Special heat treatment and aging of wrought alloys

The advent of the airplane and the dirigible balloon brought a tremendous desire for aluminium alloys that would combine the strength of steel with the lightness of aluminium. The first step along this line was taken by a German investigator, Dr. Alfred Wilm, about 1906, when he discovered that the aluminium-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 aluminium alloys are heated at a temperature near the melting point of their lowest-melting constituent (in this case the aluminium-copper-magnesium eutectic), the copper particles, which were dispersed in the form of crystalline fragments of a copper-aluminium compound throughout the mass of the wrought article, gradually dissolve and diffuse through the main body of solid aluminium. 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 aluminium, 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 aluminium or of an aluminium 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-aluminium 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 aluminium alloys appeared to be met by the development of aluminium 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.

Task 1

Give Ukrainian equivalents to the following words and word combinations:

tremendous desire, decided improvement, hard-rolled alloy, strange to say, considerable increase, resulting alloy, aircraft structural alloy, to be quite accurate, melting point, lowest-melting constituent, dispersed in the form of, dissolve and diffuse, soluble, saturated solution, to precipitate the excess of, critical dispersion, the hardening and strengthening effect, alloying constituents, suitable alloy composition, insistent demand.

Task 2	Match the words in column A with their translation from column B:
A	B
1. elongation	a) рівновага
2. dispersion	b) ковані частини
3. melting point	c) подовження
4. equilibrium	d) старіння
5. investigation	e) дисперсія
6. saturated solution	f) дослідження
7. aging	g) точка плавлення
8. wrought article	h) насичений розчин

Task 3	Fill in the correct words from the list below according to the text. Use each word only once.
alloy sheet, wrought article, quite accurate, intensity, constituent, the strength, structural alloy, solution temperature, dirigible, considerable increase, alloy

1. seems to be …	7. the most important aircraft …
2. composite strong …	8. maximum …
3. …balloon	9. …in tensile strength
4. supercooled solid …	10. soluble at the heat-treating …
5. the mass of the …	11. …of steel
6. hard-rolled …	12. lowest-melting …

Task 4	Fill in the gaps with the verbs given below used in the proper tense form.
to obtain, to consider, to be, to combine to become, to cause, to develop, to reach

1. Magnesium silicide particles … more soluble at the heat-treating temperature than at room temperature.

2. The development of aluminium alloys containing magnesium and zinc in the proportions of the compound MgZn2 … by the insistent demand of the aircraft industry for higher-strength wrought aluminium alloys.

3. When the hard-rolled alloy … the temperature of about 500°C and then quenched, both the tensile strength and elongation were markedly increased.

4. With the increase of the alloying constituents the mechanical properties … better.

5. During the World War II, a considerably stronger alloy of this type (24S) … and widely used in the defence industry.

6. Thus, the supercooled solid solution ….

7. Aluminium alloys that … the strength of steel with the lightness of aluminium became the most desirable in the airplane and the dirigible balloon production.

8. While choosing a suitable alloy composition, we … the desire for maximum mechanical properties and the necessity of fabricating the material at a reasonable cost.

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