unsuitable for structural use. Higher strength is achieved by alloying, and it is possible to attain values as high as 30 tons/in2 or more for specific purposes without any significant in. crease in weight. Gain in strength is accompanied by a lower ductility
the pure grades of aluminium in the annealed condition have a duc- tility comparable with that of lead although the metal hardens appre- ciably on working). With loss of ductility the ease of machining in- creases and all the wrought materials with the exception of the grades of pure aluminium have very good to moderate machinabilities. Cast al loys in general are more difficult to machine than wrought materials. but machining speeds and feeds are high.
Aluminium and its alloys are relatively soft metals although the hardness increases with tempering or heat-treatment. The following ty pical hardness values (Vickers Pyramid Numbers), afford a com- parison with steel:
Aluminium IA-C 18 V.P.N.
Alloy H9.W 51 Alloy H30-WP 98-109
110
Mild steel Architectural stainless
steel
160-180
The surface of aluminium can be given improved resistance to scuffing by anodising, an electrolytic process to be described in the next Digest. For purposes where resistance to abrasion is important, alloys of suitable hardness should obviously be chosen and it may be noted that the aluminium-magnesium alloys N5 and N8 resist abrasion particularly well, partly because of their high resistance to corrosion.
Table 1
B.S. PREFIX SYMBOLS FOR WROUGHT ALUMINIUM
Prefix
MUFEC@>UNAY
PC
Sheet and strip
Form
Clad sheet and strip
Drawn tube
Rivet stock
Forgings and forging stock
B. S. No.
1470
1470
1471
1472
1473
1473
Extruded round tube and hollow sections Wire
1474
1475
1476
1477
Bolt and screw stock
Bars, rods and sections Plate
Clad plate
The surface of aluminium can be burnished in production to give a very bright smooth finish of low emissivity and for this reason aluminium foil is now used exten- sively for thermal insulation purposes.
Foils vary in thick- ness up to 0.15 mm; the thicker ones may be used alone and the thinner ones supported on fibre insulation board or plasterboard. Foil is generally made from 99% pure aluminium but other alloy compositions (e.g. N3) can be used for higher strength. Layers of foil, cut end drawn into a honeycomb pattern, provide panels of high strength and very light weight.
The coefficient of thermal expan sion of aluminium is of the order of 24 x 106 per C, and allowance should be made for this in design, Joints in which aluminium is intend- ed to slide over itself often seize up,
or
move erratically. Where free movement is essential it is best to separate the metal surfaces with nylon spacers.
The elastic modulus of aluminium (9.9-10.5 x 106 lb/in2) is about one- third of that of steel; this means that an aluminium beam under a given load would deflect about three times as much as a steel beam of the
Table 2
1477
same section. For equal rigidity, therefore, aluminium beams require a greater moment of inertia, but even so they offer a net saving of some 40% in weight over steel.
A further consideration in the structural use of aluminium is the effect of fire, since aluminium alloys show a marked decrease in strength when heated. For example, alloy H30-WP loses two-fifths of its room- temperature strength when heated up to 200°C, and two-thirds of its strength at 250°C.
Structural applications
Detailed recommendations for the use of aluminium and its alloys for structural purposes are given in the report 'Structural Uses
of Aluminium', prepared by the Institu tion of Structural Engineers and revised in 1961. It includes design information, recommendations for alloys and any necessary protective
measures.
The alloys chiefly used for struc- tural purposes are H9, H20, H30 and H19. Alloy H9 lends itself well to extrusion into complicated shapes of high finish, has good resistance to corrosion and is satisfactory where high strength is not the main con sideration. Alloys H20 and H30,
B.S. SUFFIX SYMBOLS-CONDITION OR TEMPER OF WROUGHT ALUMINIUM ALLOYS
Suffix
830
M
OD
W
WP
WD
WDP
Condition of heat-treatable alloys
Solution-treated and naturally aged
Temper of non heat-treatable alloys
Annealed (fully softened) condition 'As-manufactured' condition Annealed and lightly drawn
Solution and preciptation-treated (fully heat-treated)
Drawn after solution-treatment (wire only)
As WD. with subsequent precipitation-treatment (wire only)
Air-quenched and precipitation-treated
100
P
H. H. H
H
Various hardness of temper for sheet and strip, plate. drawn tube, wire and rivet stock
THE HONG KONG & FAR EAST BUILDER -- VOLUME 17, NUMBER 6