Choosing Alloys

Understanding different alloys

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ALUMINIUM ALLOY AND ITS IMPORTANCE

Pure aluminium is soft and ductile and most commercial uses require greater strength than pure aluminium affords. So strength is achieved by the addition of other elements to produce alloys. Further strengthening is possible by means which classify the alloys roughly into two categories, non-heat-treatable and heat treatable.

NON-HEAT TREATABLE ALLOYS

The strength of alloys in this group depends upon the hardening effect of elements such as manganese, silicon, iron and magnesium. Since these alloys are work-hardenable, strength levels are controlled by various degrees of cold working, denoted by the ' H ' series of tempers. Alloys containing appreciable amounts of magnesium when supplied in strain-hardened tempers are usually given a final elevated-temperature treatment called stabilizing to ensure stability of properties

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1050/1080/1200

Far stronger than either of the above alloys. Forms readily in the intermediate tempers. Corrosion-resistant is very good, as is weldability. Better saltwater corrosion resistant than in 1200. Used for pressure vessels, tanks, fittings, or applications involving forming or welding where strength higher than 3003 is needed. Also, houseboat hulls, pontoon boats, and van bodies

5052

Far stronger than either of the above alloys. Forms readily in the intermediate tempers. Corrosion-resistant is very good as is weldability. Better salt water corrosion resistant than 1200. Used for pressure vessels, tanks, fittings or applications involving forming or welding where strength higher than 3003 is needed. Also houseboat hulls, pontoon boats and van bodies

5083

Stronger than either of the above alloys. For high strength welded applications with outstanding joint strength – rail wagons, marine components, bridges, stationery cryogenic vessels and overhead cranes. Excellent weldability and corrosion resistant. Static welded strength is second only to 5754 and 5083 is superior to 5754 in energy absorption. Restricted to temperature below 150°C = 65.5°C. The widest range of plate from stock

5086

Excellent for welded structures – strip hulls and superstructures, road tankers, pressure vessels, movable and stationery cryogenic vessels and structures requiring a high rate of energy absorption. Restricted to temperatures below 150°F = 65.5°C.

HEAT TREATABLE ALLOYS

The strength of alloy in this group is enhanced by the addition of alloying elements such as copper, magnesium, zinc, and silicon. Since these elements show increasing solid solubility in aluminium with increasing temperature, thermal treatments impart pronounced strengthening. The first step, called solution heat treatment, is an elevated - temperature process designed to put the soluble element or elements in a solid solution. This is followed by rapid quenching, usually in water, which momentarily 'freezes' the structure and for a short time renders the alloy very workable. It is at this stage that some fabricators retain this more workable structure by storing the alloys at below-freezing temperatures until they are ready to form. At room or elevated temperature, the alloys are not stable after quenching, and precipitation of the constituents from the super-saturated solution begins.

After a period of several days at room temperature, termed aging or room temperature precipitation, the alloy is considerably stronger. Many alloys approach a stable condition at room temperature. Still, some alloys, particularly those containing magnesium and silicon or magnesium and zinc, continue to age - harden for long periods of time at room temperature. By heating for a controlled time at slightly elevated temperatures, further strengthening is possible, and properties are stabilized. This process is called artificial aging or precipitation hardening. By the proper combination of solution heat treatment, quenching, cold working, and artificial aging, the highest strengths are obtained.

2014 / 2024

High strength alloys with excellent machinability widely used in aircraft. Have limited formability and only fair corrosion resistant in the heat-treated condition. Not recommended for fusion welding. Used for high-strength parts in aircraft and machinery, including gears and bolts, and for security vans where strength is critical. It tends to allow removing a greater volume of metal in a single operation for an equipment standard of finish than 6082. Durability, however, is lower and needs a protective coating. Machines better in solution treated condition than fully heat treated. Used for machined parts where these characteristics are critical.

6082 / 6061

Widely used structural alloys for light to medium strength applications. Combine good formability, weldability, brazed with fine finishing characteristics, and good corrosion resistance and strength after heat treatment. Since they lose appreciable strength when welded, the 5000 series alloys replace them in some marine applications. Availability of bars, shapes, tubing and pipe in the same alloy helps make this a popular selection. 6082 is the most common heat treated plate from stock. 6061 is similar to 6802 with better formability but more difficult to extrude and possessing lower strength. Used for couplings, hardware, hydraulic pistons, etc.

6063

Low to medium strength alloy suitable for more intricate sections requiring good corrosion resistance and high surface finish. Used in transport and all architectural applications where good anodizing characteristics are essential. The most common alloy for shape from stock.

7075

A very high strength aircraft alloy. Good machinability and hardness. Not for welding and corrosion resistant.

HOW ALLOYING ELEMENTS AFFECT ALUMINIUM?

 
 
Series    Main Alloy    Effect on Alloying Elements
 
  • 1000 ---   Non (99% Alum.)              Unalloyed aluminium is highly corrosion-resistant, low strength,                                                               workable, conductive, non-heat-treatable. 
     
  • 2000 ---   Copper Based                   Gives strength, hardness, machinability, heat-treatable
  • 3000 ---  Manganese Based              Adds moderate strength, good workability. Non-heat-treatable
  • 5000 ---  Magnesium Based              Moderate to high strength. Corrosion-resistant. Non-heat-treatable
  • 6000 ---  Magnesium & Silicon-Based  Increase strength, formability, corrosion-resistant. Heat-treatable
  • 7000 ---  Zinc Based                        For greatest strength. Heat-treatable.
 

Note: 1000, 3000, 5000, and 6000 alloys have good welding characteristics and corrosion-resistant. 2000 and 7000 alloys have higher strength and better machinability but lower weldability and corrosion-resistant.