Dissimilar metal welding method

Abstract

The invention discloses a dissimilar metal welding method, which is suitable for welding between oxygen-free copper and stainless steel, and comprises the following steps: introducing a tin component into a copper base, enabling copper and tin to form eutectic alloy solder, carrying out surface pretreatment on a base material, smearing soldering paste on a welded surface of the base material, drying, decomposing ammonia gas into nitrogen and hydrogen by adopting an ammonia decomposition furnace, nitrogen serves as protective gas, hydrogen serves as a reducing agent to reduce oxides on the welded faces of the base materials, the two base materials and the eutectic alloy welding flux are heated by the nitrogen, the eutectic alloy welding flux is melted, brazing is carried out on the base materials, in the brazing process, after the eutectic alloy welding flux is melted, a gap between the two base materials is filled through the capillary action, and the eutectic alloy welding flux is melted. And forming a compact composite metal bonding layer. Through the synergistic effect of welding flux components and hydrogen, smooth spreading of welding fluid during welding of oxygen-free copper and stainless steel is guaranteed, it is guaranteed that products have good air tightness and have no obvious welding seams, the welding quality is improved, and meanwhile it is guaranteed that the comprehensive cost is controllable.

Description

Technical Field

[0001] The present invention relates to the field of welding technology, and particularly relates to a method for welding dissimilar metals. Background Art

[0002] Some key components in new energy vehicles are made of oxygen-free copper and stainless steel. During the processing of these components, it is necessary to weld oxygen-free copper and stainless steel. Oxygen-free copper is a copper material with extremely low oxygen content, and its main characteristic is that it contains almost no oxygen or only trace amounts of oxygen elements. Compared with ordinary copper, oxygen-free copper has more excellent electrical conductivity, thermal conductivity, corrosion resistance, and higher mechanical strength. Stainless steel is a type of alloy steel, and its main components are iron, carbon, and at least 10.5% chromium (Cr). Due to the addition of chromium, stainless steel has strong corrosion resistance and oxidation resistance, which makes it widely used in many fields. According to different alloy compositions, stainless steel can be divided into multiple types, and several common types include austenitic stainless steel, ferritic stainless steel, and martensitic stainless steel.

[0003] Due to the significant differences in the physical and chemical properties between oxygen-free copper and stainless steel, the thermal conductivity coefficient of oxygen-free copper is much higher than that of stainless steel, resulting in a large temperature gradient during welding, that is, the temperature distribution in the weld area is uneven. Moreover, the difference in the thermal expansion coefficients of dissimilar materials will also cause uneven shrinkage in the welding area during the cooling process, thereby forming certain residual stresses and increasing the risk of thermal stress cracks. In addition, copper is extremely easy to oxidize at high temperatures during welding, and the relatively thick oxide film will have a negative effect on the wetting of common silver-based solders, resulting in wider and more welds. These problems make the welding quality of these two dissimilar metals, oxygen-free copper and stainless steel, poor. Moreover, the existing methods for welding dissimilar metals require the use of complex shielding gases (such as argon) or vacuum welding equipment, which are costly. Summary of the Invention

[0004] The present invention provides a method for welding dissimilar metals, which can solve the problems of poor welding quality and high cost of dissimilar metals mentioned in the background art.

[0005] A method for welding dissimilar metals, applicable to the welding between oxygen-free copper and stainless steel, includes:

[0006] Select a copper-based solder based on the melting point and introduce a tin component into the copper-based to form a eutectic alloy solder;

[0007] Perform surface pretreatment on the two base materials of oxygen-free copper and stainless steel;

[0008] Apply solder paste to the welded surfaces of the two base materials and dry it, and use a fixture to clamp the dried base materials;

[0009] Preset a temperature range, use an ammonia decomposition furnace with the furnace temperature within the preset temperature range to decompose ammonia into nitrogen and hydrogen. Nitrogen is used as a protective gas, and hydrogen is used as a reducing agent to reduce the oxides on the welded surface of the base material.

[0010] The two base materials and the eutectic alloy solder are heated by nitrogen, the eutectic alloy solder melts, and the base materials are brazed for a preset welding time.

[0011] During the brazing process, after the eutectic alloy solder melts, it fills the gap between the two base materials through capillary action to form a dense composite metal bonding layer.

[0012] Preferably, during pretreatment, the oxygen-free copper base material is mechanically polished to Ra ≤ 1.6 μm.

[0013] Preferably, during pretreatment, the stainless steel base material is wiped with a lint-free cloth dipped in ethanol, then soaked in a dilute sulfuric acid solution for 30 s, rinsed with deionized water, and dried.

[0014] Preferably, the mass fraction of the dilute sulfuric acid solution is 5%.

[0015] Preferably, the fixture uses adjustable-spacing cast iron blocks or ceramic blocks or silicon carbide.

[0016] Preferably, the furnace temperature of the ammonia decomposition furnace is 960 °C to 1000 °C.

[0017] Preferably, the ammonia decomposition furnace is provided with four functional zones, namely a preheating zone, a heating zone, a welding zone, and a cooling zone.

[0018] Preferably, a double-speed chain is arranged in the ammonia decomposition furnace, and the base material is conveyed through the double-speed chain in the four functional zones.

[0019] Preferably, during the brazing process, the eutectic alloy solder melts into a solder liquid and diffuses to the welded surface of the stainless steel base material, reacting with Fe in the stainless steel base material to form FeSn2.

[0020] Preferably, after brazing, a helium mass spectrometer leak detector is used to test the sealing performance of the welded part, and the welding area is cut open to observe the welding quality.

[0021] Advantages of the present invention:

[0022] (1) In the present invention, through the synergistic effect of the solder composition and hydrogen, the smooth spreading of the solder liquid during the welding of oxygen-free copper and stainless steel is ensured, the product has good airtightness, no obvious welds, the welding quality is improved, and the comprehensive cost is ensured to be controllable.

[0023] (2) In the present invention, the molten eutectic alloy solder fills the gap between the two base materials by capillary action to form a dense composite metal bonding layer. The composite metal bonding layer has good mechanical properties and corrosion resistance, ensuring a reliable connection between the two dissimilar metals of oxygen-free copper and stainless steel.

[0024] (3) In the present invention, by generating the ductile phase of FeSn2, the formation of the brittle phase of Fe-Cu can be effectively inhibited, thereby improving the strength and toughness of the welding layer, avoiding uneven shrinkage during subsequent cooling, and reducing the risk caused by residual stress. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 is a flowchart of a dissimilar metal welding method provided by the present invention;

[0026] Figure 2 is an image of the welding area observed by a CCD camera after cutting the welded part. DETAILED DESCRIPTION OF THE INVENTION

[0027] The following is a detailed description of the specific embodiments of the present invention, but it should be understood that the protection scope of the present invention is not limited by the specific embodiments.

[0028] The inventor has been engaged in this field for many years and found that there are significant differences in the physical and chemical properties between oxygen-free copper and stainless steel, such as melting point and thermal conductivity. Since the thermal conductivity of oxygen-free copper (380 W / m·K - 400 W / m·K) is much higher than that of stainless steel (15 W / m·K - 20 W / m·K), heat is easily dissipated from copper during welding, resulting in a large temperature gradient during welding, that is, the temperature distribution in the weld area is uneven.

[0029] Moreover, the difference in the thermal expansion coefficients of dissimilar materials will also cause uneven shrinkage in the welding area during cooling, thereby forming a certain residual stress and increasing the risk of thermal stress cracks.

[0030] In addition, the commonly used silver-based solder during welding has a large wetting angle with stainless steel, and a strongly corrosive flux needs to be added additionally. And copper is extremely easy to oxidize at high temperatures, and the relatively thick oxide film will have a negative effect on the wetting of the commonly used silver-based solder, resulting in wider and more welds.

[0031] These above problems make the welding quality of the two dissimilar metals of oxygen-free copper and stainless steel poor. And the existing dissimilar metal welding methods need to use complex protective gases (such as argon) or vacuum welding equipment, with high costs. Therefore, the inventor proposes a dissimilar metal welding method.

[0032] Such as Figure 1As shown in the figure, the present invention provides a dissimilar metal welding method applicable to the welding between oxygen-free copper and stainless steel. This dissimilar metal welding method includes:

[0033] Select a copper-based solder based on the melting point, and introduce tin components into the copper-based solder to form a eutectic alloy solder with copper and tin.

[0034] Perform surface pretreatment on the two base materials of oxygen-free copper and stainless steel. During pretreatment, the oxygen-free copper base material is mechanically polished to Ra≤1.6μm, the stainless steel base material is wiped with a dust-free cloth dipped in ethanol, then soaked in dilute sulfuric acid solution for 30s, rinsed with deionized water and dried.

[0035] Apply solder paste to the welding surfaces of the two base materials and dry it, and use a fixture to clamp the dried base materials.

[0036] Preset a temperature range, and use an ammonia decomposer with the furnace temperature within the preset temperature range to decompose ammonia into nitrogen and hydrogen. Nitrogen is used as a protective gas, and hydrogen is used as a reducing agent to reduce the oxides on the welding surfaces of the base materials.

[0037] The two base materials and the eutectic alloy solder are heated by nitrogen, the eutectic alloy solder melts, and the base materials are brazed for a preset welding time.

[0038] During the brazing process, after the eutectic alloy solder melts, it fills the gap between the two base materials through capillary action to form a dense composite metal bonding layer.

[0039] The specific operation steps are as follows:

[0040] S1. Select a copper-based solder based on the melting point, and introduce tin components into the copper-based solder to form a eutectic alloy solder with copper and tin. The melting point of the eutectic alloy solder formed by copper and tin is 227°C.

[0041] In the prior art, the most common solders for copper welding are mainly silver-based solders and copper-based solders. Among them, silver-based solders have a relatively low melting point (780 - 850°C), but they are costly, and an Ag-Fe brittle phase is likely to form at the Ag-Fe interface. Copper-based solders, on the other hand, require a higher melting point (>900°C). In order to be applicable to scenarios with a larger welding area, compared with the relatively costly silver-based solders, the present invention selects copper-based solders.

[0042] However, the high melting point of copper-based solders will result in the need to provide a relatively high temperature during the welding process, and the oxygen-free copper base material is prone to thermal deformation at a relatively high welding temperature. In order to reduce the melting point of copper-based solders and minimize the thermal deformation of the base materials caused by high temperature, tin components are introduced into the copper-based solders. Sn can not only form a low-melting eutectic with Cu to reduce the welding temperature, but also diffuse to the stainless steel interface during subsequent brazing and react with Fe to form FeSn2. FeSn2 is a ductile phase that can inhibit the Fe-Cu brittle phase.

[0043] Therefore, using a eutectic alloy solder with a melting point lower than that of the base material can reduce the thermal impact on the base material and achieve good fluidity and welding quality at low temperatures. In particular, the addition of tin can improve the wettability of the solder, helping the solder to better contact the welding surface of the oxygen-free copper base material, and then forming a solid welding layer. The eutectic alloy solder with good fluidity can penetrate into relatively small joints to ensure the integrity of the welding layer.

[0044] S2. Perform surface pretreatment on the two base materials of oxygen-free copper and stainless steel.

[0045] Specifically, during pretreatment, the oxygen-free copper base material is mechanically polished to Ra ≤ 1.6 μm to remove the surface oxidation stains. Use a dust-free cloth dipped in ethanol to wipe the stainless steel base material, then soak it in a 5% by mass dilute sulfuric acid solution for 30 s, rinse it with deionized water and dry it to remove the oxides on the welded surface, reduce the wetting angle of the subsequent solder on its surface, and facilitate the smooth spreading of the solder.

[0046] S3. Apply solder paste to the welded surfaces of the two base materials and dry it, and use a fixture to clamp the dried base materials.

[0047] Among them, a solder paste with an active ingredient including zinc chloride, sodium fluoride, and calcium fluoride can be used, which can effectively remove the oxide film on the metal surface. Since there is usually an oxide film on the surface of stainless steel and it is not easy to be wetted, using a solder paste (flux) containing fluorides, chlorides, or activators can assist in removing the oxide film to improve wettability and bonding strength.

[0048] The fixture uses cast iron blocks or ceramic blocks or silicon carbide with adjustable spacing. Preferably, cast iron blocks are used. When clamping, stack the base materials on the top of several cast iron blocks, and then use several cast iron blocks to press on the top of the base materials. The upper and lower layers of cast iron blocks are arranged correspondingly, and bolts are used to fix the upper and lower layers of cast iron blocks to each other so that the base materials are clamped.

[0049] S4. Preset a temperature range. The furnace temperature of the ammonia decomposition furnace is 960 °C to 1000 °C (i.e., the preset temperature range), which can ensure that the eutectic alloy solder is completely melted without damaging the base material. The ammonia decomposition furnace decomposes ammonia into nitrogen and hydrogen. Nitrogen is used as a protective gas, and hydrogen is used as a reducing agent to reduce the oxides on the welded surface of the base material.

[0050] Specifically, provide an ammonia decomposition furnace. The ammonia decomposition furnace is provided with four functional zones, namely a preheating zone, a heating zone, a welding zone, and a cooling zone. A double-speed chain is arranged in the ammonia decomposition furnace, and the base materials are conveyed in the four functional zones through the double-speed chain.

[0051] In an ammonia decomposition furnace, ammonia is heated and decomposed into nitrogen and hydrogen by a catalyst. Nitrogen, as a protective gas, can prevent oxidation reactions from occurring and protect the surfaces of oxygen-free copper and stainless steel during welding from reacting with oxygen in the air.

[0052] Hydrogen, as a reducing agent, acts on the surface of the base material and can remove oxides. Hydrogen can react with copper oxide and iron oxide on the surface of the base material to remove the oxide layer, providing a relatively clean metal surface to ensure the subsequent welding quality and reduce the generation of welds.

[0053] S5. The two base materials and the eutectic alloy solder are heated by nitrogen, the eutectic alloy solder melts, and brazing is performed on the base materials for a preset welding time. The preset welding time is 2.5 h, and the welding time can be adjusted based on actual conditions.

[0054] In the heating stage, the base materials and the eutectic alloy solder are gradually heated to the temperature required for brazing, and the eutectic alloy solder melts into a solder liquid. Brazing is performed for 2.5 h, and the melted eutectic alloy solder fills the gap between the two base materials through capillary action to form a dense composite metal bonding layer, which is a welding layer. Capillary action means that when the solder liquid contacts the metal surface, due to the action of surface tension, it will be attracted into the gap of the metal, fill the welding gap, and ensure that the solder liquid can completely infiltrate the joint surface (welding surface). As the solder liquid fills, the bond between the oxygen-free copper base material and the stainless steel base material is gradually formed. The composite metal bonding layer has good mechanical properties and corrosion resistance, ensuring a reliable connection between the two dissimilar metals of oxygen-free copper and stainless steel.

[0055] Among them, moreover, the eutectic alloy solder melts into a solder liquid and diffuses to the welded surface of the stainless steel base material, reacting with Fe in the stainless steel base material to generate FeSn2. FeSn2 is a ductile phase and can inhibit the Fe-Cu brittle phase.

[0056] Specifically, at high temperatures, copper and iron will form a brittle phase Fe-Cu (such as Fe3Cu), and this brittle phase has a negative impact on the mechanical properties of the welding layer and is prone to causing cracking or fracture of the welding layer. However, by generating the ductile phase FeSn2, the formation of the Fe-Cu brittle phase can be effectively inhibited, thereby improving the strength and toughness of the welding layer, avoiding uneven shrinkage during subsequent cooling, and reducing the risk caused by residual stress.

[0057] In summary, the eutectic alloy solder reacts with iron in the stainless steel base material to generate FeSn2, which can enhance the toughness of the composite metal bonding layer, reduce the formation of brittle phases, and thus improve the welding quality and extend the service life.

[0058] After brazing, the workpiece formed by welding needs to be cooled. During the cooling stage, the solder solidifies to form a composite metal bonding layer. The cooling process can be controlled by natural cooling or forced cooling using a refrigerant to ensure the stability of the grain structure and properties of the composite metal bonding layer. After cooling, the residual solder needs to be removed and the joint surface needs to be cleaned to further ensure the welding quality and appearance, that is, the brazing is completed.

[0059] S6. After brazing is completed, use a helium mass spectrometer leak detector to test the airtightness of the welded part, and cut it open to observe the welding quality of the welding area.

[0060] The leakage rate detected by the helium mass spectrometer leak detector ≤ 1×10 -5 mbar*L / S is considered qualified.

[0061] As Figure 2 shown, in this embodiment, the welded part is subjected to helium leak detection. The measured leakage amount is 7.1E-08 mbar*L / S, which meets the requirements. The operator originally cut open the welding area, polished it, and observed it through a CCD camera. Through the collected images of the CCD camera, it can be observed that the welding between oxygen-free copper and stainless steel is full and there is no obvious weld seam.

[0062] In this application, for the eutectic alloy solder formed by copper and tin, the Sn content is preferably 12% - 15%. When the Sn content is less than 10%, the formation of the Fe-Cu brittle phase cannot be effectively inhibited, while excessive Sn will cause the formation of the brittle phase of Cu6Sn5 in the Cu-Sn solder, reducing the plasticity of the joint. The Sn content in this range can form a continuous Sn diffusion layer with a moderate thickness at the welding interface, and at the same time form a metallurgical bond with the base material, effectively avoiding the generation of thermal stress cracks and weld seams.

[0063] During the welding process, through the synergistic effect of the solder composition and the reducing gas (hydrogen), ensure the smooth spreading of the solder during the welding of oxygen-free copper and stainless steel, ensure that the product has good airtightness, no obvious weld seam, improve the welding quality and ensure that the comprehensive cost is controllable.

[0064] In addition, the synergistic control of the welding temperature and welding time is the core element to achieve high-quality welding. Limit the ranges of welding temperature, welding time, gas flow rate, etc. to cover the optimal process window. By reasonably matching the heating temperature and the holding time, the flow diffusion of the solder and the thermal influence of the base material can be balanced, avoiding defects caused by improper heat input.

[0065] In some embodiments, Ag can be added to the eutectic alloy solder formed by copper and tin within the range of controllable cost to further improve the wettability of the solder. Ag forms an Ag-Cu eutectic (melting point < 780°C) with Cu, reducing the liquidus temperature and effectively improving the joint strength of the welding area.

[0066] It is understandable that in some embodiments, vacuum brazing is used to replace the brazing process in the present application. Vacuum brazing is carried out in a vacuum environment (usually with a pressure ≤ 10 -3 Pa), where a solder with a melting point lower than that of the base material is used to fill the joint gap through capillary action to achieve metallurgical connection. It can completely isolate oxygen, avoid oxidation of the base material surface, and does not require a soldering flux, ensuring a high surface cleanliness. However, the cost is relatively high, and it is not optimized in terms of cost control. Laser welding can use a high-energy laser beam to focus on the interface between copper and stainless steel, and through precise energy control, locally melt the base material to form a molten pool, and achieve metallurgical bonding after cooling. However, it is more suitable for micro-devices and not suitable for welding larger-sized devices.

[0067] The above discloses only several specific embodiments of the present invention. However, the embodiments of the present invention are not limited thereto, and any changes that can be conceived by those skilled in the art should fall within the protection scope of the present invention.

Claims

1. A dissimilar metal welding method applicable to the welding between oxygen-free copper and stainless steel, characterized in that, Including: Select a copper-based solder based on the melting point, and introduce tin into the copper-based to form a eutectic alloy solder between copper and tin; Perform surface pretreatment on two base materials of oxygen-free copper and stainless steel; Apply solder paste to the welded surfaces of the two base materials and dry it, and use a fixture to clamp the dried base materials; Preset a temperature range and welding time, use an ammonia decomposition furnace with the furnace temperature within the preset temperature range to decompose ammonia into nitrogen and hydrogen, use nitrogen as the protective gas, and use hydrogen as the reducing agent to reduce the oxides on the welded surfaces of the base materials; The two base materials and the eutectic alloy solder are heated by nitrogen, the eutectic alloy solder melts, and the base materials are brazed for the preset welding time; During the brazing process, after the eutectic alloy solder melts, it fills the gap between the two base materials through capillary action to form a dense composite metal bonding layer.

2. The method for dissimilar metal welding according to claim 1, wherein During pretreatment, the oxygen-free copper base material is mechanically polished to Ra≤1.6μm.

3. A dissimilar metal welding method according to claim 2, wherein During pretreatment, wipe the stainless steel base material with a dust-free cloth dipped in ethanol, then soak it in a dilute sulfuric acid solution for 30s, rinse it with deionized water and dry it.

4. The method for welding dissimilar metals according to claim 3, wherein, The mass fraction of the dilute sulfuric acid solution is 5%.

5. A dissimilar metal welding method according to claim 1, characterized in that, The fixture uses adjustable-spacing cast iron blocks or ceramic blocks or silicon carbide.

6. A method for welding dissimilar metals according to claim 1, characterized in that, The furnace temperature of the ammonia decomposition furnace is 960°C to 1000°C.

7. A dissimilar metal welding method according to claim 1, characterized in that, The ammonia decomposition furnace is provided with four functional zones, namely a preheating zone, a heating zone, a welding zone, and a cooling zone.

8. A dissimilar metal welding method according to claim 7, characterized in that, A double-speed chain is arranged in the ammonia decomposition furnace, and the base materials are conveyed through the double-speed chain in the four functional zones.

9. The method for welding dissimilar metals according to claim 1, characterized in that, During the brazing process, the eutectic alloy solder melts into a solder liquid and diffuses to the welded surface of the stainless steel base material, and reacts with Fe in the stainless steel base material to generate FeSn2.

10. A dissimilar metal welding method according to claim 1, characterized in that, After brazing, use a helium mass spectrometer leak detector to test the sealing performance of the welded parts, and cut it open to observe the welding quality of the welding area.

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