Metal injection moulding method

Abstract

A method for forming an article by metal injection moudling of aluminium or an aluminium alloy. The method comprises the steps of forming a mixture containing an aluminium powder or an aluminum alloy powder or both and optionally ceramic particles, a binder, and a sintering aid comprising a low melting point metal. The mixture is injection moulded and the binder is removed to form a green body. The green body is sintered. The sintering is conducted in an atmosphere containing nitrogen and in the presence of an oxygen getter.

1 . A method for forming an article by metal injection moulding of aluminium or an aluminium alloy, the method comprising the steps of:
 forming a mixture containing an aluminium powder or an aluminium alloy powder or both and optionally ceramic particles, a binder, and a sintering aid comprising a low melting point metal;   injection moulding the mixture;   removing the binder; and   sintering; wherein sintering is conducted in an atmosphere containing nitrogen and in the presence of an oxygen getter.   
   
   
       2 . A method as claimed in  claim 1  wherein the oxygen-getter comprises a metal that has a higher affinity for oxygen than aluminium. 
   
   
       3 . A method as claimed in  claim 2  wherein the oxygen-getter is selected from the group consisting of the alkali metals, the alkaline earth metals and the rare earth metals. 
   
   
       4 . A method as claimed in  claim 3  wherein the oxygen-getter is magnesium. 
   
   
       5 . A method as claimed in  claim 1  wherein blocks of the oxygen-getter are positioned around the article that is being sintered during the sintering step or powder of the oxygen-getter is placed around or on the article that is being sintered during the sintering step or the oxygen-getter is mixed in with the aluminium or aluminium powder alloy, or mixed in with the mixture that is fed to the injection moulding apparatus or the oxygen getter is present as a component of an alloy added to the mixture. 
   
   
       6 . A method as claimed in  claim 1  wherein the sintering aid is a metal that has a melting point that is lower than the melting point of aluminium and is insoluble in solid aluminium. 
   
   
       7 . A method as claimed in  claim 6  wherein the sintering aid comprises tin. 
   
   
       8 . A method as claimed in  claim 1  wherein the sintering aid is present in an amount of up to 10% by weight, based upon the total weight of the metal powder and the sintering aid. 
   
   
       9 . A method as claimed in  claim 8  wherein the sintering aid is present in an amount of from 0.1% to 10% by weight. 
   
   
       10 . A method as claimed in  claim 8  wherein the sintering aid is present in an amount of from 0.5% to 3% by weight. 
   
   
       11 . A method as claimed in  claim 1  wherein the atmosphere in which the sintering step is conducted has a low water content wherein the water vapour partial pressure is less than 0.001 kPa. 
   
   
       12 . A method as claimed in  claim 1  wherein the binder includes thermoplastic components that enable the binder to melt upon application of heat. 
   
   
       13 . A method as claimed in  claim 1  wherein the binder is made from two or more materials selected such that they are sequentially removed from the green body. 
   
   
       14 . A method as claimed in  claim 1  wherein the binder is removed by dissolution in a solvent, by thermal treatment to cause the binder to melt, evaporate or decompose, by catalytic removal of the binder or by wicking. 
   
   
       15 . A method as claimed in  claim 14  wherein two or more binder removal techniques are used in to remove the binder. 
   
   
       16 . A method as claimed in  claim 1  wherein the binder comprises stearic acid, palm oil wax and high density polyethylene. 
   
   
       17 . A method as claimed in  claim 1  wherein the sintering step involves heating the green body to a temperature at which the aluminium or aluminium alloy sinters to form a dense body. 
   
   
       18 . A method as claimed in  claim 17  wherein the temperature is within the range of about 550° C. to about 650° C. 
   
   
       19 . A method as claimed in  claim 1  wherein the mixture includes ceramic particles and the ceramic particles are selected from the group consisting of SiC, Al 2 O 3 , AlN, SiO 2 , BN and TiB 2 . 
   
   
       20 . A method as claimed in  claim 1  wherein the atmosphere comprises nitrogen or a mixture of nitrogen and an inert gas. 
   
   
       21 . A method as claimed in  claim 1  wherein the atmosphere is essentially free of oxygen or hydrogen.