Soldering is the process of joining two or more similar or dissimilar pieces of metal by the application of low melting alloy (solder) and heat. The ease with which a metal can be soldered depends upon the characteristic of the oxide and mutual solubilities of the solder and base metal. When soldering zinc or zinc coated (galvanized) iron, care must be taken in the selection of solder, flux and also the temperature employed for the operation.
The soldering of aluminium has always been surrounded by an atmosphere of mystery. It is a difficult operation, but with modern methods, soldering of aluminium is not so difficult. It requires no special equipment and can be accomplished without subjecting the metal to high temperature.
Soldering of Zinc
The solder recommended for soldering of zinc or zinc coated (galvanized) iron is a tinman’s solder containing 40 to 50% tin and remaining lead. It is very important that this solder contains very little or no Antimony. If it is present above 0.5%, hard metallic crystal is formed which makes it difficult to obtain a sound joint. The soldering operation should be completed as quickly as possible and overheating must be avoided.
Zinc soldering requires an active flux. Sometime diluted Hydrochloric Acid (HCl) also known as spirit of salts is used, but this solution is very corrosive. It is better to use a flux of chloride type such as Zinc Chloride (ZnCl). This exerts a rapid cleaning action and gives equally good soldering results with a greatly reduced risk of corrosion. To overcome this, it is advisable to wipe the joint with a clean cloth immediately after soldering or wash it in warm water.
Soldering of Aluminium
Soldering aluminium is a very difficult operation. The reason is that aluminium becomes rapidly oxidized in air and coated with a very thin oxide film which impedes the soldering process. This oxide film is removed by mechanical and chemical cleaning during soldering process. The main disadvantage of aluminum soldering is that the joints are susceptible to corrosion. The simple protective measures outlined later will ensure durable joints where moist conditions are not encountered. Aluminium soldering should be used only when welding is not possible.
Many solders are available which give good results provided sufficient care is taken in the initial tinning operation. Most aluminium solders contain 60 to 70% tin with the balance being primarily zinc.
One of the solder is ‘Fryol’. Aluminium Solder which when molten, is able to penetrate the cracks in the skin (aluminium oxide film) of Aluminium. As a result, the operation is generally simplified and amount of scrapping during soldering is reduced. Thus tinning may often be obtained merely by rubbing the stick solder on the heated surface.
When heated, Fryol commences to melt at 1700 C and becomes completely fluid at 3000 C passing through a pasty stage before melting completely. It can be applied in pasty condition and tinning can be obtained at temperatures as low as 2500 C at which there is little danger of distortion and no serious effect on heat-treated alloys. Once a tinned coating has been obtained, the joint may be completed with the aluminium solder.
The solder may be worked in the pasty form to give the desired finish. When the solder is required to flow, for example while making lap or butt joint, it is best after tinning both surfaces with aluminium solder, to complete the joint with ordinary tinman’s solder which is more fluid.
Soldering Fluxes for Aluminium
The ordinary soldering fluxes are useless for Aluminium soldering. Special fluxes have been used to assist the initial tinning. But these are not very effective and may cause corrosion. It is sometime helpful to use a non-corrosive flux, which improves the fluidity of the solder and minimizes dross formation. “Frys” zinc flux is suitable for this purpose.
Points to be Observed in Aluminium Soldering
Following points should be adhered to while carrying out soldering of aluminium :
Cleanliness is essential, clean the surfaces to be joined properly and use clean tools.
Heat the metal around the spot to be soldered and rub the solder on joint surface until it melts.
Scrap the surface of Aluminium underneath the molten solder with a scrap tool such as hacksaw blade, screw driver or a scratch brush. If a soldering iron is used a scraper can easily be attached.
When the surfaces are well tinned, complete the joint with either the special solder or ordinary tinman’s solder. Make sure that the solder has solidified before subjecting the joint to any pressure.
After soldering protect the joint against moisture attack with a coat of paint, lacquer or cover with a film of grease.