US2698230A

US2698230A - Magnesium base alloys containing zirconium

Info

Publication number: US2698230A
Application number: US210627A
Authority: US
Inventors: Doyle William Michael
Original assignee: Magnesium Elektron Ltd
Current assignee: Magnesium Elektron Ltd
Priority date: 1950-02-21
Filing date: 1951-02-12
Publication date: 1954-12-28
Anticipated expiration: 1971-12-28

Description

United States Patent MAGNESIUM BASE ALLOYS CONTAINING ZIRCONIUM William Michael Doyle, Slough, England, assignor, by mesne assignments, to Magnesium Elektron Limited, near Manchester, England, a company of England No Drawing. Application February 12, 1951, Serial No. 210,627

Claims priority, application Great Britain February 21, 1950 2 Claims. (Cl. 75--168) This invention relates to magnesium base alloys containing zirconium and has for one of its purposes to provide a method of making such alloys.

It is another object of this invention to provide means for facilitating or rendering possible the introduction of zirconium into magnesium base alloys in a particularly efiective manner.

It is a special object of this invention to provide an agent, being a fused mixture of a zirconium compound and an alkali compound, adapted to alloy zirconium with magnesium metal or with a magnesium-base alloy.

It is also an object of this invention to provide a new method of preparing such agent.

I am aware that it has been proposed to introduce zirconium into magnesium base alloys by adding to the molten magnesium under stirring such alloying salts as zirconium tetrachloride, or fused mixtures of zirconium tetrachloride and potassium and sodium chlorides. A disadvantage in either case is that serious losses of zirconium tetrachloride occur due to volatilisation. Another disadvantage of this way of proceeding is that fairly large amounts of magnesium chloride are formed, which, together with the alloying zirconium salts, being very fluid at the alloying temperatures, envelop the fine particles of the elemental zirconium forming by reduction so that the zirconium metal is not available to carry out the function of grain refining and alloying. Furthermore, separation of the alloying salt from the molten metal is very poor so that a very large percentage of the alloy has to be discarded. In general, castings made by these techniques possess inconsistent tensile properties, and they are prone to be commercially unsatisfactory due to finely disseminated magnesium chloride or alloying salt inclusions, which impair the corrosion resistance of the magnesium-zirconium alloy.

Other methods have been suggested involving the use of zirconium tetrafluoride, or double salts of zirconium tetrafiuoride and a potassium halide, but the zirconium tetrailuoride is more expensive and is less readily obtainable than zirconium tetrachloride. Also, the commercial grade of zirconium tetrafluoride is usually the monohydrate of this salt, or a complex addition compound containing the H20 radicle. It is very difficult to remove this hydroxyl radicle from the salt without oxidising the zirconium and, in any case, the procedure involved is very expensive.

As a result of considerable research, I have developed a technique whereby a compound or complex salt containing zirconium tetrachloride can be used to introduce the zirconium into the alloy while avoiding the aforementioned disadvantages.

I have found that an exothermic reaction occurs between zirconium tetrachloride and potassium fluoride, and that use can be made of this phenomenon provided precautions are taken to contain the momentary sublimation of the zirconium salt, which occurs just prior to the mixture becoming completely molten. I have also found that the addition of zirconium tetrachloride to potassium fluoride progressively lowers the melting point and that, when the weight ratio of ZrChzKF lies between approximately 0.75 :1 and 4:1, the melting points of the resulting mixtures lie between 500 and 400 0., approximately. I have further found that compound salts of such mixtures can be raised to a temperature of 650 C., without noticeable loss-of zirconium tetrachlo- 2,598,230 Patented Dec. 28, 1954 ride by volatilisation. In fact, volatflisation of the zirconium salt at this temperature is only noticeable when the ratio of ZrClrzKF is about 5:1 or greater.

Accordingly, my invention provides a compound salt comprising essentially a fused mixture of zirconium tetrachloride and potassium fluoride within the weight ratio of 0.75:1 and 4:1.

My invention further provides methods of making such a salt mixture and of making with the use of such a salt mixture magnesium base alloys containing zirconium.

I prefer to make the salt mixture or compound salt in the following manner: The potassium fluoride is first dried and a proportion of this salt and of the zirconium tetrachloride are intimately mixed in the bottom of a warm (less than C.) crucible in the ratio of approximately, two parts by weight of zirconium tetrachloride to one part by weight of potassium fluoride. This intimate mixture is used to trigger or initiate the reaction and the amount of the salts used for this purpose may be up to 25% of the whole. Then the remainder of part of the desired amounts of these salts are mixed or placed in the crucible in alternate thin layers, leaving for the final addition suflicient potassium fluoride to form a thin layer on the top. The potassium fluoride layer on the top of the mixture will automatically form a high-melting crust which will prevent excessive loss of zirconium tetrachloride during the reaction. The crucible should be fitted with a close-fitting, fairly heavy lid having a vent hole formed therein. Lientle heat is applied to the bottom of the crucible and, when the exothermic reaction occurs, the bulk of the salts decreases to about a quarter of the original volume, and forms a pool of molten mixed salts. The crust formed by the potassium fluoride on the top may then be broken and stirred in.

In making up the compound salt, according to the technique described above, and when the ratio in the final mixture of zirconium tetrachloride to potassium fluoride is to be near 4 to 1, it is desirable to ensure that in the body of the charge this ratio is not exceeded, wh ch normally would be the case after deducting the trigger portion as aforesaid. To avoid such excess, some of the zirconium tetrachloride may be withheld until the main mixture is molten. Similarly, when the ratio of zirconium tetrachloride to potassium fluoride is to be near the lower limit of 0.75 to 1, it is desirable to withhold some of the potassium fluoride from the main charge until this charge is molten, in order to ensure that the ratio in the body of the charge is near say 1.5 to 1. A mixture of two parts by weight of zirconium tetrachloride and one part by weight of potassium fluoride made up as described above will be molten at about 420 C.

For the purposes of description I shall refer to the fused mixture of only zirconium tetrachloride and potassium fluoride as the compound salt, such a salt containing from about 43 to 80% zirconium tetrachloride, the remainder being potassium fluoride, substantially as hereinafter described for the introduction of zirconium into magnesium and magnesium alloys which do not contain inhibiting elements that form high-melting compounds with zirconium. However, I refer to modify this mixture, in order to make it more suitable for the alloying of zirconium to magnesium alloys in the foundry, by making certain additions of other salts or compounds to IIIIPI'OVC its physical characteristics, and the resulting fused mixture will be called the complex salt.

The amount of zirconium tetrachloride contzfin'e'd in the fused complex salt must be adequate to ensure that this salt contains sufficient zirconium to function as a practical and economic source of this metal in the subsequent alloying operation in the foundry. I prefer the amount of zirconium tetrachloride to be between 24 and 60% by weight of the complex salt. With this object in view, when making up the complex salt, I prefer to maintain the weight ratio of ZrCltzKF in the compound salt portion of the said complex salt within the limits of 1.5 :1 and 2.5:1. Between these limits, the choice of the ratio to be used, and therefore the amount of zirconium tetrachloride in the complex salt, will be controlled by the inafter. In order to ensure that the specific gravity of the final complex salt, when molten, may be sufficiently high that it'iw ill riernaiii "st lls bouem of the 'cr'iicibledu'ring the alloying process, and give g'oodTseparatio'h' between it and th'e' molteiirnetal, Iprefer to add to the'coinpound salt aheavysalt', such as dry anhydrous barium chloride.

'Th'e preferred amount of barium chlorideis from 0 to 30% by' weight of the whole. About 10% of barium chlo ride results in the fused complex'salt having a spe- ClfiC gravity in excess of 2.0. Up to two-thirds of the requiredfamount of'barium chloride may be replaced by barium fluoride.

The complex salt may contain up to 20% of magnesium -fluoride as an'inspissating agent or thickener, in

order to decrease its'fluidity at the alloying temperature of about 8,00? C, When magnesium fluoride is used for thispurpose,l prefer to calcine it at a temperature be- Jtween 60080 0 C. for upto 3 hours in order to drive off I moisture andother volatile constituents (which 'rnay be present in commercial magnesium fluoride up to a figure ashigh as 2,0 per cent). If this calcining operation is not carried out, the addition of magnesium fluoride gives rise to frothing and bubbling of the molten salt,and the i zirconium becomes oxidised and therefore unavailable ,foralloying purposes. The grade of commercial magnesium fluoride which is stated to contain up to 15% o r more ofcalciumffluoride, may be used. Other in- PtSSatmg agents whichrnay be present, are aluminium fluorideup to 6%, and calcium fluoride up to 10%.

The presence of magnesium oxide, even when used in v the calc1ned cond1t1on, is harmful, as it oxidises the zirconium and reducesthe available zirconium content of I the salt. As an impurity, it would not be economical to found, is zirconium oxide, because it offers three advantolerate morethan 1% of-magnesium oxide.

The most useful inspissating compound which I have tages. Firstly, when used in amounts up to about 25%, it has a definite, but controllable inspissating effect which is agfunction of the amount usedf I prefer to add between 10 and 25 per cent of the oxide. Additions greater than 25 per cent tend'to make the complex salt too thick and viscous, so that it isunworkable at the alloying temperature. specific gravity of the complex salt, so that all or part R 30f the, barium chloride oribariumgchloride-barium fluweight.

must be carefully controlled, because beryllium 1n magoride addition maybe replaced. Thirdly, when added .during the make-up of the complex salt according to the technique which will be described hereafter, it controls the reactivityof the Zirconium tetrachloride during the exothermic reaction with the potassium fluoride. Within the limitsstated above, the various inspissating agents may be used together, but their total should not exceed-30 percent by=weight of thexcomplex salt. Also, i'ri'ordertoavoid'oxidationof the zirconium, they should be dried, before use, at a temperature of at least 110 C.

" Other reducible halide salts of desirable alloying additions may also'ibe added to the complex salt. For example, the presence of small amounts of beryllium in magnesium alloys considerably reduces the tendency to oxidation and burning of the molten metal at the high temperatures required for the zirconium alloying operation. Therefore, beryllium chloride or fluoride or the potassium double salts of these compounds may be added to the complex salt-to. the extent of up to'about 7% by The amount of the beryllium salt to be added nesium tends to enlarge the grains and therefore, to this .extent, counteracts the beneficial effects of zirconium.

I have found that the. reactivity of zirconium tetrachloride varies with the source of the supply and, when making up large melts of the complex salt, some of this :material may react violently with the potassium fluoride, giving rise to a loss of zirconium salt by sublimation. However, if the potassium flu'oridejexcept that portion of this salt which is used with zirconium tetrachloride to 'trigger the reaction, is mixed'with all or part of the required addition of zirconium oxide, then the complex salt may' be made upas'hereinbefore described for the compound salt and the zirconium oxide will then control or dampen the reaction which then proceeds quietly. The

trigger portion is theinixture of zirconium tetrachloride and potassium"fliioride in the ratio of 2:1 by weight,

"which is placed in th'e botto'niofthe crucible to'initiate Secondly, it is very ,dense and it increases the "this purpose between 5 per cent and 25 per cent of the required zirconium tetrachloride content of the complex salt and 10 per cent is sufficient as a rule.

When the so-called crude or unrefined grade of zirconium tetrachloride powder is used for making up the alloying salt, due allowance must be made for the zirconium oxide content and the zirconium oxide portion of the oxychloride content of this material, when deciding on the addition of zirconium oxide to be made, either for inspissation or for dampening the exothermic reaction.

When the exorthermic reaction is complete and the required amounts of zirconium tetrachloride,- potassium fluoride and zirconium oxide are in the form of a molten pool in a crucible or other suitable container, the desired amounts of the other ingredients, for example, additional inspissating agents and barium chloride or fluoride salts, are added and thoroughly mixed into the mass.

In making up the complex salt, it is desirable to maintain the temperature'as low'as possible'inorder to keep the oxidation of the zirconium salt toa minimum.

When molten and thoroughly mixed, the hereinbefore described compound or complex saltmay {be poured directly into the molten magnesium or magnesium alloy, to which it is intended'to add zirconium, and the metal temperature may be raised to between 760820 6;, the salt being stirred in for about 'l-l0minutesyafter which the metal is allowed to settle, while maintaining the temperature within this ranger'This settling period, 'which maybe up to 20 minutes,'or even longer, provided the temperature is maintained,"is necessary so that all the impurities and tracesof high meltingpoint compounds of zirconium and flux inclusions may settle to the'bottom. It is essential'that burning 'or'oxidation of the metal should be prevented as far as possible during the melting and alloying procedure by the use of a high-melting'point, magnesium chloride-free,'heavy fiux, similar to'that used for the melting of cerium-containing magnesium alloys in bale-out or sulphur-dome furnaces. This flux is care fully removed from the top'of the'metaland'from the sides of the crucible immediately before the'commencement of the final settling operation and; from then on,

oxidation of the molten metal is prevented by dusting with sulphur or suitable compound composed of a mixture of sulphur and boric' acid.

I, however, prefer to cast all the fused compound or complex saltinto thick slab moulds,'whereupon' it is broken up into suitably sized pieces. These-pieces or lumps of fused salt are then placed'in the bottom of the crucible, in which it is intended to carry out the alloying, and the magnesium ingots are placed on top, or molten magnesium is poured over the salt. After alloying in the normal way with the desired alloying metals','otl1er than zirconium, the temperature of the magnesium or magnesium alloy is raised to 760820 'CJas'before, and

the salt is stirred in by hand with a suitable stirrer, or

a mechanical device may be used. The same'type of flux as described aboveshould be used'to protect the metal from burning, and after'stirring for llO"m'inutes and removal of the flux, the'metal should be allowed to settle as before.

The desired other alloying metals referred to above may be the non-inhibiting elements zinc,'cadmium, silver, thallium, thorium, copper, bismuth, beryllium," lead, lithium, calcium, cerium or other rare-earthi'nctal or cerium-mischmetall or 'othercombinations of the rareearth metals.

I have found that it is advantageous to'repeat the stirring and settling operations',"especially when the salt is being used for the first alloying with magnesium. A definite increase in the zirconium content of the metal occurs during the second stirring after the initial stirring and settling operation. This procedure may-be repeated a third or fourth time if necessary. When a-inultiple stirring and settling technique is used, the flux may be left on the top of the metal to protectit from-burning until immediately prior to the final settling" operation, when it is carefully removed from the surface of the melt and from the sides ofthe crucible as before.

The compound or complex salt mayalso be used for revivifying magnesium'alloys containing zirconium which require to be remelted, before being. cast intoiisuitable castings or shapes's' The amount of salt required for initial alloying or revivifying purposes is a function of the available zirconium content of the salt and that required in the finished alloy, but I have found that it is preferable to arrange the available zirconium content of the salt so that an amount of salt equivalent to about 525% by weight of the metal would be required.

In order to produce the optimum reduction in the grain-size and the corresponding improvement in the tensile properties of magnesium alloys, it is necessary to have at least 0.4 per cent zirconium present in the truly alloyed or available form. However, G. A. Mellor (Journal of the Institute of Metals, 1950, 77, 163) has recently shown that the solubility of zirconium in pure magnesium is only about 0.67%, even at 900 C. I have found that the presence of zinc tends to increase slightly the solubility of zirconium in the alloy, whereas the presence of rare-earth metals tends to decrease the figure quoted by Mellor.

By way of example, an alloying salt within the scope of the present invention made up to contain:

Per cent ZrCl4 7 RF 23 ZrOz 20 BaClz 10 was used, according to the technique described hereinbefore, to introduce zirconium into a magnesium alloy containing 4.5 per cent zinc and the resulting alloy was shown, by chemical analysis, to contain 0.7 per cent of soluble zirconium. Similarly, when the same alloying salt was used with a magnesium alloy containing 4.25 per cent mischmetall, the results of the chemical analysis of the cast product showed a soluble zirconium content of 0.57 per cent. Both of the alloys possessed excellent tensile properties and were completely free from salt, flux or dross inclusions.

I wish it to be understood that various changes might be made within the scope of the appended claims, both in respect of the substances used and the steps taken to operate the invention in accordance with this specifica' Hon.

I claim:

1. Process for alloying of zirconium with magnesium, comprising intimately mixing in the bottom of a warm crucible approximately one part by weight of previously dried potassium fluoride and two parts by Weight of zirconium tetrachloride in an amount calculated on the basis of using between 5 and 25% of the total zirconium tetrachloride in the final salt for this purpose, then adding to said bottom layer successive thin alternate layers of zirconium tetrachloride and potassium fluoride, while leaving for the top layer a small amount of the potassium fluoride content, closing said crucible except for a vent hole, gently heating until exothermic reaction occurs, with the production of a molten mass, then stirring into the molten mass the top layer of potassium fluoride thereby forming an alloying salt mixture, said salt mixture comprising a fused mixture of zirconium tetrachloride and potassium fluoride, in which the content of zirconium tetrachloride ranges between 0.75 and 4 parts, by weight, per one part of potassium fluoride, and intimately mixing said salt mixture with molten magnesium.

2. The process of claim 1 wherein the alloying salt contains as an inspissating agent up to 25% of zirconium oxide and wherein at least part of the zirconium oxide is initially mixed with the potassium fluoride and the remainder of the zirconium oxide is mixed as a final step into the molten mass.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,497,538 Emley et al. Feb. 14, 1950 2,497,551 Jessup et al. Feb. 14, 1950 FOREIGN PATENTS Number Country Date 533,264 Great Britain Feb. 10, 1941 591,225 Great Britain Aug. 12, 194'! 624,304 Great Britain June 2, 1949

Claims

1. PROCESS FOR ALLOYING OF ZIRCONIUM WITH MAGNESIUM, COMPRISING INTIMATELY MIXTURE IN THE BOTTOM OF A WARM CRUCIBLE APPROXIMATELY ONE PART BY WEIGHT OF PREVIOUSLY DRIED POTASSIUM FLUORIDE AND TWO PARTS BY WEIGHT OF ZIRCONIUM TETRACHLORIDE IN AN AMOUNT CALCULATED ON THE BASIS OF USING BETWEEN 5 AND 25% OF THE TOTAL ZIRCONIUM TETRACHLORIDE IN THE FINAL SALT FOR THIS PURPOSE, THAN ADDING TO SAID BOTTOM LAYER SUCCESSIVE THIN ALTERNATE LAYERS OF ZIRCONIUM TETRACHLORIDE AND POTASSIUM FLUORIDE, WHILE LEAVING FOR THE TOP LAYER A SMALL AMOUNT OF THE POTASSIUM FLUORIDE CONTENT, CLOSING SAID CRUCIBLE EXCEPT FOR A VENT HOLE, GENTLY HEATING UNTIL EXOTHERMIC REACTION OCCURS, WITH THE PRODUCTION OF A MOLTEN MASS, THAN STIRRING INTO THE MOLTEN MASS THE TOP LAYER OF POTASSIUM FLUORIDE TURE COMPRISING A FUSED MIXTURE OF ZIRCONIUM TETRACHLORIDE AND POTASSIUM FLUORIDE, IN WHICH THE CONTENT OF ZIRCONIUM TETRACHLORIDE RANGE BETWEEN 0.75 AND 4 PARTS BY WEIGHT, PER ONE PART OF POTASSIUM FLUORIDE, AND INTIMATELY MIXING SAID SALT MIXTURE WITH MOLTEN MAGNESIUM.