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HELP ME CHOOSE

The easiest and most versatile combination is 455 alloy with EF flux.

The decision to deviate from this combination will be based on other considerations, mainly technical, sometimes financial but will certainly not be based on strength. All silver solders used on properly designed fluxed and heated joints will produce joints stronger than the parent materials. Strength is never an issue.

  • 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
Joining Aluminium

Aluminium is notoriously difficult to join because of its tenacious and very stable surface oxide. Fluxes to remove that oxide are very aggressive and can have a short life. Using the correct flux overcomes the problems allowing the use of most soft solders. Use Stayclean Aluminium Flux.


2207

is a lead free, low melting point soft solder with excellent flow characteristics and when used with Stayclean Aluminium flux will produce strong leak tight joints in a range of aluminium alloys. It also offers the benefit of a reasonable colour match. The combination will also join aluminium alloys to copper, brass, stainless & mild steel.

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Alusol

is a low temperature flux cored, lead bearing soft solder wire. It is available in 1 metre lengths at 0.9 and 1.5mm dia.
It will readily solder commercially pure aluminium and copper aluminium alloys. It is best heated with a soft propane flame.

 

Al12Si
is a 12% silicon aluminium rod suitable for general purposes. Use the same principles of brazing to get the best from this alloy although better results are often achieved if the joint is slightly "belled". Because its melting temperature (575 - 585 deg C) is close to that of pure aluminium (620 deg C) care should be taken when heating to avoid melting the surface of the aluminium. The use of propane is recommended. Bring the joint to temperature in 2 - 3 stages, allowing a short period of "soak time" to allow the joint to "soak up the heat". This will help to prevent the components collapsing during brazing. Use it with AlSi Flux.

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Technoweld
is more akin to a welding rod and tends to produce fillet joints rather than capillary ones. In simple terms, use a wire brush to clean the joint area, warm it with a propane torch to about 350 deg C, melt the alloy onto the surface, abrade the aluminium through  molten Technoweld to effect the joint. It can also be used to repair cracks etc on aluminium castings or build up studs. A pack consists of 5 rods 2.5mm dia x 200. Further details of this alloy and a video link to use it can be found on the Technoweld page under 'Other Silver Solders'.

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Joining Brass  

The joining of brass is quite straightforward (use any silver solder/flux combination) - until the question of colour match enters the equation.

Now the situation becomes more complex.


All silver solders are basically brass with silver and/or cadmium added. This addition is made to lower the melting point of the silver solder to enable you to make joints without melting the parent material.


The downside is that both of these metals are white and change the colour of the alloy. They become lighter in colour.

For a better colour match consider an alloy with less white metal eg 424 containing 24% silver. This has a melting range of 740 - 780°C, but which may be too high.

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Perhaps consider 430 containing 30% silver - melting range 665 - 755°C

If the brass is to be polished probably the best one to use is 445 containing 45% silver with a melting range 640 - 680°C.


Keep the joints small, feed the alloy internally (say with a ring or simple preform), heat externally and draw the silver solder through. This will produce the smallest fillets. The extra silver content offers greater reflectivity making the joint line less visible.

 

 

Flux Coated Rods  

All alloys and of a size of 1.5mm dia and above are available as flux coated rods. 

These rods are ideal for general maintenance work and for working in areas where water, to make up the conventional flux paste, may not be available.

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The coating is flexible but should still be treated with care to prevent it being damaged. 

Best results are obtained with the lower melting point alloys. The joint should be heated generally to a dull red heat before moving the torch to heat the joint to where you want the alloy to flow This creates a heat pattern controlling the metal flow. Apply the rod to the joint. 

The flux residues can be removed easily with water and a stiff brush or with citric acid.

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