Method for improving laser welding-brazing gap adaptability of dissimilar metal butt joint

Abstract

The invention belongs to the field of dissimilar metal laser welding, and particularly relates to a method for improving the adaptability of a dissimilar metal butt joint laser welding-brazing gap through assistance of an alternating magnetic field. After the bottom of the butt joint portion of dissimilar metal test plates with assembly gaps is suspended, a transverse alternating magnetic field and inert gas protection are applied to the lower portion of the butt joint portion, a laser welding head above a sample and a magnetic field generator below the sample are kept static in the welding process, and a laser beam is biased towards the low-melting-point metal side; and the molten low-melting-point material is wetted and spread on the surface of the non-molten high-melting-point material, and then interface metallurgical bonding is carried out to form a melt-brazed joint. The adaptability of dissimilar metal butt joint laser melt-brazing to the assembly gap is improved based on the effect that the transverse alternating magnetic field lifts the molten pool, generation of defects such as aluminum alloy weld side hot cracks caused by increase of the assembly gap is restrained, weld forming is improved, and the adaptability of dissimilar metal butt joint laser melt-brazing in engineering is improved.

Description

Technical Field

[0001] The invention belongs to the field of laser welding of dissimilar metals, and specifically relates to a method for improving the adaptability of the gap of laser brazing welding of dissimilar metal butt joints; in particular, it relates to a method for improving the adaptability of the gap of laser brazing welding of dissimilar metal butt joints by applying an alternating magnetic field. Background Art

[0002] With the continuous development and progress of industry and science and technology, environmental protection and energy consumption reduction have received more attention. In order to achieve sustainable development, all walks of life have made lightweight materials a research focus. Aluminum alloy materials have been widely used in industrial fields such as automobiles and aerospace due to their advantages such as high specific strength, good processing performance and low cost.

[0003] At the same time, titanium alloy materials are widely used in piping systems, structural parts and high-temperature parts of aircraft engines due to their excellent corrosion resistance, higher specific strength and good high temperature resistance. Copper alloys have become indispensable materials in the fields of chemical industry, refrigeration, electrical and energy electronics due to their excellent electrical and thermal conductivity and good corrosion resistance. High-strength steel has higher specific strength and better fatigue resistance. Compared with the above materials, steel materials are relatively cheap and have been widely used in aerospace, ships, automobiles and other fields. In order to further achieve lightweight, the preparation of aluminum / titanium, aluminum / copper and aluminum / steel composite components by welding or connection has become a research hotspot that people are concerned about.

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

[0005] In order to improve the adaptability of laser welding of the same metal to the assembly gap, the process methods such as swinging laser, improving welding process, and using filler wire are generally used. Chinese patent CN 110722264 A discloses a high-power laser welding method with strong gap adaptability, which pre-pile up some filler metal inside the butt groove of the workpiece, and then perform high-power laser welding after the filler material cools down. This invention combines laser self-melting welding with filling material process to improve 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 thermal, physical and chemical properties, pre-pile up filler metal by fusion welding will lead to the formation of a large amount 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, which combines the vacuum environment with the swing laser to control the energy and the temperature field of the molten pool, and controls the laser output energy during the laser swing process, thereby optimizing the welding energy distribution, improving the welding formation 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 achieve large-size workpieces and mass industrial manufacturing.

[0007] Chinese patent CN 112388164 A discloses an AC magnetic field assisted laser deep melting welding method for reducing the porosity of aluminum alloy welds. The method pre-sets an AC electromagnetic field at the bottom of the butt joint of the sample to be welded, and utilizes the Lorentz force generated by the alternating magnetic field in the molten pool to drive the bubbles to escape upward, thereby achieving the purpose of suppressing or even completely eliminating small-hole-type pores. However, this method is only applicable to the laser welding process of the same metal for partial penetration welding. For dissimilar metal butt welding, it is necessary to achieve full penetration of the entire weld to increase the bonding area of ​​the interface. At this time, the metal below the molten pool will be severely oxidized. The present invention applies a transverse alternating magnetic field to the bottom of the aluminum / titanium dissimilar metal joint, and applies inert gas protection to the back of the molten pool through a back gas protection device, which can achieve the optimization of the weld structure while improving the formation of the dissimilar metal joint and suppressing the generation of defects such as cracks.

[0008] When laser brazing dissimilar metal butt joints in engineering, the influence of assembly gap on the stability of brazing process can usually be reduced by filling a small amount of welding wire or improving laser heat input parameters. However, due to uncertain factors such as insufficient assembly precision, the gap between dissimilar material test plates is not a constant value, and it is difficult to use the same welding process parameters to adapt to welding tasks under all assembly gap conditions. At the same time, when changing the welding process parameters, the continuity of the welding process is destroyed, so the application of this process method is extremely difficult. Summary of the invention

[0009] In view of the problems existing in the prior art, the present invention provides a method for improving the gap adaptability of laser brazing welding of dissimilar metal butt joints. In order to achieve dissimilar metal butt welding, the present invention requires full penetration of the entire test plate, inert gas protection is applied to the back to inhibit oxidation of the molten pool, and a transverse alternating magnetic field is applied to assist without changing the original laser power, welding speed, laser offset distance and other welding parameters, thereby achieving dissimilar metal welding.

[0010] The method provided by the present invention can inhibit the generation of defects such as welding molten pool dripping and thermal cracks on the side of the aluminum alloy weld caused by the increase of the assembly gap, improve the weld formation, increase the joint strength, and ensure the welding quality. The present invention does not need to fill welding wire, does not need to change the brazing process parameters such as laser power, welding speed and laser offset distance, and only needs to apply a transverse alternating magnetic field below the laser brazing molten pool of the aluminum / titanium dissimilar metal butt joint, and use inert gas protection for the upper and lower surface molten pools, so as to greatly improve the adaptability of the laser brazing of the dissimilar metal butt joint to the assembly gap.

[0011] The technical solution provided by the present invention is:

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

[0013] 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-jointed manner, control the welding parts of the two workpieces to be welded to be suspended in the vertical direction, and maintain a preset assembly gap in the horizontal direction;

[0014] S2. A horizontal transverse alternating magnetic field perpendicular to the welding direction is set below the part to be welded, the center of the magnetic field corresponds 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 from the weld to the low-melting-point metal side by a predetermined distance;

[0015] S3. Setting laser welding parameters and magnetic field generation parameters, 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, using shielding gas to protect both sides of the weld;

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

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

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

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

[0020] In S2, the incident direction of the laser beam is tilted forward at a predetermined angle, pointing to the front end of the molten pool; the incident direction of the laser beam is tilted forward at an angle of 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.

[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 4000W~6000W, and brazing speed is 0.3~0.5m / min;

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

[0024] Before welding, the parts to be welded should be mechanically polished or chemically cleaned.

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

[0026] The protective gas is high-purity argon or high-purity helium, and the protective gas flow rate is 20 to 30 liters / minute.

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

[0028] The two welding sample tables do not touch each other in the vertical welding direction and maintain a certain distance, and the bottom of the to-be-welded part of the workpiece to be welded is suspended in the air;

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

[0030] The electromagnet and the electromagnetic coil are devices for generating magnetic fields. The decreasing change of the specific magnetic flux density is reflected on the workpiece to be welded. The magnetic flux density of the workpiece to be welded gradually decreases along the thickness direction of the workpiece to be welded.

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

[0032] The laser working head is used to emit a laser beam and focus on the upper surface of the sample during the welding process;

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

[0034] The back side protective gas device is located under the magnetic field generator and the sample to form a closed protective atmosphere to prevent oxidation of the back side welding area;

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

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

[0037] A pair of slide rails;

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

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

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

[0041] The front shielding gas nozzle is located near the laser working head and is used to spray shielding gas to the welding area;

[0042] The back side protective gas device is located under the magnetic field generator and the sample to form a closed protective atmosphere to prevent oxidation of the back side welding area;

[0043] The PLC control system is used to control the switching and parameter adjustment of the magnetic field generating device, the movement trajectory and speed of the movable platform, and the emission timing and power of the laser beam.

[0044] The two welding sample tables are connected by a crossbeam. The magnetic field generated by the magnetic field generator covers the welding pool area, and the laser head and the magnetic field generator remain stationary during the welding process. The welding platform is moved by controlling the motor and the transmission mechanism connected to the sample placement table 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 the AC power supply. The electromagnet gathers the magnetic field at the two poles to generate a horizontal AC magnetic field perpendicular to the welding direction. The magnetic flux density decreases from bottom to top along the plate thickness, and the distance between the poles must be kept at more than 2 to 3 times the weld width.

[0047] The laser working head is adjusted so that the incident direction of the laser beam is tilted forward at a certain angle, pointing to the front end of the welding molten pool, and the focusing position of the laser beam corresponds to the surface of the sample directly above the center of the magnetic field;

[0048] The blowing port of the shielding gas nozzle points to the molten pool of the part to be welded 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 sprays inert gas as shielding gas.

[0049] The fiber laser power supply emits a laser beam, and the AC power supply outputs a sinusoidal alternating current.

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

[0051] 1. By applying a transverse alternating magnetic field and inert gas protection below the molten pool of laser brazing welding of dissimilar metal butt joints, 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 weld after solidification are similar under the conditions of different assembly gaps, inhibiting the occurrence of defects such as thermal cracks caused by the molten pool flowing down when the gap increases and the solidification shrinkage of the aluminum alloy weld, improving the consistency of the mechanical properties of the joint under different gap conditions, and enhancing the joint strength.

[0052] 2. The external magnetic field forms induced eddy currents inside the molten pool of laser brazing of dissimilar metal butt joints, which is beneficial to homogenize the temperature field and element distribution of the molten pool, improve the distribution of intermetallic compounds at the interface, break up large-sized dendrites, refine the weld grains, and improve the joint strength.

[0053] 3. The present invention has no restrictions on the size of the workpiece. For butt joints of dissimilar metals of different thicknesses, it is only necessary to change the magnetic field parameters to increase or decrease the lifting effect on the molten pool according to demand, thereby improving the adaptability of the laser brazing gap. BRIEF DESCRIPTION OF THE DRAWINGS

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

[0055] Figure 1 Schematic diagram of the transverse alternating magnetic field assisted laser welding system provided by the present invention; 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; c is a schematic diagram of the back protection gas device;

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

[0057] Figure 3 The cross-sectional morphology and mechanical property test results of the laser welding brazing joint of aluminum / titanium dissimilar metal butt joint when no transverse alternating magnetic field is applied and the assembly gap is 0 mm for comparative example 1; wherein a is the cross-sectional morphology of the laser welding brazing joint of the butt joint; b is the mechanical property test result;

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

[0059] Figure 5 The cross-sectional morphology and mechanical property test results of the laser welding brazing joint of aluminum / titanium dissimilar metal butt joint when no transverse alternating magnetic field is applied in comparative example 2 and the assembly gap is 0.4 mm; wherein a is the cross-sectional morphology of the laser welding brazing joint of the butt joint; b is the mechanical property test result;

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

[0061] Figure 7 For comparative example 4, no transverse alternating magnetic field is applied, the assembly gap is 0 mm, and there is no inert gas protection on the back of the joint to show the macroscopic formation of the back of the aluminum / titanium dissimilar metal butt joint by laser brazing. DETAILED DESCRIPTION

[0062] The present invention is described in detail below in conjunction with embodiments. The following embodiments will help those skilled in the art to further understand the present invention, but are not intended to limit the present invention in any form. It should be noted that, for those of ordinary skill in the art, some adjustments and improvements can also be made without departing from the concept of the present invention. These all belong to the protection scope of the present invention.

[0063] In the present invention, the movable welding system used is as follows Figure 1 As shown in a, b and c, including:

[0064] A pair of slide rails;

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

[0066] A magnetic device located below the horizontal plane of the welding specimen table; used to generate a horizontal AC magnetic field perpendicular to the welding direction, and the magnetic flux density decreases along the plate thickness direction;

[0067] The laser working head is located above the welding sample table. The laser working head is adjusted to tilt the incident direction of the laser beam forward at a certain angle, pointing to the front end of the welding molten pool. The focusing position of the laser beam corresponds to the sample surface just above the center of the magnetic field.

[0068] The blowing port of the shielding gas nozzle points to the molten pool of the part to be welded 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 sprays inert gas as shielding gas.

[0069] The back side protective gas device is located under the magnetic field generator and the sample to form a closed protective atmosphere to prevent oxidation of the back side welding area;

[0070] The PLC control system is used to control the switching and parameter adjustment of the magnetic field generating device, the movement trajectory and speed of the movable platform, and the emission timing and power of the laser beam.

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

[0072] like Figure 1 (b) As shown; the magnetic poles at both ends of the electromagnet generate a magnetic field, and the distance between the magnetic poles must be kept at more than 2 to 3 times the weld width.

[0073] The electromagnetic coil is powered by an AC power supply to generate an alternating magnetic field. The electromagnet gathers the magnetic field at the poles at both ends to generate a horizontal AC magnetic field perpendicular to the welding direction. The magnetic flux density decreases from bottom to top along the thickness of the plate.

[0074] This method does not limit other methods to generate an AC magnetic field that meets the above characteristics.

[0075] like Figure 1 As shown in (c), the back protection gas device is placed below the magnetic poles at both ends of the electromagnet, and the protection of the entire back welding pool is achieved by introducing inert gas into the device.

[0076] This method does not limit other methods to generate a protective gas device that meets the above characteristics.

[0077] The two welding sample tables are connected by a crossbeam. The magnetic field generated by the magnetic field generator covers the welding pool area, and the laser head and the magnetic field generator remain stationary during the welding process. The welding platform is moved by controlling the motor and the transmission mechanism connected to the sample placement table to complete the welding process.

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

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

[0080] The two sample plates are fixed horizontally side by side on the sample table by a clamp, with the part to be welded suspended in the vertical direction and completely fitted in the horizontal direction. The laser working head is placed on the part to be welded, that is, above the weld, and the incident direction of the laser beam is adjusted to tilt forward at a certain angle to point to the front end of the welding pool. Figure 1 (b) shows that the forward tilt angle is the angle between the laser beam in the xz plane and the negative direction of the z axis, and the laser beam focusing position is the surface directly above the weld corresponding to the center of the magnetic field; a horizontal transverse AC magnetic field perpendicular to the welding direction is set below the to-be-welded parts of the two sample plates. Adjust the blowing port of the front protective gas nozzle to point to the molten pool of the to-be-welded part and form a certain angle with the horizontal plane. Introduce inert gas into the back protective gas device, turn on the laser emission and mobile platform switches, and complete the welding process.

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

[0082] Example 1

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

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

[0085] (2) The TA5 titanium alloy test plate and the AA5083 aluminum alloy test plate were fixed side by side on the welding specimen table by means of a clamp in a horizontal butt joint manner. The test plates on both sides of the butt joint were processed with a 90° straight groove. The welded part was suspended in the vertical direction and the assembly gap was controlled to be 0 mm in the horizontal direction.

[0086] (3) The position and posture of the laser welding head were adjusted, and the incident direction of the laser beam was adjusted to be tilted 7° forward. The laser beam was parallel to the butt joint plane and pointed to the front end of the molten pool. The focusing position of the laser beam was the surface directly above the weld corresponding to the center of the magnetic field, and the focus was offset from the interface to the aluminum alloy side by 2.0 mm. A horizontal transverse alternating magnetic field perpendicular to the welding direction was set below the part to be welded.

[0087] (4) Setting 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 effective value of the magnetic flux density at the lower surface of the sample is preferably 35mT (milli-Tesla), the magnetic field frequency is 200Hz, and the excitation power waveform is a sine wave.

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

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

[0090] (7) The mobile platform moves to the welding end point, the laser power is turned off, the magnetic field is applied for 2 seconds, and then the magnetic field power is turned off. The shielding gas is turned off to end the welding process.

[0091] Comparative Example 1

[0092] The difference between this comparative example and Example 1 is that during laser brazing, no horizontal transverse alternating magnetic field perpendicular to the welding direction is provided 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 and cracks in the weld. Figure 2(b) shows the test results of the mechanical properties of the joint obtained in Example 1. The tensile strength of the joint is as high as 286 MPa.

[0094] like 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. Figure 3 (b) shows the test results of the joint mechanical properties obtained in comparative example 1, and the tensile strength of the joint is 268MPa. That is, when the assembly gap of the aluminum / titanium dissimilar metal test plates is 0mm, applying a 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 embodiment and embodiment 1 is that the part to be welded is controlled to be suspended in the vertical direction and the assembly gap in the horizontal direction is controlled to be 0.4 mm. The gap is controlled by using a gasket with a thickness of 0.4 mm during the clamping of the test plate.

[0097] Comparative Example 2

[0098] The difference between this comparative example and Example 2 is that during laser brazing, no horizontal transverse alternating magnetic field perpendicular to the welding direction is provided below the part to be welded, that is, the magnetic field power supply is turned off during the welding process.

[0099] like 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 straight, and there are no obvious welding defects such as pores and cracks in the weld. Figure 4 (b) shows the test results of the mechanical properties of the joint obtained in Example 2, and the tensile strength of the joint is 277 MPa.

[0100] like Figure 5 (a) shows the cross-sectional morphology of the joint obtained in Comparative Example 2, where a large number of continuous thermal crack defects exist in the aluminum alloy weld;

[0101] like Figure 5 (b) shows the test results of the joint mechanical properties obtained in Comparative Example 2. Due to the presence of cracks in the weld, the joint broke without plastic deformation, and the tensile strength of the joint was only 37MPa. That is, when the assembly gap of the aluminum / titanium dissimilar metal test plate is 0.4mm, applying a transverse alternating magnetic field can greatly improve the adaptability of the laser melting brazing of the dissimilar metal butt joint to the assembly gap, inhibit the generation of a large number of continuous thermal crack defects in the aluminum alloy weld, and improve the joint strength several times.

[0102] Comparative Example 3

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

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

[0105] like Figure 6 (a) shows the cross-sectional morphology of the joint obtained in Comparative Example 3. There is no significant change compared with the cross-sectional morphology of the joint obtained in 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. Figure 6 (b) shows the test results of the joint mechanical properties obtained in Example 1. The tensile strength of the joint is 270 MPa, and there is no significant change in the joint mechanical properties obtained when no magnetic field is applied compared to Example 1. This indicates that when the magnetic flux density is small, the lifting and oscillation effect of the magnetic field on the molten pool is small, and the effect on the cross-interface morphology and mechanical properties of the joint is not significant.

[0106] Comparative Example 4

[0107] The difference between this comparative example and Example 1 is that the application of back welding shielding gas in step (5) is omitted. Figure 7 The figure shows the back forming of the joint obtained in Comparative Example 4. The weld shape is very irregular, the forming is very poor, and no wetting occurs on the back of the aluminum alloy and titanium alloy. When there is no inert gas protection on the back of the joint, the surface of the aluminum alloy and titanium alloy will oxidize rapidly at high temperature, the fluidity of the molten pool will deteriorate, and the wettability of the aluminum alloy will deteriorate. In the present invention, the application of back protection gas is very important for aluminum-titanium dissimilar metal laser welding.

[0108] In summary, the present invention utilizes a transverse alternating magnetic field applied to the bottom of the workpiece to generate a Lorentz force inside the molten pool. On the one hand, the Lorentz force produces a macroscopic upward lifting effect on the molten pool, suppressing the generation of defects such as thermal cracks caused by the molten pool flowing down 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 grains, and improves the joint strength. The aluminum / titanium dissimilar metal laser brazing butt joint obtained in the specific embodiment of the method can obtain a joint with a tensile strength equivalent 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. Thereby, the adaptability of the laser brazing of dissimilar metal butt joints to the assembly gap is greatly improved, the product quality is improved, and the production efficiency is guaranteed.

[0109] The above is a description of the specific embodiments of the present invention. It should be understood that the present invention is not limited to the above specific embodiments, and those skilled in the art can make various deformations or modifications within the scope of the claims, which do not affect the essence of the present invention and should be included in the protection scope of the claims of the present invention.

Claims

1. A method for improving the gap adaptability of laser brazing welding of dissimilar metal butt joints by applying an alternating magnetic field, characterized in that: The method comprises 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-jointed manner, control the welding parts of the two workpieces to be welded to be suspended in the vertical direction, and maintain a preset assembly gap in the horizontal direction; S2. A horizontal transverse alternating magnetic field perpendicular to the welding direction is set below the part to be welded, the center of the magnetic field corresponds 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 from the weld to the low-melting-point metal side by a predetermined distance; S3. Setting laser welding parameters and magnetic field generation parameters, 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, using shielding gas to protect both sides of the weld; S4, starting the magnetic field to generate a transverse alternating magnetic field to act on the workpiece to be welded, and then starting the laser beam to perform laser brazing until the welding is completed; S5. After the welding is terminated, the laser is turned off first, and the magnetic field is kept applied for a predetermined time, and then the magnetic field generating device is turned off, and the protective gas is turned off to end the welding process.

2. The method according to claim 1, characterized in that In S1, the high melting point metal workpiece to be welded includes one of titanium alloy, steel or copper, and the low melting point metal workpiece to be welded includes aluminum alloy.

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

4. The method according to claim 1, characterized in that: In S2, the incident direction of the laser beam is tilted forward at a predetermined angle, pointing to the front end of the molten pool; the incident direction of the laser beam is tilted forward at an angle of 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.

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

6. The method according to claim 1, characterized in that Among the magnetic field generation parameters in S3, the effective value of the magnetic flux density is 35 to 45 millitesla, and the magnetic field frequency is 200 to 400 Hz.

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

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

9. The method according to claim 8, characterized in that The two welding specimen tables do not touch each other in the vertical welding direction and maintain a certain distance, and the bottom of the welding part of the butt specimen is suspended; The magnetic field generating device is used to generate a horizontal alternating magnetic field perpendicular to the welding direction, and the magnetic field intensity decreases from bottom to top along the plate thickness direction; The laser working head is used to emit a laser beam and focus on 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 to the welding area; The back side protective gas device is located under the magnetic field generator and the sample to form a closed protective atmosphere to prevent oxidation of the back side welding area; The PLC control system is used to control the movement of the magnetic field generating device, the movable platform and the emission of the laser beam.

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