Project B2: Equal Channel Angular Extrusion (ECAE) of aerospace Ti alloy
•	Project Chief Investigators: Rimma Lapovok (Project Leader)
•	Senior Researchers/Research Associates: Monash: Dacian Tomus
•	Research Assistants Visiting Scholars, Hons Students etc: Monash: Daniel Curtis
The project was aimed at an investigation of the potential for cost-effective, efficient consolidation of pre-alloyed (PA) Ti-6Al-4V (HDH) powder at temperatures of 400ºC and below using Equal Channel Angular Extrusion (ECAE), with applied back pressure. The limit on processing temperature was imposed to minimise the contamination of powder and compact with gaseous constituents known to be harmful to resultant properties.
An analysis of existing published investigations of current processing techniques, most notably those involving hot isostatic pressing (HIP), reveals that relative densities of 98-100% can only be obtained at processing temperatures in excess of 800ºC. For such methods, and temperatures below 400ºC, the relative densities achievable are typically of the order of 77%, when starting with an initial ‘tape‘ density of 63%. In this context, the project goals, of reducing the processing temperature of PA powder compaction below 400ºC while achieving a relative density above 98%, are to be seen as quite challenging.
The novelty of the approach arises from the notion that severe shear deformation could prove an important factor for improving consolidation at relatively low processing temperatures. It has been shown that the use of ECAE with back pressure at 400ºC permits production of compacts with relative densities in the range 98.3-98.6% and green strengths up to 750 MPa. The improvements in density and green strength are attributed to enhancement of self-diffusion rates at low temperatures that are in turn the result of an excess of structural defects created during severe shear deformation and the effects of imposed hydrostatic pressure (back pressure).
It has also been shown that further increases in relative density (to in excess of 99%) may potentially be obtained by increasing the component of hydrostatic pressure or by manipulation of powder/compact microstructure to increase of the volume fraction of the β phase constituent and the ratio of α:β phase in the microstructure. A modest increase in the β - phase fraction from 5.5% (as-received condition) to 10% (after heat treatment) has been shown to contribute to a measurable increase in relative density from 98.3% to 98.9% at a processing temperature of 400ºC and a back pressure of 350MPa.
SEM images of unmounted powder are shown in Fig. B2. The optical micrographs of Fig. B2(b) show the existence of cracks in larger particles due to comminution of brittle hydride as part of the HDH powder processing method.
Figure B2 - Images of as-received Ti6Al4V powder: (a) Scanning Electron Micrographs (b) Optical Micrographs.
Figure B3 shows optical micrographs of billet compressed at 400C by direct compaction and by ECAE.
Figure B3 - OM micrographs of billet compressed at 400ºC (a), (b) by direct compaction and (c), (d) by ECAE (two different magnifications).
Figure B4 presents transmission electron micrographs of billet processed by ECAE.
Figure B4 - TEM micrographs of billet processed by ECAE with 262 MPa at 400ºC and sintered at 1100ºC for 4 hrs (α- and ß-phases).
Activity plan for 2008
The project goals have been achieved and this project is completed.