Bismuth bronze

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Bismuth bronze or bismuth brass is a copper alloy which typically contains 1-3% bismuth by weight, although some alloys contain over 6% Bi. This bronze alloy is very corrosion-resistant, a property which makes it suitable for use in environments such as the ocean. Bismuth bronzes and brasses are more malleable, thermally conductive, and polish better than regular brasses. The most common industrial application of these metals are as bearings, however the material has been in use since the late nineteenth century as kitchenware and mirrors. Bismuth bronze was also found in ceremonial Inca knives at Machu Picchu. [1] Recently, pressure for the substitution of hazardous metals has increased and with it bismuth bronze is being marketed as a green alternative to leaded bronze bearings and bushings.

Contents

History

The earliest known artifact containing bismuth bronze is an Inca knife from Peru, found in 1912 and likely to date from the 15th century. Whether the alloy was chosen because of metallurgical properties which facilitated casting or because of its whiter, more lustrous finish is a matter of conjecture. It is unlikely that the inclusion of the bismuth was accidental, as was likely the case for most other early bronzes which contained bismuth, which makes this the earliest evidence of intentional addition of bismuth to an alloy. [1] Bismuth bronze was rediscovered in the 1880s by James Webster for telegraph wires. Webster developed two bismuth-tin-bronze alloys. One alloy was developed as an early attempt at producing corrosion-resistant bronze, and was described as "hard, tough, and sonorous." Webster also indicated that this particular alloy was also well suited to piano wires. Another early Webster alloy was described as "durable and bright" and later used in kitchenware due to its luster and slow tarnishing. This particular alloy's ability to hold polish made it useful as a light reflector or mirror material as well, which it continued to be used for into the twentieth century [2] [3]

In the 1990s, autoparts manufacturer Federal Mogul began developing bismuth bronzes as an alternative to lead-containing bronzes because of increasing pressure on the removal of lead from both consumer and industrial applications. Bismuth is a non-toxic heavy metal, and as legislation such as the EU Restriction of Hazardous Substances or the American Reduction of Lead in Drinking Water Act continue to regulate the amount of lead that can be in a product or environment, more lead-free alternative materials have been developed which maintain properties of their lead predecessors without containing lead. Bismuth is an especially appropriate replacement for lead leaded bronze bearings because, like lead, bismuth is insoluble in copper and forms similar micro-globules that mimic lead. [4]

Characteristics

Structure

Modern "bismuth bronze" is technically neither an alloy nor a bronze, but a composite material containing brass and bismuth (though some bronzes are still used). Because bismuth, like lead, is insoluble in copper, it exists as discrete micro-globules of bismuth within the grain boundaries of the alloy which behave as bismuth particles. These particles deform easily within the crystal structure and across the surface of the metal to act as a low-shear solid lubricant in cases of low lubrication. Selenium is added to red copper-bismuth alloys because it strengthens properties of the bismuth of the material. Because liquid bismuth can lead to embrittlement in an alloy, care must be taken in processing and recycling of these materials. [5] [6] [7]

Properties

Modern bismuth bronzes on the market are developed to hold similar properties to leaded bronzes, and many of them bear nearly identical mechanical properties to common leaded alloys, such as machinability and high thermal conductivity. [8] They also have a high lubricity, which makes them ideal for outer layers in machine parts which are subject to wear such as bearings and swashplates. [9] Bismuth-tin bronze is not easily corrosive to water; preliminary bismuth is not easily oxidized.

Used in faucets, pump components, pipe fittings, plumbing goods, water pump impellers, housings and small gears, Alloy C89835 Bismuth-Tin Bronze has density 0.321 lb/in3, [10] while Alloy C89844 (which is used for fittings or valves for potable water) has a density of 0.31 lb/in3. [11]

Applications

Originally, bismuth-tin bronze was developed for telegraph wires. However, in the late nineteenth and early twentieth century, the bronze became more commonly used in mirrors, reflectors, kitchenware, and piano wires. It fell out of common use in the first half of the twentieth century. [3]

Modern bismuth bronze alloys are marketed as a green alternative to lead, often chosen over the less expensive leaded bronzes because of environmental regulation. [12] They are found as a surface layer on automotive or mechanical components which receive heavy wear such as hydraulic piston pumps. These alloys are also commonly used in bearings or bushes in tribologic systems. Also, some bismuth-bronze plumbing fittings are manufactured.

Processing

Casting

Cope & drag (top and bottom halves of a sand mold), with cores in place on the drag SandMoldCopeDragCores.jpg
Cope & drag (top and bottom halves of a sand mold), with cores in place on the drag

Lead-free bismuth bronze castings are produced by frozen molds. Sand characteristics are used to increase the rate of cooling of the bronze castings because the sand particles have a higher thermal conductivity, greater grain size and spherical shape. The mold has a high cooling potential. When the mold is frozen, the ice near the surface contracts to molten metal and thaws immediately.

Extruding and annealing

Another way bismuth brass can be processed includes the extrusion brass rod through machining to form the material into a desired shape. The material is then annealed to relieve stress that was caused by the machining. The material is annealed at a certain annealing temperature and time depending on the material’s composition. The time is selected to limit diffusional movement of bismuth. This reduces cracking of the fixture. [13]

Water atomization

High-pressure water atomization is a processing method of bismuth brass. This is a way of rapidly solidifying the metal alloy. A liquid metal is dispersed into droplets by the impingement of high-pressure jets of water. It is a low-cost process to achieve a distribution of fine particles in iron, stainless steel, and low-alloy metal powders. This is a way of rapidly solidifying the metal alloy. A limitation of water atomization is the powder purity. For metals that are inclined to oxidation, this is a major problem. [14]

Related Research Articles

<span class="mw-page-title-main">Alloy</span> Mixture or metallic solid solution composed of two or more elements

An alloy is a mixture of chemical elements of which at least one is a metal. Unlike chemical compounds with metallic bases, an alloy will retain all the properties of a metal in the resulting material, such as electrical conductivity, ductility, opacity, and luster, but may have properties that differ from those of the pure metals, such as increased strength or hardness. In some cases, an alloy may reduce the overall cost of the material while preserving important properties. In other cases, the mixture imparts synergistic properties to the constituent metal elements such as corrosion resistance or mechanical strength.

<span class="mw-page-title-main">Brass</span> Alloy of copper and zinc

Brass is an alloy of copper (Cu) and zinc (Zn), in proportions which can be varied to achieve different colours and mechanical, electrical, acoustic, and chemical properties, but copper typically has the larger proportion. In use since prehistoric times, it is a substitutional alloy: atoms of the two constituents may replace each other within the same crystal structure.

<span class="mw-page-title-main">Bronze</span> Alloy of copper and tin

Bronze is an alloy consisting primarily of copper, commonly with about 12–12.5% tin and often with the addition of other metals and sometimes non-metals, such as phosphorus, or metalloids such as arsenic or silicon. These additions produce a range of alloys that may be harder than copper alone, or have other useful properties, such as strength, ductility, or machinability.

<span class="mw-page-title-main">Metal</span> Type of material

A metal is a material that, when freshly prepared, polished, or fractured, shows a lustrous appearance, and conducts electricity and heat relatively well. Metals are typically ductile and malleable. These properties are the result of the metallic bond between the atoms or molecules of the metal.

<span class="mw-page-title-main">Metallurgy</span> Field of science that studies the physical and chemical behavior of metals

Metallurgy is a domain of materials science and engineering that studies the physical and chemical behavior of metallic elements, their inter-metallic compounds, and their mixtures, which are known as alloys.

<span class="mw-page-title-main">Solder</span> Alloy used to join metal pieces

Solder is a fusible metal alloy used to create a permanent bond between metal workpieces. Solder is melted in order to wet the parts of the joint, where it adheres to and connects the pieces after cooling. Metals or alloys suitable for use as solder should have a lower melting point than the pieces to be joined. The solder should also be resistant to oxidative and corrosive effects that would degrade the joint over time. Solder used in making electrical connections also needs to have favorable electrical characteristics.

<span class="mw-page-title-main">Tin</span> Chemical element, symbol Sn and atomic number 50

Tin is a chemical element; it has symbol Sn and atomic number 50. A silvery-coloured metal, tin is soft enough to be cut with little force, and a bar of tin can be bent by hand with little effort. When bent, the so-called "tin cry" can be heard as a result of twinning in tin crystals; this trait is shared by indium, cadmium, zinc, and mercury in its solid state.

<span class="mw-page-title-main">Brazing</span> Metal-joining technique

Brazing is a metal-joining process in which two or more metal items are joined by melting and flowing a filler metal into the joint, with the filler metal having a lower melting point than the adjoining metal.

The white metals are a series of often decorative bright metal alloys used as a base for plated silverware, ornaments or novelties, as well as any of several lead-based or tin-based alloys used for things like bearings, jewellery, miniature figures, fusible plugs, some medals and metal type. The term is also used in the antiques trade for an item suspected of being silver, but not hallmarked.

<span class="mw-page-title-main">Powder metallurgy</span> Process of sintering metal powders

Powder metallurgy (PM) is a term covering a wide range of ways in which materials or components are made from metal powders. PM processes can reduce or eliminate the need for subtractive processes in manufacturing, lowering material losses and reducing the cost of the final product.

<span class="mw-page-title-main">Phosphor bronze</span> Bronze where the oxygen is removed with phosphorus

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<span class="mw-page-title-main">Extrusion</span> Process of pushing material through a die to create long symmetrical-shaped objects

Extrusion is a process used to create objects of a fixed cross-sectional profile by pushing material through a die of the desired cross-section. Its two main advantages over other manufacturing processes are its ability to create very complex cross-sections; and to work materials that are brittle, because the material encounters only compressive and shear stresses. It also creates excellent surface finish and gives considerable freedom of form in the design process.

Historically, the term "latten" referred loosely to the copper alloys such as brass or bronze that appeared in the Middle Ages and through to the late-18th and early-19th centuries. Such alloys were used for monumental brasses, in decorative effects on borders, rivets or other details of metalwork, in livery and pilgrim badges or funerary effigies. Latten commonly contained varying amounts of copper, tin, zinc and lead, giving it characteristics of both brass and bronze. Metalworkers commonly formed latten in thin sheets and used it to make church utensils. Brass of this period is made through the calamine brass process, from copper and zinc ore. This calamine brass was generally manufactured as hammered sheet or "battery brass", and cast brass was rare.

<span class="mw-page-title-main">Gunmetal</span> Alloy of copper, tin, and zinc

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In metallurgy, non-ferrous metals are metals or alloys that do not contain iron in appreciable amounts.

In metallurgy and materials science, annealing is a heat treatment that alters the physical and sometimes chemical properties of a material to increase its ductility and reduce its hardness, making it more workable. It involves heating a material above its recrystallization temperature, maintaining a suitable temperature for an appropriate amount of time and then cooling.

<span class="mw-page-title-main">Magnesium alloy</span> Mixture of magnesium with other metals

Magnesium alloys are mixtures of magnesium with other metals, often aluminium, zinc, manganese, silicon, copper, rare earths and zirconium. Magnesium alloys have a hexagonal lattice structure, which affects the fundamental properties of these alloys. Plastic deformation of the hexagonal lattice is more complicated than in cubic latticed metals like aluminium, copper and steel; therefore, magnesium alloys are typically used as cast alloys, but research of wrought alloys has been more extensive since 2003. Cast magnesium alloys are used for many components of modern automobiles and have been used in some high-performance vehicles; die-cast magnesium is also used for camera bodies and components in lenses.

<span class="mw-page-title-main">Bismuth</span> Chemical element, symbol Bi and atomic number 83

Bismuth is a chemical element; it has symbol Bi and atomic number 83. It is a post-transition metal and one of the pnictogens, with chemical properties resembling its lighter group 15 siblings arsenic and antimony. Elemental bismuth occurs naturally, and its sulfide and oxide forms are important commercial ores. The free element is 86% as dense as lead. It is a brittle metal with a silvery-white color when freshly produced. Surface oxidation generally gives samples of the metal a somewhat rosy cast. Further oxidation under heat can give bismuth a vividly iridescent appearance due to thin-film interference. Bismuth is both the most diamagnetic element and one of the least thermally conductive metals known.

C41100 Lubaloy is a wrought copper alloy that is composed mainly of copper and zinc. Lubaloy possesses many favorable characteristics making it, and other types of brass, a popular choice in manufacturing. It is a source material in many processes including the creation of electrical components and bullet-making. There are both positive and negative health effects that are associated with the use of this material.

References

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