Aluminium in Building:- 1
DURING
17.6
Properties and uses
URING the past ten years aluminium has gained in popularity as a decorative and structual material and its use has extended in many fields. Improvements in alloys of structural significance, and major changes in the finishes available for aluminium in general, make it appropriate to review its present position as a building material. This Digest, prepared by the British Building Research Station, deals with the nomenclature, properties and uses of aluminium and its alloys; finishes will be discussed separately later.
CLASSIFICATION AND NOMENCLATURE
Aluminium has become so familiar in everyday use that it needs no general description. To the layman. the use of the name not only for pure aluminium but also for a large number of alloys of differing chemical and physical properties is of little consequence; it is sufficient for him to recognise aluminium as a light. strong, durable metal. readily available in many forms and de- corative finishes and lending itself to a wide variety of applications.
For many technological purposes, however, it is necessary to classify the different alloys and identify their distinctive properties. Pro- prietary designations are still used by aluminium producers, but the British Standards 1470 to 1477 for wrought aluminium and its alloys, and B.S. 1490 for aluminium ingots and castings, have introduced common system of nomenclature. In this, the alloys are designated by numbers relating to their composi- tion with various prefix and suffix symbols relating to the form and the condition of the material, as ex- plained below. Casting alloys are prefixed LM' to distinguish them from wrought alloys.
Composition
a
In the B.S. system of nomenclature the composition of
wrought aluminium is denoted by a number. Super-purity aluminium
(99.99%
pure) is numbered 1 and the three grades of commercially pure metal are IA (99.8%), IB (99.5%) and IC (99.0%). Wrought alloys (contain- ing various percentages of mag.
96
nesium. silicon, copper and other elements, according to the properties required) are numbered from 2 onwards.
Casting alloys are also numbered serially LM1, LM2, etc.; the num bers referring to compositions are not related to those in the wrought alloy series.
Form
which
The various forms in wrought aluminium is produced sheet, tube, wire, etc. are designat- ed by prefix letters (see Table 1).
Heat-treatment
Wrought alloys of certain com- positions develop their maximum strength only when subjected to heat- treatment; these are termed heat- treatable alloys' and are designated by the prefix H.
Other alloys are not strengthened by heat-treatment but can be hard. ened to increased 'temper' by cold working, e.g. rolling, these are term- ed 'non-heat-treatable alloys and are designated by the prefix N.
Example. The specification NS3 denotes a non-heat-treatable alloy of composition 3. prepared in sheet form
Condition
The
The hardness, ductility and strength of wrought alloys varies according to the degree of heat- treatment 01 the amount of cold working they have received. condition of heat-treated materials and the temper of non-heat-treatable alloys are identified by means of a system of suffix letters following the composition numbers (see Table 2.
Heat-treatable alloys respond to one or more stages of hardening ac- cording to their composition. The first stage, known as solution-treat- ment. consists of heating the alloy to about 500°C--the exact tempera- ture depending on the alloy and quenching in oil or water. Some
alloys will develot: the required strength by natural ageing at normal temperatures after solution-treatment and require no further heating. Others may be assisted in hardening by a second stage of heating, termed arti- precipitation-treatment. This
ficial ageing process is carried out at selected temperatures between 100 and 200°C. The wrought alloys can be cold-worked con- veniently after solution-treatment for a short time before significant ageing has occurred.
The temper of non-heat-treatable alloys can be adjusted (giving tem- pers designated as H, H, H. and H by appropriate combination of cold working and annealing.
Castings are most frequently sup plied in the 'as-cast' condition (M) which is sufficient for most engineer- ing uses, but improved properties are obtainable with some materials by solution-treatment only (W), precipitation-treatment only (P). or full heat-treatment (WP).
Table 3 and 4 list the wrought and cast alloys that are most likely to come to the attention of architects. designers and builders. The Tables show the ranges of minimum strength for different conditions, tempers and forms,
PROPERTIES AND USES
Aluminium is a light metal, which is one of the reasons for its impor
The tance as a building material. density varies from 164 to 175 lb ft3 for the wrought material and 160 to 184 lb/ft3 for castings, according to the composition of the alloy. Steel has a density of 492 lb ft3.
Pure aluminium has low strength even in its hardest temper and so is
THE HONG KONG & FAR EAST BUILDER
VOLUME 17, NUMBER 6
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