Methods for Aluminum Casting
Aluminum is amongst those metals which can be ‘cast’ by every process used in metal casting. These processes, in descending order of quantity of aluminium casting are: die casting, permanent mold casting, sand casting, plaster casting, investment casting, and continuous casting. The casting process is selected on the basis of factors such as cost, feasibility, quality, etc.
Feasibility is not a problem, as all the above methods are quite practicable. However, the most suitable casting method can be decided according to the design features or dimensions. For instance-Large products are made using sand casting. The quality factor is also important in selecting the casting process. Quality refers to both, mechanical properties (ductility and strength) and soundness (surface imperfections, cracking, and porosity freedom).
The methods most commonly used can be described as follows:
Aluminium Die Casting: As per the statistics, around $2.5m worth of die-castings of aluminum alloys are produced in the US alone every year. The process of pressure die casting utilizes almost two times the tonnage of aluminum alloys as the combination of other casting processes. Die casting is best suited for large quantity production of relatively tiny parts. Aluminum die castings upto 50 Kg of weight can be produced if casting-machine costs and high tooling are justified.
Some common applications of die cast aluminum alloys are inclusive of alloy 380.0 for Lawnmower housings, Alloy A380.0 for streetlamps housings, dental equipment, typewriter frames, Alloy 360.0 for frying skillets, instrument cases, cover plates, parts needing corrosion resistance, Alloy 413.0 for outboard parts of motor like connecting rods, pistons, housing, and Alloy 518.1 for conveyor components, escalator parts, aircraft, marine hardware.
Die castings cannot be easily heated or welded due to entrapped gases. Efforts are being on the war footing to overcome this obstacle. The die castings of aluminum alloys are generally produced using aluminum -silicon-copper alloys. This alloy family gives an excellent combination of corrosion resistance, strength, and cost, along with respite from ‘hot shortness’ and high fluidity which are mandatory for easy casting. If one desires a better resistance to corrosion, he should make use of alloys having a lower copper content.
‘Permanent Mold’ casting: ‘Permanent mold’ casting is best suited for high-volume production. Their size is larger than ‘die castings’. These castings have a very low pouring rate. They are gravity-fed. Outstanding mechanical properties are exhibited by ‘permanent mold’ castings. There is a lot of scope for further improvement if they are given heat treatment.
Some of the most common alloys of ‘aluminum permanent mold casting’ include Alloy 366.0 for automotive pistons, Alloys 355.0, A357.0, C355.0 for impellers, timing gears, compressors, missile and aircraft components, Alloys A356.0, 356.0 for aircraft wheels, parts of machine tools, pump parts, valve bodies, marine hardware, and 296.0, 333.0, 319.0.
Sand casting: This type of casting involves formation of casting mold (with sand). It is inclusive of conservative sand casting & lost-foam casting. The first one involves forming a pattern of sand, pouring the molten metal into it and breaking it once the product is formed. Lost-foam pattern involves putting a dispensable pattern of polystyrene in the mold. The rest of the procedure is the same as conservative sand casting.