Process for transforming a metal alloy into a partially-solid/partially-liquid shaped body

Abstract

A process for transforming a metal alloy ( 28 ) having a liquidus temperature and a solidus temperature into a part solid/part liquid shaped body. The metal alloy ( 28 ) is poured in the molten state at a temperature (T Mo ) into a mould ( 10 ) having an essentially cylindrical mould wall ( 12 ), whereby the mould ( 10 ) at the start of filling exhibits a starting temperature that lies below the liquidus temperature, the mould ( 10 ) is set into an eccentric rotational movement at a starting temperature for the metal alloy ( 28 ) lying above the liquidus temperature and the rotational movement maintained until the metal alloy ( 28 ) has cooled to a to a discharging temperature lying between the liquidus temperature and the solidus temperature corresponding to a desired solid/liquid ratio in the shaped body and the shaped body removed from the mould ( 10 ) at the discharging temperature. In order to set a desired cooling rate of the metal alloy ( 28 ) in the mould ( 10 ), the thickness (d 1 ) of the mould wall ( 12 ), the material and the starting temperature of the mould ( 10 ) are selected such that the change in enthalpy of the metal alloy ( 28 ), during cooling from the pouring temperature to the discharging temperature, is smaller than the change in enthalpy for an increase in temperature of the mould ( 10 ) from the starting temperature to its final temperature. The setting of the parameters influencing the cooling rate of the mould in combination with the eccentric rotation of the mould lead to an optimal, short duration of the process.

1 . A process for transforming a metal alloy ( 28 ) having a liquidus temperature (T L ) and a solidus temperature (T S ) into a partially solid/partially liquid shaped body, comprises the steps of: 
 pouring the metal alloy ( 28 ) in the liquid state at a pouring temperature (T Mo ) into a mould ( 10 ) having an essentially cylindrical mould wall ( 12 ), whereby the mould ( 10 ) at the start of the filling stage exhibits an initial temperature (T AF ) that lies below the liquidus temperature (T L ); holding the metal alloy ( 28 ) in the mould ( 10 ) until the alloy has cooled to a discharging temperature (T EM ) where (T EM ) is between the liquidus temperature (T L ) and the solidus temperature (T S ) corresponding to a desired solid/liquid ratio in the shaped body; and removing the shaped body from the mould ( 10 ) at the discharging temperature (T EM ), wherein prior to pouring the metal alloy in order to set a desired cooling rate of the metal alloy ( 28 ) in the mould is set by ( 10 ), (a) selecting the thickness (d 1 ) of the mould wall ( 12 ), (b) the material and (c) the initial temperature (T AF ) of the mould ( 10 ), wherein the change in enthalpy (ΔH M ) of the metal alloy ( 28 ) during the cooling from the pouring temperature (T Mo ) to the discharging temperature (T EM ) is smaller than the change in enthalpy (ΔH F ) for an increase in temperature of the mould ( 10 ) from the initial temperature (T AF ) to its final temperature (T EF ).    
     
     
         2 . A process according to  claim 1 , including employing the change in temperature T(t) as a function of time to determine the time for discharging, whereby the discharging takes place on reaching a given target value of temperature profile in the metal alloy ( 28 ) and a given target value of discharging temperature (T EM ).  
     
     
         3 . A process according to  claim 1 , including employing the change in temperature T(t) as a function of time at a fixed point in the mould wall ( 12 ) to determine the time for discharging, whereby the discharging takes place on reaching a given target value of temperature gradient dT W /dt and a given target value of discharging temperature (T EM ).  
     
     
         4 . A process according to  claim 1 , wherein the initial temperature (T AF ) of the mould ( 10 ) lies between room temperature and 320° C.  
     
     
         5 . A process according to  claim 1 , wherein the discharging of the shaped body ( 12 ) takes place immediately after reaching the discharging temperature (T EM ).  
     
     
         6 . A process according to  claim 1 , including holding the shaped body ( 12 ) at the discharging temperature (T EM ) on reaching the discharging temperature (T EM ) by heating the mould ( 10 ) and discharging after a holding time.  
     
     
         7 . A process according to  claim 1 , including moving the mold containing the metal alloy ( 28 ) until it has cooled to the discharging temperature (T EM ).  
     
     
         8 . A process according to  claim 7 , including, after filling of the mold, to immediately after rotating the mould ( 10 ) in an eccentric manner, wherein the rotational movement is maintained until the metal alloy ( 28 ) has cooled to the discharging temperature (T EM ).  
     
     
         9 . A process according to  claim 8 , including commencing the rotational movement when the starting temperature (T AM ) of the metal alloy ( 28 ) is between the liquidus temperature (T L ) and up to 15° C. above the liquidus temperature (T L ).  
     
     
         10 . A process according to  claim 8 , wherein the speed of rotation lies between 50 and 500 rpm.  
     
     
         11 . A process according to  claim 10 , wherein the speed of rotation is increased with progressive cooling of the metal alloy ( 28 ).  
     
     
         12 . A process according to  claim 8 , wherein the rotational movement features at least two cycles with increasing speed of rotation.  
     
     
         13 . A process according to  claim 12 , wherein a shaking cycle follows each rotational cycle.  
     
     
         14 . A process according to  claim 12 , wherein the first two rotation cycles are carried out within an overall time of 30 to 50 seconds.  
     
     
         15 . A process according to  claim 13 , wherein the shaking cycle comprises a shaking movement lasting at most 10 sec.  
     
     
         16 . A process according to  claim 1 , wherein the metal alloy ( 28 ) is an aluminium alloy.  
     
     
         17 . A process according to  claim 16 , wherein the alloy exhibits a eutectic solidus temperature with a significant volume parts ( 12   a,b ).  
     
     
         25 . A process according to  claim 1 , including shape-forming the shaped body in a pressure diecasting machine after the shaped body is removed from the mould ( 10 ).  
     
     
         26 . A process according to  claim 1 , including employing the time-dependent, non-stationary temperature field in the mould wall ( 12 ) to supervise and regulate heat extraction for determination of the optimal discharging temperature (T EM ) and with that the optimal process time from the starting temperature (T AM ) to the discharging temperature (T EM ).  
     
     
         27 . A process according to  claim 1 , including simulating and regulating the process on the basis of the Fourier coefficients for thermal conduction in the mould wall ( 12 ) using the functional relationship for a minimum process time t pr  as follows:  
       t pr =f(ΔH M , T AM , T EM , d 1 , T w  (t,d 1 ), T AF , Fo) min.  ΔH M  Change in enthalpy of the metal melt between T AM  and T EM      T AM  Temperature of the melt at the start of the rotational movement (starting temperature)    T EM  Temperature for discharging the shaped body from the mould    d 1  Thickness of the mould wall    T w  Temperature of a mould wall element during the course of the process    T AF  Initial temperature in the mould wall (pre-heat temperature)    Fo Fourier coefficient    
     
     
         28 . A process according to  claim 12 , wherein a shaping cycle is superimposed on the rotational movement.  
     
     
         29 . A process according to  claim 13 , wherein the shaking cycle comprises a shaking movement lasting between 2 to 6 seconds.