Laser ultrasonic composite welding device and application method thereof

Abstract

The invention provides a laser ultrasonic composite welding device and an application method thereof. The laser ultrasonic composite welding device comprises a rack; an ultrasonic output unit; a laser output unit; the supporting assembly comprises a first supporting unit of which the top is provided with a first adjusting block, a second supporting unit of which the top is provided with a second adjusting block, and a clamp for clamping the ultrasonic output unit; the positioning assembly comprises a base plate with an empty groove in the middle, a first pressing plate and a second pressing plate; the base plate is horizontally and detachably arranged on the first adjusting block and the second adjusting block, the first pressing plate and the second pressing plate are detachably arranged on the top of the base plate, and the first pressing plate and the second pressing plate are arranged on the front side and the rear side of the empty groove side by side; the output end of the ultrasonic output unit and the output end of the laser output unit are arranged on the upper side and the lower side of the positioning assembly respectively. The ultrasonic output unit and the laser output unit are arranged for hybrid welding, the porosity and cracks can be greatly reduced, and the welding quality is good.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of laser processing, in particular to a laser-ultrasonic composite welding device and an application method thereof. BACKGROUND

[0002] In the field of welding, copper-aluminum dissimilar welding has always been a thorny problem. Two different materials have significant differences in melting point, huge thermal conductivity and linear expansion coefficient, etc. The commonly used methods for copper-aluminum dissimilar welding at present are fusion welding and brazing composite process, brazing pressure welding, friction stir welding, single fiber laser welding and blue light composite laser welding. These methods can produce certain welding effect on copper-aluminum dissimilar welding, but they will inevitably cause brittle intermetallic compound layer (IMC), high residual stress and joint deformation, joint performance instability and a large number of crack porosity problems, etc., which limit the application and development of copper-aluminum welding.

[0003] The existing copper-aluminum dissimilar welding method has the following defects:

[0004] 1. A relatively thick brittle intermetallic compound layer (IMC) is formed, and the joint is prone to deformation and brittle fracture.

[0005] 2. The weld has many cracks and pores, and the weld bonding force is poor. SUMMARY

[0006] In view of the deficiencies of the prior art, the purpose of the present application is to provide a laser-ultrasonic composite welding device and an application method thereof, which is provided with an ultrasonic output unit and a laser output unit for composite welding, can greatly reduce the porosity and cracks, the ultrasonic wave can propagate in the molten pool, and the solid-liquid interface of the base material and the molten pool will produce violent mechanical vibration, so as to break the columnar dendrites grown through the interface, refine the grains, and improve the weld quality.

[0007] The embodiments of the present application are implemented by the following technical solutions:

[0008] A laser-ultrasonic composite welding device, comprising:

[0009] a rack;

[0010] an ultrasonic output unit for outputting ultrasonic waves to the product to be welded;

[0011] a laser output unit for outputting a laser beam to the product to be welded;

[0012] The support assembly comprises a first support unit provided with a first adjusting block at the top, a second support unit provided with a second adjusting block at the top, and a clamp for clamping the ultrasonic output unit, wherein the first support unit, the clamp and the second support unit are all arranged at the top of the rack, and the first support unit and the second support unit are respectively arranged at the left and right sides of the clamp.

[0013] The positioning assembly comprises a base plate provided with a slot at the middle, a first pressing plate and a second pressing plate; the base plate is horizontally and detachably arranged on the first adjusting block and the second adjusting block, the first pressing plate and the second pressing plate are detachably arranged at the top of the base plate, and the first pressing plate and the second pressing plate are arranged side by side at the front and back of the slot.

[0014] The output end of the ultrasonic output unit and the output end of the laser output unit are respectively arranged at the upper and lower sides of the positioning assembly.

[0015] According to a preferred embodiment, the laser output unit comprises a laser generator and a mirror group unit; the mirror group unit is arranged at the lifting end of the lifting frame to adjust the vertical height of the mirror group unit.

[0016] According to a preferred embodiment, the bottom of the clamp is provided with a clamp driving member.

[0017] According to a preferred embodiment, the bottom of the clamp is further provided with a lifting unit, which drives the clamp driving member or drives the clamp to rise and fall to adjust the vertical position of the clamp.

[0018] According to a preferred embodiment, the ultrasonic output unit comprises an ultrasonic tool head; the top of the ultrasonic tool head is provided with an output port, which is close to the slot and communicates with the slot.

[0019] According to a preferred embodiment, the ultrasonic output unit comprises an ultrasonic tool head; the top of the ultrasonic tool head is provided with an output port, which is close to the slot and communicates with the slot.

[0020] According to a preferred embodiment, the ultrasonic tool head is made of aluminum, the top surface of the aluminum structure is a flat surface, and the output port is arranged at the middle of the flat surface.

[0021] According to a preferred embodiment, the product to be welded comprises a first product to be welded and a second product to be welded, the first product to be welded is a copper product to be welded, and the second product to be welded is an aluminum product to be welded.

[0022] According to a preferred embodiment, the laser generator is a single-mode annular spot laser.

[0023] The mirror group unit comprises an inner light path transmission unit and a galvanometer unit.

[0024] According to a preferred embodiment, the laser generator can emit an inner ring laser beam and an outer ring laser beam.

[0025] The inner ring laser power range of the laser generator is 1500-2000W.

[0026] The outer ring laser power range of the laser generator is 1000-3500W.

[0027] An application method of a laser-ultrasonic composite welding device, comprising the following steps:

[0028] Step S10: The top surface of the first adjusting block and the top surface of the second adjusting block are flush with the top surface of the ultrasonic tool head, and the ultrasonic tool head contacts the base plate.

[0029] Step S20: The first product to be welded and the second product to be welded are placed on the base plate, the first product to be welded and the second product to be welded are fixed by the first pressing plate and the second pressing plate, and it is confirmed whether the first product to be welded and the second product to be welded are fixed tightly.

[0030] Step S30: The laser generator outputs a laser beam, which is transmitted to the mirror group unit through the inner light path transmission unit, the lifting end of the lifting frame is controlled to move up and down to adjust the vertical position of the mirror group unit, so as to determine the focal point position.

[0031] Step S40: The parameters of the laser generator are adjusted to confirm whether the laser output unit emits light normally, and the parameters of the ultrasonic generator are adjusted to confirm whether the ultrasonic wave output unit emits ultrasonic waves normally.

[0032] Step S50: The ultrasonic wave output unit emits ultrasonic waves to the welding position of the first product to be welded and the second product to be welded.

[0033] Step S60: After emitting ultrasonic waves, the laser output unit emits light to realize laser-ultrasonic composite welding.

[0034] According to a preferred embodiment, the welding mode of the laser-ultrasonic composite welding is lap welding, part of the first product to be welded is located above part of the second product to be welded or part of the first product to be welded is located below part of the second product to be welded.

[0035] The technical scheme of the embodiment of the application has at least the following advantages and beneficial effects:

[0036] This invention features a composite welding system with both ultrasonic and laser output units, which significantly reduces porosity and cracks. Ultrasonic waves can propagate in the molten pool, generating intense mechanical vibrations at the solid-liquid interface between the base material and the molten pool. This causes the columnar dendrites growing through the interface to break up, refining the grains and resulting in high-quality welds. Attached Figure Description

[0037] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of the present invention and should not be regarded as a limitation on the scope. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort.

[0038] Fig. 1 This is a schematic diagram of the structure of a laser-ultrasonic composite welding device provided in an embodiment of the present invention;

[0039] Fig. 2 A three-dimensional structural schematic diagram of a laser-ultrasonic hybrid welding device provided in an embodiment of the present invention;

[0040] Fig. 3 This is a partial three-dimensional structural schematic diagram of a laser-ultrasonic composite welding device provided in an embodiment of the present invention;

[0041] Fig. 4 This is a three-dimensional structural diagram of the structure after the positioning component has been removed, as provided in an embodiment of the present invention.

[0042] Icons: 1. Frame; 2. Ultrasonic tool head; 3. Ultrasonic generator; 4. First support unit; 5. First adjusting block; 6. Second support unit; 7. Second adjusting block; 8. Fixture; 9. Pad; 10. First pressure plate; 11. Second pressure plate; 12. Empty slot; 13. Laser generator; 14. Lens assembly unit; 15. Fixture drive; 16. Lifting unit; 17. Lifting frame; 18. Output port. Detailed Implementation

[0043] To better understand and implement this invention, the technical solutions in the embodiments of this invention will be clearly and completely described below with reference to the accompanying drawings.

[0044] In the description of this invention, it should be noted that the terms "upper", "lower", "front", "rear", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are only for the convenience of describing this invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on this invention.

[0045] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. The terminology used herein in the description of the invention is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention.

[0046] Example

[0047] Please refer to Figs. 1 to 4 A laser-ultrasonic hybrid welding device includes: a frame 1; an ultrasonic output unit for outputting ultrasonic waves to the product to be welded; a laser output unit for outputting a laser beam to the product to be welded; a support assembly including a first support unit 4 with a first adjusting block 5 on its top, a second support unit 6 with a second adjusting block 7 on its top, and a clamp 8 for clamping the ultrasonic output unit, wherein the first support unit 4, the clamp 8, and the second support unit 6 are all located on the top of the frame 1, and the first support unit 4 and the second support unit 6 are respectively located on the left and right sides of the clamp 8; a positioning assembly including a pad 9 with a slot 12 in the middle, a first pressure plate 10, and a second pressure plate 11; the pad 9 is horizontally and detachably mounted on the first adjusting block 5 and the second adjusting block 7, and the first pressure plate 10 and the second pressure plate 11 are detachably mounted on the top of the pad 9, and the first pressure plate 10 and the second pressure plate 11 are arranged side by side on the front and rear sides of the slot 12; the output ends of the ultrasonic output unit and the laser output unit are respectively located on the upper and lower sides of the positioning assembly.

[0048] Optionally, it also includes a lifting frame 17, and the laser output unit includes a laser generator 13 and a mirror assembly unit 14; the mirror assembly unit 14 is disposed at the lifting end of the lifting frame 17 to adjust the vertical height of the mirror assembly unit 14.

[0049] Optionally, a clamping drive 15 is provided at the bottom of the clamp 8. The clamping drive 15 can be a finger cylinder or the like, which can cause the clamp 8 to hold the ultrasonic tool head 2. The lifting unit 16 can be a telescopic mechanism such as a cylinder or an electric cylinder.

[0050] Optionally, a lifting unit 16 is also provided at the bottom of the clamp 8. The lifting unit 16 causes the clamp drive 15 or the clamp 8 to move up and down to adjust the vertical position of the clamp 8.

[0051] Optionally, the ultrasonic output unit includes an ultrasonic tool head 2; the top of the ultrasonic tool head 2 is provided with an output port 18, which is close to the empty slot 12 and communicates with the empty slot 12.

[0052] Optionally, it also includes an ultrasonic generator 3 and a laser generator 13, the ultrasonic generator 3 being connected to the ultrasonic tool head 2 and the laser generator 13 being connected to the lens assembly unit 14.

[0053] Optionally, the ultrasonic tool head 2 is made of aluminum, with a flat top surface and an output port 18 located in the center of the flat surface.

[0054] Optionally, the products to be welded include a first product to be welded and a second product to be welded, wherein the first product to be welded is a copper product to be welded and the second product to be welded is an aluminum product to be welded.

[0055] Optionally, the laser generator 13 is a single-mode ring-spot laser;

[0056] The mirror assembly unit 14 includes an internal optical path transmission unit and a galvanometer unit.

[0057] Optionally, the laser generator 13 can emit an inner ring laser beam and an outer ring laser beam;

[0058] The inner ring laser power range of laser generator 13 is 1500-2000W;

[0059] The outer ring laser power range of laser generator 13 is 1000-3500W.

[0060] A method for applying a laser-ultrasonic hybrid welding device includes the following steps:

[0061] Step S10: Align the top surfaces of the first adjusting block and the second adjusting block 7 with the top surface of the ultrasonic tool head 2, and make the ultrasonic tool head 2 contact the pad 9.

[0062] Step S20: Place the first product to be welded and the second product to be welded on the pad 9, fix the first product to be welded and the second product to be welded by the first pressure plate 10 and the second pressure plate 11, and confirm whether the first product to be welded and the second product to be welded are fixed tightly.

[0063] Step S30: Control the laser generator 13 to output a laser beam, which is transmitted to the mirror assembly unit 14 through the internal optical path transmission unit. Control the lifting end of the lifting frame 17 to move up and down to adjust the vertical position of the mirror assembly unit 14 in order to determine the focal position.

[0064] Step S40: Adjust the parameters of laser generator 13 to confirm whether the laser output unit is emitting light normally; adjust the parameters of ultrasonic generator 3 to confirm whether the ultrasonic output unit is emitting ultrasonic waves normally.

[0065] Step S50: Control the ultrasonic output unit to emit ultrasonic waves at the welding point between the first product to be welded and the second product to be welded;

[0066] Step S60: After emitting ultrasonic waves, control the laser output unit to emit light to achieve laser-ultrasonic composite welding.

[0067] Optionally, the laser-ultrasonic hybrid welding method is lap welding, where a portion of the first product to be welded is located above or below a portion of the second product to be welded. The first product to be welded is a copper structure, and the second product to be welded is an aluminum structure. Lap welding is mainly divided into two methods: copper structure on top and aluminum structure on the bottom, and copper structure on the bottom and aluminum structure on top. Different thicknesses of copper or aluminum structures can be selected, such as lap welding of a 1mm thick copper structure and a 1.5mm thick aluminum structure, or a 1mm thick copper structure and a 3mm thick aluminum structure, etc. Laser-ultrasonic hybrid welding can be performed using copper and / or aluminum structures of different thicknesses. The first pressure plate 10 can press the first product to be welded, and the second pressure plate 11 can press the second product to be welded.

[0068] Working principle of the invention:

[0069] In this embodiment, the laser generator 13 is a single-mode ring laser with an inner ring power of 2000W and an inner ring core diameter of 20μm, and an outer ring power of 4000W and an outer ring core diameter of 100μm. The laser generator 13 enters the mirror assembly unit 14 through the internal optical path transmission unit. The galvanometer unit of the mirror assembly unit 14 oscillates at different speeds, causing the laser to work on copper and aluminum plates (the first product to be welded and the second product to be welded can be copper plates and aluminum plates, respectively) and form a weld. The emitted laser beam oscillates at different speeds through the galvanometer unit, forming a spiral on the copper and aluminum plates. During the welding process, an air blowing device is installed on one side to blow away the welding dust (not shown in the attached figures in this embodiment). The welding speed can vary depending on the thickness combination, with a preferred welding speed range of 50-300 mm / s. The internal optical path transmission unit may include a laser reflector, a laser beam expander, and an optical focusing lens. The laser beam emitted by the laser generator 13 is transmitted through the laser reflector into the laser beam expander. The expanded laser beam is then transmitted through the galvanometer unit to the optical focusing lens. The optical focusing lens focuses the laser beam onto the copper and aluminum plates being welded together, forming a spiral weld. The laser spot diameter of the laser generator 13 can be selected between 20μm and 100μm. The ultrasonic generator 3 may be a 20kHz contact ultrasonic auxiliary device, whose control box can control the amplitude between 0% and 100%, and the duration between 0 and 100 seconds. The height adjustment range of the first adjusting block 5 and the second adjusting block 7 is 0-1.5mm. The preferred range for the ultrasonic generator 3 to emit ultrasonic waves is 1-2.5 seconds.

[0070] In this embodiment, the ultrasonic generator 3 adjusts the amplitude according to the actual requirements of the thickness of the experimental material. The energy output is proportional to the amplitude. The ultrasonic generator 3 emits ultrasonic waves for a duration determined by the welding time. The ultrasonic generator 3 emits ultrasonic waves for a duration longer than the actual welding time. The ultrasonic generator 3 can control the ultrasonic tool head 2 using a signal line. The ultrasonic generator 3 uses an external switch to control and transmit ultrasonic waves, and then uses a signal line to control the vibration of the ultrasonic tool head 2.

[0071] Specifically, the ultrasonic tool head 2 is installed on the top of the frame 1. The top surfaces of the first adjusting block and the second adjusting block 7 are flush with the top surface of the ultrasonic tool head 2. The ultrasonic tool head 2 contacts the pad 9. The first and second products to be welded are fixed by screws or other fasteners in conjunction with the first pressure plate 10 and the second pressure plate 11. The welding point of the first and second products to be welded is located directly above the empty groove 12. After the first and second products to be welded are installed, the laser generator 13 is adjusted. The galvanometer unit is moved up and down by adjusting the lifting frame 17 to determine the focal point position. The parameters of the laser generator 13 and the galvanometer unit are set so that a laser beam with a certain energy forms a spiral on the copper-aluminum laminated material through the galvanometer unit oscillating at different speeds. After the ultrasonic generator 3 emits ultrasonic sound and vibration, the laser generator 13 is immediately controlled to emit light. At this time, the laser-ultrasonic composite welding process is completed. The ultrasonic generator 3 can output ultrasonic waves through the output port 18 of the ultrasonic tool head 2. The ultrasonic waves can pass through the slot 12 to reach the welding point of the first and second products to be welded. The slot 12 on the pad 9 can prevent the laser beam output by the laser generator 13 from being too powerful, so that the material will not stick to the welding laser head after welding.

[0072] Extensive experimental data analysis revealed that, in dissimilar welding of copper and aluminum structures, regardless of whether the welding method is copper on top and aluminum on the bottom (copper structure abbreviated as copper, aluminum structure abbreviated as aluminum) or copper on the bottom and aluminum on top, and with different combinations of copper and aluminum thicknesses, after laser-ultrasonic hybrid welding, the metallographic structure of the weld showed that the copper-aluminum lap weld without ultrasonic assistance had a thicker brittle intermetallic compound (IMC) layer. After ultrasonic assistance (processing), the IMC layer was thinned within the amplitude range of 0%-100%. Combined with the peel force and shear force data from the bonding strength test, the joint was less prone to deformation and brittle fracture. The results indicate that the weld bonding strength is significantly improved compared to the weld without ultrasonic processing. Furthermore, the weld penetration depth is increased by 20%-30% after ultrasonic processing, and the appearance is more aesthetically pleasing. With the same welding power and welding speed, a greater weld penetration depth can be obtained, greatly improving welding efficiency. This is because the temperature and composition distribution of the molten pool are more uniform under the action of the acoustic flow effect.

[0073] Regarding weld cracks and porosity, there are many porosities and cracks without ultrasonic processing. After laser-ultrasonic hybrid welding, the porosity is significantly reduced compared to welding without ultrasonic assistance. Laser-ultrasonic hybrid welding significantly reduces porosity and cracks compared to laser welding alone. This is because when ultrasound propagates in the molten pool, the pressure at a certain particle changes periodically with time, alternating between positive and negative values. When the pressure is lower than a critical value (cavitation threshold) at a certain moment, it tears the liquid at that point, generating cavitation bubbles. As the ultrasonic frequency increases, the cavitation threshold becomes increasingly difficult to reach, requiring an increase in ultrasonic power. Subsequently, due to the periodic pressure changes, the cavitation bubbles undergo a process of vibration, growth, and collapse. During the collapse of the cavitation bubbles, due to the extremely high speed, micro-shock waves and micro-jet effects are generated. The energy from these effects in the molten pool can break up the growing dendrites. The broken dendrites can then provide new nucleation sites, which plays a positive role in improving defects such as coarse dendrites in dissimilar copper-aluminum welding. Furthermore, during laser-ultrasonic hybrid welding, due to the vibration of the base material, intense mechanical vibration will occur at the solid-liquid interface between the base material and the molten pool, thereby breaking the columnar dendrites that grow through the interface and refining the grains, thus improving the quality of the weld.

[0074] The technical means disclosed in this invention are not limited to those disclosed in the above embodiments, but also include technical solutions composed of any combination of the above technical features. It should be noted that those skilled in the art can make various improvements and modifications without departing from the principles of this invention, and these improvements and modifications are also considered within the scope of protection of this invention.

Claims

1. A laser-ultrasonic hybrid welding device, characterized in that, include: frame; Ultrasonic output unit, used to output ultrasonic waves to the product to be welded; The laser output unit is used to output a laser beam to the product to be welded. The support assembly includes a first support unit with a first adjustment block on its top, a second support unit with a second adjustment block on its top, and a clamp for holding the ultrasonic output unit. The first support unit, the clamp, and the second support unit are all located on the top of the frame, and the first support unit and the second support unit are respectively located on the left and right sides of the clamp. The positioning assembly includes a pad with a slot in the middle, a first pressure plate, and a second pressure plate; the pad is horizontally and detachably mounted on the first and second adjusting blocks, the first and second pressure plates are detachably mounted on the top of the pad, and the first and second pressure plates are arranged side by side on the front and rear sides of the slot; The output ends of the ultrasonic output unit and the laser output unit are respectively located on the upper and lower sides of the positioning component.

2. The laser-ultrasonic composite welding device according to claim 1, characterized in that, It also includes a lifting frame, and the laser output unit includes a laser generator and a mirror assembly unit; the mirror assembly unit is disposed at the lifting end of the lifting frame to adjust the vertical height of the mirror assembly unit.

3. The laser-ultrasonic composite welding device according to claim 1, characterized in that, The bottom of the clamp is provided with a clamp drive component.

4. The laser-ultrasonic hybrid welding device according to claim 1 or 3, characterized in that, The bottom of the clamp is also provided with a lifting unit, which causes the clamp drive or the clamp to move up and down to adjust the vertical position of the clamp.

5. The laser-ultrasonic hybrid welding device according to claim 2, characterized in that, The ultrasonic output unit includes an ultrasonic tool head; the top of the ultrasonic tool head has an output port, which is close to the empty slot and communicates with the empty slot.

6. The laser-ultrasonic hybrid welding device according to claim 5, characterized in that, It also includes an ultrasonic generator and a laser generator, wherein the ultrasonic generator is connected to the ultrasonic tool head and the laser generator is connected to the mirror assembly unit.

7. The laser-ultrasonic hybrid welding device according to claim 5, characterized in that, The ultrasonic tool head is made of aluminum, with a flat top surface and the output port located in the center of the flat surface.

8. The laser-ultrasonic hybrid welding device according to claim 5, characterized in that, The products to be welded include a first product to be welded and a second product to be welded. The first product to be welded is made of copper, and the second product to be welded is made of aluminum.

9. The laser-ultrasonic hybrid welding device according to claim 5, characterized in that, The laser generator is a single-mode ring spot laser; The mirror assembly unit includes an internal optical path transmission unit and a galvanometer unit.

10. The laser-ultrasonic hybrid welding device according to claim 5, characterized in that, The laser generator can emit an inner ring laser beam and an outer ring laser beam; The inner ring laser power range of the laser generator is 1500-2000W; The outer ring laser power range of the laser generator is 1000-3500W.

11. An application method of a laser-ultrasonic hybrid welding device, characterized in that, Includes the following steps: step S10: Make the top surfaces of the first adjusting block and the second adjusting block flush with the top surface of the ultrasonic tool head, and make the ultrasonic tool head contact the pad. Step S20: Place the first product to be welded and the second product to be welded on the pad, fix the first product to be welded and the second product to be welded by the first pressure plate and the second pressure plate, and confirm whether the first product to be welded and the second product to be welded are fixed tightly; Step S30: Control the laser generator to output a laser beam, which is transmitted to the mirror assembly unit through the internal optical path transmission unit. Control the lifting end of the lifting frame to move up and down to adjust the vertical position of the mirror assembly unit in order to determine the focal position. Step S40: Adjust the laser generator parameters to confirm whether the laser output unit is emitting light normally; adjust the ultrasonic generator parameters to confirm whether the ultrasonic output unit is emitting ultrasonic waves normally. Step S50: Control the ultrasonic output unit to emit ultrasonic waves at the welding point between the first product to be welded and the second product to be welded; Step S60: After emitting ultrasonic waves, control the laser output unit to emit light to achieve laser-ultrasonic composite welding.

12. The application method of the laser-ultrasonic hybrid welding device according to claim 11, characterized in that, The laser-ultrasonic composite welding method is lap welding, in which part of the first product to be welded is located above part of the second product to be welded or part of the first product to be welded is located below part of the second product to be welded.

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