ALUMINUM

Aluminum is silver-white or gray color metal. It is non magnetic and can have any surface finish from dull to highly polished. Aluminum weighs 168.5 pounds per cubic foot as compared to 487 pounds per cubic foot for steel, and has a melting point of 1220F (660C) when pure.

There are several numerical systems used to identify aluminum alloys, such as federal specifications, military specifications, the American Society for Testing Materials (ASTM), and SAE specifications. The system most used by manufacturers is one adopted by the Aluminum Association in 1954. The first digit in the aluminum alloy series indicates the alloy type. The second digit, represented by an x in the table, indicates any modifications that were made to the original alloy. The last two digits indicate the numbers of similar aluminum alloys of an old marking system, except in the 1100 series, where the last two digits indicate the amount of pure aluminum above 99 percent contained in the metal.

Aluminum Association Identifications of Alloys

Number and alloying elements

1xxx None

2xxx Copper

3xxx Manganese

4xxx Silicon

5xxx Magnesium

6xxx Magnesium and silicon

7xxx Zinc

8xxx Other elements

9xxx Unused (not yet assigned)

EXAMPLES:

An aluminum alloy numbered 2024 is an aluminum-copper alloy, where the first 2 represents the alloy copper. The 0 represents modifications to the alloy, and 24 are numbers of a similar aluminum of an old marking system. An aluminum numbered 1130 contains no major alloy, and has .30 percent pure aluminum above 99 percent.

Aluminum and its alloys are produced as castings, extrusions or as wrought (cold worked) shapes such as sheets, bars, and tubing. Aluminum alloys are harder than pure aluminum and will scratch the softer (1100 series) aluminum. Pure aluminum and the 3000 and 5000 series alloys do not respond to heat treatment. But these alloys can be strengthen by cold working. Temper refers to the level of strengthen by cold working or heat treating or both. The temper designations made by a letter represent the process and a number indicating the amount. The code follows the four digit alloy series number:

-F as fabricated. No special control over strain hardening or temper designation is noted.

-O Annealed, recrystallized wrought products only. Softest temper .

-H Strain hardened, wrought products only.

Strength is increased by work hardening. This letter -H is always followed by two or more digits. The first digit, 1, 2, or 3, denotes the final degree of strain hardening:

-H1 Strain hardened only

-H2 Strain hardened and partially annealed

-H3 Strain hardened and stabilized

The second digit denotes higher strength tempers obtained by heat treatment:

2 = 1 / 4 hard

4 = 1 / 2 hard

6 = 3 / 4 hard

8 = full hard

EXAMPLE:

3003-H18 is an aluminum-Manganese alloy, strain hardened to a full hard temper.

Some aluminum alloys can be hardened to a great extent by a process called solution heat treatment and precipitation or aging. This process involves heating the aluminum and its alloying elements until it is a solid solution. The aluminum is then quenched in water and allowed to age or is artificially aged by heating slightly. The aging produces an internal strain that hardens and strengthens the aluminum. For these aluminum alloys the letter -T follows the four digit series number. Numbers 2 to 10 follow this letter to indicate the sequence of treatment.

-T2 Annealed (cast products only)

-T3 Solution heat treated and cold worked

-T4 Solution heat treated, but naturally aged

-T6 Solution heat treated and artificially aged

-T8 Solution heat treated, cold worked, and artificially aged

-T9 Solution heat treated, artificially aged, and cold worked

-T10 Artificially aged and then cold worked

EXAMPLE

2024- T6 Aluminum-copper alloy, solution heat treated and artificially aged.

Cast aluminum alloys generally have lower tensile strength than wrought alloys. Sand castings, permanent mold, and die casting alloys are of this group. They owe their mechanical properties to solution heat treatment and precipitation or to the addition of alloys. A classification system similar to that of wrought aluminum alloys is used. The cast aluminum 108-F has an ultimate tensile strength of 24,000 PSI in the as-fabricated condition and contains no alloy. The 220-T4 copper aluminum alloy has a tensile strength of 48,000 PSI.

Cast Aluminum Alloy Designations

Code Number Major Alloy Element

1xx.x None, 99 percent aluminum

2xx.x Copper

3xx.x Silicon with Cu and/or Mg

4xx.x Silicon

5xx.x Magnesium

6xx.x Zinc

7xx.x Tin

8xx.x Unused series

9xx..x Other major alloys

Uses of Aluminum and Its Alloys

1100 Series is the most corrosion resistant of all the aluminum alloy families. It is very soft and ductile. It is used in the chemical and marine applications where corrosion resistance is of prime importance. The pure aluminum readily oxidizes forming a protective coating a few hundred atoms thick. This aluminum oxide layer forms almost instantly and is very resistant against further chemical attack. Also used is for welding rod and wire. Excellent weldability.

2000 Series is a aluminum-copper alloy commonly referred to "aircraft grade aluminum". It was developed for the aircraft industry in the 1930's for high strength and fatigue resistance. 2024 still in common use in airplanes made today. These alloys are heat treatable and very strong. Its strength is greater than most common steels. On the negative side it's corrosion resistance is only fair. The 2000 series alloys are available in a form called "Alclad". A very thin coating of pure aluminum is applied to the surface for corrosion resistance. Most of the external surfaces of aircraft use Alclad sheet. The 2000 series alloys are a poor choice for use around salt water. The copper and aluminum with the chloride in salt water setup a process called electrolysis. This causes pitting in the surface. Welding 2000 series requires special procedures to prevent high stresses and cracking. This alloy has good formability in the annealed condition.

3000 Series is aluminum-manganese alloy widely used in sheet products. This non-heat treatable alloy has good corrosion resistance and moderate strength when it is cold worked. It used in the transportation industry for trucks and marine applications. Tread plate (diamond plate) is made from this alloy. Good formability, Good weldability.

4000 Rarely used in structural applications. The silicon-aluminum alloy's main use is for welding rod and wire.

5000 This aluminum-magnesium has good corrosion resistance and moderate strength. Mostly manufactured in sheet form, non-heat treatable and good weldability. Commonly used in marine applications. Special low copper alloys are made for marine use in salt water. Moderate weldability.

6000 "The kitchen sink". This alloy has something for everyone. Good strength, good corrosion resistance, moderate weldability, good machining properties and moderate formability in the softer tempers. Magnesium and silicon are major alloying elements. Used extensively in extrusions. This alloy responds to heat treatment, but to a lesser degree than 2000 or 7000 series. This trait improves weldability by minimizing the tendency to form cracks. 6061 has high strength while 6063 has a smooth shiny finish.

7000 The major alloying element is zinc. The alloy has fantastic strength. 7075 in the full T-6 temper has a very high tensile strength, greater than many heat treated steels. Used in structural members of aircraft and rockets. The ultimate in strength, but bad seems to go with the good. The corrosion resistance is moderate to poor. Very difficult to weld or cold form. Difficult to drill and machine in the hard tempers. Requires sharp cutting tools on rigid machines.