Low shrinkage metal skeletons by three dimensional printing

Abstract

Three Dimensional Printing (3DP) is a process for the rapid fabrication of three dimensional parts directly from computer models. A solid object is created by printing a sequence of two dimensional layers. The creation of each layer involves the spreading of a thin layer of powdered material followed by the selective joining of powder in the layer by printing binder material. In the current process, metal parts are produced by printing a polymeric binder into stainless steel powder. Subsequent heat treatments de-bind and lightly sinter the printed parts. In another heat treatment operation, the printed parts are infiltrated with a molten metal alloy. The growth of the necks during the sintering step causes a shrinkage of the part which, in general, is not uniform and has a certain amount of error associated with it. As currently practiced, the magnitude of the shrinkage is 1.5% and the uncertainty is 0.2%. The 3DP process can compensate for a given amount of predicted shrinkage by beginning with a larger part. However, the uncertainty in the value of shrinkage, translates directly to the loss of dimensional control of the parts. Therefore, there is a need to improve the dimensional control of metal parts produced by 3D Printing. The current work investigates the possibility of decreasing the average shrinkage by eliminating the sintering step. The concept under investigation is an alternative method of forming the skeleton where the metal needed to create the necks between powder particles is provided by adding it through the binder rather than relying on the sintering operation. The metal is added by printing a salt solution or by printing a slurry of fine metal powder. The metal is obtained from the salt by means of a displacement reaction or by heat treatment in a reducing atmosphere. It was found that strong metallic bonding can be obtained by melting the metal or the alloy derived from these liquids resulting, essentially, in brazing of the powder particles. Injection molding tools were fabricated by printing a copper nitrate solution into a dry mixture of 66 [mu]m molybdenum and 1 [mu]m silver powder and by printing a silver slurry into 52 [mu]m molybdenum powder. The metal skeletons were infiltrated with epoxies. The shrinkage of these material systems is approximately 0.15% which compares favorably with the 1.5% shrinkage obtained by the standard sinter-based method. It was hypothesized that the shrinkage of this new binding method is due to the capillary forces induced by the molten metal necks. The traction force due to molten metal necks was modeled analytically and the compressibility of the powder was measured. The shrinkage determined by the equilibrium of these two effects match observed shrinkages well. The model suggests certain approaches to the further reduction of shrinkage, including attaining a higher packing fraction of the powder-bed.