Aluminium

Property

Constituents

Aluminium, by reason of its chemical reactivity, is not found in its pure state, but in combination with other elements. It is the most common metallic element in the earth’s crust which contains approximately 15% of Al₂O₃ (8% aluminium).

Commercial aluminium is generally 99.0% – 99.6% pure.

Typical composition of commercially pure aluminium:

Alloy Types

Aluminium is seldom used in its pure form but is normally alloyed with small proportion of other elements.

Typical of those are:

• Copper
• Manganese
• Silicon
• Magnesium
• Zinc

In the alloying furnace, the aluminum ingot is melted, and mixed with alloying metals like Magnesium, silica, copper, etc to form an aluminium alloy which offers a wide range of specific material properties. The physical properties of the alloy are very much determined by the alloy content. For example:

• Manganese offers good corrosion resistance
• Magnesium is good for welding applications
• Copper yields excellent machinability
• Zinc tends to offer very high strength

Solubility of Other Metals

Silicon, copper, iron, zinc, tin, manganese dissolve readily in aluminium. Sodium and potassium are practically insoluble in it, and titanium, vanadium, boron, nickel, and chromium have low solubility. Hydrogen is the only gas soluble in aluminium to any extent.

Manufacturing Process

Aluminium can be easily worked and can be hot or cold rolled, extruded, forged, pressed, drawn, molded, stamped, bent and shaped. It can be riveted, bolted, welded, brazed and soldered.

Used as external façade, aluminium panel can be either rolled (comes in sheets form) or extruded (for framing member). To improve the durability and strength, aluminium is also supplied in sandwich form.

One constraint on roll-formed sheet components is the availability and size of sheet materials; a roll of 1.6mm thick steel sheet is typically 430m long. The constraint is primarily the width; a typical maximum is 1250mm depending on the substrate and additional process required, although 1500mm wide sheet is available in some metals.

Sandwich Panel

Used as cladding and curtain wall panel, aluminium is supplied in sandwich form/ composite form.

Consist of 2 or more aluminium alloy sheets with either corrugated or honeycomb panel in between. For composite panel, the core can be polyethylene. This is to improve the strength and the durability of the conventional aluminium sheets. It may come as composite material with fire resistivity property.

Mechanical properties of typical aluminium sandwich panel:

* DIN 53293 ** DIN 53291 *** DIN 54451 **** DIN 52210

Dimension

For aluminium alloy strip and sheets products:

• Thickness: 1.5-4 mm (± 0.12 mm)
• Width: 1000-2100 mm (+ 3 mm)
• Length: 1000-12000 mm (+8 mm)

For aluminium alloy sandwich panel:

• Thickness: 4.7-11.2 mm (± 0.15 mm)
• Width: 1000-1500 mm (+ 6 mm)
• Length: 2000-4500 mm (+6 mm), supplied in roll form: 25,000 mm

Sizes might be varied on different manufacturers.

Workability

It can be riveted, bolted, welded, brazed and soldered.

Classification

According to BS 1490, casting alloys are designated by numbers in sequence of development which would be preceded by the letters LM (light metal), i.e. LM6.

In America, they use 4 digit system:

Examples of 4 digits numerical alloy nomenclature system:

Sheet and panel:

Finishes

Finishes of aluminium cladding affect its durability against weathering condition.

Anodized

The durability of anodising is a direct result of it fusing into the surface of the aluminium forming a hard oxide coating. It comes in a wide range of colours and recently it comes with much tighter control for colour matching.

It is difficult to obtain regularity in colour. However, anodising is being increasingly specified by architect for its durability and because it allows the metallic quality of the aluminium to remain visible.

Anodizing is an electrolytic coating process, and the metal deposit and colour is dependent upon time and current passed.

Liquid organic coating (i.e. Fluorocarbon or PVDF or PVF2)

Popularly used in Singapore, since it can promise regularity in colour matching. It can be touch-up on site.

It is hard wearing and tolerant to abuse but colour fade over time can be expected. Micro cracking can occur at the profiled bends at the time of manufacture or later in the life cycle. On aluminium sheets, these cracks can lead to an ultimate failure by corrosion at the bend.

Recommended test for liquid organic coating:

1. Detergent resistance ASTM D2248 (no loss of adhesion, no visual change)
2. Alkali resistance ASTM D1308 (no effect)
3. Humidity resistance ASTM D2247 (pass 3000 hrs)
4. Salt Spray resistance ASTM B117 (pass 3000 hrs)
5. Dry film hardness ASTM D3363 (no rupture of film)
6. Film adhesion ASTM D3359 (no loss of adhesion, no removal of film)
7. Impact resistance AAMA 605.2 (no removal of film)
8. Abrasion resistance ASTM D968 (abrasion coefficient value, min 20)

Polyester powder coating

When using polyester powder coating careful specification and monitoring is required to ensure that a thoroughly tested polymer is used and good adhesion achieved.

The powder coating process consists of three main stages:

1. Pre-treatment (involves the application of zinc phosphate conversion coatings)
2. Spraying the powder
3. Fusing the powder by heat treatment

It is possible to specify polyester powder coating, which includes anti-bacterial agents in the coating. This will kill common bacteria, such as Escherica coli, Salmonella and MSRA, which comes into contact with the coated component.

Recommended test for polyester powder coating:

1. accelerated weathering BS 3900 : Part F3 (pass 2000hrs)
2. sulphur dioxide resistance BS 3900 : Part F (pass 240 hrs, np blistering, loss of gloss or discolouration, no sign of corrosion)
3. humidity resistance BS 3900 : Part F2 (pass 1000 hrs, no blistering, softening or detachment of coating)
4. salt spray resistance ASTM B117 (pass 1000 hrs)
5. scratch resistance BS 3900 : Part E2
6. adhesion BS 3900 : Part E6
7. impact resistance BS 3900 : Part E3

Mill finished (= unfinished)

The density of aluminium is approximately one-third that of steel and copper. The density of pure aluminium is 2.6898 g/cm-3. Trace amounts of alloying elements produce cast and wrought alloys in the density range of 2.66-2.80 g/cm-3.

Modulus Elasticity

Young’s modulus of aluminium is approximately one-third that of steel. For pure aluminium, the value is 68.3 GPa. For high strength copper bearing alloys the modulus may be as high as 73 GPa.

Thermal Expansion

Pure aluminium has a coefficient of thermal expansion approximately twice that of steel at 23.5 x 10^-6 /ºC in the range 10-100 ºC.

Alloying elements have a small effect on this value. Figures in the range 20-25 x 10^-6 /ºC (for casting alloys) to 20-24 x 10^-6 /ºC for wrought alloys.

Thermal Conductivity

Aluminium is a good conductor of heat with conductivity for pure aluminium of 244 W/m ºC (about 4.5 times that of steel).

The thermal conductivity reduces with the increased alloying to 109 W/m ºC for wrought alloys and to below 100 W/m ºC for some casting alloy.

Aluminium has good reflectivity, over 90%, which means that it absorp heat slowly, and a relatively low emissivity of 0.1, which indicates low heat loss in the absence of convection.

Heat Transmission Coefficient

For composite panel, it ranges from 5 to 6 W/m² K

Service Temperature Limit

Melting point: 658.7ºC (but the melting point of some alloys can as low as 530ºC)

Boiling point: 2467ºC

At temperature below zero, aluminium and aluminium alloys exhibit higher tensile strength and elongation than at room temperatures. No alloys suffer low temperature brittleness and there is no point below which brittle fracture occurs.

Aluminium and its alloys show considerable reduction of strength at temperature above 100ºC. At 200ºC the strength is approximately half of that at room temperature. By 350ºC, most alloys will have lost most of their strength. Strength is not restored as temperature falls as the effect of work hardening will have been removed.

Specific Gravity

2.6989 at 20ºC.

Tensile Strength

48.265 MPa for 99.996% pure aluminium

145-185 MPa for aluminium alloy rolled product (sheet and strip)

Corrosion Resistance

Aluminium combines readily with oxygen and is made corrosion resistance by the transparent film of aluminium oxide that quickly forms and is relatively inert to further chemical action.

Aluminium is readily attacked by alkalis and hydrochloric acid and slowly attacked by dilute acids. It is inert to sulphur. In direct contact with metals other than zinc, cadmium, magnesium, and on magnetic stainless steel, aluminium is subject to certain types of galvanic action and should therefore be electrically insulated from other metals.

Electrical Resistance and Conductivity

Pure aluminium has a high electrical conductivity at 63% of IACS and an electrical resistivity of 2.69 µcm at 20ºC. These values increase with alloying to a resistance of over 6 µcm at 20ºC for some alloys and conductivity down to below 28% IACS.

Fire Resistance

Aluminium, being metal, does not burn and is therefore classified in fire protection regulations as non-combustible material. However, aluminium melts at about 650ºC, which can be attained during fire. When it melts, it releases no gaseoue subtances into the environment.
Where fire resistance is required, sufficient insulation must be provided to prevent the temperature of metal from rising above 200ºC.

Acid Resistance

Aluminium is found to be resistant to the attack of nitric acid, dissolves slowly in concentrated sulphuric acid and is soluble in hydrochloric acid. At normal temperature, it is not affected by sulphur, carbonic acid, carbonic oxide, vinegar, sea water, etc. But it is rapidly corroded by caustic alkalis.

UV Resistance

Also in commo with other metals, aluminium does not absorp moisture or need protection from UV radiation. Surface finishing processes such as anodizing or painting provide and maintain particular effects on aluminium.

Fatigue

Typical value of fatigue strength at 50 x 106 cycles range from 20MPa for annealed commercially pure aluminium to 124 MPa for 6082-TG material. Alloys with high manganese contents will exhibit higher values. Fatigue strength may be drastically reduced by unsatisfactory weld details.

Creep

Only commercially pure aluminium shows significant creep at room temperature. However, if the working temperature is held at around 200-250ºC, then the creep of some alloys will be significant.

Reflectivity

For pure aluminium with a clean bright surface 80%-85% of visible incident radiation and about 90% of heat will be reflected. Emissivity is typically 0.30 for a wavelength of 0.65 µmm.

However, as aluminium cladding comes in different finishes and colour tones, it is recommended to consult the finishes manufacturer for the reflectivity.