| Description |
Powder metallurgy (PM) has been considered to potentially be the ideal method to produce titanium alloy components, primarily due to cost savings. However, in terms of traditional PM approaches, using vacuum sintering, it is difficult to simultaneously obtain fine microstructure, low porosity and low oxygen concentration. In general, the use of fine powders decreases porosity, but increased oxygen content, which in turn affects mechanical properties, usually decreases ductility. A new PM process for producing titanium components, termed hydrogen sintering and phase transformation (HSPT) has been recently introduced to produce near full density components with very fine grained microstructures. In this study, the interrelation of oxygen contents, particle size and sintered part densification is discussed in order to promote the industrial application of titanium powder metallurgy. The first part of this work investigated the effects of oxygen contamination during powder metallurgy processing. It was found that the final oxygen content was principally affected by milling, where specific surface area and oxygen content of powders was controlled by the milling time and environment. The milling environments that were studied included heptane, argon, light mineral oil, heavy mineral oil, kerosene, Paraffin wax and Eicosane. The influence of milling environment on powder chemistry showed linear trends between milling time and oxygen content for specimen. Light mineral oil was determined to be the most ideal environment for protecting titanium powders from oxygen uptake during milling. Due to the strong correlation between the porosity of sintered specimens and mechanical properties, the second part of this work examined the effects of the particle size of powders on shrinkage and porosity in sintered Ti-6Al-4V specimens. The results showed that the specimens made with smaller particles were more likely to reach densification; they reached a higher final density and densification occurred at a faster rate. The porosities were lower in final sintered specimens made with finer powders than those made with coarser particle sizes. The milling environment has little effect on total porosity and the size of largest pores; instead, there appeared to be a strong correlation to specific surface area of starting powders. |
