Manufacturing of nanostructured Ti-6Al-4V alloy via closed-die isothermal multi-axial-temperature forging: microstructure and mechanical properties

Abstract

Manufacturing of nanostructured Ti-6Al-4V via a novel severe plastic deformation (SPD) method, named “multi-axial-temperature forging (MATF)” and study of its microstructure and some of its mechanical properties were the aims of this research. In this new technique, known multi-axial forging (MAF) method was developed by a multi-temperature procedure for producing nanostructure in rather brittle materials. Due to room-temperature workability limits and in order to prevent low-temperature embrittlement in Ti-6Al-4V, MATF process was started at elevated temperature of 725 °C, but in order to attain the smallest possible grain size, deformation temperature at subsequent passes was gradually decreased in steps of 75 °C. Grain refinement due to application of MATF was investigated using scanning electron microscopy (SEM). The results show that this process can severely refine the microstructure. The refinement occurred at a variable rate, so that it was significant at the first pass, but reduced substantially at subsequent passes. The average grain size of the final product obtained after the fourth pass was about 100 nm. The results of shear punch and hardness tests show that application of MATF process significantly increased the yield strength up to 50 %, ultimate strength up to 45 %, and hardness up to 26 % relative to those of the as-cast alloy, but it reduced its elongation slightly. The results of this research also show that MATF process facilitates production of nanostructure in low-temperature brittle materials like Ti-6Al-4V alloy by means of SPD techniques.