Bolton Metal Product Co., Inc.

formerly Cerro Metal


Fusible alloys are materials that readily melt at unusually low temperatures. They are generally defined as alloys that melt below 361°F which is the melting point of the lead tin eutectic solder. Bismuth is the major component of many of these alloys and influences the melting point, as well as giving these materials the unique characteristic of expansion upon solidification.  Alloys containing both bismuth and lead in optimum proportions can expand for hours or even days after solidification. Other optimum combinations of the elements that are used in fusible alloys will expand during solidification with negligible shrinkage during cooling to room temperature. These properties have proven to be useful in many processes.


Fusible alloys are classified as either eutectic or non-eutectic. In eutectic alloys it can be stated that the melting temperature coincides with its freezing temperature or has no freezing range between its liquid and solid state.  Non-eutectic alloys exhibit a range between the melting and freezing points in which the materials are slushy or pasty.


The combination of expansion or non-shrinkage, low melting temperatures, and ease of reuse has given fusible alloys their many industrial applications. A number of fusible alloys have been standardized and can be found listed in ASTM B774.


Anchoring: This application takes advantage of alloy growth after freezing. In a typical installation, a part is mounted in an oversized hole and the alloy is cast around the part. Usually, about 24 hours is required for the alloy to grow sufficiently to hold the part securely. Fusible alloys have been used in such applications as to hold bushings, bearings and non-moving parts in machinery, to hold punch heads for sheet metal piercing, permanent magnets in work holding fixtures, and anchoring hold down bolts for machinery.


Work Holding: Fusible alloys are used to hold special or irregular shaped work pieces in polishing and machining operations. For example, the extremely low melting point of Bolton Alloy 117, Bolton Alloy 136, and Bolton Alloy 158 allows them to be cast against eyeglass lenses to hold them through the grinding and polishing operations. Afterward, the alloy is melted off in warm water and re-circulated for reuse.


Similarly, Bolton Alloy 158 and Bolton Alloy 281 are used to secure thin foil sections on jet-engine turbine blades, allowing the critical “fir tree” pattern at the blade root to be machined more accurately. In a typical fixture the foil section is positioned within a hollow hard steel matrix box. A fusible alloy is then cast around the foil and it grows to secure the blade for the machining of the protruding root section. The alloy is then melted off for reuse after the machining is complete.


Cores: The low melting point of fusible alloys makes them useful for electroforming external and internal shapes of copper, iron or nickel parts. Fusible alloy mandrels can also be used when the internal part configuration will not allow the removal of a hard metal core.  A fusible alloy core is made by casting into a mold producing an electrically conductive pattern. Copper or nickel is then deposited on conductive surfaces to the required thickness. The core is melted out, leaving a dimensionally precise part with a smooth surface finish.


Tube Bending: Bending of thin-walled tubing and channels without adequate support can wrinkle, flatten, or rupture the part wall. Bolton Alloy 158 and Bolton Alloy 255 have long been used to support work pieces during bending or formatting to prevent damage. Normally the part should be lubricated before filling to prevent galling and to allow for clean alloy removal. The growth property of the alloys ensures complete part filling, and flaws in a tube wall can often be detected by bulges or leaks of molten alloy through microscopic cracks.


Bolton Alloy 158 is the most widely used for this application, and it can be melted out with hot water. Bolton Alloy 255 is used for tubes with diameters larger than 1.5 in.; however, a hot oil bath or oven heating is required to reach the 255°F melting temperature.


With Bolton Alloy 255 the tube or channel can be bent as soon as the alloy solidifies. Bolton Alloy 158 must be rapidly cooled by immersion in cold, circulating water or other quick chilling medium immediately after filling. This results in a fine grain crystalline structure that adequately supports the work piece during formation.


Encapsulation Molds: Molds for potting transformers and other electronic components are often machined from aluminum or steel. These molds are permanent and can be used an almost unlimited number of times. They also are expensive, time consuming to make and difficult to modify.


An alternative to these expensive molds is to utilize the fusible alloy for molding. Preformed plastic cups are used for this application and are available in a wide variety of shapes and sizes. After the component has been potted and the resin cured, this plastic mold becomes an integral part of the unit.


The process of dip molding (slush casting) has been widely accepted by the electronics industry, where the configuration of the potted part permits easy withdrawal of an alloy shell from the dip mandrel. Bolton Alloy 281 is best suited to this application. After the unit is positioned in the tin-shell alloy cavity, encased in resin, and the resin cured, the alloy mold is cracked off and returned to the melt pot for reuse.


Dies: Drop hammer dies for short run sheet metal forming are made by casting fusible alloys against wood or plaster patterns. Dies made of these comparatively soft alloys will survive a quick blow but will deform in a squeeze operation.  They can also be used in light sheet metal embossing dies and “lost wax” pattern dies.


Safety and Fire Protection: One of the earliest uses of fusible alloys was as the melt-out element in sprinkler heads. Several compositions are used, depending on the location of the sprinkler head in the building.  Other fire protection applications include fusible links on fire doors and safety plugs in pressure and process tanks. Alloys covering the temperature range of 117 degrees F to 520 degrees F are used depending on the application.


Soldering: In the last few years, considerable interest has developed in the use of lead free fusible alloys in soldering applications. The lower melting temperature required compared to the lead-tin solders helps prevent damage to the electronic assembly.


Radiation Therapy: Bolton 158, Bolton 203 and Bolton 174 are used as custom made shielding blocks in radiation therapy.  The Bolton 203 is a cadmium free alternative to the Bolton 158 and the Bolton 174 is a lead and cadmium free alternative.


Proof Casting:   Small cavities are proof cast with solid casting of low melting point alloys. Usually no parting medium is necessary but a thin coat of oil or graphite may be used to coat the inside of the cavity before pouring. Bolton 117 is often used for this purpose due to its low melting temperature. Bolton 160/190 is used in the proof casting of gun chambers due to its initial shrinkage and then growth to zero shrinkage or growth after one hour.



Specific application literature is available upon request.

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