2 Types of Aluminium Bronze

The copper-aluminium alloys commonly know in English-speaking countries as "aluminium bronzes" contain approximately 5% to 11% aluminium, some having additions of iron, nickel, manganese or silicon.

All the aluminium bronzes have good corrosion resistance but they vary in this respect according to their metallurgical structure which in turn depends upon the composition of the material and its manufacturing history - especially the thermal treatment to which they have been subjected.

i Metallurgical Structure

The simple aluminium bronzes containing only copper and aluminium have a single phase (alpha) structure up to about 8% aluminium. Above that level a second phase (beta) is formed producing an alpha-beta alloy. Whereas in brasses the formation of beta phase results in a substantial reduction in corrosion resistance this is not true of the beta phase in the copper-aluminium system. Consequently, while alpha-beta brasses have a much lower corrosion resistance than alpha brasses, alpha-beta aluminium bronzes have a resistance to general corrosion which is similar to that of alpha aluminium bronzes and a superior resistance to corrosion/erosion and to cavitation corrosion.

Figure 1. Influence of aluminium content and cooling rate on the corrosion resistance of binary copper - aluminium alloys.

This is true of aluminium bronzes containing only alpha and beta phases but if an alpha-beta aluminium bronze is allowed to cool too slowly from temperatures above about 600°C the beta phase converts to a mixture of alpha and gamma 2 phases at around 565°C. The gamma 2 phase has a higher aluminium content than the beta phase and shows a susceptibility to corrosion rather similar to that of the beta phase in brasses. Consequently if gamma 2 is formed as a continuous network a higher rate of penetration of corrosion through the alloy can occur and the corrosion resistance is seriously affected. Small isolated areas of gamma 2 phase will result in localised superficial corrosion but this will not penetrate into the body of the material. Research carried out in M.O.D. (Navy) laboratories established the influence of aluminium content and of cooling rate for alloys containing 8.5 to 9.5% aluminium and demonstrated that the formation of a continuous network of gamma 2 can be avoided by keeping the aluminium content below 9.1%. The corrosion behaviour of alloys containing between 8.7 and 9.1% aluminium is variable depending upon the cooling rate but even at the slowest cooling rates likely to be found in commercial practice the gamma 2 phase in this range of composition is discontinuous. Freedom from gamma 2 phase can be ensured in material of higher aluminium content by rapid cooling, for example, by water quenching, from 600°C after casting or hot working. Alternatively material containing gamma 2 as a result of too slow a cooling rate after casting, hot working or welding may be reheated to 600 to 800°C for sufficiently long to reform the beta phase and then quenched. The effect of aluminium content and cooling rate on the structure and corrosion resistance of binary aluminium bronzes is shown in Figure 1.

The comments above concerning the formation of gamma 2 phase with resultant deterioration of corrosion resistance apply only to binary aluminium bronzes, i.e. alloys of copper and aluminium without additional alloying elements. The presence of iron in sufficient quantity suppresses the formation of gamma 2 and also refines the grain structure of the alloy; consequently any gamma 2 that is present is more likely to be in a discontinuous form. About 2% iron is generally sufficient to do this in sections with a diameter up to about 75 mm, but this is not sufficient for heavier sections. Nickel has a similar effect to iron in suppressing the formation of gamma 2 in a dangerous form but is not used for that purpose since iron is almost equally effective and cheaper. Manganese additions will also suppress the breakdown of beta phase to alpha plus gamma 2 but, at the same time, modifies the character of the beta making it more susceptible to corrosion. A manganese content must, therefore, be chosen which gives the optimum balance between these two effects. Thus BS 1400 AB1 restricts the manganese content to a maximum of 1.0% and it is usual to maintain it below 0.75%.

The higher strength aluminium bronze alloys such as AB2 (castings) and CA104 (wrought) contain nominally 10% aluminium with 5% each of iron and nickel. In these alloys the beta phase breaks down during cooling through the temperature range 950 to 750°C to produce alpha plus kappa. The alloy solidifies with an all-beta structure from which the kappa phase begins to precipitate as coarse particles (often in the form of rosettes) at about 900°C. At lower temperatures the remaining beta is transformed into alpha plus kappa, the kappa commonly being of a lamellar form. Subsequent slow cooling to room temperature results in further precipitation of fine kappa within the alpha grains. The kappa phase is of variable composition containing aluminium, iron, nickel and manganese (if manganese is present) and its formation effectively increases the amount of aluminium which can be present in the alloy before the danger of gamma 2 formation arises. Gamma 2 is consequently not normally present in nickel aluminium bronzes and these alloys show very high resistance to corrosion. To ensure that the kappa phase itself is corrosion resistant the nickel content of the alloy should exceed the iron content and the manganese content should not exceed 1.3%.

Aluminium silicon bronze containing approximately 2% silicon with 6% aluminium and 0.5% iron is not at present covered by any British Standard specification but the wrought forms will be included in the next revision of BS 2872 and 2874. It is widely used in the form of forgings, extruded rods and sections and as castings under Ministry of Defence Ship Department Standards DG Ships 1044 and 129. The aluminium content of this alloy is sufficiently low for there to be no danger of gamma 2 phase formation under any normal conditions of manufacture or use.

The copper manganese aluminium alloys CMA1 and 2 are normally classified as aluminium bronzes, although they contain about 12% manganese in addition to approximately 8.5% aluminium. These alloys have an alpha-beta structure but the beta phase is of a quite different composition from that in the binary aluminium bronzes and has lower resistance to corrosion under some conditions, as discussed in Section 3(iv).

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