Effects of twist channel angular pressing on structure and properties of bimetallic Al/Cu clad composites

Effects of twist channel angular pressing on structure and properties of bimetallic Al/Cu clad composites Radim Kocich ⁎ Department of Material Forming, Faculty of Metallurgy and Materials Engineering, VŠB TU, Ostrava 17. Listopadu 15, 70833 Ostrava-Poruba, Czech Republic HIGHLIGHTS •TCAP was used for composite preparation for the first time ever. •Single pass TCAP imparted significant grain refinement and hardening. •Double pass TCAP introduced homogenisation of imposed strain. •Well bonded Al/Cu clad composites were successfully fabricated. GRAPHICAL ABSTRACT abstractarticle info Article history: Received 15 September 2020 Received in revised form 21 October 2020 Accepted 21 October 2020 Available online 24 October 2020 Keywords: Clad composite Finite element method Twist channel angular pressing TCAP Effective strain Residual stress This study documents the effects of the twist channel angular pressing (TCAP) on the structure and selected properties of Al/Cu clad composites. Processing of the designed clad composite via single, as well as double pass TCAP was performed experimentally, and simulated via the finite element method. The deformation behaviour predicted via the simulation was verified experimentally by 3D Micro-CT scanning, and by structure analyses. The results revealed that the single pass introduced significant deformation strengthening of the Al/Cu composite components, and after the second pass, the effective imposed strain in the Al sheath approached the value of 5. The composite sheath also exhibited more or less homogeneous distribution of residual stress of the lowest observed absolute values, and the smallest observed average grain size - slightly above 1.5 μm. The Cu grains in the structures of the wires within the extruded composites also refined significantly, especially after the second pass, to almost 3 μm. However, the analysed parameters exhibited slight variations across the cross-sections of the composite billets. This can be attributed to phenomena related to the (differences in the) plastic flow. The intensive imposed shear strain also imparted increase in microhardness and changes in thermal conductivity. © 2020 The Author(s). Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). 1. Introduction The trend in research and innovations in various commercial and industrial branches, including medicine, civil engineering, transportation, oil industry, aerospace industry, or energetics is to develop modern structural materials with optimized mechanical properties, especially as regards strength and ductility [1–3]. Among the methods applied during the research are technologies imposing plastic deformation, the grain size of metallic materials using which can effectively be reduced down to the ultra-fine-grained (UFG) scale, or nano-scale. The challenge is, however, finding the processing parameters providing materials featuring optimum ratios between the strength and ductility. Generally, increasing the imposed strain results in decreasing the grain size, which Materials and Design 196 (2020) 109255 ⁎Corresponding author. E-mail address: [email protected]. https://doi.org/10.1016/j.matdes.2020.109255 0264-1275/© 2020 The Author(s). Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Contents lists available at ScienceDirect Materials and Design journal homepage: www.elsevier.com/locate/matdes imparts increase in strength, but decrease in plasticity [4,5]. On the other hand, numerous works documented that this general presupposition can be overcome by modifying the processing conditions [6,7]. The up-to-date research has proven that plastic deformation can be successfully applied to control the development of structure and properties, not only for conventional alloys [8], but also for other metallic compounds [9]. Moreover, optimized SPD processing has been shown to be able to produce required structure patterns in metallic materials [10]. Although the introduction of thermomechanical processing to various production technologies enables to refine the structure very effectively, this approach has certain limitations. By this reason, the unconventional severe plastic deformation (SPD) processes are researched with the aim to prepare various UFG materials. The level of (shear) strain which can be imposed by SPD methods is far beyond that of conventional methods of plastic deformation, such as rolling, forging, drawing, and extrusion. Despite the fact that the group of SPD methods has been researched for more than 4 decades, their popularity is still high given by their positive effects on materials' structure and properties. Moreover, this „top-down“approach of structure refinement [11] can be applied to prepare void and porosity-free bulk UFG materials in volumes sufficient for structural applications [12,13]. The (sub)structure development induced by SPD is complex and can involve various deformation mechanisms (e.g. dislocation slip and climb, deformation twinning, redistribution of elements and phases and their transformation, grains rotations, grain growth, etc.) that can contribute to the final grain refinement. The activation of the particular deformation mechanisms and their extent, as well as the resulting (sub) structure development, is primarily determined by the selected process and processing history, but also by the intrinsic properties of the particular material(stacking fault energy, etc.). With the intention to (dynamically) control the substructure development of the processed materials, quite a vast number of methodsbased on the applicationof simple shear as the primary deformation mechanism has been designed (more detailed characterization of the introduced methods can be found in numerous publications, e.g. [14,15]). The general goal, regardless the applied processing method, is to fabricate the final material with the lowest possible number of processing steps while simultaneously imposing the highest possible homogeneously distributed strain. The development of new innovative materials introduces new possibilities of application of the SPD methods. Whereas the early experimental works dealt with pure metals and commercially available alloys, the contemporary research in the field of processing methods focuses on the preparation of gradient structures and laminates [16], and composite materials [17], also denoted as hybrid materials; the latter is appealing primarily due to the possibility to combine materials with rather contradictory properties within a single composite. Various metallic systems, such as Cu-Ag [18], carbon nanotubes (CNT-Al) [19], Alsteel [20,21], Ti/Cu [22], steel/Al/Cu [23], Mg/Al/steel [24], metallic glasses [25], or Al/Cu [26] can be selected to prepare UFG composite materials. The last mentioned system of composite materials is within the focus of researchers especially for its prospective usage as modern electric conductors. The primary advantage of Al/Cu composites/hybrids lies in the possibility to combine the most convenient properties of both the components, i.e. the high strength and excellent electrical conductivity of Cu with the light weight and low cost of Al, in a single modern material. Cu-based clad conductors are generally very efficient for applications using the alternating current (AC), due to the occurrence of “skin effect,”whereas Cu-based multifilament conductors can be advantageously used for applications using the direct current (DC) and low-frequency AC, due to the additional parallel paths for electrons. However, the characteristics of the composite are non-negligibly affected also by the character of the component metals mutual interface. Previously published studies have documented that increasing the temperature affecting the composite (e.g. via post-process heat treatment) above 300 °C induces formation of brittle intermetallics at mutual interfaces, which can consequently deteriorate the final properties of the composite [26,27]. Nevertheless, the selected preparation/processing technology can influence its formation and provide the possibility to optimise the width and composition of the intermixing zone, which eventually defines the properties of the prepared hybrid material. The research works reporting the effects of high shear strain and high hydrostatic pressure on the (sub)structure development, interdiffusion, and mechanisms of formation of the mutual interfaces within clad composites and hybrids are scarce. The studies documenting the application of high shear strain for preparation of composite materials reported the usage of methods based on rolling, such as asymmetrical roll bonding [28] and accumulative roll bonding [29], cyclic extrusion compression [30], or high pressure torsion (HPT) [31]. Several works have documented the application of methods based on equal channel angular pressing (ECAP) [32,33]. This work presents a detailed study characterizing processing of an Al/Cu clad composite using the twist channel angular pressing (TCAP) method. As documented by previously published studies [34–40], due to the three independent strain paths, TCAP enables to impose relatively high values of homogeneous shear strain into the processed billet within a single pass. However, the mentioned studies only dealt with single-phase materials (aluminium, copper). Since this method has not been applied to prepare composite materials before, the focus of the herein presented study is not only on the experimental preparation of the composite, but also on the experimental and numerical investigation of the deformation behaviour of both the composite components during multiple TCAP process (two subsequent passes). The (sub)structure development and mechanical and selected physical properties of the prepared composite, as well as the processing parameters such as intensity and distribution of the imposed strain, and plastic flow related to the occurrence and development of residual stress, are also characterized. 2. Material and methods The aim of the study was to provide a more detailed description of the TCAP technology and its effects on preparation of Al/Cu clad composites. The schematic depiction of the TCAP die can be seen in Fig. 1a. The die geometry was defined by the following angles: φ= 90°,ω= 90°, β= 40°, ψ= 20°. In order to investigate the effects of TCAP, two consequent passes were performed; multiple TCAP was performed with the application of deformation route A(details can be found e.g. in [40]). The composite sample consisted of a rectangular Al sheath and five circular reinforcing Cu wires (processed sample is depicted in Fig. 1b). 2.1. Numerical modelling The first part of the study deals with the numerical analysis of both the consequent TCAP passes. Forge NxT commercial software was used to analyse the deformation behaviour of the extruded composite and, at the same time, its component metals. The purpose of this part of the research work was to study the effects of the interaction of the different component metals and their plastic flows on the magnitude of effective imposed strain, (in)homogeneity of strain across the crosssection of the extruded sample, as well as possible occurrence of residual stress, its values and localization. Since the prospective application of the reinforced Al/Cu composites is in the electrotechnics, the thermal properties of the components in which are of the utmost importance, FEM, together with the Lattice Monte Carlo (LMC) method, were also applied to investigate the thermal conductivity of the composite extruded via the first TCAP pass. The partial assembly used for the thermal simulation is depicted in Fig. 2a. The predicted thermal properties were subsequently evaluated via ultrasound experimental measurements, and used as the boundary conditions for the following simulation of deformation behaviour. R. Kocich Materials and Design 196 (2020) 109255 2 To enable comparison of the simulation with the subsequent experiment, the numerical simulations were performed using an assembly the geometrical dimensions of the components and mechanical properties of the processed metals in which were identical to the experiment. Both the extruder and die were defined as rigid parts. The billet components were meshed with tetrahedral elements and the clad composite sample was characterized by a mesh with 168,742 nodes in total. Particularly, the Cu wires were meshed with a very fine mesh i.e. each of the wires was defined by 20,475 nodes. The mesh at the mutual Cu/Al interfaces was finer than in the remaining volumes of wires. Since severe shear deformation was expected to proceed during the extrusion process, automatic re-meshing was implemented. The extrusion speed of the used hydraulic press was v= 3 mm/s, and the friction was determined by the Coulomb friction of μ= 0.02. Both the parameters were selected based on our previous study [39], mutual comparison of numerical predictions and experiments in which was performed in order to acquire the values of the boundary parameters corresponding to the real experimental process. The elastic-plastic model with the Newton–Raphson convergent algorithm was applied to determine the parameters in the simulation. The stress-strain curves for the experimentally used materials (Fig. 2b), which were imported to the material flow stress database of the simulation software, were acquired using a torsion test performed at room temperature with the strain rate of 0.1 s −1 on SETARAM device –a servo-hydraulic torsion plastometer. The deformation behaviour of the components of the processed composite was described via the Haensel-Spittel equation (Eq. (1)), σf¼Aem1TTm8εm2em4=ε1þεðÞ m5Tem6εε :m3ε :m7Tð1Þ where εis equivalent strain, ε :is equivalent strain rate, Tis temperature, and Aand m 1 to m 8 are regression coefficients. The values of the individual coefficients for Cu are A= 411.19 MPa, m 1 to m 4 are −0.00121, 0.21554, 0.01472, −0.00935, respectively, and m 5 ÷m 8 are 0. The values of the individual coefficients for Al are A= 151.323 MPa, m 1 to m 4 are −0.00253, 0.21142, 0.03177, −0.00654, m 5 ÷m 8 are 0. The boundary conditions in the numerical simulation were defined by the initial temperature of 25 °C, and the values characterizing the temperature behaviour of copper, aluminium, and the die, i.e. Young's modulus, Poisson's ratio, thermal expansion coefficient, thermal conductivity, specific heat, emissivity and density, which were for Cu defined as the following constants (respectively): 111 (GPa), 0.3, 1.7 × 10 −5 (K −1 ), 394 (W/(m K)), 398 (J/kg K), 0.7 and 8960 (kg/m 3 ), and for Al as (respectively): 72 (GPa), 0.3, 2.4 × 10 −5 (K −1 ), 250 (W/(mK)), 1230 (J/kg K), 0.03 and 2800 (kg/m 3 ). Mutual friction between the two composite components at their interfaces was defined by Tresca's nolube friction (coefficient mbarre = 8.000000e-01). The deformation behaviour of the Al/Cu clad composite was monitored in two perpendicular planes within the extruded billet to be able to reliably observe the plastic flow of the material being Fig. 1. Schematic depiction of TCAP die (a), processed Al/Cu clad composite (b). Fig. 2. Model used for the thermal conductivity simulation (a); stress-strain curves used for FEM analysis (b); locations of analysed planes (c). R. Kocich Materials and Design 196 (2020) 109255 3 extruded. The first monitoring grid was superimposed on a longitudinal plane passing through the axis of the sample, whereas the second monitoring plane was located perpendicularly to the sample longitudinal axis (see Fig. 2c for both). Both the planar grids were created in fixed locations within the sample and depicted the same locations within the bulk of the material during the entire extrusion process. In other words, the undeformed grid within the original billet was deformed according to the performed deformation of the particular material volume. To allow detailed evaluation of the investigated parameters, the superimposed grids were defined by very fine square cells (0.5 × 0.5 mm 2 ). 2.2. Practical experiment The second part of the research work focuses on the experimental realization of the TCAP process. The materials selected to manufacture the composite were commercially pure Al (99.97%) with the chemical composition (in wt%) of 0.125 Fe, 0.10 Si, 0.020 Zn, 0.020 Cu, 0.015 Mn, 0.015 Mg, 0.015 Ti, and CP Cu (99.97%) with the chemical composition (in wt%) of 0.0074 Ni, 0.0058 Sn, 0.0031 Fe, 0.0030 Zn, 0.0023 Si. Prior to completing the composite, both the Al sheath and Cu wires were annealed in a furnace at 500 °C for 30 min in order to eliminate possible effects of previous deformation history. The dimensions of the assembled composite samples were identical to the dimensions in the simulation assembly, i.e. 12 mm × 12 mm square cross-section, and 130 mm length. The extrusion was carried out using a hydraulic press at room temperature (25 °C) and extrusion rate of 5 mm/ s; MoS 2 was used as the lubricant. To be able to experimentally investigate the material flow of the extruded composites during both the consequent TCAP passes and compare the results with the predicted ones, X-ray micro computed tomography (Micro-CT) was applied. The Micro-CT measurements were performed using the GE phoenix v|tome|x m 300 system equipped with a 300 kV/500 W microfocus X-ray tube, and DXR250 flat panel detector with high contrast of 2048 × 2048 pixels (200 × 200 μm 2 pixel size). The tomographic measurements were performed at room temperature of 22 °C with the exposure time of the detector of 333 ms; 2700 projections were acquired for the single 360° rotation. The applied parameters were acceleration voltage of 200 kV, and X-ray tube current of 140 μA. The voxel resolution of the acquired CT data set was 48 μm. The tomographic reconstruction [41] was performed using own CT system evaluation software (GE phoenix datos|x 2.0), and the analyses and visualization of thecomposites' inner structures were performed via the VG Studio MAX 3 software. Mechanical properties of both the TCAP-ed composites after single and double pass were primarily investigated via microhardness profile measurement on transversal cross-sectional cuts through the billets using a Zwick/Roell microhardness tester. The load was 200 g, and the loading time for a single indent was 10 s. Detailed structure analyses of the TCAP-ed clad composites were performed via scanning electron microscopy (SEM), electron backscatter diffraction (EBSD) in particular. Preparations of the samples for the analyses were carried out by grinding on SiC papers followed by mechanical, and finally electrolytical polishing. The structure investigations were performed using a Tescan Lyra 3 FIB/SEM microscope equipped with a Nordlys Nano EBSD camera. The samples were scanned under the tilt of 70° with the scan step of 0.5 μm for both the Cu and Al components. The acquired scans were further evaluated using the AZtec Crystal and ATEX software [42]. The presence of residual stress within the processed structures was evaluated via EBSD image analysis determining the presence of residual stress based on Kikuchi lines patternsfinally providing the internal grains misorientations in the scale from 0° (negligible misorientations, i.e. negligible presence of residual stress), to 15° (high misorientations, i.e. occurrence of residual stress), in rainbow colour scheme. 3. Results and discussion 3.1. Deformation behaviour The primary focus of the numerical analysis was to investigate the deformation behaviour of both the component metals, as well as imposed strain and its homogeneity, material plastic flow, and distribution of residual stress across the cross-section of the extruded billets. To enable comparison of the numerical prediction and real experiment, the deformation behaviours of the component metals during both the TCAP passes were also monitored via Micro-CT scans (see Fig. 3a and b for single and double TCAP pass, respectively). As documented by the CT scans acquired after both, single and double TCAP passes, the extruded composites exhibited differences in their deformation behaviours. Both the TCAP passes resulted in significant plastic deformation of the Al sheath and Cu wires. Nevertheless, differences can be observed, especially at the ends of the Cu wires (depending on the number of passes). After the first pass, the ends of the pair of wires situated in the upper part of the composite were bent upwards, whereas the ends of the pair of wires located in its bottom part were bent downwards (Fig. 3a). For both the upper and bottom pairs of wires, the observed bending originated during passing through the main deformation zone (MDZ), i.e. through the inlet to the horizontal channel part. As the result of the shear strain affecting the billet in the MDZ, the plastic flow in the upper half of the composite (cross-section) was faster than in the bottom half. By this reason, the bottom pair of wires was bent in the opposite direction than the upper pair. Although further passing of the composite through the remaining parts of the Fig. 3. Comparison of Al/Cu clad composite shapes via Micro-CT after: first pass (a); second pass (b). R. Kocich Materials and Design 196 (2020) 109255 4 channel resulted in slight correction of the bending of the wires, these were not eliminated. The second TCAP pass then introduced bending of the majority of the Cu wires in a single direction (Fig. 3b). The different deformation behaviours of the input and output ends of the Al/Cu clad composite can be attributed to the influencing factors affecting the individual billet ends. When passing through the MDZ, the output end of the composite is in a permanent contact with the material of the extrusion punch, which generates certain pressure on the billet and introduces friction limiting the free movement of both, the Cu wires and Al sheath. On the other hand, the free forward movement of the input end of the clad composite is not restricted during and after passing through the MDZ. However, the free flow of the Cu wires was more or less limited by the surrounding Al sheath, the limiting effect on the bending of the inserted wires of which increased during the second pass due to deformation strengthening of the Al matrix. Deformation strengthening during the second pass also occurred within the Cu wires (accumulated especially at the bent ends) and contributed to increased flow stress. The discussed material behaviour is in accordance with the characteristic plastic flows of both the component metals during the individual passes. As can be seen in Fig. 4a and b, the predicted material plastic flows for the single and double TCAP pass in which are depicted, respectively, certain differences in the plastic flow velocities originated in the primary deformation zone (twist). The twist part of the die imposed severe shear strain primarily to the Al sheath and caused its outer periphery region to flow quicker than the Cu wires. The details depicted in Fig. 4a also show the decelerating effects of friction occurring between the extruded billet and die, and between the Al sheath and individual Cu wires. The MDZ then introduced the shear strain in a greater extent also to the Cu wires. The differences in the plastic flows in the individual regions of the extruded billet (manifested as the different shapes and bending of the ends of the individual Cu wires) originated in the MDZ, the upper part of the Al sheath in the vicinity of the Cu wires in which exhibited the tendency to flow quicker than the bottom part of the composite. In other words, the shear on the individual Al/Cu interfaces manifested differently in different locations of the clad composite. These differences were most probably introduced by the twist part of the die, since materials processed via the conventional ECAP process do not typically exhibit such localization of the imposed shear strain. This supposition is supported by the further presented differences in the effective strain and residual stress distributions, as well as by the character of the mutual interfaces pointing to the tendency to form mechanical locking (see e.g. [43,44]). Latypov et al. [45] also observed unexpected (helical) shape of the reinforcing Al fibre during their numerical and experimental study of twist extrusion (TE) of a composite consisting of Cu sheath and Al fibre. According to their supposition, such behaviour can be attributed to out-of-plane distortions of the sample cross-section, especially when performing multi-pass TE. Despite the fact that the herein presented composite consists of a reverse stacking sequence (Al sheath and Cu wires compared to Cu sheath and Al wires), similar tendencies were observed –the tendency of the Cu wires to bend, and also axially rotate (see Fig. 3 non perfect circular shapes of some Cu wires cross sections in which can be seen). However, subsequent passing through the MDZ caused substantial suppression of the effects of previous passing through the twist section on the helical-like shapes of the reinforcing wires. The FEA confirmed that vortex like flow was partially observed on the cross-sections, too (seen in Fig. 4a and b). Possible reasons for these findings are the different shapes of processed samples (rounded rectangle vs. square), as well as the position and number of the Cu wires, and geometrical design of the die. Fig. 4b also shows that the first TCAP pass provoked severe degradation of the mesh superimposed through the Al sheath perpendicularly to the composite axis. In other words, the intensity of plastic flow in the inter-area between the peripheral and axial regions of the Al sheath increased substantially after passing through the MDZ. The observed plastic flow gradient, i.e. inhomogeneity, was also the most probable cause of the differences in the imposed strain observed between the Al sheath and Cu wires (see Section 3.2). The results demonstrate that the twist zone affected non-negligibly the Al sheath not only in its peripheral regions, but also in the inter-area, i.e. in the vicinity of the individual wires. Moreover, the particular area also exhibits notable plastic flow in the axial direction already in the twist zone. The subsequent passing through the MDZ resulted in the increase in the plastic flow velocity for the Al sheath. However, the flow was also affected by the friction at mutual interfaces of the Al sheath and die channel. On the other hand, the effect of this factor was reduced during the second TCAP pass by the optimized selection of thedeformation route, which also resulted in reduction in the of plastic flow inhomogeneity for the Cu wires. 3.2. Imposed strain After the first TCAP pass, the extruded billet cross-section evidently featured certain inhomogeneity of theimposed strain (Fig. 5a). Whereas the Al sheath exhibited more or less homogeneous strain distribution (Se ~ 2.5), the Cu wires exhibited variations in the imposed strain values across their cross-sections, which also manifested as strain gradient in Fig. 4. Analyses of material's plastic flow within Al/Cu clad composite during first pass: longitudinal grid (a); perpendicular grid (b). R. Kocich Materials and Design 196 (2020) 109255 5 the diagonal direction. The predicted imposed strain inhomogeneity across the Cu wires cross-sections was related to their orientations in relation to the individual deformation zones during passing through the TCAP die (i.e. the selected deformation route). The acquired results are in accordance with the above discussed differences in the plastic flows of both the component metals, by the effect of which mutual slipping of the adjoining component metals occurred (especially when passing through the MDZ). The generally more intense plastic flow of the Al sheath resulted in the localized tendency of the plastic flow of the Cu wires to delay; this difference in plastic flow velocities of the metals primarily originated in the higher flow stress of Cu. Eventually, the Cu wires featured significantly lower values of the imposed strain than the surrounding Al sheath. As can be seen in Fig. 5b, the cross-section of the Al sheath after the second TCAP pass also exhibited homogeneously distributed imposed strain, although the absolute values were significantly higher (Se ~5). Nevertheless, the distribution of the imposed strain across the Cu wires cross-sections exhibited substantial differences when compared to the first pass; the applied deformation route Aresulted in the increase in strain homogeneity within the Cu wires, as well as in the increase in the overall imposed strain values (Se ~ 2.3). In other words, rotation of the billet before entering the MDZ decreased the strain gradient across the cross-sections of the Cu wires, which also signifies that increase in the quality of the mutual contact of both the metals at their interfaces occurred (if the quality of the contact of both the metals was similar as during the first TCAP pass, the strain gradient would have been comparable to the gradient occurring after the first pass). Mutual comparison of the numerically predicted (Figs. 5)andexperimentally acquired (Fig. 3) extruded clad composite geometries showed satisfactory correlation, which documents that the boundary conditions for the simulation were reliably defined. 3.3. Stress state The predicted results showed that the effects of both the passes on the presence and distribution of residual stress within the extruded composites were different. As seen in Fig. 6a, already the first TCAP pass affected the stress state within the Al/Cu composite. During passing through the first deformation zone (twist), the composite exhibited vortex-like flow (especially in the peripheral regions of the outer Cu wires), which consequently introduced inhomogeneous stress distribution and tensile stress maxima located primarily in the outer Cu wires. These phenomena more or less aggravated the plastic flow in the axial composite region and caused the axial wire to exhibit no significant stress maxima. In other words, the slower plastic flow of the Al sheath in the axial region of the composite resulted in the axial Cu wire to Fig. 5. Predicted imposed strain after: first pass (a); second pass (b). Fig. 6. Predicted residual stress distribution after: first pass (a), second pass (b). R. Kocich Materials and Design 196 (2020) 109255 6 feature lower values of tensile stress and more homogeneous stress distribution when compared to the outer Cu wires. The stress distribution across the composite cross-section changed after passing through the MDZ, i.e. redistribution of residual stress occurred as the result of changes in the plastic flow introduced by the MDZ. After passing through the MDZ, all the Cu wires featured tensile stress with inhomogeneous distribution, especially the upper pair of wires exhibited local maxima of tensile stress (see the detail in Fig. 6a). The fact that the upper parts of billets extruded via ECAPbased methods exhibit quicker plastic flow compared to their lower parts is well known [46]. As the result of the aggravated plastic flow in the bottom region of the composite when passing through the MDZ, the bottom pair of wires exhibited higher values of tensile stress and more substantial stress inhomogeneity. The occurrence of compressive stress within the Al matrix in the vicinity of the Cu wires documents the above discussed supposition of different velocities of plastic flows of both the component metals. In other words, the Al sheath, being more susceptible to the imposed strain than the Cu wires, exhibits compressive stress in the locations adjoining to the wires. This phenomenon is the primary consequence of friction occurring at mutual Al/Cu interfaces. Cu, featuring higher flow stress, has the tendency to aggravate the plastic flow of the adjoining Al, by the effect of which characteristic cross-like shape of residual stress occurrence develops (see the detail in Fig. 6a). On the other hand, the regions in which the Al sheath is in a direct contact with the walls of the extrusion channel were characterized with relatively homogeneous distribution of stress of low values. The reason for this phenomenon was most probably the above mentioned occurrence of vortex like flow [40]. The second TCAP pass introduced significant changes not only in the residual stress distribution, but also in its values; the stress state within the Al/Cu clad composite during the second pass differed significantly already after passing through the twist zone. As can be seen in Fig. 6b, compressive stress started to dominate throughout the Al sheath cross-section, despite the fact that the outer Cu wires still featured localized maxima of tensile stress. Continuing extrusion of the billet through the MDZ resulted in certain residual stress homogenisation across the cross-sections of both the composite components, especially the Cu wires exhibited more uniform distribution of tensile stress. Fig. 6b documents that, compared to the first pass, the cross-sectional areas of the majority of Cu wires exhibited prevailing tensile stress, the maximum values of which were detected within the upper pair of wires, but also within the bottom left and axial wire. The changes were primarily imparted by the selected deformation route (A), by the effect of which the positions of all the four peripheral wires changed during extrusion. In other words, the entire cross-section was rotated by 90° when compared to the first pass, i.e. the wire located in the upper left position after the first pass was in the bottom left position after the second pass, etc., by the effect of which homogenisation of residual stress across the cross-section of the entire composite billet, i.e. both the sheath and wires, occurred. The fact that both the deformation zones, twist and MDZ, contributed to the reduction of inhomogeneity of residual stress across the crosssection of the clad composite needs to be stressed, too. 3.4. Structure analyses To verify the predicted results, detailed structure analyses were subsequently performed on the real extruded billets (e.g. Fig. 1b). Table 1 depicts the experimentally measured average grain sizes (avg. max. ferret diameter in μm) within the individual Cu wires after both, single and double TCAP passes. The table shows notable differences in grain sizes between the individual passes, but also slight differences detected between the wires within the individual extruded composites. Both thepasses evidently resulted in substantial grain refinement, as the original grain size in the pre-annealed Cu was almost 40 μminaverage (see the structure scan shown in Fig. 7a depicting large recrystallized grains featuring the majority of high angle grain boundaries – HAGBs –highlighted in black). Focusing on the wires after single TCAP pass, their average grain size varied between 6 and almost 8 μm and the largest grains were detected in the axial wire. This phenomenon can be attributed to the aggravated plastic flow in the axial composite region, as discussed in Section 3.1. The supposition is also supported Table 1 Grain sizes within individual wires after single and double TCAP. TCAP 1 pass avg. wire grain size [μm] TCAP 2 passes avg. wire grain size [μm] Upper left Upper right Upper left Upper right 6.04 6.58 3.79 4.16 Axial Axial 7.94 4.92 Bottom left Bottom right Bottom left Bottom right 6.42 6.12 4.28 3.87 Fig. 7. Original pre-annealed Cu structure with highlighted high angle grain boundaries (HAGBs) (a); OIM of structure of Al sheath after second pass with reminiscence of vortex-like flow (b). R. Kocich Materials and Design 196 (2020) 109255 7 by the value of the average grain size detected in the axial wire after the second TCAP pass, which was also the largest of all the wires within the billet, however smaller than the average axial wire grain size observed after the first pass. This behaviour demonstrates the favourable effect of the bending, i.e. MDZ, on homogenisation of the imposed strain, i.e. structure. As mentioned previously and discussed in detail e.g. in [38], the twist deformation zone primarily introduces shear strain, i.e. imparts deformation resulting in grain refinement and deformation strengthening, to the peripheral regions of the extruded billet and its effect diminishes towards the axis of the billet. On the other hand, the bend deformation zone, i.e. MDZ, primarily introduces the shear strain into the axial region of the extruded billet as the plastic flow in its peripheral regions is aggravated by the effects of friction and the geometry of the channel bend. Given by these factors, the strain imposed across the cross-section to the axial/peripheral extruded composite regions tends to equalise with increasing number of TCAP passes, which results in more homogeneous distribution of the imposed strain (see Section 3.2), and also more homogeneous grain size distribution; Table 1 depicts that the grains size range for the Cu wires after the second pass was between 3.8 and less than 5 μm, the largest grains of 4.92 μm in diameter were again detected in the axial Cu wire. As regards the individual wires, the smallest grains were observed in the upper left wire for both the extruded billets. However, the values were comparable to the grain size values detected in the bottom right wires. Similarly, the avg. grain size values detected in the bottom left and upper right wires for both the extruded composites were comparable. This minor differences occurring in diagonals can primarily be attributed to the twist deformation zone introducing the vortex-like flow, the reminiscence of which was observed in the structure of the Al sheath (see Fig. 7b the orientation image map –OIM –depicting the peripheral region of the Al sheath of the billet extruded via two TCAP passes in which is shown). Fig. 7b also depicts the severe deformed grains within the Al sheath, the avg. size of which after the second pass decreased to 1.66 μm (i.e. near to UFG structure). The fact that the observed grain size was the finest within the Al sheath (compared to the Cu wires) can be attributed to its lower flow stress and higher susceptibility to the imposed strain, both resulting in the highest effective Fig. 8. Grains misorientations, i.e. residual stress, for: upper left wire after first pass (a); second pass (b); axial wire after first pass (c), second pass (d); Al sheath after second pass (e). Fig. 9. Predicted and measured thermal conductivity values. R. Kocich Materials and Design 196 (2020) 109255 8 imposed strain (see Fig. 5b) and consequently to the finest grain size. The decreasing grain size also affected other mechanical and utility properties, as further discussed in Sections 3.6 and 3.7. 3.5. Residual stress Fig. 8a shows the grains misorientations in the range from 0° to 15° pointing to the presence of residual stress for the structure of the upper left wire after the first pass, while Fig. 8b shows the grains misorientations within the structure of the upper left wire after the second pass. Both the figures feature more or less homogeneously distributed areas with high misorientations (i.e. red colour), depicting the regions exhibiting residual stress. However, the presence of residual stress was higher in the structure of the wire subjected to two TCAP passes, which corresponds to the predicted results (see Fig. 6a and b). Fig. 8c and d then show the residual stress distribution in the axial wire after the first and second pass, respectively. The stress distribution within the axial wire was comparable after both the passes, however, its magnitude was slightly higher after the second pass. Mutual comparison of the structures of the peripheral wires and the axial wires reveals that the presence of residual stress was more evident in the peripheral wires for both the passes, which corresponds to the predicted residual stress distribution, as well as the discussed plastic flow behaviour. According to the predicted results, the Al sheath exhibited the lowest absolute values of residual stress (± relative to 0) when compared to the Cu wires, its distribution was also more homogeneous than within the wires. The acquired scans verified the prediction, as the Al sheath of the composite billet processed via two TCAP passes exhibited more or less homogeneous residual stress distribution featuring a scarce occurrence of the locations with increased misorientations (Fig. 8e). 3.6. Thermal properties The presented study also involved brief numerical and experimental evaluation of the cross-sectional thermal conductivity of the composite extruded via single TCAP pass (used model can be seen in Fig. 2a). Moreover, the conductivities of the used CP Al and CP Cu were evaluated to verify correct settings of the boundary conditions for the numerical analysis of deformation behaviour. The measured thermal conductivities of the CP Al and CP Cu were 246.8 and 399.2 W/(m·K), respectively, which corresponds well with the boundary conditions of the performed simulation (see Section 2.1). Fig. 9 depicts the comparison of the cross-sectional thermal conductivity values predicted via the LMC and FEM methods, as well as the values measured experimentally in parallel and series setting. The predicted conductivities of the composite extruded via single pass were 263.3 and 260.8 in average, depending on the selected method (see Fig. 9), and the measured values were 264.2 and 253.8 in the parallel and series setting, respectively. The measured values evidently increased when compared to the CP Al, and decreased compared to the CP Cu. The relatively low overall cross-sectional thermal conductivity value of the extruded composite was not only given by the minor content of Cu within the billet, but also by the performed processing, which resulted in accumulation of deformation strengthening and grain refinement, both introducing obstacles such as dislocations, possible formation of thermally stable precipitates, as well as lattice distortions, consequently resulting in decrease in thermal conductivity [47]. 3.7. Micro-hardness Last but not least, the Vickers microhardness of both the extruded composites was evaluated to experimentally investigate the effects of the imposed strain on the mechanical properties. The original HV values of the pre-annealed metals were 58.6 for the Cu, and 37.4 for the Al. The microhardness maps for the composites are depicted in Fig. 10aandb for the billets extruded via single and double TCAP pass, respectively. The microhardness evidently increased after both the TCAP passes; the most notable increase was observed after the first pass. The HV increase after the second pass was less significant, however, contrary to the billet after single pass, all the Cu wires within the biller after double pass exhibited the maximum HV values. This phenomenon corresponds to the predicted values of the imposed strain within the Cu wires, which were non-homogeneous after the first pass (variations of effective strain between 0.4 and 1.8, see Fig. 5a), and then equalized after the second pass (more or less homogeneous effective strain of 2.5, see Fig. 5b). The increase in microhardness is also in accordance with the observed grain refinement imparted by the imposed severe shear strain. 4. Conclusions The study presented numerical and experimental analysis of Al/Cu clad composites prepared via single and double pass twist channel angular pressing (TCAP). Both the passes imparted significant shear strain into both the composite components; the maximum effective strain reached to 5 within the Al sheath after the second pass. The Al component within this billet also featured the finest average grain size of slightly more than 1.5 μm, and the lowest occurrence of residual stress. Numerical prediction revealed that the multiple-pass processing Fig. 10. Cross-sectional microhardness map after: first pass (a); second pass (b). R. Kocich Materials and Design 196 (2020) 109255 9