Reactive binders for metal parts produced by Three Dimensional Printing

Abstract

Three Dimensional Printing (3DP) is a solid free form fabrication process which enables the construction of parts directly from computer-aided design (CAD) models. In the current process, metal parts are produced by printing a polymer binder into stainless steel powder. The parts are subsequently furnace-treated to debind, lightly sinter, and then infiltrate them with a molten metal alloy. These post-printing processes cause a total linear dimensional change of approximately -1.5 ⁺/₋ 0.2%. Experiments were conducted to investigate reactive binder systems that would improve the dimensional control of metal parts produced by 3DP. Reactive binders typically require a furnace treatment in order to be activated. To prevent the printed part from deforming before binder activation, the initial furnace treatment is carried out with the part contained in the original powder bed. The binder will remain in the part permanently. Because the part is fired in the powder bed, differentiation between the bound and unbound regions places a limitation on the types of binders that may be used. The three main categories of reactive binders investigated were carbon-based binders, metal salts, and small particles. The carbon-based binders acted to bind the part by enhancing the sintering of the stainless steel powder skeleton (binding shrinkage=0.15% when fired at 800̊C in argon, total shrinkage=0.78%). The other two categories of binders, which involved the addition of material to form the interparticle bonds, resulted in even less shrinkage. Nearly zero percent shrinkage was obtained with a silver nitrate binder, which reduced to a continuous film of silver that formed the interparticle bonds.

This reduction occurred at 400⁰C in a forming gas atmosphere. Other salts tested did not possess this desirable reduction behavior. Low shrinkage (binding shrinkage=0.0% when fired at 700⁰C in forming gas, total shrinkage<0.4%) was also obtained with a co-dispersion of small carbonyl iron and titanium carbide particles (<2 microns avg.). The carbonyl iron acted to bind the part while the TiC particles limited the total amount of shrinkage. Further investigation into these and related systems should lead to the development of a viable, high dimensional control system for metal parts produced by 3DP.