Alloys brazing

For many brazing applications there is a need for a thin, essentially two-dimensional tape that can be fitted in place using an adhesive. 

Tape-cast brazing alloys are flexible, and when laminated to an adhesive layer they can be applied to the surfaces to be brazed even in very complex and convoluted areas. The metallic powders are mixed with the organic components and the slurry is tape-cast in a very conventional manner. The thickness is controlled by the setting of the doctor blade. A wide variety of thicknesses are used for this application, although most of the tapes are in the 0.254 to 1 mm thickness range. 

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Although materials such as Portland cement (see Cement), solder (see Solder and brazing alloys), and siUcates can be considered to be adhesives, this discussion only includes organic materials such as those that form the materials presented in Table 1. 

The solder and ahoy market, including low melting or fusible ahoys, is a principal user of indium (see SoLDERS AND BRAZING ALLOYS). The addition of indium results in unique properties of solders such as improved corrosion and fatigue resistance, increased hardness, and compatibhity with gold substrates. To fachitate use in various appHcations, indium and its ahoys can be easily fabricated into wine, ribbon, foil, spheres, preforms, solder paste, and powder. 

Copper and tin phosphides are used as deoxidants in the production of the respective metals, to increase the tensile strength and corrosion resistance in phosphor bronze [12767-50-9] and as components of brazing solders (see Solders and brazing alloys). Phosphor bronze is an alloy of copper and 1.25—11 wt % tin. As tin may be completely oxidized in a copper alloy in the form of stannic oxide, 0.03—0.35 wt % phosphoms is added to deoxidize the alloy. Phosphor copper [12643-19-5] is prepared by the addition of phosphoms to molten copper. Phosphor tin [66579-64-4] 2.5—3 wt % P, is made for the deoxidation of bronzes and German silver. 

Tin [7440-31 -5] is one of the world s most ancient metals. When and where it was discovered is uncertain, but evidence points to tin being used in 3200—3500 BC. Ancient bron2e weapons and tools found in Ur contained 10—15 wt % tin. In 79 ad, Pliny described an alloy of tin and lead now commonly called solder (see Solders and brazing alloys). The Romans used tinned copper vessels, but tinned iron vessels did not appear until the fourteenth century in Bohemia. Tinned sheet for metal containers and tole (painted) ware made its appearance in England and Saxony about the middle of the seventeenth century. Although tinplate was not manufactured in the United States until the early nineteenth century, production increased rapidly and soon outstripped that in all other countries (1). 

Arsenic added ia amounts of 0.1—3% improves the properties of lead-base babbitt alloys used for beatings (see Bearing materials). Arsenic (up to 0.75%), has been added to type metal to increase hardness and castabiUty (21). Addition of arsenic (0.1%) produces a desirable fine-grain effect in electrotype metal without appreciably affecting the hardness or ductihty. Arsenic (0.5—2%) improves the sphericity of lead ammunition. Automotive body solder of the composition 92% Pb, 5.0% Sb, and 2.5% Sn, contains 0.50% arsenic (see Solders and brazing alloys). 

Silver Brazed Joints These are similar to soldered joints except that a temperature of about 600°C (1100°F) is required. A 15 percent silver, 80 percent copper, 5 percent phosphorus solder is used for copper and copper alloys, while 45 percent silver, 15 percent copper, 16 percent zinc, 24 percent cadmium solders are used for copper, copper alloys, carbon steel, and alloy steel. Silver-brazed joints are used for temperatures up to 200°C (400°F). Cast-bronze fittings and valves with preinserted rings of 15 percent silver, 80 percent copper, 5 percent phosphorus brazing alloy are available. 

Silver-brazed joints are used when temperature or the combination of temperature and pressure is beyond the range of soldered joints. They are also more reliable in the event of plant fires and are more resistant to vibration. If they are used for fluids that are flammable, toxic, or damaging to human tissue, appropriate safeguarding is required by the code. There are OSHA regulations governing the use of silver brazing alloys containing cadmium and other toxic materials. 

Maximum recommended pressure-temperature ratings for brazed joints made with copper tubing and copper or copper-alloy fittings and using representative commercial brazing alloys... 

The two main uses for gold are in settling international debts and in the manufacture of jewellery, but other important uses are in dentistry, the electronics industry (corrosion-free contacts), and the aerospace industry (brazing alloys and heat reflection), while in office buildings it has... 

In general, the rate of dezincification increases as the zinc content rises, and great care needs to be exercised in making brazed joints with copper/zinc brazing alloys, particularly if they are to be exposed to sea-water. Under these conditions, a properly designed capillary joint may last for some time, but it is preferable to use corrosion-resistant jointing alloys such as silver solders (e.g. BS 1845, Type AGJ or /4G5) . 

Joints in copper components may be a source of trouble. Copper/zinc brazing alloys may dezincify and consequently give rise to leaks . In some waters, soft solders are preferentially attacked unless in a proper capillary joint. Copper/phosphorus, copper/silver/phosphorus, and silver brazing alloys are normally satisfactory jointing materials. Excessive corrosion of copper is sometimes produced by condensates containing dissolved oxygen and carbon dioxide. Rather severe corrosion sometimes occurs on the fire side of fire-back boilers and on electric heater element sheaths under scales deposited from hard waters . 

Joining one metal to itself or to another has frequently presented many metallurgical and chemical problems. For a number of years palladium-containing brazing alloys have found widespread applications, as they offer several advantages over other conventionally used materials ... 

An interesting example of judicious choice of braze filler is to be found in the selection of silver alloys for the brazing of stainless steels to be subsequently used in a tap-water environment . Although the brazed joint may appear to be quite satisfactory, after a relatively short exposure period failure of the joint occurs by a mechanism which appears to be due to the break-down of the bond between the filler and the base metal. Dezincifica-tion is a prominent feature of the phenomenon and zinc-free braze alloys based on the Ag-Cu system with the addition of nickel and tin have been found to inhibit this form of attack. A similar result is obtained by electroplating 0-007 mm of nickel over the joint area prior to brazing with a more conventional Ag-Cu-Zn-Cd alloy. 

Upton has recently studied the marine corrosion behaviour of a number of braze alloy-parent metal combinations and has shown that compatibility was a function of the compositions of the filler and parent metals, their micro-structures and chance factors such as overheating during the brazing operation. 

Brazing Alloy filler metal used in brazing. 

About 2.47 million kg of silver are lost each year to the domestic biosphere, mostly (82%) as a result of human activities. As discussed later, the photography industry accounts for about 47% of all silver discharged into the environment from anthropogenic sources. In 1990, about 50% of the refined silver consumed domestically was used to manufacture photographic products 25% in electrical and electronic products 10% in electroplated ware, sterlingware, and jewelry 5% in brazing alloys and 10% in other products and processes. 

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