Phosphor, manganese and aluminum bronzes are among the best bronzes known, and are the most extensively used. Bronzes are used when a strong and fairly ductile non-corrosive alloy is necessary in ship and machinery parts. They are the strongest and about the most expensive alloys in engineering use. Phosphorus and manganese are used in the bronzes designated by these names merely to assist in purging the molten mixtures of metallic oxides, thus producing alloys of greater metallic purity and consequently of greater strength.
Aluminum and copper seem to form a chemical union of remarkable strength and ductility, having properties resembling those of mild steel. The alloy can be forged at a red heat, it makes excellent castings, its strength is greatly increased by hammering or rolling, and it resists the corrosive action of air and salt water.
Generally, the bronzes can be considerably strengthened by hammering or rolling, can be forged hot, (but not welded in a blacksmith forge), and their hardness increases as the per cent of copper is lessened, while ductility increases as the copper is increased. The less ductile mixtures should be rolled or worked hot. A peculiarity of the copper-tin alloys is that quick cooling in water tends to remove brittleness and to increase ductility and softness, while slow cooling from a red heat restores the original hardness.
The bronzes are less ductile than brass, and those to be rolled into sheets or drawn into wire must not contain as much tin as those to be cast. Their hardness makes them excellent for machinery bearings. Propellers and many ship fittings are made of bronze because of its strength and resistance to salt water corrosion.