Composite flux and brazing method for steel / aluminum dissimilar metal brazing / fusion brazing

Abstract

The present application relates to the technical field of welding, and provides a composite brazing flux for steel / aluminum dissimilar metal brazing and fusion brazing and a brazing method.The composite flux is prepared by mixing a certain proportion of KBF4 and KAlF4, K3AlF6 with anhydrous ethanol or acetone; the composite flux is directly coated on the surface of a steel plate during use; a welding wire is used as filler metal; and a heat source is focused on the filler metal close to the steel plate side to perform welding.The present application has the following advantages: from the perspective of toughening the interface Fe-Al intermetallic compound itself, KBF4 and molten liquid Al are used to generate an in-situ replacement reaction (KBF4+Al=KAlF4+[B]) to transition free-state B into the weld, the free-state B enters the interatomic gap of the intermetallic compound, and the intrinsic brittleness of the Fe-Al intermetallic compound is improved, thereby improving the strength and toughness of the joint.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of welding, in particular to a composite brazing flux for steel / aluminum dissimilar metal brazing / fusion brazing and a brazing method. BACKGROUND

[0002] With the development of science and technology and industry, energy shortage and environmental protection problems are increasingly prominent. In view of energy saving and environmental protection, sustainable development, various industries develop rapidly while launching a large number of researches on lightweight materials. Aluminum and aluminum alloy are widely used in the fields of aerospace, automobile, national defense, household appliances and other manufacturing fields due to the advantages of light weight, good plasticity, excellent thermal conductivity, good corrosion resistance and low price.

[0003] In current industrial design and production, some important load-bearing structural parts still use high-strength steel, and the connection of aluminum and steel has great development prospects. However, due to the great difference in physical and chemical properties between steel and aluminum, the connection between the two metals is very difficult, and a large amount of brittle Fe-Al intermetallic compounds are easily formed at high temperature, the quality of the welded workpiece is poor, although solid-phase welding and brazing such as friction welding, explosive welding, diffusion welding can realize effective welding of aluminum / steel dissimilar metals, but these methods have high cost and complex equipment, and it is difficult to weld large-sized and complex-structured workpieces, which is greatly limited in actual production and application. In recent years, fusion brazing combining the advantages of fusion welding and brazing has been developed, which can effectively reduce the generation of a large amount of brittle intermetallic compounds, therefore, fusion brazing has gradually become one of the most active research directions in the field of steel / aluminum dissimilar metal connection. However, a large number of studies have shown that the interface between the steel side base material and the weld of the fusion brazed joint will still form brittle Fe-Al intermetallic compounds, resulting in high brittleness of the interface, thereby limiting the performance improvement of the fusion brazed joint.

[0004] In view of this phenomenon, domestic and foreign scholars basically focus on the two ways of process control and metallurgical control to reduce the interface brittleness by controlling the compound morphology (shape, thickness) and type. For process control, by adjusting the welding heat input or introducing surface pretreatment and other auxiliary processes, the interface intermetallic compound layer thickness can be effectively reduced, the continuous lamellar distribution of the compound is broken, the interface is changed from thick layer to jagged, however, process control only changes the interface structure, and has little effect on the reaction of Fe and Al, and the type of interface intermetallic compound is still brittle Fe-Al intermetallic compound; for metallurgical control, the use of Si, Zn, Cu, Ti, Ni and other elements can not only improve the thickness and morphology of the interface compound, but also participate in the reaction between Fe and Al to form other compounds to replace the brittle Fe-Al compound, however, the microhardness of Fe2Al5-xZnx, CuAl2, NiAl3 and TiAl3 is still high, and the brittleness of the interface compound has not been effectively improved. Due to the brittleness of the interface compound, the strength of the steel and aluminum welded joint is severely limited, and the existing process control and metallurgical control means cannot fundamentally improve the brittleness of the interface intermetallic compound. Therefore, while changing the compound morphology and type, how to effectively reduce the intrinsic brittleness of the interface intermetallic compound is a key problem to be solved to improve the strength and reliability of the steel / aluminum dissimilar metal welded joint and obtain a steel / aluminum welded joint meeting the actual application requirements. SUMMARY

[0005] The purpose of the present application is to overcome the deficiencies of the prior art in controlling the brittleness of the interface intermetallic compound, and to provide a composite flux for steel / aluminum dissimilar metal brazing / soldering and a brazing method, thereby opening up a new way to solve the interface brittleness problem in steel / aluminum and even other dissimilar material metallurgical connection.

[0006] The present application adopts the following technical solutions:

[0007] On the one hand, the present application provides a composite flux for steel / aluminum dissimilar metal brazing / soldering, which takes KAlF4 and K3AlF6 as the matrix and adds a certain proportion of KBF4 to form a suspension solution type flux; the weight percentage of each component is as follows: KAlF4 50%-60%, K3AlF6 25%-35%, and KBF4 5%-25%.

[0008] As any possible implementation manner described above, further provided is an implementation manner, wherein the suspension solution type flux is prepared by using acetone or anhydrous ethanol.

[0009] As any possible implementation manner described above, further provided is an implementation manner, wherein the filler metal is selected from Al-based filler metals.

[0010] In another aspect, the present application also provides a brazing / braze welding method of steel / aluminum dissimilar metals, which uses the composite flux described above, and the method comprises:

[0011] S1, KAlF4 50%-60%, K3AlF6 25%-35%, KBF4 5%-25% are weighed by percentage of weight, and mixed;

[0012] S2, the mixture obtained in step S1 is added with acetone or anhydrous ethanol, and stirred to a suspended solution, so that the composite flux is obtained;

[0013] S3, the aluminum alloy plate and the steel plate are polished and cleaned on the side to be welded, and dried;

[0014] S4, the composite flux prepared in step S2 is uniformly coated on the side surface of the steel plate to be welded after the treatment in step S3;

[0015] S5, the wet flux coated on the side surface of the steel plate is dried;

[0016] S6, the steel plate and the aluminum alloy plate are butt-jointed, a welding wire is used as a filler metal, and a heat source is focused on the filler metal close to the steel plate for welding.

[0017] According to any possible implementation manner described above, further provided is an implementation manner, in step S2, the weight ratio of acetone or anhydrous ethanol to KAlF4+K3AlF6+KBF4 is 5:1.

[0018] According to any possible implementation manner described above, further provided is an implementation manner, in step S4, the coating thickness of the composite flux on the surface of the steel plate is 0.1-0.4 mm.

[0019] According to any possible implementation manner described above, further provided is an implementation manner, in step S6, the welding wire is an Al-Si welding wire with a Si content of 4.5%-6%.

[0020] According to any possible implementation manner described above, further provided is an implementation manner, in step S6, the focus of the heat source is focused on the filler metal, the welding speed is set to 10-20 mm / s, and the welding power is set to 400-1000 W.

[0021] According to any possible implementation manner described above, further provided is an implementation manner, the tensile strength of the welded joint ranges from 127 MPa to 152 MPa.

[0022] The present application has the following advantages:

[0023] (1) Break through the traditional thinking of improving the performance of steel / aluminum brazing joint by regulating the morphology and type of interface compounds, propose to use B to reduce the intrinsic brittleness of interface compounds, and toughen the interface Fe-Al intermetallic compound itself, which opens up a new way to solve the interface brittleness problem in steel / aluminum and even other dissimilar material metallurgical connection.

[0024] (2) Ingeniously use the in-situ replacement reaction of KBF4 and liquid Al (KBF4+Al=KAlF4+[B]) to transition B into the weld during brazing, avoiding the problems of oxidation and inclusion caused by the difficulty of B effectively dissolving into liquid welding material when adding elemental B, or the existence of B in the form of high melting point Al-B intermetallic phase when directly preparing B-containing welding material.

[0025] (3) In addition to B, the other product KAlF4 of the in-situ replacement reaction of KBF4 and liquid Al is one of the main components of the composite flux in the present application, and the by-product KAlF4 can supplement the consumption of the composite flux during welding, which not only has no negative effect on the welding process, but also enhances the effect of the flux.

[0026] (4) The in-situ replacement reaction of KBF4 and liquid Al may produce Al2B, and Al2B as a high melting point Al-B intermetallic alloy can exactly provide nucleation sites for aluminum melt solidification, thereby refining the aluminum alloy grains.

[0027] (5) KBF4 and KAlF4+K3AlF6 matrix are added in the form of flux, which is simple and convenient to operate and has low use cost. BRIEF DESCRIPTION OF DRAWINGS

[0028] Figure 1 Fig. 1 shows the EDS analysis diagram of the joint interface after steel / aluminum dissimilar metal butt fusion brazing in the embodiment of the present application; (a) fusion brazing joint interface based on B in-situ replacement transition; (b) traditional fusion brazing joint interface; (c) C position EDS analysis; (d) D position EDS analysis; (e) E position EDS analysis.

[0029] Figure 2 Fig. 2 shows the SEM comparison diagram of the fracture morphology of the joint after steel / aluminum dissimilar metal butt fusion brazing using the welding method of the present application; (a) fusion brazing joint interface based on B in-situ replacement transition; (b) traditional fusion brazing joint interface.

[0030] Figure 3 Fig. 3 shows the tensile strength comparison diagram of the joint after steel / aluminum dissimilar metal butt fusion brazing using the welding method of the present application. DETAILED DESCRIPTION

[0031] The specific embodiments of the present application will be described in detail below with reference to the accompanying drawings. It should be noted that the technical features described in the following embodiments or combinations of technical features should not be considered in isolation, and they can be combined with each other to achieve better technical effects.

[0032] In previous studies, KBF4 is often added in a small amount (<10%) for its special role in improving the morphological characteristics of the interfacial compound layer. According to the effect of B on toughening the interfacial metal compounds, the content of KBF4 is increased in the present application based on KAlF4+K3AlF6 as the base, and KBF4 reacts with liquid Al to transition B into the weld and interfacial intermetallic compounds.

[0033] The composite flux for steel / aluminum dissimilar metal brazing / soldering in an embodiment of the present application uses KAlF4 and K3AlF6 as the base and adds a certain proportion of KBF4 to form a suspension solution type flux. The weight percentage of each component is as follows: KAlF4 50%-60%, K3AlF6 25%-35%, and KBF4 5%-25%. The Al-Si based filler metal is used in cooperation, and the flux can be directly coated on the surface of the steel plate during use.

[0034] In a specific embodiment, the flux is a suspension solution type flux prepared by using acetone or anhydrous ethanol.

[0035] In a specific embodiment, the filler metal used during welding is an Al-based filler metal.

[0036] The brazing / soldering method for steel / aluminum dissimilar metal in an embodiment of the present application uses the composite flux described above, and the method comprises the following steps:

[0037] S1. KAlF4 50%-60%, K3AlF6 25%-35%, and KBF4 5%-25% are weighed according to the weight percentage and mixed;

[0038] S2. Acetone or anhydrous ethanol is added to the mixture obtained in step S1, and stirring is performed until a suspension solution is formed, thereby obtaining the composite flux;

[0039] S3. The aluminum alloy plate and the steel plate are polished, cleaned, and dried on the side to be welded;

[0040] S4. The composite flux prepared in step S2 is uniformly coated on the side surface of the steel plate to be welded after the treatment in step S3;

[0041] S5. The wet flux coated on the side surface of the steel plate is dried;

[0042] S6. The steel plate and the aluminum alloy plate are butt-jointed, a welding wire is used as the filler metal, and a heat source is focused on the filler metal close to the steel plate for welding.

[0043] In one embodiment, in step S2, the weight ratio of acetone or anhydrous ethanol to KAlF4+K3AlF6+KBF4 is 5:1.

[0044] In one embodiment, in step S4, the composite brazing agent is coated on the surface of the steel plate with a thickness of 0.1-0.4 mm, covering the base color of the steel plate.

[0045] In one embodiment, in step S6, the welding wire is an Al-Si welding wire with a Si content of 4.5%-6%.

[0046] In one embodiment, in step S6, the focus of the heat source is focused on the filler metal, the welding speed is set to 10-20 mm / s, and the welding power is set to 400-1000 W.

[0047] The composite brazing agent can be used for brazing / brazing of steel / aluminum dissimilar metals.

[0048] Example 1

[0049] A brazing / brazing method for aluminum / steel dissimilar metals, comprising:

[0050] Step 1: 50%-60% KAlF4, 25-35% K3AlF6 and 5-25% KBF4 are weighed and mixed in a beaker according to the weight ratio;

[0051] Step 2: The mixed powder in step 1 is mixed with acetone (or anhydrous ethanol) to form a uniform particle suspension solution, and the weight ratio of the mixed powder to acetone (or anhydrous ethanol) is 1:5;

[0052] Step 3: After the steel plate side surface is polished and cleaned, a layer of prepared particle suspension composite brazing agent is uniformly coated on the steel side surface, and the coating thickness is 0.1-0.4 mm, covering the base color of the steel plate.

[0053] Step 4: Use a hair dryer or drying box to dry the wet brazing agent, and then overlap or butt joint the steel plate and aluminum plate with a thickness of 1.5-3 mm, and use an Al-Si welding wire with a diameter of 0.8-1.6 mm as a filler metal;

[0054] Step 5: Focus the heat source on the filler metal near the steel plate, focus the focus of the heat source on the filler metal, set the welding speed to 10-20 mm / s, set the welding power to 400-1000 W, ensure that KBF4 and molten liquid Al occur in situ replacement reaction (KBF4+Al=KAlF4+[B]), and transition free B into the interatomic gap of intermetallic compounds to play a toughening role.

[0055] The results show that the existence of B can be detected in the interfacial compound, the mass fraction of B is 1.17-5.03%, the fracture morphology of the joint is changed from the traditional typical cleavage to the mixed fracture morphology, and the tensile strength of the joint is increased from 90 MPa of the traditional fusion brazing joint to 127-152 MPa.

[0056] Example 2

[0057] This embodiment is a composite flux suitable for steel / aluminum dissimilar metal brazing / fusion brazing, and the flux composition is as follows in terms of weight percentage: KAlF4 55%, K3AlF6 28%, and KBF4 17%. The base materials are 304 stainless steel and 6061 aluminum alloy plates with a thickness of 2.7 mm. The specific process of this embodiment includes the following steps:

[0058] Step 1, using an electronic balance, accurately weigh 5.5g of KAlF4, 2.8g of K3AlF6, and 1.7g of KBF4 powder respectively, and put them into a beaker for mixing;

[0059] Step 2, use a graduated cylinder to measure 50ml of acetone, then slowly pour it into the beaker containing the mixed flux powder of step 1, and stir with a glass rod until a suspended solution is obtained;

[0060] Step 3, use 800 mesh sandpaper to polish the side surfaces of the steel plate and aluminum alloy plate to be welded with a thickness of 2.7mm, and then clean them with anhydrous ethanol and dry them with a hair dryer;

[0061] Step 4, evenly coat a layer of step 2 prepared granular suspended composite flux on the side surface of the steel plate to be welded, with a coating thickness of 0.2mm, covering the steel plate base color;

[0062] Step 5, use a hair dryer to dry the wet flux coated on the side surface of the steel plate;

[0063] Step 6, butt joint the steel plate and aluminum alloy plate, and use AlSi5 welding wire with a diameter of 1.6mm as filler metal for welding;

[0064] Step 7, focus the heat source on the filler metal near the steel plate side, and focus the heat source on the filler metal, set the welding speed to 15mm / s and the welding power to 1000W.

[0065] The results show that the existence of B can be detected in the interfacial compound, the mass fraction of B is 1.17-5.03%, the fracture morphology of the joint is changed from the traditional typical cleavage to the mixed fracture morphology, and the tensile strength of the joint is increased from 90 MPa of the traditional fusion brazing joint to 127-152 MPa. Figure 1 Figure 2 The results show that the existence of B can be detected in the interfacial compound, the mass fraction of B is 1.17-5.03%, the fracture morphology of the joint is changed from the traditional typical cleavage to the mixed fracture morphology, and the tensile strength of the joint is increased from 90 MPa of the traditional fusion brazing joint to 127-152 MPa. Figure 3 ​as shown.

[0066] Example 3

[0067] This embodiment is a composite flux suitable for steel / aluminum dissimilar metal brazing / soldering, and the flux composition is as follows: KAlF450%, K3AlF625%, KBF425% by weight. The base materials are 304 stainless steel and 6061 aluminum alloy plates with a thickness of 1.8 mm. The specific process of this embodiment includes the following steps:

[0068] Step 1: using an electronic balance, accurately weigh 5g of KAlF4, 2.5g of K3AlF6 and 2.5g of KBF4 powder respectively, and put them into a beaker for mixing;

[0069] Step 2: use a graduated cylinder to measure 50ml of acetone, then slowly pour it into the beaker containing the mixed flux powder in step 1, and stir with a glass rod until a suspended solution is obtained;

[0070] Step 3: use 800 mesh sandpaper to polish the side surfaces of the steel plate and aluminum alloy plate to be welded with a thickness of 1.8mm, and then clean them with anhydrous ethanol and dry them with a hair dryer;

[0071] Step 4: evenly coat a layer of step 2 mixed granular suspended composite flux on the side surface of the steel plate to be welded, with a coating thickness of 0.2mm, covering the steel plate base color;

[0072] Step 5: use a hair dryer to dry the wet flux coated on the side surface of the steel plate;

[0073] Step 6: butt joint the steel plate and aluminum alloy plate, and use AlSi5 welding wire with a diameter of 1.2mm as filler metal for welding;

[0074] Step 7: focus the heat source on the filler metal near the steel plate side, and focus the focal point of the heat source on the filler metal, set the welding speed to 20mm / s and the welding power to 800W.

[0075] The results show that B can be detected in the interfacial compound, with a mass fraction of 5.7%, the fracture morphology of the joint changes from the traditional typical cleavage to a mixed fracture morphology, and the joint strength reaches 143MPa.

[0076] Example 4

[0077] The embodiment is a composite flux suitable for steel / aluminum dissimilar metal brazing / soldering, and the flux composition is as follows: KAlF4 60%, K3AlF6 35%, and KBF4 5% by weight; the base materials are 304 stainless steel and 6061 aluminum alloy plates with a thickness of 1.8 mm. The specific process of the embodiment comprises the following steps:

[0078] Step 1: using an electronic balance, accurately weigh 6 g of KAlF4, 3.5 g of K3AlF6 and 0.5 g of KBF4 powder respectively, and put them into a beaker for mixing;

[0079] Step 2: using a graduated cylinder, measure 50 ml of acetone, then slowly pour it into the beaker containing the mixed flux powder in step 1, and stir it into a suspended solution using a glass rod;

[0080] Step 3: using 800 mesh sandpaper, polish the side surfaces of the steel plate and aluminum alloy plate to be welded with a thickness of 1.8 mm to remove dirt, oxidation film, etc., then clean them with anhydrous ethanol, and dry them with a hair dryer;

[0081] Step 4: evenly coat the steel plate with a layer of the granular suspended composite flux prepared in step 2 on the side surface to be welded, with a coating thickness of 0.2 mm, covering the base color of the steel plate;

[0082] Step 5: using a hair dryer, dry the wet flux coated on the side surface of the steel plate;

[0083] Step 6: butt joint the steel plate and the aluminum alloy plate, and use AlSi5 welding wire with a diameter of 1.2 mm as filler metal for welding;

[0084] Step 7: focus the heat source on the filler metal near the steel plate, and focus the focal point of the heat source on the filler metal, set the welding speed to 10 mm / s, and set the welding power to 400 W.

[0085] The results show that B can be detected in the interfacial compound, with a mass fraction of 1.6%, the fracture morphology of the joint changes from the traditional typical cleavage to a mixed fracture morphology, and the joint strength reaches 127 MPa.

[0086] Although several embodiments of the present application have been described in this paper, those skilled in the art should understand that the embodiments described in this paper can be changed without departing from the spirit of the present application. The above embodiments are only exemplary and should not be used as a limitation of the scope of the present application.

Claims

1. A composite flux for steel / aluminum dissimilar metal brazing / soldering, characterized by, The composite brazing agent takes KAlF4 and K3AlF6 as a matrix, and adds KBF4 in a certain proportion to form a suspension solution type brazing agent; the weight percentage of each component is: KAlF4 50%-60%, K3AlF6 25%-35%, and KBF4 17%-25%; when the suspension solution type brazing agent is prepared, acetone or anhydrous ethanol is used; when welding, Al-based filler metal is selected; KBF4 and molten liquid Al are used to generate an in-situ replacement reaction (KBF4+Al=KAlF4+[B]) to transition free-state B into the weld, and the free-state B enters the interatomic gap of the intermetallic compound to improve the intrinsic brittleness of the Fe-Al intermetallic compound.

2. A brazing / fusion brazing method of steel / aluminum dissimilar metals, characterized by, The method uses the composite brazing agent as claimed in claim 1, and the method comprises the following steps: S1, weighing KAlF4 50%-60%, K3AlF6 25%-35%, and KBF4 17%-25% by weight percentage, and mixing; S2, adding acetone or anhydrous ethanol to the mixture obtained in step S1, and stirring to form a suspension solution, thereby obtaining the composite brazing agent; S3, polishing, cleaning, and blowing dry the surface of the aluminum alloy plate and the steel plate to be welded; S4, uniformly coating the surface of the steel plate to be welded with a layer of the composite brazing agent prepared in step S2; S5, drying the wet brazing agent coated on the surface of the steel plate; S6, butting the steel plate and the aluminum alloy plate, using a welding wire as filler metal, and focusing a heat source on the filler metal close to the steel plate to perform welding.

3. The brazing / fusion brazing method of steel / aluminum dissimilar metals according to claim 2, characterized by, In step S2, the weight ratio of acetone or anhydrous ethanol to KAlF4+K3AlF6+KBF4 is 5:

1.

4. The brazing / fusion brazing method of steel / aluminum dissimilar metals according to claim 2, characterized by, In step S4, the thickness of the composite brazing agent coated on the surface of the steel plate is 0.1-0.4 mm.

5. The brazing / fusion brazing method of steel / aluminum dissimilar metals according to claim 2, characterized by, In step S6, the welding wire is an Al-Si welding wire with a Si content of 4.5%-6%.

6. The brazing / fusion brazing method of steel / aluminum dissimilar metals according to claim 2, characterized by, In step S6, the focus of the heat source is focused on the filler metal, the welding speed is set to 10-20 mm / s, and the welding power is set to 400-1000 W.

7. The brazing / fusion brazing method of steel / aluminum dissimilar metals according to claim 2, characterized by, The tensile strength of the welded joint is in the range of 127-152 MPa.

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