Casting Alloys
Casting Alloys:
Aluminum and its alloys are used in a variety of cast and wrought form and conditions of heat treatment. Forgings, sections, extrusions, sheets, plate, strip, foils and wire are some of the examples of wrought form while castings are available as sand, pressure and gravity die-castings. e.g. Al-Si and Al-Mg alloys.
2. Wrought Aluminum Alloys:
To meet various requirements, aluminum is alloyed with copper, manganese, magnesium, zinc, nickel and silicon as major alloying elements. These alloying additions improve the properties of aluminum when added in desired percentages.
The AAA (Aluminium Association of America) has classified the wrought aluminum alloys according to a four-digit system. The classification is adopted by the International Alloy Development System (IADS) and by most of the countries in the world. Table 1-3[70] gives the basis of designation of wrought and cast aluminum alloys in the four-digit system. The first digit identifies the alloy type the second digit shows the specific alloy modification. The last two digits indicate the specific aluminum alloy or the purity level in case of pure aluminum.
3. Heat Treatable Aluminum Alloys:
Heat treating in its broadest sense, refers to any of the heating and cooling operations are performed for the purpose of changing the mechanical properties, the metallurgical structure, or the residual stress state of a metal product. When the term is applied to aluminum alloys, however, its use frequently is restricted to the specific operations employed to increase strength and hardness of the precipitation hardenable wrought and cast alloys. These usually are referred to as the "heat-treatable" alloys to distinguish them from those alloys in which no significant strengthening can be achieved by heating and cooling.
In addition the other requirements are possibility of retaining single phase supersaturated solid solution by quenching, and precipitation of coherent/partially coherent phase by decomposition of the super saturated solid solution. The examples of this group are:
1. Aluminum-copper systems with strengthening from CuAl2
2. Aluminum-copper-magnesium systems (magnesium intensifies Precipitation).
3. Aluminum-magnesium-silicon systems with strengthening from Mg2Si
4. Aluminum-zinc-magnesium systems with strengthening from MgZn2
4. Non-Heat treatable Aluminum alloys:
These alloys do not respond to heat treatment, because they consist of a homogeneous solid solution with or without non-coherent precipitate(s) and show low strength and high ductility. These alloys may be stress hardened. Commercially pure Aluminum (1100), Al-Mn (3003), Al-Mn-Mg, and Al-Si alloys are examples of this class these alloys are used as sheet, bar, plates, wire, extrusion and so on. They are readily bent, formed and welded and possess excellent resistance to corrosion.
Properties of alloy:
1. Thermal and electrical conductive of solid solutions are less those of the pure metals. According to Mathiessen’s rule when small quantities of an alloying element are added in solid solution to metal, increase in resistance does not depend upon the temperature for mixture of soluble phases; the thermal and electric resistance follows the law of mixture.
2. Density is increased by heavier metal in solid solution and is decreased by lighter metal. In case of interstitial solid solution, there is little effect on density by added element.
3. Specific heat and coefficient of thermal expansion are governed by law of mixture.
4. Melting point of metal is converted into a range by the addition of alloying element. The melting point range of an alloy can be heavier or lower than the melting point of metal. The greater is differences valences between metals of alloy the wider is the melting point.
5. Boiling point also converted into a range by the addition of alloying element.
Precautions necessary for making alloy:
When two metals which are to be mixed have got widely different melting points or when one of the element is easily oxidized or volatilized, then the minor element is first mixed is the major element in known excess quantity this mixture whose composition is known is then added in to the require quantity of the major element.
In preparing alloy is essential that oxidation be prevented are minimized as far as possible. This is done either by converting the metal with fine charcoal are by using electric induction furnaces for melting where furnaces condition can be accurately controlled. The least fusible metal is melted first and the more fusible added afterwards. Heaviest metal is added last of all in order to prevent its setting at the bottom. For casting of and alloy, suitable moulds are necessary. If moulds are made of metal surface should be coated with a mould wash for preventing the casting from sticking to the moulds surface, otherwise the smoothness of the surface of the cast will be spoiled. Mould wash should be such that it does not give off any gas metal when poured into another metal should be at very high temperature. Generally it is added at a temperature about 1000 C above the melting point of the alloy.
Aluminum and its alloys are used in a variety of cast and wrought form and conditions of heat treatment. Forgings, sections, extrusions, sheets, plate, strip, foils and wire are some of the examples of wrought form while castings are available as sand, pressure and gravity die-castings. e.g. Al-Si and Al-Mg alloys.
2. Wrought Aluminum Alloys:
To meet various requirements, aluminum is alloyed with copper, manganese, magnesium, zinc, nickel and silicon as major alloying elements. These alloying additions improve the properties of aluminum when added in desired percentages.
The AAA (Aluminium Association of America) has classified the wrought aluminum alloys according to a four-digit system. The classification is adopted by the International Alloy Development System (IADS) and by most of the countries in the world. Table 1-3[70] gives the basis of designation of wrought and cast aluminum alloys in the four-digit system. The first digit identifies the alloy type the second digit shows the specific alloy modification. The last two digits indicate the specific aluminum alloy or the purity level in case of pure aluminum.
3. Heat Treatable Aluminum Alloys:
Heat treating in its broadest sense, refers to any of the heating and cooling operations are performed for the purpose of changing the mechanical properties, the metallurgical structure, or the residual stress state of a metal product. When the term is applied to aluminum alloys, however, its use frequently is restricted to the specific operations employed to increase strength and hardness of the precipitation hardenable wrought and cast alloys. These usually are referred to as the "heat-treatable" alloys to distinguish them from those alloys in which no significant strengthening can be achieved by heating and cooling.
In addition the other requirements are possibility of retaining single phase supersaturated solid solution by quenching, and precipitation of coherent/partially coherent phase by decomposition of the super saturated solid solution. The examples of this group are:
1. Aluminum-copper systems with strengthening from CuAl2
2. Aluminum-copper-magnesium systems (magnesium intensifies Precipitation).
3. Aluminum-magnesium-silicon systems with strengthening from Mg2Si
4. Aluminum-zinc-magnesium systems with strengthening from MgZn2
4. Non-Heat treatable Aluminum alloys:
These alloys do not respond to heat treatment, because they consist of a homogeneous solid solution with or without non-coherent precipitate(s) and show low strength and high ductility. These alloys may be stress hardened. Commercially pure Aluminum (1100), Al-Mn (3003), Al-Mn-Mg, and Al-Si alloys are examples of this class these alloys are used as sheet, bar, plates, wire, extrusion and so on. They are readily bent, formed and welded and possess excellent resistance to corrosion.
Properties of alloy:
1. Thermal and electrical conductive of solid solutions are less those of the pure metals. According to Mathiessen’s rule when small quantities of an alloying element are added in solid solution to metal, increase in resistance does not depend upon the temperature for mixture of soluble phases; the thermal and electric resistance follows the law of mixture.
2. Density is increased by heavier metal in solid solution and is decreased by lighter metal. In case of interstitial solid solution, there is little effect on density by added element.
3. Specific heat and coefficient of thermal expansion are governed by law of mixture.
4. Melting point of metal is converted into a range by the addition of alloying element. The melting point range of an alloy can be heavier or lower than the melting point of metal. The greater is differences valences between metals of alloy the wider is the melting point.
5. Boiling point also converted into a range by the addition of alloying element.
Precautions necessary for making alloy:
When two metals which are to be mixed have got widely different melting points or when one of the element is easily oxidized or volatilized, then the minor element is first mixed is the major element in known excess quantity this mixture whose composition is known is then added in to the require quantity of the major element.
In preparing alloy is essential that oxidation be prevented are minimized as far as possible. This is done either by converting the metal with fine charcoal are by using electric induction furnaces for melting where furnaces condition can be accurately controlled. The least fusible metal is melted first and the more fusible added afterwards. Heaviest metal is added last of all in order to prevent its setting at the bottom. For casting of and alloy, suitable moulds are necessary. If moulds are made of metal surface should be coated with a mould wash for preventing the casting from sticking to the moulds surface, otherwise the smoothness of the surface of the cast will be spoiled. Mould wash should be such that it does not give off any gas metal when poured into another metal should be at very high temperature. Generally it is added at a temperature about 1000 C above the melting point of the alloy.
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