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Metal Casting Technologies : June 2008
Back to the Aluminium casting alloys Prof. John Hermes D. Bautista, Technical Consultant, PMAI ADVANTAGES OF ALUMINIUM CASTINGS ertain engineering advantages are inherent in the use of aluminium alloys for castings. Light weight (per unit volume) is the one most commonly cited. Some of the numerous other desirable properties include the following: 1. A wide range of mechanical properties. Strength, hardness, and other properties may be greatly altered by alloying and/or heat-treatment. Properties of the strongest alloys can be favorably compared with those of the cast irons and lower-strength steels, especially if the weight factor is considered. Suitable strength for many engineering uses is thus available. C 2. Architectural and decorative value. 3. Corrosion resistance. This property is, of course, relative; but resistance at atmospheric- and water- corrosion conditions makes possible the use of aluminium for building construction, outboard-motor parts, food-handling-equipment castings, etc. 4. Non-toxicity. The use of aluminium castings for cooking utensils and other food-handling equipment requires that no chemical reaction products toxic to humans be formed by the action of the food on the aluminium alloy. Recent developments indicate that this matter be more seriously examined for health purposes. 5. Electrical conductivity. Rotor bars in induction motors are cast of aluminium because of its desirable electrical conductivity. 6. Ease of machining. 7. Casting properties. Since aluminium has a relatively low melting point, about 650° C (1200° F), the problems of melting and pouring are greatly simplified when compared with steels and cast irons. Permanent metal molds may be used and die-casting is extensively practiced. Problems with furnace refractories and moulding sands are reduced because of the lower pouring temperatures. 8. Lower casting shipping costs per piece. LIMITATIONS Since the cost of aluminium alloys per unit weight is greater than that of cast irons and many steels, this fact is often considered a disadvantage. However, the cost per unit weight is misleading unless it is recognised that the volume per unit weight of aluminium is about 2.90 times that of the same weight of the ferrous alloys. Engineering limitations include the following: 1. Lack of resistance to abrasion and wear. 2. Absence of aluminium alloys which can develop the combination of high tensile strength, toughness, and hardness obtainable in the ferrous alloys; although recent developments seem to have overcome this. 3. Lack of resistance to severe corrosion to the degree offered by numerous copper- and nickel-base alloys and stainless steels. Obviously, the selection of aluminium as a casting material demands that its advantages outweigh its limitations in any particular application. As in the production of other castings, the basic processes of moulding, coring, melting, and cleaning are necessary. However, these processes must be modified in aluminium founding to suit the metallurgical properties that are characteristic of the alloy. ALUMINIUM CASTING ALLOYS Pure aluminium is a relatively poor casting material so that an aluminium casting is actually made of an aluminium alloy. Aluminium casting alloys are those alloys having properties peculiarly suited to casting purposes. Since there are over 100 aluminium-base casting alloys, it is evident that quite widely different properties may be obtained from the various alloys. For these alloys there are two types of properties that should be considered. There are the casting properties, those characteristics of the alloy which determine the ease or difficulty of producing acceptable castings. Then there are the engineering properties, which are those properties of interest to the designer or user of the castings. These two sets of properties can be used as a basis for studying the similarities and differences of the large number of aluminium casting alloys. METAL Casting Technologies June 2008 51