US2146331A - Aluminum-chromium-titanium alloys - Google Patents

Description

Patented Feb. 7, 1939 UNITED STATES PATENT OFFICE armors signorto'lhoTitaninmAlloy Oompany,NeIYwk.N.Y..acorporaticnol No Drawing.

Original application February 18. 1987. Serial No. 128,386. Divided and this allplioation May 21. 1938, Serial No. 200.305

3 Claims. (01. 75-138) 5 stituent, and particularly to complex light alloys containing chromium, titanium and magnesium as well as zinc and aluminum, besides the usual traces oi iron, silicon, and other impurities found in ordinary high-grade aluminum.

Castings containing the correct proportions of zinc, chromium, titanium, and magnesium in relation to aluminum, simply aged at room temperature for four or flve days, are decidedly 5 stronger than other cast aluminum alloys, and have strengths comparable to those of the more expensive quenched and tempered aluminum alloy castings. The fact that no quenching treatment, or even heating, is required for developing high strength in my. improved alumi- 2 num-zinc alloys is an important advantage. This alloy is also readily machinable and highly resistant to corrosion.

Although the examples and test-results hereinafter described apply to castings, it is obvious that my improved aluminum-zinc alloy, which does not require heat-treatment to develop high strength, would also be highly useful in wrought form. Hence my invention. is not confined to castings alone.

Aluminum-zinc alloys have also been highly recommended for their excellent mechanical properties, but in order to obtain tensile strengths as high as 30,000 pounds per square inch in sand castings, the zinc content must be up to about 35 20%. Such zinc content has been found in practice to be too high for good casting properties, strength at high temperatures, or corrosion resistance, so that these plain aluminum-zinc alloys 40 have not been very much used in industrial practice. Substantially the same objections apply s when part of the zinc is replaced by copper, which, although improving the stiffness oryield point appreciably, impairs the ductility rather seriously.

Accordingto my invention, thezinc content is held'to about or less, and the hardening and strengthening eflects of chromium, titanium, and magnesium are utilized to obtain a superior com- 50 bination of yield point, ultimate tensile strength,

and ductility. These last-mentioned elements all improve the corrosion resistance also, and in addition the titanium is advantageous in refining the grain, while the magnesium promotes agehardening at room temperature.

The preferred composition for my new alloy is 5 about 8% zinc, 0.5% chromium, 0.15% titanium, 0.8% magnesium, less than 0.8% iron, and less than 0.3% silicon, copper or manganese, with balance substantially aluminum. w

Permissible limits in composition within which the properties do not vary seriously are about as follows:

Per cent Zinc 5 to10 Chromium .2to ,6 Titanium .lto .35 Magnesium .3to 2.5

Balance substantially aluminum.

The magnesium content shouldvary inversely go with the zinc, since when both are low, the yield point is low, and when both are high the ductility is low.

To illustrate this relation, the following tensile tests results are set forth, each value being the average 0! two tests (with very few exceptions, due to flaws), made oncastings not heattreated but merely aged about four days at room temperature.

Table 1 Percent added Lbs. per sq. in.

' Percent Heat No. m T u el'oingae ens e on Zn 0r Ti Mg point strength 15 0. 47 0. 15 0 17290 21300 2. 0 12. 5 4 l 0 17250 23200 4. 0 10. 49 25 0 11900 $700 11. 3 10 5 15 0 11250 24000 12. 0 10 5 15 2 19500 28800 5. 5 10 5 15 3 23400 32500 4. 7 l0 5 l5 6 27000 32600 2. 5 9 5 .15 4 24200 32700 5. 2 8. 5 i5 6 25000 38400 3. 8 8 5 15 8 27900 35300 4. 5 7. 5 5 15 50 22400 32100 5. 5 7. 5 .5 15 84 28100 34000 4. 5 7.5 .5 .15 1.0 25800 32200 2.5 45 7. 5 5 l5 1. 5 31500 34400 1. 5 7 5 15 1.2 27300 33200 2. 8 5 5 15 1. 0 21200, 30700 5. 5 5 5 15 2. 0 27300 33400 2. 7

Oomparing heat 7 in Table 1 with heats l6 and 50 17, it will be seen that as much as 0.6% magnesium gives a rather low elongation with 10% zinc,

but with zinc, 1 or 2% magnesium may be added without reducing the elongation to an equally low value. The yield point is seen to be quite low in heats 1 to 6 with low magnesium, or in heat 16 with low zinc; but taking heat 16 as a starting point, it was raised appreciably either by increasing the zinc as in heats 7 and 10, or by increasing the magnesium as in heat 17. The best combinations of yield point and strength with reasonable elongation were obtained in heats 9, and 12 with 7.5 to 8.5% zinc and 0.6 to 0.9% magnesium, the yield points being 25,000 or more, the tensile strength over 33,000, and the elongation at least 3.8%.

The best proportions of chromium and titanium in my improved alloys were determined by some with 3 and 4, shows that higher chromium content reduced both strength and ductility with only a slight improvement in yield point. Sample 23 is very similar to 7, showing that 0.25% chromium is almost as eflective as 0.5%; but sample 24, when compared with 9, shows that in the absence of chromium the tensile properties are inferior.

It is considered advisable in these alloys to keep the chromium content close to 0.5% so as to secure the maximum benefit in resistance to corrosion without impairment of physical properties.

The addition of silicon, copper, or cadmium to these alloys is detrimental to the tensile properties of castings as illustrated by the tests reported in Table 3. Silicon was added as calcium silicide, containing about silicon, except in one instance when 90% ferrosilicon was used with inferior results.

Table 3 Per cent added Lbs. per sq. in. P

Heat No. cent Yield Tensile Zn Cr Ti Mg 81 Cu Cd point munch tion 10 0. 6 0. l6 0. 6 0 0 0 3%00 2. 6 10 6 16 6 3 0 0 27110 30700 2. 2 10 6 16 6 3(Fe8i) 0 0 25100 279(1) 2. 2 7. 6 6 l6 1. 0 0 0 0 26800 32110 2. 6 7.6 6 16 1.0 6 0 0 21000 29110 3. 6 6 6 16 1. 0 0 0 0 21200 aomo 6. 6 6 6 16 1. 0 6 0 0 21800 27700 3. 6 12. 6 4 1 0 0 0 0 14100 21000 4. 6 12. 6 4 1 0 0 3 0 20200 11726 0. 7 8. 6 46 16 6 0 0 0 250m 33400 3. 8 a. 5 46 16 6 0 2 0 20600 28000 1. t 9. 6 16 4 0 0 0 24%)0 327(1) 6. 2 9 6 16 4 0 0 2 226111 30800 0. 2 7 6 16 4 0 0 2 moo 27K!) 12. 0 6 .6 I .16 .4 0 0 6 118w 231% 16.7

preliminary experiments which indicated that with more than about 0.6% chromium, the alloys were embrittled so that both strength and ductility were reduced. Likewise I found that no particular advantage was obtained with over 0.15% titanium, although 0.1% was barely sumcient. These relations are illustrated by the following results:

. Table 2 Per cent added Lbs. per sq. in. 7

Per cent Heat No. md T i] elongaans 0 1011 Zn or T1 Mg point strength 10 0. 44 0 0 9480 21076. 11. 0 10 6 1 0 10450 21300 10. 0 10 6 16 0 11250 24000 12. 0 10 6 2 0 10300 M00 10. 8 10 49 as 0 11900 23700 11. 3 10 71 19 0 12400 18650 4. 2 10 6 0 66 26000 28600 1.0 10 6 l6 6 27000 32800 2. 6 10 26 16 66 $300 31700 2. 6 8. 6 46 16 6 26000 33400 3. 8 8. 6 0 16 6 24900 31000 3. 0 8. 6 0 ,6 moo 27900 2. 8 8 46 l6 76 24600 31300 4. 0 8 6 76 moo 29900 4; 5

The effect of variations in titanium content is shown by the first five samples listed in Table 2, in the comparison between samples 22 and 7, and by the last four samples listed. The addition of about 0.15% titanium produced a decided improvement in strength and yield point, and the grain size of the castings without titanium could be observed plainly to be coarser than the grain size of those containing titanium.

The effect of variations in chromium is also shown in Table '2. Sample 21, when compared The first seven tests listed in Table 3 show that silicon decreases the strength of these alloys, with small irregular eifects on the yield point and elongation. The addition of iron as in heat 29, which showed 1% iron on analysis, had a further detrimental effect. Heat 33 as compared with 32, and heat 34 as compared with 9, show that copper seriously decreased both strength and elongation, but with an increase in yield point. Cadmium as shown by the last four tests listed in this Table 3, whether substituted for aluminum or for zinc in the alloys, decreased the strength and yield point with an improvement in ductility.

It is evident therefore that a good grade of aluminum, low in silicon and copper, should be used for making my improved high-strength aluminum casting alloy. Iron up to 0.8% does not seem to be detrimental. The aluminum for these experiments was of at least 99% purity, and was melted in clay-bonded graphite crucibles in a gas-fired furnace. When the melt had reached a temperature of 1425" F., the chromium and titanium were added, either in the form of separate aluminum alloys containing respectively 6 to 7 chromium, and 6 to 7% titanium, or in the form of a single master alloy containing about 2.9% titanium and 8.5% chromium.

This last mentioned master alloy may be made by adding gradually to molten aluminum superheated to 2250 F. a mixture of green chromic oxide, pure white titanium oxide, and cryolite in about the following proportions: For 9 lbs. aluminum, I used lb. T102, 1 lbs. CrzOs and 2 lbs. cryolite. Each small addition of the mixed oxides and flux should be thoroughly stirred into the aluminum before the next addition is made. When all the charge is completed, an extra stirring should be given and after ten minutes the alloy may be poured into ingot molds and cut up into convenient pieces for use. This alloy may also be made by adding. the oxides, one-after the other, instead of both mixed together. Alloys made by these methods have contained 2 to 4% titanium and 8 to 10% chromium. After the titanium and chromium are added to the alumimun at about 1425 I". in the production of my new high strength alloy. and are fully dissolved. the zinc is added as the pure metal, stirred well, and finally the magnesium is added in the same way.

The molten alloy should be fluxed with a little zinc chloride, stirred,'skimmed, and allowed to cool to a suitable temperature for pouring the is castings desired. For cast-to-size tensile test bars in dry sand molds, the pouring temperature was about 1300 I".

This new aluminum-zinc alloy can be remelted repeatedly without serious deterioration, provided its composition is maintained substantially constant. With careful melting practice, small additions of magnesium should be the only adjustment required. Sodium has also been used as a preventive oi oxidation losses on remeltlng, but magnesium I have found to be preferable,

Some results obtained on remelting are presented below:

Table 4 Table m Percent added Lbs. per sq. in 5,? $23 Yibld T658116 tion 0 5; n 1 point strength.

3 0. 5 0. 0. 3 23400 W 4. 7 16 10 5 15 3 29000 348) 3. 0 3 10 5 15 5 27000 32000 2. 5 15 10 5 .15 6 31500 3521 2. 3 1 8 5 15 8 w 2981!) 4. 0 4 8 5 15 8 27900 35300 4. 5 15 8 5 15 8 am 37000 4. 0 1 -7.5 .5 .15 1.5 27000 3161]] 2.0 5 7.5 .5 15 1.5 31500 34400 1.5 15 7.5 .5 15 1.5 35500 35100 1.5 1 5 5 15 1.0 168]] 282]) 7. 5 5 5 5 15 1. 0 21200 @700 5. 5 16 5 5 15 1. 0 25100 32500 4. 5 1 5 5 15 2. 0 24400 29400 3. 5 5 5 5 15 2. 0 27300 33400 2. 7 15 5 5 15 2. 0 31200 35700 2. 0

It is evident. from Table 5 that yield points above 30,000 lbs. per sq. in. with tensile strengths over 35,000 are obtainable in these cast alloys after ageing two weeks at room temperature, with as much elongation as is shown by the common aluminum casting alloys of about 22,000 lbs. per sq. in; tensile strength. It more than 3% elongation is required after ageing, the zinc con- Original alloy charge $32 Lbs. per sq. in.

Times Percent Heat melted Yi m T us elongation e e a Zn Or Ti Mg Si Na Mg point strength 1 10 0. 5 0. 15 0. 6 0. 3 27200 30700 2. 2 3 10 5 15 6 3 0. 1 0 28800 33300 3. 5 1 15 5 15 45 0 25200 32500 3. 5 3 10 5 15 45 0 0 0 24400 35000 5. 0 3 1G 5 15 45 0 3 0 24200 32700 5. 5 5 10 5 15 45 0 0 0 21000 M50 4. 5 6 10 .5 .15 .45 0 0 .5 $2) 33400 2.8 1 8. 5 5 15 6 0 26000 34000 4. 8 3 8. 5 5 15 5 0 0 0 22900 31400 5. 3 4 8. 5 5 15 5 0 0 0 21200 32000 6. 5 5 8.5 .5 15 6 O 0 .35 27500 33700 3.0

I might state that heat 39 as given in Table 4 is really an average or heats 6 and 7 (Table 1), both of which were remelted together to obtain heats 40 to 43 inclusive. Heat 44 is likewise an average from heats 8 and 10, which were similarly remelted together in equal proportions. A gradual drop in yield points on remelting without additions may'be noticed from the above results, but this was corrected on further remelting when magnesium is added.

All the test results presented in Tables 1, 2, 3 and 4 were obtained from tests made three to tent should not be as high as 10%, nor the magnesium over 1%, and the yield point then will not be quite up to 30,000 lbs. per sq. in.

The various alloys described above have been distinguished on the basis or the materials added in the melting crucible. Owing to melting losses or absorption of impurities, the compositions of the alloys tested may have been somewhat diflerent. To obtain an idea of the degree of these differences, I made some chemical annalyses of the test bars, and the results are shown in Table 6.

Table 6 0 Percent added Chemical analysis Heat No.

Zn Cr Ti Mg Zn Gr 'li Mg Fe Si Cu Mn 3 l0 0. 49 0. 26 0 44 0. 40 0. 248 5 15 0 150 9 45 15 6 8. 08 49 172 0. 65 0. 75 .5 .15 .8 7.94 .50 .165 .74 .6 0.1 0.07 0.03 45 15 75 7. 91 49 135 69 32-.- .4 l 0 .43 .238 43 (remelt) l0 5 15 .95 10. 21 49 165 62 l5 05 five days after the alloys were cast. Immediately after casting the strength is lower, and two weeks later it is higher, without much change in ductility. Examples of these changes on ageing I will now describe:

The iron contents were found to have been derived from the aluminum ingots used as a basis for these experimental alloys. This lot of ingot contained 0.49% iron. Although that amount of chromium, titanium, or magnesium would have an important eflect on the properties of these alloys, iron is much less powerful in its influence, and may be present in the alloys in any amount from the usual low value of about 0.3% up to 0.8% without making much diil'erence.

I claim as my invention:

1. A metallurgical alloy composed of about 2% to 5% titanium, about 7% to 12% chromium with the balance commercial aluminum having impurim ties not in excess of 1.0%.

2. A metallurgical alloy composed of about 2% to 4% titanium, about 8% to 10% chromium with the balance commercial aluminum having impurities not in excess of 1.0%.

3. A metallurgical alloy composed of about 2.9% titanium, about 8.5% chromium with the balance commercial aluminum having impurities not in excess of 1.0%.

GEORGE F. COMSTOCK.