SOLDERING SOLDERING AND BRAZING Machinery's Handbook, 31st Edition
1566
Metals may be joined without using fasteners by employing soldering, brazing, and welding. Soldering and brazing utilize capillary action to join two or more adjoined fay- ing (in contact) surfaces of a component; only the filler material melts and solidifies to create a strong bond. Soldering involves the use of a nonferrous metal whose melting point is below that of the base metal and in all cases below 840 ° F (450 ° C). Brazing entails the use of a nonferrous filler metal with a melting point below that of the base metal but above 840 ° F (450 ° C). In fusion welding, abutting metal surfaces are made molten, are joined in the molten state, and then allowed to cool. The use of a filler metal and the application of pressure are considered optional in the practice of fusion welding. Soldering Soldering is used to provide a convenient joint that does not require any great mechani- cal strength. It is used in a great many instances in combination with mechanical staking, crimping, or folding, the solder being used only to seal against leakage or to assure electri- cal contact. Soldering employs lead- or tin-base alloys with melting points below 840 ° F (450 ° C) and is commonly referred to as soft soldering. Use of hard solders, silver solders and spelter solders which have silver, copper, or nickel bases and have melting points above 840 ° F (450 ° C) is known as brazing. The accompanying table, page 1567, gives some of the properties and uses of various solders that are generally available. Forms Available.— Soft solders can be obtained in bar, cake, wire, pig, slab ingot, ribbon, segment, powder, and foil form, depending on what is needed for the specific use. In bar form they are commonly used for hand soldering. The pigs, ingots, and slabs are used in operations that employ melting kettles. Ribbon, segment, powder, and foil forms are used for special applications, and the cake form is used for wiping. Wire forms are either solid or they contain acid or rosin cores for fluxing. These wire forms, both solid and core- containing, are used in hand and automatic machine applications. Prealloyed powders, suspended in a fluxing medium, are frequently applied by brush and, upon heating, con - sistently wet the solderable surfaces to produce a satisfactory joint. Fluxes for Soldering.— The surfaces of the metals being joined in the soldering operation must be clean in order to obtain an efficient joint. Fluxes clean the surfaces of the metal in the joint area by removing the oxide coating present, keep the area clean by preventing formation of oxide films, and lower the surface tension of the solder, thereby increasing its wetting properties. Rosin, tallow, and stearin are mild fluxes that prevent oxidation but are not too effective in removing oxides present. Rosin is used for electrical applica- tions, since the residue is non-corrosive and non-conductive. Zinc chloride and ammo- nium chloride (sal ammoniac), used separately or in combination, are common fluxes that remove oxide films readily. The residue from these fluxes may in time cause trouble, due to their corrosive effects, if they are not removed or neutralized. Two methods of inactivating and removing this residue are washing with water containing about 5 ounces (142 g) of sodium citrate (for nonferrous soldering) or 1 ounce (28.35 g) of trisodium phosphate (for ferrous and nonfer- rous soldering) per gallon (1 US gallon = 3.754 liters) followed by a clear water rinse, or washing with commercial water-soluble detergents. Methods of Application.— Solder is applied using a soldering iron, a torch, a solder bath, electric induction or resistance heating, a stream of hot neutral gas, or by wiping. Clean surfaces that are hot enough to melt the solder being applied or accept molten solder are necessary to obtain a good clean bond. Parts being soldered should be free of oxides, dirt, oil, and scale. Scraping and the use of abrasives as well as fluxes are techniques for prepar ing surfaces for soldering. The procedures followed in soldering aluminum, magnesium, and stainless steel differ somewhat from conventional soldering techniques and are indi cated in the material that follows.
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