Aluminum steel transition section structure based on self-adaptive gap control and vacuum brazing method thereof

Abstract

The invention discloses an aluminum steel transition section structure based on self-adaptive gap control and a vacuum brazing method of the aluminum steel transition section structure. The invention provides a solution for the problem from two aspects of structure and process. A self-adaptive gap control structure is designed structurally; a vacuum brazing mode is put forward in the process, the surface of a steel pipe section is plated with a transition layer before welding, and a three-step gradient temperature control heating mode is put forward in a welding process curve. According to the invention, a conventional soldering flux brazing mode for aluminum steel dissimilar material brazing in the industry is abandoned, the problem that a product cannot be stored for a long time due to corrosion of soldering flux to a welded joint is avoided, and efficient and reliable connection of the aluminum steel pipeline joint is realized through a mode of combining structural design with process optimization.

Description

Technical Field

[0001] This invention relates to the technical field of aluminum-steel dissimilar metal welding, and in particular to an aluminum-steel transition section structure based on adaptive gap control and its vacuum brazing method. Background Technology

[0002] In recent years, with the rapid development of the materials engineering industry, the demand for dissimilar materials has been increasing. Aluminum alloys, as a lightweight and high-strength alloy, are widely used in rocket manufacturing, especially in liquid fuel tanks. Stainless steel, due to its excellent high-temperature resistance, corrosion resistance, and high strength, is one of the commonly used alternative materials for engines. This has created a large demand for the manufacture of transition sections between aluminum and steel pipelines in rocket internal piping.

[0003] Aluminum alloys and stainless steel differ significantly in their crystal structure, microstructure, and properties. Both have low solubility, leading to the formation of a large amount of brittle Fe-Al intermediate phases in the weld joint, severely impacting its strength and toughness. The main challenge in aluminum-steel fusion welding lies in the formation of numerous brittle and hard intermediate phases at the aluminum / steel interface. Furthermore, the difference in their coefficients of thermal expansion results in substantial residual stress in the pipe joint, affecting the connection quality.

[0004] Brazing relies on adding an intermediate layer (filler metal) to achieve welding. During welding, the base material does not melt, while the filler metal melts and its liquid phase wets and fills the weld gap, reacting with the base material. After cooling and solidification, a brazed joint is formed. Because the base material does not melt, the formation of a large amount of brittle mesophase is avoided, making it a commonly used method for welding dissimilar materials. However, in existing brazing processes, flux easily remains inside the brazed joint, forming obvious flux residue defects that cannot be removed. If halogen flux is used, it can lead to significant joint corrosion during subsequent storage, affecting the storage period. Therefore, there is an urgent need to develop an aluminum-steel joint manufacturing process that can achieve long-term storage, low corrosion, dense structure, and meet the practical connection requirements, in order to meet the manufacturing needs of a large number of pipeline conversions within the internal pressurization and delivery system of rockets. Summary of the Invention

[0005] This invention provides an aluminum-steel transition section structure based on adaptive gap control and its vacuum brazing method. This invention overcomes the corrosion problem at the transition section caused by flux brazing of dissimilar aluminum and steel materials. Through a welded mating structure, and relying on a flux-free vacuum brazing process, the connection of dissimilar aluminum-steel transition sections is achieved, resulting in a manufacturing solution for a highly dense and low-corrosion aluminum-steel dissimilar material transition section.

[0006] Firstly, an aluminum-steel transition section structure based on adaptive gap control is provided, including an aluminum pipe and a steel pipe, with the aluminum pipe designed on the outside and the steel pipe placed on the inside; during welding, the brazing filler melts and enters the mating surface of the aluminum pipe and the steel pipe to form an aluminum / foil brazing filler / steel sandwich structure.

[0007] In conjunction with the first aspect, in some implementations of the first aspect, the inner bore of the aluminum pipe is designed as a conical bore with a taper of θ = 1.5° to 2.5°, and the outer wall of the steel pipe is designed as a conical surface with the same taper θ; the height h of the conical surface of the stainless steel outer wall is... sus = Height h of the inner conical surface of the aluminum alloy wall Al .

[0008] In conjunction with the first aspect, in some implementations of the first aspect, the maximum diameter D of the stainless steel outer wall conical surface is... sus satisfy:

[0009] D sus =(1+α) Al ·T max )·D Al / (1+α sus ·T max )

[0010] α Al T is the coefficient of linear expansion of aluminum alloy. max D represents the highest temperature of the workpiece. Al α is the maximum diameter of the aluminum alloy conical surface. sus is the coefficient of linear expansion of stainless steel.

[0011] In conjunction with the first aspect, in some implementations of the first aspect, before welding, the height difference between the aluminum pipe and the steel pipe is Δh = (D sus -D Al ) / tan(θ / 2); During the welding process, a load is applied to the upper part of the steel pipe so that the height difference Δh gradually decreases as the welding temperature increases, and the welding gap between the aluminum pipe and the steel pipe remains consistent.

[0012] In conjunction with the first aspect, in some implementations of the first aspect, a metal layer is plated on the outer surface of the steel pipe before welding. The plating method is to first plate copper and then nickel, and the total plating thickness is controlled at 10 to 15 μm.

[0013] In conjunction with the first aspect, in some implementations of the first aspect, a brazing filler groove is provided above the aluminum tube for placing wire-shaped brazing filler.

[0014] In conjunction with the first aspect, in some implementations of the first aspect, the aluminum pipe has a step machined inside, which is used to limit the downward displacement of the steel pipe.

[0015] In conjunction with the first aspect, in some implementations of the first aspect, the step is provided with a flow-blocking groove to accommodate excess molten solder.

[0016] Secondly, a vacuum brazing method for an aluminum-steel transition section structure as described in any of the implementations of the first aspect above is provided, comprising:

[0017] 1) Processing to form steel pipes and aluminum pipes;

[0018] 2) Place the foil-shaped brazing filler metal in the area to be welded, and insert the steel pipe into the aluminum pipe from above;

[0019] 3) Place the product into the vacuum furnace, close the furnace door, start the vacuum pump to evacuate the vacuum, set the welding process curve, and perform vacuum brazing.

[0020] 4) After welding, the parts are precision machined to remove excess material from the inner and outer walls of the product.

[0021] In conjunction with the second aspect, in some implementations of the second aspect, the welding process curve includes a low-temperature holding stage, a high-temperature holding stage, and a welding stage;

[0022] The low-temperature holding stage must meet the following requirements: temperature 350–450℃, holding time 1–1.5h, and heating rate 4–6℃ / min before the low-temperature holding stage.

[0023] The high-temperature holding stage meets the following requirements: temperature is 520-570℃, holding time is 1-1.5h, and the heating rate between the low-temperature holding stage and the high-temperature holding stage is 8-12℃ / min.

[0024] The welding stage must meet the following requirements: temperature 610–630℃, holding time 20–30 min, heating rate 8–12℃ / min between the high-temperature holding stage and the welding stage, and cooling with the furnace after the welding stage.

[0025] Compared with the prior art, the solution provided by the present invention has at least the following beneficial technical effects:

[0026] This invention addresses the connection requirements of numerous aluminum / steel dissimilar metal pipeline transition sections in rocket internal pressurization and delivery pipelines. It designs a transition section welding structure based on an adaptive gap control method and proposes a vacuum brazing process for joint connection, resulting in an aluminum-steel dissimilar metal pipeline transition joint with high connection reliability and low corrosion performance. This invention belongs to the field of aluminum-steel dissimilar metal material connection. Due to the significant differences in composition, microstructure, and properties between aluminum and steel, the welding reliability of aluminum-steel dissimilar materials is poor. Therefore, this invention proposes solutions to this problem from both structural and process perspectives. Structurally, an adaptive gap control structure is designed; process-wise, a vacuum brazing method is proposed, with a transition layer coating applied to the steel pipe section before welding. A three-step gradient temperature control heating method is proposed for the welding process curve. This invention abandons the flux brazing method commonly used in the industry for aluminum-steel dissimilar material brazing, avoiding the problem of flux corrosion of the welded joint and the resulting inability to store the product for long periods. Through structural design combined with process optimization, it achieves efficient and reliable connection of aluminum-steel pipeline joints. Attached Figure Description

[0027] Figure 1 This is a schematic diagram of the transition section structure for aluminum / steel dissimilar metal pipelines.

[0028] Figure 2 A schematic diagram of the aluminum-steel transition interface design.

[0029] Figure 3 This is a schematic diagram of the welding process curve. Detailed Implementation

[0030] The present invention will now be described in further detail with reference to the accompanying drawings and specific embodiments.

[0031] This invention comprises three aspects: the structural design of the aluminum-steel dissimilar metal transition section, the pre-welding plating pretreatment, and the vacuum brazing process.

[0032] This invention provides an aluminum-steel transition section structure based on adaptive gap control. In this transition section structure, the aluminum pipes are designed on the outside, and the steel pipes are placed on the inside. The principle is based on the principle of thermal expansion and contraction; the linear expansion coefficient of aluminum is approximately 23.5 × 10⁻⁶. -6 k -1 The coefficient of thermal expansion is greater than that of stainless steel (approximately 18 × 10¹⁰). -6 k -1 During welding, after the liquid brazing filler metal fills the gap and cools and solidifies, the outer aluminum shrinks more than the stainless steel. This causes the weld mating surface to bear compressive stress, reducing the risk of tensile stress failure at the weld and improving the stress distribution of the joint. A brazing filler metal groove can be installed above the aluminum pipe to facilitate the placement of wire-shaped brazing filler metal. During welding, the brazing filler metal in the groove melts and enters the mating surface between the aluminum and steel pipes.

[0033] In the transition section structure, the mating surfaces of the aluminum and steel pipes form small-angle conical surfaces, thus creating a small-angle conical sidewall structure. Specifically, the inner bore of the aluminum pipe is designed as a conical hole with a small angle θ (θ≈2°), and the outer wall of the steel pipe is designed as a conical surface with the same taper θ. The height h of the stainless steel outer wall conical surface is... sus = Height h of the inner conical surface of the aluminum alloy wall Al .

[0034] Maximum diameter D of stainless steel outer wall conical surface sus Based on the maximum diameter D of the aluminum alloy cone surface Al The coefficient of linear expansion of aluminum alloys is α Al The coefficient of linear expansion of stainless steel is α sus Maximum workpiece temperature T max Data such as taper θ are used to calculate the maximum diameter D of the stainless steel outer wall tapered surface. sus =(1+α) Al ·T max )·D Al / (1+α sus ·T max ); where D sus With D Al With data close together, dimensional inspection during processing becomes more difficult. Therefore, based on the small-angle conical surface of the sidewall, it is proposed to measure the height difference Δh between the bottom of the aluminum and steel after machining and fitting. sus -D Al The tan(θ / 2) is used to control the fit clearance, so as to achieve adaptive clearance control during the welding process.

[0035] This design places the bottom of the aluminum pipe on the furnace platform and applies a load to the upper part of the steel pipe. During brazing, the circumferential expansion of the aluminum pipe is greater than that of the steel pipe. Under the load, the steel pipe moves downward along the conical surface, ensuring that the welding gap remains consistent across the entire welded sidewall as the welding temperature increases (i.e., the height difference Δh gradually decreases with increasing welding temperature), achieving an adaptive adjustment. Simultaneously, the conical surface mating of the aluminum and steel pipes provides axial guidance, making it easier to ensure coaxiality and parallelism, and ensuring uniform gap across the entire circumferential mating surface. Furthermore, the conical mating method facilitates the insertion of foil-shaped brazing filler metal, allowing the entire brazing surface to be pre-filled with filler metal before welding, forming an aluminum / foil-shaped brazing filler metal / steel sandwich structure. This reduces the risk of localized incomplete brazing, such as "large encirclement" and "small encirclement," resulting from relying solely on long-distance capillary filling after wire melting, thus improving the brazing success rate.

[0036] Furthermore, steps can be machined inside the aluminum tubing to restrict the downward displacement of the steel tubing. These steps can be fitted with flow-blocking grooves to create a bottom-locking structure within the aluminum tubing. This ensures that once the aluminum alloy softens at welding temperatures, the steel tubing is restrained from further downward movement along the conical surface. Simultaneously, the presence of both the steps and flow-blocking grooves prevents excessive solder flow during welding, avoiding significant solder loss and sidewall erosion.

[0037] This invention also employs a pre-treatment process involving plating before welding. Before welding, a metal layer is plated onto the steel surface. The plating method involves first plating copper and then nickel, with the total plating thickness controlled at 10–15 μm. The purpose of this is to improve the wettability of the molten brazing filler metal on the surface of the steel pipe, ensuring sufficient spreading and filling of the brazing filler metal, while simultaneously preventing Al-Fe elements from directly contacting and forming brittle microstructures.

[0038] This invention employs vacuum brazing, with a vacuum protective atmosphere, eliminating the need for non-metallic flux. The welding process curve proposes a three-step gradient temperature control heating method involving low temperature, high temperature, and welding temperature, such as... Figure 3 As shown.

[0039] The cryogenic holding stage is the vacuum environment preparation stage, with a temperature of 350–450℃ and a holding time of 1–1.5 hours. The heating rate before the cryogenic holding stage is 4–6℃ / min. The purpose of the cryogenic holding stage is to provide a high vacuum atmosphere for the subsequent welding process.

[0040] The high-temperature holding stage is the temperature homogenization preparation stage, with a temperature of 520–570℃ and a holding time of 1–1.5 hours. The heating rate between the low-temperature holding stage and the high-temperature holding stage is 8–12℃ / min. The purpose of the high-temperature holding stage is to ensure that the temperature inside and outside the parts in the furnace is basically the same, avoiding uneven temperature in different areas of the parts.

[0041] During the welding stage, the temperature is 610–630℃, the holding time is 20–30 min, and the heating rate between the high-temperature holding stage and the welding stage is 8–12℃ / min. After the welding stage, the furnace is cooled. The rapid heating rate and strict holding time during the welding stage ensure that the brazing filler metal melts, fills, and reacts fully, while suppressing the formation and dissolution of brittle phases at the steel-aluminum interface. This staged gradient temperature control heating curve enables precise control of the welding process.

[0042] Example 1:

[0043] The present invention relates to a welding manufacturing process for transition sections between dissimilar metal pipelines made of 6A02 aluminum alloy and 316L stainless steel. The specific steps are as follows:

[0044] 1) According to Figure 1 and Figure 2The stainless steel and aluminum alloy are pre-welded in the manner shown. The outer wall of the stainless steel is machined with a small-angle conical surface with a height of 40mm. The inner wall of the aluminum alloy is machined with a small-angle conical surface with a height of 40mm on the corresponding mating surface. The bottom of the aluminum alloy is machined with a locking structure, and the upper part is machined with a brazing filler groove.

[0045] 2) Electroplating treatment is performed on the surface of the stainless steel area to be welded. Copper is electroplated on the stainless steel first, followed by pure nickel plating. The total plating thickness is controlled at 10-15 μm.

[0046] 3) Place the aluminum alloy foil brazing filler metal on the conical surface of the aluminum alloy sidewall to be welded, and then place the stainless steel ring from... Figure 1 An aluminum alloy core is inserted at the top, and then wire-shaped brazing filler metal is placed in the brazing filler groove.

[0047] 4) Place the product into the vacuum furnace, close the furnace door, start the vacuum pump to create a vacuum, set the welding process curve, and perform vacuum brazing. The welding curve adopts... Figure 3 A segmented temperature control heating method is adopted.

[0048] 5) After welding, the parts are precision machined to remove the solder groove, the bottom flow barrier groove and the excess material on the inner and outer walls of the product. The machining amount on the inner and outer walls of the product is controlled to be ≤0.5mm.

[0049] Although the present invention has been disclosed above with reference to preferred embodiments, it is not intended to limit the present invention. Any person skilled in the art can make possible changes and modifications without departing from the spirit and scope of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope defined in the claims of the present invention.

Claims

1. An aluminum-steel transition section structure based on adaptive gap control, characterized in that, It includes aluminum pipes and steel pipes, with the aluminum pipes designed on the outside and the steel pipes on the inside; during welding, the brazing filler melts and enters the mating surface of the aluminum pipes and steel pipes, forming an aluminum / foil brazing filler / steel sandwich structure.

2. The aluminum-steel transition section structure according to claim 1, characterized in that, The inner bore of the aluminum pipe is designed as a conical bore with a taper of θ = 1.5° to 2.5°, and the outer wall of the steel pipe is designed as a conical surface with the same taper θ; the height h of the conical surface on the outer wall of the stainless steel pipe is... sus = Height h of the inner conical surface of the aluminum alloy wall Al .

3. The aluminum-steel transition section structure according to claim 2, characterized in that, Maximum diameter D of stainless steel outer wall conical surface sus satisfy: D sus =(1+a Al ·T max )·D Al / (1+a sus ·T max ) α Al T is the coefficient of linear expansion of aluminum alloy. max D represents the highest temperature of the workpiece. Al α is the maximum diameter of the aluminum alloy conical surface. sus is the coefficient of linear expansion of stainless steel.

4. The aluminum-steel transition section structure according to claim 3, characterized in that, Before welding, the height difference between the aluminum pipe and the steel pipe is Δh = (D sus -D Al ) / tan(θ / 2); During the welding process, a load is applied to the upper part of the steel pipe so that the height difference Δh gradually decreases as the welding temperature increases, and the welding gap between the aluminum pipe and the steel pipe remains consistent.

5. The aluminum-steel transition section structure according to claim 1, characterized in that, Before welding, a metal layer is plated on the outer surface of the steel pipe. The plating method is to first plate copper and then nickel, and the total plating thickness is controlled between 10 and 15 μm.

6. The aluminum-steel transition section structure according to claim 1, characterized in that, A brazing filler groove is provided above the aluminum tube to hold wire-shaped brazing filler material.

7. The aluminum-steel transition section structure according to claim 1, characterized in that, The aluminum tubing has internal steps that limit the downward displacement of the steel tubing.

8. The aluminum-steel transition section structure according to claim 7, characterized in that, The step is equipped with a flow-blocking groove to accommodate excess molten solder.

9. A vacuum brazing method for an aluminum-steel transition section structure as described in any one of claims 1 to 8, characterized in that, include: 1) Processing to form steel pipes and aluminum pipes; 2) Place the foil-shaped brazing filler metal in the area to be welded, and insert the steel pipe into the aluminum pipe from above; 3) Place the product into the vacuum furnace, close the furnace door, start the vacuum pump to evacuate the vacuum, set the welding process curve, and perform vacuum brazing. 4) After welding, the parts are precision machined to remove excess material from the inner and outer walls of the product.

10. The vacuum brazing method according to claim 9, characterized in that, The welding process profile includes a low-temperature holding stage, a high-temperature holding stage, and a welding stage; The low-temperature holding stage must meet the following requirements: temperature 350–450℃, holding time 1–1.5h, and heating rate 4–6℃ / min before the low-temperature holding stage. The high-temperature holding stage meets the following requirements: temperature is 520-570℃, holding time is 1-1.5h, and the heating rate between the low-temperature holding stage and the high-temperature holding stage is 8-12℃ / min. The welding stage must meet the following requirements: temperature 610–630℃, holding time 20–30 min, heating rate 8–12℃ / min between the high-temperature holding stage and the welding stage, and cooling with the furnace after the welding stage.

Images