US1657389A - Alfbed geobge coopeb gwyeb and henby wilfbed lewis phillips - Google Patents

Description

Jan. 24, 1928. 1,657,389

A. G. c. GWYER ET AL ALLOY Filed Jan. 22. 1924 Patented Jan. 24, 1928. v 1

UNITED STATES PATENT OFFICE.

ALFRED GEORGE COOPER GWYER AND HENRY WILFRED LEWIS PHILLIPS, OF LON- DON, ENGLAND, ASSIGNOBS TO THE BRITISH ALUMINUM COMPANY, LIMITED, OF

ALLOY.

Application filed January 22, 1924, Serial No. 687,852, and in Great Britain January 22, 1988.

This invention relates to aluminium alloys containing silicon to the extent of not less than about 3 per cent as distinct from the small proportions of silicon which exist frequently as an impurity in commercial aluminium. V

The object of the invention is to provide improved or alternative methods of and means for improving the physical structure and properties of alloys and especially of aluminium alloys.

In alloys containing aluminium and silicon, the silicon, if present to the extent of about 11 to 15 per cent, may exist in the 15 form of more or less large crystals, or it may be widely dispersed in the eutectic, while in alloys with less than about 11 per cent silicon the silicon is in either case all in the eutectic, though again its dispersion may vary between very wide limits, the degree of dispersion being, of course, greatest in the completely modified and least in the completely normal alloys. The greater dispersion is accompanied by improved physical properties in the alloy.

We have made a large number of experiments in connection with the-improvementof the structure of alloys and have found as the result of our investigations that the mechanism of the process known as modification is such that the phenomena concerned are not solely and exclusively characteristic of a particular series of alloys but the process of modification is capable of being applied and extended to a large number of or many alloy systems.

We have formulated a theory which we now put forward to assist in the understanding of our invention as we find it useful to that end. whether indeed it includes the true explanation of what happens or not.

According to this theory we assume that certain alloys when in the molten state and above a certain temperature consist of homogeneous liquids of which the constituent metals are in a state of molecular dispersion i. e., true solution), but that below a certain temperature which is probably not much above the freezing point the molecules become associated in groups of colloidal" particles. With ordinary rates of solidification the colloidal groups are allowed suflicient time to coalesce into aggregates of microscopic dimensions with the result that normal structures ensue, but with ultra-rapid rates of solidification the tendency is for the colloidal state to persist after solidification, in which case the resultant structures cease to be normal and approximate to modified.

Such an ultra-rapid rate of cooling may be produced, say by quenching from the molten state in iced brine. In actual practice it is not usually possible to chill in this manner or, generally speaking, in any other manner with a degree of rapidity sufiiciently great to obtain modified structures, especially in the case of sand castings, but recourse must be had to some suitable moditying agent and in our view the true function of such a modifying agent is that of a colloid protector and the process of modification consists simply in the more or less complete stabilization of the colloidal state.

Whether this be the true explanation or not is immaterial but it assists in understanding and in predicting.

Thus on our theory it follows that the best results will only be obtained in practice if the correct quantity of modifying agent, or colloid protector, be employed and experience has proved that this is the case.

This can be illustrated by adding dprogressively increased quantities of a mo ifyinc, agent to a previously normal alloy,

whence it is seen that the structure is improved up to a certain percentage addition but after that deterioration or reversion sets in.

We have also found by a series of hardness detern'linations according to the Brinell method that accompanying the structural change there is a marlzed improvement in the mechanical properties and these experiments also emphasize the desirability of employing the colloid protector in the correct amount.

What happens in the case of one series of alloys is not necessarily a guide to the behaviour of another series because to take a specific example, aluminium-iron alloys behave differently from aluminium-silicon alloys although. they might be expected to be substantially similar.

The invention consists in a process for physically varying the structure of aluminium-silicon alloys characterized by the cially calcium.

The invention also consists in the methods of modifying alloys hereinafter described.

, The invention further consists in 1mproved or modified alloys prepared according to the processes described herein.

Referring to the accompanying diagrammatic drawings Figure 1 represents the micro-structure of an aluminium-silicon alloy containing 87 per cent aluminium and 13 per cent silicon, chill cast and of normal type, etched with hydrofluoric acid and magnified to about the scale illustrated.

Figure 2 represents a similar alloy fiuxcd with the addition of 0.1 per cent of calcium and chill cast showing the modified structure thus obtained.

In carrying the invention into effect in one form by way of example, we modify an aluminiunrsilicon alloy containing 87 per cent aluminium and 13 per centsilicon by the addition thereto of 0.1 per cent of calcium at 750 C. The appearance of a typical normal and typical modified alloy of this character is illustrated in Figures 1 and 2'.

In Figure 1 the general background (1 represents the aluminium-silicon eutectic, the structure being coarse and therefore poor: In addition, large crystals or masses of silicon b are to be seen.

In Figure 2 showing the same mixture with the addition of a modifying agent the background 0 representing the eutectic has a much better structure than the eutectic a of Figure 1. In the original photo-micrograph the grain of the eutectic is much finer and more uniform than is shown in the drawings.

The whole of the free silicon of Figure 1 forms part of the eutectic in Figure 2 so .that the eutectic in that case is richer in silicon than that of Figure 1.. The dendritic structure d of Figure 2 is due to crystals or particles of aluminium.

Modification or structural change brings with it a marked improvement in the mechanical properties.

The results of some measurements of the Brinell hardness figure with a suitable steel ball and an appropriate pressure, the figures being calculated to standard Brinell figures, are given below The invention is not limited to treatment of the above alloys.

Further, the proportions given above are not limiting proportions.

The variation of quantities of materials applies not only to the proportion of the metals in the alloys but also to the proportion of modifying agents. 7

An experiment in any given case will readily show the best quantity to employ.

In place of metallic calcium other alkaline earth metals may be used, for example barium, and the alloy need not be raised in temperature much above'the melting point.

The alloys to be modified should not contain more than a small percentage of iron, and should not be held long in the molten condition, after the addition, and should preferably be cast in Cl1i11 .m0uldS.

In the case of metallic calcium our experiments have shown that on the laboratory or small scale 0.1 per cent added is sufiicient to effect complete modification. e prefer not to work with a substantially higher percentage because while we find that, for example, 0.2 per cent effect-s com.- plete modification there is the tendency to the formation of a new constituent which in our opinion is not an advantage but'in some cases may be detrimental. Under factory or large scale conditions somewhatmore may be employed than in the laboratory scale. These remarks apply particularly to the modification of alloys of nor mal constitution and different proportions may be used for alloys not of such constitution. In the case'of alloys which are already partially modified or partially rev verted, much. smaller percentages of the modifying agents may be employed and this'should preferably be done. 7

In the case of metallic calcium especially this may be added after wrapping in aluminium foil. We find such addition. eficctsthe desired modification with no visible signs or indications of a reaction occurringv and in this respect is therefore far superior to bodies. such as metallic sodium, which react with considerable violence and with evolution of flame and smoke.

Further, since either no slag is formed or at most very little, the crucibles can be employed many times without being scraped out between each treatment with the result that the actual metal loss is comparatively low. i In place of employing calcium wrapped in aluminium, an alloy may be employed comprising, for instance, 2 per cent calcium and 98 per cent aluminium.

Alternatively, any other alkaline earth metal may be employed in the formof an alloy.

The aluminium-silicon alloys may contain combined therewith one or morev other elements, for example copper or nickel, preferably with low iron content, as distinct from the alkaline earth metals introduced according to this application which are not in combination with the substance of the alloy itself, such as the aluminium-silicon, although they are contained in the mass of the metal in a suitable form which we have called colloid protection. It is in this sense that the claims of this application which deal with aluminium-silicon alloys containing an alkaline earth metal are to be read.

Having now described our invention, What We claim as new and desire to secure by Letters Patent is:

1. A method for physically varying the structure of 21llll1ll11lUl'I1-Slll00ll alloys which consists in the addition to the molten alloy of an alkaline earth metal.

2. A method for physically varying the structure of IlUIIIlIIlUIIISlllCOH alloys which consists in the addition to the molten alloy of metallic calcium.

A method for physically varying the structure of alloys containing aluminium, silicon and a third metal which consists in the addition to the molten alloy of an alkaline earth metal.

4. A method for physically varying the structure of alloys containing aluminium,

silicon and a third metal which consists in the addition to the molten alloy of calcium.

5. A method of varying the-structure of an alloy containing aluminium and silicon which consists in the introduction into the molten alloy at a temperature only slightly above the melting oint of the alloy of an alkaline earth metal? 6. A method of var-yin the structure of an alloy containing aluminium and silicon which consists in the introduction into the molten alloy at a temperature only slightly above the melting point of the alloy of calcium.

7. An alloy containing aluminium and silicon modified by the addition of an alkaline earth metal.

8. An alloy containin aluminium and silicon modified by the a dition of calcium.

9. An aluminium silicon alloy having about 3% to silicon and containing about 0.1% of an alkaline earth metal.

10. An aluminium silicon alloy having about 3% to 15% silicon and containing about 0.1% of calcium.

In testimony whereof we have signed our names to this specification.

ALFRED GEORGE COOPER GWYER. HENRY WILFRED LEWIS PHILLIPS.

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