The fundamental difference between cast and wrought aluminum is easy to understand: Cast aluminium is the aluminium that was melted in a furnace and poured into a mold. Wrought aluminium is when the metal is worked in the solid form with the help of specific tools. These two manufacturing processes will yield two materials with very different properties.
Aluminium has a wide range of uses across every major industry, however, it is often difficult to decide which grade is best suited to a specific application. The challenge becomes greater when comparing not only between alloys but also between cast and wrought aluminium. Both are fundamentally aluminium alloys which often have the same alloying elements but in different compositions and quantities. However, the applications and material properties vary widely between the two.
Cast aluminium contains larger percentages of alloying elements when compared to wrought aluminium. Cast aluminium also has a generally lower tensile strength than wrought aluminium due to the difficulty in eliminating casting defects.
Cast aluminium alloys make use of a four-digit numbering system and include a decimal point after the third digit. This system was developed and maintained by the Aluminium Association and is the most widely used naming convention for aluminium alloys. The aluminium Association works in accordance with ANSI regulations. The first digit will indicate the principal alloying elements and is thus the most important. For more detailed information on this numbering scheme follow the link.
Table 1 – Source: Cast Aluminium Properties
Grade |
Composition (wt%) |
*Tensile Strength (MPa) |
*Yield Strength (MPa) 0.2% |
99.00% to 99.99% aluminium |
131 – 448 |
28 – 152 |
|
4% to 4.6% Copper |
131 – 276 |
90 – 345 |
|
5% to 17% Silicon |
117 – 172 |
66 – 172 |
|
5% to 12% Silicon |
117 – 172 |
41 – 48 |
|
5% to 12% Magnesium |
131 – 448 |
62 – 152 |
|
6xx.x |
Not Used |
||
6.2% to 7.5% Zinc |
207 – 379 |
117 – 310 |
|
*Average values for alloy comparison only |
Cast aluminium alloy is produced from bauxite. This is a naturally occurring mineral containing 15-20 % aluminium and is the only ore still used for commercial aluminium extraction. The process to extract pure aluminium from bauxite is highly complex and energy-intensive.
The process consists of dissolving the bauxite into caustic soda at high temperatures, once dissolved the temperature of the mixture is lowered and the alumina crystallizes while the remaining elements either settle out or recrystallise separately. This is known as the Bayer process. The alumina is then broken down in an electrolytic cell whereby an electric current separates the aluminium from the oxygen in the presence of molten cryolite. The aluminium separates out to the bottom of the cell and is removed on a regular basis and sent to the cast house where impurities are separated out. The pure aluminium is cast into billets for further processing, these billets are then smelted along with the necessary alloying elements to produce the desired grade. This smelted alloy is then either cast into billet form or into the final product shape. The casting can be done via sand casting, die casting or investment casting.
Cast aluminium alloys are not often used for structural components due to their comparatively low tensile strength. This can be overcome by specialised processing techniques but, in general, cast alloys are used for the following applications:
Cast aluminium alloys offer various advantages compared to wrought alloys as indicated in the list below:
Wrought aluminium has exceptional mechanical properties and can be formed into various standard and nonstandard shapes.
A wrought aluminium alloy can be identified by a four-digit number. The first digit will indicate the principal alloying elements and is thus the most important. The second digit, unless it is a 0, indicates a modification of the alloy and the third and fourth digits are identification numbers for the specific alloy.
Table 2 – Source: Wrought Aluminium Properties
Grade |
Composition |
*Tensile Strength (MPa) |
*Yield Strength (MPa) 0.2% |
99.00% to 99.99% aluminium |
82 – 166 |
28 – 152 |
|
2.2% to 6.8% copper |
186 – 467 |
76 – 345 |
|
0.3% to 1.5% manganese |
110 – 283 |
41 – 248 |
|
3.6% to 13.5% silicon 0.1% to 4.7% copper 0.05% to 1.3% magnesium |
172 – 414 |
45 - 180 |
|
0.5% to 5.5% Magnesium |
124 – 352 |
41 - 345 |
|
0.2% to 1.8% silicon 0.35% to 1.5% magnesium |
124 – 310.3 |
55.2 – 276 |
|
0.8% to 8.2% zinc 0.1% to 3.4% magnesium 0.05% to 2.6% copper |
228 – 572 |
103 - 503 |
|
*Average values for alloy comparison only |
Wrought aluminium is produced by smelting pure aluminium ingots with the specific alloying elements required to make a given grade of aluminium. The smelted alloy is then cast into billets or large slabs. This material is then either rolled, forged or extruded into its final shape. In some cases, the alloys are heat treated to further enhance their properties.
Wrought aluminium tends to have better tensile strength when compared to cast alloys, as can be seen in the two tables above. Their typical uses are listed below:
Wrought aluminium alloys offer numerous advantages when compared to cast aluminium as indicated in the list below;