Aluminium Casting Alloys

Posted By Tom Cropper on 09 May 2014

Posted in The Vintage Machinery Almanac

This article first appeared in Practical Engineering 1940 Vol1 No20. The article is therefore correct as of 1940 The article explores a range of issues pertaining to Materials Science and Manufacturing at the time.

In the selection of an alloy for any given purpose, two aspects must be considered. First, the particular group to be chosen. This will depend primarily upon the purpose for which the casting is to be used (and, to a lesser extent, upon the precise casting technique to be adopted), the size and complexity of the job, and, above all, the section or section variation of the work-piece will also exert pronounced influence on the final choice.

Secondly, the particular composition range within the group selected will also be largely determined by the above factors, to which may be added that of problems associated with machining and surface finishing, whether by .straightforward mechanical operations or by anodising. The number of casting alloys which may successfully be anodised for artistic effects is somewhat limited, although, for purely utilitarian ends where the colour or uniformity of the anodic film is of less importance, it may be conceded that the range of alloys suitable for treatment by the process is considerably' wider.

Alloys of Aluminium and Silicon

The use of silicon-aluminium alloys has shown a marked increase in recent years, and in many cases these alloys are superseding the copper-aluminium alloys. They possess excellent casting properties, including high fluidity (which makes possible the casting of large thin sections), they are free from hot-shortness, and take sharper impressions in the mould than any other aluminium base casting alloy.

Because of their tendency to form a dense skin, they are particularly suitable for castings which must be free from porosity, and pressure-tight. They possess, in addition, low thermal expansivity, low specific gravity, and reasonably good thermal and electrical conductivity. They are notably resistant to corrosion, and are consequently widely used for many marine and architectural purposes. The diminution in mechanical properties which takes place when heavy sections are cast is less in the case of the silicon-aluminium group than in the majority of other casting alloys.

The binary silicon-aluminium alloys, on the other hand, are not remarkable for inherent high strength or hardness, although their elongation and impact strength are superior to those of the binary copper-aluminium alloys. They may be slightly hardened by heat treatment, but the increase in hardness is rarely sufficient to justify the expense entailed. Some of them, particularly those of higher silicon content, acquire increased strength and ductility when subjected to the modification process, which refines the grain and tends to promote greater structural homogeneity.

More Complex Alloys

As a rule, however, when higher strength and hardness are demanded, more complex alloys, formed by adding small amounts of magnesium, copper, manganese or nickel, and sometimes-all of these elements to the simple silicon-bearing alloys, are employed. Such complex alloys are more responsive to heat treatment, which increases the hardness of the matrix and promotes more uniform and finer dispersion of the silicon. At the same time, the excellent foundry properties of the binary alloys are not impaired by these additions. The effects of the internal stresses sometimes produced in alloys heat-treated at high temperatures are not so important in the case of silicon aluminium alloys, and for this reason quenching may be carried out in cold water.

The most widely used binary alloy of aluminium and silicon is L33 (alloy 160), containing 11-13 per cent, of the latter element. In the modified form this alloy is known as Alpax. It is suitable for sand casting, gravity and die-casting and pressure die-casting, and because of its low specific weight and excellent corrosion-resisting qualities, finds application in a vast range of both structural and purely ornamental works.

In the unmodified form, however, sand castings, and to a lesser extent, gravity die-castings, in L33 tend to be unduly weak and brittle owing to their coarse structure. This disadvantage is entirely eliminated by the refining influence of the modification process. Because of the pronounced chilling and consequent grain refinement which occurs in pressure die-casting, modification is not always essential when the alloy is to be formed by this process.

Notes on Machining

The alloy possesses excellent machining properties, and yields a fine surface finish; nevertheless, the abrasive effect of the silicon particles is such that the best results can only be achieved by the use of diamond-tipped or hard-metal tools, or by grinding. L33 is suitable for treatment by the anodic oxidation process.

An alloy containing 5 per cent, of silicon (alloy 123) also finds extensive use on account of its excellent casting characteristics, being quite fluid at temperatures almost down to the melting point, a fact which makes it eminently suitable for work possessing thin section, and for which freedom from porosity is essential. It is, in general, not subjected to heat treatment.

Alloys of higher silicon content have somewhat greater fluidity, but that 5 per cent, of silicon feeds better, and, due to its long freezing range, is less liable to give trouble due to cold shuts. Its casting shrinkage is low, and it exhibits notable freedom from hot-shortness.

In tensile strength and proof stress it is somewhat inferior to the copper-aluminium alloys, but its impact strength is good and, in the form of sand castings, the alloy is sufficiently ductile to withstand, without fracture, a considerable degree of cold deformation by bending. By reason of its structure, which does not exhibit the heterogeneity characteristic of free-cutting alloys, the 5 per cent, silicon-aluminium alloy does not machine as well as the heat-treated silicon alloys or those of higher silicon content. The metal tends to drag, and swarf readily builds up on the tool. These disadvantages, however, may be overcome, or at least minimised, by employing tools of appropriate design, by maintaining cutting edges in a perfect condition, and by careful regulation of cutting speed and the use of appropriate cutting compounds. Again, diamond-tipped and hard-metal tools are to be recommended, as the silicon in the alloy tends to exert some abrasive effect.

Heavy-Section Castings

This alloy shows less deterioration in mechanical properties when casting heavy sections than do other. casting alloys, and the properties of specimens cut from castings need not differ very markedly from those given for separately cast test bars. The alloy anodises to an attractive light-grey shade, and is used for general automobile castings, rolling-stock fittings marine parts, chemical apparatus, and ornamental and architectural castings. The alloy containing 5 per cent, of silicon with .05 of magnesium and 1.2 per cent, of copper (alloy 125) is generally used in the heat-treated state, in order to obtain from it the best possible mechanical properties; variation in heat treatment will produce a wide range of such properties. It ranks as a high-strength alloy, as, by appropriate heat treatment, its 0.1 per cent, proof stress may be raised to 14.5 tons/sq. in. (22.8 kg.sq. mm.).

The excellent foundry characteristics of the binary silicon-aluminium alloys are not impaired by the added elements, and it is suitable for the production of castings in both sand and chill moulds. It has particularly good resistance to corrosion, as the low copper content is almost all in solid solution, and does not greatly affect the susceptibility of the alloy to chemical attack. Corrosive media, however, may cause some darkening of the surface, and for certain architectural applications, particularly for exterior work, this alloy may not be suitable.

It gives sound, dense castings, and in the heat-treated condition, possesses good machinability and may readily be tapped to give a strong lasting thread. Cast in permanent moulds, the tensile strength and elongation are about 10-15 per cent, higher than in the sand-cast form.

The alloy is outstanding among silicon-bearing alloys for its heat-resisting properties, retaining its strength well at temperatures up to 200 deg. C; partly for this  reason it is specially recommended for water-cooled cylinder heads. Other applications are cylinder blocks, water jackets, valve bodies, and other parts where intricate and pressure-tight castings are required.

Lo-Ex Alloy

The alloy containing 13 per cent, of silicon, together with small amounts- of copper, nickel and magnesium (alloy 162), is commonly known as Lo-Ex. It possesses the lowest thermal expansivity of any aluminium-base alloy used in the cast form for pistons, for which purpose it finds extensive application in the automobile industry. It is normally used in the heat-treated state, and exhibits excellent mechanical properties at elevated temperatures.

The casting properties of the alloy are good, whilst its excellent wearing qualities, satisfactory thermal conductivity, low specific weight and low coefficient of friction all commend it for use as a piston alloy. Whilst not so readily machined as the piston alloy containing 10 per cent, of copper and small controlled amounts of iron and magnesium (alloy 250), it yields an excellent surface finished with diamond-tipped or hard-metal tools. Amongst widely-used piston alloys, it is the only one which may satisfactorily be anodised without difficulty, the film obtained being, in the generally-accepted sense of the term, non-abrasive but extremely hard and wear-resisting.

(This article and the illustrations which accompany it are reproduced by courtesy of Aluminium Union, Ltd.)