Soldering, Brazing – Joining Process

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In soldering and brazing processes, the metal parts being joined are heated but not melted and molten filler material is made to flow between the two closely placed adjacent surfaces by the capillary action. A strong joint between the parts is formed on cooling to room temperature by the bond formed at the high temperature between the parent metal atoms and the filler metal atoms. These process are suitable for joining the dissimilar metals also.

Soldering

The American Welding Society (AWS) defines soldering as a joining process that takes place below 840°F, most of the brazing operations are done at temperatures ranging from 350 to 600°F.

Soldering is a metal joining process is used for making low mechanical strength joints. The filler metal used has a low melting point and is called solder.

For metallic surface to be soldered, surface must be capable of being wetted by the solder. There must be liquid solubility between the solder and one or more of the constituents metals of each part to wet a surface. The atoms of at least one of the component metals of each part to wet a surface. The atoms of at least one of the component metals of the solder may form a solid solution with the metal being soldered but combination of two metals from the liquid solution may result in the formation of intermetallic compound.

The ability of joining solder to wet a surface depends on the cleanliness of the metallic surface. After cleaning, an extremely thin film of metallic oxide immediately forms on most of the metallic surfaces and inhibits its wetting by solder. Flux is used to dissolve the oxide film and also to protect the metallic surfaces thus uncovered until it has been effectively wetted by the solder.

In soldering joining process, the heat is supplied to the joint by soldering iron. The soldering iron may be heated electrically or by other means. The function of soldering iron is to heat the joint. The flat face of the soldering iron is held directly against the joint assembly so that the heat is transferred effectively to the parts being soldered.
Simple soldering joint
In soldering joining process, the heat is supplied to the joint by soldering iron. The soldering iron may be heated electrically or by other means. The function of soldering iron is to heat the joint. The flat face of the soldering iron is held directly against the joint assembly so that the heat is transferred effectively to the parts being soldered.

Solder:
Most of the solders used in soldering joining process are made of lead and tin alloys. Some solders also contain small amounts of cadmium and antimony. The amount of percentage composition of tin and lead determines the physical and chemical properties of joints made with solder. Bar, stick, fill, wire, strip are the different forms solder is available. Solder can be obtained in circular or semi-circular rings or any other desired shape. sometimes the flux can be included with the solder.

Fluxes:
The function of fluxes is to remove the non-metallic oxide film from the metal surface during the heating and soldering operations, so that clean metals may make mutual metallic contact. The flux does not constitute a part of the soldered joint. Commonly used fluxes in soldering joining process are Zinc chloride (Zncl2), ammonium chloride (NH4cl and hydrochloric acid (Hcl).

Brazing

The American Welding Society (AWS) defines brazing as a joining process that takes place above 840°F but below the melting point of the base metals. Most of the brazing operations are done at temperatures ranging from 1100 to 1500°F.

Since, brazing joining process is done at high temperature, brazing is useful for joining thick metal parts for making relatively stronger joints. Both similar and dissimilar parts can be joined. The success of brazing operation depends upon that a fact that a molten metal of low surface tension will flow easily and evenly over the surface of a properly heated and chemically clean base metal, just as water flows over a clean glass plate.

During brazing the base metal of two pieces to be joined is not melted. An important requirement is that, similar to soldering, the filler metal must be wet the base metal surfaces to which it is applied. Some diffusion or alloying of the filler material with the base metal takes place even through the base metal does not reach its solidus temperature.

The surfaces to be joined must be made chemically clean before brazing operation is started. however the fluxes are applied to remove oxides from the surfaces. Borax is the most commonly used flux during brazing process. It will dissolve the oxides of most of the common metals.

Brazing process is similar to soldering but the main difference between brazing and soldering is that brazing requires higher temperature than soldering.

Methods of Brazing:

Based upon the method of heating used in brazing process, different brazing methods have evolved. Two commonly used methods of brazing are:

  • Torch Brazing:
    Torch brazing is 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 brazing purpose as it produces sufficiently high temperature needed for brazing.
  • Furnace Brazing:
    Furnace brazing 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.

Advantages of Soldering and Brazing:

  • Low temperature. Since the base metal does not have to melt, a low-temp heat source can be used. This minimized distortion and creates a smaller heat-affected zone.
  • Joints can be made be permanently or temporarily. Since the base metal is not damaged, parts can be disassembled at a any time by simply supplying heat. The parts then can be reused. The joint made by soldering or brazing process is solid enough to be permanent.
  • Metals of dissimilar can be joined. By using soldering and brazing process dissimilar metals can be easily joined, such as aluminum to brass, copper to steel and cast iron to stainless steel. It is also possible to join nonmetals, i.e. ceramics can be easily brazed to each other or to metals.
  • Speed of joining. Parts can be preassembled and furnace soldered or brazed in large quantities. A lower temperature means less time in heating.
  • Less chance of damaging parts. A heat source can be used that has a maximum temperature below that which may cause damage to the base material.
  • Parts of varying thickness can be joined. Very thin parts or a thick part and a thin part can be easily joined without burning through or overheating them.
  • Easy realignment. Parts can be easily realigned by reheating the joint, re-positioning the parts and allowing the filler metal to solidify.

References:

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