Brazing is a process of making joints where in coalescence is produced by heating to suitable temperatures above 5000 C and by using a non-ferrous filler metal having a melting point (up to 9000 C) below that of the base metal, the filler metal being distributed between the closely fitted surfaces of the joint by capillary action. Brazing gives a much stronger joint than soldering. The principal difference is the use of a harder filler material commercially known as spelter. Filler metals used in this process may be divided into copper base alloys and silver base alloys. The spelter is usually an alloy of copper, zinc and tin. Both similar and dissimilar metals can be joined.
The clearance between the two parts to be joined should be critically controlled. If there is too much of clearance, the capillary forces may not be sufficient to draw the filler metal into the joint, whereas insufficient clearance may have too small an amount of filler metal to give rise to any effective strength. Another important factor to be considered is the temperature at which the filler metal enters the joint. While designing a brazed joint, care has to be taken to account for the differences in the coefficients of thermal expansion of the two pieces to be joined.
The end of the parts which are to be joined must be chemically clean. The flux along with spelter (filler metal) is applied to remove oxides from the surfaces. Borax is the most widely used flux. It will dissolve the oxides of most of the common metals. The parts to be joined are either clamped or held together through some other suitable means and heated. The spelter, together with the flux melts and flows along the contacting surfaces, unites with them and solidifies on cooling to form the joint. It is a good practice to prepare the brazing mixture in the form of a paste and than apply it to the surface. This paste is made by mixing the spelter and borax (flux) is equal proportions and adding proper amount of water to it to form the paste. Other fluxes used are mixtures of borax, boric acid, fluorides and chlorides.
Brazing is a much widely used joining process in various industries because of its many advantages. Dissimilar metals, such as stainless steel to cast iron can be joined by brazing. Almost all metals can be joined by brazing except aluminium and magnesium which cannot easily be joined by brazing.
Because of the lower temperatures used there is less distortion in brazed joints. Also, in many cases the original heat treatment of the plates being joined is not affected by the brazing heat. The joint can be quickly finished without much skill. Because of the simplicity of the process it is often an economical joining method with reasonable joint strength. The brazed joints are reasonably stronger, depending on the strength of the filler metal used.
Silver brazing makes use of a silver based filler metal. Silver brazing is used to give high strength joints. Though originally used for jewellery applications, silver brazing is now extensively used in industrial applications. They can be used with a large range of materials, but because of its high cost it is used in only special applications requiring high strength and high temperature service.
Applications of Brazing:
Brazing has been used to manufacture a wide variety of products such as Honey comb sandwich panels for aircraft missiles, motor cycle frames, air plane propellers, Hydraulic fitting, refrigerator evaporators, manufacture of cutting tools etc.,
The use of pressure-vacuum brazing has found wide spread acceptance in the general application of brazing joint in nuclear, aerospace engineering.
The advantages bounded to the use of the induction technology are the inexpensiveness of the working phase, the carrying out speed, the high qualitative standard you can get and the perfect monitoring of all possible parameters at stake.
