Precipitation hardening (age hardening) – strengthening by precipitation of fine particles of a second phase from a supersaturated solid solution.
The precipitation hardening heat treatment involves the following stages:

•    Solution treatment

During solution treatment a part is heated to a temperature above the solvus temperature in order to dissolve the second phase in the solid solution.
The part is held at this temperature for a time varying from 1hour to 20 hrs. until the dissolving has been accomplished.
The temperature and the soaking time of solution treatment should not be too high to prevent excessive growth of the grains.

•    Quenching

Quenching is carried out in water, water-air mixture or sometimes in air.
Object of the quenching operation is obtaining supersaturation solid solution at room temperature.
Since the second phase retains dissolved at this stage, hardness of the quenched alloy is lower than after age precipitation, however higher than hardness of the alloy in annealed state.

•    Aging

Depending on the temperature at which this operation is carried out aging may be artificial or natural.
o    Artificial aging.
At this stage the part is heated up to a temperature below the solvus temperature, followed by soaking for a time varying between 2 to 20 hours.
The soaking time depends on the aging temperature (the higher the temperature, the lower the soaking time).
The aging temperature and the soaking time are also determined by the desired resulted combination of the strength and ductility of the alloy.
Too high aging temperature and time result in overaging – decrease of the strength and increase of ductility.

o    Natural aging.

Natural aging is conducted at room temperature and it takes a relatively long period of time (from several days to several weeks).
Precipitation hardening heat treatment is commonly used for the following alloys:
      Aluminum alloys: Al-Cu, Al-Mg-Si, Al-Cu-Mg, Al-Zn.
      Copper alloys: beryllium bronze,aluminum bronze, aluminum-nickel bronze, chromium copper.
      Stainless steels: iron-chromium-nickel alloys with additions of copper, aluminum, titanium and niobium. The precipitation hardening stainless steels are either austenitic or martensitic.
      Nickel alloys: nickel-copper alloys with additions of titanium, aluminum and iron.
      Titanium alloys: titanium-aluminum alloys, titanium-vanadium alloys.