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Composite Casting

Composite Casting refers to a large number of processes in which molten metal is cast on to a solid metal component, so that the two subsequently form one integral unit. In this way the favorable mechanical or other technological properties of one metal can be combined with those of another. For example, bearings may be made from an outer shell of strong metal with a lining of special low-friction metal cast inside it, or a metal possessing high strength may be given cast-on covering of corrosion-resistant metal, or an expensive metal may be combined with a cheaper metal for economy.

The mechanical connection of the two metals can be made by means of interlocking devices such as dovetailing, grooves, recesses, etc., which form a physical key, or by shrink fitting or by bond established as a result of diffusion at the interface of the two metals so that local interpenetration occurs. The bond may be further strengthened by heat treatment (annealing) or by the interposition of special bonding layers of metal at the junction.

The bearing comprising an outer steel shell and a metal alloy lining, the shell is pre-heated and the molten lining metal is cast inside it. In the integral casting process developed by Mascher (Fig.1) the steel shell is first given a coating of zinc and then, while it is still hot, is filled with molten lining metal (lead bronze) in a compound iron-and-sand mold. The zinc coating serves to establish a strong bond between the lining and the shell. Another technique is represented by the salt-bath displacement process, in which the bearing shell and the mold for forming the lining within it are together preheated in a salt bath, which also has the function of protecting the metal against oxidation. When the required temperature has been reached, the bearing metal is poured, displacing the salt. In the immersion process (Fig.2), which is used, for example, for making bushings provided with bearing metal on both sides or for the production of relatively large castings, the bearing shell enclosed within the mold is heated in a salt bath and is then lowered into a bath of molten bronze in which the bronze displaces the salt in the mold.

Centrifugal casting techniques are extensively used for the production of thick walled composite bearings (Figs.3 & 4). The steel outer shell is heated in a salt bath or is zinc-coated. It is then gripped in a centrifugal casting machine, and when the latter has reached the requisite speed of rotation, the molten metal is introduced into it. In the case of a composite bearing built up from different metals in concentric layers, an intermediate bonding layer of tin is applied to the first layer of metal after it has solidified. Then the second layer is cast. Another technique for the casting of bearings the appropriate quantity of casting metal in the form of chips or granules, together with a fluxing agent, is fed into the mold, which is closed at both ends. The mold is then rotated while it is heated externally, so that the casting metal melts and is distributed in the mold by centrifugal action

In A1-Fin process, which is a patented American method for composite casting of light metal alloys in combination with steel or cast iron, is becoming increasingly important in the motor industry and other industries. In this process the steel basic component, whose surface has been thoroughly cleaned and degreased, is heated in molten aluminum at about 750 oC, so that it becomes coated with an iron-aluminum compound (Fe2Al5) which in turn becomes covered with a layer of pure aluminum. The component is then placed in the mold and metal is cast so quickly that the aluminum coating is still liquid, so that this coating and any adhering oxides are, as it were, washed away by the molten casting metal, which can now bond itself firmly to the underlying layer of Fe2Al5.