Rehardenable materials system with diffusion barrier for three-dimensional printing

Abstract

Three-Dimensional Printing (3DP) is a solid freeform fabrication process being developed for the direct manufacture of functional tooling and prototypes from a computer solid model. One of its many important applications is the fabrication of metal tooling for plastic injection molding. In order to achieve a fully dense 3DP metal tool, the sintered powder skeleton is infiltrated with a molten alloy, which has a melting point lower than the skeleton material. However, the choices of materials systems are limited by the interactions of the metal powders and infiltrants during the infiltration process. Currently, the materials system with the best wear resistance for 3DP metal tooling consists of 420 stainless steel powder and bronze infiltrant. However, it only has an overall hardness of 25 HRC because the bronze infiltrant is soft and not hardenable. A hardenable 3DP metal system is desirable. The main goals of this thesis research are: 1) to improve the flexibility of choice of metal powders and infiltrants by using a diffusion barrier to isolate them; and 2) to demonstrate the diffusion-barrier approach with steel and hardenable copper-alloy infiltrant. The model materials systems in this study consist of stainless steel and tool steel powder skeletons with Cu-20Ni-20Mn infiltrant. It was demonstrated that TiN coating deposited on steel substrates by CVD successfully prevented the reaction between the steel and molten Cu-20Ni-20Mn at 1200° C. In general, TiN coating on tool-steel substrates demonstrated better diffusion-barrier behavior than the stainless-steel substrates. The optimum thickness of the TiN coating was determined to be in the range of 0.5 to 1 [mu]m. Fracture strength of the TiN coating as high as 560 MPa can be achieved for a 0.5 [mu]m thick TiN coating on 440 C stainless steel bar. A 0.8 [mu]m TiN-coated H13 tool-steel powder skeleton was successfully infiltrated with Cu-20Ni-20Mn infiltrant. Age hardening at 450° C for one day resulted in a high macrohardness of 40 HRC, surpassing the current 3DP metal system for injection molding tooling. Future works on the study of other diffusion barrier materials and fluidized-bed CVD for metal powders could result in more 3DP metal systems with better properties and ease of processing.