US2507713A - Fluxing process for magnesium alloys - Google Patents

Description

Patented May 16, i950 rwxme BRQQESS FORMAGNESIUM OYS Alf ed H He se wqri zion fibie ass aq by mesne assignments, to diathieson Qhemieal Corporation, a.,.corporatign,,of v ir gini'a No Drawing. Application July "7, 1945,- ;Serial No. 603 149 .2 Claims.

l This invention relates to magnesium alloys and has for one of its objects the provision of an improved method of treating magnesium a1- loys including the production, refining or casting of such alloys, in operations involving the use of an improved flux. The invention provides a method of treating magnesium alloys with a .flux of less density than the magnesium alloy which will float, on the metal and advantageously involves the incorporation of the metal in a previously prepared and molten --fiux. In ;the preparation of magnesium-base lithium alloys, the lithium is introduced into the bath of magnesium beneath the .flux and isprotected;during its solution in the ,magnesiumby the-overlying layer of low density flux. In accordance with nthe method of the invention, magnesium-alloys, especially magnesiumebase lithium alloys, v are treated with a flux ,having the capacity to pro- .duce an alloy relatively low in sodium;.and;-of greatly improved properties.

.In another of, ,its; ,aspeots, the'inventienmro- ".vides an improved halide flux for the treatment of; magnesium alloys, especially effective in the :treatment of .magnesium-baserglithium alloys, 3.

which gives improvements in boththetreatment operations and in the properties;1of-; the;;zneta1 produced. More particularly, -the invention pro- .vides a-fiux formed largely of a mixture-pt lithium chloride and a suitable fluoride -;which has the capacity to remove sodium from the; magniesium alloy being treatedto such low percentages as to give an alloy having SLlPGIlOIyiOIOIJBl'iEiFBS. IFluxes of theinvention may have such -low denisity that they float over molten magnesium. They zmay also have a higher density ;than certain ;magnesium-base lithium alloys andacco i xingly sink through such molten alloys. Nevertheless, :the flux has such unusual viscosity or; surtace 'ltension characteristics. that it forms atenacious film over the surface. of the magnesium-base Elithium alloy protecting it from reactive g ases even though the bulk ofythe flux;has-settled.;to the bottom of the confining vessel.

A flux-embodying my inventin;con prises:60% T possible {that in -the presence of high percent alkaline earth fluoride. or mixtures Pheregflth total proportion of alkaline earth halidesb least-Ian 1 th il Lithium chloride ,is of particular advantage in a-flux for the treatment oi ina'ggnesiumfbase lithium alloys. Fusedlithimnchloride is oi rel ativelydovv density compared .with many 'ofthe other salts used. in fluxes .andlmay be adrnpgeld yvitirother 10W .d e nsi ty halides, such aspensium chloride up to about half theweight' of the lithium chloride .to obtain fluxes which are lighter than the magnesium alloys to ten d. .As a result these fluxes float on. the alloys a d protect themfrom theflaction of theatmosp he'r'e. The addition ,of fluorides, and particularly li hium fiuoride, to lithium chloride in than es of the invention serves. toinbrease significantly the refinin 'properties of the'flux Without??- preciably changing itsdensity. Suitable fluorides fpr use in ,thefiuxes of the invention to obtain :their desirable prgpertiesihclude the. alkali metal flu r de l ith u a ipdiassimfl e h areline earth metal fiuor id'es. Lithium iluoridfppears to be theJnost satisfactory, particularly .for magnesium base -lithiiim 'alloys but Tether fluorides rnay in somelinstances be used. calcium or, barium iluorides may ,be substituted g-for ;a1l or part of the lithium fluoride." it is es -of lithium chloride in the flux, easement iluor-ide may loe converteddnto lithium jde but this is in v no way disadvantageous. itjisl'an advantage to use the cheap er calcium Tfiuor'itlie and by -interact ion .in the flux obtainltlh'e Tdei-s ab refinin a io a ciated, W lthi i fluoride. yIt is advantageous, however in stituting calcium-fluoride for lithium ilnoride'not ii r eve a io alabeui 1.0% of caleiumfllwride in the flux. Thev chlorides .of theiallra'line 'e irtiis ;s hould be used with regard to their distinctive characteristics. 'lhusthe chlorides, and also" he titer-i Warm e feei v an ba u m bemused wherethe entry oithe metals. ntofthe alloy is recognized. :lylagnesium chloride"; is .e s-

rem-1 a n e ve; ifilfiixee i t a loys .-free .of lit lum, .for where itlis ,desired not .to lin oducedithium into the alloy 1 Av particularlyv advantageous co'mlQQSitio cording to the present"invention' 'is a? t which -co ntains .a[bout of lithium jeliilor'ide ,and ,-about 25% of lithium, fluoride. it'fis liespecially, ,desiralile .to. .use. {fluire's'foontainixig such relatively l iigh percentages of. fluorides", of their refining action ion the alloys 1' fluxes. Even though some ultra-light alloys, such as magnesium alloys containing large proportions of lithium may be lighter than some of the fluxes of the invention, the latter, because their density is not greatly different from that of the alloy, are very readily stirred into the alloy. It is easy even with such alloys to maintain a liquid cover over the molten alloy and. thus protect the metal from the action of the atmosphere until the alloying and refining processes have been completed. The film of the flux of the invention remains fluid throughout the alloying process in contrast to fluxes which dry and thus fail to protect the magnesium.

Molten magnesium has a density of about 1.57 which is considerably less than the densities of the fluxes composed of the chlorides and fluorides of magnesium, potassium, sodium, calcium, and barium which are commonly used in commercial fluxes. As a result, any flux prepared from these materials which is not foamed up by the use of carbonaceous or other ingredients sinks to the bottom of the molten magnesium and frequently will not maintain a satisfactory fluid flux cover over the melt. Lithium chloride has a liquid density of slightly more than 1.46 and, when mixed with other low-density chlorides such as potassium chloride in about equal parts, the resulting fluid flux has a lower density than molten magnesium. The fluorides added in the amounts above set forth do not appreciably change the density of the flux. The low density of these fluxes permits the formation of a liquid cover over the molten magnesium and maintains complete protection of the metal from the atmosphere until the alloying process has been completed.

In designating a suitable flux for use with magnesium and lithium alloys, it is to be remembered that they are both very reactive metals and, when molten, must be protected from both oxygen and nitrogen. The protection by means of a flux is complicated by the fact that specific gravity of the molten alloy may be as low as 1.4, lower than any possible salt flux. Furthermore, lithium is one of the most electropositive of the metals and a suitable flux must be free from ingredients which would react with it to cause excessive loss or introduce harmful impurities, including the very prevalent sodium, in the alloy. Since lithium is an expensive metal, a suitable flux must permit high recoveries and avoid losses due to formation of sludge and of emulsions of metal in the flux. Of course, the flux must separate cleanly from the metal and not remain dispersed to form flux inclusions and it is very desirable that it be capable of repeated use, be stable in contact with air and other gases, and be substantially non-volatile at the temperatures used in treating the alloy.

In carrying out a method of the invention using the improved flux, any suitable procedure may be followed. The conventional operations are applicable but I have found it particularly advantageous to use the fluxes of the invention in a preferred procedure in which all the necessary flux is added to the crucible and melted before any metal is added. No further addition of flux is usually necessary. In this way it is possible to maintain a protective covering of a flux on the surface of the metal during melting and during subsequent additions of alloying elements. It is not necessary to remove the flux during alloying or before pouring. After the flux is melted it is my preferred procedure to add first the desired amount of magnesium which sinks below the surface of the flux as it is added, thus thoroughly protecting the metal during melting. Any other alloying elements, such as zinc, aluminum, manganese, lead, etc., are then added, and, like the magnesium, sink below the surface of the flux. During their addition the molten mass is advantageously agitated in a suitable manner, such as by stirring, to insure uniform distribution of the alloying elements. It is preferable to add the lithium metal last in preparing magnesium-base lithium alloys. The lithium, being by far the lightest of the alloying elements, may float on top of the magnesium and the flux. It is, therefore, important to maintain suflicient agitation to distribute the lithium homogeneously in the magnesium. I have found it particularly effective to introduce the lithium by means of an inverted perforated steel cup, or its equivalent, in order to hold the lithium under the surface of the other metals until it has melted and alloyed. Particularly when relatively large amounts of lithium have been added to the molten alloy, the density may eventually become less than that of the flux, much of which will then go to the bottom of the crucible. Even in such cases, however, a thin film of flux is retained on the surface of the molten alloy which is of sufficient thickness to protect the molten alloy from air. A thin film of flux also surrounds the alloy along the sides of the crucible so that there is, in effect a ball of molten alloy in the center of a mass of fluid flux. When the alloy is stirred or worked in any suitable way, the flux is sufficiently light and fluid to work up to the surface and protect the metal. Such agitation is desirable in order to obtain the advantageous refining action of the fluxes and to more thoroughly incorporate the lithium in the alloy. Under these conditions, the flux is readily distributed through the alloy, even though the majority of the flux is at the bottom of the crucible.

When the alloy is cast, the flux may remain in the crucible to be used for the next batch of metal. The very satisfactor separation of metal from flux results in a minimum of slag inclusions and high metal recovery. One reason for the good flux-metal separation is that the dispersed particles of this flux remain fluid throughout the entire refining process and coalesce with other small particles as well as the main body of the flux. Consequently small dispersed particles of this fluid flux tend to work themselves out of the alloy and thus free the metal from slag inclusions.

I have found that temperatures of 1450" F. appear to cause excessive oxidation of the metals of the alloys and are unnecessary in order to obtain homogeneous distribution of the alloying element. I prefer to operate at temperatures above 1100 F. and I have found it particularly advantageous to use the improved fluxes in the temperature range of 1l00 to 1400 F.

A remarkable result in using the improved fluxes of the invention is that magnesiumbase lithium alloys treated with it have certa'in physical properties "far superior to the:

same alloys refined with other fluxes. It has been found that refining with the improved-fluxes of the invention reduces .the amount of sodium in the magnesium-base lithium alloys and that the amount of sodium present very markedly affects the physical characteristics of these particular alloys. For example, about 0.1% sodium in an alloy containing 87% magnesium, 9% lithium, and 4% zinc decreases the elongation of the relatively sodium-free alloy from 36% to often less than 10%.

The amount of sodium which is present and which is detrimental to these magnesium lithium alloys is so small that it is not possible to obtain consistent quantitative results by chemical methods. The spectrographic method first used was not quantitative but it was possible thus to determine reliably the relative amounts of sodium in the alloys before, during and after treatment with gases according to the present invention. The relative sodium content was determined by measuring the density of the strongest lines in the sodium spectrum which occur in the visible range at wave lengths of 5890.0 and 5895.9 Angstrom units. Subsequently by the useof standard alloys, it has been determined that high, medium and low contents referred to herein correspond respectively to above about 0.2%, about 0.1% and below about 0.055% of sodium. In the following examples, the sodium content of the alloys was measured spectrographically:

Example I To 360 parts of a flux consisting of 75% lithium chloride and lithium fluoride which had been melted in a crucible were added 870 parts of magnesium. When the whole was in the fluid state,

Example II A mixture of scrap alloy comprising approximately 87% of magnesium, 9% of lithium-and 4% zinc was re-refined-using flux having a composition of 75% lithium chloride and 25% lithium fluoride. Aboutone-third of the starting .scrap alloy had previously been made withfiux'No. :1 of Example I and the remaining two-thirds had beenmade with flux No. 2 of Example I. The alloy made with flux No. 2 contained lithium having about 2.5% of sodium. '360 parts ofthe 75% lithium chloride-25% lithium fluoride fiu-x was agitated with 1181 parts by weight of the scrap alloy at a temperature of 1380 F. for thirty minutes. The refined alloy was cast and subsequently given a secondtreatmentunder thesame conditions and for the same time with additional quantities oithe'lithium chloride-lithiumfluoride flux. The properties of the alloy after the sec- -ond refining were also'measured. The following example shows the fiuxing material'used, sodium content foundand tensile properties obtained in the-alloy (87% magnesium, 9% lithium, and 14% zinc) when subjected to the three successive treatments with various fluxes:

40 parts of zinc were added and dissolved while the mixture was agitated by stirring. Finally, 90 parts of lithium wereintroduced by means of aninverted perforated steel cup. These operations were conducted at a temperature of about 1380 F. Using the same procedure and conditions, three alloys of the same composition were prepared using in No. lamixture of commercial fluxes containing approximately KCl 37.5%, MgClz 42.0%, BaClz 4.5%, CaF2'8.5%, MgO 7.5%, inNo. 2 a commercial flux as shown and in No. 3 a flux of the invention.

The data of the following table shows the properties of the three resulting alloys andclearly' demonstrates the advantages of the flux of the invention:

It will be seen that as-the alloy was treated successively with the fluxof this invention the sodium content decreased, the tensile strength and yield strength increasedand the elongation and reduction of area were increased by 8- to 10- fold.

Example III A heat was prepared under-a flux comprising 49 lithium chloride, 49% potassium chloride and 2% calcium fluoride. The alloy had thecomposition shown'inExample I and the procedure there described was'followed. The resulting casting was extruded'ancl, while cold,'bent upon itself around a radius of less'than A its thickness to without breaking orcracking.

Where it is desired similarly to reduce the potassium content of such alloys, 'it'is desirable to use components of the'flux which are substantially free of potassium salts. Where thisaction is less important, potassium chloride .or other potassium salts may be used in the flux with some advantage in the reduction of the cost thereof. I

The fluxes-of my invention are very suitable for i use with the present commercial magnesium alloys. ,Heatsof 'ASTM Alloy N0. .18 (13% :Al,,

1% Zn, 0.2% Mn, balance Mg) and ASTM No. 8' (6% A1, 0.7% Zn, 0.2% Mn, balance Mg) prepared under a flux containing 75% LiCl and HF were found to extrude readily and fully as well as alloys prepared withthe present commercial fluxes. The relatively high stability of the fluxes to air and combustion gases and the fact that they are specifically lighter than many magnesium alloys make them suitable for melting such alloys in a reverberatory furnace, for example. These flux properties also make them suitable for remelting magnesium alloy scrap. The scrap immediately sinks into the flux when charged and is therefore completely protected from the action of the atmosphere.

The fluxes of the invention serve effectively to decrease the sodium content of the alloys, a particular advantage with respect to magnesiumbase lithium alloys but also notably improving other light-metal alloys.

One of the unusual properties of my flux is that it can contain relatively large amounts of sodium without causing serious contamination of the alloy by this metal. The high capacity of the flux for sodium is of great advantage since this element is a very harmful impurity in magnesium-lithium alloys even in very small quantities. This affinity of the flux for sodium permits the repeated use of the flux without fear of contamination and also allows me to use less highly refined flux and alloying ingredients, especially with respect to their sodium contents. Avoidance of the inclusion of sodium in the flux is desirable but not necessary.

I may supplement the treatments herein described by effecting a removal of harmful alkali metals, notably sodium, from the alloy by subjecting the molten alloy to the action of certain gases which have the capacity to diminish the sodium content of the alloy. The alloy may be treated while molten in any suitable manner and preferably while in contact with the improved flux with a reactive gas such as chlorine, or mixtures thereof, or with gaseous diluents such as air or chlorine-nitrogen mixtures. The gas is introduced near the bottom of the fusion in any convenient manner as, for example, through a steel tube with the outlet at the bottom of the container. After the gas has been passed through the molten alloy for a suitable time at a suitable temperature, the mixture is allowed to settle and the alloy is poured or decanted in any conventional way to separate it from the flux.

When using these gases it is desirable to use a relatively low temperature for the operation so asto control their chemical reactivity. I prefer, however, to use somewhat less reactive gases than these extreme examples. Such relatively less reactive gases include ammonia and nitrogen. I have found it particularly advantageous to use nitrogen gas. Under certain conditions it will form; nitrides of magnesium and lithium but a considerable loss of metal in this way can be tolerated in view of the reduction of the content Of deleterious elements and resulting improvement in the alloy properties.

. Other compounds and elements which are gaseous at the operating temperature, say, above about 1200 F., but which are liquid or even solid at room temperature, may be suitable for use in practicing this operation. Thus, bromine may be substituted for chlorine and even iodine may be used. The added cost of these halogens may be disadvantageous but this does not affect their efliciency. Several of the Freons, such as clichlorodifluoromethane, may be used in particular instances.

Example IV g g fi 5? c i t t i i Elongation gi u on 0 tea Metal ggg ggfi f Per cent Per Cent None- High 8.0 14.0 5 minutes Medium 19. 5 25. 4 30 minutes Low-.. 27.9 39.2

As a check, the same alloy was fused under the same flux and maintained at the same temperature without the use of nitrogen. Samples removed at the end of five and of thirty minutes showed substantially the same sodium content, elongation and reduction of area as the original sample.

Example V Another sample of alloy having substantially the same composition as that used in Example IV was subjected to treatment with nitrogen gas for twenty minutes. Before the treatment the alloy showed an elongation of 11% and a reduction of area of 16.3%. After the twenty-minute treatment with nitrogen, the elongation was 26% and the reduction of area 34.8%.

Example VI An alloy containing 8.8% of lithium and 4% zinc, balance magnesium, showed before treatment an elongation of 16% and reduction in area of 20.9%. Chlorine gas was passed through the melted alloy under a flux consisting of 50% lithium chloride and 50% potassium chloride for 1 minutes. After the treatment the mixture was allowed to separate and the alloy poured.-

Test pieces showed an elongation of 27% and a reduction of area of 44.4%, increases of 69% and 112%, respectively based on the original values.

The high capacity of the flux for sodium and the eifect of nitrogen on the flux is illustrated in the following example:

Example VII 1200 grams of an alloy having approximately the composition: 87% magnesium, 9% lithium and 4% zinc, were melted under 375 grams of a flux of lithium chloride and 25% lithium fluoride to which 4% by weight of sodium chloride had been added. The fusion was treated by bubbling nitrogen gas therethrough for 30- minutes after which the alloy was cast and extruded. Sufficient sodium chloride was available-in the flux to introduce about 0.5% of sodium in the alloy. Such a concentration of sodium is known to result in an inferior alloy having low elongation and reduction of area of about 9 and 13 respectively. However, average to good values were In another important adaptation of the in- I 'vention. I may add lithium fluoride to an alloy prepared with lithium chloride flux and efiect a clean separation of flux from the metal which prior to the addition of lithium fluoride could not have been cast.

The life of a flux, which has become contaminated with sodium and other impurities to the extent that it no longer exerts a suilicient puritying action on a magnesium-base lithium alloy may be increased by treating the molten flux in the presence of molten alloy, for example, scrap alloy with nitrogen or a nitrogen-containing gas, such as ammonia. This treatment may be suitably carried out by introducing the gas beneath the surface of the bath by means of an iron pipe and allowing it to bubble through the flux until the desired degree of purification has been obtained. The time of gas treatment will vary between wide limits, depending on, among other things, the size of the bath, the amount of impurities present, and the rate of gas flow. However. given moderate amounts of impurities and a gas flow not great enough to cause excessive splashing, thirty minutes is usually a suilicient time.

In using the fluxes of the invention, e'ven unskilled workmen are able to prepare satisfactory alloys free from slag inclusions, whereas using prior fluxes, a high degree of skill is necessary in order to obtain satisfactory castings free or inclusions.

In addition to the operations herein described, the term treating" includes the preparation of the alloys or combination of the components, modifying the composition by the addition of further components, or increased quantities of components already present, refining and casting such alloys.

It was noted in one of my operations that, when the amount of sodium chloride equivalent to 0.6% sodium in the alloy was added to the flux, 42% of the sodium was lost by evaporation from the slag during addition of lithium.

I claim:

1. In a method of treating molten magnesiumbase lithium alloy, the improvement which comprises agitating the alloy in the molten state with a flux consisting essentially of from 60% to 98% of a chloride component and from 40% to 2% of a fluoride component, said chloride component consisting of from to 50% of potassium chloride, from 0 to of alkaline earth metal chloride of the group consisting of calcium, barium and strontium chlorides and the balance lithium chloride and said fluoride component consisting of at least one fluoride of the group consisting of potassium fluoride, lithium fluoride and alkaline earth metal fluoride of the group consisting of calcium, barium and strontium fluorides, the total of the alkaline earth metal chloride and fluoride being less than 15% of the flux, said flux as initially used being practically free of sodium, and, if sodium be present in the alloy, absorbing sodium of the alloy in the flux producing an alloy containing less than 0.1% of sodium.

2. In a method of treating molten magnesiumbase lithium alloy, the improvement which comprises agitating the alloy in the molten state with a flux consisting of about 7 5% of lithium chloride and about 25% of lithium fluoride, said flux as initially used being practically free 01'; sodium, and, if sodium be present in the alloy, absorbing sodium of the alloy in the flux producingan alloy containing less than 0.1% of sodium.

ALFRED H. HESSE.

REFERENCES CITED The following references are of record in the flle of this patent:

UNITED STATES PATENTS Number Name Date 1,626,292 Lund Apr. 26, 1927 2,066,579 Schichtel Jan. 5, 1937 2,160,812 Alden et al June 6, 1939 2,231,881 Burkhardt et al Feb. 18, 1941 2,261,905 Nelson et al Nov. 4, 1941 2,283,884 Nelson May 19, 1942 2,370,935 Bushrod Mar. 6, 1945 FOREIGN PATENTS Number Country Date 524,113 Great Britain July 30, 1940 509,024 Germany Oct. 6, 1930 OTHER REFERENCES Publication Serial No. 340,402, Alien Property Custodian, Hans Grothe, Method of Electrolytically Obtaining Magnesium; published May 18, 1943. 1

Henry et al., Institute of Metals Division, A.I.M.E., vol. III, 1934; pages 319-332.

Claims (1)

1. IN A METHOD OF TREATING MOLTEN MAGNESIUMBASE LITHIUM ALLOY, THE IMPROVEMENT WHICH COMPRISES AGITATING THE ALLOY IN THE MOLTEN STATE WITH A FLUX CONSISTING ESSENTIALLY OF FROM 60% TO 98% OF A CHLORIDE COMPONENT AND FROM 40% TO 2% OF A FLUROIDE COMPONENT, SAID CHLORIDE COMPONENT CONSISTING OF FROM 0 TO 50% OF POTASSIUM CHLORIDE, FROM 0 TO 10% OF ALKALINE EARTH METAL CHLORIDE OF THE GROUP CONSISTING OF CALCIUM, BARIUM AND STONTIUM CHLORIDES AND THE BALANCE LITHIUM CHLORIDE AND SAID FLUROIDE COMPONENT CONSISTING OF AT LEAST ONE FLUORIDE OF THE GROUP CONSISTING OF POTASSIUM FLUORIDE, LITHIUM FLUORIDE AND ALKALINE EARTH METAL FLUORIDE OF THE GROUP CONSISTING OF CALCIUM, BARIUM AND STONTIUM FLUORIDES, THE TOTAL OF THE ALKALINE EARTH METAL CHLORIDE AND FLUORIDE BEING LESS THAN 15% OF THE FLUX, SAID FLUX AS INITIALLY USED BEING PRACTICALLY FREE OF SODIUM, AND, IF SODIUM BE PRESENT IN THE ALLOY, ABSORBING SODIUM OF THE ALLOY IN THE FLUX PRODUCING AN ALLOY CONTAINING LESS THAN 0.1% OF SODIUM.