The six basic steps in brazing are as follows :
1. Good fit and proper clearance
The highest tensile strength of 1,35,000 psi can be achieved when the joint clearance is 0.0015 inch. Higher the gap lesser would be the tensile strength. The surface alloy formed has a very high tensile strength. Higher the gap the tensile strength decreases to that of filler alloy. It is very hard to achieve such a clearance in day-to-day working. We can still get a higher tensile strength of 100,000 psi with a gap of 0.005 inch. Translated into shop practice - an easy slip fit will give a perfectly brazed joint between tubular assembly. If two flat parts are joined, the milling finish will be sufficient to give the required clearance and metal-to-metal contact is the sufficient clearance. For dissimilar metal joining in tubular assembly we have to take into account the coefficient of expansion of both the base metals.
2. Cleaning the base metals
Capillary action will work best when the surfaces of the metals are clean. The following sequence is recommended. Degreasing the component, acid pickle for oxide layer, cleaning in hot water and drying the component. Component can be enclosed if immediate brazing is not possible.
3. Protection from oxidation
The heating of metal surface accelerates the formation ofoxide as an outcome of chemical reaction between hot metal and oxygen from the atmosphere. A coating of flux on the joint will safeguard the surface from the air, preventing oxide formation and dissolve and absorb any oxide that may be present on the metal. It promotes wetting and spreading of molten filler metal. It protects base metal and filler alloy during brazing process. Efficiency of flux is indicated by removal of oxide layer on base metal and filler metal, wetting properties, spread ability and capillarity, temperature resistance and easily replaced by filler alloy. It should be easy to apply or to dispense with and should indicate the brazing temperature and clear visibility of joining area. Flux should have minimum negative influences that are minimum post-corrosion of workpiece and minimum health hazards.
How to apply flux?
Brushing, dipping, spray gun, vapour flux through flame. Connecting a device between fuel cylinders and brazing torch apply vapour flux. The device converts the liquid flux into fine vapour, which reaches brazing area through flame. The colour of the vapour flux flame is green due to complex boroxides for the application of copper to copper joining and steel to copper joining. The joint is very clean and minimum post brazing cleaning operation is needed on using vapour flux generator.
4. Assembly for brazing
We want to hold the part and maintain the correct alignment during heating and cooling cycle, so that capillary action can do the job. The simplest way to hold part is by gravity or we can give a helping hand by additional weight. We use clamps, fixtures, etc to achieve the desired result. The clamps and fixtures should have minimum contact area with job and be made of poor conductor of heat like stainless steel, Inconel and refractory material. This will help in taking less heat away from the job.
5. Brazing the assembly
We have discussed good fit and proper clearance, cleaning the metal, protection against oxidation, and assembly. These were preparatory steps in the brazing process. The fifth steps is the actual accomplishment of the brazed joint. It involves heating the assembly to brazing temperature and flowing the filler metal through joints. The heating method most commonly used is the hand held torch. Other heating processes are furnace, induction and electrical resistance, etc. We have to take care that both the parts are consistently heated. In general, the material joined is a very good conductor of heat and carries heat to cooler area and needs more heat as it dissipates the heat very fast. We have heated the assembly uniformly to brazing temperature. Now we are ready to deposit the brazing alloy. In case of manual brazing all it involves is careful holding of the rod or wire against the joint area. The heated assembly will melt part of filler alloy which will instantly be drawn by capillary action throughout the joint area. Filler metal always flows towards the high heat. Care should be taken so that the uniform temperature is there inside the joint area. You can use preform like rings, washers, shims, and special shape of filler metal, paste and powder. We can pre-place them at the joint area before heating the assembly. In case of furnace, induction and resistance heating the brazing alloys are generally pre-placed.
6. Cleaning the brazed joint
The flux residues are chemically corrosive in nature and if not removed properly can fail the joint in the long run. Common complain of service engineer is leaking of joints within one year. It is mainly due to non-removal of flux and poor quality of brazing. We have to remove the oxide layer formed during heating cycle in near the joint area. The most fluxes are water-soluble and can be washed at 70 degree Centigrade. It can be further removed by metal brush. Overheating in the brazing operation burns the flux and it becomes difficult to remove and we need mild acid solution. A 25% hydrochloric acid at 70°C will remove the most tenacious flux residues. Acid pickle will also remove the oxide layer.
By following all the above steps properly, one can achieve brazed joints that will meet best of quality standards and delight the customers.