Metal sheet resistance spot welding device and welding process thereof

Abstract

The invention relates to the technical field of metal welding, and discloses a sheet metal resistance spot welding device and a welding process thereof.The welding process comprises the following steps that S1, matched containing grooves are formed in the welding face of a first sheet metal and the welding face of a second sheet metal; s2, the first metal sheet containing groove is filled with intermediate powder for the first time, and the powder is completely molten through the laser melting technology to form a metal film; s3, secondary powder filling is carried out in the containing groove where the metal thin film is formed in the step S2, the surface of the metal thin film is covered, and a thickened powder layer is formed; s4, gradient laser treatment is carried out on the secondary powder filling layer, and laser parameters are controlled to enable surface layer powder to form a semi-molten adhesion layer; and S5, the containing groove of the second metal sheet and the first metal sheet are assembled in an aligned mode, and the semi-molten adhesion layer and the second metal sheet are welded through resistance spot welding. According to the method, the combination effect of the two dissimilar metal sheets is improved by improving the nugget number and fluidity of the intermediate through the combination of secondary intermediate filling and the partitioned laser melting technology.

Description

Technical Field

[0001] The present invention relates to the technical field of metal welding, and particularly relates to a resistance spot welding device for thin metal plates and its welding process. Background Art

[0002] When spot welding thin metal plates with significantly different physical and chemical properties, due to the huge differences in thermal conductivity and melting point between the thin plates, intense metallurgical reactions are likely to occur at the interface during welding, and intermetallic compounds are formed at the interface. For example, when directly spot welding aluminum and titanium, brittle IMCs layers such as TiAl3 and Ti3Al will be formed. These compounds have high hardness (above 1000 HV) and extremely low plasticity, and will become the preferred paths for crack initiation and propagation, resulting in low tensile shear load of the joint and poor reliability.

[0003] Common methods to solve such problems include the intermediate layer transition method, the current control method, and the multi-pulse welding method.

[0004] Among them, the current control method can precisely control the heat distribution required for the formation of the fusion core by real-time monitoring and dynamically adjusting the welding current, effectively avoiding burn-through or collapse of the low-melting-point metal due to overheating, and ensuring full fusion of the high-melting-point metal at the same time. However, for dissimilar metal combinations, a large number of tests are required to determine the optimal current curve, and the parameter optimization is difficult. The multi-pulse welding method can precisely adjust the heat distribution of metals with different melting points by releasing pulse energy in stages. However, during welding, the formation of intermetallic compounds cannot be completely avoided, resulting in reduced reliability. The intermediate layer transition method uses a transitional material to form a transitional liquid phase layer at the interface of dissimilar metals, which can effectively block the direct contact of metals with large differences in crystal structure and significantly reduce the formation thickness of brittle intermetallic compounds. However, the bonding strength between the intermediate layer and the base material is often lower than the strength of the base material itself.

[0005] Therefore, a new solution is needed to make up for the deficiencies in each process. Summary of the Invention

[0006] The present invention overcomes the deficiencies of the prior art and provides a resistance spot welding device for thin metal plates and its welding process.

[0007] To achieve the above object, on the one hand, the present invention provides a technical solution: a resistance spot welding process for thin metal plates, including the following steps:

[0008] S1. Pretreat the welding surfaces of the first thin metal plate and the second thin metal plate with a melting point difference > 400 °C, and open matching accommodation grooves on the two welding surfaces.

[0009] S2. First fill the accommodation groove of the first thin metal plate with intermediate body powder, and use laser melting technology to completely melt the powder to form a metal film.

[0010] S3. Perform secondary powder filling in the groove where the metal thin film has been formed in step S2, covering the surface of the metal thin film and forming a thickened powder layer;

[0011] S4. Perform gradient laser treatment on the secondary powder filling layer, controlling the laser parameters to make the surface powder form a semi-molten adhesive layer;

[0012] S5. Align and assemble the groove of the second metal thin plate with the first metal thin plate, and weld the semi-molten adhesive layer to the second metal thin plate by resistance spot welding.

[0013] In a preferred embodiment of the present invention, when the first metal thin plate and the second metal thin plate are aluminum alloy and stainless steel, the intermediate powder, by mass percentage, has the following components: Al 86 - 92%, Si 6 - 10%, Mg 1 - 3%, Ce 0.1 - 0.5%;

[0014] When the first metal thin plate and the second metal thin plate are copper alloy and carbon steel, the intermediate powder, by mass percentage, has the following components: Cu - Sn - Ti composite powder. The components of the first powder filling are Cu 85 - 90%, Sn 10 - 15%, and the components of the second powder filling are Cu 95 - 97%, Ti 3 - 5%;

[0015] When the first metal thin plate and the second metal thin plate are magnesium alloy and titanium alloy, the intermediate powder, by mass percentage, has the following components: Zn 70 - 85%, Al 12 - 25%, Ga 0.5 - 3%;

[0016] In a preferred embodiment of the present invention, the particle size of the intermediate powder is 45 - 75 μm.

[0017] In a preferred embodiment of the present invention, the gradient laser treatment in step S4 specifically includes the following steps:

[0018] S41. Apply a laser power of 1.2 - 1.8 kW with a pulse width of 5 - 10 ms to the central area of the intermediate powder layer using an annular light spot to form a weld nugget with a diameter of 0.08 - 0.12 mm;

[0019] S42. Apply a laser power of 0.6 - 1.0 kW with a pulse width of 10 - 20 ms to the periphery of the weld nugget to form a semi-molten adhesive layer.

[0020] In a preferred embodiment of the present invention, in steps S2 and S3, the thickness of the first powder filling layer is: 0.1 - 0.3 mm, and the thickness of the second powder filling layer is: 0.2 - 0.5 mm.

[0021] In a preferred embodiment of the present invention, in steps S2 and S3, ultrasonic vibration is used to assist the dispersion of the intermediate powder during powder filling, and the ultrasonic frequency is controlled to be 20 - 40 kHz and the amplitude is 5 - 10 μm.

[0022] In a preferred embodiment of the present invention, the setting range of the resistance spot welding process parameters in step S5 is as follows:

[0023] Welding current:

[0024] When it is a combination of aluminum alloy and stainless steel: control the welding current at 6 - 8 kA;

[0025] When it is a combination of copper alloy and carbon steel: control the welding current at 10 - 12 kA;

[0026] When it is a combination of magnesium alloy and titanium alloy: control the welding current at 8 - 10 kA;

[0027] Electrode pressure: 2 - 6 kN, controlled in matching with the welding current;

[0028] Welding time: 100 - 300 ms.

[0029] On the other hand, the present application provides a resistance spot welding device for metal sheets, including a frame base, and the following are integrally arranged on the frame base:

[0030] A plate body support mechanism, including a support, a transfer platform arranged on the support, and positioning pins detachably installed on both sides of the transfer platform;

[0031] A powder filling mechanism, including a double powder feeder arranged vertically, a transverse sliding member for the feeder to move along the length direction of the transfer platform, and a position adjusting screw rod;

[0032] An ultrasonic auxiliary device, including a piezoelectric ceramic ultrasonic probe fixedly installed on the side wall of the nozzle of the powder feeder;

[0033] A laser melting device, including a longitudinal support column fixed on the top of the frame base, a transverse cantilever beam hinged to the longitudinal support column, and a pulsed laser head installed at the end of the cantilever beam;

[0034] A resistance spot welding mechanism, including adjustable pressure type electrode heads symmetrically arranged above and below the transfer platform.

[0035] In a preferred embodiment of the present invention, a hydraulic telescopic component is arranged inside the cantilever beam, and the pulsed laser head is fixedly connected to the end of the hydraulic telescopic component for adjusting the position of the pulsed laser head on the frame base.

[0036] In a preferred embodiment of the present invention, the electrode head realizes vertical movement control through a hydraulic drive unit.

[0037] The present invention solves the defects existing in the background technology, and the present invention has the following beneficial effects:

[0038] (1) The present invention provides a resistance spot welding process for metal sheets, combined with a secondary intermediate powder filling process, using a zone laser melting technique to solve the problem of local thickness reduction caused by the solidification shrinkage of the molten pool in the first filling of the intermediate, while reducing the generation of pores in the intermediate. It not only solves the formation of metal compounds during the welding of dissimilar metals but also improves the bonding strength of the welded metal sheets. Compared with the traditional single intermediate layer transition method, this application uses a secondary filling of the intermediate layer combined with a zone laser melting process to form multiple fusion nuclei between the first metal sheet and the intermediate, between the intermediate and the second metal sheet, and on both sides of the intermediate near the two interfaces, improving the fluidity of the intermediate fusion nuclei, thereby improving the bonding effect between the two metal sheets and the intermediate. In addition, the formation of multiple fusion nuclei can make the stress on the welded part more uniform, avoid local stress concentration, reduce the generation of welding defects such as cracks and pores, and thus improve the tensile strength and shear strength between the first metal sheet and the second metal sheet.

[0039] (2) This application provides different types of intermediate powder solutions for the combination of various dissimilar metals, effectively suppressing the formation of brittle intermetallic compounds. For the combination of aluminum alloy and stainless steel: Al-Si-Mg-Ce intermediate is used. Si preferentially combines with Fe to form a Fe-Si-Al ternary phase, avoiding the formation of the Fe-Al binary brittle phase; the Ce element inhibits the generation of the oxide film by forming CeO2, enhancing the wettability of the molten pool. For the combination of copper alloy and carbon steel: The first filling of Cu-Sn alloy reduces the melting point and enhances the wettability. The second filling of Cu-Ti composite powder generates a Fe-Ti solid solution through the reaction of Ti with Fe, blocking the direct contact between Fe and Cu and reducing the low-melting-point Cu-Fe brittle phase. For the combination of magnesium alloy and titanium alloy: In the Zn-Al-Ga intermediate, Ga reacts with MgO / TiO2 to form Ga2O3, purifying the interface and improving the wettability. At the same time, Zn / Al diffuse into the magnesium and titanium matrices respectively to form a gradient solid solution, avoiding the formation of Mg-Ti intermetallic compounds. Compared with the traditional single intermediate layer, the gradient composition design of the present invention enables the interface to form a continuous solid solution transition, improving the shear strength of the joint.

[0040] (3) This application also provides a resistance spot welding device for metal sheets, which realizes high-precision welding through multi-process collaborative control, realizes the automation of the welding process, ensures the sufficient heating of the intermediate powder during the welding process through an adjustable position pulse laser head, and uses ultrasonic waves to assist in the dispersion of the intermediate powder to achieve spreading during the filling of the intermediate powder. Compared with the prior art, high-precision welding is achieved through multi-process collaborative control, significantly reducing the defect rate. Description of the Drawings

[0041] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for the description of the embodiments or the prior art. Obviously, the drawings in the following description are only some embodiments recorded in the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings;

[0042] Figure 1 is the process flow chart of the spot welding process of the present invention;

[0043] Figure 2 is the structural schematic of the spot welding device of the present invention Figure 1 ;

[0044] Figure 3 is the structural schematic of the spot welding device of the present invention Figure 2 ;

[0045] In the figure: 1, frame base; 2, plate support mechanism; 21, support member; 22, transfer platform; 23, positioning pin; 3, powder filling mechanism; 31, powder feeder; 32, transverse sliding member; 33, position adjustment screw rod; 4, ultrasonic auxiliary device; 41, piezoelectric ceramic ultrasonic probe; 5, laser melting device; 51, longitudinal support column; 52, cantilever beam; 53, pulsed laser head; 6, resistance spot welding mechanism; 61, electrode head. Detailed implementation manners

[0046] The following will clearly and completely describe the technical solutions in the embodiments of the present invention with reference to the drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts belong to the protection scope of the present invention.

[0047] Many specific details are set forth in the following description in order to fully understand the present invention, but the present invention can also be implemented in other ways different from those described herein. Therefore, the protection scope of the present invention is not limited by the specific embodiments disclosed below.

[0048] In the description of this application, it should be understood that the orientation or positional relationships indicated by the terms "center", "longitudinal", "transverse", "upper", "lower", "front", "rear", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", etc. are based on the orientation or positional relationships shown in the drawings. These are only for the convenience of describing this application and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation. Therefore, it should not be construed as a limitation on the protection scope of this application. In addition, the terms "first", "second", etc. are only used for descriptive purposes and cannot be construed as indicating or implying relative importance or implicitly specifying the quantity of the indicated technical features. Therefore, the features defined with "first", "second", etc. may explicitly or implicitly include one or more of such features. In the description of this invention, unless otherwise stated, the meaning of "a plurality" is two or more.

[0049] In the description of this application, it should be noted that unless otherwise clearly specified and limited, the terms "install", "connect", and "couple" should be understood in a broad sense. For example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be directly connected or indirectly connected through an intermediate medium, and it can be the communication inside two elements. For those of ordinary skill in the art, the specific meanings of the above terms in this application can be understood through specific circumstances.

[0050] Exemplary process:

[0051] As Figure 1 shown, an exemplary resistance spot welding process for metal sheets includes the following steps:

[0052] S1. Clean the surfaces of the welding surfaces of the two metal sheets, and machine matching accommodation grooves with a depth of 0.5 - 1.0 mm on the two welding surfaces. The cross-section of the accommodation groove can be arc-shaped or square for accommodating the intermediate body powder.

[0053] S2. First fill the accommodation groove of the first metal sheet with intermediate body powder having a thickness of 0.1 - 0.2 mm, and use a pulsed laser with a wavelength of 1064 nm and a frequency of 50 - 100 Hz to completely melt the powder to form a dense metal film.

[0054] S3. Secondarily fill the surface of the melted film with intermediate body powder having a thickness of 0.3 - 0.5 mm to form a thickened layer.

[0055] S4. Apply a laser with a power of 1.2 - 1.8 kW to the central area through an annular light spot to form a fusion core, and apply a laser with a power of 0.6 - 1.0 kW to the periphery to form a semi-molten adhesion layer.

[0056] S5. Align and assemble the two metal plates, apply a pressure of 2 - 6 kN through the upper and lower electrodes, select a welding current of 6 - 12 kA and a power-on time of 100 - 300 ms according to the metal combination, and bond the semi-molten adhesive layer with the second metal sheet.

[0057] Based on the above process steps, in step S2, the intermediate powder is first filled onto the metal sheet, and the powder is melted using pulsed laser. Based on the wettability of the melted intermediate powder, a continuous and dense metal film is formed by spreading on the surface of the metal sheet, effectively blocking the formation of intermetallic compounds between dissimilar metals.

[0058] Then, secondary powder filling is carried out. On the one hand, the secondary powder filling provides additional material to ensure sufficient volume of the intermediate in the welding area, avoiding the problem that the dense film formed by laser melting becomes locally thinner due to the solidification shrinkage or volatilization of the molten pool after the first powder filling, compensating for the volume of the intermediate and enhancing the interfacial strength, and reducing the problem of porosity generation in the intermediate by staged melting.

[0059] On the other hand, the flow of the weld nugget during resistance spot welding of metals is jointly driven by electromagnetic force (Lorentz force) and temperature gradient. The better the fluidity of the weld nugget, the more reliable the bonding of the two metals. In this application, using the semi-molten adhesive layer formed after secondary powder filling, through laser heating, a temperature gradient difference is formed between the peripheral semi-molten layer and the central weld nugget, strengthening the clockwise flow of the weld nugget inside the semi-molten adhesive layer, promoting the concentration of heat towards the contact interface with the metal sheet, forming a more uniform weld nugget, significantly promoting the flow of the weld nugget, and optimizing the bonding effect between the two metal sheets and the intermediate. And the secondarily filled intermediate with a certain thickness can form multiple weld nuggets between the first metal sheet and the intermediate, between the intermediate and the second metal sheet, and on both sides of the intermediate near the two interfaces. Applying more intermediate powder during the second coating helps to reach a sufficient melting temperature at the contact surface between the intermediate and the second metal sheet during welding, thus forming weld nuggets. The formation of multiple weld nuggets can make the stress on the welded part more uniform, avoid local stress concentration, reduce the generation of welding defects such as cracks and pores, and thereby improve the tensile strength and shear strength between the first metal sheet and the second metal sheet.

[0060] Specifically, in step S1, matching accommodation grooves are opened on the metal sheet. Specifically, traditional water jet cutting method, stamping forming method or laser cutting method can be adopted, which is not limited here.

[0061] Preferably, when the first metal sheet and the second metal sheet are aluminum alloy and stainless steel, the components of the intermediate powder by mass percentage are: Al 86 - 92%, Si 6 - 10%, Mg 1 - 3%, Ce 0.1 - 0.5%.

[0062] During the resistance spot welding of aluminum alloy and stainless steel, on the one hand, since the affinity of Si with Fe in aluminum alloy and stainless steel is higher than that of Al, the Fe-Si-Al ternary phase will be preferentially formed, reducing the formation of the brittle Fe-Al binary phase; and introducing rare earth element Ce, Ce will combine with oxygen to form stable oxides (such as CeO2), reducing the oxidation of the molten pool and lowering the hindrance of the formed Al2O3 film to wettability. On the other hand, the Al-Si-Mg system will form a eutectic with a low melting point (the eutectic point of Al-Si is 577°C) at 580 - 630°C, reducing the melting temperature and enhancing the fluidity of the liquid metal, thus making the molten metal concentrate towards the interface.

[0063] In summary, using the above intermediate as the intermediate for the resistance spot welding of aluminum alloy and stainless steel, after the first powder filling, the intermediate powder is completely melted by laser, forming a dense Al-Si-Mg-Ce film to block the direct contact between Fe and Al. Then, the semi-molten adhesive layer formed after the second powder filling realizes the viscosity gradient control through the low melting point characteristic of Al-Si, forming a low-viscosity fusion core in the central area to promote flow and a high-viscosity semi-molten layer in the periphery to inhibit spatter.

[0064] Preferably, when the first metal sheet and the second metal sheet are copper alloy and carbon steel, the intermediate powder, by mass percentage, consists of: Cu-Sn-Ti gradient composite powder. The first powder filling is 85 - 90% Cu and 10 - 15% Sn, and the second powder filling is 95 - 97% Cu and 3 - 5% Ti;

[0065] During the resistance spot welding of copper alloy and carbon steel, during the first powder filling, Sn reduces the melting point of the Cu alloy (the eutectic temperature of Cu-Sn is 227°C), enhancing the wettability of the molten metal to the surface of the carbon steel and ensuring the uniform thickness of the dense metal film. After the second powder filling, Ti reacts with Fe in the steel to form Fe-Ti solid solutions (such as FeTi, Fe2Ti), further blocking the direct contact between Fe and Cu.

[0066] Preferably, when the first metal sheet and the second metal sheet are magnesium alloy and titanium alloy, the intermediate powder, by mass percentage, consists of: 70 - 85% Zn, 12 - 25% Al, 0.5 - 3% Ga;

[0067] During the resistance spot welding of magnesium alloy and titanium alloy, in the above intermediate powder, by using the reducibility of Ga, Ga reacts with the oxides (such as MgO, TiO2) formed on the magnesium / titanium surface to generate low-melting-point Ga2O3 to purify the interface and ensure the wettability of the intermediate. Subsequently, when the first metal sheet and the second metal sheet are welded, Zn diffuses into the magnesium matrix to form a Mg-Zn solid solution, and Al diffuses into the titanium matrix to form an Al-Ti solid solution, achieving gradient bonding and improving the welding reliability.

[0068] Further, control the particle size of the intermediate powder to be 45 - 75 μm to ensure uniform heating of the intermediate powder by laser melting and facilitate the dispersion of the intermediate powder on the metal sheet.

[0069] Preferably, in steps S2 and S3, ultrasonic vibration is used to assist the dispersion of the intermediate powder during powder filling. Control the ultrasonic frequency to be 20 - 40 kHz and the amplitude to be 5 - 10 μm. Through ultrasonic waves, further improve the uniformity of the intermediate powder, thereby improving the uniformity of the dense metal film and the secondary semi - molten adhesive layer formed during the first powder filling and the second powder filling.

[0070] Preferably, the setting range of the resistance spot welding process parameters in step S5 is as follows:

[0071] Welding current:

[0072] When it is a combination of aluminum alloy and stainless steel: control the welding current to be 6 - 8 kA;

[0073] When it is a combination of copper alloy and carbon steel: control the welding current to be 10 - 12 kA;

[0074] When it is a combination of magnesium alloy and titanium alloy: control the welding current to be 8 - 10 kA;

[0075] Electrode pressure: 2 - 6 kN, controlled in matching with the welding current;

[0076] Welding time: 100 - 300 ms.

[0077] Exemplary device:

[0078] As Figure 2 shown, a metal sheet resistance spot welding device includes a frame base 1, and the frame base 1 is integrally provided with:

[0079] A plate support mechanism 2, including a support member 21, a transfer platform 22 provided on the support member 21, and positioning pins 23 detachably installed on both sides of the transfer platform 22 by bolts;

[0080] A powder filling mechanism 3, including a double powder feeder 31 vertically arranged, a transverse sliding member 32 for the feeder to move along the length direction of the transfer platform 22, and a position - adjusting screw rod 33;

[0081] An ultrasonic assistance device 4, including a piezoelectric ceramic ultrasonic probe 41 fixedly installed on the side wall of the nozzle of the powder feeder 31;

[0082] A laser melting device 5, including a longitudinal support column 51 fixed on the top of the frame base 1, a transverse cantilever beam 52 hinged to the longitudinal support column 51, and a pulsed laser head 53 installed at the end of the cantilever beam 52;

[0083] The resistance spot welding mechanism 6 includes adjustable pressure electrode heads 61 symmetrically arranged above and below the transfer platform 22.

[0084] Furthermore, a hydraulic telescopic component is arranged inside the cantilever beam 52, and the pulsed laser head 53 is fixedly connected to the end of the hydraulic telescopic component. The position of the pulsed laser head 53 on the frame base 1 can be adjusted through the hydraulic telescopic component, so that the pulsed laser head 53 is arranged on the metal thin plate located on the transfer platform 22.

[0085] Furthermore, the electrode head 61 realizes vertical movement control through a hydraulic drive unit, so as to adjust the contact position between the electrode heads 61 on the upper and lower sides of the metal thin plate and the metal thin plate.

[0086] This equipment realizes high-precision resistance spot welding of dissimilar metal thin plates through the coordinated control of multiple mechanisms, and its core principle is as follows:

[0087] In the plate support mechanism 2, the positioning pins 23 ensure the alignment and assembly of the two metal thin plates, and the positions of the receiving grooves are accurately matched; the transfer platform 22 is driven by a servo motor to automatically convey the thin plate to the powder filling, melting, and welding stations.

[0088] The double powder feeder 31 can adjust the position of the nozzle of the powder feeder 31 through the horizontal sliding member 32 and the position adjusting threaded rod 33 to adapt to the first powder filling and the second powder filling of the intermediate powder of the thin plate at different positions. Combined with the piezoelectric ceramic ultrasonic probe 41 arranged on the side wall of the nozzle of the powder feeder 31, the uniform feeding of the intermediate powder is realized;

[0089] In the laser melting device 5, the horizontal position of the pulsed laser head 53 is adjusted by means of a hydraulic drive unit, and the longitudinal height of the pulsed laser head 53 is adjusted through the longitudinal support column 51;

[0090] The electrode head 61 can not only adjust the height position of the electrode head 61 through the hydraulic drive unit, but also apply different pressures to the metal thin plate to ensure that the semi-molten adhesive layer of the first metal thin plate is in close contact with the second metal thin plate.

[0091] Example 1

[0092] This example uses the welding process of this application as Figure 1 shown to weld two aluminum alloy plates (grade: 3A21 (LF21)) and stainless steel plates (grade: 1Cr18Ni9Ti (321)) with a length of 300 mm, a width of 200 mm, and a thickness of 5 mm respectively. The specific implementation steps are as follows:

[0093] S1. Clean the welding surfaces of the two metal thin plates, and use the mechanical cutting method to open a square cross-section receiving groove with a length of 200 mm, a width of 5 mm, and a depth of 0.5 mm at both welding end faces;

[0094] S2. First, fill the groove of the aluminum alloy plate with intermediate powder with a thickness of 0.2 mm. The intermediate powder, by mass percentage, consists of: Al 90%, Si 8%, Mg 1.5%, Ce 0.5%. Use a pulsed laser with a wavelength of 1064 nm and a frequency of 85 Hz to completely melt the powder to form a dense metal film.

[0095] S3. Second, fill the surface of the already melted film with intermediate powder with a thickness of 0.5 mm. The intermediate powder, by mass percentage, consists of: Al 90%, Si 8%, Mg 1.5%, Ce 0.5%, to form a thickened layer.

[0096] S4. Apply a laser with a power of 1.8 kW and a pulse width of 5 ms to the central area through an annular light spot, and apply a laser with a power of 1.0 kW and a pulse width of 10 ms to the periphery to form a semi-molten adhesive layer.

[0097] S5. Assemble the two metal plates in alignment, apply a pressure of 6 kN through the upper and lower electrodes, set a welding current of 6 kA and a power-on time of 200 ms, and use the electrode heads to perform resistance spot welding on the upper and lower sides of the groove of the metal plate to completely bond the semi-molten adhesive layer with the second metal thin plate.

[0098] Example 2

[0099] This example uses the welding method of the present invention as Figure 1 shown to weld two copper alloy plates (grade: B10) and carbon steel plates (grade: Q235B) both with a length of 300 mm, a width of 200 mm, and a thickness of 5 mm. The specific implementation steps are as follows:

[0100] S1. Clean the welding surfaces of the two metal thin plates, and use the mechanical cutting method to open grooves with a square cross-section, a length of 200 mm, a width of 5 mm, and a depth of 0.5 mm at both welding ends.

[0101] S2. First, fill the groove of the copper alloy plate with intermediate powder with a thickness of 0.2 mm. The intermediate powder, by mass percentage, consists of: Cu 85%, Sn 15%. Use a pulsed laser with a wavelength of 1064 nm and a frequency of 90 Hz to completely melt the powder to form a dense metal film.

[0102] S3. Second, fill the surface of the already melted film with intermediate powder with a thickness of 0.5 mm. The intermediate powder, by mass percentage, for the second powder filling is: Cu 95%, Ti 5%, to form a thickened layer.

[0103] S4. Apply a laser with a power of 1.8 kW and a pulse width of 5 ms to the central area through an annular light spot, and apply a laser with a power of 1.0 kW and a pulse width of 10 ms to the periphery to form a semi-molten adhesive layer.

[0104] S5. Align and assemble the two metal plates, apply a pressure of 6 kN through the upper and lower electrodes, set a welding current of 12 kA and a power-on time of 200 ms, and perform resistance spot welding on the upper and lower sides of the metal plate groove using electrode tips to completely bond the semi-molten adhesive layer with the second metal sheet.

[0105] Example 3

[0106] This example uses the welding method of the present invention as Figure 1 shown to weld two magnesium alloy plates (grade: AZ91D) and titanium alloy plates (grade: Grade9) with a length of 300 mm, a width of 200 mm, and a thickness of 5 mm each. The specific implementation steps are as follows:

[0107] S1. Clean the welding surfaces of the two metal sheets, and use the mechanical cutting method to open a square cross-section groove with a length of 200 mm, a width of 5 mm, and a depth of 0.5 mm at both welding end faces.

[0108] S2. First, fill the magnesium alloy plate groove with an intermediate powder with a thickness of 0.2 mm. The intermediate powder, by mass percentage, consists of: 78% Zn, 20% Al, and 2% Ga. Use a pulsed laser with a wavelength of 1064 nm and a frequency of 100 Hz to completely melt the powder to form a dense metal film.

[0109] S3. Second, fill the surface of the molten film with an intermediate powder with a thickness of 0.5 mm. The intermediate powder, by mass percentage, consists of: 78% Zn, 20% Al, and 2% Ga, to form a thickened layer.

[0110] S4. Apply a laser with a power of 1.8 kW and a pulse width of 5 ms to the central area through an annular light spot, and apply a laser with a power of 1.0 kW and a pulse width of 10 ms to the periphery to form a semi-molten adhesive layer.

[0111] S5. Align and assemble the two metal plates, apply a pressure of 6 kN through the upper and lower electrodes, set a welding current of 10 kA and a power-on time of 200 ms, and perform resistance spot welding on the upper and lower sides of the metal plate groove using electrode tips to completely bond the semi-molten adhesive layer with the second metal sheet.

[0112] Comparative Example 1

[0113] This comparative example uses the traditional intermediate layer transition method to weld two aluminum alloy plates (grade: 3A21 (LF21)) and stainless steel plates (grade: 1Cr18Ni9Ti (321)) with a length of 300 mm, a width of 200 mm, and a thickness of 5 mm each. The specific implementation steps are as follows:

[0114] S1. Clean the surfaces of the welding faces of the two metal sheets, and use the mechanical cutting method to open a square-sectioned groove with a length of 200 mm, a width of 5 mm, and a depth of 0.5 mm on both welding ends;

[0115] S2. Fill the groove of the aluminum alloy sheet with intermediate powder with a thickness of 0.5 mm. The intermediate powder, by mass percentage, consists of: 90% Al, 8% Si, 1.5% Mg, 0.5% Ce. Use a pulsed laser with a wavelength of 1064 nm and a frequency of 85 Hz to completely melt the powder to form a dense metal film;

[0116] S3. Align and assemble the two metal plates, apply a pressure of 6 kN through the upper and lower electrodes, set a welding current of 6 kA and a power-on time of 200 ms, and use the electrode heads to perform resistance spot welding on both sides of the groove of the metal plate to completely bond the semi-molten adhesive layer with the second metal sheet.

[0117] Comparative Example 2

[0118] This comparative example uses the traditional intermediate layer transition method to weld two copper alloy plates (grade: B10) and carbon steel plates (grade: Q235B) with lengths of 300 mm, widths of 200 mm, and thicknesses of 5 mm. The specific implementation steps are as follows:

[0119] S1. Clean the surfaces of the welding faces of the two metal sheets, and use the mechanical cutting method to open a square-sectioned groove with a length of 200 mm, a width of 5 mm, and a depth of 0.5 mm on both welding ends;

[0120] S2. Fill the groove of the copper alloy sheet with intermediate powder with a thickness of 0.5 mm. The intermediate powder, by mass percentage, consists of: 90% Cu, 7.5% Sn, 2.5% Ti. Use a pulsed laser with a wavelength of 1064 nm and a frequency of 90 Hz to completely melt the powder to form a dense metal film;

[0121] S3. Align and assemble the two metal plates, apply a pressure of 6 kN through the upper and lower electrodes, set a welding current of 12 kA and a power-on time of 200 ms, and use the electrode heads to perform resistance spot welding on both sides of the groove of the metal plate to completely bond the semi-molten adhesive layer with the second metal sheet.

[0122] Comparative Example 3

[0123] This comparative example uses the traditional intermediate layer transition method to weld two magnesium alloy plates (grade: AZ91D) and titanium alloy plates (grade: Grade9) with lengths of 300 mm, widths of 200 mm, and thicknesses of 5 mm. The specific implementation steps are as follows:

[0124] S1. Clean the surfaces of the welding joints of the two metal sheets, and use the mechanical cutting method to open a square-sectioned groove with a length of 200 mm, a width of 5 mm, and a depth of 0.5 mm at both welding ends;

[0125] S2. Fill the groove of the magnesium alloy sheet with intermediate powder with a thickness of 0.5 mm. The intermediate powder, by mass percentage, consists of: 78% Zn, 20% Al, and 2% Ga. Use a pulsed laser with a wavelength of 1064 nm and a frequency of 100 Hz to completely melt the powder to form a dense metal film;

[0126] S3. Assemble the two metal plates in alignment, apply a pressure of 6 kN through the upper and lower electrodes, set a welding current of 10 kA and a power-on time of 200 ms, and perform resistance spot welding on both the upper and lower sides of the metal plate groove using electrode tips to completely bond the semi-molten adhesive layer with the second metal sheet.

[0127] Comparative Example 4

[0128] This example uses the welding method of the present invention as Figure 1 shown to weld two magnesium alloy plates (grade: AZ91D) and titanium alloy plates (grade: Grade9) each with a length of 300 mm, a width of 200 mm, and a thickness of 5 mm. The specific implementation steps are as follows:

[0129] S1. Clean the surfaces of the welding joints of the two metal sheets, and use the mechanical cutting method to open a square-sectioned groove with a length of 200 mm, a width of 5 mm, and a depth of 0.5 mm at both welding ends;

[0130] S2. First, fill the groove of the magnesium alloy sheet with intermediate powder with a thickness of 0.2 mm. The intermediate powder, by mass percentage, consists of: 78% Zn, 20% Al, and 2% Ga. Use a pulsed laser with a wavelength of 1064 nm and a frequency of 100 Hz to completely melt the powder to form a dense metal film;

[0131] S3. Second, fill the surface of the already melted film with intermediate powder with a thickness of 0.1 mm. The intermediate powder, by mass percentage, consists of: 78% Zn, 20% Al, and 2% Ga, to form a thickened layer;

[0132] S4. Apply a laser with a power of 1.8 kW and a pulse width of 5 ms to the central area through an annular light spot, and apply a laser with a power of 1.0 kW and a pulse width of 10 ms to the periphery to form a semi-molten adhesive layer;

[0133] S5. Assemble the two metal plates in alignment, apply a pressure of 6 kN through the upper and lower electrodes, set a welding current of 10 kA and a power-on time of 200 ms, and perform resistance spot welding on both the upper and lower sides of the metal plate groove using electrode tips to completely bond the semi-molten adhesive layer with the second metal sheet.

[0134] Comparative Example 5

[0135] In this embodiment, the welding method of the present invention as Figure 1 shown is used to weld two magnesium alloy plates (grade: AZ91D) and titanium alloy plates (grade: Grade9) both with a length of 300 mm, a width of 200 mm, and a thickness of 5 mm. The specific implementation steps are as follows:

[0136] S1. Clean the surfaces of the welding surfaces of the two metal thin plates, and use the mechanical cutting method to open a square-sectioned groove with a length of 200 mm, a width of 5 mm, and a depth of 0.5 mm at both welding ends;

[0137] S2. First, fill the groove of the magnesium alloy plate with intermediate powder with a thickness of 0.2 mm. The intermediate powder, by mass percentage, consists of: 78% Zn, 20% Al, and 2% Ga. Use a pulsed laser with a wavelength of 1064 nm and a frequency of 100 Hz to completely melt the powder to form a dense metal film;

[0138] S3. Second, fill the surface of the already melted film with intermediate powder with a thickness of 0.8 mm. The intermediate powder, by mass percentage, consists of: 78% Zn, 20% Al, and 2% Ga, to form a thickened layer;

[0139] S4. Apply a laser with a power of 1.8 kW and a pulse width of 5 ms to the central area through an annular light spot, and apply a laser with a power of 2.0 kW and a pulse width of 10 ms to the periphery to form a semi-molten adhesion layer;

[0140] S5. Assemble the two metal plates in place, apply a pressure of 6 kN through the upper and lower electrodes, set a welding current of 10 kA and a power-on time of 200 ms, and use the electrode heads to perform resistance spot welding on both sides of the metal plate groove to completely bond the semi-molten adhesion layer with the second metal thin plate.

[0141] Take the welded plate specimens of the above Examples 1 - 3 and Comparative Examples 1 - 5, conduct a tensile test on the metal material according to the experimental standard ASTM E8 to obtain the tensile strength of the specimen, conduct a bending test on the metal material according to the experimental standard ASTM E290 to obtain the bending strength of the specimen, and conduct a resistance spot welding shear test according to the experimental standard ISO 14272 to obtain the joint shear strength of the specimen. The experimental data are shown in Table 1:

[0142] Table 1 Experimental data table

[0143] Example Tensile strength (MPa) Flexural strength (MPa) Shear strength (MPa) Example 1 213 176 178 Comparative Example 1 152 121 125 Example 2 245 202 210 Comparative Example 2 178 152 143 Example 3 201 148 151 Comparative Example 3 144 105 103 Comparative Example 4 148 111 114 Comparative Example 5 164 127 128

[0144] From the above test data, it can be seen that the tensile, bending and shear strengths of the Example 1-3 groups are significantly higher than those of the Comparative Example 1-3 groups. The reason is that in Comparative Example 1-3, due to the use of only a single-layer filling, the porosity at the interface is high, and brittle intermetallic compounds are generated, resulting in performance deterioration. In the Example 1-3 groups, the secondary filling of intermediate powder is used to compensate for the insufficient thickness during the first filling of intermediate powder, reduce the number of pores at the interface, and effectively block the formation of brittle intermetallic compounds. At the same time, after the secondary filling, a semi-molten adhesion layer is formed, and a temperature gradient difference is formed between the outer semi-molten layer and the central fusion nucleus, strengthening the fluidity of the semi-molten adhesion layer, promoting the concentration of heat towards the contact interface with the metal sheet, and forming a more uniform fusion nucleus. The formation of multiple fusion nuclei can make the stress on the welded part more uniform, avoid local stress concentration, reduce the generation of welding defects such as cracks and holes, and thus improve the tensile strength and shear strength between the first metal sheet and the second metal sheet, achieving the effect of significantly improving the fluidity of the fusion nucleus, thereby optimizing the bonding effect between the two metal sheets and the intermediate body, and improving the mechanical properties of the welded plate.

[0145] Comparing Example 3 with Comparative Example 3 and Comparative Example 4, it can be seen that Comparative Example 3 and Comparative Example 4 adjusted the thickness of the secondary filling of intermediate powder. The too-thin intermediate layer in Comparative Example 4 led to insufficient tensile strength and low shear strength when welding two metal sheets. The reason is that after the secondary powder filling, due to the insufficient thickness, a fluid fusion nucleus could not be effectively formed. Therefore, after the secondary powder filling, the porosity decreased compared with the single-layer filling, but the decrease was limited. At the same time, the too-thin thickness led to a temperature difference when the intermediate layer after the secondary powder filling was connected to the dense film formed by the first powder filling. The temperature difference resulted in a lower connection strength between the metal plates, manifested as low bending strength. In Comparative Example 5, a too-thick intermediate layer was formed during the secondary powder filling. The thick layer led to a steep increase in the temperature gradient of the molten pool, and coarse columnar crystals were formed during cooling. The difference in thermal expansion coefficient between the intermediate layer and the base material was amplified in the thick layer, resulting in an increase in the interfacial residual stress, causing local stress concentration, manifested as a significant decrease in bending strength. Moreover, the too-thick powder absorbed too much laser energy, and an effective temperature gradient could not be formed in the outer semi-molten adhesion layer. The number of generated fusion nuclei was limited and the fluidity decreased, manifested as limited bending strength and shear strength.

[0146] Therefore, according to the above experiments, it can be known that the secondary powder filling process adopted in the process of the present invention can significantly improve the mechanical properties of metal plates. Compared with the traditional technology, it improves the performance after welding between different metal plates by the principle of forming more fusion nuclei and improving the fluidity of the fusion nuclei.

[0147] Based on the inspiration of the ideal embodiments of the present invention, through the above description, relevant personnel can completely make various changes and modifications without departing from the technical idea of this invention. The technical scope of this invention is not limited to the content in the specification, and the technical scope must be determined according to the scope of the claims.

Claims

1. A metal sheet resistance spot welding process, characterized in that: The specific steps include: S1. Pre-treating the welding surfaces of the first metal sheet and the second metal sheet with a melting point difference of more than 400° C., and opening matching grooves on the two welding surfaces; S2, first filling the intermediate powder into the first metal sheet container, and using laser melting technology to completely melt the powder to form a metal film; S3, filling powder for the second time in the container where the metal film has been formed in step S2, covering the surface of the metal film and forming a thickened powder layer; S4, performing gradient laser treatment on the secondary powder filling layer, and controlling the laser parameters so that the surface powder forms a semi-molten adhesive layer; S5, aligning the receiving groove of the second metal sheet with the first metal sheet, and welding the semi-molten adhesive layer to the second metal sheet by resistance spot welding.

2. A metal sheet resistance spot welding process according to claim 1, characterized in that: When the first metal sheet and the second metal sheet are aluminum alloy and stainless steel, the intermediate powder has the following components by mass percentage: Al86-92%, Si6-10%, Mg1-3%, Ce0.1-0.5%; When the first metal sheet and the second metal sheet are copper alloy and carbon steel, the intermediate powder has the following composition by mass percentage: Cu-Sn-Ti composite powder, the first powder filling composition is Cu85-90%, Sn10-15%, and the second powder filling composition is Cu95-97%, Ti3-5%; When the first metal sheet and the second metal sheet are magnesium alloy and titanium alloy, the intermediate powder has the following components in mass percentage: Zn 70-85%, Al 12-25%, Ga 0.5-3%.

3. A metal sheet resistance spot welding process according to claim 2, characterized in that: The particle size of the intermediate powder is 45-75 μm.

4. A metal sheet resistance spot welding process according to claim 1, characterized in that: The gradient laser processing in step S4 specifically includes the following steps: S41, applying a laser power of 1.2-1.8 kW to the center of the intermediate powder layer with a circular spot and a pulse width of 5-10 ms to form a molten core with a diameter of 0.08-0.12 mm; S42, applying 0.6-1.0kW laser power to the periphery of the molten core with a pulse width of 10-20ms to form a semi-molten adhesive layer.

5. The metal sheet resistance spot welding process according to claim 1, characterized in that: In the steps S2 and S3, the thickness of the first powder filling layer is 0.1-0.3 mm, and the thickness of the second powder filling layer is 0.2-0.5 mm.

6. A metal sheet resistance spot welding process according to claim 5, characterized in that: In the step S2 and step S3, ultrasonic vibration is used to assist the dispersion of the intermediate powder during powder filling, and the ultrasonic frequency is controlled to be 20-40 kHz and the amplitude is 5-10 μm.

7. A metal sheet resistance spot welding process according to claims 1-6, characterized in that: The setting range of the resistance spot welding process parameters in step S5 is: Welding current: When it is a combination of aluminum alloy and stainless steel: control the welding current to 6-8kA; When copper alloy and carbon steel are combined: control welding current to 10-12kA; When magnesium alloy and titanium alloy are combined: control the welding current to 8-10kA; Electrode pressure: 2-6kN, controlled to match welding current; Welding time: 100-300ms.

8. A metal sheet resistance spot welding device, comprising a frame base, characterized in that: The frame base is integrated with: The plate support mechanism comprises a support member, a conveying platform disposed on the support member, and positioning pins detachably mounted on both sides of the conveying platform; The powder filling mechanism comprises a vertically arranged double powder feeder, a transverse sliding member for allowing the feeder to move along the length direction of the conveying platform, and a position adjustment threaded rod; An ultrasonic auxiliary device, comprising a piezoelectric ceramic ultrasonic probe fixedly mounted on the side wall of the powder feeder nozzle; The laser melting device comprises a longitudinal support column fixed to the top of the frame base, a transverse cantilever beam hinged to the longitudinal support column, and a pulse laser head installed at the end of the cantilever beam; The resistance spot welding mechanism comprises an adjustable pressure electrode head symmetrically arranged above and below a conveying platform.

9. A metal sheet resistance spot welding process according to claim 8, characterized in that: A hydraulic telescopic assembly is arranged in the cantilever beam, and the pulse laser head is fixedly connected to the end of the hydraulic telescopic assembly to adjust the position of the pulse laser head on the frame base.

10. A metal sheet resistance spot welding process according to claim 8, characterized in that: The electrode head is controlled to move vertically via a hydraulic drive unit.

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