Aluminium Casting Alloys

Aluminum (Al) is found in large quantities in the earth’s crust. The third most common element only oxygen and silicon are present in a greater quantity than aluminum. From 2001 to 2011 the total world production of aluminum has increased by 83%.

 

 

World Production of Aluminium, 2001 vs 2011 (Australian Government, 2013)

 

Aluminum never occurs in free state for it is too reactive and forms compounds with oxygen, silicon, alkali and alkaline earth metals (Shakhashiri, 2008).  The main ore of aluminum is bauxite which is a mixture of hydrated aluminum oxide (Al₂O₃. x H₂O) and hydrated iron oxide (Fe₂O₃. x H₂O). The other ores of aluminum are  kaolinite, nepheline and alunite. With developments in the field of manufacturing technologies, the use of aluminum and its alloys has also increased.

 

Aluminum Alloys

 

Alloys in which aluminum is the predominant material are termed as aluminum alloys. The alloying elements that are typically used with aluminum are – copper, magnesium, manganese, silicon and zinc (Polmear, 1995) .  There are many advantages associated with aluminum alloys making them a preferred material over iron-steel products in electrical, construction, automotive, aviation, packaging, chemical and medicine industries (Arun P N, 2010). The properties of aluminum alloys that contribute for their popularity are:

• High corrosion resistance
• Excellent machining properties
• Light weight
• High thermal/electrical conductivity
• High ductility/easily deformable

 

Alloys can be primarily classified as casting alloys and wrought alloys (Benedyk, 2009) October 10)). Incorporating casting process for aluminum alloys is relatively a recent development. Two important incidents have been responsible for changing the way aluminum is used. The first is the discovery of the electrolytic process of reduction of aluminum oxide by Charles Martin Hall and Paul Heroult in France and second was development of alloys suitable for the foundry process  ( Heine et al, 1967). 

 

 

Hall-Héroult Electrolytic Cell (Udomphol, 2007)

 

Wrought Aluminum Alloys

 

The composition of wrought aluminum alloys is regulated by internationally agreed classification system (IADS). Classification of wrought aluminum alloys is listed in Table 1 given below.

 

4-digit series	Aluminum content or main alloying elements
1 xxx	99% minimum purity
2 xxx	Al – Cu alloys
3 xxx	Al – Mn alloys
4 xxx	Al – Si alloys
5 xxx	Al – Mg alloys
6 xxx	Al – Mg – Si alloys
7 xxx	Al – Zn – Mg alloys
8 xxx	Others

 

Table 1: Classification of Wrought Aluminum Alloys

 

Each registered alloy is described using a four digit number, a letter (indicating the basic treatment) and a further double digit number (1^st number represents secondary treatment used for influencing properties and 2^nd number represents residual properties). For example, 5152 H36 = Aluminum-magnesium alloy, cold worked and stabilized to develop a ¾ hard condition.

 

Cast Aluminium Alloys

 

The casting process offers multiple advantages for aluminum alloys. Aluminum is one of the most versatile of the foundary metals. Cast products consume nearly 20% of the metal produce. Some of the characteristics that make aluminum suitable for casting are: light weight, low melting temperature, insignificant solubility in all gases (except hydrogen) and fine surface finish. In addition mechanical properties like hardness, strength, architectural and decorative value, resistance to corrosion, electrical conductivity, non toxic make it a preferred material for casting.

 

Some of the disadvantages associated with aluminum casting are – high shrinkage (3.5% – 8.5%) during solidification (Heine et al, 1967) , high solubility in hydrogen in the molten state, toughness and hardness and inability to resist corrosion in severe conditions in comparison to stainless steel (Polmear, 1995). 

 

Classification of cast aluminum alloys is listed in Table 2 given below.

 

4-digit series	Aluminum content or main alloying elements
1xx.x	99% minimum purity
2xx.x	Al – Cu alloys
3xx.x	Si with added Cu and/or Mg
4xx.x	Al – Si alloys
5xx.x	Al – Mg alloys
7xx.x	Al – Zn alloys
8xx.x	Al – Sn alloys
9xx.x	Others
6xx.x	Unused series

 

Table 2: Classification of Cast Aluminum Alloys (Lee, 2003)

 

The most commonly used casting techniques for aluminum are:

• Sand casting

 

Sand Casting is a versatile process allowing formation of complex shapes and designs. The sand that can be used for this process can be silica sand, zirconia sand, olivine. What makes this an expensive method is that the mold is destroyed after every use. The shape and size of sand play an important role in determining the quality of the cast.

 

• Die casting

 

There are various processes that can be adopted in die casting. The various processes are – gravity casting, high pressure die casting (CAE, 2007), low pressure die casting, vacuum die casting and squeeze casting which can be direct or indirect.

 

 

Low Pressure Die Casting

 

 

High Pressure Die Casting

 

 

 

Squeeze Casting (Direct)

 

Squeeze Casting (Indirect) (Udomphol, 2007)

 

 

Most Commonly Used Aluminum Alloys and Additives

 

Discussing some of the most commonly used aluminum alloys produced using casting process are discussed below in detail:

 

• Aluminum‐silicon

This includes EN AC‐44 000 and EN AC‐47 000 alloys and the most important alloy in this category is AlSil2. Al-Sl alloys have good casting properties, resistance to corrosion and medium strength. These can be rewelded and are suitable for complex thin walled and pressure tight components. Lowering the silicon levels produces denser castings. 

 

• Aluminum-silicon-copper

This includes EN AC‐46 000 alloys. The copper in the alloy improves strength, cutability and hardness. With good casting properties alloys in this category for large scale production of castings that need to be cut. These alloys are not heat treated but can be under suitable conditions (Kelechukwu et al, 2012).

 

• Aluminum‐magnesium

This includes EN AC‐42 000 and EN AC‐43 000 alloys. Alloys falling in this category exhibit good resistance to corrosion in salt water making them suitable for use in armatures in the shipyard, pipes and other components to be used in chemical industry. They exhibit good polishing and anodized properties making them suitable for use in decorative purposes. 

 

• Alloy Additives

Some of the commonly used additives for achieving the desired qualities are listed in the table given below:

 

Alloy Additive	Advantages and Disadvantages
Silicon (Si)	·         Lowers the melting point; ·         Increases melt flow and fluidity; ·         Increases hardness and strength
Copper(Cu)	·         Increases hardness and strength; ·         Enables certain heat treatments; ·         Improves cutability; ·         Reduces resistance to corrosion
Magnesium (Mg)	·         Increases hardness and strength; ·         Improves resistance to corrosion; ·         Reduces melt flow; ·         Increases potential for oxidation
Iron (Fe)	·         Reduces occurrence of heat fissures; ·         Increases formation of harmful ·         intermetallic phases which leads to reduction in ductility; ·         Leads to nucleation of pores.
Manganese (Mn)	·         Counters the negative effects of iron
Zinc (Zn)	·         Increases hardness and strength; ·         Reduces resistance to corrosion
Nickel(Ni)	·         Increases hardness and strength

 

Casting Defects

 

While casting aluminum alloys the most commonly observed defects are (Turbalioglu & Sun, 2011)

• Formation of pores as a result of trapped gas, water vapor or burnt lubricant
• Shrinkage resulting in 4 – 5% reduction in volume
• Formation of oxide film when aluminic reacts with atmospheric oxygen
• Surface defects owing to coming together of two metals
• Laminations resulting from inability of two layers of metals to combine homogenously

 

Aluminum and Other Factors

Aluminum industry is present in Australia for over 50 years. It contributes billions of dollars towards the economy by way of income from exports (valued around $ 4 billion), wages, salaries and capital work. The industry is present in the form of bauxite mines (5), refineries (7), smelters (5), extrusion mills (12) and rolled product plants (2). 450,000 tonnes of aluminum is consumed within the country  (Australian Government, 2013).

Aluminum and Energy Consumption

Electrolysis is used for extraction of Aluminum from its ore (bauxite). An energy intensive process it requires nearly 15,000 kWh pertonne of energy. Resmelting is a process that consumes comparatively lesser energy.

 

Aluminium can result in savings in transport related energy demands. Aluminum when used for making cars and recyclable containers in place of heavier materials results in lowering of vehicle weight which further reduces CO₂ emissions. For every 25 kilogram reduction in weight carbon emission can be reduced by 3.0 grams.

 

Use of aluminum can also result in saving of energy when used in electric wiring in place of copper.

 

Aluminum and Health

 

As mentioned earlier Aluminum is one of the most commonly found elements in nature. The fact that it is non-toxic allows it to be used in packaging of food items. The belief that aluminum causes Alzheimer’s has not been substantiated.  Aluminum that is consumed by the human body is eliminated through urine within a period of 24 hours.

 

Aluminum can also be absorbed by inhalation. This can lead to aluminosis, a respiratory illness. Long term exposure to aluminum can build up in the skeleton. Eliminating aluminum from the skeleton is an extremely slow process.

 

Aluminum and Environment

The primary environmental issue associated with aluminum is production of sludge. Simply dumping of aluminum in garbage fills has led to some problems. The disposal scene has also gone through considerable changes. Awareness, regulation and effective monitoring has gone a long way in reducing unmanaged disposal of garbage.  

 

Concentration of aluminum in ground water is related to the pH level. At a level below 5.5 it rarely crosses 100 ml/l. As the pH level decreases the solubility of aluminum increases.

 

Another issue with aluminum is related to mining of Bauxite. It is usually mined in open pit mines.  As per statistics 25 km² of land is opened for bauxite mining all over the world annually. Efforts are on to reduce this area. Measures like mining in separate sections are adopted in order to establish  the eco systems once again on the mined area.

 

Recycling of Aluminum

Recycling is an important part of the aluminum industry. The industry has played an instrumental role in developing technologies for recycling of aluminum. Can collection centers have been established. The motivating factor behind recycling of aluminum is the fact that much less energy is consumed in recycling as compared to energy consumed for producing one ton of aluminum. Recycling saves as much as 95% of energy required for producing molten aluminum from bauxite. A tone of recycled aluminum also translates to saving 7 tons of bauxite. In Australia 10% of the total aluminum consumption is met by recyclable material (Aluminum and the Australian Economy, 2000).

 

Conclusion/Summary

 

The report focused on the properties of aluminum casting alloys and the various uses in the industry. Some of the commonly used aluminum casting alloys were discussed and the aspects of aluminum related to environment, health etc were also discussed. It can be finally stated that aluminum casting alloys continue to play an important role in everyday life and industry and are expected to occupy this position in the future as well.

 

References:

 

Australian Government (2013) The Australian Aluminium Industry. Retreived from http://www.innovation.gov.au/Industry/AustralianAluminiumIndustry/Documents/TheAustralianAluminiumIndustry.pdf

 

Benedyk, J.C. (2009, October 10). International Temper Designation Systems For Wrought Aluminum Alloys: Part I Strain Hardenable Aluminum Alloys. Light Metal Age. Retrieved from: http://www.lightmetalage.com/PDFs/LMA-2009-10-026TempersPartI.pdf

 

CAE DS (2007) Aluminium Alloys for High Pressure Die Casting, Retrieved from http://webhotel2.tut.fi/projects/caeds/tekstit/metals/metals_aluminum.pdf

 

Heine, R.W., Loper, C.R. & Rosenthal, P.C. (1967). Principles of Metal Casting. New York: Mc Graw Hill Publications.

 

Kelechukwu, O., Israel, O., Emeka, O. & Ihebrodike, M. (2012). Effects of heat treatment on the electrochemical corrosion behaviour of aluminum alloy AA3003 in an aqueous acid media. Journal of Chemistry and Materials Science Vol. 1(3) pp. 055-062. Retrieved from: http://garj.org/garjcms/pdf/2012/september/Okeoma%20et%20al.pdf

 

Lee, J.A. (2003) Cast Aluminum Alloy For High Temperature Applications. Retrieved from: http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20030106070_2003122249.pdf

Gurcan, A.B. & Baker, T.N. (1995). Wear Behaviour Of AA6061 Aluminium Alloy And Its Composites. Wear:Elsevier. 188, 185-191

 

Shakhashiri, B. Z. (2008). Chemical of the Week: Aluminium. SciFun.org. Retrieved from http://scifun.chem.wisc.edu/chemweek/PDF/Aluminum.pdf

 

Turbalioglu, K. & Sun, Y. (2011) The improvement of the mechanical properties of AA 6063 aluminum alloys produced by changing the continuous casting parameters. Scientific Research and Essays Vol. 6(13), pp. 2832-2840. doi: 10.5897/SRE11.471

 

Udomphol, T. (2007). Aluminium and its Alloys. Suranaree University Of Technology. Retrieved from: http://eng.sut.ac.th/metal/images/stories/pdf/02_Aluminium%20and%20aluminium%20alloy.pdf