AU2915899A - Magnesium alloying - Google Patents

Description

WO 99/49089 PCT/AU99/00189 - 1 MAGNESIUM ALLOYING FIELD OF THE INVENTION The present invention relates to a method for producing an alloy containing magnesium (Mg) and 5 aluminium (Al) and to alloys produced by the method. It is to be understood that the alloy may contain constituents other than Mg and Al (for example, zinc (Zn)) and, throughout the remainder of this specification, the alloy will be referred to as Mg-Al 10 alloy. The present invention also relates to a master alloy suitable for use in the method for producing Mg-Al alloy. BACKGROUND ART In the production of Mg-Al alloy, manganese (Mn) is 15 routinely added to molten material in an alloying vessel to reduce the level of iron (Fe) impurity in the resulting Mg-Al alloy. The Mn and Fe form an insoluble Fe-Mn containing precipitate which settles to the bottom of the alloying vessel and is thereby removed from the 20 subsequently cast Mg-Al alloy. The Mn has conventionally been added as manganese chloride (MnCl 2 ) salt. Such an approach is problematic because, in addition to reacting with Fe to produce Fe-Mn containing precipitate, the MnC1 2 reacts with molten Mg to form magnesium chloride 25 (MgCl 2 ) which precipitates and thereby effects loss of Mg and also results in the formation of gaseous hydrochloric acid (HC1) when MnCl 2 or MgCl 2 hydrolyse in the presence of atmospheric moisture. US patent no. 5248477 (issued 28 September 1993) 30 teaches a process for producing Mg-Al alloy in which Mn is added to molten Mg or molten Mg alloy as molten elemental Mn or a molten mixture of elemental Mn and Al. The process is thus a liquid-liquid alloying technique. In discussing prior art, US patent no. 5248477 notes 35 that "Mn can be added as elemental Mn or it can be added in the form of a commercially available mixture of metals in particulate or powder form, usually in the form of a briquette, comprising about 75% Mn and about 25% Al" (see WO99/49089 PCT/AU99/00189 - 2 column 3 lines 10-14) but teaches that "the addition of the elemental Mn in solid form has little effect on the reduction of Fe content in the melt" (see column 3 lines 18-20). US patent no. 5248477 goes on to teach that the 5 "reason for the addition of the MnCl 2 as opposed to the addition of elemental Mn either in pure or mixed form is that the effectiveness for Fe precipitation is significantly greater and the Mn alloying efficiency itself is significantly greater as well. It has been 10 observed repeatedly that in primary Mg, the Mn content can be raised to a significantly higher level with MnCl 2 additions than can be achieved with the addition of elemental Mn in the form of electrolytic flake, for example" (see column 6 line 68 - column 7 line 9) and US 15 patent no. 5248477 concludes that "the addition of elemental Mn, in the solid state, to a Mg melt results in inefficient alloying of the Mn and the Fe content is poorly controlled, if at all" (see column 7 lines 52-54). SUMMARY OF THE INVENTION 20 In a first aspect, the present invention provides a method for producing an Mg-Al alloy in an alloying vessel containing molten Mg or molten Mg alloy, the method including the steps of establishing the temperature of the molten Mg or Mg alloy in the range of 650-750 0 C and 25 thereafter adding a solid master alloy containing Al and Mn to the alloying vessel whereby Mn is released for reaction with Fe in the alloying vessel. In a second aspect, the present invention provides Mg-Al alloy prepared by a method according to the first 30 aspect of the present invention. The Al-Mn master alloy is added to molten Mg or molten Mg alloy at temperature in the range 650-750 0 C. Preferably, the temperature is in the range 650-7100, more preferably 680-700 0 C. 35 Some of the Al alloying component and possibly other alloying components (for example, zinc (Zn)) can be added to the alloying vessel separately from the Al-Mn master alloy. Preferably however, all alloying components are WO99/49089 PCT/AU99/00189 - 3 added to the alloying vessel by way of the Al-Mn master alloy. The Mg-Al alloy may be produced by adding the master alloy to molten primary Mg and, where that is the case, 5 the primary Mg may have been produced by electrolysis of anhydrous MgC1 2 in an electrolytic cell. Molten Mg typically leaves the electrolytic cell at a temperature of about 655 0 C. The Mg-Al alloy may be produced by adding the master alloy to molten Mg or Mg alloy derived 10 from recycling of scrap material. Typically, the conventional process where MnCl 2 is used requires heating the Mg or Mg alloy to 730-750 0 C, adding Al and any other alloying components (for example, Zn) over a period of 5-10 minutes, stirring in MnCl 2 over 15 a period of about 20 minutes, and cooling the contents of the alloying vessel to about 670 0 C over a period of about 20 minutes prior to transferring the resulting Mg-Al alloy to a settling furnace. Advantageously, the batch time for producing Mg-Al 20 alloy in accordance with the first aspect of the present invention can be reduced as compared with the conventional MnCl 2 process because the alloying vessel does not need to be heated to 730-7500C, the Al and Mn can be added in a single step, and less time is required 25 for cooling. Reduction of batch time in accordance with the present invention is desirable in a Mg smelter because energy consumption is reduced and capital costs associated with requirement for a plurality of alloying vessels can be reduced. It is also to be noted that in 30 contrast to the liquid-liquid alloying technique of US patent no. 5248477, the requirement for a separate vessel for melting the alloying components and the energy consumption associated therewith are avoided. The present invention is also believed to be 35 advantageous in relation to the chemical composition of Fe-Mn intermetallic compound that is formed. Without wishing to be bound by theory, it is believed that Fe-Mn intermetallic compounds formed at higher temperatures WO99/49089 PCT/AU99/00189 - 4 contain a greater proportion of Fe than those formed at lower temperatures and that corrosion problems are lessened in Mg-Al alloys in which the Fe-Mn intermetallic compounds formed during their production contain lesser 5 proportions of Fe. The Al-Mn master alloy used in the process according to the first aspect of the present invention must be capable of releasing Mn for reaction with Fe when added to the Mg or Mg alloy at a temperature in the range of 10 650-750 0 C. Any Al-Mn master alloy that meets this requirement falls within the scope of the present invention but various properties of the master alloy are preferred. The Al-Mn master alloy may include other alloying 15 components, for example, Zn. Preferably, the master alloy contains a minority of Mn, for example, less than 10 percent by weight Mn. This is to be contrasted with the Mn-Al briquette referred to in US patent no. 5248477 which contained about 75 percent Mn. 20 Preferably, the majority of the Mn in the Al-Mn master alloy is present in the form of an Al-Mn intermetallic compound (for example, Al 6 Mn) with a minority of the Mn present as elemental Mn. The Al-Mn intermetallic compound is preferably in the form of fine 25 needles or thin platelets. The balance of the Al in the master alloy is preferably e-Al. As previously mentioned, in the method according to the first aspect of the present invention, the solid master alloy is added to molten Mg or Mg alloy at temperature in the range of 650 30 710 0 C which is cooler than in prior art techniques. Without wishing to be bound by theory, preferred A1 6 Mn containing solid master alloys are believed to enable lower Mg temperatures to be used because: (1) A1 6 Mn is much more soluble in Mg than 35 elemental Mn, (2) A1 6 Mn is surrounded by C-Al when formed with the result that the surface of A1 6 Mn particles are not coated with an oxide WO99/49089 PCT/AU99/00189 -5 layer which would inhibit dissolution in Mg, and (3) Al 6 Mn melts at about 705 0 C compared to elemental Mn which melts at 1246 0 C. 5 Preferably the Al-Mn master alloy has a nickel (Ni) content less than 30ppm and a copper (Cu) content less than 50ppm. Preferably, the master alloy is produced by cooling a molten master alloy precursor, for example, by quench casting. 10 The Al-Mn master alloy may be of granular form. Granules of the Al-Mn master alloy may be produced using a water-cooled wheel in a manner analogous to that used for producing aluminium granules. The use of granular Al-Mn master alloy enables the master alloy to be added 15 to the alloying vessel from an overhead hopper under gravity. Preferably, the hopper is mounted on load cells or the like and has a gate enabling a predetermined mass of Al-Mn master alloy to be added to the alloying vessel. Advantageously, such an arrangement enables the 20 requirement to open and close a lid of the alloying vessel to make additions to be avoided with consequential reduction in the level of operator involvement required. Preferably, the hopper is heated to drive any moisture off granular Al-Mn master alloy which obviates any 25 requirement for an alloying pre-heat furnace. The master alloy may be in the form of cast ingots in which case the master alloy is preferably added to the alloying vessel from an overhead conveyor. The conveyor is preferably arranged to be heated to drive moisture 30 from the master alloy and the ingots are preferably of a consistent mass whereby the mass of master alloy added to the alloying vessel is controllable by addition of a pre determined number of the ingots. In a third aspect, the present invention provides an 35 Al-Mn master alloy containing less than 10% by weight Mn, wherein a majority of the Mn is in the form of an Al-Mn intermetallic compound and a minority of the Mn is elemental Mn. The master alloy is suitable for use as WO99/49089 PCT/AU99/00189 -6 the master alloy in the process according to the first aspect of the present invention. EXAMPLES The ensuing examples are illustrative of embodiments 5 of the present invention and should not be construed as limiting the scope of the present invention in any way. Example 1 - Preparation of Solid Al-Mn Master Alloy Solid master alloy was prepared in two stages. In the first stage master alloy precursor was 10 prepared by addition of a source of Mn to molten Al at 800 0 C to produce Al-Mn alloy containing 5.5% by weight Mn. Three sources of Mn were used, namely (a) Mn-Al splatter containing 60% by weight Mn, (b) Mn-Al Altabs containing 75% by weight Mn, and (c) Mn coarse injection 15 powder. In all cases the master alloy precursor was prepared within 10 minutes of addition of the source of Mn by moderate stirring. In the second stage master alloy precursors from the first stage were re-melted and stirred at 800 0 C for one 20 hour and then cooled at a variety of cooling rates to achieve a temperature of 500 0 C in times ranging from 40 to 400 seconds. In all cases the resulting Al-Mn master alloy was largely a mixture of acicular Al 6 Mn intermetallic phase and t-Al although the size of the 25 Al 6 Mn intermetallic phase was larger at slower cooling rates. Example 2 - Preparation of Mq-Al Alloy Mg-Al alloys (AM60) were prepared by addition of solid Al-Mn master alloys from Example 1 to molten 30 primary Mg at 680 0 C and 700 0 C. Only 5-10 minutes was required to achieve the ASTM minimum Mn level of 0.26% in the AM60 alloy at both 680 0 C and 700 0 C. After two hours of settling and a reduction in temperature of the melt to 660 0 C, the Fe level was reduced from approximately 300ppm 35 to less than 40ppm and Mn recovery was greater than 80%.

WO99/49089 PCT/AU99/00189 - 7 The ensuing examples are not in accordance with the present invention and are provided for comparative purposes only. Comparative Example 1 - Preparation of Mq-Al Alloy 5 Example 2 was repeated using Mn-Al splatter containing 60% by weight Mn in lieu of solid Al-Mn master alloys from Example 1. Mn from the Mn-Al splatter could not be readily released into the primary Mg at temperatures below 710 0 C. Even at 730 0 C with 20 minutes 10 of stirring, a Mn level of only 0.20% was achieved at a Mn recovery of less than 50%. Comparative Example 2 - Preparation of Mq-Al Alloy Example 2 was repeated using Mn-Al Altabs containing 75% by weight Mn in lieu of solid Al-Mn master alloys 15 from Example 1. Mn from the Mn-Al Altabs could not be readily released into the primary Mg at temperatures below 710 0 C. Stirring for 20 minutes at 730 0 C was required to achieve the ASTM minimum Mn level with a Mn recovery of less than 80%.

Claims (11)

1. A method for producing an Mg-Al alloy in an alloying vessel containing molten Mg or molten Mg alloy, the method including the steps of establishing the 5 temperature of the molten Mg or Mg alloy in the range of 650-750 0 C and thereafter adding a solid master alloy containing Al and Mn to the alloying vessel whereby Mn is released for reaction with Fe in the alloying vessel. 10

2. A method as claimed in claim 1 wherein the Mg or Mg alloy is established at a temperature in the range of 650-710 0 C prior to addition of the solid master alloy.

3. A method as claimed in claim 1 wherein the Mg or Mg 15 alloy is established at a temperature in the range of 680-700 0 C prior to addition of the solid master alloy.

4. A method as claimed in any one of the preceding claims wherein the master alloy contains a minority 20 of Mn.

5. A method as claimed in claim 4 wherein the master alloy contains less than 10% by weight Mn.

6. A method as claimed in any one of the preceding claims wherein a majority of the Mn in the master 25 alloy is in the form of an Al-Mn intermetallic compound and a minority of the Mn in the master alloy is elemental Mn.

7. A method as claimed in any one of the preceding claims wherein a majority of elemental Al in the 30 master alloy is X-Al.

8. A method as claimed in any one of the preceding claims wherein the master alloy contains less than 30ppm Ni and less than 50ppm Cu.

9. An Mg-Al alloy prepared by a method as claimed in 35 any one of the preceding claims. WO99/49089 PCT/AU99/00189 -9

10. An Al-Mn master alloy containing less than 10% by weight Mn wherein a majority of the Mn is in the form of an Al-Mn intermetallic compound and a minority of the Mn is elemental Mn. 5

11. A master alloy as claimed in claim 10 containing one or more alloying components in addition to Al and Mn.