Al-alloy plate heat ex-changer vacuum brazing leak reason analysis (Part 1)
Vacuum brazing is under vacuum condition, heating and heat preservation was carried out on the structure, make the solder melted in the appropriate temperature and time scale, under the effect of capillary force and solid metal infiltration, dissolution, diffusion, welding, an advanced welding method to reach the purpose of welding. Prominent advantage of vacuum brazing is can connect different metals, complex structure and welding, after welding of the welding head is bright and clean density, small deformation and good mechanical properties and corrosion resistance. However, vacuum brazing quality of heat exchanger structure design, assembly, the chemical composition of aluminum composite panels (hereinafter referred to as the composite board), the thickness of solder layer, brazing process system, the requirements of the work environment is strictly, otherwise, appear easily fin bending lodging, brazing seam discontinuous, virtual welding, corrosion, leakage and other defects. Among them, the leakage is a major quality defects.
1.2 production process
(1) material: the composite panels, fin, seals, etc. To finalize the design, scale processing.
(2) the surface treatment process: the caustic washing – water – acid to water, hot water washing and drying.
(3) assembly: composite plate, side panel, fin, seals, etc. For composite molding machinery.
(4) vacuum brazing, vacuum of vacuum brazing furnace after three stages of heating and heat preservation.
The first stage
(a) preheat temperature, heat preservation;
The second stage
(b) energy storage and temperature, heat preservation;
The third stage
(c) brazing temperature, heat preservation;
Power cuts. For furnace temperature drop to specified temperature.
(5) plastic deformation of heat exchanger after vacuum brazing, mechanical method is used to correct.
(6) guide plate welding: adopts argon arc welding method of welding guide plate heat exchanger, the seal position on both ends of the heat exchanger.
(7) pressure inspection: the inspection by blowing air way heat exchanger bearing capacity, the leakage test.
Spray (8) : for heat exchanger cleaning, drying, spraying, drying, improve the appearance quality.
(9) package delivery.
2 Vacuum furnace leaks cause analysis
2.1 heat exchanger assembly
(1) the structure of the surface pretreatment
All heat exchanger structure in front of the assembly must be made via surface treatment of the acid and alkali wash, to remove surface dirt, grease, oxide film, and so on. Dirt can hinder effective contact between the components; Oil in high temperature vacuum will decompose gasification, reduce the vacuum degree in vacuum brazing furnace; Because of the compact aluminum alloy surface oxidation film, the melting temperature is far higher than the substrate material, especially composite board solder layer of oxide film during brazing filler metals melt layer is not sufficient, cause cannot be weld metal completely fusion, and thus affect the brazing quality. For this, must strictly control the surface pretreatment of raw materials, including the necessary mechanical cleaning, and shortening assembly time before soldering.
(2) the structure size tolerance
Composite panels, big fin, fin, seals, small seal in shape, the size of the scale after processing deviation must get effective guarantee. Fin should be controlled in the range of deviation, seals should be controlled in negative deviation range. Otherwise there is no guarantee that after assembling composite board after work closely with seals, again with fin with appropriate contact area between the brazing gap, easy to create virtual welding, brazing seam discontinuous or welded phenomenon.
(3) the structure surface roughness and shape
Structure on the surface roughness affect the capillary force. Generally speaking, the surface is too smooth, solder to uniform distribution on the contact area, the resulting hole will reduce the intensity of brazing, especially the surface roughness of seals. In order to ensure the solder uniform distribution on the contact of weld, structure of the brazing surface should have appropriate coarsening. Usually, the reference value of surface roughness of optional Ra0, 7 mm ~ 2, 0 the square root of microns is advisable. In addition, the inside of the seals shall be made into 30 ° chamfer, is beneficial to reduce the surface tension of the solder during vacuum brazing, increase the wettability, reduce soldering defects.
(4) fixture clamping force
Due to the stainless steel heat exchanger is assembled fixture clamping, and the thermal expansion coefficient of the stainless steel clamp is less than aluminum alloy products, so the clamping force is too big, easy create after brazing fin bending lodging; Clamping force is too small, fin is loose. So should be set according to the specific heat exchanger structure design for clamping force.