The term brazing often leaves beginner welders curious and a little put off. After all, it's a strange sounding word that means something much simpler than you would think. Brazing is simply a metal joining process. Filler metal is heated above and then distributed by capillary action between two or more close-fitting parts. It is brought slightly above its melting point and is protected by a flux, or suitable atmosphere, and then it flows over the base metal (wetting) and joins the workpieces together as it is cooled. It's not as difficult to master as it may sound and there are seven steps you can take to make sure that your brazing work is successful.
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1. Designing and Fitting
The design is the first step to a successful and strong brazed joint. In order to assure that the joint is as strong as it can be, the joint needs to be designed with an overlap that is three or four times thicker than the thinner member. The lap joint is the easiest way to do this. A scarf join can be the next best choice. Joint clearance – distance between faying surfaces – is another consideration. It's important because brazing works through capillary action and the clearance will define the capillary. Most brazing alloys in the B-Ag and B-CuP families need a clearance of between .001” and .005”. If you're using metals that are dissimilar, don't forget that the rates of thermal expansion may be different when you are deciding about joint clearance.
The metals being joined must be as clean as possible for the best capillary action. Always remove grease and oil using a chemical solvent or vapor degreasing. Abrasive cleaning should be used to remove surface oxides.
When the metals are cleaned, you have to protect them from oxidation while they are being heated. The oxygen in the gas flame will result in oxide formation on unprotected metals and prevent capillary action. Proper fluxing (which is a protective atmosphere) protects the filler metals and the base from oxide formation.
It is very important to maintain the alignment of the base metals while the brazing process is being done so that you know you are getting capillary action that is effective. Gravity alone can often accomplish this but sometimes you need to add a little bit of weight. When parts need fixturing, design the fixtures with the smallest mass possible and with materials that are poor heat conductors. This prevents heat from being taken away from the braze while it is being heated.
5. Selecting Filler Metal
Whenever you select a filler metal for any application, a number of factors come into play. The main ones are the joint clearance, the compatibility with the metals you are joining, the heating method and brazing temperature, appearance, environment, and service temperature. Learn about the different alloys you have available and what each can do and you shouldn't have too much difficulty deciding what is the best one for the job.
There are many different kinds of heating methods available for brazing, with torch brazing being the most common one. The entire assembly should be brought evenly up to brazing temperature. When the parts are at the right brazing temperature (you usually will see a dull red glow) add the brazing alloy to the joint. Capillary action draws the alloy into the joint as long as you have followed the other steps (keeping it clean, etc).
7. Cleaning Post-Braze
Once the metal has been brazed, allow it to air cool and solidify. Flux is corrosive, so remove any flux left by immersing the piece in hot water and brushing it off. If left, it can attack and weaken the joint. When the flux is saturated a residue might result and if so, an acid bath will be needed to remove it. Once it is all removed, you're done and your piece is brazed and strong.