CN113369668B

CN113369668B - Stirring friction brazing preparation method for composite pipe/pipe bank and composite board

Info

Publication number: CN113369668B
Application number: CN202110560980.0A
Authority: CN
Inventors: 张贵锋; 刘旭; 杨小辉; 石盛; 范蒙
Original assignee: Xian Jiaotong University
Current assignee: Xian Jiaotong University
Priority date: 2021-05-20
Filing date: 2021-05-20
Publication date: 2022-08-09
Anticipated expiration: 2041-05-20
Also published as: CN113369668A

Abstract

The invention discloses a stirring friction brazing preparation method of a composite pipe/calandria and a composite board, which comprises the following steps: a square tube is used as an inner tube, and two groove-shaped preformed pieces are used as materials for forming an outer tube; solder is preset between the inner pipe and the outer pipe; welding a tank bottom plane/finished square tube plane lapping interface by stirring friction brazing of a needle-free tool, and welding a slot wing side plane/finished square tube plane lapping interface and a slot wing/slot wing butt interface by stirring friction brazing of a needle-containing tool, so that a composite square tube is obtained by welding a composite plate on the side surface of the square tube; and (3) welding a plurality of composite square pipes by friction stir brazing or argon arc welding to obtain the composite calandria. The invention utilizes the aluminothermic reaction induced by stirring friction to improve the welding temperature of the lap joint interface, ensures that the Si-containing aluminum-based brazing filler metal is smoothly melted, and simultaneously realizes the film removal by a mechanical and metallurgical double mechanism, inhibits the excessive thickening of intermetallic compounds and inhibits the thermal stress.

Description

Stirring friction brazing preparation method for composite pipe/pipe bank and composite board

Technical Field

The invention relates to application of friction stir brazing, in particular to a friction stir brazing preparation method of a composite pipe/pipe bank and a composite board.

Background

The Friction Stir Brazing (FSB) technology is to solve the problems of narrow Welding width (only the diameter of a stirring pin), hook-shaped defects at the interface of a stirring zone (the original surface of a lower plate easily vertically extends into an upper plate base material or horizontally extends into a stirring core zone to cause the effective bearing thickness of the upper plate to be reduced), difficulty in vertical mixing inside the stirring zone, key hole and tool abrasion and the like existing in Friction Stir Lap Welding (FSLW), and the Brazing technology (Zhang Guifen, et Al. Friction Stir layer composite and material dense joint of Al to Steel. metallic and Materials transformations A,2011,42 (289): 2850). The technical points of the FSB are as follows: 1) a needle-free tool is adopted; 2) and solder capable of obviously dissolving the base material is preset. Wherein, the purpose of adopting the 'needleless' tool is to eliminate the keyhole, avoid using a guide plate, eliminate the hook defect at the periphery of the stirring area, and eliminate the malignant abrasion and broken needle of the tip of the stirring needle by a hard lower plate; the purpose of introducing the brazing filler metal capable of generating eutectic reaction with the base metal is to make up for the loss of mechanical film removing capability of the stirring pin caused by a needleless tool, namely, the surface layer of the base metal is liquefied through the eutectic reaction between the base metal and the brazing filler metal or the obvious dissolution of the brazing filler metal to the base metal, and then the eutectic liquid phase is extruded by the extrusion action of the shaft shoulder to bring out an oxide film, so that the interface film removal is realized by replacing plastic flow, and the welding amplitude is greatly expanded to the diameter of the shaft shoulder, thereby solving the problems of difficult up/down mixing and narrow welding amplitude in a stirring area in friction stir lap welding. Compared with the traditional furnace brazing, the FSB has strong mechanical film removing capacity, can improve the wettability and can be welded in the atmospheric environment.

The FSB inherits the advantages of FSW (surface friction heat) as a heat source and the mechanical action (extrusion and torsion) of a shaft shoulder, and particularly can strengthen the interface metallurgical reaction and the film removal through the following mechanisms by means of the mechanical action of a rotating shaft shoulder: (1) the mechanical torsion membrane rupture establishes a plurality of microchannels for eutectic reaction; (2) the eutectic reaction and the eutectic liquid phase flow can be strengthened by mechanical torsion, and the oxide film is driven to move away from the original position; (3) the eutectic liquid phase is extruded at the later stage of the shaft shoulder to take away oxide film scraps. Therefore, the film removing effect of the FSB interface is superior to that of FSLW (FSW only depends on plastic flow singly, although an oxide film can be broken, the interface is difficult to remove), the film removing range can be greatly widened to be close to the diameter of a shaft shoulder, and even if welding is carried out in an atmospheric environment (protective gas and brazing flux can be avoided), excellent and wide-range interface wettability can be ensured. The diameter range of the FSB common stirring tool is 30 +/-10 mm; if the power of the spindle motor and the power of the feeding motor are allowed to adopt the diameter of 60 +/-20 mm, the efficiency is further improved, and the torsional stripping is strengthened.

The traditional explosion cladding technology is not suitable for cladding thin parts and narrow-width small parts due to the reasons of explosion impact force, work hardening, edge effect and the like, and is particularly not suitable for preparing a composite pipe with smaller size; the rolling compounding requires heating and is accompanied by large plastic deformation, and is not suitable for the preparation of hollow composite pipes (the pressure bearing capacity of hollow pipes is limited).

For a circular tube, the arc-shaped surface to be welded (non-planar) makes heating and pressurizing very difficult when FSB is applied to composite tube preparation, and the friction heating area is too small, so that the brazing filler metal is difficult to melt. Theoretically, the plane of the shaft shoulder is in line contact with the arc-shaped outer surface of the circular tube, only the contact line is possibly heated, but other parts are still in a cold state and quickly dissipate heat, so that the composite tube taking the circular tube as the base material is difficult to prepare by the FSB method. For the square tube, because four sides of the square tube are flat, the square tube can theoretically form large-area contact friction with the shaft shoulder, so that enough friction heat is generated to melt the brazing filler metal. At present, narrow, small and thin Al/X (X ═ Al, Fe, Cu, Ti and SUS) composite plates or composite tubes can be prepared by utilizing a plurality of reciprocating FSBs, and the composite plates or composite tubes have the advantages of convenience, flexibility, energy conservation and environmental protection. However, due to the limitation of the upper limit of the heating temperature of the friction stir welding, the FSB generally adopts brazing filler metal such as Zn powder, Zn foil and the like, and the reasons are as follows: firstly, the melting point of Zn is low (420 ℃), and the Zn can perform eutectic reaction with Al at 381 ℃, so that the Zn is easy to melt by using friction heat, has stronger dissolving capacity on an Al base material, and is beneficial to breaking an oxide film on the surface of the solid Al base material; secondly, Zn and Al do not form brittle intermetallic compounds.

However, from the perspective of practical application and requirements, it still belongs to the technical problem to prepare the aluminum-clad steel composite pipe (for example, the thickness of the aluminum layer is 1-3 mm, and the thickness of the steel layer is more than 6mm) applied in the fields of permanent corrosion prevention and the like. In the stirring friction brazing (FSB) composite process flow adopting soft solder (the melting point is lower than 450 ℃) such as Zn foil and the like, the problem that the prepared aluminum/steel composite square pipe is cracked automatically at a welding interface after being welded exists. The reasons are related to poor interface film removal (secondary cause), excessive thickness of brittle intermetallic compound (IMC) (first main cause 1), large steel pipe restraint (second main cause), thermal stress, and the like. Due to the inherent contradiction between torsional stripping and heat input, a firm joint which does not crack after welding cannot be obtained only by optimizing the standard parameters, and a new breakthrough path must be searched.

Disclosure of Invention

Aiming at the technical problems, the invention provides a stirring friction brazing preparation method of a composite pipe/pipe array and a composite plate, which has the comprehensive advantages of thin interface IMC and low interface thermal stress. The idea of improving the welding temperature of a lap joint interface by using the heat release of the 'stirring friction induced aluminum-induced thermal reaction (fractional-induced thermal reaction) between the Al-Si brazing filler metal and the surface oxide film of the stainless steel powder' is utilized to ensure that the brazing filler metal is smoothly melted and simultaneously improve the film removal of the high-strength substrate (scraping of the stainless steel powder is utilized; the heat release of the aluminum thermal reaction is utilized to soften the high-strength substrate and reduce the deformation resistance of the high-strength substrate); and the excessive thickening of IMC (Si element is used), and the CTE mismatch and the thermal stress of the coating and the base pipe are regulated and controlled (stainless steel metal powder is used) in the preparation process of the composite square pipe.

In order to achieve the purpose, the invention adopts the following technical scheme:

the invention optimizes the design of the FSB brazing filler metal from the perspective of inhibiting the excessive thickening of intermetallic compounds (IMC) of a welding interface and the thermal stress caused by mismatching of Coefficient of Thermal Expansion (CTE), and provides a two-component or multi-component (more than three components) composite brazing filler metal:

(1) si-containing aluminum-based brazing filler metal is used as a carrier, and a non-metallic Si element is introduced into the interface layer, so that excessive thickening of an Fe-Al binary intermetallic compound (IMC) of the interface is inhibited.

(2) In order to ensure that the Si-containing aluminum-based brazing filler metal can be smoothly melted by friction stir welding (the peak temperature of friction stir welding is about 500 ℃ C., and the friction temperature is hard to rise again due to the reduction in the friction heating pressure after yielding of the aluminum base material as the coating layer), metal particles (for example, stainless steel powder) are added, and an oxide film (for example, an oxide film whose component is Cr) is dense and stable on the surface of the metal particles is used ₂ O ₃ 、Fe ₂ O ₃ Etc.) and Si-containing aluminum-based brazing filler metal, as an auxiliary heat source other than frictional heat to melt Si-containing aluminum-based brazing filler metal having a high melting point (the brazing filler metal having a melting point of 450 ℃ or higher is a brazing filler metal), thereby constituting a technical scheme of "friction stir brazing".

For the aluminum-clad steel composite square tube, in order to inhibit the excessive growth of an Al-Fe binary intermetallic compound (IMC), one or more of Si-containing aluminum-based brazing filler metals Al-Si, Al-Si-Cu, Al-Si-Mg and Al-Si-Zn are used as carriers to introduce a non-metallic element Si; further, in order to make up for the defect that the upper limit temperature of friction stir welding is difficult to melt the Si-containing aluminum-based brazing filler metal due to the fact that the aluminum base metal is softened and yielded and the interface temperature is lower than the Al-Si eutectic temperature (577 ℃), the used brazing filler metal is a composite brazing filler metal, namely a mixture consisting of the following components (i) and (ii) or a mixture consisting of the following components (i), (ii) and (iii); and the heat released by the aluminothermic reaction of the Si-containing aluminum-based brazing filler metal induced by the stirring friction of the rotating shaft shoulder and the stainless steel powder surface oxide film is used as an auxiliary heat source, so that the interface temperature is improved, and the Si-containing aluminum-based brazing filler metal with higher melting point is ensured to be smoothly melted:

the method comprises the following steps of firstly, providing metal powder (an oxide source for carrying out aluminothermic reaction with aluminum in the aluminum-based brazing filler metal by utilizing a dense oxide film on the surface of the metal powder) such as stainless steel powder and nickel powder, wherein the particle size of the metal powder is 10-500 microns.

② Al-Si binary or Al-Si-X (X ═ Cu, Mg, Zn) ternary hard solder powder or foil.

③ Zn powder or Zn foil.

Compounding hard brazing filler metal: the mass fraction of the first component is 5-50%, and the mass fraction of the second component is 50-95%.

Compounding hard brazing filler metal: the mass fraction of the component I is 5-40%, the mass fraction of the component II is 35-85%, and the mass fraction of the component III is 10-30%.

In the composite hard solder, the hard solder of the component II comprises 12 to 50 mass percent of Si, and the balance of Al/X is not limited.

In the composite brazing filler metal, the metal powder specifically has the following functions (taking stainless steel powder as an example):

on the other hand, a dense "oxide film" on the surface of stainless steel powder is used.

The peak temperature of Friction Stir Welding (FSW) of aluminum is generally about 500 ℃ (edited by the society of fusion and welding of japan: friction stir welding, published by japan, january 2006, P76), and a high temperature as high as 600 ℃ cannot be obtained, so that an aluminum-based brazing filler metal containing Si (for example, an Al-12Si eutectic brazing filler metal) cannot be melted (the theoretical melting point of Al-12Si is 557 ℃, which is a brazing filler metal, but it requires 600 ℃ or more to be melted because actual brazing heating is not balanced heating). In order to solve the problem that the brazing filler metal cannot be directly melted due to low interface temperature of conventional friction stir welding, the friction stir-induced aluminothermic reaction is provided"(the brazing filler metal contains aluminum and oxide), the brazing filler metal is melted by utilizing heat release in the process of reducing the oxide by the aluminum, namely, an auxiliary heat source is formed by utilizing the heat release reaction of hot aluminum and an oxide film on the surface of stainless steel powder, and the interface welding temperature is increased to ensure that the brazing filler metal is smoothly melted (the first effect of the thermit reaction is heating); meanwhile, the purpose of cleaning the surface of the stainless steel powder is realized, and the metallurgical bonding between the stainless steel powder metal and the brazing filler metal or the aluminum base metal is facilitated (the second effect of the thermite reaction is cleaning). When the invention realizes the aluminothermic reaction by stirring friction, the used oxide mainly comes from a compact and stable oxide film (passive film) on the surface of the stainless steel powder, therefore, the oxide powder (such as Fe) can not be directly added into the brazing filler metal ₂ O ₃ Or Cr ₂ O ₃ Powder).

On the other hand, after the oxide film on the surface of the stainless steel powder is consumed by thermite reaction, the coefficient of thermal expansion of the brazing seam is regulated and controlled by utilizing the metal particle body under the original oxide film, and the mismatching degree of the Coefficient of Thermal Expansion (CTE) between the aluminum clad material/brazing seam and the interface/steel substrate is inhibited; and the welding zone structure is divided into a plurality of sections by the separation function, the smaller shrinkage (the base number of the shrinkage section is small) of each section is utilized to replace the larger total shrinkage generated by the continuous brazing seam structure (the base number of the shrinkage range is large, and the total width of the original interface) in cooling, and the thermal stress is favorably reduced. In addition, the metal particles also have the beneficial effects of scraping and removing films of the high-strength substrate and hindering crack propagation.

In the composite brazing filler metal, the Si-containing aluminum-based brazing filler metal has the following specific functions:

(1) si element is introduced to convert the intermetallic compound of the aluminum/steel interface from an Al-Fe binary phase to an Al-Fe-Si ternary phase (tau phase), thereby inhibiting the growth of Al-Fe binary IMC from being too thick.

(2) Because the aluminum-based brazing filler metal has a lower melting point than that of the aluminum plate, has small deformation resistance and is easy to crush and deform, the activation of the aluminum-based brazing filler metal and the full contact between the aluminum-based brazing filler metal and the oxide film on the surface of the stainless steel powder are easy to realize, the aluminothermic reaction between the ignited aluminum-based brazing filler metal and the oxide film on the surface of the stainless steel powder is easy to induce, the defect of friction heat during stirring can be overcome, and the aluminum-based brazing filler metal can be smoothly melted. And the aluminum plate parent metal is difficult to fully contact with the surface of the stainless steel powder due to large deformation resistance, so that ignition of the aluminothermic reaction between the aluminum plate parent metal and the oxide film on the surface of the stainless steel powder becomes difficult.

(3) After the aluminum-based brazing filler metal is melted by comprehensively utilizing the reaction heat (which is used as an auxiliary heat source to supplement the friction heat) of the stirring friction heat and the aluminothermic reaction, the aluminum-based brazing filler metal can play roles in dissolving a base metal, removing a film, wetting and expanding the welding range.

The invention also provides a process flow for preparing the composite pipe (for example, the thickness of the clad aluminum plate is 2-8 mm, and the wall thickness of the finished square pipe is less than or equal to 8mm) by stirring friction brazing based on the composite brazing filler metal, and the stirring friction brazing welding procedure in the process flow is also suitable for preparing the composite plate. The process flow comprises the following steps:

1) material preparation and assembling process

Selecting a finished square tube with relatively high hardness as a base material of the composite square tube, and selecting a groove-shaped preformed piece with relatively low hardness as a coating material of the composite square tube; combining a pipe body surrounded by the two groove-shaped preformed pieces and the finished square pipe in a mode of sleeving the outer pipe with the inner pipe, and placing composite brazing filler metal between the inner surface of the groove-shaped preformed pieces and the outer surface of the corresponding side of the finished square pipe.

The composite brazing filler metal is a mixture of one or more of the Si-containing aluminum-based brazing filler metals and metal particles (e.g., stainless steel powder) having a stable oxide film; the multi-component composite brazing filler metal can induce the stable oxide film on the surfaces of the Si-containing aluminum-based brazing filler metal and metal particles to generate thermit reaction by using stirring friction, release heat of the thermit reaction is used as an auxiliary heat source to make up for the deficiency of frictional heat, melting of the Si-containing aluminum-based brazing filler metal is guaranteed, excessive thickening of intermetallic compounds (introduction of Si) can be inhibited, mechanical membrane removal on the surface of a high-strength substrate is strengthened (deformation resistance of a substrate interface is reduced due to rising of heating temperature of the thermit reaction), and the damage of thermal stress caused by strong restraint of a finished product square tube is reduced, so that the problem that the welded square tube prepared by compounding the conventional stirring friction brazing (adopting soft solder) with the aluminum-clad steel is cracked is solved.

A low melting point solder (e.g., a solder such as Zn powder or Zn foil) may be added to the mixture.

In addition, solid square steel or square copper can be placed in the finished square tube to balance the stirring friction pressure, so that the situation that the finished square tube cannot bear the stirring friction pressure and is crushed and deformed under the condition that the wall thickness is less than or equal to 2mm is avoided.

2) Welding process

According to the set parameters of the friction stir brazing process, firstly welding along the bottom surface of one groove-shaped preformed piece assembled on a finished square pipe by using a needleless tool, and then welding along the bottom surface of the other groove-shaped preformed piece assembled on the finished square pipe by using the needleless tool (namely welding of a groove bottom plane/a finished square pipe plane lapping interface), wherein when the single-side width of the finished square pipe is larger (larger than the diameter of a tool shaft shoulder), the two sides of the needleless friction stir brazing can be welded according to multiple times of friction stir brazing, namely when the groove bottom plane/the finished square pipe plane lapping interface is welded, the number of welding beads can be adjusted according to the ratio of the size of the finished square pipe to the diameter of the friction stir tool and the lapping width between adjacent welding beads; and then, respectively welding along the side surfaces (namely, a groove wing side plane/finished square tube plane lapping interface) of the two groove-shaped preformed pieces by using a needleless tool (the number of welding tracks can be adjusted according to the ratio of the size of the finished square tube to the diameter of the stirring tool and the overlapping width between adjacent welding tracks) and welding along the butting position (namely, a groove wing/groove wing butting interface) of the two groove-shaped preformed pieces on the finished square tube by using a needle tool (if the size of the finished square tube is small, for example, the single-side width of the finished square tube is smaller than the diameter of a tool shaft shoulder, welding along the groove wing/groove wing butting interface can be carried out and the welding of the groove wing side surface/square tube plane lapping interface and the groove wing/groove wing butting interface can be simultaneously completed under the condition of using the needle tool).

The invention also provides a process flow for preparing the composite calandria by utilizing the composite pipe (such as the aluminum-clad steel composite square pipe), which comprises the following steps:

the method comprises the steps of taking a single small-sized composite square pipe as an element, assembling a plurality of composite square pipes in a building block mode to form composite square pipe assemblies with different shapes such as a larger-sized straight shape, a T shape, a cross shape, an X shape, a square shape, a rectangular shape or a circular shape, placing brazing filler metal and outer pipe material plates (for example, aluminum plates) on the outer surface of the assembled large-sized composite square pipe assemblies, and welding the elements into a whole through the outer pipe material plates covering the outer surface of the elements by means of friction stir brazing, so that the composite exhaust pipes with different specifications (large sizes and various shapes) are obtained. The composite calandria made by combining different composite square pipes can increase the bearing area and improve the bearing capacity.

When the requirements on the bearing area and the bearing capacity of the composite calandria are not high, a plurality of composite square pipes can be used as raw materials, after the assembly is completed, the composite square pipes arranged at adjacent positions are welded by Tungsten Inert Gas (TIG) welding, namely, only aluminum coatings on the surfaces of the corresponding composite square pipes are melted (continuous welding beads or intermittent welding beads are welded as required), and the composite calandria can be obtained; or the adjacent side surfaces of the two composite square pipes are welded together through short-needle type friction stir butt welding, so that the composite calandria can be obtained; or the welding between the adjacent composite square pipes is realized by stirring friction brazing after the brazing filler metal is preset or coated between the adjacent composite square pipes, and the composite calandria can also be obtained.

The invention has the beneficial effects that:

(1) according to the invention, through the designed composite brazing filler metal, the aluminothermic reaction between the aluminum-based brazing filler metal (which has a low melting point relative to the coating and is easy to yield) and the oxide film (or the oxide powder) on the surface of the metal powder is induced by the friction stir heat, the heat released by the aluminothermic reaction makes up the deficiency of the friction stir heat, the problem that the temperature of the friction stir welding interface is not enough to melt the brazing filler metal is solved, the friction stir brazing is promoted to the brazing field from the soft brazing field, and the thermal stress caused by CTE mismatch is favorably eliminated.

(2) According to the invention, while the aluminothermic reaction is induced by using the stirring friction to improve the interface temperature, the liquid nonmetal Si element is introduced, and Al-Fe-Si ternary IMC (tau phase) is used for replacing Al-Fe binary IMC, so that the inhibition of the excessive thickness growth of the binary IMC is realized.

(3) The invention further reduces the harm of thermal stress by utilizing the low thermal expansion coefficient and the separation effect of the stainless steel powder.

(4) The invention utilizes the beneficial effects of the three items (1) to (3), realizes film removal by a mechanical and metallurgical double mechanism, inhibits IMC and thermal stress, and effectively solves the problem that the composite pipe is cracked automatically after being welded in the friction stir brazing composite process adopting soft solder such as Zn foil and the like. The composite pipe prepared by the invention has excellent mechanical property (the interface shear strength of the pure aluminum-clad steel composite square pipe obtained by the experiment can reach 56MPa), and the application field of the composite pipe prepared by friction stir brazing is expanded.

Drawings

Fig. 1 is a schematic diagram of an assembly and welding process of a composite square tube prepared by friction stir brazing, wherein: (a) stirring friction brazing with two sides free of needle; (b) friction stir brazing with needles on both sides; RS denotes the backward side, and AS denotes the forward side.

FIG. 2 shows the fracture morphology that cracks immediately after FSB recombination using conventional solder (Zn foil).

FIG. 3 is a joint interface through microcrack (crack upon brazing) after low heat input (475rpm) FSB compounding with pure Zn foil braze; wherein: (a) weld bead center area (10000 times) microcrack location and IMC thickness; (b) bead edge cracking (2000 times).

FIG. 4 shows two-component composite solder powder (Wt) using "pure Zn powder + stainless steel (SUS)304 powder _sus 30%) were subjected to low heat input friction stir brazing compounding and the joint interface penetrated cracks (cracked immediately after welding, parallel to the interface).

FIG. 5 is a graph showing the combined interface rotation speed-shear strength of 1060Al/Q235 composite square tubes (welding parameters: 150 mm/min-1.5-0.7 mm) obtained by using two components of [ (Al-12Si) +30 (wt%) SUS304] composite hard solder powder.

FIG. 6 shows a two-component "Al-12 Si powder + SUS powder" composite hard solder powder (Wt) _sus 30%) of stainless steel particles in the joint after friction stir welding (welding parameters: 475rpm-150 mm/min-1.5-0.7 mm); in the back-scattered photograph, the upper black area was an aluminum clad material, the lower white area was a Q235 steel base material, and the white particles were stainless steel powder.

FIG. 7 shows a two-component "Al-12 Si powder + SUS powder" composite hard solder powder (Wt) _sus 30%) of the weld center backscatter photographs (crack-free structure) after friction stir rubbing, where: (a) IMC thickness 1.6 μm (475 rpm); (b) IMC thickness 2.5 μm (750 rpm); (c) the IMC thickness was 3.5 μm (1500 rpm).

FIG. 8 shows the original appearance (950rpm) of an aluminum-clad steel composite square pipe prepared by friction stir brazing in which the thermite reaction is induced by friction stir.

FIG. 9 is a graph showing the combination of the interface rotation speed-shear strength (a: 150 mm/min-1.5-0.7 mm) and the welding speed-shear strength (b: 950 rpm-1.5-0.7 mm) of 1060Al/Q235 composite square tube obtained by using three components of [ (Al-12Si) +17.5 (wt%) Zn +30 (wt%) SUS304] composite hard solder powder.

FIG. 10 is a diagram illustrating the position of thermocouple temperature measurement.

Fig. 11 shows the effect of adding no SUS particles on the temperature measured at the same point. FIGS. 11a and 11b show the effect (475rpm) of the addition of SUS particles on the temperature measured at the same temperature measuring point. FIGS. 11c and 11d show the effect (750rpm) of the temperature measurement at the same temperature measurement point when no SUS particles were added or added. FIGS. 11e and 11f show the effect (1500rpm) of the addition of SUS particles on the temperature measured at the same temperature measuring point.

Detailed Description

The present invention will be described in further detail with reference to the accompanying drawings and examples.

Process flow design

Based on the idea that the square tube type substrate is flat in periphery and can theoretically form large-area contact friction with a shaft shoulder of a friction stir welding tool (stirring head), and sufficient friction heat is expected to be generated to melt brazing filler metal to prepare the composite tube, a process flow that two preformed U-shaped coating layers (for example, bent by an aluminum plate) are welded on a square base tube (for example, a square steel tube) by a Friction Stir Brazing (FSB) method is firstly provided, so that a single composite square tube (for example, an aluminum-clad steel composite square tube) is prepared.

The technological process of preparing the composite square pipe by using the friction stir brazing mainly comprises the steps of material preparation and assembly and the welding step consisting of the steps of the friction stir brazing with two sides free of needles and the friction stir brazing with two sides provided with needles.

The material preparation and assembly process specifically comprises the following steps: the inner pipe (base layer) adopts a finished square pipe, and has relatively high hardness; the outer tube (cladding) consists of two trough-like preforms (e.g. a flat-bottomed U-shaped structure pre-bent with aluminium sheet) and is relatively stiff; the two groove-shaped preformed pieces are tightly sleeved on the inner pipe along two opposite side planes of the inner pipe in a mouth-to-mouth mode, and brazing filler metal is preset at all lap joint interfaces of the two groove-shaped preformed pieces and the inner pipe.

The welding procedure specifically comprises the following steps: when the single side of the square pipe is narrow, welding the bottom plane of one groove-shaped preformed piece and the plane of one side of the inner pipe overlapped with the groove-shaped preformed piece by using needleless stirring friction brazing, and welding the bottom plane of the other groove-shaped preformed piece and the plane of one side of the inner pipe overlapped with the groove-shaped preformed piece by using needleless stirring friction brazing to finish the needleless stirring friction brazing of two sides; and then carrying out friction stir butt welding between the two groove-shaped preformed pieces by using pin friction stir brazing, and simultaneously realizing the friction stir brazing between the wing side plane of each groove-shaped preformed piece and the lap joint position of the wing side plane of each groove-shaped preformed piece on the corresponding side plane of the inner pipe, namely completing the friction stir brazing of the two-sided pins. And welding the four lateral outer surfaces of the internal square pipe by the two groove-shaped preformed pieces to obtain a single composite square pipe with four lateral surfaces being metallurgical bonding interfaces. When the width of one side of the square pipe is wide (>40mm), the pin-less friction stir brazing performed on each side can be decomposed into multi-pass friction stir brazing.

Referring to fig. 1, the welding process is to weld the bottom surfaces of two preformed U-shaped cladding layers on two planes of a square base tube by a friction stir brazing process with a needleless tool; and then performing friction stir welding (realizing butt joint between the wing side edges of the two U-shaped coatings) and friction stir brazing (realizing lap welding between the wing side planes of the two U-shaped coatings and the plane interface of the base tube) on the two remaining planes by using a tool with a needle. However, when the "steel-in-aluminum" composite square pipe is produced using a soft solder such as a Zn foil, the pipe cracks by itself after welding (the sound of continuous crack propagation can be heard after welding). Observing the self-cracking fracture, and observing that a white metal layer is adhered to the surface of the square steel pipe; in addition, the crack propagation sound of 'crack and crack' is generated from time to time in the cooling process after welding, and the reason of self-cracking is confirmed not to be film removal and poor wettability but to be that the interface intermetallic compound (IMC) is fractured under the action of thermal stress. The specific analysis is as follows:

compared with plate compounding, the reason that the thermal stress is large when the square pipe is compounded is that the steel pipe has strong rigidity, the free shrinkage of aluminum materials is severely limited, and the damage of the thermal stress in the welding and cooling process is increased, which shows that the shrinkage of aluminum on any side surface after welding is limited by the edge of the steel pipe connected with the aluminum pipe and the edge of other sides of the steel pipe not connected with the aluminum pipe, so that the restriction of the square steel pipe when the aluminum materials and the interface brittleness shrink is far more than that of the steel plate base metal.

Reasons for excessive thickening of the "aluminum-clad steel" composite square tube interface intermetallic compound (IMC) include: 1) after the Zn liquid is extruded, the Fe-Al binary intermetallic compound grows faster. 2) The square tube is hollow (not a solid round rod or thick plate), and has poor heat dissipation, so that the interface temperature is easy to rise; experiments show that the aluminum-clad steel composite square pipe can crack under the same FSB welding standard parameters, and the aluminum-clad steel solid core composite block can not crack; when the flat plate is compounded, the steel plate is fixed on the workbench and is fully contacted with the workbench with large thickness and size, the workbench is utilized to radiate interface heat in a heat conduction mode, the interface temperature is lower, and the IMC is correspondingly thinner. 3) When the side length of the square tube is larger, the diameter of the tool shaft shoulder is correspondingly increased, and the friction heat production is obviously increased.

The phenomenon that the welded aluminum-clad steel composite square pipe is cracked when the stirring friction brazing process is applied to prepare the aluminum-clad steel composite square pipe is caused by the combined action of adverse factors in two aspects of excessively thickened IMC layer (interface Fe-Al intermetallic compound grows too fast and excessively thick) and rigidity constraint of the square steel pipe.

(II) composite brazing filler metal design

The invention comprehensively considers the four aspects of wettability, IMC excessive thickening inhibition, thermal stress reduction and smooth melting of the brazing filler metal to design the brazing filler metal components. The friction stir brazing can obtain a mechanical film breaking effect by virtue of the torsion action of the rotating shaft shoulder, so that the choice of the wettability is wide, the base material aluminum has a certain dissolving capacity, and the difficulty lies in the realization of the latter three aspects. The invention specifically designs the following components aiming at the three aspects of inhibiting the easy growth of an interface Fe-Al intermetallic compound, improving the interface temperature to melt brazing filler metal and inhibiting the strong restraint hazard under the composite condition of a square tube:

(1) in terms of suppressing excessive thickening of interfacial intermetallic compound (IMC): aiming at aluminum/steel compounding, the Si-containing aluminum-based brazing filler metal is adopted. The introduction of non-metal Si element is beneficial to inhibiting the growth of Fe-Al intermetallic compound. For example, Si element is introduced into the brazing filler metal in the form of Al-Si (typical mark HL 400: Al-12Si), Al-Si-Cu, Al-Si-Mg or Al-Si-Zn brazing filler metal or 1 series cast aluminum alloy (ZL1 x), and the Al-Fe-Si ternary intermetallic compound with slow growth is formed by adding non-metallic element Si to inhibit the overgrowth and embrittlement of the Al-Fe binary intermetallic compound.

(2) In the aspect of raising the interface temperature to ensure the smooth melting of the Si-containing aluminum-based brazing filler metal: it is proposed to introduce an auxiliary heat source by means of "stirring friction induced limited thermite reaction". That is, by adding metal (for example, stainless steel) powder containing a dense stable oxide film and not prone to collapse and yield, and by inducing an "aluminothermic reaction" between the Si-containing aluminum-based brazing filler metal (which has a lower melting point than the aluminum plate base metal, is prone to yield, and is liable to be brought into sufficient contact with the oxide film on the surface of the stainless steel metal powder) and the "metal surface oxide film" by "stirring friction", the exothermic heat from the aluminothermic reaction is used as an auxiliary heat source to increase the interface temperature, thereby making up for the shortage of the stirring friction heat and ensuring smooth melting of the Si-containing aluminum-based brazing filler metal having a high melting point. On the other hand, the existence of the metal particles can also increase the friction coefficient between interfaces, thereby being beneficial to interface scraping and film removal and the increase of the interface temperature. In addition, the cavity in the pipe reduces the heat dissipation of the interface and is beneficial to the rise of the interface temperature.

(3) In terms of suppressing the interface residual thermal stress, the properties of the metal material itself in the metal (e.g., stainless steel) powder are mainly utilized, and the mechanism of action can be divided into the following two (taking the added metal powder as stainless steel (304SS) powder as an example): in one aspect, the thermal expansion coefficient of stainless steel (17.3 × 10) ^-6 A thermal expansion coefficient between aluminum (23.6 x 10) ^-6 K) and coefficient of thermal expansion of steel (11.76X 10) ^-6 K), it is advantageous to mitigate the degree of mismatch of the thermal expansion coefficients. When the stainless steel powder is embedded into the aluminum plate and forms metallurgical bonding with the aluminum plate, the thermal expansion coefficient of the aluminum plate near seam area is lower than that of original pure aluminum, so that the shrinkage of an aluminum base material in the near seam area is reduced, the constraint degree of steel on the interface along with the aluminum shrinkage is reduced, and the harm of thermal stress formed by the steel on the aluminum shrinkage is reduced. On the other hand, the stainless steel metal powder which is consumed by the aluminothermic reaction of the oxide film on the surface of the stainless steel has the function of separating the whole brazing seam, namely, the brazing seam, the interface phase and continuous structures of each part of a base material (such as stainless steel particles embedded in the soft aluminum base material) close to the interface can be separated into a plurality of small sections by utilizing the metallurgical bonding of stainless steel particles with clean surfaces (without oxide films) and the surrounding aluminum materials and the dispersity of the clean stainless steel powder, so that the total shrinkage of uniform structures in each section is reduced (compared with the original full-width interface which is not divided), the shrinkage peak value of the corresponding section and the corresponding thermal stress peak value are inhibited, and the function of inhibiting the thermal stress is achieved.

(4) In addition, in order to reduce heat input, improve repeatability (meaning that the composite square tubes prepared in different batches have consistent performance and reproducible results), widen a parameter window (meaning that the adjustable range of friction stir process parameters is larger), and add a small amount of low-melting-point solder (such as Zn powder or Zn foil).

The compounding process for preparing the aluminum-clad steel composite square pipe by stirring friction brazing has the following characteristics:

(a) compared with the aluminum mother plate with high melting point and larger rigidity, the aluminum in the brazing filler metal is easier to generate the aluminothermic reaction with the oxide film on the surface of the stainless steel powder. This is because the Si-containing aluminum-based brazing filler metal (e.g., Al — Si brazing filler metal) has a low melting point, is easily yielding, is easily activated, and easily achieves sufficient contact with the oxide film on the surface of the stainless steel powder, thereby lowering the ignition temperature of the thermite reaction.

(b) The reduced oxide in the thermite reaction is a finite, dense "surface oxide film" (passivation film):

compared with the traditional thermite reaction using oxide powder (the whole powder is oxide from the surface layer to the inside), the oxide source is only an oxide film on the surface of the stainless steel powder, and metal material particles (not all oxides) are under the surface. This has the advantage that, on the one hand, the desired oxides (e.g. Cr) for the thermite reaction are compatible ₂ O ₃ ) (ii) a On the other hand, excessive thermite reaction caused by excessive oxide is avoided, so that the interface temperature is excessively increased (the thickening of interface brittle phase is aggravated), and Al in a reaction product is avoided ₂ O ₃ Too much to cause excessive inclusion.

The aluminothermic reaction initiated with the stainless steel "surface oxide film" is as follows:

2Al+Cr ₂ O ₃ →2Cr+Al ₂ O ₃

2Al+Fe ₂ O ₃ →2Fe+Al ₂ O ₃

stainless steel powder is chosen for two reasons: firstly, the stainless steel powder can provide an obvious and sufficient oxidation film which is concentrated on the surface, and the phenomenon that the interface plasticity is influenced by overhigh temperature rise and excessive slag inclusion caused by completely adopting the chromium oxide powder does not occur; second, stainless steel thermal expansion coefficient (17X 10) ^-6 K) centered between the aluminum (23.6X 10) ^-6 K) and mild steel (12X 10) ^-6 and/K), the relaxation of thermal stress is simultaneously considered.

(c) The induction of the thermite reaction comes from the friction stir of the rotating tool:

the traditional aluminothermic reaction needs additional energy (such as heating by using a burning magnesium strip or a resistance wire) to improve the temperature of the powdery oxide and the aluminum powder, but the invention omits the additional energy, directly utilizes the frictional heat of a shaft shoulder to induce the aluminothermic reaction, and has the advantages of simplicity, effectiveness and less emission.

(d) The ignition temperature of the thermite reaction is greatly reduced:

under the friction and extrusion action of the shaft shoulder, the aluminum-based hard brazing filler metal is easy to yield due to the lower melting point than that of the aluminum base metal, and an oxide film on the surface of the aluminum-based hard brazing filler metal can be broken along with the aluminum-based hard brazing filler metal; the aluminum-based brazing filler metal powder or foil is easy to be in compaction contact with the oxide film on the surface of the stainless steel powder; in addition, aluminum atoms in the aluminum-based brazing filler metal are easy to activate along with crushing deformation; so that the generation of thermite reaction can be induced early. Then the aluminum parent metal with higher temperature and flowing deformation can participate in the aluminothermic reaction of the oxide film on the surface of the stainless steel. The friction stir can reduce the ignition temperature of the thermite reaction from 900 c, which is typical of traditional loose fill pharmaceuticals, to about 490 c.

(e) Semi-solid brazing:

in the welding process, stainless steel (SUS) powder is always in a solid state, so that the brazing filler metal is in a semi-solid state from the melting point of the brazing filler metal. The stainless steel powder is always in a solid state, which is beneficial to the overhead aluminum material, avoids the long-time excessive diffusion of the interface caused by the premature and excessive close contact of Al/Fe, and inhibits the excessively rapid and thick increase of IMC. In addition, the method is beneficial to alleviating the mismatch degree of the thermal expansion coefficient between Al and Fe, and separating (breaking up) the total shrinkage size range.

(f) The method avoids using any equipment and materials for igniting thermite reaction, avoids the operation of preheating the matrix, and does not add any additional operation procedure.

(III) composite calandria preparation process

After being arranged and combined, a plurality of composite square pipes are welded with plates (the outer pipe materials of the composite pipes) covering the outer surfaces of the composite square pipes into a whole by friction stir brazing to obtain the composite calandria, so that the bearing area is increased, and the bearing capacity is improved.

The composite calandria can also be obtained by welding the adjacent single composite pipes one by one, the welding method can adopt one or more of FSB, TIG and FSW for mixed use, and the specific technological process is selected as follows:

(1) after brazing filler metal is preset or coated on the surfaces of the aluminum coating layers of the adjacent composite square pipes, welding (short welding pass or long welding pass can be used) between the two adjacent composite square pipes is achieved through Friction Stir Brazing (FSB), and the composite calandria is manufactured.

(2) For the adjacent single composite square pipe with thicker cladding (such as aluminum cladding), only the cladding (aluminum) can be melted by short-pass argon tungsten arc welding (TIG) to prepare the composite calandria.

(3) And welding the side planes of the adjacent single composite square tubes together through short-needle type friction stir butt welding to obtain the composite calandria.

(IV) examples of welding

The selected square base tube is a commercial low-carbon steel (Q235) hollow square tube with the wall thickness of 3mm and the side length of a single side of 30 mm; the outer U-cladding was a 3mm thick pure aluminum (1060 aluminum) plate pre-bent into a corresponding channel. The stainless steel powder is specifically SUS304 powder (the surface of the stainless steel powder is provided with an oxide film). The material of the stirring head is H13 with the diameter

(a needle-free tool and a needle-containing tool are adapted according to the working procedures, the root diameter of the stirring needle is 10mm, and the tip diameter is 6 mm); the FSB experimental equipment is an X52K vertical lifting milling machine; the inclination angle is 1.5 degrees, and the pressing depth is 0.7 mm. The comparison test is carried out by mainly changing the rotating speed and the welding speed. After welding, according to the standard of GB/T6396-; and the interface tissue and defects are analyzed using the backscatter image.

In order to verify the correctness and the effectiveness of the FSB of the composite hard solder which is provided by the invention and utilizes the aluminothermic reaction induced by stirring friction to assist in melting, the following 5 types of solders are designed: (1) pure Zn foil soft solder; (2) pure Zn powder and stainless steel powder; (3) al-12Si braze powder; (4) al-12Si braze powder + stainless steel powder (30 wt%); (5) al-12Si brazing filler metal powder, pure Zn powder and stainless steel powder. Sample assembly reference is made to figure 1.

For the traditional Friction Stir Brazing (FSB) composite square pipe adopting pure Zn foil soft solder, the composite square pipe can automatically crack after being welded, and the appearance of the fracture is shown in figure 2. The fracture morphology indicates that significant adhesion of aluminum to the steel surface has occurred, not poor wetting. Microscopic structure observation shows (figure 3), even under the minimum heat input working condition (475rpm-150 mm/min-1.5-0.7 mm), the intermetallic compound FeAl with the thickness of 5-6 μm is formed on the whole interface ₃ (Zn). Cracks initiate from the advancing side and the retreating side at the edge and have propagatedTo the center of the weld (when magnified to 10000 times, microscopic cracks that have propagated to the center of the joint can be identified). The conventional FSB composite square tube adopting soft solder (pure Zn foil) is cracked after being welded mainly by two aspects: (1) the Al-Fe binary IMC grows too fast and thick; (2) the deformation of the welding surface along with the shrinkage of aluminum is limited by the strong restraint of the steel pipe, the stress is difficult to release, and finally the IMC cracks.

For two-component composite soft solder powder (Wt) composed of pure Zn powder and stainless steel powder _sus 30%), even under low heat input conditions, the FSB joint interface still exhibits through cracks parallel to the interface (see fig. 4), and cracks immediately after welding, demonstrating that the "solder powder + stainless steel powder" two-component composite solder powder is not effective in inhibiting IMC and cracking.

For single Al-12Si brazing filler metal powder, the brazing filler metal cannot be melted due to single frictional heat, and assembled samples cannot be combined through welding, namely, the composite square tube cannot be obtained.

For two-component composite brazing filler metal (Wt) composed of Al-12Si brazing filler metal powder and stainless steel powder _sus 30%). As can be seen from the curve of the rotating speed-the shearing strength (the specification parameter: 150 mm/min-1.5-0.7 mm) in FIG. 5, the shearing strength can reach 56MPa at the rotating speed of 600rpm and 750rpm, and the reproducibility is good. The observation of the backscattering macrostructure (fig. 6) shows that the stainless steel particles are mainly distributed inside the brazing seam close to the interface (see region a in fig. 6), and a few stainless steel particles are embedded in the aluminum base material used as the coating, which is beneficial to reducing the thermal expansion coefficient of the brazing seam, separating the original larger shrinkage region and reducing the damage of thermal stress. The observation result of the back scattering microstructure (fig. 7) shows that the central structure of the weld joint has no cracks after stirring and rubbing by adopting the two-component composite hard solder powder containing Si-aluminum-based hard solder powder and stainless steel powder; IMC thickness at different rotational speeds is much lower than the thickness of conventional FSB using solder at minimum heat input (6 μm); the intermetallic compound formed at 750rpm was 2.5 μm thick τ ₅ -Al _7.1 Fe _1.6 The Si phase (containing Si at 10 at.%) and part of Fe-Al intermetallic compound are broken by the twisting action of the stirring head and enter into the aluminium mother material. The above results indicate that the composition "containsThe Si aluminum-based hard solder powder and stainless steel powder combined hard solder powder has obvious effects of inhibiting excessive increase of IMC thickness and improving performance, and the obtained composite square tube has no cracking phenomenon after welding (see figure 8).

For the three-component composite brazing filler metal composed of Al-12Si brazing filler metal powder, pure Zn powder and stainless steel powder, as can be seen from the rotating speed-joint shear strength curve (150 mm/min-1.5-0.7 mm) shown in FIG. 9(a) and the welding speed-joint shear strength curve (950 rpm-1.5-0.7 mm) shown in FIG. 9(b), the joint shear strength can reach 30-47 MPa, which is obviously superior to the cracking result after the welding of the traditional soft solder (pure Zn foil).

In addition, in order to further confirm the level of the increase in the interface temperature, the temperature near the welding interface was measured by a thermocouple (see fig. 10), and it was confirmed that the heating temperature was increased by 90 to 100 ℃ after the addition of the stainless powder (by exothermic thermite reaction) compared to the case where the stainless powder was not added (see fig. 11a and 11b, 11c and 11d, 11e and 11 f). The test result proves that under the working condition that the stirring friction temperature is close to the melting point of the aluminum-based brazing filler metal but can not reach the melting point of the aluminum-based brazing filler metal, the aluminothermic reaction induced by the stirring friction is utilized to assist in heating, and considerable contribution is made to supplement of the deficiency of the friction heat.

In conclusion, the present invention utilizes the thermal reaction of aluminum materials to increase the interface temperature by replacing the conventional soft solder (e.g., pure Zn foil) with a two-component or multi-component composite brazing filler metal to ensure the melting of the brazing filler metal; the damage of excessive thickening and thermal stress of IMC is inhibited, and the phenomenon of cracking after welding is eliminated; the shearing strength of the interface of the obtained composite square tube 1060Al/Q235 can reach 56 MPa. The invention expands the application field of FSB from composite plate preparation to composite square tube preparation, and the prepared composite square tube completely meets the use requirements of steel tube structures such as ocean platform pile legs, wind power generation rod barrels and the like, thereby fully playing the role of corrosion resistance of the coating. The invention can also be used for preparing the composite board, has the comprehensive advantages of thin interface IMC and low interface stress compared with the traditional Zn foil brazing filler metal, and is more suitable for the working condition of large thermal stress caused by strong, long and thick substrates.

Claims

1. A method of friction stir brazing a composite pipe, comprising: the method comprises the following steps:

1) material preparation and assembly process

Overlapping and assembling a tubular base material with a square or rectangular circumferential outer contour and groove-shaped coating materials corresponding to the shapes of two sides of the tubular base material according to an internal and external composite structural form, and presetting brazing filler metal at an internal and external overlapping interface;

2) welding process

Stirring, rubbing and brazing by adopting a needle-free tool or a needle-containing tool, so that the bottom surface of the overlapped groove-shaped coating material and the corresponding side surface of the tubular base material are welded together, and the wing side surface of the overlapped groove-shaped coating material and the corresponding side surface of the tubular base material are welded together; stirring, rubbing and brazing by adopting a tool with a needle to enable the wing side edges of the butted groove-shaped coating materials on the corresponding side surfaces of the tubular base material to be welded together;

the stirring friction brazing includes a stirring friction brazing process of using a multi-component composite brazing filler metal containing Si-containing aluminum-based brazing filler metal and metal powder as a brazing filler metal preset at an inner and outer lap joint interface, inducing an aluminothermic reaction between the Si-containing aluminum-based brazing filler metal and an oxide film on the surface of the metal powder through stirring friction, and melting the Si-containing aluminum-based brazing filler metal by using heat released by the aluminothermic reaction as an auxiliary heat source.

2. The method of friction stir brazing a composite pipe according to claim 1, wherein: the composite brazing filler metal adopts Si-containing aluminum-based brazing filler metal which is low in melting point relative to a groove-shaped coating material and easy to soften and deform as one of the components, the Si-containing aluminum-based brazing filler metal is in full contact with an oxide film on the surface of metal powder through stirring friction, an aluminothermic reaction is induced between the Si-containing aluminum-based brazing filler metal and the oxide film on the surface of the metal powder, the Si-containing aluminum-based brazing filler metal is melted, and the melted Si-containing aluminum-based brazing filler metal and the metal powder which is dispersed in a liquid phase formed by melting the Si-containing aluminum-based brazing filler metal in a solid-phase particle form are utilized to realize the film removal, the intermetallic compound thickening inhibition and the thermal stress inhibition by a double mechanism.

3. The method of friction stir brazing a composite pipe according to claim 1, wherein: the step 1) specifically comprises the following steps: selecting a finished square tube with relatively high hardness as a base material of the composite square tube, and selecting a groove-shaped preformed piece with relatively low hardness as a coating material of the composite square tube; and assembling a pipe body surrounded by the two groove-shaped preformed pieces and the finished square pipe in a mode of sleeving the outer pipe with the inner pipe, and placing the composite brazing filler metal between the inner surface of the groove-shaped preformed pieces and the outer surface of the corresponding side of the finished square pipe.

4. The method of friction stir brazing a composite pipe according to claim 3, wherein: the finished square tube is made of steel, the groove-shaped preformed piece is of a flat-bottom U-shaped structure bent by an aluminum plate, and the composite square tube is of an aluminum-clad steel double-layer structure; wherein the thickness of the aluminum plate is 1-8 mm, and the wall thickness of the finished square tube is less than or equal to 8 mm.

5. The method of friction stir brazing a composite pipe according to claim 3, wherein: the step 2) specifically comprises the following steps: according to the set parameters of the friction stir brazing process, firstly welding along the bottom surface of one groove-shaped preformed piece assembled on the finished square tube by using a needle-free tool, and then welding along the bottom surface of the other groove-shaped preformed piece assembled on the finished square tube by using the needle-free tool; and then welding along the butt joint position of the two groove-shaped preformed pieces on the finished square pipe by using a needle tool.

6. A method for preparing a composite calandria by stirring friction brazing is characterized in that: the method comprises the following steps:

1) material preparation and assembling process

2) composite pipe welding process

the stirring friction brazing comprises a stirring friction brazing process of using multi-component composite brazing filler metal containing Si-containing aluminum-based brazing filler metal and metal powder as brazing filler metal preset at an inner and outer lap joint interface, inducing an aluminothermic reaction between the Si-containing aluminum-based brazing filler metal and an oxide film on the surface of the metal powder through stirring friction, and melting the Si-containing aluminum-based brazing filler metal by using heat released by the aluminothermic reaction as an auxiliary heat source;

3) assembling a plurality of composite pipes obtained by welding in the steps 1) and 2) into a base material according to a preset calandria shape, and then welding the base material and an outer covering plate into a whole to obtain a composite calandria; or assembling a plurality of composite pipes obtained by welding in the steps 1) and 2) according to a preset pipe array shape, and then welding adjacent composite pipes into a whole to obtain the composite pipe array.

7. The composite brazing filler metal is characterized in that: the composite brazing filler metal comprises one or more of metal powder and Si-containing aluminum-based brazing filler metal, wherein after the composite brazing filler metal is stirred and rubbed, an aluminothermic reaction is initiated between the Si-containing aluminum-based brazing filler metal and an oxide film on the surface of the metal powder, and the exothermic reaction is used as an auxiliary heat source to melt the Si-containing aluminum-based brazing filler metal;

the composite brazing filler metal is a mixture of a first component, a second component and a third component; the first component is one or more of stainless steel powder or nickel powder; the second component is one or more of Al-Si binary brazing filler metal or Al-Si-X ternary brazing filler metal with the melting point of more than 450 ℃, wherein X is Cu, Mg or Zn; the third component is Zn powder or Zn foil; in the mixture, the mass fraction of the first component is 5-40%, the mass fraction of the second component is 35-85%, and the mass fraction of the third component is 10-30%;

or the composite brazing filler metal is a mixture of a first component and a second component; the first component is one or more of stainless steel powder or nickel powder; the second component is one or more of Al-Si binary brazing filler metal or Al-Si-X ternary brazing filler metal with the melting point of more than 450 ℃, wherein X is Cu, Mg or Zn; in the mixture, the mass fraction of the first component is 5-50%, and the mass fraction of the second component is 50-95%.

8. A method of friction stir brazing to produce a composite sheet, comprising: the method comprises the following steps:

presetting multi-component composite hard solder containing Si-containing aluminum-based hard solder and metal powder between base metals, and utilizing aluminothermic reaction induced by stirring friction to assist in melting the Si-containing aluminum-based hard solder;