Effects of powder conditioning on the quality, microstructure and mechanical properties of sintered titanium alloys
A small amount of internal lubricant (Stearic acid (SA) or Magnesium stearate (MgSt)) was added to Ti powder for the purpose of improving the compressibility and the green density homogeneity. Lubrication improved the compressibility of Ti powder compacts, especially in the low pressure region (<400MPa), yet at higher pressures, lubricants hindered further improvement. An addition of up to 0.3wt.% SA improved the green density. MgSt showed a better lubrication effect and an addition of up to 0.6wt.% improved the density. The difference in the lubrication effect between SA and MgSt was due to their mixing or blending characteristics. A homogenous mixing of lubricant led to an increased lubricated area and the effectiveness of the lubrication. Lubricants had a strong influence on shape retention, green strength and ejection behaviour of Ti powder compacts. The green density distribution was improved by adding a lubricant. The density profiles were measured experimentally using a coloured layer method. SA gave better results compared with MgSt, with 0.6wt.% of SA considered to be the optimum addition. The mixing or blending characteristics of SA additions accounted for the better improvement in performance. By adding a lubricant, the sintered density distribution in Ti compacts was improved by controlling the pore morphology with respect to their size, aspect ratio and orientation. But 1wt.% SA created many pores ranging in size from 50-100μm, both in the top and bottom regions, and this led to very bad ductility. The consistency in mechanical properties of a sintered Ti Φ40 mm compact was significantly improved by adding 0.6wt.% SA. Such improvement was achieved because of a lower sintering mismatch initiated from a more homogenously distributed green density, both in the horizontal and transverse directions. MgSt was not recommended because of higher oxygen pick-up. Rare earth elements (RE) were added to Ti metal and alloy for the purpose of scavenging oxygen and to promote better microstructural control. Three additive forms were studied. Er and LaB6 additions were added to Ti and Ti6Al4V alloys directly. The Ti(Ti6Al4V)-Y alloy was made by mechanical milling (MM). But direct Er additions caused a segregated microstructure and processing involving the MM of Ti(Ti6Al4V)-Y leaded to oxygen pick-up. These methods are therefore not recommended. Direct LaB6 additions achieved excellent results. The reinforcement was uniformly distributed with various orientations and the microstructure was refined. The TiB reinforcement gave excellent strength and good ductility. The acicular TiB phase seemed to be the only problem, because while it significantly improved the strength it decreased the ductility. Theoretical research was carried out on the compaction process for internally lubricated Ti powder. A modified Cooper-Eaton formula was employed to analyse the compaction behaviour of Ti powder. There was a good fit between the simulation results and the experimental data. The theoretical research indicated that cold compaction of titanium powder could be separated into two stages: a particle rearrangement (PR) stage, which occurred in a compacting pressure range of 0-200MPa, followed by a plastic deformation (PD) initiated (PR) stage from 200-1000MPa. The existence of stage II was due to the low plastic deformability of titanium and the low density achieved at the end of stage I. Ti particles needed to be plastically deformed or even cracked into fragments to fill the gaps in-between Ti particles. At pressures between 200-600MPa, the use of an internal lubricant improved consolidation, leading to better densification because a lubricant facilitates particle motion. On the basis of this discussion, the approaches for modifying the cold compaction behaviour of Ti powder were studied.
Power metallurgy, Titanium, Powder conditioning, Microstructure, Mechanical properties
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