A modification on ECAP process by incorporating torsional deformation

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Abstract

In the present study, integration of equal channel angular pressing (ECAP), as a well known severe plastic deformation (SPD) technique, and torsion deformation, is studied by using three dimensional finite element analysis. This process is to be named as torsional-equal channel angular pressing (T-ECAP). In this modification a part of the exit channel in the ECAP die is rotating around its axis, to impose extra shear strains to the samples. To study deformation behavior in the T-ECAP process, three-dimensional finite element analysis (FEA) was carried out by using the elasto-plastic finite element analysis ABAQUS/Explicit Simulation. To investigate the validity of the simulation results, experimental studies were furthermore performed on commercially pure aluminum (AA 1050). Vickers hardness test was used to determine the distribution of hardness on both normal and longitudinal sections of the deformed samples with respect to the exit channel of the die. The hardness test results showed more uniform distribution of hardness in both sections of the T-ECAP processed samples regarding the ones produced by ECAP process. The load requirement comparison for performing both processes showed lower value for the T-ECAP with respect to the ECAP process. The simulation results for the strain values showed higher magnitude and more uniform distribution for the T-ECAP with respect to the ECAP process.

Research highlights

► T-ECAP process is introduced as a modified SPD technique using 3-D finite element analysis. ► Process load is lower in T-ECAP process in comparison to ECAP process. ► T-ECAP process shows more uniform strain distribution in comparison to ECAP process. ► Higher amount of strain is imposed to the sample during each pass in T-ECAP process. ► The exit channel rotation is diffused to the samples’ center.

Introduction

Ultra fine grained (UFG) materials found lots of attention due to their unique physical and mechanical properties. There are two main approaches to process these materials, including bottom-up and top-down approaches. In the bottom-up approach the UFG materials are fabricated by assembling individual atoms or by consolidation of nano particles. This approach includes processes in which nano sized particles produced by inert gas condensation [1] or cryogenic ball milling and consolidated subsequently [2]. The top-down approach is different since it starts with materials with conventional microstructure. Microstructural refinement is introduced through heavy straining, as in severe plastic deformation (SPD) techniques. The main advantages of the top-down approach are the reduction of contamination and remained porosity, which are inherent features in the materials produced using bottom-up approach [3], [4], [5].

It has been shown that UFG or sometimes nano crystalline (NC) materials can be obtained using severe plastic deformation [6], [7], [8]. Up to now different SPD processes have been proposed such as equal channel angular pressing (ECAP) [9], high pressure torsion (HPT) [10], multiaxial compression/forging (MAC/F) [11], twist extrusion (TE) [12], and simple shear extrusion (SSE) [13] for processing bulk materials. Also constrained groove pressing (CGP) [14] and accumulative roll bonding (ARB) [15] are proposed for processing sheet materials. To produce UFG materials using these methods, several passes must be carried out which are time consuming and increase the total cost. Some practices were carried out to overcome these problems e.g. twist channel angular pressing [16], dual equal channel lateral extrusion (DECLE) [17], multi pass equal channel angular pressing [18] and torsion extrusion [19]. In these processes the specimen subjected to intense strains just in one pass, but they have carried some limitation including higher load requirement with respect to the processes mentioned at first. To overcome these problems, in the present study the feasibility of combination of well known SPD process, ECAP, with torsional deformation is studied by using three dimensional finite element analysis. This method which is an integration of conventional ECAP and torsion deformation can be named as torsional-equal channel angular pressing (T-ECAP) process.

In both ECAP and T-ECAP techniques the material bent through an angle (channel intersection angle; φ) from vertical channel into horizontal (exit) channel. The main difference of these two processes is that a part of the exit channel in the T-ECAP process is rotating around its axis in contradiction with the ECAP process in which the exit channel is stationary. This rotation results in some shear strain which is imposed on the material during single pass and it might be expected to achieve faster grain refinement than what can be obtained in ECAP process. It should be noted that, in the present work, the effect of applied modification on the grain refinement is not investigated.

However, in the current study, different aspects of the T-ECAP process, in comparison with the conventional ECAP process, are investigated, which can be categorized as follows: (1) load requirement for performing both processes, (2) the possible magnitude of effective strains, which can be applied in one pass, (3) the distribution of strains in the produced samples and, (4) mechanical properties such as hardness to determine its distribution on different paths of the processed sample.

To study some parts of the above aspects, finite element analysis (FEA) was carried out by utilizing commercial elasto-plastic finite element analysis ABAQUS/Explicit Simulation [20]. This attempt was performed to evaluate the plastic deformation behavior of material during ECAP and T-ECAP processes and also to calculate the load and strain conditions after applying the mentioned processes.

Section snippets

Experimental procedures

Commercially pure aluminum (AA1050) was used as a starting material. Billets of 14 mm in diameter and 70 mm in length were machined out from an AA1050 thick sheet. The billets were annealed at 450 °C for 2 h and then furnace cooled at a rate of 25 °C/h. The annealed hardness of the aluminum was 23 HV. To perform the T-ECAP process, a special die was designed. Fig. 1 shows a two dimensional schematic view of the die.

The channel intersection angle was 90°(φ = 90). As it can be seen, a part of the exit

Finite element analysis procedure

Plastic deformation behavior of the specimens during the ECAP and T-ECAP processes was simulated by using the commercial elasto-plastic finite element analysis ABAQUS/Explicit Simulation. The simulations were performed by using 3D models in which the geometrical dimensions and mechanical properties of the specimens were chosen as the same as those of the experiments. This selection makes it possible to compare the simulation results with those obtained by the experiments.

To determine the

Comparison of ECAP and T-ECAP

As discussed in the previous sections, the only difference between ECAP and T-ECAP is the rotation of a parts of the exit channel around its axis in the T-ECAP. This rotation may lead to some advantages.

Fig. 3 shows the experimental load–displacement curve for the ECAP and T-ECAP processes performed at the mentioned conditions. As it can be seen, lower load is required for the T-ECAP process in comparison with the ECAP process. Mathieu et al. show that with a special design, in which the entry

Conclusions

An integration of equal channel angular pressing and torsion deformation is developed using three dimensional finite element analysis, named as T-ECAP process. In this method a part of the exit channel in well known ECAP process is rotating around its axis. The experimental results on commercially pure aluminum show that the required load for performing the T-ECAP process is lower than that of the ECAP process in the same conditions. This phenomenon arises from the change in friction mode in

Acknowledgment

The authors would like to acknowledge the financial support of Shiraz University through the grant number of 88-GR-ENG-16.

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