Question

Compare Alpha Brasses and Duplex Brasses?

Summarize each of the advantages (lead, tin, silicone, arsenic)

Answer 1

Alpha brasses

The range of alloys, termed ‘alpha brasses’, or ‘cold working brasses’, contain a minimum 63% of copper. They are characterised by their ductility at room temperature, and can be extensively deformed by rolling, drawing, bending, spinning, deep drawing, cold heading and thread rolling. The best known material in this group contains 30% zinc and is often known as ‘70/30’ or ‘cartridge’ brass, CuZn30 - due to the ease with which the alloy can be deep drawn for the manufacture of cartridge cases. The cases (up to 100mm diameter) start as flat discs blanked from strip or plate and are successively formed to the final shape by a series of operations, carried out at room temperature, which progressively elongate the sidewalls and reduce their thickness. CuZn30 possesses the optimum combination of properties of strength, ductility and minimal directionality which make it capable of being severely cold drawn. Its ductility allows cold manipulation and the alloy has better corrosion resistance than the brasses with a higher zinc content. For long production runs of deep-drawn components it is essential to keep the process well monitored. The tooling and lubrication must be well maintained and arrangements made to ensure a consistent supply of feedstock. Deep drawing properties are controlled by alloy composition and trace impurities (lead and iron) and mechanical and thermal history during manufacture. Good agreement should be reached with reputable suppliers regarding quality assurance. Tubes for heat exchangers are frequently manufactured from the alpha brasses, normally of 70/30 composition but often containing alloying additions which enhance corrosion resistance. Substantial quantities of alpha alloys are also used for the manufacture of fasteners such as wood screws, rivets and zip fasteners. For less demanding fabrications such as spring contacts in a domestic electrical socket, an alloy with a higher zinc content (and hence lower price) can be used, such as CuZn33 (2/1 brass), CuZn36 and CuZn37 (common brass). These alloys are not quite as ductile as CuZn30, although other mechanical properties are similar. They are perfectly adequate for all but the most severe cold working operations.

Duplex brasses

The ‘alpha-beta brasses’, ‘duplex brasses’ or ‘hot working brasses’ usually contain between 38% and 42% zinc. In contrast to the alloys of the first group, their ability to be deformed at room temperature is more limited. They are, however, significantly more workable than the alpha brasses at elevated temperatures and can be extruded into bars of complex section, either solid or hollow, and hot forged in closed dies (hot stamped) to complex shapes. The ideal hot working temperature range is whilst the brass is cooling, between 750ºC and 650ºC, during which the alpha phase is being deposited . The mechanical working process breaks down the alpha phase into small particles as it is deposited, resulting in good mechanical properties.

Note the need for careful control of annealing temperature and cooling rate if it is required to obtain a single-phase alpha structure in a brass of high zinc content such as common brass and dezincification-resistant brass. Current use of continuous annealing techniques for sheet, strip, wire and tube gives a much quicker cooling rate than previous batch annealing in controlled atmosphere bell furnaces. For brasses of the CuZn37 type this resulted in a greater tendency to retain some of the beta phase and the standard composition has therefore now been adjusted to CuZn36. These brasses are available as extruded rods, bars and sections, which in turn are the starting stock for the manufacture of a vast range of engineering components and accessories . Hot stampings are used in virtually every industry: pipe fittings,domestic taps, radiator valves, gas appliances, window and door furniture being merely a few typical examples of the products which can be manufactured by this process . Good tolerances are maintained during manufacture, minimising the need for machining during the final component production. The addition of lead to these alloys aids chip breakage during machining, producing short broken chips which are easily cleared from the cutting area to improve machinability. Since the cost of zinc is lower than that of copper, brasses of higher zinc content have a lower first cost. This may be significant in assessing manufacturing and total-lifetime costs.

EFFECT OF ALLOYING ADDITIONS

Alloying additions are made to the basic copper-zinc alloys for a variety of reasons:-

• to improve machinability

• to improve strength and wear resistance

• to improve corrosion resistance

• for other special reasons

The very wide variety of standard brass compositions that are available reflect the many ways in which an optimum combination of properties can be tailored to ensure fitness for the desired application

. Effects of alloying elements

Lead

The addition most commonly made to brasses to modify their properties is lead, up to 3% of which may be added to alpha-beta brasses to provide free-machining properties. The lead does not form a solid solution with the copper and zinc but is present as a dispersed discontinuous phase distributed throughout the alloy. It has no effect on corrosion resistance. Lead is not added to wrought alpha brasses since, in the absence of sufficient beta phase, it gives rise to cracking during hot working.

Tin

1% tin is included in the composition of Admiralty brass CW706R (CZ111) and Naval brass CZ112 (nearest CW712R). As their names indicate, these brasses were developed originally for seawater service, the tin being added to provide improved corrosion resistance. Nowadays Aluminium brass CW702R (CZ110) has replaced Admiralty brass for marine service but Admiralty brass is used for fresh water. Naval brass retains some important applications in seawater service.

Silicon

Silicon increases the strength and wear resistance of brass and is also sometimes included in die casting brasses and in filler alloys for gas welding to reduce oxidation of the zinc and to assist fluidity. Its principal effect from the corrosion point of view is to increase the beta phase content.

Arsenic

Arsenic is often added in small amounts to alpha brass alloys to provide protection against dezincification corrosion as discussed