CN1639389A

CN1639389A - Magnesium-zirconium alloying

Info

Publication number: CN1639389A
Application number: CN03804793.4A
Authority: CN
Inventors: 马前; 戴维·圣约翰; 马尔科姆·T·弗罗斯特
Original assignee: Cast Centre Pty Ltd
Current assignee: Cast Centre Pty Ltd
Priority date: 2002-01-18
Filing date: 2003-01-20
Publication date: 2005-07-13
Anticipated expiration: 2023-01-20
Also published as: EP1466038B1; CN100393912C; EP1466038B9; TW200302285A; EP1466038A1; AU2003201396B2; US20050161121A1; DE60330309D1; EP1466038A4; WO2003062492A1; ATE450634T1

Abstract

Zirconium sponge can be chemically depassivated by treatment with hydrofluoric acid to improve the ability of molten magnesium/magnesium alloy to dissolve zirconium from the treated zirconium sponge and to form a melt containing substantially evenly distributed particles of zirconium.

Description

Magnesium-zirconium alloy

Technical Field

The present invention relates to the addition of zirconium to pure magnesium or magnesium alloys, and to the preparation of magnesium-zirconium (Mg-Zr) alloys including Mg-Zr master alloys.

Background

Zirconium is an effective grain refiner for magnesium alloys containing negligible amounts of elements such as Ai, Si, Fe, Ni, Co, Sn and Sb that form stable compounds with zirconium. The addition of about 1 wt.% zirconium to such magnesium alloys can easily result in 80% or more reduction in grain size at typical cooling rates. This particular grain refining capability makes zirconium an important alloying element of magnesium alloys that do not alloy with Al and Si. For example, zirconium-containing Mg-RE-Zn alloys such as EZ33(Mg-3.3RE-2.7Zn-0.6Zr) and ZE41(Mg-1.2RE-4.2Zn-0.7Zr) provide a specific combination of high temperature and room temperature properties that are not achievable for Mg-Al-Zn alloys.

The solubility of zirconium in molten pure magnesium is about 0.6%, which increases slightly with increasing melting temperature. It has been reported that: the most remarkable feature of the microstructure of magnesium alloys containing more than a few tens percent of soluble zirconium is the presence of a zirconium-rich core in the majority of the magnesium particles. This zirconium-rich core is considered to be a peritectically solidified product. To achieve excellent grain refinement in large-scale production, it is desirable to dissolve all of the zirconium content (i.e., about 0.6%) in the magnesium melt.

For decades, various means of introducing zirconium into molten magnesium have been developed, including:

(a) alloying with various forms of zirconium metal;

(b) alloying with sponge zirconium;

(c) alloying with Zn-Zr mother alloy;

(d) and ZrO₂Melting into alloy;

(e) with halides or complex halides of various zirconium or with various kinds of ions such as NaCl, KCl, BaCl₂Halides and/or mixtures of complex halides of salts of NaF, KF, etc. are alloyed together; and

(f) and melting the alloy and Mg-Zr mother alloy into alloy.

The advantages and disadvantages of each of these approaches have been discussed in detail by Saunders and Streiter (W.P. Saunders and F.P. Streiter, "zirconium fused to magnesium", Proc. Natl. Engineers, 1952, Vol. 60, p. 581-. Since about 1960, only the Mg-Zr master alloy has gained widespread industrial application as a zirconium source for alloying with magnesium. This Zr rich Mg-Zr master alloy is made by chemical reduction of magnesium based on a salt mixture of zirconium fluoride or zirconium chloride. The two master alloys are essentially the same and contain about one-third by weight of zirconium. One of these two, obtained in about 1945 by Magnesium Elektron Ltron (MEL), by chemical reduction of complexed zirconium fluoride with molten magnesium, has long been known as Zirmax (trade mark). In the united states, a similar variety of Mg-Zr master alloys was developed based on the reduction process of chloride salts.

Zirmax type master alloys hold the basic zirconium alloy material for the industrial production of zirconium containing magnesium alloys. Zirmax contains about 33% zirconium and 67% magnesium, and the majority of the zirconium is present in the magnesium matrix in the form of zirconium particles of various sizes, predominantly in the sub-micron to 10 μm range.

Before Zirmax-type master alloys became the standard zirconium source for zirconium-containing magnesium alloys, alloying with various forms of zirconium metal was investigated.

Work published by Sauerwald in 1947 on the fusion of zirconium metal powder into Magnesium (v.f. Sauerwald, "Dus zustshandsdiagramim Magnesium-zirkonium", zeitschrift furarannische chemie, 1947, volume 255, page 212-. 5% by weight zirconium metal powder was added to magnesium at various temperatures between 680 and 1100 c in an argon atmosphere. Soluble zirconium contents of more than 0.5% by weight (digestion of the sample in HCl acid) were obtained at all temperatures tested. In the same year, Ball reported work (C.J.P.ball, "Metallurgia", 1947, Vol.35, 125-&129; page 211) which states: zirconium metal is dissolved in magnesium at 900-. Operation at such temperatures is not feasible on a large scale due to the evaporation of magnesium. Emley was reported in 1948 (E.F. Emley, "discussion of Faraday society", 1948-49, Vol.47, No.4, p.219): since zirconium metal powder is expensive and very flammable, the possibility of alloying by reducible zirconium compounds is naturally considered.

In 1952, Saunders and Streiter reported their research, in which various forms of metallic zirconium (i.e., sponge zirconium, molten zirconium, iodide-decomposed ductile zirconium, and zirconium powder) were studied as zirconium alloy additive materials for magnesium at 760 ℃ (1400F). The zirconium frit was added in 6.35mm (1/4 inch) blocks to a small ladle and stirred in the ladle with steel rods. After stirring for 30 minutes, no clearly visible solution appeared. Analysis of the melt showed the result of having a soluble zirconium content of 0.03% for 1% zirconium addition. The iodide process zirconium pieces rolled to 127-. It was stirred in a ladle for several minutes. The following are found: after holding the temperature for 65 minutes, the soluble zirconium content produced only reached 0.1% for 1% zirconium addition. The use of zirconium powder was evaluated by making the additions in different ways, since zirconium powder is pyrophoric and some method of protecting the powder from oxidation had to be applied. The method comprises the steps of preparing zirconium powder and various binders into pills, sealing the zirconium powder into a sealed magnesium capsule, compacting the zirconium powder and the magnesium powder, and using the zirconium powder in the form of sintered zirconium powder compacts. Generally, the zirconium content of the product is reported to vary between 0.7 and 0.85% for 3% zirconium addition to the Mg-5Zn melt. The solubility of zirconium in magnesium is affected by the presence of a third element. Reportedly: for the presence of about 3.4% Zn, the solubility of Zr in magnesium can increase from 0.6% to slightly more than 0.7%, and 5% zinc increases the solubility of Zr in magnesium to about 0.8%.

The fusion of alloys with zirconium sponge according to the zirconium test of various metal morphologies, as taught by Saunders and striester, illustrates the most promising results. The sponge zirconium used was prepared by Kroll (Kroll) reduction:

((a) use of chlorine and carbon in zirconium oxides, e.g. zircon (ZrSiO)₄) Baddeleyite (ZrO)₂) To prepare zirconium tetrachloride by the reaction

Or is or

；

(b) The resultant zirconium tetrachloride is separated from the ferric trichloride (from iron impurities) and silicon tetrachloride (if present) by fractional distillation; and

(c) the purified zirconium tetrachloride is reduced by reaction with molten magnesium in argon to produce "zirconium sponge" - )。

In their experiments, the sponge was ground essentially to reduce the average particle size to about 12.7 μm or 0.0005 inches. The results show that: for 3% zirconium additions, the sponge zirconium produced a soluble zirconium content of about 0.62-0.66% in the Mg-5Zn alloy after 3-4 minutes of stirring. Soluble zirconium contents in the range of 0.32 to 0.52% were obtained for 1% zirconium sponge addition. Furthermore, the authors found that: as the particles become finer powders, the efficiency of alloying decreases as the sponge pieces decrease in size, since the material burns off before it can be submerged under the melt. Therefore, some method of protecting the powder from oxidation has to be applied.

Although the Saunders and striester work on zirconium sponge by the Kroll reduction process demonstrated excellent alloying results, alloying zirconium sponge into magnesium is generally limited to laboratory scale. As recognized by Saunders and striester and noted: "A significant drawback of the industrial application of zirconium sponge in the field of magnesium alloys is: the melting of the material into an alloy requires a rather laborious effort ". This laborious effort is clearly directed to the milling process, which is clearly simple due to the applied fusion method, i.e. 3-4 minutes of stirring. Furthermore, the inevitable problem of contamination occurring in the grinding process is another significant drawback in the industrial use of zirconium sponge.

In magnesium principles, Emley makes similar comments on alloying zirconium metal into magnesium: "pure zirconium metal obtained in any way is expensive and very flammable in powder form, and for these reasons it is easily combined with it to contaminate it with oxygen, hydrogen and nitrogen, the route through zirconium metal is not clearly the best".

Tests have shown that: in the mass production of magnesium alloys, when Zirmax is dissolved into a magnesium melt at a large-scale advantageous addition rate, insoluble zirconium particles can be easily observed in the microstructure of the resulting magnesium alloy. (Ma Qian, L.Zheng, D.Graham, D.H.StJohn and M.T.Frost, "precipitation of insoluble zirconium particles in a pure magnesium melt", journal of light metals, 2001, volume 1, No.3, 157-165 pages; and Y.Tamura, N.Kono, T.Motegi and E.Sato, "grain refining mechanism and casting structure of Mg-Zr alloy", journal of the Japan light metals Association, 1998, volume 48, No.4, 185-189 pages). Many of these residual (insoluble) zirconium particles have an average particle size of about 5 μm.

The density of zirconium was 6.5gcm^-3And the density of the molten magnesium is 1.6gcm^-3. The zirconium particles therefore have a strong tendency to settle in the magnesium melt unless strongly stirred. The larger the particle, the faster it will settle to the bottom of the melt. For example, it has been found that at 780 ℃, a 15 micron particle of zirconium sinks to the bottom of the magnesium melt at a rate of about 40 mm/min, and thus it is difficult to keep such particles suspended in the melt at this temperature. Conversely, at the same temperature, when the particle size is less than 3 microns, it can be easily suspended in the magnesium melt.

The Chambers science and technology dictionary (1991) defines: "passivity" is the lack of response of a metal or mineral surface to chemical attack occurring on a clean, newly exposed surface. Due to various factors, including: insoluble films produced by aging, oxidation or fouling; the surface energy of the discontinuous lattice is reduced; absorbent layer … ". Throughout the specification, the terms "depassivation", "depassivated", "depassivating" are understood to have meanings derived from the definition of "passive" above.

Summary of The Invention

In a first aspect, the present invention provides a method for treating zirconium metal, the method comprising chemically depassivating the zirconium metal. The zirconium metal is preferably zirconium sponge in a process to form treated zirconium sponge. The zirconium sponge may be chemically depassivated by treatment with a fluoride ion source. The fluoride ion source may be hydrofluoric acid. The fluoride ion source may be a mixture of hydrofluoric acid and nitric acid.

The hydrofluoric acid preferably has a concentration of between 0.10% and 50.0%, more preferably between 0.50% and 5.0%, most preferably between 1.5% and 2.5%, calculated as indicated hereinafter for the acid concentration. These acid concentrations range from 0.05 to 50.0 molar, from 0.25 to 2.63 molar, and from 0.76 to 1.28 molar, respectively, which can be approximated as 0.05 to 50.0 molar, from 0.25 to 3.0 molar, and from 0.75 to 1.5 molar.

In a second aspect, the present invention provides a method of treating zirconium sponge, the method comprising treating the zirconium sponge with a solution comprising fluoride ions to form treated zirconium sponge.

The zirconium sponge is preferably a porous agglomerate of zirconium particles. Preferably, the sponge is formed by a kroll reduction process.

Preferably, the sponge comprises zirconium and unavoidable impurities. Hafnium is a common impurity in zirconium. In contrast, Fe, Ni, Al, Si, C, Co, Sn, and Sb are not desirably contained as inhibitors of alloying, and their total concentration is preferably less than 1%, more preferably less than 0.5%.

Preferably, the zirconium sponge is in the physical shape of small particles and each particle has a porous structure. Preferably, these zirconium sponge particles have the following properties:

the mean particle size of the particles is between 0.1 and 10mm, more preferably between 0.5 and 5mm

The smallest dimension of the granule is 0.5mm, more preferably 1mm, and the largest dimension is 10mm, more preferably 5mm

-density of sponge 5.2-6.3g/cm³More preferably 5.5 to 5.8g/cm³

The porosity of the sponge (1-density of sponge/density of solid zirconium) is 0.08 to 0.2, more preferably 0.11 to 0.15.

The pore size in the polished cross section of each zirconium sponge particle is generally between 5 and 60 μm.

In a third aspect, the present invention provides treated zirconium sponge produced by the method according to the first or second aspect of the present invention.

Compared to untreated (original sample) zirconium sponge, it has been found that: treatment of zirconium sponge in accordance with the present invention increases the ability of the molten magnesium/magnesium alloy to dissolve zirconium and form a melt containing a substantially uniform distribution of zirconium particles.

In a fourth aspect, the present invention provides zirconium sponge comprising agglomerates of zirconium particles and having a surface layer comprising a fluorine-containing compound at least partially coating at least some of the particles. The fluorine-containing compound is preferably a fluoride of zirconium and may be of the formula Zr_xF_y·nH₂A compound of O.

In a fifth aspect, the present invention provides a method of producing a magnesium-zirconium master alloy, the method comprising the steps of:

(a) mixing the treated zirconium sponge according to the third aspect of the invention or the zirconium sponge according to the fourth aspect of the invention with molten magnesium/magnesium alloy to form a magnesium-zirconium melt containing dissolved zirconium and zirconium particles; and

(b) the magnesium-zirconium melt is cast to solidify as a magnesium-zirconium master alloy.

Preferably, the sponge is mixed with the molten magnesium/magnesium alloy by stirring.

In a sixth aspect, the present invention provides a magnesium-zirconium master alloy produced by the method of the fifth aspect of the invention. Preferably, the master alloy contains 10% to 50%, more preferably 20% to 40% zirconium in the magnesium/magnesium alloy. Preferably, at least 90% of the zirconium particles in the master alloy are less than 5 μm in size, more preferably less than 3 μm in size. Preferably, the average particle size is less than 5 μm.

In a seventh aspect, the present invention provides a magnesium-zirconium master alloy comprising dissolved zirconium and zirconium particles substantially free of halide impurities, wherein 90% of the zirconium particles have a size of less than 5 μm, preferably less than 3 μm.

Preferably, the master alloy is cast as a billet, which term is understood to include billets, pellets and the like.

In an eighth aspect, the present invention provides a method of adding zirconium as an alloying element to a molten magnesium/magnesium alloy, the method comprising: the treated zirconium sponge of the third aspect of the invention or the zirconium sponge of the fourth aspect of the invention is mixed with a magnesium/magnesium alloy.

In a ninth aspect, the present invention provides a method of adding zirconium as an alloying element to a molten magnesium/magnesium alloy, the method comprising: the magnesium-zirconium master alloy according to the sixth or seventh aspect of the present invention is mixed with a molten magnesium/magnesium alloy.

Preferably, the amount of zirconium added to the molten magnesium/magnesium alloy is greater than the amount required to saturate the magnesium/magnesium alloy with zirconium at the temperature of the melt.

In a tenth aspect, the present invention provides a zirconium-containing magnesium alloy produced by the method of the eighth or ninth aspect of the invention.

Brief Description of Drawings

In order that the invention may be more fully understood, reference will now be made to the accompanying drawings, which are mentioned below, by way of example, to preferred embodiments and other elements of the invention.

Fig. 1(a) - (c) are photomicrographs illustrating the grain refining ability of raw and untreated zirconium sponge when added to pure magnesium at 730 ℃. The three micrographs were at the same magnification. Fig. 1(a) is pure magnesium, fig. 1(b) is a manual stirring followed by 30 minutes after the addition of 1 wt% untreated zirconium sponge, and fig. 1(c) is a manual stirring followed by 30 minutes after the addition of an additional 1 wt% untreated zirconium sponge.

Fig. 2(a) - (c) are photomicrographs illustrating the grain refining ability of raw and untreated zirconium sponge when added to pure magnesium at 780 ℃. The magnification of these three micrographs is the same as in FIGS. 1(a) - (c). Fig. 2(a) is pure magnesium, fig. 2(b) is a manual stirring at 2 minutes followed by the addition of 1 wt% untreated zirconium sponge and then holding at 780 ℃ for 30 minutes, and fig. 2(c) is a further holding at 780 ℃ for 210 minutes.

Fig. 3(a) - (c) are photomicrographs illustrating the grain refining ability of the treated zirconium sponge of the present invention when added to pure magnesium at 680 ℃. The three micrographs were at the same magnification as in the above figure. Fig. 3(a) is pure magnesium, fig. 3(b) is manual stirring at 20 minutes following the addition of 1 wt% of the treated zirconium sponge, and fig. 3(c) is after 10 minutes of further manual stirring.

Fig. 4(a) - (c) are photomicrographs illustrating the grain refining ability of the treated zirconium sponge of the present invention when added to pure magnesium at 730 ℃. The three micrographs were at the same magnification as in the above figure. Fig. 4(a) is pure magnesium, fig. 4(b) is manual stirring at 30 minutes after addition of 1 wt% of treated zirconium sponge, and fig. 4(c) is further manual stirring for 2 minutes after holding for 30 minutes.

Fig. 5(a) - (c) are photomicrographs illustrating the grain refining ability of the treated zirconium sponge of the present invention when added to pure magnesium at 800 ℃. All micrographs were at the same magnification as in the above figure. Fig. 5(a) is pure magnesium, fig. 5(b) is manual stirring at 30 minutes after addition of 1 wt% of treated zirconium sponge, and fig. 5(c) is further manual stirring for 2 minutes after holding for 30 minutes.

Fig. 6 is a photograph showing the appearance of untreated (original sample) zirconium sponge used in one embodiment of the present invention.

Fig. 7 is a photomicrograph showing a view of a typical microstructure of the treated zirconium sponge particles of the invention shown in fig. 6.

Fig. 8 is a photomicrograph showing a view of another microstructure of the zirconium sponge particles shown in fig. 6 after treatment according to the invention.

FIG. 9 is a schematic diagram illustrating a method of adding treated zirconium sponge to molten magnesium.

Fig. 10 and 11 show exemplary views of the microstructure of a billet of a master alloy made according to the present invention.

Figures 12 and 13 show typical views of commercially available Zirmax master alloys.

Fig. 14 and 15 show exemplary views of the microstructure of a billet of a master alloy made according to the present invention.

FIG. 16 is a photomicrograph showing reaction products remaining on the zirconium sponge particles after treatment according to the invention.

Examples

Comparative test

Untreated (original sample) zirconium sponge with zirconium sponge particles having a physical shape and a diameter size of 1-10mm was selected. The main impurity in the sponge is hafnium. The impurity concentration is:

hf is about 0.8%

Fe+Cr＝0.1％

C＝0.004％

H＝0.001％

N＝0.002％

Without the treatment of the present invention, sponges were added to two samples of molten magnesium at 730 and 780 ℃, respectively. Conical samples (Φ 30 × Φ 20 × 25mm) were collected at different times and examined, showing little evidence of grain refinement (see fig. 1 and 2), even when the melt was held at 780 ℃ for 2 to 6 hours. Wet chemical analysis of the soluble zirconium content in the samples using 15% HCl acid showed negligible zirconium content (<0.05%).

Preparation of treated zirconium sponge

The same zirconium sponge used in the comparative test described above was first immersed in an acidic solution prepared in the following way:

45ml of concentrated nitric acid (68.5% -69.5%) and 45ml of concentrated hydrofluoric acid (50%) were mixed and diluted in water to a total of 1000 ml. This gave a composition containing about3% HNO₃And an acidic solution of 2% HF, which corresponds to about 1.1 molar HF and 0.5 molar HNO₃。

The zirconium sponge was placed in this acid solution for 5 minutes. Bubbles were observed, indicating that the acid may have at least partially removed ZrO₂The layer and some of the zirconium metal below it is dissolving. After removal from the acid solution, the zirconium sponge was washed in ethanol and dried under a heating lamp at about 50 ℃ for 60 minutes. Water is also considered a suitable cleaning agent.

Treated zirconium sponge used to prepare the alloys described in fig. 3-5 was prepared by immersing zirconium sponge in the same HF solution used for the comparative experiment for 5 minutes, followed by rinsing in water and drying. The HF solution was prepared by diluting 45ml of concentrated HF (50%) in water to a total of 1000ml to give about 2.25% HF, which corresponds to about 1 molar HF.

An alternative acid solution that can be successfully used is 0.07-0.25% HF. This is a very dilute HF acid solution and is easy to handle.

The treated zirconium sponge was prepared by immersing zirconium sponge in the same 2% HF solution used for the comparative test for 4 minutes, followed by rinsing in water and drying. The reaction product of this treatment is evident as a white color on the sponge particles in fig. 16.

While successful treatments are believed to include dissolution or some physical removal of oxides from the Zr surface, we do not wish to be bound by any theory as to why such treatments are effective.

XPS analysis of the treated and untreated sponge particles gave the results shown in table 1 below. The treated particles were immersed in a 0.5% HF solution for 4 minutes, washed with water and dried. For each analysis given, data was collectedfrom a layer 5 nm or 10 atoms deep on the surface of six different sized particles.

Table 1 XPS analysis of treated and untreated sponge particles

Sponge particle	Surface composition in atomic percent
Sponge particle	Surface composition in atomic percent										C	O	Zr	Fe	Si	F	Cl	Mg	Hf
Untreated	30.5	49.2	16.2	1.1	3.0	0	0	0	0		C	O	Zr	Fe	Si	F	Cl	Mg	Hf
Untreated	30.5	49.2	16.2	1.1	3.0	0	0	0	0	Treated granule (Black)	10.3	43.3	14.9	1.3	2.3	27.8	0	0	0
Treated granules (Gray)	26.0	41.1	15.6	1.1	1.7	27.4	0	0	0	Treated granule (Black)	10.3	43.3	14.9	1.3	2.3	27.8	0	0	0

O was in the form of ZrO in all three cases studied, depending on the measured energy levels for each element₂And F measured in the treated particles is in the form of, for example ZrF₄Zr (b) of_xF_y·nH₂The form of O exists.

In all the tests carried out, it was observed that: the weight of the sponge particles decreased with increasing treatment time in the acid solution until all particles disappeared or the HF was consumed.

While we do not wish to be bound by any theory, we presently believe that: the mechanism is that the treatment results in partial or complete removal of the oxide film on each sponge particle and the formation of a continuous or discontinuous layer of the reaction product (possibly Zr) on the particle_xF_y·nH₂O). Due to Zr formation on the sponge particles_xF_y·nH₂O patches or coatings, so that after treatment, oxidation of the zirconium under these patches or coatings can be prevented. Consider that: the patch then dissolves into the molten magnesium, leaving the portion of the zirconium surface exposed to the molten magnesium to provide a fresh contact site for the molten magnesium. BSE images have revealed: in thatThere are many tiny channels in each sponge particle, which provide an explanation for the breakdown of the sponge particle.

The mechanism presented above is only provided as a possible explanation of the test results and other mechanisms may be present. The present invention may be combined with the described mechanisms but should not be considered as being limited thereto.

Preparation of magnesium-zirconium alloy

A hole is machined in a small piece of magnesium ingot and the desired pieces of treated zirconium sponge are placed into the hole as shown in fig. 9. The sheet blank is then rapidly immersed below the surface of the magnesium melt. This allows the treated zirconium sponge to be introduced directly into the melt without the possibility of remaining on the surface of the melt. The method avoids the treated sponge being trapped in the slag and the treated sponge not being wetted by the melt. The sponge may be added to the melt in various other ways, such as by adding a mass of sponge particles, as long as it is successfully introduced below the surface.

In addition, it has been found that: in some cases the zirconium sponge particles may be added directly to the melt. One example is:if the surface of the magnesium melt is coated with, for example, 1% SF₆(balance: 49.5% CO)₂And 49.5% dry air) and thus the oxygen concentration above the surface of the magnesium melt is very low, the sponge particles can be smoothly added directly, as long as the operation is rapid. For example, the zirconium sponge particles may be added through a steel funnel at a height of 800mm from the melt surface, with the bottom of the funnel being placed just above the melt surface. This allows the sponge particles to rapidly enter the melt without oxidation. This has proven to be a very convenient way of adding fine (<5mm) sponge zirconium particles to a magnesium melt.

After the treated zirconium sponge was added at a rate of 1 wt%, the melt was left for a few minutes to reheat to the correct temperature, and then stirred for 30 minutes. Three temperatures were used: 680 ℃, 730 ℃ and 800 ℃. After the treated zirconium sponge was added, conical samples were collected at different times.

Figures 3-5 show typical diagrams of the particle structures obtained from these three experiments, respectively.

The results of the wet chemical analysis are summarized in table 2, in which the results of the wet chemical analysis of the soluble zirconium content in the samples taken from all three alloy tests are listed.

TABLE 2 soluble and Total zirconium content (%)

	Temperature of fusion to alloy
	Temperature of fusion to alloy						680℃ Adding 1 wt% of sponge		730℃ Adding 1 wt% of sponge		800℃ Adding 1 wt% of sponge
		Soluble in^*	General description of the invention^**	Soluble in	General description of the invention	Soluble in	680℃ Adding 1 wt% of sponge		730℃ Adding 1 wt% of sponge		800℃ Adding 1 wt% of sponge		General description of the invention
Before adding		Soluble in^*	General description of the invention^**	Soluble in	General description of the invention	Soluble in	＜0.005	＜0.005	＜0.005	＜0.005	＜0.005	＜0.005	General description of the invention
Before adding	Stirring for 30 minutes	0.48	0.93	0.56	0.76	0.54	＜0.005	＜0.005	＜0.005	＜0.005	＜0.005	＜0.005	0.92
Further holding for 30 minutes	Stirring for 30 minutes	0.48	0.93	0.56	0.76	0.54			0.56	0.85	0.56	0.66	0.92
Further holding for 30 minutes	Stirring for another 2 minutes			0.56	0.97	0.57			0.56	0.85	0.56	0.66	0.74

^*Soluble: 15% HCl^**In total: 50% HCl + 6% RF

As can be seen, after 30 minutes of stirring, the soluble zirconium content reached 0.56% at both temperatures 730 ℃ and 800 ℃. This is very close to the solubility limit of zirconium in molten pure magnesium, namely: 0.6% by weight. It has been reported that: the soluble zirconium content obtained by adding the same amount of zirconium from Zirmax master alloy is generally less than 0.5% and is typically about 0.4% at 720 ℃. (see: Y.Tamura, N.Kono, T.Motegi and E.Sato, "grain refining mechanism and cast structure of Mg-Zr alloy", journal of the light metals Association of Japan, 1998, volume 48, No.4, page 185-. The use of the pretreated zirconium sponge showed better recovery than the use of Zirmax master alloy.

Stability of treated zirconium sponge

A0.4% HF solution was prepared by adding 40ml of 10% HF to 960ml of water. The same zirconium sponge as used for the comparative experiment was immersed in 0.4% HF at room temperature for 5 minutes, then washed in water and dried. The dried treated zirconium sponge was stored in a plastic bag.

After four weeks of storage, 300g of the treated zirconium sponge particles were added to 30kg of pure magnesium at 680 ℃ and stirred. After 30 minutes of stirring, and after 60 minutes of stirring, the treated zirconium sponge particles were cast into chilled test bar samples (25 mm in diameter) prior to addition.

Good grain refinement was obtained after 30 minutes of stirring. After 30 minutes of stirring, the soluble and total zirconium contents were 0.38% and 0.69%, respectively, which increased to 0.42% and 0.81%, respectively, after 60 minutes of stirring.

Preparation of magnesium-zirconium master alloy

A total of 150g of treated zirconium sponge particles ranging in size from about 1 to 3mm were added to 550g of magnesium melt at 730 ℃. The nominal addition of zirconium was about 25 wt%. These zirconium particles were added in two batches. Stirring was carried out throughout the entire fusion process. The melt was cast into a steel ingot mold after stirring for 60 minutes.

Figures 7 and 8 show typical views of the microstructure of the treated zirconium sponge. Each zirconium sponge particle will eventually break down into many fine zirconium particles of about 2-3 μm in size due to the gradual dissolution of the porous structure. Maintaining gentle stirring throughout promotes the formation of a suspension of fine zirconium particles in the melt.

The magnesium-zirconium melt prepared as described above can be cast into different moulds, preferably into chill moulds. It is preferred that the height of each billet is not much greater than 500mm unless the die used has an excellent chilling effect. Low casting temperatures, such as 680 c or less, are preferred if possible. A blanket gas must be used during casting.

Fig. 10 and 11 show typical views of the microstructure of a billet made according to the above description, with the addition of 25% zirconium. The white phase was zirconium particles. Figures 12 and 13 show typical views of Zirmax master alloys for MEL. As can be seen, the zirconium particles present in the master alloy of the present invention are generally smaller than those present in Zirmax. As discussed earlier, small zirconium particles are always highly preferred.

A magnesium-zirconium master alloy containing about 50% by weight of zirconium was prepared by adding 440g of the treated zirconium sponge particles to 440g of molten magnesium at 700 ℃ with slow manual stirring for 90 minutes. Fig. 14 and 15 are representative illustrations of the microstructure of the as-cast ingot after completion of stirring, wherein the gray particles are zirconium and the white phase is magnesium.

Preparation of magnesium-zirconium alloys from magnesium-zirconium master alloys

A magnesium-zirconium master alloy (made according to the present invention) containing about 25 wt% zirconium was added to a crucible containing 30kg of molten magnesium at 730 c. The master alloy was preheated to about 175 ℃ prior to addition to the crucible and enough master alloy was added to give an addition of about 1 wt% zirconium.

After the addition of the master alloy, the melt was stirred with a mechanical stirrer at 150rpm for 5 minutes. Thereafter, the melt may be left for 15 minutes and then sand cast into 30mm thick plate-like samples (160 mm. times.140 mm) at 730 ℃. Prior to the addition of the master alloy, plate-like samples of pure magnesium were also sand cast at 730 ℃. The plate-like sample of pure magnesium had an average particle size of about 10,000 μm. After alloying with the master alloy, the resulting plate-like sample had an average particle size of 98 μm, a soluble zirconium content of 0.49%, and a total zirconium content of 0.58%.

It should be understood that: the use of the term "comprising" and variations thereof such as "comprises" and "comprising" in this specification is intended to imply the inclusion of stated features but not the exclusion of any other features.

The reference to any prior art in this specification is not, and should not be taken as, an: this prior art forms part of the common general knowledge in australia or elsewhere, or is suggested in any form.

Claims

1. A method of treating zirconium metal, the method comprising chemically depassivating the zirconium metal.

2. A method of treating zirconium sponge, the method comprising chemically depassivating the zirconium sponge to form treated zirconium sponge.

3. A method according to claim 2 wherein the zirconium sponge is chemically depassivated by treatment with a fluoride ion source.

4. A method according to claim 3 wherein the fluoride ion source is an acidic solution containing fluoride ions.

5. A method according to claim 3 wherein the fluoride ion source is hydrofluoric acid.

6. A method of treating zirconium sponge, the method comprising treating the zirconium sponge with a solution comprising fluoride ions to form treated zirconium sponge.

7. Treated zirconium sponge produced according to the process of any one of claims 2 to 6.

8. A zirconium sponge comprises agglomerates of zirconium particles and has a surface layer comprising a fluorine-containing compound at least partially coating at least some of the particles.

9. The zirconium sponge according to claim 8 wherein the fluorine-containing compound is a zirconium fluoride compound.

10. The zirconium sponge according to claim 9 wherein the compound of zirconium fluoride has the formula Zr_xF_y·nH₂O。

11. A method of producing a magnesium-zirconium master alloy, the method comprising the steps of:

(a) mixing treated zirconium sponge according to claim 7 or zirconium sponge according to any one of claims 8 to 10 with molten magnesium/magnesium alloy to form a magnesium-zirconium melt containing dissolved zirconium and zirconium particles; and

12. A magnesium-zirconium master alloy produced according to the method of claim 11.

13. A magnesium-zirconium master alloy according to claim 12, which contains 10 to 50% by weight of zirconium.

14. A magnesium-zirconium master alloy according to claim 12, which contains 20 to 40% by weight of zirconium.

15. A magnesium-zirconium master alloy according to any one of claims 12 to 14, wherein 90% of the zirconium particles are less than 5 μm in size.

16. A magnesium-zirconium master alloy comprising dissolved zirconium and zirconium particles substantially free of halide impurities, wherein 90% of the zirconium particles are less than 5 μm in size.

17. A magnesium-zirconium master alloy according to claim 15 or claim 16, wherein 90% of the zirconium particles are less than 3 μm in size.

18. A method of adding zirconium as an alloying element to a molten magnesium/magnesium alloy, the method comprising: mixing a treated zirconium sponge according to claim 7 or a zirconium sponge according to any one of claims 8 to 10 with a molten magnesium/magnesium alloy.

19. A method of adding zirconium as an alloying element to a molten magnesium/magnesium alloy, the method comprising: mixing a magnesium-zirconium master alloy according to any one of claims 12 to 17 with a molten magnesium/magnesium alloy.

20. A zirconium-containing magnesium alloy produced by the method of claim 18 or claim 19.