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Bronze Welding

Bronze welding is a process of joining metals in which the edges of the metal are heated to a temperature below their melting point and a bronze alloy is simultaneously melted and caused to flow over the edges and join with them. By this means a strong and ductile union is produced between high melting point metals, i.e. cast iron, steel, copper and malleable iron etc.

Bronze welding resembles brazing up to a point, the application of brazing is generally limited to joints where a close fit or mechanical fastening serves to consolidate the assembly. In bronze welding the filler alone provides the joint strength, and is applied by the manipulation of a heating flame in the same manner as gas welding. The heating flame to serve the dual purpose of melting off the bronze filler and simultaneously heating the surface to be joined. the operator in this manner controls the work, hence the term "bronze welding".

Almost any copper-zinc alloy can be used as a medium for such welding, but the consideration of costs, flowing qualities, strength and ductility of the deposit, have led to the adoption of one general purpose 60-40 copper-zinc alloy with minor constituents incorporated to prevent zinc oxide fuming and to improve fluidity and strength.

Silicon is the most important of these minor constituents, it is useful in three ways :-

  1. Silicon in the manner with which it readily unites with oxygen to form silica, provides a covering for the molten metal which prevents zinc volatilisation and also serves to maintain the balance of the constituents of the alloy to be carried through to the deposit.
  2. The coating of silica combines with the flux used in bronze welding to form a very fusible slag, which material assists the tinning operation, which is an essential feature of any bronze welding process.
  3. Silicon prevents the formation of gas holes and porosity in the deposited weld metal, which would reduce the strength of the weld, by its capacity for retaining gases in solution during solidification of the alloy.

Silicon introduces valuable properties to the bronze welding rod, other elements, such as lead, have a detrimental effect on the deposit. A welding rod with a reduced lead content is essential if porosity, brittleness and low-tensile strength is to be avoided.

Welding Technique

The type of metal being welded has no effect on the welding technique. It is essential that the surfaces are cleaned and degreased, the temperature is raised to approximately 850C, or dull red in colour. The bronze rod is melted by the heating flame and allowed to flow over these surfaces.

The types of flame which can be used are :-

  • Oxy-acetylene
  • Oxy-hydrogen
  • Oxy-coal gas
  • Air-acetylene

The Oxy-acetylene flame has the highest flame intensity and is most generally used; the use of Air-acetylene is limited to work on light gauge material. The size of the flame depends on the thickness and mass of metal to be welded, and upon the extent of pre-heating. To reduce volatilisation of the Zinc from the bronze alloy by forming a skin of oxide on the surface of the deposit, a slightly reducing (oxidising) flame should be used.

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