for thick plate aluminum alloy based on laser-assisted perforation. Background Art

[0002] Variable polarity plasma arc piercing welding forms a through-thickness hole on the workpiece, and during the welding stage, the liquid metal flows through the pool wall and solidifies on the solidification side to form a weld seam. VPPA has the advantages of high energy beam and has significant advantages in the welding of medium and thick plate aluminum alloys. Due to the high energy density, the piercing molten pool can exist stably, which is beneficial to the discharge of gas in the molten pool. At the same time, the variable polarity square wave is adopted to ensure the cathode cleaning around the molten pool and obtain high-quality aluminum alloy weld seams. It is hailed as a "zero-defect" welding process by experts at home and abroad. In view of the urgent need for transformation and upgrading in the field of advanced manufacturing in China, aiming at the specific problems of high-precision spacecraft structure manufacturing and the further increase in the size of welded structural parts, the development of welding technology to ensure the welding quality of workpieces, improve welding efficiency, and enhance the plasma arc piercing ability is imminent. In order to solve the problem of thick plate welding and give full play to the process advantages of VPPA piercing welding in aluminum alloy welding, it is necessary to propose a targeted aluminum alloy thick plate piercing welding scheme without introducing interference factors.

[0003] The characteristics of variable polarity plasma arc (VPPA) with high energy density and active cleaning of the oxide film play a crucial role in the field of aluminum alloy welding. Its unique piercing welding technology has become an excellent method for high-efficiency welding of aluminum alloys. VPPA piercing welding has good flexibility, low requirements for the operating environment, and low dependence on tooling, and has an absolute advantage in the in-situ welding and installation of large aluminum alloy structures. The piercing phenomenon can only occur under sufficient energy density, and the required energy density increases as the plate thickness increases. With the development of industries such as shipbuilding, military, and aerospace, the application of large-thickness metal plates is becoming more and more extensive, which puts higher and higher requirements on thick plate welding. In order to improve the thick plate welding ability of the plasma arc, a method of variable polarity plasma arc welding for thick plate aluminum alloy based on laser-assisted perforation is proposed, which can improve the welding stability of the plasma arc and the thermal efficiency. Summary of the Invention

[0004] The object of the present invention is to overcome the defects and deficiencies of the existing welding methods in thick plate welding, and to provide a variable polarity plasma arc welding method for thick plate aluminum alloy based on laser-assisted piercing. Plasma arc welding has the advantages of high energy density, large weld depth-to-width ratio, and small heat-affected zone. Moreover, PAW can form a small hole channel in the molten pool, and full penetration welding can be achieved with only one weld pass for medium and thick workpieces, which has irreplaceable advantages compared with other welding methods. However, the energy distribution of the plasma arc is somewhat insufficient for large thick plate welding, and the heat source is difficult to simultaneously meet the heat requirements of the bottom and top of the molten pool. When the thickness of the base material exceeds 15 mm, it is difficult to complete the piercing. As the arc moves downward to the lower part of the small hole, the attenuation of the arc energy increases, the energy at the top is sufficient while the energy at the bottom is insufficient. Using large current welding will result in excessive energy at the top, and the molten pool is prone to instability. Moreover, large current welding is also likely to damage the tungsten electrode. During the thick plate plasma arc welding process, the dynamic stability of the molten pool and the small hole is poor, and the welding process window is narrow. It is difficult to meet the thick plate welding requirements by changing the welding parameters. A laser-assisted welding method is proposed by adding a laser heat source timely on the back of the workpiece. With its high energy density and concentrated heat effect, the laser heat source promotes the melting and deformation of the back metal, precisely acts on the molten pool, and provides a guiding channel for the plasma, thus realizing the penetration of the workpiece. Once the piercing is formed, the welding process changes to maintaining the stability of the small hole and controlling the shape of the molten pool. At this time, the acting direction of the force in the welding process changes from downward penetration to supporting the molten pool around to keep the metal on the hole wall stable. Maintaining the stability of the small hole no longer requires excessive arc force, and the plasma arc itself can complete the welding process. The welding reliability of the aluminum alloy thick plate structure is greatly improved.

[0005] In order to achieve the above object, the present invention adopts the following technical solutions:

[0006] A variable polarity plasma arc welding method for thick aluminum alloy plates based on laser-assisted piercing. On the basis of VPPA welding, a laser heat source and a plasma heat source are coaxially arranged vertically below the aluminum alloy base material to be welded. First, the welding power supply is preheated to the set temperature. After the preheating is completed, the welding power supply is started, and the plasma arc is ignited. As the welding current and voltage increase, the plasma arc transfers heat and force to the base material, causing the base material metal to melt and form a molten pool due to heat transfer. When it is observed that the plasma arc cannot continue to dig downwards and the blind hole depth reaches the plasma arc penetration limit, a laser beam is emitted vertically by the laser welding equipment to irradiate the surface below the base material, generating extremely high local heat, which promotes the melting and deformation of the back metal, precisely piercing the molten pool and providing a guiding channel for the plasma, thereby achieving the penetration of the workpiece. Once the piercing is formed, the welding process changes to maintaining the stability of the small hole and controlling the shape of the molten pool. At this time, the direction of the force action in the welding process changes from downward penetration to supporting the molten pool around to keep the metal on the hole wall stable. Maintaining the stability of the small hole no longer requires excessive arc force, and the plasma arc itself can complete the welding process. It breaks through the dilemma of VPPA thick plate welding piercing and provides a scientific basis for optimizing the welding process.

[0007] Preferably, the laser heat source and the plasma arc heat source are coaxially placed, where the laser beam can have a maximum deflection angle of 30 degrees relative to the axis of the plasma arc. The plasma arc is located above the plate, responsible for preheating and melting the plate and maintaining the stability of the molten pool during the welding process. The laser heat source is located below the plate, and its main function is to provide heat input with a high energy density during the piercing stage to promote the formation of the piercing.

[0008] Preferably, the welding current is between 300 A and 700 A, and the current waveform is an alternating current rectangular wave with a period of 25 ms, where the duration of the negative half-wave is controlled within the range of 2 ms to 4 ms. With the high arc force of the plasma arc, single-pass welding of the piercing molten pool for plates with a thickness of 15 mm to 25 mm is achieved.

[0009] Preferably, the power setting range of the laser heat source is 4000 W to 6000 W to ensure that it can effectively play the role of assisting in piercing.

[0010] Preferably, when the plasma arc piercing reaches a certain depth and cannot continue to penetrate, the laser heat source is started to pierce the blind hole. Immediately after the penetrating small hole is formed, the laser heat source is turned off, thereby realizing the laser-assisted piercing process.

[0011] Preferably, after the formation of the penetrating small hole, maintaining the stability of the small hole no longer requires an excessive arc force, and the plasma arc itself can complete the welding process. The plasma arc starts to move above the plate; relying on the stiffness characteristics of the plasma arc itself, the stability of the liquid metal in the small hole molten pool is maintained; through the smooth movement of the plasma arc, stable flow-around welding of thick plates is achieved.

[0012] In view of the problem that it is difficult to penetrate holes in the welding of thick aluminum alloy plates, the present invention uses laser-assisted welding to achieve smooth hole penetration of thick plate base materials. A variable polarity plasma arc is ignited between the tungsten electrode and the aluminum alloy workpiece. Under the thermal-mechanical action of the constrained arc, the aluminum alloy is melted to form a molten pool. At the same time, the free surface of the molten pool deforms. As the welding current and voltage increase, the arc force gradually increases. At the same time, under the action of the arc force, the liquid surface gradually sinks to form a blind hole. When it is observed that the plasma arc cannot continue to dig downwards, the depth of the blind hole reaches the penetration limit of the plasma arc. Then, the laser welding equipment is turned on. Using the high energy density of the laser, the metal is melted from the back of the workpiece, and the blind hole is "pierced" under the action of the evaporation reaction force to form a penetrating "small hole", guiding the arc plasma to flow out of the small hole. After the hole penetration is formed, the plasma arc forms a stable flow pattern inside the small hole, and the liquid metal surrounds the plasma arc. The laser is turned off, and the quasi-rigid characteristics of the plasma arc are used to maintain the stability of the small hole wall-attached molten pool, realizing high-quality hole-penetrating welding of thick aluminum alloy plates. The present invention innovatively proposes a method of using laser-assisted variable polarity plasma arc welding. On the premise of maintaining the stability of the molten pool, smooth hole penetration of thick plate base materials is achieved, meeting the high-quality manufacturing requirements of high-alloy materials and complex large-scale structures.

[0013] Compared with the prior art, the advantages of the method of the present invention are as follows.

[0014] The present invention provides a brand-new welding method for aluminum alloy plates, breaking the dilemma that it is difficult to penetrate holes in thick aluminum alloy plates and broadening the welding application field. Compared with traditional hole-penetrating methods, both a laser and an arc are used as heat sources at the same time. The high energy density of the laser can quickly melt and evaporate the material, precisely pierce the molten pool, and provide a guiding channel for the plasma, so as to smoothly penetrate the hole. The high-temperature and high-speed plasma jet provides sufficient rigidity. Together with the solid-liquid interface of the penetrated molten pool, the liquid metal is controlled between the two. The synergistic effect of the plasma jet and the laser beam will not allow a large amount of the melt to flow out of the molten pool surface, ensuring a stable transition of the penetrated molten pool and obtaining better hole-penetrating quality. Laser-assisted hole penetration is applicable to the welding of thick aluminum alloy plate base materials, which means that the hole-penetrating technology can be widely applied to high-strength and high-difficulty welding fields such as aviation, aerospace, and automobiles. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Figure 1 Schematic diagram of variable polarity plasma arc welding for laser-assisted hole penetration of thick plate base materials

[0016] Figure 2 Schematic diagram of the laser-assisted plasma arc piercing process

[0017] In the figure: 1: Aluminum alloy base material, 2: Plasma arc, 3: Plasma welding torch, 4: Variable polarity plasma arc welding power source, 5: Laser generator, 6: Laser emission controller, 7: Adjustable laser source, 8: Laser beam, 9: Control computer, 10: Welding direction, 11: Longitudinal section of the molten pool Specific implementation mode

[0018] The embodiments of the present invention will be specifically described below with reference to the accompanying drawings.

[0019] The present invention provides a variable polarity plasma arc welding method based on laser coaxial assistance for piercing thick plate base materials. A variable polarity plasma arc welding power source capable of outputting a rectangular wave current is used. A variable polarity plasma arc is ignited between the tungsten electrode and the aluminum alloy workpiece, and a plasma arc is formed between the plasma welding torch and the workpiece. Under the thermal-mechanical action of the constrained arc, the aluminum alloy is melted to form a molten pool. At the same time, the free surface of the molten pool deforms. As the welding current and voltage increase, the arc force gradually increases. When the blind hole depth reaches the plasma arc penetration limit, a laser beam is emitted vertically by the laser welding equipment and irradiated on the surface below the base material, generating extremely high local heat, which promotes the melting and deformation of the back metal and precisely pierces the molten pool, providing a guiding channel for the plasma, thereby realizing the penetration of the workpiece. Once the piercing is formed, the welding process changes to maintaining the stability of the small hole and controlling the shape of the molten pool. At this time, the acting direction of the force in the welding process changes from downward penetration to supporting the molten pool around to keep the metal on the hole wall stable. Maintaining the stability of the small hole no longer requires too much arc force, and the plasma arc itself can complete the welding process. This welding technology that combines laser and plasma arc is called a variable polarity plasma arc welding method for thick plate aluminum alloy based on laser-assisted piercing. The laser-assisted piercing variable polarity plasma arc welding method for thick plate base materials of the present invention can successfully pierce aluminum alloy plates with a thickness of 15 mm to 25 mm.

[0020] The laser heat source and the plasma arc heat source are coaxially placed. The laser beam can have a maximum deflection angle of 30 degrees relative to the axis of the plasma arc. The plasma arc is located above the plate, responsible for preheating and melting the plate and maintaining the stability of the molten pool during the welding process. The laser heat source is located below the plate, and its main function is to provide heat input with a high energy density during the piercing stage to promote the formation of the piercing.

[0021] The welding current is between 300 A and 700 A, and the current waveform is an AC rectangular wave with a period of 25 ms. The duration of the negative half-wave is controlled within the range of 2 ms to 4 ms. Single-pass welding of the piercing molten pool for plates with a thickness of 15 mm to 25 mm is achieved by means of the high arc force of the plasma arc.

[0022] The power setting range of the laser heat source is from 4000W to 6000W, ensuring that it can effectively play the role of assisting in piercing.

[0023] When the plasma arc piercing reaches a certain depth and cannot continue to penetrate, the laser heat source is activated to pierce the blind hole. Immediately after a penetrating small hole is formed, the laser heat source is turned off, thereby realizing the laser-assisted piercing process.

[0024] After the penetrating small hole is formed, maintaining the stability of the small hole no longer requires excessive arc force, and the plasma arc itself can complete the welding process. The plasma arc starts to move above the plate; relying on the stiffness characteristics of the plasma arc itself, the stability of the liquid metal in the small hole molten pool is maintained; through the stable movement of the plasma arc, stable flow-around welding of thick plates is achieved.

[0025] The specific operation steps of this forming manufacturing method are as follows:

[0026] (1) Preparation before processing: The laser heat source and the plasma arc heat source are arranged coaxially on the upper and lower sides of the aluminum alloy plate to be welded, ensuring that the axes of the laser beam and the plasma arc are aligned. Other gas paths and water paths are connected according to the conventional connection methods. At the same time, clean the oxides and oil stains on the surface to be welded to improve the welding quality.

[0027] (2) Preheating and melting: Adjust the distance between the end face of the welding torch and the workpiece to keep the arc height between 4mm and 6mm. First, start the pilot arc between the tungsten electrode and the nozzle, and then start the main plasma arc after stabilization. Perform fixed-point preheating and melting on the aluminum alloy plate to be welded. Under the action of the plasma arc, the surface of the plate starts to melt, and the liquid metal gradually sinks under the action of the arc force to form a blind hole. Continue heating to gradually increase the depth of the blind hole until it reaches the maximum depth that the plasma arc can penetrate.

[0028] (3) Laser intervention: Turn on the power supply of the laser welding equipment. Before starting the laser, the equipment needs to be preheated for 5 - 10 minutes to ensure the stable operation of the laser. Observe the flow state of the free surface of the molten pool using high-speed photography. When the depth of the blind hole reaches the penetration limit of the plasma arc, start the laser heat source. The high energy density of the laser melts the metal from the back of the plate, generating steam.

[0029] (4) Piercing process: The generation of steam produces an outward evaporation reaction force, which helps to "pierce" the blind hole to form a penetrating small hole. Once the small hole is formed, the plasma of the plasma arc will flow out through this small hole to achieve piercing.

[0030] (5) Completion of welding: After perforation, a stable circulation mode is formed inside the small hole by the plasma arc, and the liquid metal surrounds the plasma arc. Turn off the laser, and utilize the quasi-rigid characteristics of the plasma arc to maintain the stability of the molten pool attached to the wall of the small hole. Under the continuous action of the plasma arc, move the welding torch along the predetermined welding path to complete the welding process of the entire thick aluminum alloy plate.

[0031] Example 1:

[0032] As Figure 2 shown, prepare the aluminum alloy base material 1, connect the water circuit and gas circuit of the whole system, connect the variable polarity plasma arc welding power source 4, the welding torch 3 and the base material 1 into a loop through a cable, connect the laser generator 5 to the laser emission controller 6, and connect the laser emission controller 6 and the adjustable laser source 12 and then connect to the control computer 9. Preheat the welding power source to the set temperature. After the preheating is completed, start the variable polarity plasma arc welding power source 4 to ignite the plasma arc. Turn on the pilot arc. After the pilot arc is stable, turn on the main arc 2. As the welding current and voltage increase, the plasma arc 3 transfers heat and force to the base material 1. Under the thermal-mechanical action of the constrained arc, the aluminum alloy is melted to form a molten pool. When it is observed that the plasma arc cannot continue to dig downwards and the depth of the blind hole reaches the penetration limit of the plasma arc, the laser beam 8 emitted by the IPG YLG-2000 fiber laser 5 irradiates vertically on the surface below the base material 1, generating extremely high local heat and precisely piercing the molten pool to provide a guiding channel for the plasma, thereby realizing the penetration of the workpiece. The synergistic effect of laser welding and variable polarity plasma arc welding completes the perforation of the thick base material 1. Once the perforation is completed, maintaining the stability of the small hole no longer requires excessive arc force, and the plasma arc itself can complete the welding process.

[0033] For the method of welding and perforating the aluminum alloy thick plate base material of the present invention, first, the aluminum alloy is melted to form a molten pool under the thermal-mechanical action of the constrained arc. In order to ensure the complete perforation of the thick plate base material and make the welding proceed smoothly, a method of fixed-point laser-assisted welding and perforation is proposed. The laser beam is emitted by the laser welding equipment and irradiates vertically on the surface below the base material, generating extremely high local energy and precisely piercing the molten pool. Under the synergistic action of the laser welding equipment and variable polarity plasma arc welding, the workpiece is penetrated and a small hole is formed. Overcome the defects and deficiencies of the existing welding methods in thick plate welding, solve the problem of difficult perforation in aluminum alloy thick plate welding, avoid instability in perforation welding, ensure the welding quality of the workpiece, improve the welding efficiency, enhance the plasma arc perforation ability, and realize variable polarity plasma arc perforation welding of large and complex aluminum alloy structures.

Claims

1. A method for variable polarity plasma arc welding of thick plate aluminum alloy based on laser-assisted perforation, characterized in that: On the basis of variable polarity plasma welding of aluminum alloy, the laser heat source and the plasma arc heat source are coaxially placed on the upper and lower sides of the plate to be welded; at the starting point of welding, the plasma arc first performs point melting on the aluminum alloy plate, and at the same time, the liquid level gradually sinks under the action of the arc force to form a blind hole. When the depth of the blind hole reaches the penetration limit of the plasma arc, the laser is turned on, and the metal is melted from the back of the workpiece by the laser. Under the action of the evaporation reaction force, the blind hole is pierced to form a penetrating small hole, and the arc plasma is guided to flow out of the small hole; after the perforation is formed, the plasma arc forms a stable flow pattern inside the small hole, and the liquid metal surrounds the plasma arc. The laser is turned off, and the plasma arc is used to maintain the stability of the molten pool attached to the wall of the small hole.

2. The thick plate aluminum alloy variable polarity plasma arc welding method based on laser-assisted perforation according to claim 1 is characterized in that: The laser heat source is coaxially placed with the plasma arc heat source, wherein the laser beam can have a maximum deflection angle of 30 degrees relative to the plasma arc axis. The plasma arc is located above the plate and is responsible for preheating and melting the plate and maintaining the stability of the molten pool during the welding process; the laser heat source is located below the plate and its main function is to provide heat input during the perforation stage to promote the formation of perforations.

3. The thick plate aluminum alloy variable polarity plasma arc welding method based on laser-assisted perforation according to claim 1, characterized in that: The welding current is between 300 A and 700 A, and the current waveform is an AC rectangular wave with a period of 25 ms, in which the duration of the negative half-wave is controlled in the range of 2 ms to 4 ms. The high arc force of the plasma arc is used to achieve single-pass perforated molten pool welding of 15 mm to 25 mm thick plates.

4. The thick plate aluminum alloy variable polarity plasma arc welding method based on laser-assisted perforation according to claim 1, characterized in that: The power setting range of the laser heat source is 4000W to 6000W.

5. The thick plate aluminum alloy variable polarity plasma arc welding method based on laser-assisted perforation according to claim 1, characterized in that: When the plasma arc perforation reaches a certain depth and cannot continue to penetrate, the laser heat source is started to pierce the blind hole. After the small hole is formed, the laser heat source is immediately turned off, thereby realizing the laser-assisted perforation process.

6. The thick plate aluminum alloy variable polarity plasma arc welding method based on laser-assisted perforation according to claim 1, characterized in that: After the penetrating small hole is formed, the plasma arc itself can complete the welding process, and the plasma arc starts to move above the plate; through the stable movement of the plasma arc, stable flow around the thick plate welding is achieved.

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