Brazing Joints and Surface Preparation

Common categorization of joint is butt joint and lap joint is also applicable to brazing joints. Normally a butt joint provides very limited area for brazing. We know the strength of the joint depends on the brazing area and so limited brazing area is responsible for week joint formation. To increase the brazing area the mating parts are often scarified or stepped by altering them through extra processing. This are demonstrated in figure below. The extra processing makes the alignment of parts, during brazing, slightly difficult.

Brazed Butt Joints

The other type of joint used in brazing is lap joint. Lap joint can provide relatively larger overlapping area and so better strength. The parts (workpieces) to the joined are kept so that some of their contact area should remain overlapped. Brazing is done on the overlapped edges of both the parts. Some examples of lapped brazed joints are shown in figure below.

Examples of Lap Joints by Brazing

In case of lapped joints over lap of at least three times the thickness of the thinner part is recommended. An advantage of brazing over welding while making lap joints in that the filler metal is bonded to the workpieces throughout the entire interface area between the parts rather than only at the edges. Clearance between the mating surfaces should be large enough so that molten filler metal can flow throughout the entire overlapped area. At the same time clearance should be small enough so that capillary action can exists to facilitate the flow of molten filler metal between the overlapped areas. Recommended clearance is upto 0.25 mm. Other important instruction for making brazing joint is cleanliness of mating surfaces. The mating surfaces should be free of oxides, oils,  grease, etc. to make wetting and capillary action comfortable. Cleaning may be done using mechanical means or by chemical treatments depending on the situation.

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BRAZING FLUXES, EQUIPMENTS AND FILLER METAL

Main property of brazing filler metal is its fluidity, its capability of penetration into the interface of surfaces. Melting point of filler metal must be compatible with workepiece metal. Molten filler metal should also be chemically insensitive to the workpiece metal. Filler metal can be sued in any form including powder or paste.

Purpose of brazing flux is same it is in case of welding. It prevents formation of oxides and other unwanted by products making the joint weaker. Characteristics of a good flux are :

  • low melting temperature,
  • less viscosity so that filler metal (molten) can displace it, and’
  • adhering to the workpiece.

Common fluxes are borax, borates, chlorides and florides.

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