PSI - Issue 21

Nathaniel Mupe et al. / Procedia Structural Integrity 21 (2019) 73–82 Mupe et al./ Structural Integrity Procedia 00 (2019) 000 – 000

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1. Introduction Magnesium alloys are attractive as structural materials since they have low density, high stiffness and specific strength. Grain refinement of metals and alloys through SPD methods which employ simple shear have been extensively used to improve their mechanical properties. At room temperatures Mg alloys exhibit poor ductility and formability because of the hexagonal close-packed (HCP) structure. The activation of non-basal slip systems in Mg alloys needs elevated temperatures to avoid shear crack formation [Han et al. (2016)]. Some of the most developed SPD techniques studied on magnesium alloy AZ31 are equal channel angular pressing (ECAP) [ Hilšer et al. (2017 ); Peláez et al. (2015)], high-pressure torsion (HPT) [Huang et al. (2013)], accumulative back extrusion (ABE) [Fatemi Varzaneh et al. (2015)] and multi-directional forging (MDF) [Miura et al. (2008)]. Despite achieving homogenous and refined grains, they suffer from discontinuous production that can be feasible in laboratory scale, but not appropriate for commercial production. Thus, a recent method called twist extrusion (TE), with relatively low strain, was developed by Y. Beygelzimer et al. (2002). The convectional TE which is linear is based on pressing out a rectangular prism specimen through a die. The die profile consists of three zones; at the entry zone 1 is prismatic, in the middle zone 2 is a twist part and at the exit zone 3 is prismatic. The mode of deformation in zones 1 and 3 is simple shear in the transverse plane. The shear in zone 3 is in opposite direction to zone 2 and thus straightens the formerly twisted specimen into its original shape. The zones 1 and 2 provide the deformation along the extrusion axis where the equivalent strain is zero. The twisted part of the die in zone 2 has two distinct sections. The first section consists of most of the specimen volume and deformation occurs through simple shear in the rotating longitudinal plane. The second section comprises of most of the specimen periphery layer and deformation is mode is severe simple shear along the cross-section contour [Y. Beygelzimer et al. (2002)]. Salehi et al. (2014) states that this method has limited material waste and can easily be used for commercial production since the direction of specimen flow is the same. A drawback of linear TE is that strain is localized at the inlet and outlet of the twisting channel. The billet is subjected to high strain and rigid body rotation resulting to a high punching force and perhaps inconsistent strain distribution. Chengpeng et al. (2012) states that other challenges include low life mould, complex die designs, process continuity breaks and low efficiency. This study proposes Nonlinear twist extrusion (NTE) technique. It is purported to overcome the challenges faced in linear TE and significant efficacy is expected compared to other conventional SPD methods. The NTE die profile has three parts as illustrated in Fig. 1. Part I cross-section consists of the circular channel of radius r that transforms into an elliptical shape channel whose major and minor axis are a and b , respectively. Part II consists of the nonlinear twisting channel. The billets is twisted according to the parabolic relation = while in conventional TE, it is assumed that the rotation angle increases linearly with displacement, = [Yalçinkaya et al. (2019)]. This shows that the billets rotate more in NTE than TE thus being subjected to more deformation. In part III, the cross-section transforms from elliptical back to initial circular shape but the twist profile maintains a constant rotation angle. The rate of change of twisting in part I to II is moderate to neutralize stress concentration resulting from rapid increase of shear strain. This study focuses on the following areas: efficacy of NTE method in improving the mechanical properties; the relationship between microstructure evolution and the mechanical properties. The specific objective was to investigate the effect of low temperature condition during NTE since other SPDs studies indicated failure due to specimen fractures [Agnew et al. (2004)]. Nomenclature θ rotation angle of twist channel a major axis of elliptical shape cross section of the channel r radius of the circular cross section of the channel L length of the linear twisting geometry x displacement along the longitudinal axis n parameter, which is 1 for TE and 1,2, 3..., n for NTE 2. Material Wrought magnesium alloys AZ31 with composition shown in table 1 was used in the study. The commercial AZ31 alloy was supplied as extruded rods. Each rod was measuring Φ 9.5 mm by 210 mm. Billets sizes Φ 9.5 mm by 30