Of the five metals that now show, the highest figures for annual tonnage production in the world, three (iron, copper, and lead) have been known and used by man for many thousands of years. The fourth (zinc) did not come into general use until the Middle Ages. Only the fifth (aluminum) is of ultramodern origin, as far as mankind is concerned. Discovered in 1825, and first publicly exhibited at the Paris Ex­position in 1855, aluminum was still so difficult to obtain that it was more expensive than silver, until the development of the dynamo provided cheap electricity and the development of the present electrolytic process for its production made it possible to make large quantities cheaply. Even today, the world production of aluminum is only about one percent of the world production of iron, although about half that of copper by weight.

The fundamental properties upon which the usefulness of aluminum depends are its lightness, workability, resistance to corrosion, high electrical and thermal conductivity, silvery luster, and high power of reflecting light and radiant heat. With these, there must, of course, be combined a strength adequate for the purpose; and since pure aluminum is not very strong (as compared with iron, for example), the development of the required strength with a minimum reduction in its other desired properties has been a basic problem of the fabricating industry. Since, when the metal was new, the established metal-working industry was broadly content with the metals and practices available to it, and disinclined to spend money in developing a new and untried metal, the whole burden of both technological and commercial development had to be shouldered by the producers of the metal, in order to sell their product, and they became fabricators as well as producers.

Castings. For many purposes, the simplest process of producing metal articles is that of casting the molten metal into a suitable mold. The relatively low melting point of aluminum permits the use of a variety of casting processes that are not suitable for metals like iron and copper. The cheapest type of mold is one made of moist ("green") sand, which is rammed around a wooden or metallic pattern. Where only a limited number of castings are to be made, or where the casting is very large or intricate, sand molds produce the cheapest castings. If very large numbers of the same casting are to be produced, and if the casting is not too large, a permanent mold (usually iron) may be used, because of the moderate casting temperatures employed, and may pro­duce castings that are both cheaper and metallurgically superior. By rapidly forcing metal under pressure into a suitable permanent mold, "die castings" are produced. They have very high surface smoothness and dimensional accuracy. A special type of plaster used for molds produces castings with surface smoothness and dimensional accuracy comparable with those of die castings, but with somewhat lower mechan­ical properties.

Of the alloying elements used with aluminum, zinc was the cheapest and was the one earliest used. It very decidedly increases the strength of the aluminum and at least 15 or 20 pct of it may be added with beneficial results, as far as the strength and ease of casting are con­cerned. It was soon found, however, that binary aluminum-zinc casting alloys were quite inferior to pure aluminum, from the standpoint of hot shortness and resistance to corrosion as well as specific gravity, and that they had a marked tendency to become brittle with age, so they have not been used extensively for many years.

Zinc was very largely replaced more than 40 years ago by copper, and for years the standard aluminum casting alloy was the binary aluminum-copper alloy containing 8 pct of the latter metal. This is a very stable alloy with good casting characteristics and better corrosion resis­tance than the type of zinc alloys previously used. Its strength was adequate for most purposes (about 22,000 psi sand-cast) although its elongation was low. Some improvements were made over a series of years by the addition of 1 or 2 pct of zinc and an increase in the iron content to around 1.2 pct, and later by adding up to 2.5 pct of silicon; and such alloys have produced many of the aluminum castings used in the automobile field. One of their strong points is their great ease of machining. This is particularly advantageous in automobile castings, where large numbers of castings are put through complicated machining operations and the time required for each operation must be reduced to a minimum. This time is considerably less for these aluminum-alloy castings than for iron castings.

Silicon and iron are the two most common impurities in aluminum, and traditionally both were generally looked upon as undesirable., Iron, indeed, in amounts up to perhaps 1.5 pct or a little more, may improve the tensile strength of some of the alloys, but if there is much more than this the high-melting iron-aluminum constituent increases the vis­cosity of the alloy at the pouring temperature and also tends to make the castings brittle. In the early days of the aluminum industry, metallic silicon was not available as an alloying ingredient, and although a number of investigators showed that aluminum alloys containing considerable amounts of silicon had reasonably good properties, it was not until after World War I that serious attempts were made to introduce such alloys.

Wrought Aluminum Alloys. For many purposes, castings are not suitable and worked or wrought articles are required. In general, their production starts with a casting operation in which an ingot of suitable size and shape is formed. This ingot is then rolled, pressed, extruded, or otherwise deformed by the application of severe mechanical stresses so as to produce the article desired.

Of all the working processes, hammer forging is the simplest in its essence and probably was the first to develop in the history of metal working. In connection with aluminum, however, it was one of the later developments. One of the difficulties to be overcome is that the temperature range over which aluminum can be forged satisfactorily is rather narrow, particularly for the strong alloys. Also, at their forging temperature these alloys are much stronger than steel is at its forging temperature, so that the size of hammer and the power required to forge a connecting rod (for example) out of aluminum are greater than that required to forge the same article out of steel.

Any mechanical working, such as rolling, breaks down the cast structure of the original ingot and tends to produce a fibrous structure, which very much improves the physical properties of the article. Working under a hammer — the simplest method of forging — may sometimes be advantageously replaced by a press-forging operation in which the lump of metal, preferably previously broken down by rolling, is formed into the desired shape while hot, by successive strokes of a press provided with suitable dies. In any forging operation, care must be taken to control and direct the flow of the metal during the working operation, so that the direction of the resulting "fiber" gives the results desired in the finished article. The dominant characteristic of the forging process, as compared with any casting process for producing the same article, is the marked metallurgical superiority of the product. Not only are the mechanical properties higher but forgings are also inherently much freer of hidden defects, such as dross or porosity, and at the same time have nearly the dimensional accuracy of permanent-mold castings.

A special kind of permanent-mold casting process is spoken of as "die casting", or sometimes "pressure die casting". In this, the watercooled steel mold or "die" is mounted adjacent to a crucible of molten metal, and by an ingenious device successive "shots" of molten metal are squirted into the die, so that the die is filled at each "shot" with the molten metal under pressure. In the subsequently developed "cold-chamber" process, the charge of molten metal is cooled to partial solidification and then forced into the die by heavy hydraulic pressure. By both processes, it is possible to fill the die in a fraction of a second. Consequently, castings with extremely thin walls can be made, with the assurance that the metal will not freeze before the narrow cavities of the mold have been properly filled.

The oldest of the wrought aluminum alloys now in production is the one called 3S, which contains about 1.25 pct of manganese added to commercial aluminum. The addition of this small amount of manga­nese increases the tensile strength of the sheet or other wrought article by about one third without very seriously reducing its elongation. Moreover, manganese is one of the few elements that can be added to aluminum without decreasing its corrosion resistance. This alloy is said to have been developed under stress in the early days to meet a very serious condition threatening the loss of a large amount of business because of the inferior corrosion resistance of the binary copper-alloy sheet that was then being produced, and it was extensively made and sold for many years before it was introduced abroad. Its most common use is in cooking utensils, where it increases the hardness and strength of the utensil without increasing its weight, and without unduly increasing its fabrication cost.

Attempts to add copper and zinc in alloys for the production of sheet were at first unsuccessful because of the poor corrosion resistance of such sheet. It was only after the introduction of the heat treatment of copper alloys that their corrosion resistance became satisfactory. Early attempts to produce aluminum-magnesium alloys, on the other hand, failed because of rolling difficulties. The alloys themselves had excellent properties but their economical fabrication was beyond the skill of the industry at that time. As the result of intensive study and improvements in fabricating technique, these alloys are now coming into their own, and are among our most corrosion-resistant aluminum alloys.

Special Heat Treatment and Aging of Wrought Alloys. The advent of the airplane and the dirigible balloon brought a tremendous desire for aluminum alloys that would combine the strength of steel with the lightness of aluminum. The first step along this line was taken by a German investigator, Dr. Alfred Wilm, about 1906, when he discovered that the aluminum-copper alloys containing about 4 pct of the latter element were susceptible to a decided improvement in physical properties by a simple heat treatment. If the hard-rolled alloy was heated for some time to a temperature of about 500°C and then quenched, both the tensile strength and elongation were markedly increased. The properties were still better if about 0.5 pct each of magnesium and manganese was added to the alloy; and, strange to say, if this alloy was allowed to rest for a few days after heat treatment, a further considerable increase in tensile strength took place spontaneously. The resulting alloy, when properly fabricated, had substantially the strength of mild steel — a tensile strength of 55,000 to 60,000 psi and an elongation of about 20 pct in 2 inches.

A few years before World War II, a considerably stronger alloy of this type (24S) was developed, which became the most important aircraft structural alloy used during the war.

At that time, no one knew just what was happening during the heat treatment or during the aging. Later investigations have given a picture of these phenomena, which seems to be quite accurate. Apparently, when the aluminum alloys are heated at a temperature near the melting point of their lowest-melting constituent (in this case the aluminum-copper-magnesium eutectic), the copper particles, which were dispersed in the form of crystalline fragments of a copper-aluminum compound throughout the mass of the wrought article, gradually dissolve and diffuse through the main body of solid aluminum. Magnesium silicide particles behave similarly, all being much more soluble at the heat-treating temperature than at room temperature. A nearly saturated solution is formed, and when the alloy article is quenched in cold water this solution does not have time to precipitate the excess of dissolved copper. The result, therefore, is a supercooled solid solution.

If only copper is present, the gradual precipitation of the excess and the attainment of equilibrium takes place very slowly at ordinary temperatures, but if some magnesium is also present it is practically completed in about four days. This excess of copper, probably in the form of a compound of copper and aliminum, appears to be precipitated in minute particles throughout the mass of the object, and the size of these particles depends upon the conditions under which they are precipitated. If these are correct, we have what is known as a critical dispersion and the hardening and strengthening effect of the precipitated particles is of maximum intensity.

In general, the mechanical properties are improved (for example, the tensile and yield strengths are increased) as the amounts of the alloying constituents are increased, up to a certain limit. However, as has already been indicated, each increase in the amount of copper, magnesium, or manganese increases markedly the difficulty and expense of working the alloy, and particularly the amount of scrap produced. The choice of a suitable alloy composition thus always involves a compromise between the desire for maximum mechanical properties and the necessity of fabricating the material at a reasonable cost.

For this field, a composite strong alloy sheet (called "Alclad") was developed in which the core of strong alloy was provided on each side with an external layer of pure aluminum or of an aluminum alloy electrolytically negative to the core. The thickness of this layer was only about 5 pct of the total thickness of the sheet, and it was so efficient a protector that a 1/16-in. sheet of this material has been exposed for more than five years to the continuous spray of a strong salt solution without any loss in its tensile strength or elongation.

A striking example of the use of this composite material is found in the all-aluminum dirigible constructed some years ago, where the outer envelope served both as envelope and as gas container and was made of Alclad 17S-T less than 0.01 in. thick. This sheet was strong enough so that the workmen walked around inside the envelope freely during its construction, and yet light enough to give the dirigible an adequate lifting power. After 12 years of service, tests showed that corrosion had not affected the properties of the material sufficiently to reduce them below the guaranteed minimum values upon which the material originally was sold.

The insistent demand of the aircraft industry for higher-strength wrought aluminum alloys appeared to be met by the development of aluminum alloys containing magnesium and zinc in the proportions of the compound MgZn₂. However, it was soon found that wrought alloys of this type were subject to serious stress corrosion and consequently were unsuitable for use. It was not until near the close of World War II that a practical composition of this type (75S), satisfactorily free of susceptibility to stress corrosion, was developed and commercialized. It has a tensile strength about 20 pct above that of 24S, in the heat-treated and aged condition, and is available in both bare and Alclad forms.

Extrusion and Other Working Operations. Another method of working that has special applications is extrusion. In this process, a heated ingot of the metal is squirted through a die under very great pressure. Its purpose is to produce special shapes, such as moldings, small structural shapes, hollow cylinders, and so forth, at a moderate cost. It is very often possible to thus produce shapes that could not be produced by rolling or any other method, and the possibility of making these   odd   shapes has   considerably increased the usefulness of aluminum alloys in architectural work and in the con­struction of trucks, railroad cars, and other equipment. The extrusion of hollow cylindrical blooms, from which tubing can be drawn, is also very important.

Like the other working processes, extrusion is not without its trou­bles. Surface scratches caused by defects in the die, foreign particles that get into the metal, and other faults, may be very troublesome. Sometimes the combination of the amount of working and the tempera­ture of extrusion produces peculiar grain-size effects. A considerable proportion of the grains may be oriented in a definite direction, generally so as to increase the tensile strength and decrease the elongation of theextruded article in the longitudinal direction. This may be important in connection with certain articles made of extruded heat-treatable alloys.

There are two forming operations that are termed "drawing", of which the simplest is wire drawing. Rod as small as 3/8-in. diameter can be most easily and cheaply produced by hot-rolling an ingot of suitable shape on special rolls, but to produce smaller rod or wire this hot-rolled rod must be drawn through dies made of chilled cast iron or special steels, or even of diamonds for the smaller sizes. By repeat­edly drawing or pulling this wire through successively smaller holes in these dies, the diameter of the wire may be reduced as desired. Here the control of the process involves an adjustment of the successive die sizes, so that internal fractures of the wire may be avoided, and suitable lubrication to reduce the die friction to the minimum.

Cooking utensils and articles of similar shape are also "drawn", but this drawing is accomplished in a press, and generally starts with a circle cut from sheet of the proper thickness. A plunger pushes the center of the circle downward into a hollow die having rounded upper corners, and the outer portions of the circle are drawn in over these rounded corners to form the cylindrical side of the article. The metal circle being drawn is thus subjected to tension radially and compression tangentially. Here again, lubrication is important, as well as die design and fine-grained material is necessary in order to avoid the "eggshell" effect on the drawn portions.

"Spinning" is a very useful metal-working operation for the forma­tion of hollow aluminum utensils of shapes that cannot be drawn in a die, or of sizes that are not feasible to make in a draw press. In the spinning operation, the blank may be either a flat circle cut from a sheet of suitable thickness or a drawn cuplike article made from such sheet. In either case, the blank is fastened in a spinning lathe in contact with a wooden or steel form having the shape of the article desired, and the blank and form are rapidly rotated together. By pressing on the outside of the blank with a suitable well-lubricated tool, the metal may be gradually forced or "spun" in, until it makes contact with the form. A familiar article produced in this way is the ordinary aluminum teakettle, where a drawn cuplike blank has its top spun over a collapsible form to make the top of the kettle. In forming certain articles, such as pitchers or cuspidors, where it would be difficult, if not impossible, to withdraw the internal form, the article may be spun "free hand" by the workman, who then adjusts the pressure and motion of the spinning tool by eye to give the desired shape to the article. After spinning or drawing, there are, of course, necessary trimming and beading operations, which do not require explanation.

The development of free-cutting aluminum alloys (such as US) which machine as easily as free-cutting brass or steel, permitted the use of aluminum to make screw-machine products. A "screw machine" is an automatic lathe provided with a variety of tools on an automatic turret. It is fed long lengths of rod, which it automatically machines into small articles such as bolts, screws, pipe inserts. In a "free-cutting" alloy, the tools must produce short, brittle chips, and not long turnings that may wind around and clog the machine. This is accomplished by means of appropriate alloying additions.

Another interesting type of working performed on aluminum is the stamping operation by which bronze powder was formerly made. "Bronze" powders may be made of aluminum, copper, or some grade of brass, but at present most of the bronze powder is made of aluminum. Sheet scrap or foil scrap or other small pieces of this metal, together with a suitable amount of lubricant, may be fed to batteries of stamps. By continual stamping, they are gradually hammered down into fine flakes, which are extremely thin in proportion to their diameter. In the ordi­nary aluminum powder, most of these flakes will pass through a 120-mesh sieve, while their average thickness may be on the order of 0.00002 in. The very great reduction in cross section produced by hammering the metal into such thin flakes reduces the elongation to the point where the flakes break up very easily, and that is the reason why these small flakes are produced instead of larger sheets. These little flakes are then polished by rotating brushes in a special machine and are ready for use in the manufacture of aluminum paint.

The stamping process has now been largely replaced by a special sort of grinding operation. Ball mills with a multitude of small steel balls "grind" atomized aluminum powder with the lubricant and min­eral spirits. The small balls flatten the powder to flakes like those produced by the stamping process. The paste that remains after the excess of mineral spirits has been removed can be used directly for the manufacture of paint. Abroad, the grinding is reported to be carried out dry, originally in a special type of beater mill, with an inert atmosphere. The dry powder easily forms an explosive mixture with air.

It will be apparent that while aluminum is a metal with a multitude of uses, it is also one presenting a multitude of problems, some of them being the common problems of the metal industry and others peculiar to itself. The steady progress of investigation and development, however, has carried the Industry a long way in its 60 years of life, and may be expected to make this metal increasingly useful to mankind as the years go by.