RU2754134C1 - Method for diffusion welding of steel and aluminum products - Google Patents

Abstract

FIELD: metal materials production.

SUBSTANCE: invention relates to methods for producing permanent welded joints of products from dissimilar metal materials, in particular to a method of diffusion welding of products from steel and aluminum. A galvanic nickel coating is applied to the surface of the steel part cleaned from oxides, after which the steel part is heat treated at a temperature of 700-710°C and the subsequent polishing of the nickel surface. The time to reach the specified temperature of heat treatment is 25-30 minutes, further exposure of the part is carried out for 40-45 minutes. Surface preparation of the aluminum part is carried out. The surfaces to be welded are heated, squeezed with a given force and held in this state until mutual diffusion between the metals to be welded is formed.

EFFECT: high strength of welded joints production.

4 cl, 2 dwg, 2 ex

Description

The invention relates to methods for producing permanent welded joints of products from dissimilar metal materials, in particular, to a method of diffusion welding of products from steel and aluminum.

Diffusion welding of materials in the solid state is a method of obtaining a monolithic joint formed as a result of the appearance of bonds at the atomic level, which appeared as a result of the convergence of contact surfaces due to local plastic deformation at an elevated temperature, which ensures mutual diffusion in the surface layers of the materials to be joined. Usually, this type of welding is carried out in a vacuum or an oxygen-free environment, as a result of which a monolithic joint is formed.

In diffusion welding, a welded joint is formed as a result of the combined effect of pressure and heat. A distinctive feature of diffusion welding is the use of relatively low heating temperatures and low specific pressures, usually not exceeding the yield strength of the materials being welded at the welding temperature. In diffusion welding, there are two main stages in the formation of a high-quality welded joint: the creation of physical contact, in which all points of the materials to be joined are at a distance of interatomic interaction from each other, and the formation of the structure of the welded joint under the influence of relaxation processes.

There is a method of diffusion welding of products made of dissimilar metals, including preparation of the surface of a steel part, applying a galvanic nickel coating to the cleaned surface of a steel part, preparing the surface of an aluminum part, heating the surfaces to be welded, pressing them with a given force at a temperature of 0.5 ÷ 0.7 of temperature melting aluminum and holding in this state until the formation of mutual diffusion between the metals being welded. (Diffusion welding of materials: Handbook / Edited by N.F. Kazakov. - M .: Mashinostroenie, 1981. - 271 p.) It is known that the interaction of aluminum with iron forms a number of intermetallic compounds: FeAl ₃ , Fe ₂ Al ₂ , Fe ₂ Al ₅ , FeAl ₂ , etc., which cause the fragility of the joint. Therefore, it is impossible to obtain a sufficiently strong connection of aluminum and its alloys directly with steel by diffusion welding. A strong connection of aluminum and its alloys with steels is achieved by using intermediate layers that can be applied to steel parts by electroplating or chemical methods, vacuum spraying, etc. slowing down the formation of brittle intermetallic compounds. Before diffusion welding, always prepare the surfaces of steel and aluminum parts.

The task of the proposed technical solution is to create an improved method of diffusion welding, as well as to expand the arsenal of methods for diffusion welding.

The technical result consists in ensuring the strength of the welded joints obtained by the proposed method.

To solve the problem and ensure the technical result, a method is proposed for diffusion welding of steel and aluminum products, including preparation of the surface of a steel part, applying a galvanic nickel coating to the cleaned surface of a steel part, preparing the surface of an aluminum part, heating the surfaces to be welded, squeezing them with a given force and holding for this state until the formation of mutual diffusion between the metals being welded. Moreover, after applying a galvanic nickel coating to the cleaned surface of the steel part, it is heat treated at a temperature of 700-710 ° C, followed by polishing the nickel surface. The time to reach the set temperature is 25-30 minutes, its further exposure is carried out for 40-45 minutes.

Surface preparation of the steel part may include acid pickling.

After heat treatment and polishing, the nickel surface can be degreased.

Surface preparation of an aluminum part can include degreasing, etching in alkali solution, rinsing with hot and cold water, and blowing with air.

Compression with a given force during welding is carried out at a temperature of 0.5 to 0.7 of the melting point of aluminum.

FIG. 1 shows a diagram of the connection of dissimilar metals at the initial stage (on the left is an aluminum part, on the right is a steel part).

FIG. 2 shows a diagram of the connection of metals at the final stage.

Surface preparation of the steel part includes acid etching to remove the oxide film. Then a nickel coating 2 with a thickness of 3-5 microns is galvanically applied to the cleaned surface (Fig. 1). The heat treatment carried out then in vacuum at a temperature of 700-710 ° C makes it possible to achieve diffusion bonding of nickel with a steel substrate and to compact the nickel due to the self-diffusion process.

After heat treatment, polishing is carried out on the nickel surface, using pastes, for example, "GOI", degreased and the part is placed in a closed room. Thorough cleaning and polishing of the surface before welding promotes intimate contact between the steel-aluminum surfaces during welding.

The aluminum part is degreased and etched in an alkali solution, then washed with hot and cold water and blown with high pressure air. In no more than three hours, the parts are assembled and sent for welding.

The welding process includes:

- convergence of parts (vacuum environment);

- parts heating and holding;

- creation of tight contact and pressure between the parts to be contacted, temperature 0.5-0.7 from the melting point of aluminum, holding;

- cooling under pressure.

It is known that free electrons 1 easily move inside the metal and cannot leave its surface due to the existing barrier. An electric field is such a barrier that keeps electrons near the metal surface. It is believed that in the surface layer of the metal, electrons are in a potential well and its overcoming is possible only with sufficient energy acting on them, or when the contacting surfaces come closer to a distance of at least 4 atomic ones. Due to the impossibility of going out from the metal surface, they form an electron cloud in it, which, on the one hand, pushes the electrons deep into the metal, and on the other hand, creates conditions for the formation of a near-surface ionic layer with an excess positive charge. Therefore, a double electric layer of ions and an electron cloud is formed near the metal surface.

Ensure the convergence of the metals to be joined during welding until their contact (to a level of less than 4 atomic distances, Fig. 1), and then they are heated, ensuring the mutual movement of electrons between the metals and near-surface ions of nickel with aluminum. The joining of the surfaces leads to the formation of a common structure between the metals (Fig. 2).

The splicing of nickel with steel, in addition to the molecular splicing during the deposition of a nickel coating, is carried out due to the volumetric diffusion of chemical elements of steel into the coating. Chromium and iron are such elements. Along with them, traces of manganese and silicon are noted in the nickel coating. The penetration of these elements into the coating is caused, as is known, by their low affinity for nickel.In addition, their diffusion into nickel is facilitated by:

- temperature of heat treatment (700-710 ° С) and long exposure at it for 40-45 minutes;

-molecular bond of nickel with steel, achieved during coating;

- a certain porosity of the coating;

It is known that there are always voids in the structure of metals. Their volume in the structure of body-face-centered (BCC) and face-centered (FCC) steel is 32 and 26%, respectively. In stainless steel, as you know, its structure is mainly fcc interspersed with bcc - residual, which after hardening of steel, no more than 5-7%. The formation of voids is mainly based on octahedral and tetrahedral pores in the structure. And the pores, as you know, due to surface tension draw in the above-mentioned elements, contributing, together with their diffusion, to the formation of a complex chemical composition in the coating.

Examples of implementation of the method.

Example 1.

After turning, rods with a diameter of 20 mm made of 12Kh18N10T steel were electroplated with nickel without removing the oxide film from the steel surface. Then they were electroplated with nickel. The nickel thickness was 4 µm. Then, the bar with nickel was heat treated at a temperature of 700 ° C. The heating time was 30 minutes. The holding time of the bar at this temperature is 45 minutes. Heating and cooling of the rod was carried out in a vacuum of 5.0 × 10 ^-4 mm. Hg Nickel was clarified by cleaning with a soft CB wheel using an abrasive paste. After that, the surface of the nickel coating was wiped with a solvent - spray 3M. Further blowing of the bar with compressed air provided the required purity of the nickel coating.

Preparation of an aluminum rod AD1 with a diameter of 20 mm was carried out as follows. At the first stage, the oxide film was removed from the sample using AF-4A flux containing 28% sodium chloride, 50% potassium chloride, 14% lithium chloride and 8% sodium fluoride. Then, the steel + AD1 rods were assembled and placed in a diffusion welding installation. The samples to be welded were heated using a TESLINE 40T-9E2 high-frequency frequency converter in a vacuum. The welding temperature was 490 ° C. The temperature was measured with a potentiometer using a chromel-alumel thermocouple placed near the welding site. The axial pressure on the samples was 0.9 of the yield point of the AD1 alloy. The heating time of the samples to the welding temperature was 20 minutes, and when the temperature reached 490 ° C, holding was carried out for 2 minutes at an axial pressure of 0.9 from the yield point of the AD1 alloy. The welded sample was cooled in the volume of the installation.

Example 2

Bars with a diameter of 20 mm made of steel 12X18H10T after turning were subjected to removal of the oxide film by chemical etching in an aqueous solution of 4% nitric acid (density 1.35), 36% hydrochloric acid (density 1.19). The temperature of the solution was 40 ° C, the duration of the etching was 4 min. Then they were electroplated with nickel. The nickel thickness was 4 µm. Then, the bar with nickel was heat treated at a temperature of 700 ° C. The heating time was 30 minutes. The holding time of the bar at T = 700 ° C was 45 minutes. Heating and cooling of the rod was carried out in a vacuum of 5.0 × 10 ^-4 mm. Hg Nickel was clarified by cleaning with a soft CB wheel using an abrasive paste. After that, the surface of the nickel coating was wiped with a solvent - spray 3M. Further blowing of the bar with compressed air provided the required purity of the nickel coating.

Preparation of an aluminum rod AD1 with a diameter of 20 mm was carried out as follows. At the first stage, the oxide film was removed from the sample using AF-4A flux containing 28% sodium chloride, 50% potassium chloride, 14% lithium chloride and 8% sodium fluoride. Then, the steel + AD1 rods were assembled and placed in a diffusion welding installation. The samples to be welded were heated using a TESLINE 40T-9E2 high-frequency frequency converter in a vacuum. The welding temperature was 490 ° C. The temperature was measured with a potentiometer using a chromel-alumel thermocouple placed near the welding site. The axial pressure on the samples was 0.9 of the yield point of the AD1 alloy. The heating time of the samples to the welding temperature was 20 minutes, and when the temperature reached 490 ° C, holding was carried out for 2 minutes at an axial pressure of 0.9 from the yield point of the AD1 alloy. The welded sample was cooled in the volume of the installation.

In the process of loading the welded specimen, example 1, during mechanical tests, its destruction occurred at a load of no more than 10% of the strength of the AD1 alloy. The results of metallographic analysis showed that the rupture of the samples occurred at the steel - nickel coating interface. The presence of iron oxide FeO was found under the coating. Whereas in the test of the sample, example 2, with the oxide film removed from the steel before applying a nickel coating to its surface, the strength before fracture was 0.95 of the strength of the AD1 alloy. The weld is tight and there are traces of nickel diffusion into the steel and simultaneously into the AD1 alloy.

The proposed method is carried out in an installation for thermal diffusion welding, containing a vacuum chamber in which the parts to be welded are placed, a heating system with the energy of a high-frequency field or a glow discharge or in another way. A vacuum is created in the chamber using a vacuum system. The necessary pressure in the contact area of the parts is created by a compression system, for example, a hydraulic one.

Advantages of the proposed welding method.

• precision processing precision;

• solidity of the connection;

• the ability to connect materials that are not connected in any other way (obtaining complex composites);

• the ability to create products of complex design and configuration;

• low energy consumption;

• no need to use additional elements, such as additives, electrodes, solders and fluxes.

• environmental friendliness of the welding process - no splashes of melt, harmful radiation and gas emissions, no emission of ultraviolet radiation;

• very high bond strength;

• convenience of creating hollow structures, including composite ones;

• cost-effective - several dissimilar metals can be joined in one operation

Claims (5)

1. A method of diffusion welding of steel and aluminum products, including preparation of the surface of a steel part, applying a galvanic nickel coating to the cleaned surface of a steel part, preparing the surface of an aluminum part, heating the surfaces to be welded, pressing them with a given force and holding in this state until mutual diffusion is formed between the metals being welded, characterized in that after applying a galvanic nickel coating to the cleaned surface of the steel part, it is heat treated at a temperature of 700-710 ° C, followed by polishing the nickel surface, and the time to reach the specified temperature is 25-30 minutes with further holding it for 40-45 minutes. 2. The method according to claim 1, characterized in that the surface preparation of the steel part includes acid etching. 3. The method according to claim 1, characterized in that after heat treatment of the nickel-plated steel part and polishing, its surface is degreased. 4. The method according to claim 1, characterized in that the preparation of the surface of the aluminum part includes degreasing, etching in an alkali solution, rinsing with hot and cold water, and blowing with air. 5. The method according to claim 1, characterized in that the squeezing with a given force is carried out at a temperature of 0.5 ÷ 0.7 of the melting point of aluminum.

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