Precise welding method for dense welding seams of complex structure

Abstract

The invention relates to a precise welding method for dense welding seams of a complex structure, and relates to the technical field of heat exchange equipment manufacturing. The process comprises the steps of welding of the interior of a core body, surface treatment of a brazing seam and welding of an end socket. The inner part of the core body adopts BNi-2 brazing filler metal to perform vacuum brazing welding on the assembled core body, the surface treatment of the welding seam is that local anodic oxidation treatment is performed on a brazing seam area after welding, a NiO protective layer is selectively generated, and the welding of the end socket is that silver-copper-titanium active brazing filler metal is pre-arranged in a connecting area of the core body and the end socket. And the lower melting point can prevent a brazing seam in the core body from melting. By selecting brazing filler metal components, welding parameters and a post-treatment process, the problems that in traditional brazing, interface brittle phases, pore defects, weld joints are prone to corrosion and the like are solved, the sealing performance of joints is remarkably improved, and the method is suitable for manufacturing of high-performance heat exchangers in the fields of aviation, chemical engineering and the like.

Description

Technical Field

[0001] The invention relates to the field of welding of a heat exchanger core and a head, in particular to a precision welding method for dense welds of a complex structure. Background Art

[0002] Plate heat exchangers, as highly efficient and compact heat exchange equipment, are widely used in the fields of petrochemicals, energy and power, refrigeration and air conditioning, etc. Its core component, the heat exchange core, is usually made of multiple layers of stainless steel sheets composited through a vacuum brazing process. Nickel-based brazing filler metals containing silicon and boron are generally used as filler materials during the brazing process. During equipment assembly, a head component needs to be welded to the end of the core to achieve the medium distribution function. However, when the same brazing filler metal is used to weld the head, the heat-affected zone of the weld is very likely to overlap with the existing brazing seam inside the core. The resulting material metallurgical problems have become a key technical bottleneck restricting product reliability.

[0003] The traditional process is to directly perform argon arc welding on the brazing area, but the high temperature of the molten pool will cause the original brazing weld to melt, and the low-melting-point silicon and boron elements in the brazing filler metal will migrate into the welding molten pool. The silicon element in the molten pool tends to aggravate the dendritic segregation during the metal solidification process, significantly reducing the weld's resistance to thermal cracking; the boron element reacts with the chromium element in the base metal to form brittle compounds, resulting in deterioration of the weld toughness. These problems will further cause stress concentration and corrosion cracking risks under dynamic thermal load conditions, seriously affecting the service life of the equipment. Existing processes usually use mechanical grinding to remove surface residues of the brazing weld, but this method is difficult to completely remove the silicon-boron phase that has penetrated into the matrix, and excessive grinding will destroy the integrity of the matrix surface, which will increase the sensitivity of welding defects. Summary of the Invention

[0004] In response to the welding problem of the plate heat exchanger core, the conventional plate heat exchanger core assembly and welding process is complicated and tedious, and the space utilization rate is low. The present invention aims to use stacked assembly and welding cores to precisely stack and form a dense flow channel network; in response to the problem of metallurgical defects caused by residual elements in brazing during the welding of plate heat exchanger heads, the present invention aims to break through the technical limitations of traditional processes. Under high temperature, existing surface cleaning methods can neither completely eliminate the risk of element migration nor damage the structural integrity of the matrix due to excessive processing. The core goal of the present invention is to develop a synergistic protection process. By constructing a functional protective layer in the brazing area, the physical barrier of silicon and boron elements and the simultaneous optimization of avoiding reaching the melting point during the welding process are achieved, thereby ensuring the mechanical properties and long-term service reliability of the head welding joint, while avoiding the cost increase or equipment compactness loss caused by structural design compromise.

[0005] The technical solution adopted by the present invention to solve the above technical problems is:

[0006] To achieve the above objectives, the present invention provides a precision welding method for complex structures with dense welds, which adopts a step-by-step processing process: first, using BNi-2 brazing filler metal foil, the heat exchanger core is assembled and fixed in the stacking order of partition-brazing filler metal foil-fin-partition, and the core is welded by vacuum brazing at 1040-1150°C; after welding, the core is subjected to a heat treatment at 1050-1150°C for 3-4 hours to ensure that silicon and boron elements diffuse as evenly as possible into the substrate; after the heat treatment is completed, the exposed weld of the core welded to the head is exposed, the remaining position is covered with acid and alkali resistant tape, and the exposed area is anodized to form a ceramic NiO layer; Ag-Gu-Ti brazing filler metal foil is evenly laid along the overlap gap at the pre-welded head, and the core is placed in a brazing furnace and welded at 850-900°C.

[0007] Compared with the existing welding method preparation technology, the present invention has the following significant advantages:

[0008] 1. Compared with other heat exchanger cores, the plate-fin heat exchanger core is precisely stacked with partitions, fins, and seals. Vacuum brazing can complete the welding of dozens of layers of structure at one time, forming a dense flow channel network. The volume is only 1 / 5 to 1 / 10 of that of the shell and tube heat exchanger.

[0009] 2. Heat treatment can diffuse part of silicon and boron elements into the substrate as evenly as possible, reduce the interface concentration gradient, and effectively reduce the possibility of diffusion;

[0010] 3. In the pretreatment before welding the head, the protective layer formed by anodic oxidation, the thicker oxide layer can physically block the diffusion of silicon and boron elements in the brazing weld, and can effectively combine with the secondary brazing filler metal;

[0011] 4. During the welding of the head, vacuum brazing is performed at a temperature of 850-900°C, which is higher than the melting point of Ag-Gu-Ti brazing filler metal and lower than the melting point of BNi-2 brazing seam. It can also activate the activity of Ti and promote Ti to fully reduce NiO, thus improving wettability and facilitating direct welding of the protective layer and the brazing filler metal at the brazing seam.

[0012] 5. Through the coordinated optimization of material surface modification and welding process, the interference of residual elements in brazing on the head welding quality in traditional processes has been overcome, providing an innovative solution for the design and manufacture of high-reliability plate heat exchangers. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Figure 1 This is a diagram of the stacking, assembly and disassembly of a plate heat exchanger core according to the present invention.

[0014] Figure 2 This is a front view of a plate heat exchanger core after welding is completed.

[0015] Figure 3 This is a front view of a plate heat exchanger of the present invention after partial anodization. DETAILED DESCRIPTION

[0016] The present invention will be described in further detail below with reference to the accompanying drawings.

[0017] Combine Figure 1 、 Figure 2 、 Figure 3 The present invention provides a precision welding method for complex structure dense welds, specifically comprising a heat exchanger having a partition 1, a brazing foil 2, a fin 3, and a seal 4. On the left side of the front end of the heat exchanger, Figure 3 The middle dotted line is the reserved head welding place. The brazing welds exposed during the welding process are surface treated. First, anodizing is used to form a nickel oxide protective film, and then another low-melting-point brazing material is used to achieve the purpose of welding the head.

[0018] The present invention provides a precision welding method for complex structure and dense welds. The specific method is as follows: first, BNi-2 brazing material is used to lay foil on the contact interface between the partition and the fin, and the heat exchanger core is assembled and fixed in the order of partition-brazing material foil-fin-partition. The brazing temperature is 1040-1150°C, and the core is welded by vacuum brazing with a vacuum degree of ≤5×10 -3 Pa; After the core is welded, the core is subjected to heat treatment, with a gradual temperature increase of 10°C / min, and needs to be kept in the temperature range of 1050-1150°C for 3-4 hours, with a vacuum degree of ≤1×10 -3 Pa, so that silicon and boron elements diffuse into the matrix as evenly as possible; after the heat treatment is completed and before anodizing, use acid and alkali resistant tape (such as polytetrafluoroethylene tape) to cover and shield the welds of non-head welding parts, with a voltage of 20-25V and a current density of 2-5A / dm 2 , a ceramic NiO layer with a thickness of about 200 to 300 nm is generated in an alkaline electrolyte; a silver-copper-titanium brazing foil (0.1 mm) is laid on the weld of the pre-welded head and placed in a brazing furnace with a brazing temperature of 850-900 °C and a vacuum degree of ≤5×10 -3 Pa, and the welding is completed. The result is a high-quality weld with no cracks and low segregation. This overcomes the technical bottleneck of secondary brazing, which is caused by brazing element contamination in traditional processes, and achieves controllable performance in secondary brazing.

[0019] Example 1

[0020] Combine Figure 1 、 Figure 2 、 Figure 3316L steel is used as the partition and seal. The thickness of the partition is 50mm, the length is 900mm, and the width is 500mm. The core welding brazing material is BNi-2. After assembling 10 layers with a high temperature resistant fixture, put it into a vacuum brazing furnace and pump it to ≤5×10 -3 Pa, apply slight pressure (0.1MPa), heat to 800℃ and keep it for 10min, then heat to 1040℃ and keep it for 30-60min, cool to 900℃ and cool quickly; perform heat treatment, vacuum degree ≤1×10 -3 Pa, heated to 1050℃ and kept warm for 4h, slowly cooled to 800℃ and then quickly cooled to room temperature, and the core welding was completed; polytetrafluoroethylene tape was covered in the non-head welding area, and 0.3MNaOH+0.05M Na2SiO3 (PH≈12.5) alkaline electrolyte was used at 25℃, with the anode being the brazing weld and the cathode being the nickel plate. The constant voltage mode was adopted, the voltage was increased to 25V, and the current density was 5A / dm 2 Oxidation was performed for 30 min, then the pressure was increased to 50 V for 60 min, and finally the pressure was increased to 80 V for 60 min to form a nickel oxide layer with a thickness of 2 to 3 μm. After cleaning and drying, silver-copper-titanium brazing foil (thickness 0.1 mm) was laid on the pre-welded head. The head was fixed with a fixture and placed in a brazing furnace with a vacuum degree of ≤5×10 -3 Pa, the room temperature was raised to 850°C (10°C / min), then to 900°C (5°C / min), held for 15 minutes, cooled from 900°C to 500°C (5°C / min), and then allowed to cool naturally. This not only activated the activity of Ti to reduce NiO, but also avoided melting of the BNi-2 brazing seam and diffusion of silicon and boron elements, completing the welding of the plate heat exchanger core and head.

Claims

1. A precision welding method for complex structure dense welds, characterized in that: The specific steps of this method are: Step 1: Assemble the core in the stacking order of partition-brazing material-fin-brazing material-partition, and place seals at both ends. Use a fixture to assemble and fix them in place, leaving a head position at one end. Place the core in a vacuum furnace, evacuate the vacuum, apply slight pressure, and heat to above the melting point of the brazing material BNi-2 for vacuum brazing. Step 2: After welding, the core is kept at high temperature for a long time to allow B / Si to diffuse evenly into the 316L matrix and reduce the interface concentration gradient; Step 3: Use local area anodic oxidation to form a dense nickel oxide NiO protective layer on the surface of the brazing weld at the reserved head position; Step 4: Evenly lay the second brazing material Ag-Gu-Ti foil along the overlap gap of the pre-welded head weld, assemble and fix the head in the reserved welding area, and place it in a vacuum brazing furnace to complete the welding.

2. The precision welding method for complex structure dense welds according to claim 1, characterized in that: During the core assembly process, BNi-2 brazing material is used to lay foil 0.05-0.1 mm on the contact interface between the partition and the fin. The brazing temperature is 1040-1150 ° C and the vacuum degree is ≤5×10 -3 Pa.

3. The precision welding method for complex structure dense welds according to claim 1, characterized in that: During the heat treatment process, the temperature was gradually increased at 10°C / min and kept within the temperature range of 1050-1150°C for 3-4 hours. The vacuum degree was ≤1×10 -3 Pa.

4. The precision welding method for complex structure dense welds according to claim 1, characterized in that: Before anodizing, use acid and alkali resistant tape to cover the welds at both ends of the non-head welding area, and gradually increase the voltage to 20-25V and the current density to 2-5A / dm 2 , a ceramic NiO layer with a thickness of 200 to 300 nm is generated in an alkaline electrolyte.

5. The precision welding method for complex structure dense welds according to claim 4, characterized in that: Choose polytetrafluoroethylene tape for acid and alkali resistant tape.

6. The precision welding method for complex structure dense welds according to claim 1, characterized in that: During the head welding process, silver-copper-titanium brazing foil 0.1mm is laid on the weld and placed in the brazing furnace. The brazing temperature is 850-900℃ and the vacuum degree is ≤5×10 -3 Pa, welding is completed.

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