Binder removal in selective laser sintering

Abstract

A method of fabricating an article, such as a prototype part or a tooling for injection molding, by way of selective laser sintering, using a composite powder system of a metal and/or ceramic powder with a polymer binder comprising thermoplastics and thermoset polymers, and a metal hydride powder to form a “green” article. After removal of unfused material from the green article it is placed in an oven or furnace in a non-reactive atmosphere such as, for example, nitrogen or argon, for subsequent heat treatment to decompose and drive off the binder and sinter the metal substrate particles prior to infiltration by a metal with a lower melting point. During the critical step of decomposing the binders, the metal hydride begins to decompose also and releases an in-situ concentration of hydrogen gas that creates the reducing conditions necessary to thoroughly decompose the polymer fragments so that the hydrocarbon fragments can escape the skeleton structure of the article. It has been found that even with higher loadings of binders, leading to higher desired green strengths, the decomposition of the metal hydride eliminates the blistering phenomena associated with high loadings of some binders.

We claim:  
     
         1 . A method of fabricating an article, comprising the steps of forming a green article by the selective laser sintering of a composite powder, wherein said composite powder comprises metal and/or ceramic particles, polymer particles, and particles of a metal hydride.  
     
     
         2 . The method of  claim 1  wherein said metal hydride particles are selected from the group consisting of titanium hydride, nickel-metal-hydride, magnesium hydride, lithium aluminum hydride, calcium hydride, sodium hydride, and sodium borohydride and combinations thereof.  
     
     
         3 . The method of  claim 1  further comprising using a steel powder in said composite powder.  
     
     
         4 . The method of  claim 3 , further comprising using titanium hydride as the metal hydride.  
     
     
         5 . The method of  claim 2  wherein said polymer particles are selected from a group consisting of thermoplastic and thermoset polymers.  
     
     
         6 . The method of  claim 5  further comprising using polyamide polymers.  
     
     
         7 . The method of  claim 5  further comprising using phenolic polymers.  
     
     
         8 . The method of  claim 1 , further comprising after said forming step, heating said green article in a first heating step to a first temperature to decompose said polymer binder and said metal hydride.  
     
     
         9 . The method of  claim 8 , further comprising after said first heating step heating said article in a second heating step to a second temperature, the second temperature being above said first temperature, to sinter said composite particles to one another, forming a brown article.  
     
     
         10 . The method of  claim 9 , further comprising during said second heating step, infiltrating said brown article with a second material.  
     
     
         11 . The method of  claim 10  further comprising using a copper alloy as said second material.  
     
     
         12 . A preform green article formed by selective laser sintering comprising a composite powder, the composite powder being fused and comprising metal and/or ceramic particles, polymer particles and metal hydride particles.  
     
     
         13 . The green article according to  claim 12  wherein the metal hydride particles are selected from the group consisting of titanium hydride, nickel-metal-hydride, magnesium hydride, lithium aluminum hydride, calcium hydride, sodium hydride, and sodium borohydride and combinations thereof.  
     
     
         14 . The green article according to  claim 12  wherein said composite metal powder further comprises a steel powder.  
     
     
         15 . The green article according to  claim 14  wherein said composite powder further comprises titanium hydride.  
     
     
         16 . The green article according to  claim 13  wherein said polymer particles are selected from the group consisting of thermoplastic and thermoset polymers.  
     
     
         17 . The green article according to  claim 16  wherein said thermoplastic polymers further comprise polyamides.  
     
     
         18 . The green article according to  claim 17  wherein said polymer particles further comprise phenolics.