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1) Determine the joint spacing:
In the brazing process, the filler metal is drawn into the joint by a pulling force known as capillary action during the heat cycle. So it is particularly important to maintain the right amount of space between the parts to allow this to happen. Usually, the strongest joints are made by allowing just enough space for the filler metal to flow into the joint area, typically in the range of .001” to .005” (0.25 mm to .127 mm). When the clearance is narrower than this, it's harder for the filler metal to distribute itself adequately throughout the entire joint - and joint strength is reduced. Wider spacing will generally result in a weaker joint.
It is also important to remember that metals expand and contract at different rates when heated and cooled. Particularly when joining dissimilar metals, expansion/contraction rates must be allowed for when the parts are positioned.
2) Clean the base metals to be joined:
The braze material will not flow properly if oil, grease, dirt or rust blocks its path. First remove any oil or grease with a degreasing solvent or other method. Then remove rust and scaling with a chemical bath, stainless steel wire brush or Emory cloth. The joint area must be clean.
3) Apply flux:
It is essential to protect the base metals from oxidation during the heating cycle. Oxygen from the gas flame will quickly result in oxide formation on the surface of unprotected metals and prevent effective capillary action. It is very important to use good amount of flux to prevent oxidation. Insufficient amount of flux will quickly become saturated and lose its effectiveness.
4) Position parts carefully:
Before applying heat to the parts, make sure they are properly-positioned and braced to remain in proper alignment. Particularly with lap joints, the laws of gravity help in this regard. Clamps, additional weights and supports are sometimes needed. While choosing support materials, select those that are poor conductors of heat, such as stainless steel or ceramics. These will draw minimal heat away from the joint and preserve the efficiency of heating process.
5) Turn on the heat:
For a strongest braze joint, the metals that are being joined together need to be at close to the same temperature. Slow heat cycles generally produce better results than fast heat cycles. In many brazing operations, the filler metal is applied to the joint after the proper temperature is reached. The melting filler metal will tend to flow toward areas of higher temperature, so it is good practice to apply heat to the side of the assembly opposite to where the filler metal is positioned. The heat then draw the molten metal down into the joint area.
6) Clean the joint:
Parts which are brazed in an open-air atmosphere require a two-step cleaning operation. Flux residues are chemically corrosive and may weaken the joint if not removed. After the filler has solidified, a hot water quench after the heat cycle is recommended. To remove residual oxidation, the parts can be dipped in diluted sulphuric or hydrochloric acid. Care should be taken to avoid etching the joint with too strong an acid solution.