AU731066B3 - Method of production of magnesium alloy - Google Patents

Description

AUSTRALIA

PATENTS ACT 1990 COMPLETE SPECIFICATION FOR A PETTY PATENT

ORIGINAL

Name of Applicant: Actual Inventors: Address of Service: Invention Title: STATE TITANIUM RESEARCH AND DESIGN INSTITUTE Ivan A. Brannik, Ellen L. Kaluzskaya, Alexander M. Bashmakov and Andrey P. Gerb BALDWIN SHELSTON WATERS MARGARET STREET SYDNEY NSW 2000 "METHOD OF PRODUCTION OF MAGNESIUM ALLOY" The following statement is a full description of this invention, including the best method of performing it known to us:la METHOD OF PRODUCTION OF MAGNESIUM ALLOY Field of the Invention This invention relates to the metallurgy of non-ferrous metals, and in particular to a method of production of magnesium alloys.

Description of the Prior art Methods of production of magnesium alloys in the electrolytic furnaces having stationary or removable crucibles are known in the prior art see, for example, A.V. Kurdiumov et. al. "Casting production of non-ferrous and rare metals", Moscow, Metallurgy 1982, pages 261-263).

The prior art method includes the step of melting of crude magnesium with alloy forming components under a layer of protective flux or in an atmosphere of protective gas accompanied by the subsequent step of refining; The protective flux of the prior art consists of the halogenides of alkali, alkali-earth metals and magnesium.

The step of refining is carried out. in the same crucible 2 previously used in the step of melting and occurs upon the settling of the alloy under the flux.

Because the covering fluxes are highly hygroscopic, the utilization of the above discussed method often results in the formation of flux inclusions in the melt. It is known that such inclusions ultimately reduce the quality of the alloys. Furthermore, the prior art refining process is frequently carried out in crucibles which are insufficiently sealed. Thus, melting losses of the metal and the decrease of alloy quality are frequent occurrences.

After prolonged cooling, the metal reaches the casting temperature and the molten metal is poured into casts in the open air. The cooling and casting result in further losses of the metal. In addition, the rather small capacity of the crucibles does not allow for the casting of large-sized metal ingots with a homogeneous composition and stable temperature.

Summary of the Invention One of the main objects of the invention is to improve the quality of the alloys subjected to the step of refining and to reduce the metal losses. This is accomplished by isolating of the surface of magnesium alloy from contact -3with the outside environment and thereby preventing chemical interaction between the surface of the alloy and components of surrounding atmosphere. The process of the invention is carried out above a salt melt layer in the continuous refining furnace.

One aspect of the invention provides a method of production of magnesium alloy comprising: fusing of a crude magnesium with alloy forming components in furnace above a layer of protective flux or in an atmosphere of a protective gas; and refining of the fused composition above a layer of a salt melt in a furnace of continuous refining, wherein said fused composition supplied to the step of refining has a temperature between 6500 and 690' and in said step of refining said fused composition passes through a plurality of successively connected refining chambers with a gradual increase of the temperature thereof to a range between 680' and 700 C.

In a first embodiment, the components of the salt melt are preferably selected in the following proportions (percent by mass): MgCI 2 5.0-15.0 NaCI 15.0-40.0 CaCI 2 0.5-15.0 NaBr 0.05-20.0 BaCI 2 0.05-15.0 KC1 the remaining In another embodiment, the furnace preferably used in the step of fusing is a crucible furnace and the plurality of interconnected refining chambers consists of between 2 and 4 successively connected refining chambers.

Unless the context clearly requires otherwise, throughout the description and the claims, the words 'comprise', 'comprising', and the like are to be construed in an inclusive sense as opposed to an exclusive or exhaustive sense; that is to say, in the sense of "including, but not limited to".

Description of the Preferred Embodiment The method of production of magnesium alloys of the invention includes the step of fusion of crude magnesium with alloy-forming components in a crucible furnace above a layer of protective flux or an atmosphere of protective gas accompanied by the subsequent step of refining. The step of refining is carried out over a layer of salt melt in a furnace utilizing a continuous refining process. An alloy having the temperature in the range between 650 and 6900 C is supplied to the step of refining. During this step, the alloy passes through between 2 and 4 refining chambers of a continuous refining furnace. The refining chambers are successively connected to each other. The temperature of the alloy is gradually increased until reaching the 6800 700C range. The ingredients of the utilized salt melt 5 are as follows (in percent of mass): MgC12 between 5.0 and 15.0; NaCI between 15.0 and 40.0; CaC1 2 between 0.5 and 15.0; NaBr between 0.05 and 20.0, BaC1 2 between 0.05 and 15.0; and KC1 the remaining portion.

The production of different grades of magnesium alloys is carried out by varying characteristics of the process.

The production temperature varies between 7000 C and 7800 C.

However, the step of refining of all alloys is carried out at the temperatures approaching the temperature of alloy casting which is within the range between 680 and 7000C.

The main harmful impurities that are removed during the refining step are: oxides, nitrides and excess iron. Such impurities are typically introduced into the alloy during the step of fusing. The impurities from flux inclusions are also avoided.

Purification of the produced magnesium alloy from the excess iron occurs during settling and cooling of the metal in the crucible. After that, the magnesium alloy having the temperature between 650 and 6900 C is supplied to the furnaces for continuous refining. The refining step proceeds continuously in the hermetically sealed furnace, over the layer of the melt of halogenides of alkali, alkaliearth metals and magnesium. During the refining process any contact of magnesium alloy with the atmosphere is excluded.

6 This ultimately results in the reduction of melting losses of the metal and the metal losses due to oxidation. Upon passing through the plurality of refining chambers, the alloy is gradually cleansed of harmful impurities. The chemical composition of the alloy is blended and the temperature thereof reaches the level required for casting.

The inert gaseous are utilized to protect the magnesium alloy in the furnace from oxidation and the inclusion of non-metallic fragments. Although, the refining step can be carried out in the furnace having any reasonable number of refining chambers, in the preferred embodiment of the invention, the furnace having 2 4 of successively connected refining chambers is utilized.

The composition of the salt melt of this invention is significant as it provides the requisite differential in density between that of the salt melt and that of the refined alloy. Additionally, the composition of the salt melt provides the required level of viscosity and of electrical conductivity. The composition of the salt melt also optimizes the heat transfer characteristic of the salt melt. The basic ingredients of the salt melt are the chlorides of sodium, potassium and magnesium. To maintain the desired density differential calcium chloride, barium 7 chloride and sodium bromide are utilized as weighting components.

The method of the invention is carried out in the following manner: Crude magnesium is supplied to the crucible of the furnace. Then, alloy forming components are introduced into the melt. Melting of such components occurs under the layer of covering flux or in the atmosphere of protective gas at the temperature between 7200 and 750 0

C.

During the step of settling, the alloy is cooled down to the temperature between 650 and 6900 C and is transferred into the receiving chamber of the continuous refining furnace.

The salt melt is situated at a lower portion of this furnace and contains halogenides of alkali metals and halogenides of alkali-earth metals as well as magnesium. In the refining process, the salt melt acts as a heat transfer medium. Salt melt formulations are derived for each alloy to ensure that the respective salt melt has the required level of consistency, viscosity, homogeneity and the ability to absorb impurities and magnesium oxide. When the refined alloy passes successively through the multiple chambers of the furnace of continuous refining, the refined alloy separates from and is freed of oxides, nitrides and excess iron impurities. As the alloy moves through the continuous refining furnace, the temperature of the alloy is gradually.

8 increased to the range between 680 and 700 0 C. This is the required temperature level for the production of highquality alloy castings.

Example 10 kg of crude magnesium and alloy forming components, such as for example: 0.85 kg of Al; 0.22 kg of Zn and 0.17 kg of Mn are introduced into the crucible of the laboratory furnace. Then, these ingredients become molten at the temperature of about 7500 C under the layer of covering flux, (grade VI-2). The produced alloy is permitted to settle and cool down to a temperature of approximately 670± 20 0 C. Then the alloy is placed in a laboratory furnace having a salt heating arrangement and containing four refining chambers. The salt melt contained the following components (percent by mass): MgCl 2 12.0; NaCl 37.0; CaC12 3.0; NaBr 10.0; BaCl 2 12.0; KC1 25.0. Argon gas is supplied under the cover of the furnace. During passage of the alloy through the refining chambers its temperature is increased up to 690t10° C. The duration of the refining process is approximately 45 minutes. Small slabs are produced from the refined alloy. The results of the analysis of the slabs are presented in the accompanying Table 1. Slime is removed from the last chamber and the 9 loss of magnesium alloy entrapped in the slime is calculated.

As shown in the Table i, the production of magnesium alloys by the method of this invention is improved by reducing the content of entrained gases and by reducing the losses of alloy with the slime. The measurement of gases entrained within the alloys is determined by the analysis of the oxygen and nitrogen content of the alloys. Expressed in parts per million (ppm), the oxygen content is reduced from a range of 70 to 90 ppm to 37 to 40 ppm; and similarly nitrogen content from a range of 50 to 80 ppm to 28 to 31 ppm. The Table also shows that the losses of magnesium alloy with the slime are reduced from the prior art range of from 0.4 to 0.6 percent to the respective losses in the present invention in the range of from 0.260 to 0.292 percent.

The method of the invention consists of the steps of fusing of crude magnesium with the alloy forming components in the crucible furnace above the layer of protective flux or in the atmosphere of protective gas. The produced alloy having the temperature within the range between 650 and 690 o C is supplied to the step of refining. During the step of refining the alloy passes above the layer of salt 10 melt and through the plurality of successively connected refining chambers. The temperature of the alloy is gradually increased to the range between 680 and 700° C.

11 TABLE 1 No.

Experiment.

1.

2.

Content of entrained gases (in ppm) Losses of 02 N2Magnesium 02

N

2 Alloy with Slime at refining by mass) 0.1260 0.262 0.280 0.292 0.263 Prior Art 70-90 7-9050-80 0.4-0.6

Claims (3)

1. A method of production of magnesium alloy comprising: fusing of a crude magnesium with alloy forming components in furnace above a layer of protective flux or in an atmosphere of a protective gas; and refining of the fused composition above a layer of a salt melt in a furnace of continuous refining, wherein said fused composition supplied to the step of refining has a temperature between 6500 and 690' and in said step of refining said fused composition passes through a plurality of successively connected refining chambers with a gradual increase of the temperature thereof to a range between 6800 and 7000 C.

2. The method of claim 1, wherein components of the salt melt are being selected in the following proportions (percent by mass) MgCl 2 5.0-15.0 NaCI

15.0-40.0 CaCl 2 0.5-15.0 NaBr 0.05-20.0 BaCI 2 0.05-15.0 KCI the remaining 3. The method of claim 1 or 2 wherein said furnace used in the step of fusing is a crucible furnace having between 2 and 4 successively connected refining chambers. DATED this 21 st Day of September, 2000 STATE TITANIUM RESEARCH AND DESIGN INSTITUTE Attorney: PAUL G. HARRISON Fellow Institute of Patent and Trade Mark Attorneys of Australia of BALDWIN SHELSTON WATERS