____________________ * Corresponding author: Karlsplatz 13/E308, A-1040, Vienna, Austria, +43-(0)1-5880130820, fwarchom@pop.tuwien.ac.at MICROSTRUCTURE EVOLUTION DURING HOT DEFORMATION OF Ti-6Al-4V DOUBLE CONE SPECIMENS F. Warchomicka 1* , C. Poletti 1 , M. Stockinger 2 , T. Henke 3 1 Vienna University of Technology – Institute of Materials Science and Technology 2 Böhler Schmiedetechnick GmbH & Co KG 3 RWTH Aachen University – Institute of Metal Forming ABSTRACT: Double cone specimens of Ti-6Al-4V alloy with a bimodal starting structure have been heat treated and then compressed at temperatures near to the beta transus (T β ±70K) with strain rates between 0.1 and 5s -1 . The deformed specimens were immediately quenched to conserve the phase distribution. Alpha grains maintain their morphology during deformation in the alpha + beta region up to 0.2 of strain. Dynamic recrystallization of the alpha grains is observed using EBSD technique at strains of 1 for all the strain rates. Dynamic recrystallization of beta phase takes place in samples deformed above the beta transus temperature. There, the beta grains’ shape does not change up to 0.3 of strain. The grade of recrystallization depends on the previous heat treatment, and the recrystallized grain size on the strain rate and the temperature of deformation. KEYWORDS: Hot deformation, microstructure, forging, titanium alloys, recrystallization, compression tests 1 INTRODUCTION Titanium alloys are attractive for structural applications in the aerospace industry due to their high specific strength in comparison with other engineering materials. Their strength and ductility are strongly related to the microstructure obtained during thermomechanical processes. The control of the alpha phase (hcp) morphology in wrought alpha + beta titanium alloys is of great importance in achieving desired combinations of static and dynamic mechanical properties. The presence of annealed equiaxed alpha grains increases ductility at low temperature [1], as well the resistance to fatigue crack initiation [2]. The beta processing is useful due to the good formability of the beta phase (bcc) and it assumes importance in the kinetics of phase transformation in subsequent heat treatments [3] to achieve the required mechanical properties. The alpha + beta alloy Ti-6Al-4V (Ti64) is one of the most investigated titanium alloy. Several works are related to the modification of the microstructure during hot working. Semiatin’s studies determined the effect of initial alpha plate colonies on the flow behaviour and the kinetic of globularization with the temperature and strain [4] as well as the evolution of the cavities [5]. Dynamic recrystallization [6] and lamellae kinking [7] was also observed with initial Widmanstätten alpha plate structure while an equiaxed (alpha + beta) microstructure resulted in superplasticity, dynamic recovery and adiabatic shear bands during subtransus deformation [7,8]. During deformation in the beta region dynamic recovery and dynamic recrystallization were reported [7,9] In this work, the microstructure evolution is analyzed after hot deformation of pre-forged and annealed Ti64 near to the beta transus temperature. Double cone specimens have the advantage of a wide range of strains within the specimen at a constant temperature and almost constant strain rate. A strain distribution within the deformed specimen is determined by finite element method (FEM) in order to correlate the microstructure with the local deformation parameters. 2 MATERIAL AND EXPERIMENTAL PROCEDURE Commercial Ti-6Al-4V alloy containing 6.45Al, 4.33V, 0.18Fe, 0.18O, 0.006N, 0.03C and balance Ti (all compositions in wt%) was used in this work. The beta transus temperature (T β ) for this material is approximately 1000 °C. The material was pre-forged in different steps by cogging in beta and alpha + beta field, followed by an annealing heat treatment at 730°C, 30min and air cooled. In this condition, the alloy presents a bimodal microstructure with the primary alpha grains oriented in the cogging direction and partially interconnected. Double cone specimens (25mm height, 10 and 25mm minor and major diameter, respectively), were machined with the plane of orientated alpha grains normal to the load. DOI 10.1007/s12289-010-0745- © Springer-Verlag France 2010 9 Int J Mater Form (2010) Vol. 3 Suppl 1:215 218 –