|
 |
Testimonials |
|
|
|
 |
|
| Home > Services > Aluminum Alloy | |
| Aluminum Alloy | |
Today there are many popular grades of primary and secondary alloys across the globe. The same can be streamlined by a few common grades to capture the majority of the requirement
Bagla group uses several aluminium alloys with different mechanical properties and chemical compositions for high pressure die casting parts. The success of the pouring operation depends partly upon certain qualities of the metal itself, for example its composition and temperature, which influences flow, and partly upon properties and design of the mould, including the nature of the moulding material and the gating technique used to introduce the metal into the mould cavity.
The first requirement of aluminium casting alloys is adequate mechanical strength for the intended use. Other important factors are machinability, fluidity (to ensure that within a millisecond the entire cavity with various counters/shapes is filled in without air entrapment, shrinkage which needs detailed thought about air vent position), ability to be surface treated, corrosion resistance, conductivity, leak tightness, appearance and cost. Selection of a particular alloy depends usually on the optimum compromise of all the factors and the intended application.
The production of good castings requires that the aluminium alloys possess favourable properties. Those considered of importance for aluminium alloys are :
| • |
Minimum solidification shrinkage |
| • |
Adequate Fluidity |
| • |
Minimum difficulty in producing pressure tight castings |
| • |
Minimum problem in drosing and gas absorption |
| • |
Freedom from hot tearing and cracks |
To achieve the above properties following are the principal alloying elements or impurities in the aluminium die cast alloys.
Silicon:
Silicon up to 12% increases fluidity and freedom from hot tearing, reduces external shrinkage, improves pressure tightness and improves dimensional stability. It is evident that silicon improves many of the casting properties of the aluminium alloys. Shrinkage problems are minimal in the aluminium alloys containing 8 to 13 % Silicon. This is the reason that general purpose casting alloys contain substantial amount of silicon with some percentage of copper and other elements.
Copper:
Copper within 2-3% increases tensile strength and hardness. It can be effective in reducing internal shrinkage and it improves machinability by increasing the hardness of alloy matrix. AlSiCu alloys, ones having 10-12% Silicon and 2- 3% copper have superior casting qualities.
Magnesium:
Magnesium in the AlSiCu alloys improve hardness and tensile properties with as little as 0.30% addition of magnesium and also improves machinability. Higher percentage of magnesium causes the extensive drossing.
Iron:
Iron is the natural level impurity in the aluminium alloys due to its association with the elements in Bauxite ore and the solvent action of aluminum in melting tools and accessories with which it comes in contact. Although considered an impurity in the alloy, it performs a useful function in die casting alloys. Its existence upto 1% increases strength and hardness of alloys and reduces the tendency to hot shortness. Iron content of 0.9 to 1.1 % is the most desirable level. Iron below 0.8% may cause welding or soldering of the alloy to the surface of the die.
Zinc:
When Zinc exceeds 1.5% in die casting alloy, shrinkage and hot shortness increases and decreases strength and hardness. In larger section thickness castings , this effect may be noticed.
Manganese:
Manganese increases strength and ductility of aluminium alloys in small amounts by combining with iron to change the micro-structural constitution from needle-like to plates. Manganese imparts some minor improvement to strength and corrosion resistance in aluminium die casting alloys.
Strontium, Sodium, Calcium:
Strontium, Sodium and Calcium are “modifier” alloys. The microstructure of AlSi casting alloys contains several phases, including a “silicon” phase. In slow solidification processes such as sand or permanent mould casting, the phase solidifies as needle-like, which is harmful to mechanical properties. Here these “modifier elements” may be added to change the shape of the silicon phase to a less detrimental “fibrous” structure.
The die cast process, however, achieves such fast solidification of the metal that the silicon phase does not have as sharp as needle-like structure. Consequently,addition of sodium, strontium, or calcium is usually not necessary.
However, strontium is being explored today as an additive in die cast alloys with some success, to achieve mechanical property improvement by dispersing micro-porosity.
Titanium, Boron:
Titanium and Boron additions are not usually necessary for die casting aluminium alloys but do give definite benefits for low-pressure and permanent mould casting. Titanium and Boron are grain refiners and they can improve strength and elongation and tend to reduce shrinkage.
As in the case with all alloying elements, the percentages present, and combinations thereof, have a considerable effect in all the properties mentioned.
Shrinkage problems are minimal in the aluminium alloys containing 8 to 13 % Silicon. Accordingly yield is at a maximum and difficulties with hot tearing and micro-porosity are minimised.
Silicon rich aluminium alloys favour fluidity and resistance to tearing. It is evident that Silicon improves many of the casting properties of the aluminium alloys. This is the reason that general purpose casting alloys contain substantial amount of silicon with smaller percentages of Copper and other elements.
The more oxidisable element, magnesium, causes extensive drosing in high magnesium content alloys. However, such alloys can be safely cast if good melting, pouring and casting practices are followed.
| | |
|