Original Research Article Friction Stir Radial Backward Extrusion (FSRBE) as a new grain refining technique Hossein Jafarzadeh a, * , Alireza Babaei b , Firooz Esmaeili-Goldarag b a Department of Mechanical Engineering, Tabriz Branch, Islamic Azad University, Tabriz, Iran b Department of Mechanical Engineering, University College of Nabi Akram (UCNA), Tabriz, Iran 1. Introduction Unique physical and mechanical properties of ultra-fine grained (UFG) materials have attracted many interests in processing such materials with different severe plastic deformation (SPD) methods [1]. The most challenging part of the SPD is that the processed UFG materials are often exhibit reduction in the elongation and have very low ductility [2]. On the other hand, to the best of our knowledge, Friction Stir Processing (FSP) is the only SPD method in which the dynamic recrystallization [3] in the presence of friction-induced heating causes fine-grained materials with proper ductility [4]. With regard to this unique capability, Abu-Farha [5] presented Friction Stir Back Extrusion (FSBE) as a novel SPD technique for processing tubes with a fine-grained substructure. Besides proper ductility, since the temperature is increased at the deformation zone, the material processing is done with much a r c h i v e s o f c i v i l a n d m e c h a n i c a l e n g i n e e r i n g 1 8 ( 2 0 1 8 ) 1 3 7 4 – 1 3 8 5 a r t i c l e i n f o Article history: Received 28 October 2017 Accepted 21 April 2018 Available online Keywords: Friction Stir, Radial Backward Extrusion Fine-grained structure Microstructure modeling Cellular automaton a b s t r a c t A new method entitled Friction Stir Radial Backward Extrusion (FSRBE) is presented for processing fine-grained tubes. In FSRBE technique, an initial pure copper billet is placed inside a cylindrical chamber. The billet is pushed toward a rotating tool which results in radial and backward flow of the material while is frictionally stirred. The microstructure evolution during FSRBE was investigated through experimental observations and cellular automaton (CA) modeling. The observations reveal that the microstructure with initial grain size of 75 mm was refined to a fine-grained structure with an average grain size of 12 mm. The results of tensile tests demonstrate slight improvement in the value of yield and ultimate strength, elongation and microhardness. The microstructural evolution during FSRBE pro- cessing in the micro-level was studied using a coupled cellular automaton algorithm and finite element model. First, the macroscopic plastic flow behavior of material during FSRBE was calculated using FEM simulation method. Next, by tracing the plastic strain, the strain rate and temperature, in the deformation domain of cellular automaton, the DRX kinetics of pure copper is obtained in a devised post-processing step. The microstructure observations showed that the proposed model predictions were in reasonably good agreement with the experimentally obtained results. © 2018 Politechnika Wroclawska. Published by Elsevier B.V. All rights reserved. * Corresponding author. E-mail address: h.jafarzadeh@iaut.ac.ir (H. Jafarzadeh). Available online at www.sciencedirect.com ScienceDirect journal homepage: http://www.elsevier.com/locate/acme https://doi.org/10.1016/j.acme.2018.04.006 1644-9665/© 2018 Politechnika Wroclawska. Published by Elsevier B.V. All rights reserved.