The heat treatment of cast / wrought aluminium and extruded alloys is carried out to change the mechanical properties of the castings or extrusions. The heat treatment consists of one or more thermal cycles being applied to the aluminium alloy components. The thermal treatments are designed by suffixes and main ones are as follows
Castings or extrusion are heated to a temperature just below the alloy melting point (this varies depending on the chemical compositions), for a time (this varies with alloy and section of the component) to allow the alloying constituents to enter into a solid solution. This solid solution is maintained by rapid cooling either by polymer quenching or hot water quenching in order to reduce distortion and reduce internal stresses.
This treatment is carried out in order to increase the strength and hardness of solution treated /as cast / as manufactured extrusions. The times and temperature vary depending upon the alloy and are usually in the temperature range 150/200°c. Times vary between 2/24 hours. Lower temperatures and longer times give better precipitation and thus better mechanical properties.
After solution treatment the casting can be brought into a stable state by heating to a temperature between 200/250°c. Times and temperature vary with different alloys and mass of the components.
The highest mechanical properties are obtained by solution treatment followed by a precipitation treatment as described in the section on precipitation.
Some components that are used at elevated temperatures such as pistons require a solution treatment and stabilization between 200/250°c in order that further heating of the component in service will not affect the mechanical properties.
These processes are used to remove casting stresses of soften alloys for subsequent shaping or mechanical working. Stress relieving can be carried out between 200/250°c and annealing from 350/400°c.
Dezincification Resistant Brass Treatment (DZR)
This is used where dezincification type of corrosion may occur. This type of corrosion is prominent in hot water systems. The resistance is obtained by careful control of the alloy composition and the subsequent heat treatment. The heat treatment consists of heating between 500 - 540°c for 2 - 5 hours and slow air cooling.
Following the solution process parts can be set to customer tolerances