Fri, 13 Jul 2018 02:33:23 +0000
Aluminum vacuum brazing process
I. vacuum brazing preparation
1) vacuum brazing furnace boiler stop more than 24 hours, before the vacuum brazing required in no less than 450 + 20 ℃ temperature bake for an hour (or 700 + 50 ℃ heat preservation 60 minutes).
2) the preheating of diffusion pump shall not be less than 1 hour before the opening of vacuum brazing furnace, and shall not be less than 70 minutes in winter;
3) the dosage of magnesium powder should be 15g/m3.
4) according to the operating rules of aluminum vacuum brazing, make the debugging of all the systems, keep the water, electricity, gas and other instruments display normal, and the data are consistent.
Ii. Supporting data
1) open the water cooling system of the cut-off valve, keep the water pressure in 0.2 ~ 0.3 MPa, keep clear of drainage, working in the furnace during import water temperature is not lower than 25 ℃, the temperature is not higher than 30 ℃.
2) when the vacuum brazing furnace is stopped at ordinary times, the furnace needs to be vacuumed, and the vacuum should be maintained at no less than 5 x 10* -2pa.
3. Vacuum furnace installation
1) send the clamped beam assembly to the central thermostat area of the furnace with a forklift and put it in order;
2) the workpiece should be stackable, reliable and tidy;
3) the workpiece is not allowed to contact the heating element. If found, the workpiece should be immediately removed and adjusted.
4) wipe the sealing ring and contact surface of the furnace door with alcohol or acetone, close the furnace door and tighten the buckle.
Vacuum brazing mechanism
Vacuum brazing process is in a vacuum environment (reaches 6. 5 x 10-3 pa), as the temperature rises to above 577 ℃, the oxide film on aluminum alloy surface under the action of magnesium (Mg) element gradually shedding and molten solder brazing seam formation, during this period, there are two process, summarized as to membrane process and the weld forming process, can be respectively as follows:
4. 1 vacuum brazing film removal process
Magnesium (Mg) in addition to eliminate the oxygen in the environment and the harmful effect of moisture, also penetrate membranes under the parent metal surface, with diffusion into silicon, the surface layers of the formation of low melting point of the Al – Si – Mg alloy melt, thus destroy the surface oxide film and the combination of the parent metal, enable the molten solder wetting parent metal, along the parent metal spread out under membrane, and float to the surface membrane and remove. But the amount of Mg element of solder is established, the oxygen content in certain conditions on membrane ability can satisfy the requirement of brazing, according to the test results, aluminum plate-fin heat exchanger cleaning qualified parts had better finish brazing production within 24 h, the longest must not exceed 72 h. Parts over 72 hours need to be cleaned again. It should be avoided that the parts are placed for too long after cleaning, resulting in the formation of oxide film on the surface of aluminum alloy, which will affect the quality of vacuum brazing. Aluminum fin type radiator parts cleaning is completed, with the increase of spare parts storage time, the parts surface oxygen content gradually to increase, will affect the vacuum brazing seam quality, eventually resulting in a decline in the strength and the weld quality and performance of the product, can not meet the requirement of the product design.
4. 2 forming process of vacuum brazing weld
Brazing seam is the key to obtain good performance makes the molten liquid solder fully into the brazing gap filling, and with the parent metal very good physical chemistry, satisfy requirement of brazing seam after condensing crystallization. The formation of aluminum alloy vacuum brazing joints can be roughly divided into three steps:
The first is the removal of oxide film on aluminum alloy surface.
Second, filler metal filling weld;
The third is the interaction between the filler metal and the base metal.
Vacuum brazing process
1) according to the corresponding temperature change curve, the control procedures of the temperature of each period and the rate of rise and fall were prepared;
2) when the vacuum reaches 1.6 ~ 2 * 10-2pa, the heating procedure of the vacuum brazing furnace can be opened;
3) during the whole brazing process, nitrogen is filled to accelerate the heating and cooling rate. The instruments should be observed frequently and the time interval recorded.
Prevent abnormal occurrence;
4) brazing temperature curve and the melt temperature under 500 ℃ to strictly abide by, allow based on what furnace charging quantity and the change of environment conditions