SOLUTION HEAT TREATMENT AND AGE HARDENING

Aluminum, as well as some alloys, such as high-nickel steels, is subject to a two-stage phenomenon known as solution heat treatment and aging.

Figure 1. Age -hardening heat treatment phase diagram

Solution Heat Treating:

Consider the age-hardening process of aluminum-copper alloys. The process involves a solution heat treatment at about 950 ^oF in which

1. the CuAl₂ phase is dissolved in the matrix,

2. the alloy is quenched to room temperature to retain the solid solution formed at high temperature

3. following the quench, the alloy is either aged naturally (i.e., held at room temperature) or aged artificially by heating at a relatively low temperature (200 to 400 ^oF).

Figure 1 shows the solubility line PS, which indicates a decrease in solubility of the q phase in the a phase as the temperature decreases. If a metal with composition X is heated above the solubility line to a temperature T₁, the q phase will dissolve and uniformly disperse into the homogeneous solid a solution. Upon slow cooling, the phase will reform, and below PS solubility line the metal will once again consist of two distinct phases, q and a .

If the metal with composition X is heated to T₁, and quenched, the dispersed submicroscopic phase is trapped in the a solution. The solution a is said to be supersaturated, because it contains more q particles at room temperature than it can hold in its lattice structure. This process is called solution heat-treating. Figures 2 and 3 show the changes in microstructure as a result of quenching.

Aging:

The supersaturated solid solution is unstable and if, left alone, the excess q will precipitate out of the a phase. This process is called aging.

There are three types of aging:

Natural aging

Artificial aging

Abnormal aging

Natural Aging:

When the process occurs at room temperature, it is called natural aging.

Artificial Aging:

If the material that has been solution heat treated requires a heating to speed up the precipitation, the process is called artificial aging. It should be noted that freezing the solution heat treated material will retard the aging process.

Abnormal Aging:

In many instances aging will occur without precipitation. The particle may actually diffuse within the lattices and distort them. This type of aging is called abnormal aging.

Figure 2
Figure 3

After solution heat treatment the material is ductile, since no precipitation has occurred. Therefore, it may be worked easily. After a time the solute material precipitates and hardening develops. As the composition reaches its saturated normal state, the material reaches its maximum hardness. The precipitate, however, continues to grow. The fine precipitates disappear. They have grown larger, and as a result the tensile strength of the material decreases. This is called overaging. The hardness and tensile strength variation during aging and overaging are shown in Figure 4.

Figure 4. Hardness and Tensile Strength and Aging stages.

Table 1 shows the effect of aging on the properties of Aluminum Alloy 2014.

ALLOY AND CONDITION	TENSILE STRENGTH (PSI)	YIELD STRENGTH (PSI)	ELONGATION % IN 2 IN.	HARDNESS BHN 500 KG 10 MM
Annealed	27,000	14,000	18	45
Solution treated. Naturally aged	62,000	42,000	20	105
Solution treated , artificially aged	70,000	60,000	13	135

Table 1. Effect of aging on the properties of 2014 aluminum alloy.

As mentioned earlier, refrigeration retards the aging process. At 32^oF the beginning of the aging process is delayed for several hours and freezing at temperatures of up to -100 ^oF retards aging for an extended period. Use is made of this fact in the aircraft industry when aluminum-alloy rivets, which normally age at room temperature, are kept in deep-freeze refrigerators until they are driven. The rivets have previously solution-treated, and as a single phase they are very ductile. After being driven, aging takes place at room temperature, with a resulting increase in strength and hardness.