AT THE HEART OF THE ROAD TRANSPORT INDUSTRY.

Call our Sales Team on 0208 912 2120

INTRODUCTORY

20th April 1951, Page 76
20th April 1951
Page 76
Page 76, 20th April 1951 — INTRODUCTORY
Close
Noticed an error?
If you've noticed an error in this article please click here to report it so we can fix it.

Which of the following most accurately describes the problem?

Extraction

Aluminium is the most abundant metal in the earth's crust. Its most useful natural form, however, is the hydrated oxides that largely compose the mineral bauxite. This does not respond to simple smelting methods, and the metal was not isolated until 1825. Thereafter, small amounts of aluminium were produced by reduction by sodium, but it remained a very costly metal. In 1886, however, a very much cheaper way of reducing aluminium oxide (alumina), by electrolysis, was discovered, and this process is the one now in usc.

Briefly, the ore is first treated to produce pure alumina; this oxide is dissolved in molten cryolite and a heavy electric current is passed through the solution from carbon anodes, near the surface, to the carbon lining of the bath. The oxide is broken down electrolytically, the oxygen combining with the anodes to escape as carbon dioxide, and metallic aluminium sinks to the bottom, whence it is tapped from time to time. Little change takes place in the cryolite ; the carbon anodes arc consumed and have to be regularly replaced.

Four pounds of high-grade bauxite are needed to produce two of alumina, which will in turn yield one pound of aluminium of over 99 '„ purity. Roughly to kWh of electricity are consumed for each pound of metal obtained from the reduction cell ; this makes cheap and plentiful power essential, and the world's main reduction plants are sited on specially built hydroelectric schemes.

Fabrication

Of the aluminium produced, more is marketed as sheet than in any other form. Sheet rolling is not basically different from that of most other metals, and similar equipment is used : a prepared cast ingot is passed through a succession of pairs of hard steel rolls and its thickness reduced step by step to that desired. It is first rolled hot, but the final reductions are made on the cold sheet to obtain a good finish and perhaps, by work hardening, a desired degree of hardness. The production of accurately-gauged, flawless sheet and strip demands very precise and elaborate equipment and much experience.

Equal in importance is the extrusion process, by which lengths of metal of constant cross-section, solid or hollow, are made. Very high pressures arc used to force hot plastic aluminium out of a container through a steel die of the required shape. This process produces, in a single operation, sections that could be made in no other way. Aluminium is one of the few structural metals that can be extruded, and the variety of intricate sections possible encourages ingenious design in many products.

Seamless light-gauge tube is made by drawing down extruded thick-walled tube through hard steel dies. Rolled sections and rod are formed by passing a cast billet or heavy extruded bar through a series of shaped rolls; by drawing the rod through dies it is further reduced into wire. A principal use of rod and wire is for making rivets.

Although now accounting for a lesser volume of metal than sheet rolling or extrusion, casting and forging are practised on a large scale.

Casting was the earliest way of making things in aluminium, and was in use from the first days of the motor industry, which is still a major consumer in crankcases, pistons, and other engine parts. Sand moulds, or iron or steel " permanent " moulds (gravity dies), are used.

Forging is suitable for highly stressed but more simple parts, such as engine connccting rods or propeller blades. A piece of hot aluminium is simply hammered or pressed to shape between a pair of hard steel dies.

Why it is important to the engineer The properties of aluminium are to be discussed in later articles of this series, but we should mention two of the characteristics that made aluminium worth developing, and that lie behind its wide acceptance as a primary structural material.

The first is well-known : that the specific gravity of the metal is low (2.70), about a third of that of steel. This gives it first place where lightness is sought in structures or mechanisms, for its strength, slight in the pure state, can by alloying and by mechanical and thermal treatment be raised to equal and surpass that of structural steel

Not, perhaps, so widely appreciated is aluminium's power of resisting damaging attack by the atmosphere. This, probably as much as low weight, has promoted its use in building for the sake of long life and freedom from recurrent painting costs. Marine conditions are adequately withstood by suitable alloys, which are in use in many sea-going craft. Its reluctance to corrode, and the harmlessness of its salts, make the metal of value in the processing and packing of food.

In its short life, aluminium has become established as a material useful to most branches of engineering, indispensable to some.

Tags


comments powered by Disqus