Characterization of titanium powder: Microscopic views and macroscopic flow

Abstract

Powder metallurgy is a very popular and cost effective process due to improvements in powder availability and the ability to produce a part close to the desired net shape. With the introduction of the Armstrong Process®, which produces commercially pure titanium powder, it is necessary to understand the flowability characteristics of the irregular particles produced. This paper reports the physical properties of the raw powder and a characterization of the inter-particle forces present in the poly-disperse size mixture. The titanium powder size distribution and particle morphology are determined using sieve analysis and quantitative SEM image analysis, respectively. The results demonstrate that attempts to improve the shape of the particles simply yield smaller irregular shaped particles, but with a high concentration (≈20%) of extremely fine particles that are less than 45μm. However, the microstructure of all particle sizes remains consistent with an irregular outline and a porous interior, much like coral. The inter-particle force properties are approximated by measuring the dynamic angle of repose within a rotating tumbler. Tumbler dimensions and rotation rate of the system are non-dimensionalized via the Froude number, which relates input energy to gravity driving the flow. The results confirm the increase of the angle of repose with increasing Froude number. However, the high concentration of very fine particles in mixtures that are processed to generate more round titanium powder are found to be a detriment to uniform, repeatable flow that is necessary in manufacturing processes. The overall conclusion is that narrower size ranges of titanium powders produced by the Armstrong Process® are necessary to provide useful source materials in powder metallurgy applications.