Process for nodulizing silicon in casting aluminum silicon alloys

Abstract

A method for nodulizing silicon crystals in casting Aluminum-(8-12%)-Silicon alloys is described. An initial alloy melt is refined by addition of a master alloy containing elements of titanium, phosphorous, boron, zirconium and rare earths to the initial molten alloy, and then through conventional heat treatment and aging. Testing the alloys shows that the nodulized silicon crystals are blunted in shape and well distributed, resulting in a high resistance to wear and high ultimate tensile strength at room temperature and at 300° C. Machinability of the Al—Si alloys is also greatly improved by the process.

1 . A method for nodulizing silicon in Al—Si alloy castings having a silicon (Si) content in the range of 8.0-23.0%, which comprises: 
 a) forming a melt by heating a selected start Al—Si alloy mass to its alloy liquidus temperature;  
 b) adding a master alloy mixture to the melt at a master alloy temperature of 150° C. above the start alloy liquidus temperature, said master alloy mixture comprising quantities of Ti, B, Zr and rare earth elements sufficient such that the Al—Si alloy casting comprises 0.02-0.30% Ti, 0.001-0.10% B, 0.02-0.10% Zr, and 0.03-0.3% rare earth elements by weight;  
 c) forming a treated molten mass by degassing the melt with nitrogen;  
 d) producing a casting by pouring and casting the treated molten mass;  
 e) treating the casting further in solution by: 
 (1) heating the casting at a temperature of 500-530° C., and holding that temperature for a time sufficient to obtain a solid solution Al—Si alloy; and  
 (2) quenching the solid solution alloy casting at high temperature in water; and  
 
 f) aging the new Al—Si alloy casting by heating for a time sufficient to obtain desired alloy mechanical characteristics.  
 
     
     
         2 . The method as recited in  claim 1 , wherein the added master alloy mixture comprises metal salts of Ti, B, Zr, and the rare earth elements.  
     
     
         3 . The method as recited in  claim 1 , wherein the added master alloy mixture comprises: an Al—Ti alloy having 1-10% by weight titanium and 90-99% by weight aluminum; an Al—B alloy having 0.2-3.0% by weight boron and the balance being aluminum; an Al-RE alloy having 4.0-10% by weight rare earth elements and 90-96.0% by weight aluminum; and an Al—Zr alloy having 1.0-5.0% by weight zirconium and the balance being aluminum and incidental impurities.  
     
     
         4 . The method as recited in  claim 1 , wherein the added master alloy mixture comprises: titanium alkalifluoride, alkali boron fluoride, zirconium alkalifluoride, rare earth chloride.  
     
     
         5 . The method as recited in  claim 1 , further comprising stirring the master alloy mixture in the melt and holding for a period of time.  
     
     
         6 . The method as recited in  claim 1 , wherein the casting heating is maintained for 6-8 hours.  
     
     
         7 . The method as recited in  claim 1 , wherein the aging step is carried for 6-9 hours.  
     
     
         8 . The method as recited in  claim 1 , wherein the aging step is carried out at a temperature of 130-230° C.  
     
     
         9 . The method as recited in  claim 1 , wherein the overall weight of the master alloy is about 3% of the weight of the molten mass.  
     
     
         10 . The method as recited in  claim 1 , wherein the start Al—Si alloy further comprises: Cu, Mg, Mn and Fe.  
     
     
         11 . The method as recited in  claim 1 , wherein the start Al—Si alloy further comprises: 0.8-1.5% Cu, 0.6-1.0% Mg, <0.5% Mn, and <0.7% Fe by weight.  
     
     
         12 . The method as recited in  claim 1 , wherein the master alloy mixture further comprises quantities of P sufficient such that the Al—Si alloy casting comprises 0.005-0.1% P.  
     
     
         13 . The method as recited in  claim 1 , wherein the added master alloy mixture further comprises quantities of metal salts of P sufficient such that the Al—Si alloy casting comprises  0 . 005 - 0 . 1 % P.  
     
     
         14 . The method as recited in  claim 1 , wherein the added master alloy mixture further comprises quantities of phosphide sufficient such that the Al—Si alloy casting comprises 0.005-0.1% P.  
     
     
         15 . The method as recited in  claim 1 , wherein the added master alloy mixture further comprises a Cu—P alloy having 5.0-8.0% by weight phosphorous and 92.0-95.0% by weight copper.