Both the soldering and brazing processes are similar to welding in so far as they are methods of uniting metals, but they are different in that the filling or joining material is usually of different composition from the pieces joined. The work of brazing brass and soldering lead resembles the welding process without the hammering but it is not welding in the proper sense of the term; it is, rather, a sort of "metallic gluing" process.
Soldering is the process of joining pieces of metal by filling the space between them with material known as "solder", and doing it in such a way that the solder will adhere to both of them and hold them together. The first requirement of a good solder is that it will "wet" the surfaces, or amalgamate with the pieces to be joined. Alloys of lead and tin, with the addition at times of other metals, are the usual solders, although special solders are made without either of them. Soldered joints are not so strong as welded or brazed joints because of the material used but they are frequently used because they are easily and cheaply made.
The process requires less heat than welding or brazing and the joints may be cooled rapidly. An ordinary gas flame with a foot pump for the blast, or even a candle and a blowpipe, may be used for soldering. If the piece worked upon is so large that it rapidly conducts away the heat, a gasoline or kerosene torch is used to preheat the piece and prevent chilling the solder. The blowpipe is used for small work like jewelry and the only point to watch is to be sure the flame is hot enough to burn all of its carbon. The blue center of a flame is highly oxidizing whereas the yellow outside portion causes a reducing action.
Fluxes are essential to successful soldering and there are several kinds on the market, although most operators soon learn to make their own. The fluxing materials generally used are salammoniac zinc -chloride solution, rosin in alcohol, borax, and ' tallow. Flux is used to dissolve grease and to remove any oxide present after cleaning. It combines chemically with.the oxide and leaves a clean surface of metal which the solder will wet. Sometimes the flux is combined with the golder when making up the sticks, and this is known as "self-fluxing" solder.
The tools required for soldering are very simple and few in number. The source of heat for the operation is usually a soldering "copper" or "bit," Fig. 36, which has in turn been heated in a gas or charcoal stove, Figs. 37, 38, and 39. It is made with a comparatively large head, small shaft, and wooden handle and has a pointed tip on the head for working. The tip should be "tinned" before using, and this is done by first cleaning and sandpapering, fluxing with zinc chloride, and heating. While hot it is rubbed well with a stick of tin which adheres to the surface, and it will remain tinned for a long time unless heated to a red heat. A type of grooved soldering copper is shown in Fig. 40, and an electric soldering tool is shown in Fig. 41. Solders. Most solders melt at about 200 degrees centigrade (392° F.), the softer solders melting at about 180 degrees centigrade (356° F.) and harder solders at about 330 degrees centigrade (626° F.).
Fig. 36. Soldering Copper Courtesy of Central Electric Compony.
Fig. 37. Soldering Iron Heating Stove Courtesy of Central Electric Company.
There are so many special solders on the market that no closer figures can be given, but these will be a safe guide for most of the ordinary kinds of work. Ordinary solder is half tin and half lead. Hard solder is two parts lead and one part tin. Antimony is sometimes added to harden and stiffen solder, and arsenic is added to make it flow more freely. Bismuth makes the solder brittle and cadmium makes it soft. When bismuth and cadmium are both added they reduce the melting point, and "Wood's metal", which contains two parts of tin, two of lead, two of cadmium, and eight of bismuth, melts at 70 degrees centigrade (158° F.). Copper strengthens solder but raises the melting point very rapidly.
Fig. 38. Charcoal Stave.
Courtesy of M. Klein and .Son.
Fig. 39. Gaeline Furnuce Courtesy of Central Electric Company.
Fig. 40. Grooved Soldering Copper Courtesy of central Electric Company.
Fig. 41. Electric Soldering Tool Courtesy of Central Electric Company.
The process of soldering consists in first scraping the surfaces clean; heating the pieces to the soldering temperature by any suitable means; fluxing the surfaces to be joined; applying the solder with the solder bit; and finishing the joint by smoothing off. Rust and grease must be scraped or washed off with some alkaline solution. With liquid fluxes it is best to apply them after heating the surfaces to be joined, in order to eat away the film of oxide and keep the surface clean but, with borax or rosin solutions, it is best to apply them cold. After the surfaces are properly cleaned, heated, and fluxed, the solder is melted on with a flame or by rubbing with a hot bit and then the joint is smoothed up.
The most important points to watch are the temperature and the flux. Too much heat causes oxidation, makes the solder run too freely, and burns the tinning off the soldering tips. Poor fluxing prevents the solder from amalgamating with the pieces and, hence, they will not join. Platinum, silver, gold, tinned sheet iron, and most other metals may be soldered but it has thus far been practically impossible to solder aluminum commercially. A number of processes have been devised for soldering aluminum but none of them are in very general use and most of them have been failures.