The key to Successful Brazing and Soldering.
If you experience difficulties producing sound strong joints re-examine your techniques to check that you are adhering to the basic principles of the process.
You will also find that you use less material!
Brazing can be defined as: "a process of joining generally applied to metals in which, during or after heating, molten filler is drawn into or retained in the space between closely adjacent surfaces of the parts to be joined, by capillary attraction".
The key words are by capillary attraction. Everything that you do, joint design, fluxing, heating is aimed at promoting capillary flow. If this is not achieved, you are not brazing! You are not getting all the inherent benefits and advantages of the process. You are simply using a very expensive filler rod to block a hole!
Everything that you do should be aimed at maximising capillary flow;
Controlling Joint Gaps.
Cleaning of the joint at 600 deg C - the function of the flux.
Developing heat patterns to encourage the flow of alloy.
The alloy melts but just goes into a ball and doesn't flow
There is a flux problem. The flux being used either:
does not have an active life long enough for the heating cycle or;
is not removing all the oxides present.
the joint is being overheated.
The alloy flows everywhere
Look at your heating technique and the positioning of where the alloy is applied. The alloy always flows to where it is hottest. Consider using a smaller burner that enables you to control the heat and create the heat pattern that promotes capillary flow. Metal flow can be controlled by using a blocker over which the alloy will not flow. A cheap method is to paint on Tippex.
It is imperative that all joints are perfectly clean if an alloy is to flow properly and produce sound joints. It is, however, immaterial as to the cleanliness at room temperature, it is the state at soldering or brazing temperature that is all important.
Excessive attention to joint cleanliness during assembly is un-necessary. No matter how much you clean the components, you are going to create more oxide from the heat source than you have so painstakingly removed.
Oxide removal is the function of the flux!
Certainly all parts should be free of oil and grease. If appropriate, de-grease using warm soapy water, solvent, wire wool or a stiff wire brush. This is probably more relevant to soft soldering because the lower temperatures involved may not drive off or burn off grease from handling the components.
Do not use emery cloth or grit based products. These can leave deposits behind that the flux cannot remove leading to porous joints.