Silver Solders
Page 49
If you've noticed an error in this article please click here to report it so we can fix it.
—Their Composition, Application and Use
SUITABLE for use with ferrous and non-ferrous metals, silver solders have a field of application that may be defined as that in which soft solders are net strong enough, or have too low a melting point.Indeed, to describe them as solders is somewhat misleading, for the melting point renders it impossible for them to be manipulated by means of a soldering iron.
• The heat is usually • supplied by -a blow-pipe flame and the function of the solder is thus more nearly that of a brazing medium, although the ease with which the solder melts and flows, and the strength and reliability of the finished joint is superior to that obtained by spelter brazing.
The Silver-content Range.
In view of the fact that their composition has a vital bearing on the working and application of these solders, it will be of interest to examine this matter before proceeding to invesfigate physical properties and mani pulation. Whilst silver solders are esgentially alloys of silver, copper and zinc, within that definition they vary considerably, the percentage of silver ranging. from under 10 to over 80, and the zinc being present in amounts from 40 per cent. downwards.
For general-engineering use, the silver-content is usually about 40-50 per cent., the colour being light yellow, which deepens or lightens according to the proportion of silver. An advantage of this is that it is possible to match the solder to the 'metals to be joined, and thus to make the joint in. visible. In certain spheres this is an important consideration.
Effects of Other Metals.
Other constituents than silver, copper and zinc may be present in small quantities, with varying results. A small amount of nickel, for instance, improves the strength and the corro
sion-resisting properties. Tin lowers the flow point but makes for brittleness and, for that reason, is undesirable, as also is lead. Cadmium, in general circumstances, is to be welcomed, for, whilst being of assistance in making the solder, when substituted for some of the zinc in low-silver/content solders, it reduces the flow point appreciably.
Silver solders are notably ductile, and joints made with them are admirably suited for service where vibration and shocks are paramount, The fact that these solders are now being adopted for
aeroplane work, wherever possible, is a certain proof of their reliability. Their tensile strength varies from 18 tons per sq. in. °for a 75-silver, 22copper and 3-zinc mixture to about 29 tons per sq. in. for a 60-silver, 25copper and 15-zinc alloy.
Resistance to corrosion is of great importance in a jointing medium. Silver solders possess it to a more marked degree than the general run of nonferrous materials. Corrosion resistance increases with the silver-content.
The melting points of silver solder vary, with the compositions, from about 700 degrees C to 870 degrees C. In working them, an oxy-acetylene torch generally gives the best results so long as the flame is kept moving in circles and is not permitted to dwell, as the heat is intense. The parts to be joined should be thoroughly cleaned, chemically, if necessary.
The Importance of Pressure.
The solder is intended to be used as a thin film and not as a filling medium, therefore it is essential that the edges of the joint should fit closely together. Equally important is pressure, and if the two components of the joint can he mounted in a fixture to press them tightly together, the improved results will more than justify the extra trouble.
As a flux for general work, borax in a hot saturated solution, in a paste or in a powder form, will be found satisfactory. The whole if the work face should be well covered with flux, for insufficient flux is the prime cause of failure in silver-solder work.
It is almost as important that the joint and surrounding surfaces should be slowly and thoroughly pre-heated, so that heat may not be rapidly conducted away when soldering is commenced. At the same time, the joint should not be heated for Icing at a temperature just below the melting point of the solder as oxidation at this tern
perature is rapid, and although the flux will dissolve the oxides, its efficiency is unnecessarily reduced thereby before soldering is commenced.
If, on completion, the joint be allowed to cool slowly, the flux will form a film on the work which is difficult to remove either by scraping or filing. A means of overcoming this is to wash the joint in a mild sulphuricacid solution.
Applications of the Process
Although non-ferrous metals take silver solder better than ferrous, good results can be obtained on iron and steel. Stainless steel is among the most difficult metals to work on owing to the formation of an oxide film which prevents the solder from running properly. Moreover; the. temperature necessary is unsuitable for such steels and difficulty is likely to arise from minute cracks.
however, by applying a .special flux Of equal parts of borax and boric acid, mixed to a paste with a Saturated zinc. chloride. solution,' it is possible,. with care to get good results, Pressure is imperative in making these joints. The lower the silver-content the better the results, is the general experience.
Aluminium bronze is one of the few other materials: likely to give trouble, and the above remarks apply to it.
Little difficulty arises in dealing with brasses and bronzes, one point to observe being that the low melting points of some brasses require the use of high-grade low-melting-point solders. Because of its high heat conductivity and tendency to oxidation, high-silver-content solders are preferable for copper.
Soldering Nickel Alloys.
Solders with 50 per cent, or more of silver are again desirable for Monetmetal, and care is needed to avoid overheating, which may cause cracking. Oxidation may prove a difficulty, but this can be overcome with a plentiful use of flux. The same applies to nickel and nickel silver.
The choice of silver solders is a wide one, and it is unlikely that anything will require soldering for which a suitable solder is not forthcoming. Continual research is being carried out as to the effects of small additions of manganese, phosphorous and silicon and this phase of jointing practice is one that it should repay the progressive maintenance engineer to watch.
F.Y. 1136