Adiabatic Shearing Mechanism Of Ti-6Al-4V

Ti-6Al-4V alloy is an important alloy of titanium industry widely used in aerospace, marine industry, the weapons industry and other important fields because of its good performance. However, it is unstable deformation under high strain rate, Ti-6Al-4V alloy’s shear mechanism and adiabatic shearing sensitivity of different microstructure under high strain rate was study in this article. There was an obviously engineering significance for the study of the evolution of the adiabatic shear bands in the material.This topic was mainly used by Split Hopkinson Pressure Bar(SHPB) device, the dynamic compression and dynamic shear experiments of the cylindrical and hat shaped specimens were carried out respectively under high strain rate, the dynamic behavior of Ti-6Al-4V alloy was quantitatively investigated by analyzing the real stress-strain curve of cylindrical specimens and the time-voltage curve of hat shaped specimens obtained by SHPB, and analyzing the adiabatic shear sensitivity of the specimens with different shapes of three kinds of microstructure(such as equiaxed microstructure, bimodal microstructure and lamellar microstructure). In addition, the microstructure of the sample fracture and the adiabatic shear band(ASB) was observed by SEM and metallographic microscope, and the mechanism and the micro-hardness of the ASB was studied. The dynamic compression results of Ti-6Al-4V alloy were simulated by establishing Johnson-Cook constitutive equation. The results show that:(1) cylindrical specimens of Ti-6Al-4V alloy’s mechanical properties showed a continuous yielding behavior and showed a certain strain rate hardening effect with the increasing of strain rate. Fracture strain rate, fracture strength, fracture strain of equiaxed microstructure were relatively better; Under uniaxial compressive stress state. The adiabatic shear sensitivity of three kinds microstructure arrange from strong to weak: lamellar microstructure, bimodal microstructure, equiaxed microstructure, while under combined compression and shear stress state, three microstructure adiabatic shear sensitivity from strong to weak: equiaxed microstructure, bimodal microstructure and lamellar microstructure.(2) The microstructure of the adiabatic shear band was characterized by the typical "white bright band", and was accompanied by the phenomenon of micro-cracks and micro-hole. The fracture of the adiabatic shear band was due to the micro-cracks in the shear zone and the macro material damaged caused by the expansion of the micro-hole and the growth of the micro-hole. When the stress state was different, the plastic deformation of Ti-6Al-4V alloy under different loading conditions and the extension of the adiabatic shear band in different microstructure was demonstrated by the adiabatic shear sensitivity of Ti-6Al-4V alloy. The micro-hardness in the core of the adiabatic shear band was higher than that of the microstructure around the adiabatic shear band.(3) The constitutive equation of Ti-6Al-4V alloy was established, and when the dynamic compressive strain rate was 1100s-1~3500s-1.The dynamic plastic deformation of the alloy produced an adiabatic temperature rise in the range of 32K~193K. By the calculation of adiabatic temperature rise, the formation of the small equiaxed grains in ASB was not formed by dynamic recrystallization, but by the grain fragmentation. And it affected the theoretical curve and the actual curve fitting of the Johnson-Cook model.