A method for improving gap adaptability of laser brazing of dissimilar metal butt joint

Abstract

The present application belongs to the field of dissimilar metal laser welding, and particularly relates to a method for improving the gap adaptability of dissimilar metal butt joint laser brazing by alternating magnetic field. By setting the butt joint part of the dissimilar metal test plate with assembly gap suspended at the bottom, applying transverse alternating magnetic field and inert gas protection below it, and setting the laser welding head above the sample and the magnetic field generating device below the sample to remain stationary during the welding process, and the laser beam is offset to the low melting point metal side, the molten low melting point material wets and spreads on the surface of the un-molten high melting point material, and then interface metallurgical bonding occurs to form a brazing joint. The present application improves the assembly gap adaptability of dissimilar metal butt joint laser brazing based on the effect of transverse alternating magnetic field lifting the molten pool, suppresses the generation of aluminum alloy weld side heat cracks and other defects caused by the increase of assembly gap, improves the weld forming, and improves the adaptability of dissimilar metal butt joint laser brazing in engineering.

Description

TECHNICAL FIELD

[0001] The application belongs to the field of dissimilar metal laser welding, and particularly relates to a method for improving the gap adaptability of dissimilar metal butt joint laser brazing. BACKGROUND

[0002] With the continuous development of industry and science and technology, environmental protection and energy consumption reduction are valued. In order to realize sustainable development, all walks of life take the lightweight of materials as the research focus. Aluminum alloy materials have been widely used in the fields of automobile and aerospace industry due to their high specific strength, good processing performance and low cost.

[0003] Meanwhile, titanium alloy materials are widely used in pipeline systems, structural parts and high-temperature parts of aircraft engines due to their excellent corrosion resistance, higher specific strength and better high-temperature resistance. Copper alloy has become an indispensable material in the fields of chemical industry, refrigeration, electrical and energy electronics due to its excellent electrical and thermal conductivity and good corrosion resistance. High-strength steel has high specific strength and good fatigue resistance, and compared with the above-mentioned materials, the price of steel material is relatively low, and it has been widely used in the fields of aerospace, ship, automobile and the like. In order to further realize lightweight, the preparation of aluminum / titanium, aluminum / copper and aluminum / steel composite components through welding or connection has become a research hotspot.

[0004] As a new laser brazing technology suitable for connecting high-melting-point metal materials and low-melting-point metal materials, it utilizes the advantages of fast heating and cooling speed, accurate energy input and action position control and low residual stress of the workpiece after welding. In the welding process, the laser irradiates on the low-melting-point aluminum alloy material to make it melt to form a molten pool, and then the molten low-melting-point liquid metal wets and spreads on the un-melted high-melting-point metal and occurs interface metallurgical bonding to form a filler seam. Laser brazing technology is currently considered as an excellent choice for preparing aluminum / titanium, aluminum / copper and aluminum / steel composite components. However, the laser brazing of dissimilar metal butt joints is extremely sensitive to the assembly gap of the workpiece, and the existence of the assembly gap will seriously affect the stability of the laser brazing process and the final joint quality.

[0005] To improve the adaptability of laser welding of the same metal to the assembly gap, generally use swing laser, improve welding process, use filler wire and other process means, Chinese patent CN 110722264 A discloses a high-power laser welding method with strong gap adaptability, by pre-accumulating part of the filler metal inside the workpiece butt joint groove, after the filler material cools, high-power laser welding is carried out. The invention combines laser self-fusion welding and filler material process, improves the adaptability of high-power laser welding to the assembly gap of the workpiece. However, for laser welding of dissimilar metals with large differences in thermophysical and chemical properties, pre-accumulating filler metal by fusion welding will lead to the generation of a large number of brittle intermetallic compounds, which greatly deteriorates the joint performance.

[0006] Chinese patent CN 115008002 A discloses a vacuum environment swing laser welding method and system, by combining vacuum environment with swing laser to control energy and molten pool temperature field, and at the same time, the laser output energy is adjusted during the swing of the laser, thereby optimizing the welding energy distribution, and then improving the welding forming and quality, and improving the welding adaptability. However, the welding process equipment used in this method is extremely complex, and the vacuum chamber limits the size of the workpiece, which cannot realize large-size workpiece and batch industrial manufacturing.

[0007] Chinese patent CN 112388164 A discloses an alternating magnetic field assisted laser deep penetration welding method for reducing aluminum alloy weld pores, by prepositioning an alternating electromagnetic field at the bottom of the butt joint of the sample to be welded, the Lorentz force generated by the alternating magnetic field drives the bubbles to escape upwards, achieving the purpose of suppressing or even completely eliminating small hole type pores. However, this method is only suitable for laser welding of the same metal in the process of partial penetration welding, for dissimilar metal butt welding, the whole weld needs to be fully penetrated to increase the bonding area of the interface, at this time the metal below the molten pool will be severely oxidized. This invention applies a transverse alternating magnetic field at the bottom of the aluminum / titanium dissimilar metal joint, and applies inert gas protection to the back of the molten pool through the back gas protection device, which can improve the forming of dissimilar metal joints and optimize the weld structure, and suppress the generation of cracks and other defects.

[0008] In the laser fusion brazing of dissimilar metal butt joints in engineering, the influence of assembly gap on the stability of fusion brazing process can be reduced by filling a small amount of welding wire or improving the laser heat input parameters, but due to the existence of uncertain factors such as insufficient assembly accuracy, the gap between dissimilar material test plates is not a constant value, it is difficult to adapt to all assembly gap conditions with the same welding process parameters. At the same time, when changing the welding process parameters, the continuity of the welding process is destroyed, so the application difficulty of this process method is extremely great. SUMMARY

[0009] In view of the problems in the prior art, the application provides a method for improving the gap adaptability of laser brazing of dissimilar metal butt joints.

[0010] The method provided by the application can inhibit the generation of defects such as weld pool dripping and aluminum alloy weld seam side heat cracks caused by the increase of assembly gap, improve weld seam forming, improve joint strength, and ensure welding quality. The application does not need to fill welding wire, and does not need to change the laser power, welding speed and laser offset distance and other brazing process parameters. Only the transverse alternating magnetic field is applied below the laser brazing pool of the aluminum / titanium dissimilar metal butt joint, and the inert gas protection is used on the upper and lower surface pools, and the adaptability of the laser brazing of the dissimilar metal butt joint to the assembly gap can be greatly improved.

[0011] Under the technical scheme provided by the application:

[0012] A method for improving the gap adaptability of laser brazing of dissimilar metal butt joints by applying an alternating magnetic field, the method comprising the following steps:

[0013] S1, the welding parts of the high-melting-point metal workpiece and the low-melting-point metal workpiece are fixed side by side in a horizontal butt joint manner, and the welding parts of the two workpieces are controlled to be suspended in the vertical direction and to keep a preset assembly gap in the horizontal direction;

[0014] S2, a horizontal transverse alternating magnetic field perpendicular to the welding direction is arranged below the welding part, the center of the magnetic field corresponds to the weld joint between the two workpieces, the focusing position of the laser beam is the surface directly above the weld joint, and the laser beam focal point is offset from the weld joint to the low-melting-point metal side by a predetermined distance;

[0015] S3, laser welding parameters and magnetic field generation parameters are set, the magnetic field generation parameters include magnetic flux density and magnetic field frequency, and the magnetic flux density gradually decreases along the thickness direction of the sample; in the welding process, the two surfaces of the weld joint are protected by the protective gas;

[0016] S4, the magnetic field is started to generate a transverse alternating magnetic field acting on the workpiece, and then the laser beam is started to perform laser brazing until the welding is completed;

[0017] S5, after the welding is terminated, the laser is first turned off, the magnetic field continues to be applied for a predetermined time, then the magnetic field generating device is turned off, the protective gas is turned off, and the welding process is ended.

[0018] In S1, the high melting point metal to be welded workpiece comprises one of titanium alloy, steel or copper, and the low melting point metal to be welded workpiece comprises aluminum alloy.

[0019] In S1, the assembly gap is 0-0.5mm.

[0020] In S2, the incident direction of the laser beam is inclined by a predetermined angle, and points to the front end of the molten pool; the incident direction of the laser beam is inclined by an angle of 7-10°, and the offset distance of the laser beam focal point from the weld seam to the low melting point metal side is 1.5-2.5mm.

[0021] In S3, the laser form can be continuous laser or pulsed laser.

[0022] When continuous laser is used, the laser welding parameters are: laser power is 4000-6000W, and the brazing speed is 0.3-0.5m / min.

[0023] When pulsed laser is used, the pulse frequency is 10-200Hz.

[0024] Before welding, the to-be-welded part is mechanically polished or chemically cleaned.

[0025] In S3, in the magnetic field generation parameters, the effective value of magnetic flux density is 35-45mT, and the magnetic field frequency is 200-400Hz.

[0026] The protective gas is high-purity argon or high-purity helium, and the protective gas flow is 20-30L / min.

[0027] The laser welding system used in the method comprises a pair of sliding rails, two welding sample tables arranged on the sliding rails, a magnetic field generating device below the horizontal plane of the welding sample tables, a laser working head above the welding sample tables, a front protective gas nozzle, a back protective gas device, and a PLC control system.

[0028] The two welding sample tables are not in contact and maintain a certain distance in the vertical welding direction, and the bottom of the to-be-welded part of the butt joint to-be-welded workpiece is suspended.

[0029] The magnetic field generating device is used to generate a horizontal alternating magnetic field perpendicular to the welding direction, and the magnetic field strength decreases along the thickness direction.

[0030] The electromagnet and the electromagnetic coil are the magnetic field generating device, and the gradual decrease of the magnetic flux density is reflected on the to-be-welded workpiece, and the magnetic flux density of the to-be-welded workpiece gradually decreases along the thickness direction of the to-be-welded workpiece.

[0031] The above-mentioned perpendicular to the welding direction refers to the relative position of the to-be-welded workpiece, and the thickness direction refers to the change of the magnetic flux density from bottom to top.

[0032] The laser working head is used for emitting a laser beam focused on the upper surface of the sample during the welding process;

[0033] The front protective gas nozzle is located near the laser working head and is used for spraying protective gas to the welding area;

[0034] The back protective gas device is located below the magnetic field generating device and the sample and forms a closed protective atmosphere to prevent the back welding area from being oxidized;

[0035] The PLC control system is used for regulating the movement of the magnetic field generating device and the movable platform and the emission of the laser beam.

[0036] As an embodiment of the present application, the laser welding system comprises:

[0037] A pair of sliding rails;

[0038] Two welding sample tables arranged on the pair of sliding rails, wherein the two welding sample tables are not in contact with each other and maintain a certain distance in the vertical welding direction, so that the two sample plates of the butt joint sample can be respectively loaded on the corresponding welding sample tables, and the bottom of the to-be-welded part of the butt joint sample is suspended;

[0039] A magnetic field generating device located below the horizontal plane of the welding sample table, used for generating a horizontal alternating magnetic field perpendicular to the welding direction, and the magnetic flux density decreases from bottom to top along the plate thickness direction;

[0040] A laser working head located above the welding sample table, and the focusing position of the laser beam is the surface above the weld corresponding to the center of the magnetic field;

[0041] A front protective gas nozzle located near the laser working head and used for spraying protective gas to the welding area;

[0042] A back protective gas device located below the magnetic field generating device and the sample and forming a closed protective atmosphere to prevent the back welding area from being oxidized;

[0043] A PLC control system used for regulating the switching and parameter adjustment of the magnetic field generating device, the movement trajectory and speed of the movable platform, and the emission time and power of the laser beam.

[0044] The two welding sample tables are connected through a cross beam. The magnetic field generated by the magnetic field generating device covers the welding pool area, and the laser working head and the magnetic field generating device remain stationary during the welding process. The movement of the welding platform is realized by controlling the motor and the transmission mechanism connected with the sample placing table, so as to complete the welding process.

[0045] The magnetic field generating device is an electromagnet and an electromagnetic coil.

[0046] The electromagnetic coil generates an alternating magnetic field through alternating current power supply, the electromagnet concentrates the magnetic field at the two end poles, a horizontal alternating magnetic field perpendicular to the welding direction is generated, the magnetic flux density decreases from bottom to top along the plate thickness direction, and the pole distance needs to be kept more than 2 to 3 times of the weld width;

[0047] The laser working head is adjusted to be inclined at a certain angle in the laser beam incidence direction, is directed to the front end of the welding molten pool, and the laser beam focus position corresponds to the sample surface directly above the center of the magnetic field;

[0048] The blowing port of the protective gas nozzle is directed to the molten pool of the to-be-welded part and forms a preset included angle with the horizontal plane, the blowing port is at a certain distance from the center of the molten pool, and inert gas is sprayed as the protective gas.

[0049] The optical fiber laser power supply emits a laser beam, and the alternating current power supply outputs an alternating current in a sinusoidal wave form.

[0050] Compared with the prior art, the present application has the following beneficial effects:

[0051] 1. By applying a transverse alternating magnetic field and inert gas protection below the laser brazing and welding molten pool of the dissimilar metal butt joint, the induced Lorentz force generated in the molten pool produces an upward lifting effect, so that the shape and size of the aluminum alloy molten pool and the solidified weld under different assembly gaps are similar, the generation of defects such as hot cracks caused by the molten pool dripping down and the shrinkage of the aluminum alloy weld during solidification when the gap increases is inhibited, the consistency of the mechanical properties of the joint under different gaps is improved, and the joint strength is improved.

[0052] 2. The induced eddy current formed by the external magnetic field in the laser brazing and welding molten pool of the dissimilar metal butt joint is beneficial to homogenize the molten pool temperature field and element distribution, improve the distribution of interfacial intermetallic compounds, and can break large-size dendrites, refine the weld grain, and improve the joint strength.

[0053] 3. The present application has no limitation on the size of the workpiece, for the butt joint of dissimilar metal plates of different thicknesses, only the magnetic field parameters need to be changed, the lifting effect on the molten pool is increased or decreased according to the requirements, and the effect of improving the gap adaptability of laser brazing and welding can be obtained. BRIEF DESCRIPTION OF DRAWINGS

[0054] Other features, objects and advantages of the present application will become more apparent from the following detailed description of non-limiting embodiments made with reference to the accompanying drawings:

[0055] Figure 1 The transverse alternating magnetic field assisted laser welding system provided by the present application is shown in the schematic diagram, wherein a is a schematic diagram of the transverse alternating magnetic field assisted laser welding device, b is a schematic diagram of the interaction between the magnetic field and the molten pool, and c is a schematic diagram of the back protective gas device.

[0056] Figure 2The cross-sectional morphology and mechanical property test results of the aluminum / titanium dissimilar metal butt joint laser brazing joint when the assembly gap is 0 mm for Example 1 are applied with a transverse alternating magnetic field with a magnetic flux density of 35 mT and a magnetic field frequency of 200 Hz; wherein a is a cross-sectional morphology diagram of the butt joint laser brazing joint; b is a mechanical property test result;

[0057] Figure 3 The cross-sectional morphology and mechanical property test results of the aluminum / titanium dissimilar metal butt joint laser brazing joint when the assembly gap is 0 mm for Comparative Example 1 are not applied with a transverse alternating magnetic field; wherein a is a cross-sectional morphology diagram of the butt joint laser brazing joint; b is a mechanical property test result;

[0058] Figure 4 The cross-sectional morphology and mechanical property test results of the aluminum / titanium dissimilar metal butt joint laser brazing joint when the assembly gap is 0.4 mm for Example 2 are applied with a transverse alternating magnetic field with a magnetic flux density of 35 mT and a magnetic field frequency of 200 Hz; wherein a is a cross-sectional morphology diagram of the butt joint laser brazing joint; b is a mechanical property test result;

[0059] Figure 5 The cross-sectional morphology and mechanical property test results of the aluminum / titanium dissimilar metal butt joint laser brazing joint when the assembly gap is 0.4 mm for Comparative Example 2 are not applied with a transverse alternating magnetic field; wherein a is a cross-sectional morphology diagram of the butt joint laser brazing joint; b is a mechanical property test result;

[0060] Figure 6 The cross-sectional morphology and mechanical property test results of the aluminum / titanium dissimilar metal butt joint laser brazing joint when the assembly gap is 0 mm for Comparative Example 3 are applied with a transverse alternating magnetic field with a magnetic flux density of 15 mT and a magnetic field frequency of 200 Hz; wherein a is a cross-sectional morphology diagram of the butt joint laser brazing joint; b is a mechanical property test result;

[0061] Figure 7 The back macroscopic formation of the aluminum / titanium dissimilar metal butt joint laser brazing joint when the assembly gap is 0 mm and the joint back is not protected by inert gas for Comparative Example 4 is not applied with a transverse alternating magnetic field. DETAILED DESCRIPTION

[0062] The application will be described in detail below with reference to the examples. The following examples will help those skilled in the art to further understand the application, but do not limit the application in any form. It should be noted that those skilled in the art can make several adjustments and improvements without departing from the concept of the application. These all belong to the protection scope of the application.

[0063] In the present application, the movable welding system used is, for example,Figure 1 a, b and c, comprising:

[0064] A pair of slide rails;

[0065] Two welding sample tables arranged on the pair of slide rails, wherein the two welding sample tables are not in contact and maintain a certain distance in the vertical welding direction, so that the two sample plates of the butt joint sample can be respectively loaded on the corresponding welding sample tables, and the bottom of the to-be-welded part of the butt joint sample is suspended;

[0066] A magnetic device below the horizontal plane of the welding sample table; for generating a horizontal alternating magnetic field perpendicular to the welding direction, and the magnetic flux density decreases along the plate thickness direction;

[0067] A laser working head above the welding sample table, the laser working head is adjusted to a certain angle forward of the laser beam incidence direction, pointing to the front end of the welding pool, and the laser beam focus position corresponds to the sample surface directly above the center of the magnetic field;

[0068] The blowing port of the shielding gas nozzle points to the molten pool of the to-be-welded part, and forms a preset angle with the horizontal plane, the blowing port is a certain distance away from the center of the molten pool, and inert gas is sprayed as shielding gas.

[0069] A back shielding gas device below the magnetic field generating device and the sample, forming a closed shielding gas atmosphere, for preventing oxidation of the back welding area;

[0070] A PLC control system for regulating the switching and parameter adjustment of the magnetic field generating device, the motion trajectory and speed of the movable platform, and the emission timing and power of the laser beam.

[0071] The magnetic device adopts an electromagnet and an electromagnetic coil, and the magnetic field is specifically: a horizontal alternating magnetic field perpendicular to the welding direction, and the magnetic field strength decreases from bottom to top along the plate thickness direction.

[0072] As shown in Figure 1 (b), the two end poles of the electromagnet generate a magnetic field, and the pole distance needs to be maintained more than 2 to 3 times the weld width.

[0073] The electromagnetic coil generates an alternating magnetic field through an alternating current power supply, and the electromagnet concentrates the magnetic field at the two end poles to generate a horizontal alternating magnetic field perpendicular to the welding direction, and the magnetic flux density decreases from bottom to top along the plate thickness direction.

[0074] The method does not limit other methods to generate an alternating magnetic field meeting the above characteristics.

[0075] As shown in Figure 1 (c), the back shielding gas device is arranged below the two end poles of the electromagnet, and the protection of the entire back welding pool is realized by introducing inert gas into the device.

[0076] The method does not limit other methods to generate the shielding gas device meeting the above characteristics.

[0077] Two welding sample tables are connected by a cross beam. A magnetic field generated by a magnetic field generating device covers the welding pool area, and the laser working head and the magnetic field generating device remain stationary during the welding process. The movement of the welding platform is achieved by controlling the motor and the transmission mechanism connected to the sample placement table, thereby completing the welding process.

[0078] The movable welding system further comprises a control motor and a transmission mechanism connected to the welding sample table to achieve its movement.

[0079] The principle of the movable welding system is as follows:

[0080] Two sample plates are fixed horizontally side by side on the sample table by clamps, with the welding parts hanging vertically and completely adhering horizontally. The laser working head is placed above the welding part, i.e. the weld, and the incident direction of the laser beam is adjusted to a certain angle forward, pointing to the front end of the welding pool. Figure 1 As shown in (b), the forward angle is the included angle between the laser beam in the x-z plane and the negative direction of the z axis, and the focusing position of the laser beam is the surface directly above the center of the magnetic field. The welding parts of the two sample plates are provided below with a horizontal transverse alternating magnetic field perpendicular to the welding direction. The blowing port of the front protective gas nozzle is directed to the welding pool of the welding part and forms a certain angle with the horizontal plane. Inert gas is introduced into the back protective gas device, the laser emission and the moving platform switches are turned on, and the welding process is completed.

[0081] In the following examples and comparative examples, the mechanical property test method is as follows: according to GB / T 228.1-2010 metal material tensile test standard, a standard tensile sample with a width of 15mm, a thickness of 6mm and a gauge length of 65mm is cut in the middle of the joint length, and a uniaxial tensile test is carried out on a universal electronic material testing machine. The tensile test is carried out at room temperature, and the tensile test speed is 0.2mm / min.

[0082] Example 1

[0083] The embodiment provides a method for improving the gap adaptability of laser brazing of dissimilar metal butt joints by using an alternating magnetic field, comprising the following steps:

[0084] (1) Prepare two high-melting-point metal sample metal parts to be welded and low-melting-point metal sample metal parts to be welded with the same thickness of 5-12mm, and chemically clean the surfaces of the welding parts. In this embodiment, TA5 titanium alloy and AA5083 aluminum alloy are used for butt joint, and the sizes of the titanium alloy and aluminum alloy test plates are the same, with a length of 150mm, a width of 100mm and a thickness of 6mm.

[0085] (2) Fix the TA5 titanium alloy test plate and the AA5083 aluminum alloy test plate side by side on the welding test plate in a horizontal butt joint manner using a clamp. The test plates on both sides of the butt joint are machined with a straight bevel of 90°. The part to be welded is suspended in the vertical direction and the assembly gap is controlled to be 0mm in the horizontal direction.

[0086] (3) Adjust the position and posture of the laser welding head, adjust the incident direction of the laser beam to tilt forward by 7°, the laser beam is parallel to the butt joint plane and points to the front end of the molten pool, the laser beam focus position is the surface directly above the weld corresponding to the center of the magnetic field, and the focal point is offset from the interface to the aluminum alloy side by 2.0 mm; set a horizontal transverse alternating magnetic field perpendicular to the welding direction below the part to be welded.

[0087] (4) Set the laser welding parameters and magnetic field generation parameters. In this embodiment, the laser mode is continuous laser, the laser welding power is preferably 4200W, and the welding speed is preferably 0.4m / min. The magnetic field generation parameters include magnetic flux density and magnetic field frequency. In this embodiment, the preferred effective value of the magnetic flux density at the lower surface of the sample is 35mT (millitalas), the magnetic field frequency is 200Hz, and the excitation power supply waveform is a sine wave.

[0088] (5) Turn on the welding shielding gas. In this embodiment, the shielding gas is high-purity argon. The shielding gas is applied to both sides of the weld at the same time, and the flow rate is 20L / min.

[0089] (6) The laser power supply and magnetic field power supply are turned on using the robot integrated control system. First, the electromagnet generates a transverse alternating magnetic field that acts on the test plate to be welded for 2 seconds. After 2 seconds, the laser welding head emits a laser. The laser welding head above the sample and the magnetic field generating device below the sample remain stationary. The aforementioned welding speed is controlled by the movable welding platform to complete the laser brazing of the dissimilar metal joint.

[0090] (7) Move the mobile platform to the welding termination point, turn off the laser power supply, maintain the magnetic field for 2 seconds, and then turn off the magnetic field power supply. Turn off the shielding gas to end the welding process.

[0091] Comparative Example 1

[0092] The difference between this comparative example and Example 1 is that, during laser brazing, a horizontal alternating magnetic field perpendicular to the welding direction is not set below the part to be welded, that is, the magnetic field power supply is turned off during the welding process.

[0093] like Figure 2 (a) shows the cross-sectional morphology of the joint obtained in Example 1. The lower part of the aluminum alloy weld tends to be straight, and there are no obvious welding defects such as pores or cracks in the weld; Figure 2(b) shows the mechanical property test results of the joint obtained in Example 1, the tensile strength of the joint is up to 286 MPa.

[0094] As shown in Figure 3 (a) shows the cross-sectional morphology of the joint obtained in Comparative Example 1, the aluminum alloy weld is slightly concave, and there are no obvious welding defects such as pores and cracks in the weld; as shown in Figure 3 (b) shows the mechanical property test results of the joint obtained in Comparative Example 1, the tensile strength of the joint is 268 MPa. That is, when the assembly gap of the aluminum / titanium dissimilar metal test plate is 0 mm, the application of the magnetic field can improve the concave defect of the aluminum alloy weld, and the tensile strength of the joint is increased by nearly 7%.

[0095] Example 2

[0096] The difference between this example and Example 1 is that the assembly gap in the horizontal direction is controlled to be 0.4 mm, and the control method is to use a gasket with a thickness of 0.4 mm during clamping of the test plate.

[0097] Comparative Example 2

[0098] The difference between this comparative example and Example 2 is that the horizontal transverse alternating magnetic field perpendicular to the welding direction is not arranged below the to-be-welded part during laser brazing, that is, the magnetic field power is turned off during welding.

[0099] As shown in Figure 4 (a) shows the cross-sectional morphology of the joint obtained in Example 2, the lower part of the aluminum alloy weld tends to be flat, and there are no obvious welding defects such as pores and cracks in the weld; as shown in Figure 4 (b) shows the mechanical property test results of the joint obtained in Example 2, the tensile strength of the joint is 277 MPa.

[0100] As shown in Figure 5 (a) shows the cross-sectional morphology of the joint obtained in Comparative Example 2, there are a large number of continuous hot crack defects in the aluminum alloy weld;

[0101] As shown in Figure 5 (b) shows the mechanical property test results of the joint obtained in Comparative Example 2, due to the existence of cracks in the weld, the joint is broken without plastic deformation, and the tensile strength of the joint is only 37 MPa. That is, when the assembly gap of the aluminum / titanium dissimilar metal test plate is 0.4 mm, the application of the transverse alternating magnetic field can greatly improve the adaptability of the laser brazing of the dissimilar metal butt joint to the assembly gap, and suppress the generation of a large number of continuous hot crack defects in the aluminum alloy weld, and the strength of the joint is increased by several times.

[0102] Comparative Example 3

[0103] The difference between this comparative example and Example 1 is that the parameters of the magnetic field are different, specifically in step (6):

[0104] The magnetic flux density effective value is 15 mT, the magnetic field frequency is 200 Hz, and the excitation power waveform is a sine wave.

[0105] As shown in Figure 6 (a) is the joint cross-sectional morphology obtained by Comparative Example 3, which has no obvious change compared with the joint cross section obtained by Example 1, the lower part of the aluminum alloy weld is still concave, and no obvious welding defects such as cracks and pores are observed in the weld; as shown in Figure 6 (b) is the mechanical property test result of the joint obtained by Example 1, the tensile strength of the joint is 270 MPa, which has no obvious change compared with the mechanical property of the joint obtained by Example 1 without applying a magnetic field. It is indicated that when the magnetic flux density is small, the lifting and oscillation effects of the magnetic field on the molten pool are small, and the effects on the joint cross-sectional morphology and mechanical property are not significant.

[0106] Comparative Example 4

[0107] The difference between the present comparative example and Example 1 is that the application of the back welding protective gas in step (5) is omitted. Figure 7 As shown in the figure, the back forming of the joint obtained by Comparative Example 4 is extremely irregular, the forming is poor, and the aluminum alloy and titanium alloy back do not wet. When there is no inert gas protection, the surfaces of the aluminum alloy and the titanium alloy will be rapidly oxidized at high temperature, the fluidity of the molten pool will be poor, and the wettability of the aluminum alloy will be poor. In the present application, the application of the back protective gas is crucial to the laser welding of aluminum and titanium dissimilar metals.

[0108] In summary, the present application utilizes the transverse alternating magnetic field applied below the bottom of the workpiece to generate the Lorentz force in the molten pool. The Lorentz force, on the one hand, produces the macroscopic upward lifting effect of the molten pool, and inhibits the generation of defects such as thermal cracks caused by the downward flow of the molten pool and the shrinkage of the aluminum alloy weld when the gap increases; on the other hand, it stirs the molten pool, homogenizes the temperature field and element distribution of the molten pool, refines the weld grain, and improves the joint strength. The aluminum / titanium dissimilar metal laser fusion brazing butt joint obtained by the method in the specific embodiments can obtain a joint with a tensile strength comparable to that when the gap is 0 mm even when the gap is 0.4 mm, and the joint forming is improved to a certain extent. Thus, the adaptability of the laser fusion brazing butt joint of dissimilar metals to the assembly gap is greatly improved, the product quality is improved, and the production efficiency is ensured.

[0109] The specific embodiments of the present application are described above. It should be understood that the present application is not limited to the above specific embodiments, and those skilled in the art can make various modifications or modifications within the scope of the claims, which do not affect the essential content of the present application, and should be included in the protection scope of the claims of the present application.

Claims

1. A method for improving the gap adaptability of laser brazing of dissimilar metal butt joints by applying an external alternating magnetic field, characterized in that, The method includes the following steps: S1. Fix the welding parts of the high-melting-point metal workpiece and the low-melting-point metal workpiece side by side in a horizontal butt joint manner, control the welding parts of the two workpieces to be suspended in the vertical direction, and maintain a preset assembly gap in the horizontal direction; the high-melting-point metal workpiece includes one of titanium alloy, steel or copper, and the low-melting-point metal workpiece includes aluminum alloy. S2. A horizontal alternating magnetic field perpendicular to the welding direction is set below the area to be welded, with the center of the magnetic field corresponding to the weld between the two workpieces to be welded. The laser beam is focused on the surface directly above the weld, and the laser beam focus is offset by a predetermined distance from the weld towards the low-melting-point metal side. S3. Laser welding parameters and magnetic field generation parameters are set. The magnetic field generation parameters include magnetic flux density and magnetic field frequency, and the magnetic flux density gradually decreases along the thickness direction of the sample. During the welding process, a shielding gas is used to protect both sides of the weld. S4. Start the magnetic field to generate a transverse alternating magnetic field that acts on the workpiece to be welded. Then start the laser beam to perform laser brazing until the welding is completed. S5. After welding is terminated, first turn off the laser, then keep the magnetic field applied for a predetermined time before turning off the magnetic field generating device and the shielding gas to end the welding process. In S2, the incident direction of the laser beam is tilted forward by a predetermined angle, pointing towards the front end of the molten pool; the forward tilt angle of the incident direction of the laser beam is 7° to 10°, and the offset distance of the laser beam focus from the weld to the low melting point metal side is 1.5 to 2.5 mm.

2. The method according to claim 1, characterized in that, In S1, the assembly gap is 0 to 0.5 mm.

3. The method according to claim 1, characterized in that, In S3, the laser form is either continuous laser or pulsed laser; When using continuous laser, the laser welding parameters are: laser power of 4000W to 6000W and brazing speed of 0.3 to 0.5m / min. When using pulsed lasers, the pulse frequency is 10–200 Hz.

4. The method according to claim 1, characterized in that, In S3, the effective value of the magnetic flux density is 35 to 45 millitalas, and the magnetic field frequency is 200 to 400 Hz.

5. The method according to claim 1, characterized in that, The protective gas is argon or helium, and the flow rate of the protective gas is 20-30 L / min.

6. The method according to claim 1, characterized in that, The laser welding system used in this method includes a pair of slide rails, two welding sample stages set on the slide rails, a magnetic field generating device located below the horizontal plane of the welding sample stage, a laser working head located above the welding sample stage, a front protective gas nozzle, a back protective gas device, and a PLC control system.

7. The method according to claim 6, characterized in that, The two welding test stands do not contact each other in the vertical welding direction and maintain a certain distance, and the bottom of the part to be welded of the butt test specimen is suspended. The magnetic field generator is used to generate a horizontal alternating magnetic field perpendicular to the welding direction, and the magnetic field strength decreases from bottom to top along the plate thickness direction. The laser working head is used to emit a laser beam that is focused onto the upper surface of the sample during the welding process; The front shielding gas nozzle is located near the laser working head and is used to spray shielding gas into the welding area; The back protective gas device is located below the magnetic field generating device and the sample, forming a closed protective atmosphere to prevent oxidation of the back welding area; The PLC control system is used to regulate the movement of the magnetic field generating device, the movable platform, and the emission of the laser beam.

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