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Analysis of Leakage Causes of Aluminum Alloy Plate Heat Exchanger

Aluminum alloy plate heat exchanger (hereinafter referred to as heat exchanger) is a pressure vessel, and must meet the corresponding pressure requirements during its application. Due to its structural characteristics, its welding process can only be achieved by vacuum brazing.

Vacuum brazing is to heat and heat the structural parts in a vacuum state, so that the brazing filler metal melts within a suitable temperature and time range, and fully infiltrate, dissolve, diffuse and weld with the solid metal under the action of capillary force, so as to achieve An advanced welding method for welding purposes. The outstanding advantages of vacuum brazing are that it can connect different metals and realize simultaneous welding of complex structures. The welded joint after welding is smooth and compact, with small deformation and excellent mechanical properties and corrosion resistance. However, vacuum brazing has very strict requirements on the structural design and assembly quality of the heat exchanger, the chemical composition of the aluminum alloy clad plate (hereinafter referred to as the clad plate), the thickness of the brazing filler metal layer, the brazing process system, and the working environment. , It is easy to appear fin bending and lodging, and the brazing seam is discontinuous. Quality defects such as virtual welding, erosion, and leakage. Among them, leakage is a major quality defect

1. Analysis of the cause of leakage

1.1 Assembly of the heat exchanger

(1) Surface pretreatment of structural parts

All structural parts of the heat exchanger must undergo surface treatment, that is, acid-base washing, before assembly to remove surface dirt, oil stains, oxide films, etc. Dirt will hinder the effective contact between components; oil stains will decompose and gasify at high vacuum temperatures, reducing the vacuum degree in the vacuum brazing furnace; due to the dense oxide film on the surface of the aluminum alloy, its melting temperature is much higher than that of the base material, especially The oxide film of the brazing filler metal layer of the clad plate is not fully melted during brazing, so that it cannot be fully fused with the metal to be welded, thus affecting the quality of brazing. To this end, the surface pretreatment of the raw materials must be strictly controlled, including the necessary mechanical cleaning, and the assembly time before brazing must be shortened.

(2) Needle layer thickness

The setting of the thickness of the clad plate and the thickness of the brazing material should match the pressure requirements of the heat exchanger, and must be formulated in combination with theoretical calculation and production practice. It is specially pointed out here that when the thickness of the solder layer is too thin, it is easy to cause welding defects such as low welding strength, weak welding and substandard pressure; Leakage occurs due to erosion. Therefore, the thickness of the solder layer and its uniformity are important indicators to measure its quality, and also one of the important factors affecting the quality of brazing. In practical applications, the thickness of the solder layer is generally controlled at ( 10+ 3)% of the thickness of the composite plate.

(3) Other quality requirements for composite panels

Another function of the composite plate in the heat exchanger is to act as a channel partition, and it also has pressure requirements. Therefore, there should be no internal and external defects that affect its pressure. Intrinsic defects such as pores, slag inclusions in the core layer alloy, poor welding with the brazing material layer, etc.; in addition to the above-mentioned unclean surface treatment, there are also bumps and scratches during processing. When the thickness of the solder layer is increased, the continuity of the metal will be directly destroyed, resulting in a decrease in the pressure bearing capacity.