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Metals -- Bronze

Bronze

The term bronze generally applies to any copper alloy that has as the principal alloying element a metal other than zinc or nickel. Originally the term was used to identify copper-tin alloys that had tin as the only, or principal, alloying element. Some brasses are called bronzes because of their color, or because they contain some tin. Most commercial copper-tin bronzes are now modified with zinc, lead, or other elements.

 

The copper-tin bronzes are a rather complicated alloy system. The alloys with up to about 10% tin have a single-phase structure. Above this percentage, a second phase, which is extremely brittle, can occur, making plastic deformation impossible. Thus high-tin bronzes are used only in cast form. Tin oxide also forms in the grain boundaries, causing decreased ductility, hot workability, and castability. Additions of small amounts of phosphorus, in production of phosphor bronzes, eliminate the oxide and add strength. Because tin additions increase strength to a greater extent than zinc, the bronzes as a group have higher strength than brasses-- from around 60,000 to 105,000 lb/in² (413 to 723 MPa) in the cold-worked high-tin alloys. In addition, fatigue strength is high.

 

Bronzes containing more than 90% copper are reddish; below 90% the color changes to orange-yellow, which is the typical bronze color. The maximum strength is with 80% copper and 20% tin. Ductility rapidly decreases with the increase of tin up to 20%, after which it practically disappears until 80% is reached, when it again increases. Above 20% tin the alloy rapidly becomes white in color and loses the characteristics of bronze.

·        The family of aluminum bronzes is made up of alpha-aluminum bronzes (less than about 8% aluminum) and alpha-beta bronzes (8 to 12% aluminum) plus other elements such as iron, silicon, nickel, and manganese. Because of the considerably strengthening effect of aluminum, in the hard condition these bronzes are among the highest-strength copper alloys. Tensile strength approaches 100,000 lb/in² (689 MPa). Such strengths plus outstanding corrosion resistance make them excellent structural materials. They are also used in wear-resistance applications and for non-sparking tools.

·        Phosphor bronzes have a tin content of 1.25 to 10%. They have excellent mechanical and cold-working properties and a low coefficient of friction, making them suitable for springs, diaphragms, bearing plates, and fasteners. Their corrosion resistance is also excellent. In some environments, such as salt water, they are superior to copper.  Leaded phosphor bronzes are available with improved machinability.

·         Silicon bronzes are similar to aluminum bronzes. Silicon content is usually between 1% and 4%. In some, zinc or manganese is also present. Besides raising strength, the presence of silicon sharply increases electrical resistivity. Aluminum-silicon bronze, an important modification, has exceptional strength and corrosion resistance and is particularly suited to hot working.

·         Gear bronze may be any bronze used for casting gears and worm wheels, but usually means a tin bronze of good strength deoxidized with phosphorus and containing some lead to make it easy to machine and lowering the coefficient of friction. A hard gear bronze, or hard bearing bronze, of the U.S. Navy, contains 84 to 86% copper, 13 to 15% tin, up to 1.5% zinc, up to 0.75% nickel, and up to 0.5% phosphorus. Hard and strong bronzes for gears are often silicon bronze or manganese bronze.