The connection of the tube and the tube sheet connector is the key process of heat exchanger manufacturing. There are three connection methods of expansion joint, welding and expansion welding, but the connection method of tube and tube sheet expansion welding is often used. It is still controversial whether it is welding first and then expanding or first expanding and then welding.
1. The application and analysis of the welding first expansion process and the advantages of the welding first expansion process and the heat exchanger manufacturers have traditionally used the welding first expansion process, and rarely used the expansion first welding process. The reason is that it is closely related to the use of mechanical roll expansion as the main method of tube expansion. Because in the process of rolling the expansion tube, there is friction and a lot of heat is generated, it must be lubricated and cooled with engine oil. The oil penetrates into the gap of the expansion joint, and it is very difficult to completely clean it. The presence of oil, water and other debris in the cracks is easy to form gases during welding, and these gases exist in the welds before they can escape. On the other hand, the expanded tube area often blocks the exhaust passage, increasing the possibility of generating pores in the weld. Adopting the welding first and then expanding process can avoid the above disadvantages, especially for titanium and some non-ferrous metals, the basic requirements for welding are very strict, and oil and water and iron ion pollution are not allowed. It is easier to ensure welding by selecting the welding first and expanding process. Seam quality.
Analysis of the shortcomings of the welding first expansion process There are inherent shortcomings in the mechanical roll expansion method. The length of each tube is different, and the connection strength and tightness are uneven; the inner surface of the expansion tube interface hardens, which brings repeated expansion. Difficult; the compatibility of the expansion joint of the tube-tube sheet material is limited, such as the expansion joint of the titanium tube and the carbon steel, the expansion joint of the aluminum tube and the carbon steel, etc. are restricted; the labor productivity is low, and the small tube It is difficult to expand the diameter or thick-walled pipes.
The annular weld bead of the nozzle is uneven. Because there is an assembly gap of 0.2 ~ 0.5mm between the pipe and the tube sheet, and it is always eccentrically arranged, plus the processing deviation of the hole of the tube and the tube sheet, resulting in the annular welding of each nozzle The road is uneven. It is easy to weld through for thin-walled pipes.
The distance between the expanded area and the weld is 15mm, the purpose is to avoid the damage of the weld caused by the expansion force. There is gas in this non-expanded tube area. When the heat exchanger is heated, its volume expands, generating a strong pressure, which may cause damage to the weld or the expansion tube. In addition, in order to make full use of the design thickness of the tube sheet, the tube expansion area within the thickness of the tube sheet is always as long as possible. The 15-mm-long non-expansion zone is not obvious for thick tube sheets, but it cannot be underestimated for thin tube sheets.
The elongation of the pipe damages the weld, and the mechanical rolling expansion of the pipe thins the wall of the pipe, and the pipe is elongated, which damages the weld.
Welding knots are formed at the nozzle during welding, and the shrinkage and deformation of the nozzle cause difficulties for future expansion operations. In order to make the pipe joint into the pipe hole smoothly, it is necessary to put higher requirements on the welding of the nozzle.
Second, the application and analysis of the first expansion and then the welding process. The analysis of the shortcomings of the first expansion and then the welding process. When analyzing the first expansion and then the welding, because the lubricating oil during expansion is difficult to clean with an economic method, it is easy to form gas during welding, and these gases are too late The escape will exist in the weld, especially in the sealing welding of argon arc welding, there are often air holes at the arc closing seal, which affects the quality of the weld. Therefore, an arc starting plate is used at 18 Umin. The width of the welding seam and Yu Zhe are the same.
After welding, smooth the weld seam. Grind the surface with sand cloth step by step from coarse to fine, and finally polish it with water sandpaper. When grinding, it must be noted that the coarser sand particles cannot be brought to the next level of grinding process. When grinding with fine emery cloth, the weld seam and the base metal are polished uniformly, and the weld seam and the base metal must maintain the same roughness. The surface of the welded seam after grinding shall be free from defects such as cracks, porosity, and ballast.
All parts of the whole escalator to be oxidized will be anodized in the same batch. This will contribute to the uniform color of the oxide film. Special attention should be paid to protect the surface after oxidation treatment.
The above analysis and production practice proves that LD31 aluminum alloy profile parts are made of SAlMg5 welding wire, and the parts to be welded are carefully processed to ensure the same gap between the seams. Appropriate welding process specifications are used to ensure that the weld seam is finely and evenly polished. Polishing makes the parts have a good surface condition, and the anodizing process is controlled according to the general anodizing treatment specifications, so that the color of the weld and the base material anodizing film is basically the same. It is best to treat the aluminum parts that need to be oxidized in the same batch to make the aluminum parts appear uniformly bright silver white.
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