PART 1 : TYPES OF ALUMINUM AND THEIR CHARACTERISTICS
General
Aluminum used to be considered unweldable because of its refractory oxide layer (alumina). Later on, a flux (stripper) was used to remove the alumina, thus enabling gas welding of aluminum alloys. Through electronic stripping, it is now possible to weld aluminum using the GTAW process or even with coated electrodes. Aluminum can readily be welded these days, and it is easy to create a high-quality, good-looking bead using the SMAW, OFW, GTAW or GMAW processes.
Classification
There are a wide variety of aluminum alloys. They are usually classified according to the Aluminum Association Alloy Number for wrought alloys. The four-digit number indicates :
XXXX alloy series (see below) based on the major alloy element ;
XXXX if other than ”0”, denotes a variant of the basic alloy;
XXXX in the 1XXX series, indicates the purity of the aluminum; for series 2XXX through 8XXX, indicates a specific alloy within the series
Example Alloy series Major element
1100 1XXX Al-99% or more
2219 2XXX Copper (Cu)
3003 3XXX Manganese (Mn)
4043 4XXX Silicon (Si)
5356 5XXX Magnesium (Mg)
6061 6XXX Mg + Si
7005 7XXX Zinc (Zn)
8006 8XXX Other elements
The 1XXX, 3XXX, 4XXX, 5XXX series and some alloys of the 8XXX series, cannot be heat-treated; i.e., their mechanical properties cannot be significantly altered through heat treatment. However, the 2XXX, 6XXX, 7XXX series, and other alloys of the 8XXX, series are heat treatable. Their mechanical properties can be altered through heat treatment because they contain certain alloy elements that dissolve in the alloy at high temperatures and can then be naturally or artificially aged to improve their mechanical properties.
Series 1XXX alloys are often used for their good thermal and electrical conductivity. They are thus used in the manufacture of electrical overhead conductors (1350), heat exchangers and cookware (1050).
Series 2XXX includes the aluminum-copper alloys primarily found in aeronautics or public and military transportation because of their strong mechanical properties and high machinability (2017).
The aluminum-manganese alloys of the 3XXX series are valued for their corrosion resistance, weldability and formability. They can be found in the manufacture of the roof sheeting, storage tanks and cooking vessels (3003).
For engine parts, or engine blocks themselves, aluminum-silicon alloys of the 4XXX series are used since they are very easy to cast and stand up well under forging (4356).
Because of their excellent weldability, corrosion resistance and good mechanical properties, aluminum-magnesium alloys of the 5XXX series are used in the manufacture of dump truck bodies, bottle caps and scuba diving air tanks (5356).
The 6XXX series includes aluminum-magnesium-silicon alloys used for camping trailers, streetlight fixtures, and truck bodies, and certain marine applications (6061).
The aluminum-zinc alloys of the 7XXX series are used for special applications; 7020 is used in the first and second stages of the Ariane rockets, for example. These are the best all-around alloys. They are corrosion resistant, easy to cast and machine, and have good mechanical properties because of their zinc content. They are even used to make armor plate for some types of military aircraft (7075).
Metallurgical state designation
The metallurgical state designator is separated from the alloy designation by a dash. This designation (”Aluminum Association Temper Designation System) is used for wrought or cast aluminum and its alloys. It reflects the various treatment sequences used to obtain the desired metallurgical characteristics.
It is comprised of one letter indicating the basic state, and one or more numbers for cold worked or heat treated states. The following four basic states can be found :
-F, As fabricated. The physical state of these materials has not been controlled during the manufacturing process. There are no specific mechanical property requirements for such materials.
-O, Annealed or re-crystallized. Materials in which ductility and dimensional stability are more important than hardness and mechanical strength. These are the softest and weakest of the hot-worked alloys.
-H, Cold worked (hard drawn). Alloys in which mechanical properties have been improved by cold working, with or without heat treatment. The symbol ”H” is always followed by two or more numbers. The first number represents a well defined sequence of operations.
-H1X. Cold worked. Applied to products whose mechanical properties have been obtained solely through cold working. The second number defines the level of cold working : 1 is the lowest, 8 is the highest, and 9 is used for ultra hard materials.
-H2X. Cold worked and partially annealed. Products hardened to a high level through cold working followed by partial annealing to restore the desired properties. The residual cold working level is indicated by the second number, the same as for H1X.
-H3X. Cold worked and stabilized. Applies to alloys containing magnesium whose mechanical properties have been stabilized through low-temperature heat treatment. The residual cold working level is indicated in the same manner as the preceding types.
-T, Heat treated. Product that underwent various heat treatments, with or without hardening through cold working, to obtain a stable state other than ”F”, ”O” or ”H”. The symbol ”T” is followed by a number between 2 and 10 inclusively that indicated the thermo-mechanical treatment sequence to which the alloy has been subjected.
-T2 Annealed (cast product only)
-T3 Solution annealing followed by cold working
-T4 Solution annealing followed by natural aging to a state considered stable
-T5 Artificial aging following hot forming
-T6 Solution annealing followed by artificial aging
-T7 Solution annealing followed by overaging or stabilization
-T8 Solution annealing, cold working and artificial aging
-T9 Solution annealing, tempering and cold working
-T10 Artificial aging as in T5 followed by cold working
Physical properties of aluminum alloys
Heat capacity
The heat capacity of aluminum (the quantity of heat required to increase by 1°C one gram of aluminum) is twice as high as iron. That is why it takes more heat or energy to raise the temperature of aluminum than it does when welding steel.
Thermal expansion coefficient
This parameter is also about double that of steel. However, steel deforms to about the same extent as aluminum because aluminum’s melting point, 1220°F (660°C), is about half that of steel, 2800°F (1540°C).
Thermal conductivity
Aluminum conducts heat three to five times as much as steel. This is why heat dissipates very quickly in the areas adjacent to the weld. More heat must therefore be provided because of the heat capacity, but it must also be provided quickly to minimize heat losses on each side of the weld.
Solidification range
Some alloys have a high solidification range; as aluminum-copper alloys with a 140°F (60°C) solidification range. Within this range, the alloy is still liquid and has no mechanical resistance. This range must be passed through as quickly as possible to prevent burn through within the zone.
Next part : Weldability of aluminum alloys