Temperature melting, fusible alloys

Fusible Safety Devices. Low melting bismuth alloys, especially those that are eutectic have found numerous uses in safety devices. These alloys can be cast into any shape necessary in order to be used in a plug or switch that must function at a given temperature. 

The above techniques are not satisfactory for solid sample containing light fractions that will be lost if exposed to ambient conditions of pressure and temperature. Samples like these can best be handled by cooling the sample and encapsulating them in small sealed metal—fusible alloy—containers (capsules) or glass capillaries. The metal must have a low melting point. 

An alloy known as Wood s Fusible Alloy, which melts at a much lower temperature even, is composed of tin, two parts lead, two parts bismuth, seven parts cadimiun, one to two parts. This melts at about 68 centigrade, boiling point loo . 

Historically, fusible alloys are low-melting-point metallic alloys with a temperature of fusion below the boiling point of pure water (100°C). By extension, other alloys with a melting point below 200°C are also included in this group. 

FIGURE 484 A spot-type combination rate-ol-rise, fixed-temperature device. The air in chamber A expands more rapidly than it can escape from vent B. This causes pressure to close electrical contact D between diagram C and insulated screw E. Fixed-temperature operation occurs when fusible alloy F melts releasing spring G which depresses the diaphragm, closing the contact points. 

Because of its low melting point, extreme softness and ductility, and excellent wetting characteristics, indium has been widely used as an alloying element for many low-temperature solders and fusible alloys [53,63]. However, because of its high price (several times more expensive than silver), it is not economically feasible to use In-rich alloys for replacing Pb n solders, which are... 

Low Melting Alloys. Lead ahoys having large amounts of bismuth, tin, cadmium, and iadium that melt at relatively low (10—183°C) temperatures are known as fusible or low melting ahoys. The specifications of many of these ahoys are Hsted ia ASTM B774-87 (7). 

High temperatures may be obtained also with the aid of baths of fusible metal alloys, e.g. Woods metal - 4 parts of Bi, 2 parts of Pb, 1 part of Sn and 1 part of Cu - melts at 71 °C Rose s metal - 2 of Bi, 1 of Pb and 1 of Sn - has a melting point of 94 °C a eutectic mixture of lead and tin, composed of 37 parts of Pb and 63 parts of Sn, melts at 183 °C. Metal baths should not be used at temperatures much in excess of 350 °C owing to the rapid oxidation of the alloy. They have the advantage that they do not smoke or catch fire they are, however, solid at ordinary temperature and are usually too expensive for general use. It must be remembered that flasks or thermometers immersed in the molten metal must be removed before the metal is allowed to solidify. 

Heat is the most obvious choice of a characteristic by which a fire can be automatically recognized. In the section on fire suppression systems, the fusible links in the sprinkler heads represented one type of heat detector. Alloys have been developed that will have reproducible melting points. When the temperature at the detector site exceeds the melting point of the alloy, contacts are allowed to move so that the device can either make or break a circuit, just as with a manual alarm system. There are plastics which can perform in the same manner. Fixed temperature systems are very stable and not prone to false alarms, but are relatively slow to respond. There are several other versions of these fixed temperature detectors, including bimetalhc strips, where the differential rate of expansion of two different metals causes the strip to flex or bend to either make or break the contact. Others depend upon the thermal erqransion of hquids. 

Fusible Plugs A fusible plug is a plug made of a metal that will melt at a selected temperature and relieve pressure in a container. Fusible plug applications include boilers, compressed gas cylinders, and other pressure vessels. They allow overpressure to vent when a container becomes heated from fire or other sources. A plug may consist entirely of the alloy... 

Fuse wire A fine-gauge wire made of an alloy that overheats and melts at the relatively low temperatures produced when the wire carries overload currents. When used in a fuse, the wire is called a fuse (or fusible) link. 

The core material to be used depends on the actual processing requirements, particularly temperature. It can range from a wax to different ratios of zinc-aluminum eutectic mixtures (alloys) to special fusible eutectic alloys. The core material has to melt below the melt temperature of the plastic. These shaped cores are usually inserted in a mold cavity where it is retained by the mold (such as is used with a mold core puller) or by spiders (as used in certain metal core supports for extrusion dies). After processing, the core material... 

One example of the utilization of adsorption-induced strength lowering in these processes is the use of small amounts of fusible surface-active metals in the treatment of hardened steels and solid alloys. The powder of a fusible metal is introduced into the polymeric binder of grinding wheels along with the diamond powder (Figure 7.50). Due to the increase in temperature in the course of the grinding, the microamounts of active metal are melted and lower the strength of hard metals and... 

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