HEAT TREATMENT
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Napplied to aluminium and its alloys, the term "heat treatment" usually implies those processes, more specifically called solution treatment and precipitation treatment, by which the mechanical properties of certain alloys may be improved.
With these should logically be considered annealing, which has the effect of softening all forms of aluminium, and is often necessary, during fabrication, to remove the effect of work-hardening as a prelude to further cold-forming operations. This is the only thermal treatment applied to the simpler work-hardening type of aluminium alloy, and may well be described first.
Annealing
Aluminium, like other metals, has a structure of innumerable crystals or grains. Cold working, as in rolling or drawing, distorts this structure; the individual grains are deformed, movement taking place along slip planes, and there is as a consequence an increased resistance of the metal as a whole to further deformation. It is then said to be strainor work-hardened, and this state is permanent at ordinary temperatures.
If the metal is heated, some relief of the internal stresses begins; this is recovery. As the temperature rises, the distorted original grains disappear and new grains grow to form a stress-free system.This recrystallisation brings the metal to its softest state.
These effects depend on time as well as temperature; so that the change may be completed quickly, it is usual to heat in air at a much higher temperature (340' C to 45o'C) than the minimum necessary for recrystallisation. Apart from convenience,
this is done to avoid the merging of crystals to form larger ones, which is encouraged by, among other factors, long heating time. Grain growth, as it is called, impairs mechanical properties and surface finish. Alloys that are especially prone to grain growth are sometimes annealed more rapidly in molten salts at about 500 C.
Annealing, which is normally carried out in electrical or gas-fired air furnaces, is used to make further cold working possible, and several anneals may be needed before the metal reaches its final shape. A variation is partial-, back-, or temperannealing, where fully hard material of some compositions may be allowed to soften only to the intermediate degree of hardness, or temper, required.
Solution Treatment-
Certain metals, of which copper is the best example, are capable, in the solid state, of true solution in aluminium. The amount of such elements that the aluminium will accept is limited, but increases with temperature. If the temperature is lowered again, the excess proportion of the alloying element forms compounds, such as Cu Al,, which are deposited within the grains, and on their quantity, and more especially on their size, depends the resistance of the structure to deformation and hence the mechanical properties of the alloy.
Solution treatment, sometimes called normalising (from a faulty analogy with steel metallurgy) or simply "heat-treatment ", is a process by which this phenomenon is exploited. The temperature is raised to about 500 C, usually in a bath of molten sodium nitrate, until the alloying elements are taken fully into solid solution. The alloy is then cooled, by quenching in water, too quickly for the dissolved elements to be precipitated; the solution remains supersaturated and the metal is temporarily soft. This condition is unstable and precipitation gradually takes place in the form of extremely line particles. This small particle size (about to,000,000 in. dia.) enhances the tenacity of the metal, which is then said to have age-hardened or aged.
Refrigeration is sometimes used to delay precipitation from the supersaturated
state. In rivets, for example, the soft, formable condition that prevails for an hour or so after solution treatment may be maintained for about twenty hours, if need be, by storing at a low temperature.
Precipitation Treatment
In some alloys, the spontaneous ageing process just described is complete after a few days at room temperature. A greater degree of precipitation and hardening than occurs naturally is, in certain alloys, induced by heating to about 170 C for ten hours or so (time and temperature depending on the composition). This is called precipitation treatment or artificial ageing. Excess treatment, either in temperature or time, causes undesirable growth of the precipitated particles, reducing their effectiveness in strengthening the metal.
Temper Designation --
The condition of an alloy is conveniently indicated by a symbol placed after its specification number. The system adopted by the British Standards Institution is as follows : M — as manufactured, with no heat treatment and without regard to workhardening.
0 — annealed, soft.
141-1, ' 2H. 41-I or H,— the degree of workhardening resulting from rolling or drawing.
W — solution heat treated.
P — precipitation heat treated (artificially aged).
T — solution heat treated and naturally aged.
The fourth article in this series will be concerned with tensile properties.