Abstract:
Magnesium alloys are heated to a molten state in preparation for hot-working thereof, for example, die-casting. The presence of inclusions within the magnesium alloys results in metallurgical defects therein and will adversely affect the quality of the castings produced from the magnesium alloys. An embodiment of the invention processes the magnesium melt containing impurities through a non-reactive filter under an environment filled with protective gas for substantially purifying the magnesium melt and to reduce the presence of inclusions in castings formed therefrom.

Description:
FIELD OF INVENTION  
       [0001]     The present invention relates generally to a filtration system for purifying magnesium. Specifically, the present invention relates to a filtration system for recycling and purification of scrap magnesium and magnesium ingots with impurities.  
       BACKGROUND  
       [0002]     Magnesium alloys are heated to a molten state in preparation for hot-working thereof. Molten magnesium alloys easily oxidise and react with impurities, especially when scrap magnesium alloys are reused. As a result, magnesium alloys are contaminated by non-metallic and metallic inclusions, for example oxides or intermetallic compounds, when melted.  
         [0003]     The magnesium alloys are typically melted for producing castings. The presence of inclusions within the magnesium alloys results in metallurgical defects therein and will adversely affect the quality of the castings produced from the magnesium alloys.  
         [0004]     A known method for removing the impurities is to send the magnesium alloys to a smelter for smelting. However, smelting is a costly process. Another known process uses impediment plates disposed within a furnace for removing top and bottom sludge from magnesium melts. However, the impediment plates do not remove inclusions suspended in the magnesium melts.  
         [0005]     Hence, this clearly affirms a need for a filtration system for purifying magnesium melts.  
       SUMMARY  
       [0006]     In accordance with a first aspect of the invention, there is disclosed a filtration system for magnesium recycling and purification, the filtration system comprising: 
        a first chamber for containing magnesium melt, the magnesium melt containing impurities; and     a second chamber for receiving purified magnesium melt,     wherein said first and second chambers have disposed therebetween a filter, the filter being a silicon-free medium.        
 
         [0010]     In accordance with a second aspect of the invention, there is disclosed a filtration method for magnesium recycling and purification, comprising the steps of: 
        receiving magnesium melt into a first chamber, the magnesium melt containing impurities within the first chamber containing impurities;     providing a second chamber, the second chamber being in fluid communication with the first chamber; and     substantially removing the impurities from the magnesium melt flowing from the first chamber into the second chamber using a filter, the filter being disposed between the first chamber and the second chamber and the filter including a silicon-free medium.        
 
         [0014]     In accordance to a third aspect of the invention, there is disclosed a filtration method for magnesium recycling and purification, comprising the steps of: 
        receiving magnesium into a first chamber, the magnesium within the first chamber containing impurities and the magnesium being one of a magnesium melt or solid magnesium ingot;     providing a second chamber, the second chamber being in fluid communication with the first chamber;     heating the magnesium contained in the first chamber by a heating apparatus for melting the magnesium and for maintaining the magnesium melt in a molten state, and the magnesium melt in the first chamber thereby flowing into the second chamber; and     substantially removing the impurities from the magnesium melt flowing from the first chamber into the second chamber using a filter, the filter being disposed between the first chamber and the second chamber and the filter including a silicon-free medium.       
 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0019]     Embodiments of the invention are described hereinafter with reference to the following drawings, in which:  
         [0020]      FIG. 1  shows a partial front sectional view of a filtration system according to an embodiment of the invention;  
         [0021]      FIG. 2  shows a partial side sectional view of the filtration system of  FIG. 1 ;  
         [0022]      FIG. 3  is an illustration of a filter of the filtration system of  FIG. 1 ;  
         [0023]      FIG. 4  is an illustration of a first chamber and the filter of  FIG. 3 ;  
         [0024]      FIG. 5   a  shows a low magnification light optical microscope (LOM) micrograph of a tensile specimen made from magnesium melt obtained from the filtration system of  FIG. 1 ;  
         [0025]      FIG. 5   b  shows the LOM micrograph of  FIG. 5   a  under high magnification;  
         [0026]      FIG. 6   a  shows a low magnification light optical microscope (LOM) micrograph of a mobile phone case specimen made from magnesium melt obtained from the filtration system of  FIG. 1 ;  
         [0027]      FIG. 6   b  shows the LOM micrograph of  FIG. 6   b  under high magnification;  
         [0028]      FIG. 7   a  shows a graph plotting the tensile strength (ultimate tensile strength and yield strength) of test samples as a function of cross-head speed with the test samples being cast from a magnesium melt purified using the filtration system of  FIG. 1 ; and  
         [0029]      FIG. 7   b  shows a graph plotting the percentage elongation of the test samples of  FIG. 7   b , as a function of cross-head speed. 
     
    
     DETAILED DESCRIPTION  
       [0030]     An embodiment of the invention, a filtration system  20  is described with reference to  FIG. 1 , which shows a partial front sectional view of the filtration system  20  and  FIG. 2  which shows a partial side sectional view of the filtration system  20 . The filtration system  20  is for use in substantially purifying magnesium.  
         [0031]     As shown in  FIG. 1 , the filtration system  20  includes a crucible  22  being divided into two parts by a filter adapter  24  disposed therein, the two parts of the crucible being namely a first chamber  26  and a second chamber  28 . The first chamber  26  is in fluid communication with the second chamber  28  through an opening  30 , as shown in  FIG. 2 , in the filter adapter  24  that forms a passageway therebetween.  
         [0032]     As shown in  FIG. 2 , the filter adapter  24  is for receiving a filter  32  therewithin and for removably engaging thereto.  FIG. 3  is an illustration of the filter  32  and  FIG. 4  is an illustration of the first chamber  26  and the filter  32 . When the filter  32  is engaged to the filter adapter  24 , the filter  32  blocks the opening  30  of the filter adapter  24 , thereby intersecting the passageway between the first chamber  26  and the second chamber  28 , as shown in  FIGS. 1, 2  and  5 . The filter adapter  24  is preferably a steel structure that shaped and dimensioned for holding the filter  32  at the periphery thereof.  
         [0033]     The filtration system  20  further includes a heating apparatus coupled to the crucible  22 . The heating apparatus is preferably integrated with the crucible  22  for providing heat to the crucible  22  and its contents. The heating apparatus is electrically connected to a controller (all not shown).  
         [0034]     The filtration system  20  is for providing substantially purified magnesium melts to downstream systems or machineries, for example, a die-casting machine. For obtaining purified magnesium melts from the filtration system  20 , magnesium ingots or scraps are provided to the first chamber  26  of the filtration system  20 . The controller activates the heating apparatus to provide heat to the crucible  22  and the magnesium therein, thereby melting the magnesium.  
         [0035]     Alternatively, magnesium melt can be provided to the first chamber  26  of the crucible  22 . The heating apparatus provides heat to the crucible  22  to maintain the magnesium melt in its molten state and to melt the magnesium scraps and ingots added to the magnesium melt thereafter.  
         [0036]     The filter  32  of the filtration system  20  is preferably made of a silicon-free material. Conventional filters, for example a filter for aluminium alloys, are made of silicon-based materials. The silicon-based materials readily react with magnesium to cause contamination therein and are therefore undesirable. The filter  32  is made of one of steels or ceramic material which comprises of one or more material selected from a group consisting of Al 2 O 3 , MgO, AlPO 4  and Mg 3 (PO 4 ) 2 .  
         [0037]     The filter  32  comprises of an array of apertures (not shown). Each of the apertures is shaped and dimensioned for preventing the passage of a particle having a size greater than 5 microns therethrough. Preferably, each pair of adjacent apertures are spaced apart a distance of 5 to 250 microns. The magnesium melt  38  in the first chamber  26  passes through the filter  32  and into the second chamber  28  of the crucible  22 . Therefore, the impurities suspended in the magnesium melt  38  contained in the first chamber  26  is substantially removed by the filter  32  before entering the second chamber  28  as purified magnesium melt  40 . The magnesium melt  38  contained in the first chamber  26  contains bottom sludge that has settled at the bottom of the first chamber  26 . In most situations, top sludge can also be found floating at the surface of the magnesium melt  38  contained in the first chamber  26 . The filter adapter  24  functions to substantially impede the top sludge and bottom sludge in the first chamber  26  from entering the second chamber  28  (all not shown).  
         [0038]     With reference to  FIG. 1 , the magnesium melt  38  in the first chamber  26  is drawn into the second chamber  28  by hydrostatic forces acting on the magnesium melt  38 . The magnesium melt  38  in the first chamber  26  continues to be drawn into the second chamber until the hydrostatic pressures of magnesium melt  38  in the first chamber  26  and the purified magnesium melt  40  in the second chamber  28  are in equilibrium.  
         [0039]     Preferably, each of the first chamber  26  and the second chamber  28  has a thermocouple  42  disposed therewithin. Both the thermocouples  42  are electrically connected to the controller for transducing temperature of the magnesium melt  38  in the first chamber  26  into first temperature signals (not shown) and the temperature of the purified magnesium melt  40  in the second chamber  40  into second temperature signals (not shown). The first and second temperature signals are transmitted to the controller. From the first and second temperature signals, the controller uses a control function (not shown) to determine and control the heat output of the heating apparatus, thereby maintaining the magnesium melt  38  and the purified magnesium melt  40  in a molten state and to prevent overheating thereof. In the molten state, the viscosities of both the magnesium melt  38  and the purified magnesium melt  40  are greatly reduced, thereby facilitating flow thereof through the filter  32 .  
         [0040]     The crucible  22  is preferably enclosed for receiving and retaining protective gas therein. A gas feed system (not shown) is connected to the crucible for supplying the protective gas thereinto. The protective gas prevents both the magnesium melt  38  and the purified magnesium melt  40  from reacting with the atmosphere by forming a screen therebetween.  
         [0041]     An extractor  44 , as shown in  FIG. 1 , extends from within the second chamber  28  to a die-casting assembly  46 . The extractor  44  is for extracting the purified magnesium melt  40  from the second chamber  28  and providing the purified magnesium melt  40  to the die-casting assembly  46 . The extractor  44  shown in  FIG. 1  uses a piston and a goose-neck chamber assembly for extracting the purified magnesium melt  40 .  
         [0042]     Extracting the purified magnesium melt  40  from the second chamber  28  reduces the level of the purified magnesium melt  40  contained therein. The reduction of the level of the purified magnesium melt  40  in the second chamber  28  further draws the magnesium melt  38  from the first chamber  26  and into the second chamber  28 . Magnesium melt and magnesium ingots or scraps can be further provided to the first chamber  26  for replenishing the second chamber  28  and thereby the filtration system  20  with purified magnesium melt  40 .  
         [0043]     The purified magnesium melt  40  supplied from the filtration system  20  to the die-casting assembly  46  provides the die-casting assembly with a substantially inclusion-free purified magnesium melt  40  supply for use in a die-casting process.  
         [0044]     Two types of parts, hand-phone case and tensile test specimens, were cast during tests. The alloy used in the tests was AZ91 HP having a composition of Al 8-9.5%, Zn 0.3-1.0%, Mn≧0.17%, Si≦0.05%, Fe≦0.004%, Cu≦0.015%, Ni≦0.01%, others ≦0.01%, others ≦0.01%, Mg (remaining). For comparison the tests were conducted using 100% fresh ingot and ingot including 10% scraps material.  
         [0045]     After casting, the microstructure and chemical properties of the specimens were analyzed. The microstructure analysis was conducted with light optical microscopy (LOM) while mechanical properties were determined using an Instron tensile testing machine. The cross-head speed was varied from 0.1 to 20 mm/min during tensile tests.  
         [0046]     Typical microstructures for the tensile test specimen, having a diameter of 10 mm-thick walled part and a mobile phone case, having a wall thickness of 0.6 mm-thin wall part. In both thick and thin walled parts, the microstructures consist mainly of α-Mg, intermetallic-Al 2 Mg 17 , eutectic composition and some fine precipitate. However, the thin walled parts showed much finer structure, as shown in  FIGS. 7   a  and  7   b , when compared to the thick walled parts shown in  FIGS. 6   a  and  6   b . The reduced α-Mg grain size is due to rapid solidification rate occurred in the thin walled part.  
         [0047]     Mechanical properties of cast samples were determined after casting.  FIGS. 8   a  and  8   b  show the tensile strength, comprising the ultimate tensile strength (ITS) and yield strength (YS) and elongation as a function of the cross-head speed. As the cross-head speed is directly related to the strain rate of the testing, the results obtained indicated that the strain rate has a very slight influence on the flow stress and strain of magnesium castings when tested at room temperature. The UTS and YS are about 142 and 119 MPa respectively with an elongation of about 1%.  
         [0048]     A comparison was made between the mechanical properties of castings made with filtered magnesium (with about 10% scraps) and castings made with fresh magnesium (100% new ingot). The strength of the castings is increased when filtered magnesium is used as compared to when 100% new magnesium ingot is used. However, the ductility of the magnesium alloys is reduced. The decreased ductility is typically due to the presence of more intermetallic compounds and less magnesium in the alloy having scrap parts therein although the alloy has already been filtered. Therefore, the composition of the alloy should be adjusted when using scrap parts.  
         [0049]     In the foregoing manner, a filtration system is described according to an embodiment of the invention for addressing the foregoing disadvantages of conventional filtration devices. Although only one embodiment of the invention is disclosed, it will be apparent to one skilled in the art in view of this disclosure that numerous changes and/or modification can be made without departing from the scope and spirit of the invention.