Microstructural evolution and mechanical properties of Ti6Al4V alloy prepared by the multi-pass hot caliber rolling at 700 °C and 800 °C with different reductions

Abstract

This study adopts a promising technique named multi-pass hot caliber rolling (MP-HCR) to manufacture the Ti6Al4V bars with a ultra-fine fibrous (UFF) microstructure to improve the comprehensive properties of material. However, this kind of special microstructure is sensitive to temperature and could result in a complex evolution, and even heavily influence the service performance. Thus, an exact understanding of the MP-HCR processing - microstructure - properties relationship is a necessary step for precisely tailoring this UFF microstructure and taking its potential advantage. For this purpose, the Ti6Al4V bars were firstly manufactured by the MP-WCR method, then high-temperature tensile and impact tests were carried out to obtain the strength, plasticity, and toughness indicators at different temperatures, and finally scanning electron microscope (SEM), electron backscattered diffraction (EBSD) and transmission electron microscope (TEM) were utilized to analyze the UFF evolution. To uncover the formation mechanism of the fibrous structure and very ultra-fine grains, a thermo-mechanical coupled finite element method, combining with the microstructural characterization, was used to analyze the MP-HCR deformation, and it is concluded that the effects of elongation along the rolling direction and mechanical fragmenting are mainly responsible for the formation of UFF. It is found that the ultra-fine grains coarsening reduces the strength while the interfaced zones between the fibrous structure becomes a potential crack source deteriorating the impact toughness. To obtain a desired combination of microstructure and properties, a proposed processing scheme of rolling temperature at 700 °Cwith a reduction of 51 % is selected for MP-WCR in this work.