Study on the feasibility of the torsion extrusion (TE) process as a severe plastic deformation method for consolidation of Al powder
Introduction
Synthesis of bulk materials from particles has been practiced 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. Powder consolidation is the second vital step in the development of the material with controlled microstructure after the powder fabrication technique in the powder metallurgy route. The last step of conventional powder metallurgy route is sintering process in which the compacted sample keeps at high temperature to remove it porosities and make it nearly dense. In this step the fired microstructure of the sample may be coarsen which is not suitable. Usually it is desirable to employ a special consolidation process that allows lower processing temperature.
An effective and promising method of achieving good consolidation without destroying the microstructure is by using severe plastic deformation (SPD). Recently many efforts were led to achieving a fully dense product through consolidation of metallic or metallic based composite powders by SPD processes, e.g., ECAP [1], high pressure torsion straining [2], back pressure equal channel angular pressing [3] and ECAP-Forward Extrusion [4].
One of the above technique used widely is high-pressure torsion (HPT) in which plastic strain in the order of 5–10 is applied under a pressure in the order of 5 GPa [5], [6]. Consolidation of powders of Al [7], Fe [8], Ti [9], Co [10], Ni [11], amorphous Al [2], [12], nano-metal matrix composites (nano-MMCs) [13], [14] and others has been carried out by this technique. Another process used is equal channel angular pressing (ECAP) in which very large plastic deformation can be accumulated with no cross-section reduction [15]. Some of the powders that have been consolidated using ECAP include Al [16], Cu [17], amorphous Al [18], [19], and amorphous Cu [20].
The advantages of all these processes are the presence of the shear strains leads to obtain products with densities near their theoretical values. The intense shear strain in HPT and ECAP is shown to disrupt the surface oxide layer and create good bonding between particles. Lapavok et al. [1] investigated the effects of shear strain in omission of the porosities in ECAP. The most remarkable effects of using shear strain for powder consolidation were seen to be the change in the size, shape, and distribution of the pores when shear deformation is implemented [1]. As a result, consolidation can be carried out at much lower temperatures. In fact, HPT is often carried out at room temperature thanks to the huge hydrostatic pressure applied [5], [6].
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 pre-compacting and canning of particles and as the effective strains per pass are usually equal or less than one (ɛ ≤ 1). Therefore enough strains would not be created for final consolidation through each single pass [21]. Furthermore, in the above mentioned processes, the presence of back pressure is necessary for obtaining fully dense products. In addition, each pass requires billet preparation, pre-heating and lubrication. As a result, such processes are both time and labor consuming with a high production cost. 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.
The aim of the present study is to introduce a method to put aside those disadvantages while maintaining the advantages. To reach this aim, the feasibility of torsion extrusion (TE) process, which is also known as shear extrusion (SE) process, was investigated for demonstrating the potential for cost effective, enhanced consolidation of atomized commercially pure Al powder at 350 °C. This method comprises the conventional forward extrusion process (high pressure) and a steadily rotating die for achieving the desired shear strain during one pass production. Moderate shear strain due to material twisting was also generated in torsion extrusion process [22]. This method provides these potential advantages: (i) elimination of multi pass processing, (ii) obtaining long rods of full density during single pass production, (iii) no need for further instruments for providing back pressures (iv) capability of performing the desired strains in a specimen by changing the angular velocity and extrusion ratio and (v) a decrease in the required extrusion load as the die or punch is rotating.
Section snippets
Materials and experimental procedures
In the present study, air atomized commercially pure aluminum powder was used as a starting material with an average particle diameter of 45 μm. For comparison, a commercially pure Al Ingot (referred as Ingot sample) and a bulk sample of Al powder consolidated by conventional hot extrusion (referred as FE sample) were used. The bulk material produced by TE process is also referred to as TE sample. The Ingot sample was annealed at 500 °C for 2 h and an average grain size of about 85 μm was obtained.
The pore elimination mechanism
A mechanism suggested schematically for pore shape change and closure under the combination of shear deformation and hydrostatic pressure during torsion extrusion is shown in Fig. 2. As it was explained before, the upper part of the billet which is in the container does not rotate while its lower part, which is in contact with the rotating die, is rotating. In result, intensive simple shear strain arises inside a narrow layer S [21] as shown in Fig. 2, which is designated by simple shear band
Conclusions
Commercial pure aluminum powder was successfully consolidated by using torsion extrusion process at 350 °C. Torsion extrusion includes the conventional forward extrusion process while using a steadily rotating die. The effects of the hydrostatic stress during extrusion with enhanced shear strain by torsion results in significant improvements in the relative density and mechanical properties. As a consequence, the TE process is a good candidate for producing a bulk rod with high percentage of
Acknowledgment
The authors would like to acknowledge the financial support of Shiraz University through the grant number 88-GR-ENG-16.
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