Back pressure equal channel angular consolidation of pure Al particles
Introduction
Synthesis of bulk materials from particles has been practised in powder metallurgy and ceramic manufacturing for a long time. It avoids large scale solidification processing which often results in composition segregation and casting defects, and is the only viable method of manufacturing for metals hard to melt or deform and for ceramics. However, fully dense materials are difficult to produce using conventional powder metallurgy. More recently, it has been increasingly realised that unusually high mechanical properties may be obtained in compositions and microstructures far-off-equilibrium, as exemplified by the extremely high tensile strength of over 1500 MPa achieved in an Al alloy of nano face-centred cubic structured particles in an amorphous matrix [1]. However, such far-off-equilibrium compositions and microstructures can be most readily produced only in very small volumes or dimensions in the form of particles and ribbons by such processes as rapid solidification [2] and mechanical milling [3]; consolidation of these particles into bulk materials becomes necessary especially for structural applications. In conventional powder metallurgy, consolidation is accomplished through sintering at high temperatures, which tends to change the special microstructures of the particles. It is therefore desirable to develop a new process to synthesise bulk materials which can achieve full density and strength at lower temperatures.
High pressure torsion (HPT) [4], [5], [6], [7], [8], [9] and equal channel angular (ECA) deformation [10], [11], [12], [13], [14], [15], [16] have been used to consolidate particles of a variety of alloys. The severe shear deformation involved in these processes is believed to be able to disrupt the surface oxide layer and create good contact between particles. Consequently, consolidation can be carried out at much lower temperatures. However, HPT produces a material too small for meaningful mechanical testing and practical use. On the other hand, ECA deformation, although capable of producing large volume materials, usually requires compacting and canning of particles and full density may not be achieved after one or even multiple passes.
It is therefore desirable to combine the high pressure available in HPT and the ability to produce uniform shear in a large volume material displayed by ECA deformation. In this investigation, a back pressure was applied in ECA deformation to achieve such a combination. The bulk material made by back pressure equal channel angular consolidation (BP-ECAC) of pure Al particles at a low temperature of 100 °C showed good bonding between particles, full density and excellent mechanical properties, compared to that processed using pure Al ingot. It demonstrates that BP-ECAC is a viable new process for synthesising bulk materials from particles.
Section snippets
Experimental materials and procedures
The pure Al powder used was produced by atomisation and supplied by ECKA Granules Australia. The specified composition of the as-received powder was as follows: Al > 99.7 wt.%, Si < 0.10 wt.%, Fe < 0.20 wt.%, other each <0.02 wt.% and other total <0.10 wt.%. The analysed composition of the as-received powder is shown in Table 1, conforming to the specified ranges. The particle size distribution was determined using a Coulter LS130 laser diffraction analyser and the results are displayed in Fig. 1. The
Microstructures
The optical microstructure of the PM material after BP-ECAC at 100 °C is shown in Fig. 4. Good bonding between particles was obvious with no pores observed. The grains had an average size of 7.2 μm, considerably finer than the as-received particles (Fig. 1). For comparison, the microstructure of the IM material after BP-ECAD under the same conditions is shown in Fig. 5. The average grain size was 535 μm. Although the IM material was also refined by ECAD, compared to the as-cast ingot with an
Discussion
What has been accomplished in this investigation is of great significance to both science and technology of particle consolidation. It has demonstrated beyond doubt that excellent bonding between particles can be achieved at low temperatures by forcing them to undergo severe shear deformation under moderate compressive stress. Shearing exposed fresh and clean surfaces of the particles and compressive stress ensured good contact between them. Under these conditions bonding occurred spontaneously
Summary
- (1)
Pure Al particles were successfully synthesised into fully dense bulk material using back pressure equal channel angular consolidation (BP-ECAC) at temperatures much lower than those used in normal sintering.
- (2)
BP-ECAC was superior to ECA processes without back pressure as full density and good bonding throughout volume were achieved in just one pass with no need to pre-compact and can the loose particles.
- (3)
The PM material resulting from BP-ECAC possessed strength and ductility comparable and
Acknowledgements
This project was supported in part by the Australian Research Council and in part by the University of Melbourne under the Melbourne Research Grants Scheme. Assistance received from Comalco Research and Technical Support and ECKA Granules Australia is gratefully acknowledged.
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