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 is 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 corroded by 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 in most cases at least one is a metallic element, although it is also sometimes used for mixtures of elements; herein only metallic alloys are described. Most alloys are metallic and show good electrical conductivity, ductility, opacity, and luster, and may have properties that differ from those of the pure elements 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 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 and zinc, in proportions which can be varied to achieve different colours and mechanical, electrical, acoustic and chemical properties, but copper typically has the larger proportion, generally 66% copper and 34% zinc. 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 polished or fractured, shows a lustrous appearance, and conducts electricity and heat relatively well. These properties are all associated with having electrons available at the Fermi level, as against nonmetallic materials which do not. Metals are typically ductile and malleable.

<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">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 are sometimes used to reduce or eliminate the need for subtractive processes in manufacturing, lowering material losses and reducing the cost of the final product. This occurs especially often with small metal parts, like gears for small machines. Some porous products, allowing liquid or gas to permeate them, are produced in this way. They are also used when melting a material is impractical, due to it having a high melting point, or an alloy of two mutually insoluble materials, such as a mixture of copper and graphite.

<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.

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<span class="mw-page-title-main">Bismuth</span> Chemical element with atomic number 83 (Bi)

Bismuth is a chemical element with the 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.

<span class="mw-page-title-main">Soldering</span> Process of joining metal pieces with heated filler metal

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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.

<span class="mw-page-title-main">Diehl Metall</span>

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Friction extrusion is a thermo-mechanical process that can be used to form fully consolidated wire, rods, tubes, or other non-circular metal shapes directly from a variety of precursor charges including metal powder, flake, machining waste or solid billet. The process imparts unique, and potentially, highly desirable microstructures to the resulting products. Friction extrusion was invented at The Welding Institute in the UK and patented in 1991. It was originally intended primarily as a method for production of homogeneous microstructures and particle distributions in metal matrix composite materials.

References

  1. 1 2 Gordon, Robert B.; Rutledge, John W. (1984). "Bismuth Bronze from Machu Picchu, Peru". Science. 223 (4636). American Association for the Advancement of Science: 585–586. Bibcode:1984Sci...223..585G. doi:10.1126/science.223.4636.585. JSTOR   1692247. PMID   17749940. S2CID   206572055.
  2. ↑ Materials of Engineering. John Wiley & Sons. 1890. Retrieved 2013-04-13.
  3. 1 2 "Henley's Twentieth Century Formulas, Recipes And Processes". The Norman W. Henley Publishing Company. 1916. Retrieved 2013-05-06.
  4. ↑ Vetterick; et al. (2010). "NOVEL LEAD FREE BRONZE BEARING MATERIALS PRODUCED BY POWDER METALLURGY PROCESSING" (PDF). MPIF. p. 2. Retrieved 2013-04-13.
  5. ↑ "Novel Lead Free Bronze Bearing Materials Produced by Powder Metallurgy Processing" (PDF). Mpif.org. Retrieved 2013-11-26.
  6. ↑ "Resources: Standards & Properties - Copper & Copper Alloy Microstructures: Leaded Brasses". Copper.org. 2010-08-25. Retrieved 2013-11-26.
  7. ↑ Davis, Joseph R. (January 2001). Copper and Copper Alloys - Google Books. ASM International. ISBN   9780871707260 . Retrieved 2013-11-26.
  8. ↑ "Bronze Bars: Lead Free Bronze and Continuous Cast Bronze". Reliablebronze.com. Retrieved 2013-11-26.
  9. ↑ "Lead-free copper-based coatings with bismuth for swashplate compressors". Osdir.com. Archived from the original on 2013-06-30. Retrieved 2013-11-26.
  10. ↑ "C89835 Lead-Free Replacement for C932, C836, C844". CONCAST METAL PRODUCTS CO. n.d.
  11. ↑ "C89844 Lead-Free Replacement for C844". CONCAST METAL PRODUCTS CO. n.d.
  12. ↑ "Plumbing Alloys in the Scrap Stream". Copper.org. Archived from the original on 2013-05-22. Retrieved 2013-05-06.
  13. ↑ "Patent US8211250 - Method of processing a bismuth brass article - Google Patents" . Retrieved 2013-11-26.
  14. ↑ Yefimov, N. A. (2009-02-24). Handbook of Non-Ferrous Metal Powders: Technologies and Applications - Oleg D Neikov, Stanislav Naboychenko, Irina B Mourachova, Victor G Gopienko, Irina V Frishberg, Dina V Lotsko - Google Books. Elsevier. ISBN   9780080559407 . Retrieved 2013-11-26.
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