Explain the mechanism of precipitation hardening what types

Explain the mechanism of precipitation hardening, what types of precipitates are most effective in producing hardening? Explain the terms aging and overaging? Aluminum alloys are often supplied in with a temper grade? Explain the meaning of the temper. With reference to the Al-Cu phase diagram above, design an age hardenable alloy. Specify its composition and the exact heat treatment process you will use for age hardening.

Solution

Explain the mechanism of precipitation hardening. What types of precipitates are most effective in producing hardening?

Precipitation hardening relies on changes in solid solubility with temperature to produce fine particles of an impurity phase, which impede the movement of dislocations, or defects in a crystal\'s lattice.

The precipitation-hardening process involves three basic steps: solution treatment, quenching and aging.

The most effective precipitates used to produce hardening are for the aluminum, Cooper (Cu) and Magnesium (Mg).

Explain the therms aging and overaging.

Aging is the process in which the metal alloy is heated until a certain temperature and is kept at that temperature for a certain time, until the precipitation process happens.

Overaging is what happens after the time threshold of the aging process is surpassed, in the overaging process the aluminum alloy hardness decreases because of an excessive diffusion of the precipitate.

Aluminum alloys are often supplied with a temper grade. Explain the meaning of the temper.

The temper designation indicates the treatment done to a given aluminum alloy. It has two main components, a letter which designates the the treatment (“-F” As fabricated, ”-H” Cold Worked, “-O” Annealed, “-T” Heat treated, “-W” solution heat treated), and a number from 1 to 3 digits, who gives information about the conditions of the treatment.

Temper refers to the treatment given to the alloy.

Design an age hardenable aluminum-cooper alloy.

Remembering that the maximum amount of copper that can contribute to precipitation strengthening is about 5.45 % Cu, thus for a 4%Cu aluminum-cu alloy:

Step 1: Solution treat at a temperature about 823K (550 C) up to the alpha phase.

Step 2: Quench to room temperature fast enough to prevent the formation of the theta phase.

Step 3: Rise the temperature to something about 373K (it has to be bellow the solvus line), and age the alloy for about 1-2 hours (depends of the hardeness wanted).