Material used in the present study was a cold rolled Al7075 alloy with chemical composition shown in Table 1. Cylindrical samples with 40 mm length and 14 mm diameter were cut from as received plate so that the centerline of specimens lied parallel to the rolling direction. Annealing was performed by heating at 415˚C for 240 minutes and then cooling at furnace to achieve full recrystallized structure. Microstructure consisted of equiaxed grains with 94 µm in size and dispersed Fe and Mg rich inclusions was developed after annealing. Annealed sample was then solutionized at 480˚C for 30 minutes and quenched in water.

Cylindrical sample was placed inside the channel of special designed die and pressed at room temperature with the punch travelling at 5 mm/s. Figure 1 shows the 2D schematic representation of the die used in the present study. As it is seen, material is subjected to two different deformation steps during flow inside die channel. At first, material is extruded and its diameter is decreased from 14 to 7 mm. The amount of equivalent plastic strain during

Table 1. Chemical composition of alloy used in the present study (wt%).

extrusion is calculated as 1.38 (ε = 2ln (D_f/D₀)) without considering the effect of friction and assuming uniform deformation. Extruded material flows inside vertical channel and approaches to the intersection of two channels where ECAP step is executed. Intense shear deformation is imposed on sample during passing the ECAP region. Imposed plastic strain by one pass of ECAP is calculated by the expression proposed by Iwahashi et al. [17]:

where is die channel angle and is an outer curvature angle of two intersected channels. The die channel and outer curvature angles are 120˚ and 0˚, respectively. Therefore, the strain imposed during ECAP step is calculated about 0.8.

Deform 3D^TM software was used for finite element investtigation of material flow and damage prediction during Ex-ECAP process. For this reason, die and punch were assumed as rigid parts due to higher strength and remaining undeformed during process. Only half of die, punch and sample were considered in the simulation because of the existence of mirror symmetry in the geometry of processing technique. Sample was assumed as deformable part and meshed with 45,000 tetrahedron elements with minimum size of 0.375 mm. Mechanical properties of Al7075 alloy were imported in the form of true stress-true strain curves at room temperature with different strain rates up to strain of 4. Figure 2 shows the true stress-true strain curves imported as a mechanical behavior of material. These stress-strain curves were obtained considering the Johnson-Cook plasticity model [18]. Also, the amount of friction coefficient between specimen and die channel wall is selected as 0.12 which is a typical value for deformation of aluminum alloys.

Figure 3 represents the material flow during different steps of FEM simulation of Ex-ECAP process. As it is seen, at step 38 (t = 1.5 s) extrusion has been carried out on front head of sample. At step 52 (t = 2.1 s), ECAP stage is executed on previously extruded part and material behind front head of sample is extruded and is in the second deformation step. At the end of process (step 100, t = 4 s), one part of specimen is subjected to two deformation steps by extrusion and ECAP. One part is only extruded, one part is in the extrusion zone, and the other has not been subjected to any deformation.

Figure 4 shows the variations of Von Misses equivalent strain and strain rate on symmetry plane of Ex-ECAPed sample. In Figure 4(a), it is observed that plastic strain is inhomogeneous in the inside of deformed sample. This inhomogeneity is related to the different amounts of deformation imposed on different regions and also the existence of friction between sample and die channel wall resulting in redundant shear deformation at near surface regions of sample. Processed sample can be divided to six regions (A to F). Strain rate is zero at most of the sample except extrusion and ECAP zones (B and D). It is