Foundry alloy

Abstract

An aluminium-based alloy having 6.5-7.5 wt. % silicon and 0.35-0.50 wt. % magnesium as the major alloying elements and a method of manufacturing an article from the alloy are disclosed. The alloy is characterised by a microstructure in which β phase (Al 5 SiFe) that forms during heat treatment as a transformation product of π phase (Al 8 Si 6 Mg 3 Fe) is the sole or predominant iron-containing phase in the alloy.

1 . An ally which comprises: 
 Si: 6.5-7.5 wt %    Fe: up to 0.20 wt %    Cu: up to 0.05 wt %    Mn: up to 0.05 wt %    Mg: 0.3 to 0.50 wt %    Zn: up to 0.05 wt %    T 1 : up to 0.20 wt %    Balance: Al and other components, the other components comprise a total of not more than 0.15 wt % and any single component of the other components does not exceed 0.05 wt %, the alloy having a microstructure which includes a primary aluminium-containing matrix and one or more iron-containing phases dispersed in the matrix, and wherein the sole or predominant iron-containing phase is β phase that formed as a transformation product of π phase.    
     
     
         2 . The alloy defined in  claim 1 , wherein when the alloy includes more than one iron-containing phase, the iron-containing phases also include π phase.  
     
     
         3 . The alloy defined in  claim 2 , wherein the π phase is up to 30 vol % of the iron-containing phases.  
     
     
         4 . The alloy defined in any one of the preceding claims, wherein the Mg content of the alloy is 0.40-0.45 wt %.  
     
     
         5 . A method for manufacturing an alloy article which comprises: 
 (a) providing a malt heaving a composition of: 
 Si: 6.5-7.5 wt %  
 Fe: up to 0.20 wt %  
 Cu: up to 0.05 wt %  
 Mn: up to 0.05 wt %  
 Mg: 0.35 to 0.50 wt %  
 Zn: up to 0.05 wt %  
 Ti: up to 0.20 wt %  
 Balance: Al and other components, the other components comprising a total of not more than 0.15 wt % and any single component of the other components not exceeding 0.05 wt %,  
   (b) casting said melt and solidifying a casting at a cooling rate that produces a microstructure of an aluminium-containing matrix and π and β iron-containing phases dispersed in the matrix;    (c) solution heat treating the casting to at least partially transform π phase to β phase; and    (d) quenching the casting to form the alloy article.    
     
     
         6 . The method defined in  claim 5 , wherein the cooling rate is sufficient to produce a dendrite arm spacing in the matrix of between 10 and 4.5 μm.  
     
     
         7 . The method defined in  claim 5  or  claim 6 , wherein the sole or predominant iron-containing phase in the alloy article is β phase.  
     
     
         8 . The method defined in  claim 5 , wherein when the alloy includes more than one iron-containing phase in the alloy article, the iron-containing phases also include π phase.  
     
     
         9 . The method defined in  claim 8 , wherein the π phase is up to 30 vol % of the iron-containing phases.  
     
     
         10 . The method defined in  claim 5  or  claim 6 , wherein the step of solidifying the casting produces iron-containing phases that include a substantial proportion of π phase and the subsequent solution heat treatment step is effective to convert a majority of the π phase to β phase to give a microstructure in the alloy article that includes iron-containing phases which are predominantly β phase.  
     
     
         11 . The method defined in any one of  claims 5  to  10 , wherein prior to casting the melt is at a temperature above the liquidus temperature of the alloy.  
     
     
         12 . The method defined in any one of  claims 5  to  12 , wherein the quenching step is in hot water having a temperature of 70-80° C.  
     
     
         13 . The method defined in any one of  claims 5  to  12 , further includes an ageing heat treatment of the alloy article.  
     
     
         14 . The method defined in  claim 13 , wherein the ageing heat treatment includes heating the alloy article to a temperature of 140-170° C., holding the alloy article at that temperature for 1-10 hours, and air cooling the alloy article to room temperature.