Aluminium is a chemical element, which makes up about 8% of the Earth’s crust, making it the most abundant metal and the third most abundant element after oxygen and silicon. Aluminium is well known for having a low density (about 2.7 g/cm3) and, through the phenomenon of passivation, excellent corrosion resistance.
As pure aluminium is relatively soft, small amounts of alloying elements are added to produce a range of mechanical properties. The alloys are grouped according to the principal alloying elements. Specific commercial alloys have a four-digit designation according to the international specifications for wrought alloys or the ISO alpha-numeric system. Table 1 provides further details on the make up of these classifications.
The first digit of the series indicates the principal alloying element added to the aluminium alloy and is used to describe the series, i.e. 1000 series or 5000 series, etc. The second digit represents the modification of the specific alloy within the series; i.e. x1xx represents the first modification to the specified alloy, while x2xx represents the second modification. The third and fourth digits identify the alloy within a specific series. To summarise, alloy 2024, is in the 2000 series of alloys, has zero modifications and is specified alloy type 24.
There is, however, an exception to this numbering system and that is for 1000 series aluminium; the last two digits provide the minimum aluminium percentage above 99%. For example, 1050 means 99.50% minimum aluminium content.
Aluminium alloys will also include a temper designation, these define additional processing steps (if implemented). The temper designations are detailed in table 2. Further to the basic temper designations detailed in table 2, there are two sub-designations for ‘H’ – strain hardening, and ‘T’ – thermally treated. Table 3 and 4 describe these ‘H’ and ‘T’ designations, respectively.
Table 1 – Wrought aluminium alloy series
| Series |
Principle alloying element |
Tensile strength (Mpa)*1 |
Heat treatable |
Applications |
| 1xxx |
99% minimum aluminium (pure) |
70 - 185 |
X |
Corrosion resistance, piping, electrical conductivity |
| 2xxx |
Copper |
185 - 430 |
✔ |
Universal, aerospace, forgings |
| 3xxx |
Manganese |
110 - 280 |
X |
Pots and pans, heat exchangers, corrosion resistance |
| 4xxx |
Silicon |
170 - 380 |
X/✔ |
Filler wire (welding) |
| 5xxx |
Magnesium |
125 - 350 |
X |
Marine, automotive, pressure vessels, bridges, buildings |
| 6xxx |
Magnesium and Silicon |
125 - 400 |
✔ |
Extrusions, decorative, automotive, universal |
| 7xxx |
Zinc |
220 - 750 |
✔ |
Universal, aerospace, armour plate, competitive sports equipment |
*1 Dependent on composition and subsequent processing steps
Table 2 – Temper designations
| Temper designation |
Meaning |
| F |
As fabricated – Applies to products of a forming process in which no special control over thermal or strain hardening condition is employed |
| O |
Annealed – Applies to product that has been heated to produce the lowest strength condition to improve ductility |
| H |
Strain hardened – Applies to products that are strengthened through cold working. The strain hardening may be followed by supplementary thermal treatment, which produces some reduction in strength. Two or more digits always follow the ‘H’ |
| W |
Solution heat-treated – An unstable temper applicable only to alloys which age spontaneously at room temperature after solution heat treatment |
| T |
Thermally treated – To produce stable tempers other than F, O, or H. Applies to product that has been heat-treated, sometimes with supplementary strain hardening to produce a stable temper. One or more digits always follow the ‘T’ |
Table 3 – Sub-divisions of ‘H’ Temper designations
| H temper designation*2 |
Meaning |
| H1x |
Strain hardened |
| H2x |
Strain hardened and partially annealed |
| H3x |
Strain hardened and stabilised |
| H4x |
Strain hardened and lacquered or painted |
*2 The second ‘x’ digit indicates the degree of strain hardening: x2 – quarter hard, x4 – half hard, x6 – three-quarters hard, x8 – full hard, x9 – extra hard
Table 4 – Sub-divisions of ‘T’ Temper designations
| T temper designation*3 | Meaning |
|---|
| T1 |
Naturally aged after cooling from an elevated temperature shaping process |
|---|
| T2 |
Cold worked after cooling from an elevated temperature shaping process and then naturally aged |
|---|
| T3 |
Solution heat treated, cold worked and naturally aged |
|---|
| T4 |
Solution heat treated and naturally aged |
|---|
| T5 |
Artificially aged after cooling from an elevated temperature shaping process |
|---|
| T6 |
Solution heat treated and artificially aged |
|---|
| T7 |
Solution heat treated and stabilised (overaged) |
|---|
| T8 |
Solution heat treated, cold worked and artificially aged |
|---|
| T9 |
Solution heat treated, artificially aged and cold worked |
|---|
| T10 |
Cold worked after cooling from an elevated temperature shaping process and then artificially aged |
|---|
*3 Additional digits may be added to the ‘Tx’ designation and indicate stress relief. TX51 or TXX51 – stress relieved by stretching, and TX52 or TXX52 – stress relieved by compressing
Aluminium alloys are ubiquitous in transport applications because they provide engineering materials with good strength-to-weight ratios at reasonable cost. Further applications make use of the corrosion resistance and conductivity (both thermal and electrical) of some alloys. Although normally low strength, some of the more complex alloys can have mechanical properties equivalent to steels. Owing to the many benefits of aluminium alloys offered to industry, there is a need to identify best practices for joining them.
Aluminium alloys pose a range of difficulties when welding, including:
- High thermal conductivity. This results in excessive dissipation of heat, which can make welding difficult and/or result in unwanted distortion of the parts, owing to a larger heat input being required.
- Hydrogen solubility. Hydrogen is very soluble in molten aluminium, resulting in the weld pool absorbing hydrogen during processing. Once the molten material solidifies, the hydrogen bubbles become entrapped, creating porosity.
- Oxide layer. Aluminiums have an oxide layer (aluminium oxide), which has a much higher melting point (2060 C°) than the parent aluminium alloy (660 C°). When welding, this can result in the oxide layer being included in the weld region, potentially causing lack of fusion defects and reducing the strength of the weld. Consequently, the workpieces should be cleaned with a wire brush or chemical etching prior to welding to prevent oxide inclusion.
There are numerous processes that can be used for welding aluminium and its alloys, which are detailed below:
Arc Welding
Arc welding is commonly used for joining aluminium alloys. Most of the wrought grades in the 1xxx, 3xxx, 5xxx, 6xxx and medium strength 7xxx (e.g. 7020) series can be fusion welded with arc-based processes. The 5xxx series alloys, in particular, have excellent weldability. High strength alloys (e.g. 7010 and 7050) and most of the 2xxx series are not recommended for fusion welding because they are prone to liquation and solidification cracking.
- Can you weld aluminium with MIG? MIG welding can be successfully used to join aluminium alloys. The process is best suited for thinner gauges of material, such as aluminium sheet, because the amount of heat required is less when compared to thicker plates. Pure Argon is the preferred shielding gas for this process and the welding wire/rod used should be compositionally as similar as possible to the parts being welded
- Can you weld aluminium with TIG? TIG welding can also be used for joining aluminium alloys. Owing to the high thermal conductivity of bulk aluminium, the TIG process enables sufficient generation of heat to keep the weld region hot enough to create a weld pool. TIG welding can be used to join thick and thin sections. Similarly to MIG welding, pure argon is the preferred shielding gas and the welding wire/rod used should be compositionally similar to the parts being welded.
Laser Welding
Like other fusion based processes, including arc welding, laser beams can be used to weld many series of aluminium alloys. Laser welding is typically a faster welding process compared to other welding processes due to its high power density at the material’s surface. Keyhole laser welding is capable of producing high aspect ratio welds (narrow weld width: large weld depth), resulting in narrow heat-affected zones. Laser beam welding can be used with crack sensitive materials, such as the 6000 series of aluminum alloys when combined with an appropriate filler material such as 4032 or 4047 aluminum alloys. Sheilding gases used are selected dependent on the aluminium grade to be joined.
Electron Beam Welding
Similarly to laser welding, electron beams are good at producing fast welds and small weld pools. Electron beams are also better at producing welds in very thick sections of aluminium. Unlike other fusion-based processes, electron beam welding occurs in a vacuum, meaning that a shielding gas is not required, resulting in very pure welds.
Proper filler metal selection (filler wire or filler rod), carefully selected welding parameters and joint design are essential in order to minimise the risk of hot cracking in aluminium alloys when using fusion welding processes like arc, electron beam and laser welding.
Friction Welding
Friction welding is a solid-state joining process (i.e. no melting of the metal occurs), which is particularly suitable for joining aluminium alloys. Friction welding is capable of joining all series of aluminium alloys, including 2xxx and 7xxx, which are difficult with fusion-based processes. Moreover, owing to the nature of the solid-state process, the need for shielding gas is eliminated and superior mechanical performance of the weld region is obtained when compared to fusion welding processes. There are several friction processing variants:
- Friction stir welding (FSW). FSW was developed at TWI Ltd in 1991. FSW works by using a non-consumable tool, which is rotated and plunged into the interface of two workpieces. The tool is then moved through the interface and the frictional heat causes the material to heat and soften. The rotating tool then mechanically mixes the softened material to produce a weld. The process is typically used for joining aluminium sheet/plate material
- Refill friction stir spot welding (RFSSW). RFSSW is a development of the FSW process and is used as a spot welding technique to replace rivets in aluminium sheet metal applications
- Linear friction welding (LFW). LFW works by oscillating one workpiece relative to another while under a large compressive force. The friction between the oscillating surfaces produces heat, causing the interface material to plasticise. The plasticised material is then expelled from the interface, causing the workpieces to shorten (burn-off) in the direction of the compressive force. During the burn-off the interface contaminants, such as oxides and foreign particles, which can affect the properties and possibly the service life of a weld, are expelled into the flash. Once free from contaminants, pure metal to metal contact occurs, resulting in a weld. The process is used for joining bulk aluminium components to produce near-net-shapes
- Rotary friction welding (RFW). RFW is similar to LFW with the exception that the bulk aluminium parts are cylindrical and rotated to generate frictional heat instead of linearly oscillated