Strengthening Mechanisms: Introduction

Pure, untreated aluminium is a soft metal with insufficient strength for most engineering applications. In order to take advantage of its low density, aluminium has to be strengthened by one or more mechanisms. These are considered in det

The 4 mechanisms

In general, four different strengthening mechanisms are used to strengthen aluminium. These are summarised below:

Softening Mechanisms in Aluminium Alloys

Alloys that develop their shape and properties by cold-working often have to be re-softened at regular intervals before further deformation can take place.

Other examples of softening include the re-solution of second phases in age-hardening alloys, and the controlled grain growth of alloys with small grain sizes. An interesting exception is provided by solute-hardened materials which cannot be softened since this strengthening mechanism is determined solely by composition and not by thermal or mechanical processing.

Aluminium Processing

From bauxite extraction to the final aluminium products, several processes are needed.

Firstly, alumina is extracted from bauxite through the Bayer process.
Then, alumina is reduced by electrolysis into molten metallic aluminium through the Hall-Heroult process. This molten aluminium (also called primary aluminium) is then cast into ingots for subsequent remelting or more usually into cylindrical extrusion billets or rectangular rolling slabs.

Besides primary production, aluminium recycling is also an important source of aluminium, especially for ingot production.
Ingots are used to produce cast products like engine blocks.

Extrusion billets are pushed through shape dies to give extruded profile used, for example, in structures while rolling slabs are hot rolled and usually cold rolled into sheet, plate or foil used, for example, for facade panels or packaging applications.

Extruded profiles and sheets, frequently called wrought products, as well as cast products usually need subsequent transformations and treatments to become useful components or products.

Those transformations (like surface treatment, forming, joining, etc.), frequently called enabling technologies, are gathered as downstream processes.

Rolling: Introduction

Rolled products, i.e. sheet, plate and foil constitute almost 50% of all aluminium alloys used.

In North America and Western Europe, the packaging industry consumes the majority of the sheet and foil for making beverage cans, foil containers and foil wrapping. Sheet is also used extensively in building for roofing and siding, in transport for airframes, road and rail vehicles, in marine applications, including offshore platforms, and superstructures and hulls of boats.

Also, while relatively little is currently used in the manufacture of high volume production automobiles, it is expected that the next decade will see an increase of aluminium sheet used for body panels.

The starting stock for most rolled products is the DC (Direct Chill semi-continuous cast) ingot. The size of the ingot depends on the size of the DC unit available, the hot rolling mill capacity, volume required for a particular end use and to some extent the alloys being cast. Ingots up to over 20 tons in weight, 500-600 mm thick, 2000 mm wide and 8000 mm long are produced.

The DC ingot is usually cooled after casting to room temperature and then re-heated to around 500 °C prior to successive passes through a hot rolling mill where it is reduced in thickness to about 4 - 6 mm

The strip from the hot rolling mill is coiled for transport to the cold mill which might be on the same site or elsewhere. Cold mills, in a wide range of types and sizes are available; some are single stand, others 3 stands and some 5 stand. Cold rolling speeds vary but modern mills operate at exit speeds as high as 3000 m per minute and alloys may be cold rolled to thickness of around 0.05 mm.

Applications of Rolled Products

Here are some typical applications of rolled aluminium sheet and plate alloys.

Heat treatment

Different aluminium alloys can be subjected to a range of heat treatments:
Homogenisation: after casting, alloys are heated to remove any segregation, i.e. to obtain a homogenous composition throughout the alloy.
Annealing: strain-hardening alloys (1xxx, 3xxx and 5xxx) can be softened after cold working.
Precipitation or age hardening: 2xxx, 6xxx and 7xxx alloys can be strengthened by precipitation hardening, or 'ageing'.
Solution heat treatment of precipitation hardening alloys prior to ageing in order to take alloy elements into solution.
Stoving (e.g. to 'cure' a paint or lacquer coating)

Process Routes

Although we often tend to consider processes such as casting, rolling, extruding and heat treatment as separate processes, it is very important also to think in terms of entire process routes.

A process route will be developed for a given application in order that all the required properties are achieved or optimised.On the following pages, you can see some different process routes for different semi-finished products.

Recycling

Aluminium has been recycled since the days it was first commercially produced and today recycled aluminium accounts for one-third of global aluminium consumption world-wide.

Recycling is an essential part of the aluminium industry and makes sense economically, technically and ecologically.

At the end of their useful life, all aluminium products retain some worth which guarantees that it is possible to create value by recycling them into new products.

Efficiency of aluminium recycling thus translates into high recycling rates for the various applications.
ail in this section.

Applications

The main properties which make aluminium a valuable material are its low density, strength, recyclability, corrosion resistance, durability, ductility, formability and conductivity.

Due to this unique combination of properties, the variety of applications of aluminium continues to increase. It is essential in our daily lives. We cannot fly, go by high speed train, high performance car or fast ferry without it. We cannot get heat and light into our homes and offices without it. We depend on it to preserve our food, our medicine and to provide electronic components for our computers.

Reference
Web site: http://aluminium.matter.org.uk/

Back