Economical furnace processing for 3-D printed metal parts

Abstract

Three-Dimensional Printing is a layer based manufacturing process which uses powdered material to build parts with complicated geometries directly from a 3-D computer model. Metal parts fabricated by this method are initially held together with a polymer binder and are somewhat fragile. To achieve full density and desirable mechanical strength, the green part must undergo two furnace operations, sintering and infiltration. During sintering, the part is heated to near the powder's melting temperature and necks form at the contact points between particles. During infiltration, a lower melting temperature alloy is melted while in contact with the porous skeleton and capillary forces wick the alloy into the skeleton and fill the void space. The furnace conditions currently used for these processing steps are expensive, involve the use of vacuum and pure hydrogen, and therefore prohibit commercialization. This research focuses on the reduction of cost and complexity of the post-processing steps. A furnace which avoids using vacuum or pure hydrogen, and utilizes less expensive materials of construction, is designed in order to significantly reduce the cost of post-processing and make it easier to scale up for larger part sizes. Based on a series of experimental furnaces that were built and tested, a design for an economical furnace was completed. The furnace consists of a gas-tight outer shell, alumina blanket insulation, and metallic wire heating elements embedded in ceramic plates. The shell is kept at temperatures low enough to allow the use of Teflon and silicone for various seals including the door. A forming gas mixture of 5% hydrogen in argon is used to provide a reducing atmosphere and a slight positive pressure is maintained to prevent contamination of the atmosphere in the case of a leak in the shell. Trace water vapor from the insulation and heating element supports must be removed from the atmosphere through various means to prevent oxidation. Several additional features were incorporated in the design to allow a future study of the cause of residual porosity after infiltration.