US3167425A

US3167425A - Method of producing a magnesium base alloy

Info

Publication number: US3167425A
Application number: US103220A
Authority: US
Inventors: James H T Petch; Fisher Philip Andrew
Original assignee: Magnesium Elektron Ltd
Current assignee: Magnesium Elektron Ltd
Priority date: 1960-04-29
Filing date: 1961-04-17
Publication date: 1965-01-26
Anticipated expiration: 1982-01-26
Also published as: GB950628A

Description

United States Patent ()1 3,167,425 Patented Jan. 26, 1965 ice 3,167,425 METHOD OF PRGDUCHNG A MAGNESEUM BASE ALLOY James H. T. Petch and Philip Andrew Fisher, Manchester, England, assignors to Magnesium Elehtron Limited, Swinton, England No Drawing. Filed Apr. 17, 1961, Ser. No. 103,220 Claims priority, application Great Britain Apr. 29, 1960 11 Claims. (Cl. 75-435) This invention relates to magnesium base alloys of the kind containing from 0.1 to 1.0 percent zirconium with or without other alloying constituents such as zinc, rare earth metals, thorium, silver, cadmium, lead and other elements which do not cause zirconium to precipitate from the molten magnesium alloy, and also to alloys containing minor amounts of manganese and beryllium commensurate with the zirconium content.

Among the alloys of this kind which have been commercially produced are (l) wrought alloys containing besides zirconium no other intentional alloying elements except possibly to 0.005 percent beryllium and 0 to 0.4 percent manganese and 0 to 1 percent rare earth metals, (2) wrought alloys containing besides zirconium about one and one-quarter percent zinc, and (3) cast and wrought alloys containing besides zirconium 2.5 to 7.0 percent zinc.

The first mentioned of these two kinds of alloys have not previously been used on a large scale commercially, although they have been studied scientifically since they comprise the binary system from which more complex commercially used alloys such as those containing magnesium, zirconium, zinc, rare earth metals, silver, thorium, etc. have been developed. The binary alloys are becoming of increasing importance in the nuclear energy industry where they are employed in the Wrought form to sheath the radioactive fuel element and so protect it from possible attack by the cooling medium. These alloys are particularly suitable for this purpose in so far that they are characterised by a very small grain size, a property imparted to them by their zirconium content.

The quality of such protective sheaths must be very high since considerable distortion may occur in service. The sheath must be able to accommodate such distortion without cracking since this might lead to exposure of the radioactive material to the coolant. One requirement of quality in the sheath is that it must be as free as possible from discontinuities such as oxide films, inter metallic compounds or other discrete particles since it would be expected that inclusions of this type could initiate crack formation during deformation.

During the study of alloys containing magnesium and zirconium, it has been observed that such alloys are characterised by the presence of inclusions comprising intermetallic compounds of zirconium with elements present in the metal as impurities such as iron, silicon, aluminium, manganese and hydrogen. If the alloy is worked these intermetallic particles may become spread out to form hard stringers in the workpiece, which may have a detrimental effect on its properties and in particular its fatigue endurance. They are therefore highly undesirable in wrought alloys. In the unworked alloys however, these intermetallic particles appear to be of little practical significance.

In the case of the magnesium zirconium alloys containing more than 2.5% zinc, a different type of intermetallic inclusion can arise which is composed primarily of zirconium and zinc. The zinc-containing alloys have been used on a considerable scale for purposes requiring high tensile strength and resistance to fatigue at normal temperatures, e.g. for cast aircraft wheels. These large wheels and other castings are commonly examined radiographically and it is found that clouds of dense intermetallic particles are disclosed on the radiographs of a proportion of such castings. The profusion of the Zn-Zr particles is often such as is likely to affect adversely the properties of even the unworked alloy. Accordingly it is highly desirable to enablelarge castings to be produced which can be relied on regularly to be free from such particles whilst maintaining their good properties such as high tensile strength and low degree of microporosity.

Castings made for subsequent working by plastic deformation (e.g. by extrusion, rolling, forging, etc.) in these alloys with zinc contents over 2.5% should also. be free of these intermetallic particles, since the stringers to which the particles give rise will adversely affect the quality of the product. i

In producing both shaped castings and castings for Working in alloys containing magnesium and zirconium of the type covered by the present application, it has always been considered that a temperature of at least 760 C. should be used to alloy the zirconium into the magnesium in order to achieve adequately fine grain and high tensile strength. Accordingly the practice has long been to puddle the melt in the presence of excess zirconium at a temperature of 760 C.-780 C., for wrought stock or 780800 C. for shaped castings and to pour the melt at a temperature of 690 C.-740 C. for wrought stock or at a temperature of 760 C.-820 C. for shaped castings. The zirconium may be added in the form of a master salt consisting of a mixture of salts including a halide of zirconium (eg as described in British patent specification No. 715,967) or as a master alloy including metallic zirconium (e.g. as described in British patent specification No. 652,222 or patent application No. 857,709 to which U.S. Patent No. 2,970,904 corresponds). The excess zirconium remains at the bottom of the crucible and is puddled by means of an iron tool such as an iron disc on along iron red, the excess zirconium being stirred by the disc to introduce the maximum amount of zirconium solubly into the alloy.

In attempting to provide material satisfactorily free from inclusions, we have carried out many experiments with a view to eliminating the inclusions from alloys of both the low zinc (or zinc free) and the high zinc types. Thus We have tried the use of filtration using various known devices and we have also inserted various types of bafiles in the metal stream with the object of preventing the inclusions from passing into the castings. We have also tried a number of alloying temperatures between 740 and 800 C. and various zirconium contents in the range (LS-0.7%. Also in the case of the high zinc alloys, tests have beenmade on various kinds of moulding sands, oxidation inhibitors, mould treatment, core composition, mould washes and even variation of alloying and casting temperatures over a range of 720 to 920 C. By none of these methods has it been possible to obtain magnesium-zirconium alloy consistently free from deleterious zirconium rich particles.

We have found that the Zn-Zr particles are not directly connected with impurities in the melt but arise when a large body of the alloy is held molten at a temperature considerably below the alloying temperature. Thus they will form in large castings where the freezing rate is sufliciently slow. They can form in the crucible itself if the pouring temperature is substantially below the alloying temperature and can then appear in small as well as large castings.

We have now found that, contrary to accepted belief (J and teaching, it is possible to introduce a full zirconium content into magnesium at temperatures as low as 650 to 700 C. and to obtain very good mechanical propertiestherefrom. For this purpose, a master salt containing zirconium fluoride is unsuitable, as also is zirconium metal and it is necessary to use a master alloy in which the melting point of the entrained salt phase is below 600 C. An example is provided by the invention of British patent application No. 857,709. When zirconium is introduced at these low tempenatures, we find that the rate of pick up of impurities by the melt, e.g. iron from the crucible and hydrogen from moist air, 'fiuxes, etc., is so low that castings poured in a known manner from the melt at temperatures not exceeding about 725 C. show very low incidence of zirconium rich particles.

Moreover, if a high zinc alloy is puddled at these temperatures and cast at say 760 C., the level of soluble zirconium achieved in the melt corresponds to saturation at the low temperature and not at 760 C., so that insufficient zirconium can separate out on cooling to give rise to extensive ZIk-ZI' particle formation in large castings.

The zirconium may be introduced into the magnesium using a zirconium alloying substance consisting either of (a) a master alloy made by reduction with magnesium of salt mixture containing zirconium halides, provided the meltingpoint of the entrained salt phase is sufficiently low, e.g. below 600 C. or of (b) a mixture of zirconium chloride ZrCl with one or more chlorides of the alkali metals and/ or alkaline earth metals. Thus master alloys in accordance with British Patent No. 652,230 and British Patent 857,709 are suitable, whereas salt mixtures in accordance with British Patents 642,243 and 715,967 and master alloys made therefrom are unsuitable. Compacts of magnesium and zirconium metal powder, and other forms of zirconium metal and also unsuitable.

The zirconium master alloy for the purpose of the present invention preferably consists of the following 7 British patent specification 857,709 viz.:

(1) A salt phase forming 1 to 15 percent by weight of the master alloy and containing MgCl and MgF the latter being from 25 to 40 percent by weight of the MgCl together with alkali metal chloride in quantity by weight from one-third to two-thirds the weight of the MgCl with or without BaCl (2) A matrix phase consisting wholly or mainly of magnesium with zirconium and with or without permissible elements.

(3) Metallic zirconium embedded in phase (2) and constituting 25 to 45 percent by weight of the master alloy.

The master alloy willpreferably be made by the reduction by magnesium of salt mixtures containing at least one zirconium halide.

The particular example quoted in specification 857,709 is suitable for the purpose of the present invention (i.e. a master alloy made by adding magnesium together with 160 lbs. fluozirconate to a mixture of 364 lbs. MgCI 150 lbs. KCl, and 136 lbs. BaCI If the zirconium alloying substance is a mixture of chlorides this may be as described in the specification of British Patent No. 652,230.

According to the present invention, therefore, a methd of producing a casting in a magnesium base alloy containing 0.1 to 1.0 percent by weight of zirconium comprises puddling the alloy at a temperature above the liquidus and not exceeding 700 C. in the presence of a zirconium alloying substance capable of introducing zirconium into magnesium at these temperatures and casting at a temperature at least equal to the puddling temperature.

The casting temperature should preferably be slightly above the alloying temperature. 7

Generally We prefer to puddle the alloy at a temperature of 660 to 690 C. The casting temperature may be 690 to 725 C. for alloys containing up to 2% zinc but may be 700 to 790 C. for alloys containing 2.5 to 7 percent zinc.

For an alloy containing 0.4 to 1 percent zirconium and 4 to 5 percent zinc, and used for shaped castings, the alloy may be puddled at 660 and 680 C. and cast at a temperature not less than 720 C. For an alloy containing 0.4 to 1 percent zirconium and 5 to 7 percent zinc, and intended for shaped castings, the alloy may be paddled at a temperature of 660 to 680 C. and cast at a temperature of from 720 to 790 C.

For producing a casting for subsequent plastic deformation in an alloy containing 5 to 7 percent zinc the puddling temperature may be 64-0 to 680 C. and the casting temperature may be 680 to 725 C. whereas if the alloy contains 2.5 to 4 percent zinc the puddling temperature may be 660 to 700 and the casting temperature may be 680 to 725 C.

The process of alloying the zirconium at these low tem peratures may be combined with filtration or similar steps to provide additional control over the incidence of zirconium rich particles in the casting.

The magnesium zirconium alloy may contain manganese in accordance with British Patent No. 806,104. Beryllium may also be incorporated in the melt by any known method applicable to magnesium-zirconium alloys.

The alloy may contain:

Percent by weight Zinc From 0 to 7 Manganese From: 0 to 0.4 Rare earth metals From 0 to 1.0 Beryllium From 0 to 0.005

with or without other permissable elements known to be compatible with magnesium-zirconium alloys e.g. as specified in U.S. Patent No. 2,788,272.

We claim:

1. A method of producing a casting of a magnesium base alloy containing 0.4 to 1.0 percent by weight zirconium which comprises puddling the alloy at a temperature above the liquidus and not exceding 700 C., in the presence of a zirconium master alloy capable of introducing zirconium into the magnesium at this tempera ture, ringing the temperature of the alloy to a casting temperature at least equal to the puddling temperature, and thereupon casting the alloy at said casting temperature.

2. A method as claimed in claim 1 wherein the alloy is puddled at a temperature of 660 to 690 C., and is cast at a temperature of 690 to 725 C.

3. A method as claimed in claim 1, in which the alloy contains at least one of the following elements in the amounts stated below: 7

Percent by weight 4. A method as claimed in claim 1, for producing a casting for working by plastic deformation in which the alloy contains zinc up to 2 percent by weight.

5. A method for producing a casting as claimed in claim 1, in which the alloy contains 2.5-7 percent zinc and the melt is cast at 700 to 790 C.

6. A method for producing a shaped casting according to claim 1 in which the alloy contains 0.4 to 1 percent zirconium together with over 4 and up to 7 percent zinc and is puddled at a temperature of from 640 to 680 C. and is cast at a temperature of from 720 to 790 C.

7. A method as claimed in claim 1 for producing a casting for working by plastic deformation wherein the alloy contains 2.5 to 4 percent zinc and is puddled at a temperature of 660 to 700 C. and poured at a temperature of from 680 to 725 C.

8. A method as claimed in claim 1 in which the zirconium master alloy is made by reduction with magnesium of a salt mixture containing at least one zirconium halide.

9. A method as claimed in claim 1, wherein said master alloy comprises three phases, the first of which is a mixture of halides forming a salt phase and constituting 1 to 15 percent by weight'of the master alloy, said salt phase having a melting point not exceeding 600 C.

10. A method as claimed in claim 9, wherein said second phase comprises a matrix phase consisting of magnesium with zirconium and with elements which do not precipitate zirconium from molten magnesium-zirconium alloy.

11. A method as claimed in claim 10, wherein said third phase comprises metallic zirconium embedded in said second phase.

References Cited in the file of this patent UNITED STATES PATENTS 2,497,540 Emley Feb. 14, 1950 2,664,353 Saunders et a1 Dec. 29, 1953 2,698,230 Doyle Dec. 28, 1954 2,906,619 Roberson et a1 Sept. 29, 1959 2,919,190 Whitehead et a1 Dec. 29, 1959 FOREIGN PATENTS 652,230 Great Britain Apr. 18, 1951 134,221 Australia Sept. 21, 1949

Claims

1. A METHOD OF PRODUCING A CASTING OF A MAGNESIUM BASE ALLOY CONTAINING 0.4 TO 1.0 PERCENT BY WEIGHT ZIRCONIUM WHICH COMPRISES PUDDLING THE ALLOY AT A TEMPERATURE ABOVE THE LIQUIDUS AND NOT EXCEDING 700*C., IN THE PRESENCE OF A ZIRCONIUM MASTER ALLOY CAPABLE OF INTRODUCING ZIRCONIUM INTO THE MAGNESIUM AT THIS TEMPERATURE, BRINGING THE TEMPERATURE OF THE ALLOY TO A CASTING TEMPERATURE AT LEAST EQUAL TO THE PUDDLING TEMPERATURE, AND THEREUPON CASTING THE ALLOY AT SAID CASTING TEMPERATURE.