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Materials Science on CD-ROM User Guide

Aluminium Alloys: Processing

Version 2.1

Graeme Marshall, Alcan International Ltd. Paul Evans, Alcan International Ltd. Andrew Green, MATTER

October 1997

Assumed Pre-knowledge

Before starting this section, it is important that you have a basic understanding of binary phase diagrams. You should be familiar with the following terms: phase, equilibrium, Gibbs' free energy, dislocation, solid solution, stress, strain.

It is also suggested that you have referred to the MATTER module Aluminium Alloys: Systems.

Module Structure

This module comprises 3 sections:

• Casting
• Homogenisation
• Rolling and Extrusion

Casting

The casting of ingots is one of the first stages in the production of wrought alloys. In this section, you will study:

• how grain structure may be controlled at the casting stage;
• how segregation of the components can occur; and
• the formation of intermetallic compounds.

The section starts with a general introduction to the direct chill (DC) casting technique which is used to produce the majority of ingots. The importance of controlling grain size is covered in an exercise in which the user can add different amounts of grain refiner to a melt, and see how it affects the resultant grain structure.

The section then proceeds by looking at the phenomenon of microsegregation - i.e. the formation of a non-uniform composition due to solute partitioning. The influence of the partition coefficient, k is examined.

This section is completed with a brief look at the formation of intermetallic compounds in aluminium alloys.

Homogenisation

As-cast aluminium alloys tend to have very non-uniform microstructures. Wrought alloys for rolling or extrusion are given an ingot heat treatment to modify the cast structure. This is often referred to as homogenisation.

Features of the cast structure that affect subsequent processing and/or properties can be altered by soaking for several hours at a temperature in the range 500-620°C.

This section examines two features of particular relevance to aluminium alloys: the elimination of microsegregation and the elimination of supersaturation.

The time required for homogenisation to occur depends on these factors:

• the cell size, usually denoted by 2l;
• alloy element(s). Elements with low diffusivity in aluminium, such as Fe will take longer to homogenise than those with high diffusivity;
• homogenisation temperature, T.

An interactive simulation allows the user to select different cell sizes and temperatures to see how the homogenisation time is affected. For a more detailed study of homogenisation, the user is referred to the MATTER module Atomic Diffusion in Metals and Alloys.

In certain situations, it is not possible to homogenise above an alloy's solvus temperature. In this case, the alloy may be heated to a temperature somewhat below the solvus temperature to allow the removal of supersaturation via the formation of dispersoids.

Rolling and Extrusion

This short section gives a brief overview of these two important processes.

Bibliography

The student is referred to the following resources in this module: