Patent Publication Number: US-2023145566-A1

Title: Method of eliminating microstructure inheritance of hypereutectic aluminum-silicon alloys

Description:
INTRODUCTION 
     The present disclosure relates to methods of processing casting aluminum alloys, more particularly to a method of eliminating microstructure inheritance of hypereutectic aluminum-silicon alloys. 
     Hypereutectic aluminum-silicon (Al—Si) alloys are widely used in the automobile industries due to their low density, excellent wear and corrosion resistance, low coefficient of thermal expansion, good strength, and excellent castability. They are used in applications that typically require a combination of light weight and high wear resistance, including, but not limited to engine blocks, pistons, transmission casings, and transmission clutch housings. The performance of Al—Si alloys depends on the microstructure inheritance of these alloys. Hypereutectic Al—Si alloys having uniform distribution of fine silicon particles have higher strength and better wear resistance. 
     Typical Al—Si alloys used for the casting of transmission clutch housings include a B390 Al—Si alloy. B390 Al—Si alloy used for casting has a microstructure inheritance of relatively large Si particles that can lead to coarse primary Si particles in the completed cast workpieces. Coarse primary SI particles may significantly reduce the alloy ductility of the transmission clutch housing. 
     Thus, while Al—Si alloys used for casting transmission clutch housings achieve their intended purpose, there is a need for a method of eliminating microstructure inheritance of relatively large primary Si particles to improve the strength of the transmission clutch housings. 
     SUMMARY 
     According to several aspects, a method of eliminating microstructure inheritance in a hypereutectic aluminum-silicon (Al—Si) alloy is disclosed. The method includes, heating a first amount of the Al—Si alloy to a predetermined temperature above a liquidus temperature of the Al—Si alloy to form a first amount melt; holding the first amount melt at the predetermined temperature for a first predetermined amount of time; and stirring the first amount melt for a second predetermined amount of time. The first predetermined amount of time is between 0.1 hour to 0.5 hour. The second predetermined amount of time may be equal to or less than the first predetermined amount of time. 
     In an additional aspect of the present disclosure, the method further includes heating a second amount of the Al—Si alloy above the liquidus temperature of the Al—Si alloy to form a second amount melt, and mixing a stirred first amount melt with the second amount melt to form a processed Al—Si alloy. 
     In another aspect of the present disclosure, the predetermined temperature is greater than 800° C., preferably between about 750° C. to about 850° C., and more preferably between about 790° C. to about 810° C. 
     In another aspect of the present disclosure, wherein stirring the first amount melt includes contact-less magnetic stirring. 
     In another aspect of the present disclosure, wherein the processed Al—Si alloy includes a first amount melt from about 25 weight percent (wt %) to about 50 w %. 
     According to several aspects, a method of casting a workpiece is disclosed. The method includes heating an Al—Si alloy to a processing temperature between about 750° C. to about 850° C. to form an Al—Si alloy melt; maintaining the Al—Si alloy melt at the processing temperature for a processing time between about 0.1 hour to about 0.5 hour to form a processed Al—Si alloy melt; and pouring the processed Al—Si alloy melt into a mold cavity defining the workpiece. 
     In an additional aspect of the present disclosure, mixing a non-processed Al—Si alloy melt to the processed Al—Si alloy melt to form a casting alloy mixture; and pouring the casting alloy mixture into the mold cavity defining the workpiece. 
     In another aspect of the present disclosure, the method further includes stirring the Al—Si alloy melt at the processing temperature for the processing time between about 0.1 hour to about 0.5 hour to form the processed Al—Si alloy melt 
     In another aspect of the present disclosure, the casting alloy mixture includes about 30 weight percent (wt %) to about 40 wt % of the processed Al—Si alloy melt, preferably 35 wt %. 
     According to several aspects, a method of processing a hypereutectic aluminum-silicon (Al—Si) alloy for casting. The method includes heating an Al—Si alloy to form an Al—Si alloy melt; stirring a first portion of the Al—Si alloy melt for a predetermined time at a predetermined temperature to form a processed Al—Si alloy melt; and mixing a second portion of the Al—Si alloy melt to the processed Al—Si alloy melt to form a processed Al—Si casting alloy. The predetermined time is from about 0.1 hour to about 0.5 hour. The predetermined temperature is from about 750° C. to about 850° C. The processed Al—Si casting alloy comprises about 30 weight percent (wt %) to about 40 wt % of the processed Al—Si alloy melt. Stirring the first portion of the Al—Si alloy melt includes contact-less magnetic stirring. 
     Further areas of applicability will become apparent from the description provided herein. It should be understood that the description and specific examples are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The drawings described herein are for illustration purposes only and are not intended to limit the scope of the present disclosure in any way. 
         FIG.  1    is a micrograph of a hypereutectic aluminum-silicon alloy before processing with a method of eliminating microstructure inheritance; 
         FIG.  2    is micrograph of the hypereutectic aluminum-silicon after processing with the method of eliminating microstructure inheritance, according to an exemplary embodiment; 
         FIG.  3    is a graph showing a frequency distribution of Si particle sizes in the hypereutectic aluminum-silicon alloy of  FIG.  2   , according to an exemplary embodiment; 
         FIG.  4    a graph showing a frequency distribution of roundness of Si particles in the hypereutectic aluminum-silicon alloy of  FIG.  2   , according to an exemplary embodiment; 
         FIG.  5    is a block flow diagram of the method of eliminating microstructure inheritance of hypereutectic aluminum-silicon alloys, according to an exemplary embodiment; 
         FIG.  6 A  is a side view of an exemplary automotive component cast from a hypereutectic aluminum-silicon processed by the method of  FIG.  5   , according to an exemplary embodiment; 
         FIG.  6 B  is a perspective top view of the exemplary automotive component of  FIG.  6 A , according to an exemplary embodiment; and 
         FIG.  7    is a diagrammatic cross-section of a contact-less magnetic stirring apparatus  700  configured to facilitate the method of  FIG.  5   , according to an exemplary embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     The following description is merely exemplary in nature and is not intended to limit the present disclosure, application, or uses. The illustrated embodiments are disclosed with reference to the drawings, wherein like numerals indicate corresponding parts throughout the several drawings. The figures are not necessarily to scale and some features may be exaggerated or minimized to show details of particular features. The specific structural and functional details disclosed are not intended to be interpreted as limiting, but as a representative basis for teaching one skilled in the art as to how to practice the disclosed concepts. 
       FIG.  1    shows a micrograph  100  of a B390 hypereutectic aluminum-silicon alloy casting, also referred to as a B390 Al—Si alloy, before processing with a method of eliminating microstructure inheritance, which is disclosed in detail below. The micrograph  100  shows a microstructure of the B390 Al—Si alloy having an Aluminum (Al) matrix  102  surrounding primary Silicon (Si) particles  104 , eutectic Si particles  106 , α-Fe(Al 15 (Fe,Mn) 3 Si 2    108 , and β-Fe(Al 5 FeSi)  110 . The Fe(Al 15 (Fe,Mn)3Si 2    108 , and β-Fe(Al 5 FeSi)  110  are also referred to as Alpha Phase  108  and Beta Phase  110 , respectively. 
       FIG.  2    shows a micrograph  200  of the B390 Al—Si alloy casting after processing with the method of eliminating microstructure inheritance. The micrograph  200  shows a microstructure of the processed B390 Al—Si alloy having an Al matrix  202  surrounding primary Si particles  204 , eutectic Si particles  206 , α-Fe(Al 15 (Fe,Mn) 3 Si 2    208 , and β-Fe(Al 5 FeSi)  210 . The Fe(Al 15 (Fe,Mn)3Si 2    208 , and β-Fe(Al 5 FeSi)  210  are also referred to as the Alpha iphase  208  and Beta phase  210 , respectively. 
     Referring to  FIG.  1   , the relatively large primary Si particles of micrograph  100 , as compared to micrograph  200 , can lead to coarse primary Si particles and eutectic Si particles in a completed cast workpiece. Coarse primary SI particles may significantly reduce the alloy ductility of cast workpiece. The Alpha phase  108  is plate-like shape in 3D and needle shape in 2D. The alpha phase is very brittle and easy crack, which significantly reduce material properties such as ductility and fatigue performance. Referring to  FIG.  2   , the smaller and more uniformly dispersed primary Si particles  204  and eutectic Si particles enable the completed cast workpiece to have higher strength and better wear resistance. 
       FIG.  3    is a graph showing a frequency distribution of primary Si particle sizes in the processed B390 Al—Si alloy. The primary Si particles in the processed B390 Al—Si alloy have a smaller diameter than the non-processed B390 Al—Si alloy. The nominal size of primary Si particle in the non-processed alloy is around 40-80 microns. The processed B390 Al—Si alloy has a large frequency (%) of Si particles having diameters ranging from 5 to 20 microns, particularly between 10 to 15 microns.  FIG.  4    is a graph showing a frequency distribution of roundness of the Si particles in the processed B390 Al—Si alloy. The processed Al—Si alloy has a large frequency (%) of roundness ranging between 1 to 5, particularly between 2 to 3. The roundness is represented with an aspect ratio of an Si particle. A perfectly round particle is represented by a roundness of 1, which is unitless. 
       FIG.  5    shows a block flow diagram of a method of eliminating microstructure inheritance of hypereutectic aluminum-silicon alloys (Method  500 ). The method  500  begins in Block  502 , where a first amount of an Al—Si alloy is heated to a predetermined temperature above a liquidus temperature of the Al—Si alloy to transform the Al—Si alloy into a liquid state. The liquidus temperature of B390 Al—Si alloy is approximately 600° C. The Al—Si alloy in liquid state is referred to as an Al—Si alloy melt. It is preferable that the predetermined temperature is between 750° C. to 850° C., preferably 790° C. to 810° C., and more preferably 800° C. 
     Moving to Block  504 , the Al—Si alloy melt is held at the predetermined temperature for a predetermined amount of time, preferably between 0.1 hour to 0.5 hour. Within the predetermined amount of time, the Al—Si alloy melt is continuously agitated by stirring to break down the short-range element clusters and segregation of the Si particles, which includes primary Si particle and eutectic Si particle element clusters. The Si particles are broken down to have diameters ranging from 5 to 20 microns, preferably between 10 to 15 microns, and have roundness ranging from 1 to 5, preferably between 2 to 3. The Al—Si alloy melt may be stirred by one or more of: a mechanical stirring, an ultrasonic stirring, a magnetic stirring, and a contact-less magnetic stirring, to break down the short-range element clusters and segregation of the Si particles. Contact-less magnetic stirring means the Al—Si alloy melt is stirred using a rotating magnetic field acting on the iron (Fe) in the alloy to stir the mixture without the use of a traditional magnetic stir bar disposed in the Al—Si alloy melt. An exemplary contact-less magnetic stirring apparatus is shown in  FIG.  7    and disclosed in detail below. 
     Moving to Block  506 , a second amount of the Al—Si alloy is heated above the liquidus temperature of the Al—Si alloy to form a second amount Al—Si alloy melt. The second amount of Al—Si alloy melt is not processed by a mechanical stirring, an ultrasonic stirring, a magnetic stirring, or a contact-less magnetic stirring to break down the short-range element clusters and segregation of the Si particles. The non-processed second amount of Al—Si alloy melt is blended with the processed first amount of Al—Si alloy melt to form an Al—Si casting alloy mixture. It is preferable that the weight percentage of the first amount of the Al—Si alloy melt in the Al—Si casting alloy cast mixture is between 25 to 50 weight percent (wt %), preferable between 30 to 40 wt %, and more preferably 35 wt %. 
     Moving to Block  508 , the molten Al—Si casting alloy mixture is poured or injected in to casting mold having a predefined form factor defining an automotive work piece, such as a transmission clutch housing. The molten Al—Si casting alloy mixture is cooled and solidified to form the automotive workpiece. 
     Shown in  FIG.  6 A  is a sideview of an exemplary cast workpiece  600 , which is a cast clutch housing for a transmission of a motor vehicle. Shown in  FIG.  6 B  is a perspective top view of the exemplary cast workpiece  600  of  FIG.  6 A . While a cast transmission clutch housing is shown as an exemplary cast workpiece, it should be appreciated that the workpiece may include any automotive or non-automotive cast component that requires excellent wear and ductility properties. 
       FIG.  7    is a diagrammatic cross-section of contact-less magnetic stirring apparatus  700  configured to facilitate the method of  FIG.  5   . The apparatus  700  includes an insulated crucible  702  configured to contain a molten alloy  704 , a heating element  706  to melt and maintain the molten alloy  704  at a predetermined temperature, and a plurality of magnets  708  configured to generate a rotating magnetic field sufficient to magnetically stir the molten alloy  704  within the crucible  702  about a center axis-A. The magnets  708  may be that of permanent magnets fixed to a rotating platform  710  or electric magnets configured to generate a rotating magnetic field. 
     Method  500  may be applied to B390 hypereutectic aluminum alloy as well as to 392, 393 hypereutectic aluminum alloys, and to near-eutetic alloys such as 336, 339, 360, 369, 383, 384, A356, A357, etc. alloys. Method 500 may be applied to other metallic alloy systems such as hypoeutectic or hypereutectic Mg alloys, and to any alloys with formation of secondary phase particles in the microstructure during solidification. 
     Numerical data have been presented herein in a range format. The term “about” as used herein is known by those skilled in the art. Alternatively, the term “about” includes +/−0.05% by weight”. It is to be understood that this range format is used merely for convenience and brevity and should be interpreted flexibly to include not only the numerical values explicitly recited as the limits of the range, but also to include all the individual numerical values or sub-ranges encompassed within that range as if each numerical value and sub-range is explicitly recited. While examples have been described in detail, those familiar with the art to which this disclosure relates will recognize various alternative designs and examples for practicing the disclosed method within the scope of the appended claims. 
     The description of the present disclosure is merely exemplary in nature and variations that do not depart from the general sense of the present disclosure are intended to be within the scope of the present disclosure. Such variations are not to be regarded as a departure from the spirit and scope of the present disclosure.