US1860947A - Aluminum alloy casting and process of making the same - Google Patents

Description

Patented May 31, 1932 UNITED STATES PATENT oerce ALADAR PACZ, OF CLEVELAND, OHIO,- ASSIGNOR OF ONE-HALF TOALUMINUM COI- PANY OF AMERICA, 01 PITTSBURGH, PENNSYLVANIA, A. CORPORATION OF PENN- SYLVANIA, AND ONE-HALF TO METALLGESELLSCHAFT AKTIENGESELLSCHAFT, OF

FRANKFORT-ON-TEE-MAIN, GERMANY,

A CORPORATION OF GERMANY.

AI UMINUM ALLOY CASTING AND PROCESS OF MAKING THE SAME No Drawing. Application filed February 24,

1928, Serial No. 256,820, and in- Germany larch 22, 1927.

Renewed.- March 25, 1932.

This invention relates primarily to the production of heat-treated castings of aluminum-base alloy, and its chief object is to provide a more economical and more flexible process for producing such articles. A'further object is to provide a method of accelerating the heat-treatment of aluminum alloy castings, whereby the expected results of heat-treatment, and in some cases even better results, can be obtained in less time than by prior processes. In some cases useful results can be obtained at lower temperatures. Another object is to provide an improved article of the kind indicated, having high tensile strength and elongation and increased hardness. To these and other ends the in vention comprises the novel article and process hereinafter described.

In the making of heat-treated castings of aluminum-copper alloys as heretofore carried out to obtain maximum results it has been necessary to heat the castings for a prolonged period. For castings of small or medium dimensions the minimum time was about seven hours, and for pieces of'larger dimensions the time required in practice was often as long as twenty hours. These long periods necessarily added to the cost of manufacture. It is also known that to develop the best pro erties of the casting the temperature of the heat-treatment must be close to the melting point of the lowest-melting eutectic. This requirement demandsaccurate control of temperature, which is a matter of some difliculty in the case of long periods of heat-treatment, and need has therefore been felt for a simpler, more economical and more flexible method, which. can be carried out with ease and certainty, and this need my present invention is designed to meet.

The invention is based on the discovery that the presence of a suitable amount of titanium in aluminum-base casting alloys containing copper affects the susceptibility of the casting alloys to heat-treatment in a way not hitherto recognized, despite the fact that the art contained numerous examples of aluminum-copper-titanium alloys for making articles which were to be worked before heat-treatment. No practical advantage, however, resulted from such suggestions, and indeed, so far as I have been able to learn, the titanium was not used for the purpose of improving or in any way modifying the heattreating properties of the alloy but only for hours and in some cases enabling lower temperatures to be used, thus permitting wider variations of furnace temperatures without disadvantageous results. In fact as good results are possible with my invention in three hours heat-treatment with the best prior practice in a period more than twice that long. Moreover, the results obtained by theuse of the titanium in my invention is to a large extent independent of the pouring temperature and the thickness of the cast section, thus permitting the pouring temperature to be adjusted to suit casting conditions without loss of desirable physical properties. tor of great commercial importance.

According to my invention the ran 0 of titanium content is from 0.05 five undredths) of .0116 per cent to 0.5 one-half) approximately, but the preferred range is from about 0.1 per centto about 0.3 per cent; whereas the prior roposals for the use of titanium in alloys w 'ch were to be worked before heat-treatment have called for about 1 per cent of titanium or even more.

" gin preparing the alloy I have found it adof one per cent,

as could be obtained.

This latter phenomenon isa fee-- presenceof' an alkali metal fluoride. One

convenient method of doing this is to first prepare an aluminmn-titanium alloy by the introduction of titanium oxide along with alumina in the well-known method of pro- .atthee ducing aluminum by electrolytic reduction of alumina in a fused-bath of natural or artificial cryolite, so that titanium oxide is reduced also. If titanium bearing aluminais available, it can be advantageously used for this purpose. The desired copper content can be su plied later by the addition of a suitable a uminum-copper alloy containing sa from 20 to 50 per cent of copper. Anot er method of introducing titanium which I have found useful is to allow a double fluoride of titanium, such as sodium orpotassium-titanium fluoride. to react with molten aluminum or molten aluminum-copper alloy. This method is more expensive, but has the advantage that it can be carried out in any foundry without the use of the electrolytic cell. Titanium may also be supplied in the .form of a copper titanium alloy, such as the cupro-titanium of commerce, which contains about 5 to 10 per cent titanium. In the latter case the titanium is added without contact with molten fluoride. While a substantial effect is secured by the introduction of-titanium in this way, I have found in general that better results are secured when the titanium is alloyed with the aluminum or the aluminum-copper alloy through the medium of or in association with alkali metal fluorides. Why this should be so, I am unable at present to state, but nevertheless I have found that method distinctly advantageous. In practice it has been found convenient to prepare first an aluminum-titanium alloy containing about 0.25 per cent titanium, and

. then make up the final alloy by adding in the proper amount an aluminum-copper alloy rich in the latter metal. together with magnesium, manganese,'-or any other constituent that may be desired in, the finished casting.

The copper content of the alloy is preferably held between about 3 and 5.51per cent/- If a. greater proportion of copper-is em-- ployed, as for example to increase the hardness and raise the yield point of the casting, nse, perhaps. of ductility, it; will be dificu t if not impossible to dissolvelthe excess copper constituent (CuAl,) by heating; Even under these conditions, howeveigf the presence of titanium in suitably controlled amounts is efiective in improving the properties of thealloy and accelerating the heat treatment. Magnesium in amounts up to about 0.5 per cent can be added to the alloy with a consequent increase of strength find hardness but with lessened ductility.

The alloy may be cast in sand or other suitable molds and the subsequent heattreatment may be carried out by any known means, as for example a gas fired or an electrically heated furnace. As will appear hereinafter, one is in general permitted considerable variations in pouring temperatures instead 'of being held strictly to narrow limits.

In addition to the soluble elements, such as copper, magnesium, zinc, and silicon, which are added to aluminum alloys for the purpose of making them susceptible to improvement by heat-treatment, relatively insoluble elements have been added, such as manganese and chromium, for example. These elements form hard intermetallic compounds with the aluminum, which when disseminated throughout the aluminum matrix produce some further hardening of the alloy, although the apparently have no particular effect upon the susceptibility of the alloy to heat-treatment. Titanium, which is quite insoluble, has been previously used in aluminum alloys for its hardening effect, just as have the elements iron, manganese and chromium. In my invention, however, the function of the titanium is quite different from that of thehardening elements just referred to. The amount employed is small and its effect is not only different but is out of all proportions to the amount added. Its

presence apparently produces a solidi ficationstructure which makes the casting peculiarly susceptible to heat-treatment, perhaps by causing the copper and other soluble constituents to take a form in which they go into solid solution more readily; and as before stated, the effect is to a large extent independent of the temperature at which the alloy is poured and the thickness of the cast section.

As an example of the. invention, the following table shows the tensile strength and elongation of sand-cast half-inch cylindrical test bars of alloy having the composition: copper, 4.4 per cent; iron, 0.7 per cent; silicon. 0.7 per cent, and titanium 0.23 per cent. For comparison the table gives the tensile strength and elon ation of similar bars of the same alloy .wit out titanium. The bars were poured at a temperature of 1400 F. (760 0.), then heat-treated at 960 F. (515 C.) forthe indicated periods, quenched and aged at 212 F. (100 C.) for one-half our.

wan 0.23%tltlnlum Without titanium Toma Elonaatktm Tomm Elonntiqn lbsJaq dn 131$... Ilia/sq. m. m ''nih m 36,376 8.0 D, 140 5.5 34,8) 10.0 ,m 6.0 35, 10.5 31, 6.8 aaoao 10.2 new 1.8

' It is seen from the above that substantially maximum properties of the test bars containing 0.23 per cent of titanium were developed after only three hoursi; high temperature treatment, whereas without the titanium the properties were less satisfactory, even after eating for thirty hours.

. tures of about 212 F. (100 C. to 300 F..

. In the case of bars of similar composition but cast in 1% inch sections instead of inch sections, the superiority of the bars containing titanium was especially marked. After heating for three hours, the bar with 0.2 per cent added titanium was about 4120 pounds stronger per square inch, and after thirty hours heating (followed by quenching and aging as before) was still 3300 pounds stronger than the bar without titanium, though in neither case was the tensile strength per unit cross section as high as in the inch sections. Examination of the polished cross-section of these 1% inch bars showed marked differences in structure between those with and those without added titaniunL, The former were sound and substantially free from the minute cavities expected in thick sand-cast sections of the alloy, while such cavities were present in large numbers in the bar poured without titanium. Even with only four hours heating.

the titanium-containing bars .were as nearly free from undissolved intergranular CuAl as were the others after much longer treatment.

Even when poured at 1250 F. (675 (1.), the alloy with added titanium shows its superiority, especially in the thicker section, as for example, the 1% inch bars just cited. In this case bars with added titanium were 4,000 pounds stronger per square inch after four hours heating, quenching and a ing, and

were still 2,000 pounds stronger a ter thirty.

hours heat treatment.

The presence of the titanium makes it possible in some cases to obtain useful improvement of physical properties with lower temperatures in heat treatment than heretofore, for example 925 F. (495 C.). It is generally preferable, however, to heat-treat at hi her temperatures, say from 950 F. (510 C. to 970 F. (520 (1), but for only a short period as compared with the time required for castings of the same aluminum-copper alloy without titanium. After heating and quenching or other rapid cooling, the casting may be allowed to age naturally, or it may be aged artificially by heating at tempera- 150 C.) in the usual way.

Although the advantages of my invention are apparent even when the iron content of the alloy is relatively high, as evidenced by the results exhibited in the table, still more improvement in the physical properties of the casting can be obtained if the iron content is lessened. For instance, sand-cast test bars A; inch in diameter were cast at 1400 F. (760 C.) from an alloy of the following composition: copper, 5.1 per cent; iron, 0.15 per cent; silicon, 0.15 per cent, and titanium 0.22 per cent. Similar bars were cast from an alloy of the same composition but without the titanium. The latter bars showed a tensile strength of only 28,300 pounds per square inch and an elongation of only 2.8 percent in 2 inches, after heating for twenty hours at a temperature of 960 F. (515 0.), quenching, and aging for two days at room temperature. 011 the other hand, the titanium-containing bars showed a tensile strength of 36,820 pounds per square inch and an elongation of 5.5 per cent in 2 inches after heating for only five hours, quenching and aging, and a tensile strength of 42,200 pounds per square inch and elongation of 8.2 per cent after twenty hours heating, quenching and aging.

Although my invention is particularly directed to the production of aluminum-copper alloy castings and their heat treatment, nevertheless it is not necessarily limited thereby. The alloy can be'rolled, forged or otherwise worked as circumstances may require.

I claim 1. Process of producing articles of aluminum-copper alloy, comprising incorporating in the molten alloy at an appropriate.

stage of its production, titanium in amounts sufiicient to accelerate subsequent heat treatment, casting the alloy, and heat-treating the casting under conditions sub-normal. for a similar casting of a similar alloy devoid of titanium.

2. In a process of producing'aluminumcopper-titanium alloy castings containing between about 3 and 5.5 per cent copper and 0.05 to 0.5 per cent titanium, the steps comproduced by incorporating titanium in the molten alloy at an appropriate stage of its production, casting the alloy and heat treating the casting under conditions subnormal for a similar casting of a similar alloy devoid of titanium. Y

1'11 testimony whereof I hereto afiix my signature.

ALADAR PACZ.