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Cast vs. wrought aluminium: standards, properties and applications

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 Alloys

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.

Properties of Cast Aluminium

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%

1xx.x

99.00% to 99.99% aluminium

131 – 448

28 – 152

2xx.x

4% to 4.6% Copper

 

131 – 276

90 – 345

3xx.x

5% to 17% Silicon

 

117 – 172

66 – 172

4xx.x

5% to 12% Silicon

117 – 172

41 – 48

5xx.x

5% to 12% Magnesium

131 – 448

62 – 152

6xx.x

Not Used

7xx.x

6.2% to 7.5% Zinc

207 – 379

117 – 310

*Average values for alloy comparison only

Cast Alloy Production

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 crystalizes 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 Alloy Applications

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: 

  • Machine tools
  • Engine cylinder heads
  • Gearbox housings
  • Axle housings
  • Cast wheels
  • Window fittings
  • Farm equipment
  • Garden tools

Cast Alloy Advantages

Cast aluminium alloys offer various advantages over wrought alloys as indicated in the list below:

  • Lower price per kilogram when compared to wrought aluminium
  • Wide range of shapes can be achieved due to casting flexibility
  • Some specialised alloys are only available as castings due to their low ductility
  • Parts can be produced that require limited post-cast machining

Wrought Aluminium

Wrought aluminium has exceptional mechanical properties and can be formed into various standard and nonstandard shapes. 

Wrought Aluminium Properties

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%

1xxx

99.00% to 99.99% aluminium

82 – 166

28 – 152

2xxx

2.2% to 6.8% copper

 

186 – 467

76 – 345

3xxx

0.3% to 1.5% manganese

 

110 – 283

41 – 248

4xxx

3.6% to 13.5% silicon

0.1% to 4.7% copper

0.05% to 1.3% magnesium

 

172 – 414

45 - 180

5xxx

0.5% to 5.5% Magnesium

 

124 – 352

41 - 345

6xxx

0.2% to 1.8% silicon

0.35% to 1.5% magnesium

 

124 – 310.3

55.2 – 276

7xxx

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 Alloy Production

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 Alloy Applications

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: 

  • Extrusions
  • Electrical conductors & bus bars
  • Aircraft Airframes
  • Cooking utensils
  • Welding rods
  • Pressure vessels
  • Motorcycle frames
  • Sheeting

Wrought Alloy Advantages

Wrought aluminium alloys offer numerous advantages when compared to cast aluminium as indicated in the list below;

  • Excellent mechanical properties
  • Structural integrity i.e. no defects from casting
  • Better surface finishes
  • Ease of manufacture for example welding and machining
  • Ease of forming, for example, aluminium can be extruded into an almost endless range of cross sections that can be custom designed to suit a specific application

Gallery

Sources