Aluminium is the choice metal for making light weight parts of vehicles, aerospace and transport industries. Casting of liquid aluminium alloys into metal moulds utilizing systems like gravity, low pressure and high pressure die casting is an economical way of making difficult shapes which need minimum machining. Australia's auto industry supports a large local die casting business, manufacturing parts that include cylinders, pistons and engine sumps etc.
Rising demand in world auto market for aluminium die cast parts is producing great opportunity and challenge for Australian business which wants to emerge as a global player. By setting partnerships between Nissan and Ford, CAST has developed and produced new and latest technology that has been benefiting our partner's products. In turn these skills have created IP that is poised on the verge of commercialization.
To increase production of high pressure die casting by lessening its time cycle by 30%. The cycle time has been reduced by more than 20% on certain parts at two industry partner plants. The project involves identifying places where cycle time may be reduced, and doing it practically to prove the findings of research. This made it essential to involve shop floor staff to implement the changes needed in systems. These trails are generally in variation with day to day production and due to true co-operation of shop floor staff it become possible to achieve targets.
The third year of project has seen the true spirit of co-operation between researches and industrial partners in which latest research results got through simulated trails have been done practically on shop floor with help and support of Ford and Nissan staff. The changes once tested during trials have been incorporated in production systems giving benefits of reduced costs by reducing time of each part manufactured. An example of reduced time achieved is at Nissan on a gearbox side cover made in twin cavity die has given successful results after many months; from an initial cycle time of 75 seconds down to 60 seconds. While research at Ford on a changer housing casting, has been successfully implemented by lessening cycle time from 90 seconds to 74 seconds.
In future the reduction of time cycle will be tried for other parts as well.
Automatic fault detection in aluminium die casting:
This involves developing a system to detect surface and sub surface defects.
A fully automatic fault detecting machine named CAST vision has been produced and a prototype process is in place for extended in-plant on-line trials. This is the third year of this project and it is giving good results. The result of algorithm which was designed and developed in 2nd year has been put to test now. By prototyping the CAST team had designed and developed a working system CAST vision. This can discriminate between good and defective parts.
The prototype system has capacity to detect blocked holes on any of holes on this complex casting. Offline processes have also been readied which will detect hot tears and cold shuts on Ford's structural sump casting.
Work at Nissan on their pump cover casting has led to a CAST vision type process for in-line fault detection. The process can take images and find certain types of defects on the surface part. This project has shown that advances in mechanical vision applied for finding faults of aluminium castings can be converted from project stage to a working prototype successfully. The next stage is take concepts from single stage to multistage processes capable to handle more complex shapes and surfaces. This result will become a strong contender for future commercialization.
Aluminium Pressure Die Casting
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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