Back pressure equal channel angular consolidation—Application in producing aluminium matrix composites with fine flyash particles

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Abstract

Back pressure equal channel angular consolidation (BP-ECAC) is an innovative process to synthesise bulk materials from particles. It enables particles from nano to micro scales to be consolidated into fully dense materials at much lower temperatures and under much simpler conditions. It is particularly useful in consolidating particles with non-equilibrium microstructures and in producing metal matrix composites (MMCs). As an example, BP-ECAC was used in this investigation to produce aluminium matrix composites with flyash particles. Compared to the conventional powder metallurgy (PM) and ingot metallurgy, BP-ECAC is more efficient and capable of incorporating very fine particles at higher volume fractions with more uniform particle distribution. In particular, it can be used to produce master composite billets for subsequent melting and casting.

Introduction

Consolidating particles into bulk materials for various applications has evolved from an ancient art to modern science and engineering over the history of human civilisation. Although melting and casting of metals had overtaken clay shaping and firing as the most important manufacturing technology for a long time, fabrication using particles has many advantages over using melt or bulk solid. First of all, materials that are difficult to melt or form have to be shaped using particles, as in the case of ceramics to which clay belongs. Solidification is often accompanied by severe segregation of alloying elements, causing undesirable consequences such as brittleness and lowered strength. More importantly, many non-equilibrium phases such as nano and amorphous structures can be most economically produced in the form of particles; these non-equilibrium structures are critical to the enhancement of many materials properties such as strength and stiffness. It is increasingly desirable to form a new material using a variety of phases which cannot be as easily produced using solidification and heat treatment alone, e.g. metal matrix composites (MMCs).

The conventional powder metallurgy (PM) relies on sintering at elevated temperatures to achieve consolidation. Bonding between particles is achieved through diffusion which requires both high temperature and long time. This not only makes the process expensive but also tends to destroy any non-equilibrium structures existing in particles. Without any assistance from pressure or liquid, full density is also hard to obtain. It is thus desirable to develop a consolidation process at much lower temperatures and for much shorter times.

One of the most promising methods makes use of severe plastic deformation (SPD) to achieve consolidation. Both high-pressure torsion (HPT) [1], [2], [3], [4] and equal channel angular (ECA) deformation [5], [6], [7] have been used. The particles can be consolidated at much lower temperatures (even room temperature). However, the thickness of material produced by HPT is very limited and shear is not uniform from the centre to the edge. With ECA deformation, much larger samples and uniform shearing across the volume can be achieved. However, full density may not be reached without multiple passes [6].

Recently, it has been demonstrated that application of a back pressure during ECA deformation is effective to produce fully dense materials from consolidating Al, Ti and nanometre sized Al particles [8], [9], [10]. The so-called back pressure equal channel angular consolidation (BP-ECAC) process was carried out at temperatures much lower than those used in sintering, with no need to can and degas the particles. Full bonding was achieved between particles instantaneously as they passed through the shearing zone. Nanometre sized grains were achieved from consolidating nano particles.

One obvious application of BP-ECAC is to synthesise MMCs. Particulate reinforced MMCs may be solidification processed by stirring ceramic particles into metal melt [11]. However, it is difficult or impossible to incorporate very fine particles (typically ceramic particles have to be >∼5–10 μm) and the volume fraction is limited to around 20–30%. The process and the compositions of the alloy and the particles have to be tightly controlled to avoid excessive reaction between particles and the melt. The manufacturing is often expensive. Alternatively, MMCs can be produced by powder metallurgy [12] in which the ceramic and metal particles are mixed before sintering although the presence of ceramic particles makes the consolidation process even more difficult and secondary process is often necessary to eliminate pores.

In the present investigation, BP-ECAC was used to synthesise Al based composites containing flyash particles, ULTALITE™.1 Flyash is a by-product of coal power stations. Much of the flyash is presently treated as a waste product for uses such as land fill although a small quantity is utilised as fillers in concrete and other materials. In an earlier project [13], flyash particles were successfully incorporated into aluminium alloys through solidification processing and their uses in automotive applications such as brake components were demonstrated [14]. It is shown here that BP-ECAC is capable of producing ULTALITE™ with more uniform particle distributions, using finer particles or as master billets of high volume fractions for diluting and casting.

Section snippets

Experimental materials and procedures

For BP-ECAC, the raw materials used were pure Al powder and flyash particles. The pure Al powder was supplied by ECKA Granules Australia. The particles were produced by atomisation with the following specified composition: Al > 99.7 wt.%, Si < 0.10 wt.%, and Fe < 0.20 wt.%. The particles were analysed using inductively-coupled plasma atomic emission spectroscopy which revealed the following composition: Al–0.02 wt.%Si–0.11 wt.%Fe, conforming to the specification. Analysis using a LECO RO-416DR instrument

Results and discussion

The microstructures of the as-consolidated MMC billets are shown in Fig. 6, Fig. 7 for materials containing the coarse and fine flyash particles, respectively. It is observed that individual particles were all well surrounded by Al matrix and the interfaces appeared good, as illustrated by the SEM microstructure in Fig. 8.

The microstructures of the as-cast diluted MMC ingots with the coarse and fine flyash particles are shown in Fig. 9, Fig. 10, respectively. It is noted that the MMC ingot with

Summary

BP-ECAC was successfully employed to produce the ULTALITE™ aluminium composites containing flyash particles.

Very fine particles with a significant proportion <2 μm and high volume fraction (50%) were BP-ECA consolidated into MMC billets with uniform particle distribution and good interface between particles and matrix. The processing temperature was much lower and time much shorter than those used in conventional sintering. The master MMC billet containing concentrated flyash might be diluted in

Acknowledgements

This project was supported in part by the Australian Research Council under the Linkage Project scheme with Cyco Tech as the industry partner, and in part by the University of Melbourne under the Melbourne Research Grants Scheme. We are grateful to ECKA Granules Australia for kindly providing the Al particles used and to Comalco Research and Technical Support for chemical analysis.

References (14)

  • J. Sort et al.

    Acta Mater.

    (2003)
  • Z. Lee et al.

    Scr. Mater.

    (2004)
  • A. Parasiris et al.

    Scr. Mater.

    (2000)
  • O.N. Senkov et al.

    J. Alloys Compd.

    (2004)
  • K. Xia et al.

    Scr. Mater.

    (2005)
  • J.M. Torralba et al.

    J. Mater. Proc. Technol.

    (2003)
  • I.V. Alexandrov et al.

    Metall. Mater. Trans. A

    (1998)
There are more references available in the full text version of this article.

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