High green density metal parts by vibrational compaction of dry powder in three dimensional printing process

Abstract

The material properties and dimensional accuracy of metal tooling produced by the Three Dimensional Printing process can be enhanced by increasing the green density of the 3D printed part. Green density is the ratio of metal powder volume to the external volume of the printed part, and is a measure of how tightly packed the powder particles in the printed part are. The central goal of this thesis was to increase the green density of metal parts from the current level of 58% to levels greater than 75%. Two approaches were taken for increasing green density. The first was to utilize bimodal mixtures of metal powders which could be packed to significantly higher densities than the monomodal powders which had been previously used. Three bimodal powder mixtures, with tap densities near 80%, were studied. The second approach was to develop a new powder layering device which could pack these bimodal powders to the tap density during layer creation. New understandings about the relationship between the stresses applied to the powder layer and the resulting packing density changes were required to design this device. Shear cell and unconfined compression tests were performed to characterize the metal powder stress / strain behavior. Particulate stress / strain models, such as the Mohr-Coulomb failure law and the Jenike yield locus theory, were used to interpret the packing behavior of the metal powders under various stress conditions.

(cont.) A simple frictional model of powder behavior was proposed for the low stress levels permissible in the 3DP process. The application of a small static normal stress, in combination with an oscillatory horizontal shear stress, was found to be the most effective means of reducing particle interlocking and provided the best layer densification results. A new layer densification mechanism was constructed and successfully used to generate printed parts with green densities in excess of 75%. Photographic analysis techniques used to analyze part microstructures indicated significant improvements in packing homogeneity. Packing defects between the printed layers were reduced or eliminated. Compositional analysis indicated no significant segregation of the bimodal components during layer spreading.