Welding supplies welding aluminum to steel
(1) cleaning before welding is very important because of the existence of high stable oxide film Al203. In general, the complex process of alkali washing (NaOH aqueous solution, 20-30 ℃, 2-5min) + photochemical (HNO3 aqueous solution, 2-5min) + water washing is used. For cleaning, 60-70% Na2CO3 aqueous solution can be used to clean 5-10min, and then washed and rinsed. The cleaned joint shall be brazed within 6-8h.
(2) brazing method
1) soft soldering: it mainly uses flame soldering, iron soldering and furnace soldering. All need to use flux, and have strict requirements on heating temperature and holding time:
① during flame soldering and soldering iron soldering, avoid heating flux directly by heat source to prevent flux from overheating and failure.
② for the flux with high zinc content, in order to prevent the base metal from corrosion due to the melting of zinc, the heating shall be stopped once the joint is formed.
③ sometimes, the film can be removed by ultrasonic wave or hexagram wiping method, but not necessarily by using flux.
2) brazing: it mainly uses flame brazing, furnace brazing (including brazing in protective atmosphere and vacuum brazing), and penetration brazing.
① flame brazing is suitable for small parts and single parts. Because impurities in acetylene may cause solder failure, it is recommended to use gasoline compressed air flame with slight reducibility to prevent oxidation of base metal.
② when brazing in the air furnace, the filler metal should be preset, and the filler metal should be dissolved in distilled water to prepare a thick liquid with a mass fraction of 50% - 75%. Coating or spraying on the brazed surface can also make a proper amount of powder flux hard cover the solder and the surface to be brazed, and then put the assembled weldment in the furnace for heating and brazing. The heating temperature shall be strictly controlled to prevent the base metal from overheating.
③ generally, paste or foil solder is used for immersion brazing. The assembled weldment shall be preheated to near the brazing temperature, and then immersed in the flux for brazing. Brazing temperature and time should be strictly controlled. If the brazing temperature is too high, the base metal is easy to dissolve, and the brazing filler metal is easy to lose; if the brazing temperature is too low, the brazing filler metal is not melted enough, and the brazing penetration rate is low. Generally, the brazing temperature should be between the liquidus and the solidus of the base metal, and should be determined according to the type and size of the base metal and the composition and melting point of the filler metal. The immersion time must ensure the full melting and flow of the solder, but if the immersion time is too long, it may lead to the diffusion of Si from the solder to the base metal and embrittlement of the near seam area.
In the vacuum furnace, the metal magnesium is often used as the activator to modify the oxide film on the surface and facilitate the wetting and spreading of the solder. Iron can be directly placed on the weldment in the form of particles, or introduced into the brazing area in the form of vapor, or added to the aluminum silicon solder in the form of alloy composition. In order to ensure the full effect of magnesium vapor on the base metal, the local shielding measures of putting the weldment in the stainless steel box first and then brazing in the vacuum furnace can be taken. The advantages of brazing in vacuum furnace are compact joint, smooth surface and no need to clean after brazing, but the disadvantage is that magnesium vapor pollutes the furnace, which needs to be cleaned and maintained frequently, and the equipment price is high.
⑤ when brazing in neutral or inert gas furnace, both magnesium activator and flux can be used to remove the film. The amount of magnesium used in the former is much lower than that of brazing in vacuum furnace. The NOCOLOK brazing method with fluoride flux and nitrogen protection has good results. The fluoride flux does not absorb moisture and has no corrosiveness, so it is free of cleaning after brazing.
(3) residue treatment: except for fluoride flux, there are residue treatment problems in flux brazing.
L) the residue of organic flux for aluminum can be cleaned with methanol, trichloroethylene and other organic solvents.
2) the residue of reaction flux for aluminum can be pickled with hydrochloric acid, neutralized with NaOH aqueous solution, and finally washed with hot water and cold water.
3) the residue of brazing flux for oxide based aluminum can be soaked in hot water of 60-80 ℃ for 10min, then cleaned by brush, washed with cold water, soaked in nitric acid solution with volume fraction of 15% for 30min, and finally cleaned with cold water.