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.
I want to join stainless steel
455 gives a reasonable colour match but the first alteration is to the flux. EF flux will not remove chromium oxide. Use HT5 with the 455.
A potential problem after silver soldering stainless steel is the phenomenon of "crevice corrosion" when water attacks the joint interface leading to joint failure. This is prevented by either
a) using a silver solder containing nickel, eg 449, which are normally more expensive and less free flowing or
b) using a grade of stainless steel that contains nickel. This range of steel is quite commom. They are in the 300 series eg 316. Not sure what grade of stainless you have? Use a magnet. All the nickel bearing grades are non-magnetic.
If you want a lower temperature alloy use the solder 2207 flux cored wire or any soft solder using Staybrite flux. 2207 offers an excellent colour match.
I want to make two joints in close proximity at different times
With skill and care this can often be done using the same silver solder. When it melts, it dissolves some of the parent materials so altering its' composition and melting temperature.
Alternatively initially use an alloy of lower silver content and higher melting range eg 438, 424, or 418 although the latter two will need the use the longer life flux HT5.
I want a cheaper alloy
Select one with a lower silver content. The down side to this is that the alloy will take longer to melt (more heat) and the joint gaps may need opening (more alloy) to to achieve good penetration. Often the cheapest joints are made using 455 and small gaps.
I want a good colour match on brass
A question that is impossible to answer! There are so many grades each with its' own colour. Try reducing the silver content of the alloy eg 430.
Alternatively keep the visual amount of silver solder to a minimum. Design the joints such that alloy is drawn from the inside to the outside by preplacing a ring or slug of solder and heating from the outside. The result is a fine witness that will be almost invisible when the joint is polished. The reflectivity of 445 adds to the illusion.
I want to join cast iron
Silver solder will not flow over the graphite on the surface of the iron. Remove it by heating the iron to a red heat (no flux) with the oxidizing part of the flame. That is the area outside the blue zone. Allow to cool and brush with a stiff wire brush. Apply HT5 flux and treat normally.
I want to join aluminium bronze
Conventional fluxes will not remove the aluminium oxide if the aluminium content is above 2%. Increase the activity of EF flux by adding 25% by weight of kitchen salt (sodium chloride) to the flux. Treat normally using 455.
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.
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.
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'.
The joining of brass is quite straightforward (use any silver solder/fluxcombination) - 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.
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.
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 toheat 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.