Elsevier

Computational Materials Science

Volume 81, January 2014, Pages 79-88
Computational Materials Science

Influence of outer corner angle (OCA) on the plastic deformation and texture evolution in equal channel angular pressing

https://doi.org/10.1016/j.commatsci.2013.07.006Get rights and content

Highlights

  • Strain decreases obviously with the increasing OCA.

  • 20° Of OCA leads to optimal strain distribution.

  • Texture evolution is dependent on the OCA during ECAP process.

Abstract

Equal channel angular pressing (ECAP) is one of the most promising severe plastic deformation (SPD) techniques for fabrication of bulk ultrafine-grained (UFG) materials. There are many factors influencing the ECAP process which consist of the ECAP die geometries, strain paths, processing numbers, frictional conditions, processing temperatures, materials properties and so on. In this study, a crystal plasticity finite element method (CPFEM) model was used to investigate the influence of outer corner angle (OCA) of an ECAP die on the plastic deformation and texture evolution during the ECAP process. The simulation results revealed that Ψ = 20° resulted in the optimal distribution of effective plastic strain in the deformed sample for a fixed die channel angle of 90°. It should be noted that the angle of Ψ  30° lead to similar textures after ECAP due to the development of corner gaps. Therefore, this study can offer valuable guidelines for the design and fabrication of ECAP dies in the future work, particular from the view of texture evolutions.

Introduction

As one of the major severe plastic deformation (SPD) techniques, equal channel angular pressing (ECAP) process has been successfully used to produce ultrafine-grained (UFG) bulk metallic materials with special properties [1]. ECAP process is different to the earlier ‘bottom-up’ fabrication techniques of UFG materials, such as gas condensation, ball milling and electrodeposition, which are limited to the production of fairly small samples, together with some degree of residual porosity and a low level of contamination [2]. ECAP process is based on the microstructure refinement in bulk billets by accumulating plastic strain without changing their cross-sectional shapes during the multi-pass processing. Under the ideal conditions, this deformation was assumed as simple shear along the intersecting plane of two channels [3].

The final size of grains produced by ECAP depends on the materials and processing features [4]. Iwahashi and co-authors [5] have processed high purity aluminum with an initial grain size of ∼1.0 mm and their results revealed that the grain size could be reduced to ∼4 μm after a single pass. In addition, a rotation of the sample between two consecutive passes lead to a more rapid evolution of the subgrain boundaries into the high angle boundaries (HABs) and the final grain size was about 1 μm after 10 passes of ECAP process. According to their later experiments [6], the subgrains evolved more rapidly using route B, less rapidly using route C and the evolution was slowest using route A. By contrast, the equilibrium grain size was much smaller (∼0.27 μm) due to the lower stacking fault energy and the associated lower rate of recovery after processing to a total strain of ∼10 [7].

During the ECAP process, major influencing factors on the accumulated plastic strain are the die channel angles and processing numbers. At the very early stage, the effective strain was calculated from the following relationship [8]ε=cosh-1NcotΦ2where N is the number of ECAP passes and Φ is the die channel angle. However, Iwahashi et al. [9] suggested that Eq. (1) was based on the incorrect assumptions incorporated into the analytical treatments and he proposed a new equation in a later report [3] to alleviate the deficiency of Eq. (1) as following for the sharp die shown in Fig. 1(a)ε=2N3cotΦ2

The die channel angle can be specifically designed for different materials, varying from 60° to 150°. For pure aluminum, an angle of Φ = 90° is the best choice for achieving excellent grain refinement and a large fraction of HABs [10]. However, the finite element analysis of the ECAP die with Φ = 90° and Ψ = 0° reveals that, it is very difficult to completely fill the die corner for less ductile materials except under conditions where a back pressure is applied in the exit channel [11]. Therefore, many experimental works were conducted with the die having an OCA instead of a sharp one to improve the material filling [12], [13], [14]. According to the analytical analysis [9], the effective strain estimation for the billets through the ECAP die with an outer corner angle (Ψ) between two channels as shown in Fig. 1(b) can be expressed byε=N32cotΦ2+Ψ2+ΨcosecΦ2+Ψ2where Ψ varies from 0° to π  Ψ. According to this equation, the strain in each pass only depends on the values of Φ and Ψ, and it increases while these two parameters decrease. For a single pass of ECAP process, the effective plastic strain increases from the minimum 0.605 to the maximum 0.667, with decreasing the OCA from Ψ = 60° to 0° when Φ is fixed at 120°, while the maximum strain and minimum strain are 1.155 and 0.907 respectively for Ψ = 0° and 90° when Φ is fixed at 90°.

In the last two decades, a number of studies have been devoted to understand the influence of OCA on the material flow behavior and deformation inhomogeneity during the ECAP process based on the classic finite element modeling [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], physical modeling [16], [18], [27], [31] and experimental [18], [27], [31], [32] techniques. For example, Suh et al. [15] observed that a bigger Ψ lead to a more severe deformation inhomogeneity. Park and Suh [16] compared the deformation mode in a sharp corner die (Ψ = 0°) with a round corner die (Ψ = 90°), and they concluded that the uniform deformation in the sharp die was governed by simple shear deformation while a gradual bending of the material occurred instead of shear deformation in the round corner die. Xu and Langdon [32] processed two aluminum alloys through the ECAP dies having the same die channel angle of Φ = 90° but with different OCA, namely Ψ = 0° and 28°, respectively. They have found that both samples exhibited essentially identical homogeneity after four passes and no additional microhardess inhomogeneity was introduced using the die with a round corner. Based on the FEM simulations, Nagasekhar and Tick-Hon [24] found that 10° was the best OCA for the 90° ECAP die to produce the optimal strain homogeneity in the sample. However, a totally different observation was obtained in a later FEM simulation conducted by Xu and co-authors [26] which revealed that the deformation distribution was uniform when the OCA is smaller than Ψ = 28° and their result was consistent with the study [18] but contradictory with the investigation [33]. Besides, another recent study [19] showed that the optimum outer corner angle for the 90° die should be 3°. Therefore, a careful and systematical understanding on the influence of OCA is still necessary. It should also be noted that all the above mentioned studies only focused on the influence of outer corner angle on the material flow and deformation behavior, and none of them studied its influence on the texture evolution during the ECAP process which is essential in the metal forming fields.

In our previous studies, we have studied the influences of frictional conditions between the sample and ECAP die channels [34], and the sample dimensions [35] on the deformation heterogeneity and texture evolution during the ECAP process using a CPFEM model. This study was designed specifically aiming to extend the same approach to the investigation of the influence of OCA on the plastic deformation and texture evolution of aluminum single crystals after one pass of ECAP process. This work will be very useful and is able to offer systematical information on the ECAP die design and fabrication in the future work, particular from the view of texture evolutions.

Section snippets

Numerical analysis

According to the literature [36], the simulated texture can be significantly influenced by the degree of accuracy in describing the deformation history during the ECAP process. The predicted textures based on the deformation histories provided by the FEM simulations [37], [38], [39] or analytical flow line models [40], [41] are in better agreement with the experimental results than the simple shear model [3], [42], [43] which is only suitable for the ideal conditions.

In this regard, a fully

Deformed mesh and load curve

The CPFEM simulations have been terminated when the processing time t equals 450 s. Deformed mesh of the samples during the ECAP process under different OCAs has been shown in Fig. 2. It is obvious that the distortion is not uniform along the sample thickness for all the OCAs which represent the inhomogeneous material flow during ECAP. Fig. 2(a) shows the mesh of the sample with Ψ = 0°. As can be seen, three regions can be distinguished along the sample axis (ED), namely the leading head,

Conclusions

  • 1.

    It is the first attempt to apply a crystal plasticity FEM model to study the influence of outer corner angle during the ECAP process and all the simulations have been successfully finished. The simulated die channel angle was fixed to 90° with the OCA varying between 0° and 90°.

  • 2.

    Significant influence of OCA on the material flow, PDZ shape, Mises stress distribution and the deformation heterogeneity has been observed. Plastic deformation was found to deviate from the ideal simple shear along the

Acknowledgements

This work was supported by an Australia Research Council Discovery Grant. Simulations were performed using the HPC cluster of University of Wollongong and the computing facilities provided by NCI National Facility of Australia.

References (50)

  • R.Z. Valiev et al.

    Prog. Mater. Sci.

    (2006)
  • Y.T. Zhu et al.

    Scr. Mater.

    (2004)
  • V.M. Segal

    Mater. Sci. Eng. A

    (1995)
  • Y. Iwahashi et al.

    Acta Mater.

    (1997)
  • Y. Iwahashi et al.

    Acta Mater.

    (1998)
  • Y. Iwahashi et al.

    Scr. Mater.

    (1996)
  • K. Nakashima et al.

    Acta Mater.

    (1998)
  • P.B. Prangnell et al.

    Scr. Mater.

    (1997)
  • L.H. Su et al.

    Mater. Lett.

    (2011)
  • L.H. Su et al.

    Acta Mater.

    (2012)
  • J.Y. Suh et al.

    Scr. Mater.

    (2001)
  • W. Wei et al.

    Scr. Mater.

    (2006)
  • S.C. Yoon et al.

    J. Mater. Process Technol.

    (2007)
  • C.J. Luis-Perez et al.

    J. Mater. Process Technol.

    (2004)
  • H.J. Hu et al.

    Trans. Nonfer. Metal Soc. China

    (2010)
  • A.V. Nagasekhar et al.

    Comp. Mater. Sci.

    (2004)
  • Y.L. Yang et al.

    J. Mater. Process Technol.

    (2003)
  • S.B. Xu et al.

    J. Mater. Process Technol.

    (2007)
  • F. Djavanroodi et al.

    Prog. Nat. Sci. Mater. Int.

    (2012)
  • G.M. Stoica et al.

    Mater. Sci. Eng. A

    (2005)
  • C. Xu et al.

    Scr. Mater.

    (2003)
  • Y. Wu et al.

    Scr. Mater.

    (1997)
  • G.Y. Deng et al.

    Compos. Mater. Sci.

    (2013)
  • I.J. Beyerlein et al.

    Prog. Mater. Sci.

    (2009)
  • S. Li et al.

    Acta Mater.

    (2005)
  • Cited by (21)

    • EBSD study of the microstructure and texture evolution in an Al–Si–Cu alloy processed by route A ECAP

      2021, Journal of Alloys and Compounds
      Citation Excerpt :

      Equal channel angular pressing (ECAP) is one of the most promising techniques to refine microstructures of ductile metals and alloys by severe plastic deformation (SPD) for structural applications [7–9]. There has been a great interest over the last decade to investigate the effects of different ECAP parameters including die channel angle [10], outer angle [11], number of ECAP passes [12], rotational route [13], back pressure [14], friction [15], pressing speeds [16], and temperature of ECAP process [17]. Texture is the distribution of crystallographic orientations of polycrystalline materials.

    • 3D FEM simulation of Al-Zn-Mg-Cu alloy during multi-pass ECAP with varying processing routes

      2021, Materials Today Communications
      Citation Excerpt :

      At present, the finite element method (FEM) has become a powerful technique to analyze complicated engineering problems [20]. The deformation behaviour, distribution of strain or stress, metal flow and simulated load-stroke curves can be easily obtained by FEM for ECAP and other metal forming processes [21]. Agwa et al. [2] observed that inner corner angle (Φ), outer corner angle (Ψ), and friction have a great role in strain homogeneity, strain coefficient of variance, and load-punch curve evolution.

    • Crystal rotation and microstructures in an aluminum single-slip system under tensile loading

      2018, Materials Characterization
      Citation Excerpt :

      In recent years, crystal plasticity modeling has been studied for application to cold rolling [1,2].

    • Theoretical and experimental investigation of thermal and oxidation behaviours of a high speed steel work roll during hot rolling

      2017, International Journal of Mechanical Sciences
      Citation Excerpt :

      Even though there are several similar measurements on hot rolling of aluminium alloy were conducted in laboratory [6–8], however, those results were not comparable with the industrial cases because a lot of practical influencing factors were not been able to be considered in laboratory. Except the experimental tools, computational model is fortunately nowadays a powerful and reliable tool for simulating different thermo-mechanical-metallurgical processes from macro-, micro- to nano-scale size, with quick development of computer skills [9–11]. To date, a large number of investigations have already been successfully conducted on modelling thermal behaviours of work rolls during hot rolling.

    View all citing articles on Scopus
    View full text