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The Zinc-Aluminium Die Casting Alloys
Tue 10 Jul 2007 - 15:39
The Zinc-Aluminium Die Casting Alloys
By [http://ezinearticles.com/?expert=Aman_Kumar]Aman Kumar
New high performing zinc-aluminium ZA casting alloys (ZA-8, ZA-12, ZA-27) give superior mechanical properties which designers can apply utilizing die casting technology. In general the ZA alloys are stronger, harder and offer more creep resistance than standard zinc alloys and can be used where bearing properties are important.
Aluminium alloys with 0.5-0.9% Fe content have largely replaced 1350 EC alloy for making electrical circuits because the latter continuously suffered from gradual loosening at terminals, which led to overheating. This problem has been totally removed in new conductor alloys without sacrifice of conductivity.
To get economic benefit of weight advantage of aluminium wire should be capable of attaching securely to standard fixtures without special handling techniques. But EC wire on binding screw terminals tightened to a standard torque may become loose, when the wire heats due to being overloaded. The wire gets expanded more than the Cu-alloy fixture and creeps to relax the added stress.
On getting cool it contracts to a smaller dimension, whereby the area of contact is reduced and it permits oxide to form at interface. On a subsequent current overflow, the overheating increases which leads to further loosening of wire. EC wire annealed for adequate bend ability gets sub structurally loosened at 200°C and ultimately fails due to repetitions of these cycles.
The new alloys (800 series) of 0.5-0.9% Fe have much better microstructural stability and creep resistance and, therefore, they are not prone to these failures.
While annealed to the same ductility or bend ability, the high Fe alloys are double strong. This capability has been established by practical field use of many years in USA, Europe and South Africa after these alloys were introduced in 1968.
Better and latest alloys which not only provide high integrity to terminations but are suitable for magnet wire after normal hot annealing have been made after adding a third alloy to improve its performance examples are 0.5% Fe with 0.5% Co and 0.5% Fe with 0.2-0.4% Si.
Processing and microstructure:
In continuous casting a bar of 50cm2 is made at 16 m/min on a 2.5m diameter copper wheel. The quick solidification results in a 20 μm dendrite arm spacing and eutectic red cpacing of about 0.2 μm with a supersaturation of about 0.1% Fe. These very fine particles play a significant role in giving stability to substructure while being incapable of nucleating crystallization.
The presence of sub grains has been known in hot worked aluminiums but without quantitative determinations of the dimensions or the effects on properties. As the temperature rises from 200-450°C, the cold yield strength of the hot worked product decreases greatly from the strengthening made by 97.5% cold rolling.
As has been seen in many hot worked metals, the yield strength is inversely proportional to sub grain diameter. Because the temperature is less and strain rate is high in a given pass than those in the previous one, substructure "inherited" from i.e., carried forward from, the latter is altered by dislocations to the existing walls to raise their density and by formation of new walls to subdivide the sub grains lessening their size.
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Article Source: http://EzineArticles.com/?expert=Aman_Kumar http://EzineArticles.com/?The-Zinc-Aluminium-Die-Casting-Alloys&id=633013
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