Brazing: Like soldering, brazing is a process of joining metals without melting the base metal. Filler material used for brazing has liquidus temperature above 450°C and below the solidus temperature of the base metal. The filler metal is drawn into the joint by means of capillary (entering of fluid into tightly fitted surfaces). Brazing is a much widely used joining process in various industries because of its many advantages. Due to the higher melting point of the filler material, the joint strength is more than in soldering. Almost all metals can be joined by brazing except aluminum and magnesium which cannot easily be joined by brazing. Dissimilar metals, such as stainless steel to cast iron can 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. But the brazed joint is generally not useful for high temperature service because of the low melting temperature of the filler metal. The color of the filler metal in the brazed joint also, may not match with that of the base metal. Because the filler metal reaches the joint by capillary action, it is essential that the joint is designed properly. The clearance between the two parts to be joined should be critically controlled. Another important factor to be considered is the temperature at which the filler metal is entering the joint. During brazing, the base metal of the two pieces to be joined is not melted. An important requirement is that the filler metal must wet the base metal surfaces to which it is applied. The diffusion or alloying of the filler metal with the base metal place even though the base metal does not reach its solidus temperature. The surfaces to be joined must be chemically clean before brazing. However, fluxes are applied to remove oxides from the surfaces. Borax is the most widely used flux during the process of brazing. It will dissolve the oxides of most of the common metals.
Methods of Brazing: Torch Brazing: It is the most widely used brazing method. Heat is produced, generally, by burning a mixture of oxy-acetylene gas, as in the gas welding. A carbonizing flame is suitable for this purpose as it produces sufficiently high temperature needed for brazing.
Furnace Brazing: It is suitable for brazing large number of small or medium parts. Usually brazing filler metal in the granular or powder form or as strips is placed at the joint, and then the assembly is placed in the furnace and heated. Large number of small parts can be accommodated in a furnace and simultaneously brazed.
Braze Welding: In welding processes where the joint of the base metal is melted and a joint is prepared having higher joint strength, it is likely to cause metallurgical damage by way of phase transformations and oxide formation. In this process, the base metal is not melted, but the joint is obtained by means of a filler metal.
SOLDERING: Soldering is a method of joining similar or dissimilar metals by heating them to a suitable temperature and by means of a filler metal, called solder, having liquidus temperuatre not exceeding 450°C and below the solidus of the base material. Though soldering obtains a good joint between the two plates, the strength of the joint is limited by the strength of the filler metal used. Solders are essentially alloys of lead and tin. To improve the mechanical properties and temperature resistance, solders are added to other alloying elements such as zinc, cadmium and silver in various proportions. Soldering is normally used for obtaining a neat leak proof joint or a low resistance electrical joint. The soldered joints are not suitable for high temperature service because of the low melting temperatures of the filler metals used. The soldering joints also need to be cleaned meticulously to provide chemically clean surfaces to obtain a proper bond. Solvent cleaning, acid pickling and even mechanical cleaning are applied before soldering. To remove the oxides from the joint surfaces and to prevent the filler metal from oxidizing, fluxes are generally used in soldering. Rosin and rosin plus alcohol based fluxes are least active type and are generally used for electrical soldering work. Because of the content of acids, these are corrosive at soldering temperature. They can be easily cleaned after the soldering. The organic fluxes such as zinc chloride and ammonium chloride are quick acting and produce efficient joints. But because of their corrosive nature the joint should be thoroughly cleaned of the entire flux residue from the joint. These are to be used for only non-electrical soldering work. Fluxes are normally available in the form of powder, paste, liquid or in the form of core in the solder metal. It is necessary that the flux should remain in the liquid form at the soldering temperature and be reactive to be of proper use. The most commonly used soldering methods include soldering iron (flame or electrically heated), dip soldering, and wave soldering. A soldering iron is a copper rod with a thin tip which can be used for flattening the soldering material. The soldering iron can be heated by keeping in a furnace or by means of an internal electrical resistance whose power rating may range from 15 W for the electronic applications to 200 W for sheet metal joining. This is the most convenient method of soldering but somewhat slower compared to the other methods. In dip soldering, a large amount of solder is melted in a tank which is closed. The parts that are to be soldered are first cleaned properly and dipped in a flux bath as per the requirement. These are then dipped into the molten solder pool and lifted with the soldering complete. The wave soldering is a variant of this method wherein the part to be soldered (e.g.” an electronic printed circuit board, PCB) is not dipped into the solder tank, but a wave is generated in the tank so that the solder comes up and makes a necessary joint.
Basic Operations in Soldering: For making soldered joints, following operations are required to be performed sequentially.
1. Shaping and fitting of metal parts together: Filler metal on heating flows between the closely placed adjacent surfaces due to capillary action, thus, closer the parts the more is solder penetration. This means that the two parts should be shaped to fit closely so that the space between them is extremely small to be filled completely with solder by the capillary action. If a large gap is present, capillary action will not take place and the joint will not be strong.
2. Cleaning of surfaces: This is done to remove dirt, grease or any other foreign material from the surface pieces to be soldered, in order to get a sound joint. If surfaces are not clean, strong atomic bonds will not form.
3. Flux application: Soldering cannot be done without a flux. Even if a metal is clean, it rapidly acquires an oxide film of submicroscopic thickness due to heat and this film insulates the metal from the solder, preventing the surface to get wetted by solder. This film is broken and removed by the flux. The flux is applied when parts are ready for joining.
4. Application of heat and solder: The parts must be held in a vice or with special work holding devices so that they do not move while soldering. The parts being soldered must be heated to solder-melting and solder-alloying temperature before applying the solder for soldering to take place the assembly
so that the heat is most effectively transmitted to the being soldered. As soon as the heat is applied, the flux quickly breaks down the oxide film (the insulating oxide layer barrier between the surface and solder). Now solder is applied which immediately melts and metal to metal contact is established through the medium of molten solder. Finally, the surplus solder is removed and the joint is allowed to cool. Blow torches dipping the parts in molten solder or other methods are also used for soldering.
Solders: Solders are alloys of lead and tin. Solder may also contain certain other elements like cadmium, and antimony in small quantities. The percentage composition of tin and lead determines the physical and mechanical properties of the solder and the joint made. Most solder is available in many forms-bar, stick, fill, wire, strip, and so on. It can be obtained in circular or semi-circular rings or any other desired shape. Sometimes the flux is included with the solder. For example, a cored solder wire is a tube of solder filled with flux.
Solder Fluxes: The flux does not constitute a part of the soldered joint. Zinc chloride, ammonium chloride, and hydrochloric acid are the examples of fluxes commonly used in soldering. The function of fluxes in soldering is to remove oxides and other surface compounds from the surfaces to be soldered by displacing or dissolving them. Soldering fluxes may be classified into four groups-
(1) Inorganic fluxes (most active)
(2) Organic fluxes (moderately active)
(3) Rosin fluxes (least active), and (4) Special fluxes for specific applications