Learn About the Different Brass Types
'Brass' is a generic term that refers to a wide range of copper-zinc alloys. In fact, there are over 60 different types of brass specified by EN (European Norm) Standards. These alloys can have a wide range of different compositions depending upon the properties required for a particular application. Brasses can also be classified in a variety of ways, including by their mechanical properties, crystal structure, zinc content, and color.
Brass Crystal Structures
The essential distinction between different types of brasses is determined by their crystal structures. This is because the combination of copper and zinc is characterized by peritectic solidification, an academic way of saying that the two elements have dissimilar atomic structures, making them combine in unique ways depending upon content ratios and temperatures. Three different types of crystal structure can form as a result of these factors:
- Alpha Brasses: Alpha brasses contain less than 37% zinc melted into copper and are named for their formation of a homogenous (alpha) crystal structure. The alpha crystal structure occurs as zinc dissolves into copper forming a solid solution of uniform composition. Such brasses are softer and more ductile than their counterparts and, therefore, more easily cold worked, welded, rolled, drawn, bent or brazed.
The most common type of alpha brass contains 30 percent zinc and 70 percent copper. Referred to as '70/30' brass or 'cartridge brass' (UNS Alloy C26000), this brass alloy has the ideal combination of strength and ductility for being cold drawn. It also has a higher resistance to corrosion than brass with greater zinc content. Alpha alloys are commonly used to make fasteners, such as wood screws, as well as for spring contacts in electrical sockets.
- Alpha-Beta Brasses: Alpha-beta brasses - also known as 'duplex brasses' or 'hot-working brasses' - contain between 37-45% zinc and are made-up of both the alpha grain structure and a beta grain structure. Beta phase brass is atomically more similar to that of pure zinc. The ratio of alpha phase to beta phase brass is determined by zinc content, but the inclusion of alloy elements such as aluminum, silicon or tin can also increase the amount of beta phase brass present in the alloy.
More common than alpha brass, alpha-beta brass is both harder and stronger and has a lower cold ductility, than alpha brass. Alpha-beta brass is cheaper due to the higher zinc content, but more susceptible to dezincification corrosion.
While less workable than alpha brasses at room temperature, alpha-beta brasses are significantly more workable at high temperatures. Even when a lead is present to improve machineability such brasses are resistant to cracking. As a result, alpha-beta brass is usually hot worked by extrusion, stamping or die-casting.
- Beta Brasses: Although much more rarely used than alpha or alpha-beta brasses, beta brasses make up a third group of the alloy that contains greater than 45% zinc content. Such brasses form a beta structure crystal and are harder and stronger than both alpha and alpha-beta brasses. As such, they can only be hot worked or cast.
In contrast to crystal structure categorization, identifying brass alloys by their properties allows us to consider the effect of alloying metals on brass. Common categories include:
- Free machining brass (3% lead)
- High tensile brasses (aluminum, manganese and iron inclusions)
- Naval brasses (~1% tin)
- Dezincification resistant brasses (arsenic inclusion)
- Brasses for cold working (70/30 brass)
- Casting brasses (60/40 brass)
The terms 'yellow brass' and 'red brass' - often heard in the US - are also used to identify certain types of brasses. Red brass refers to a high copper (85%) alloy that contains tin (Cu-Zn-Sn), which is also known as gunmetal (C23000), while yellow brass is used to refer to a brass alloy with a higher zinc content (33% zinc), thereby making the brass appear a golden yellow color.
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