Brazing of Aluminium
Pure Aluminium and certain aluminium alloys can be joined by aluminium brazing. A filler material or a brazing rod, usually, an alloy of Aluminium and Silicon of a lower melting point than that of parts to be brazed is used for aluminium brazing. One of the filler materials contains 10 to 13% silicon and melts at 575C. This is 85C below the melting point of aluminium. It is clean and smooth flowing in action with a good penetrative and wetting power, which causes it to flow into and around the joint readily as soon as its melting point is reached.
A flux which removes the surface film oxide and melts at lower temperature than that of the filler rod is essential for easy operation. When heated to a liquid state, flux should flow smoothly and cleanly ahead of molten filler metal. ALDA flux is provided for brazing of ground equipment and general engineering works.
Parts to be brazed must be cleaned and free from oil and grease. Sharp edges should be removed by filing and light level of chamfer at 45 should be provided as a recess, or lead for the filler metal. Slight clearances are better than close fits so that brazing metal can penetrate freely and spread around the joint.
Standard oxy-acetylene welding equipment is used for aluminium brazing. Care must be exercised in adjusting the flame strictly to neutral. To have this neutral flame high-pressure blow pipe and two stage regulator are used. After the parts are assembled and set for brazing, the work should be pre heated with envelop of the flame. Blue cone of the flame should be two inches away from the metal. End of the filler rod is then heated and dipped into the flux, small amount of which adheres to the rod. The ‘tuft’ of flux is then touched on the joint to check the temperature. At the correct temperature the flux will begin to flow smoothly and rapidly along the joint. Then small amount of filler metal is added and the rod is withdrawn. As the rod melts, capillary attraction together with forward sweeping movement of the flame causes the molten metal to flow quickly along the joint.
To prevent corrosion, all traces of flux residue must be removed from joints after brazing. Dipping the parts in water when it is still hot removes greater amount of the flux residue. Another method is to scrub the parts with a stiff bristle brush during immersion in hot soapy water, and then rinse in clean running water. A more positive method is to dip the parts into a solution of 5 to 10 % of nitric acid (HNO3) in water and slightly scrub afterwards. Finally parts are washed in clean running water.
Aluminium brazing should not be used for joining parts, which by virtue of their size or shape, land themselves to welding. Neither can it be used for the repair of casting. Long seams are not suitable for brazing, as deformation during heating may separate the parts.
Application. The process is best suited for the fabrication of sheet metal articles such as radio & radar chassis parts, brackets & joints in extruded sections, handles & spouts in domestic works and small parts for electrical appliances.
Brazing of Cast Iron
It is very difficult to braze cast iron on account of more carbon percentage in metal and the oxide formation during heating. Principle employed to braze cast iron is to heat the work first and to cause alloy to melt by placing it in contact with the hot work. The process is simple provided certain fundamental principles are followed.
The brazing spelter used for cast iron brazing consists of Cu 60% and Zn 40%. The parts to be joined are first cleaned either by filing, grinding, scrapping or sand blasting. A special flux, comprising of boric acid 16 ozs, pulverized potassium chloride 4 ozs and carbonate of irons 3 ozs thoroughly mixed and kept dry is used for brazing.
It is very essential to clean the cast iron parts before brazing and then to secure the parts in position by means of clamps, bolts, wires, or pegs to prevent movement during brazing operation.
To make a joint, the parts are heated to bright yellow colour. It is important not to exceed a temperature of 1100C in this process. After joints have been filled with molten spelter, remove the work from heat source and allow it to cool down slowly. Covering the work with the fine ashes will prevent sudden cooling. Casting may be severely stressed internally and may ultimately fracture if the work is cooled quickly.