The alloy melts but just goes into a ball and doesn't flow
There is a flux problem. The flux being used either;
a) does not have an active life long enough for the heating cycle or
b) is not removing all the oxides present.
c) the joint is being overheated
Change the flux
How do you control the joint gap?
The important joint gap is that at brazing temperature. When the joint is heated the components expand. If the components are of the same material they will expand to the same extent. The joint gap set at room temperature is maintained.
Different materials expand at different rates. This is known as the co-efficient of expansion.opper and its alloys have broadly similar values. In general terms the model engineer should experience no difficulties when joining copper to brass or bronze.
However there are exceptions.
For example lead bearing, free machining brasses can expand 20% more than copper. This can have an adverse effect on the joint gap. In tube to tube or tube through sheet joints the gap can increase or decrease depending on which material forms the outer component. Ideally the material with the higher co-efficient should be the outer material.
Having made the joint as it cools the outer component (with the higher co-efficient) contracts more. The thermal stresses on the filler metal in the joint are in compression which the filler metal can readily absorb. If the materials are reversed, the inner material contracts more than the outer material. The thermal stresses now put the filler metal in tension. This can result in the filler metal tearing itself apart or separating from one of the components. As the brazing temperature, or the differential between the co-efficients of expansion increases, then so does the risk. Two common material combinations that needs consideration are copper to mild steel and steel to tungsten carbide.
The risk of thermal cracking can be reduced by three simple adjustments to your brazing technique.
Use a brazing alloy with a lower melting range.
Increase the joint gap to the maximum tolerated by the alloy. This physically increases the volume of alloy which will allow more “give” to absorb the stress.
Cool the joint slowly. Do not quench.
Typical relative co-efficients are;
Copper - 9.9
Brass - 10.6
Leaded Brass - 11.3
Al Bronze - 9.0
Mild Steel - 6.0
316 Stainless - 8.9
Cast Iron - 6.5
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.
There are special alloys for brazing copper. Why can't I use them for boiler making?
These alloys contain phosphorus. In a hot sulphur bearing atmosphere, like in a firebox, there is rapid corrosion of the joint leading to joint failure.
For advice on joining different common materials see "Help Me Choose"
Or for more information click here for advice on Joint Cleanliness