Method and apparatus for making metal alloy castings

Abstract

A method and apparatus are provided for fabricating of continuous castings with fine and uniform microstructure, which can be used as feedstock for secondary processing routes, such as thixoforming, forging and machining or direct application in industry. An overheated liquid alloy is fed into a high shear device (for example, a twin-screw extruder) and sheared intensively to produce a sheared liquid alloy or a semisolid slurry, wherein the sheared liquid alloy is at a temperature close to its liquidus and the semisolid slurry is then transferred to a shaping device for production of continuous castings with fine and uniform microstructures through a solidification process. The shaping device is any device capable of forming continuous (i.e. infinite length) products, such as a direct chill (DC) caster (DC rheocasting) for production of continuous billets, an extrusion die (rheo-extrusion) for production of continuous bars or wires, or a twin-roll caster (twin-roll rheocasting) for producing of continuous strips. In all those cases, the cross-section of the continuous castings exhibits a microstructure in which a controlled volume fraction of fine and spherical primary particles are uniformly distributed in a fine structured matrix.

1 . A method for forming a continuous product from a liquid metal alloy, comprising the steps of: 
 cooling the alloy to about the liquidus or from the solidus to the liquidus of the alloy,    applying shear to the alloy at a sufficiently high shear rate and intensity of turbulence to convert the alloy into its thixotropic state, and    transferring the sheared liquid alloy or sheared semisolid slurry into a shaping device in order to form a solid product,    wherein the shaping device is capable of forming a continuous product.    
     
     
         2 . A method as claimed in  claim 1 , wherein shear is applied to the alloy by means of an extruder having at least one screw, the screw having at least one vane thereon, the vane at least partially defining a helix around the screw to propel the alloy through the extruder, wherein the at least one screw can be rotated to shear said liquid alloy at a rate sufficient to inhibit complete formation of dendritic structures therein while the alloy is in a semisolid state.  
     
     
         3 . A method as claimed in  claim 1  or  2 , wherein shear is applied to the alloy by means of a twin screw extruder having at least two screws which are at least partially intermeshed.  
     
     
         4 . A method as claimed in  claim 3 , wherein the screws are substantially fully intermeshed.  
     
     
         5 . A method as claimed in any preceding claim, wherein the shaping device is a direct chill caster, and extrusion die, or a twin roll caster.  
     
     
         6 . A method as claimed in any preceding claim, wherein the alloy comprises at least two immiscible components, and wherein said components are provided separately in liquid form, and pre-mixed before being subjected to shear.  
     
     
         7 . A method as claimed in  claim 6 , wherein the alloy is subjected to sufficient shear to be converted into a liquid suspension in which the minor immiscible component is dispersed in the major immiscible component in the liquid phase.  
     
     
         8 . A method as claimed in  claim 7 , wherein the liquid suspension is cooled to its monotectic temperature or below whilst being sheared in order to form a semisolid slurry.  
     
     
         9 . A method as claimed in  claim 8 , wherein the viscosity of the slurry is sufficiently high to prevent coarse segregation of the immiscible system.  
     
     
         10 . A method as claimed in any of  claims 6  to  9 , wherein the shaping device is not a pre-heated metal band.  
     
     
         11 . A method as claimed in any of  claims 1  to  6 , wherein the alloy does not include any immiscible components.  
     
     
         12 . An article formed from a metal alloy, which article is obtainable by means of a method as claimed in any preceding claim.  
     
     
         13 . Apparatus for forming a continuous product from a liquid metal alloy, comprising a temperature-controlled shearing device capable of imparting sufficient shear and intensity of turbulence to a liquid metal alloy to convert it into its thixotropic state, and a shaping device in fluid communication with the shearing device, wherein the shaping device is capable of forming a continuous product.  
     
     
         14 . Apparatus as claimed in  claim 13 , wherein the shearing device is an extruder having at least one screw, the screw having at least one vane thereon, the vane at least partially defining a helix around the screw to propel the alloy through the extruder, wherein the at least one screw can be rotated to shear said liquid alloy at a rate sufficient to inhibit complete formation of dendritic structures therein while the alloy is in a semisolid state.  
     
     
         15 . Apparatus as claimed in  claim 13  or  14 , wherein the shearing device is a twin screw extruder having at least two screws which are at least partially intermeshed.  
     
     
         16 . Apparatus as claimed in  claim 15 , wherein the screws are substantially fully intermeshed.