Plastic deformation induced nano-scale twins in Ti-6Al-4V machined surface with high speed machining

High speed machining (HSM) is one of the plastic deformation processes in which deformation twinning is generated. The mechanism of deformation twinning formation and its effects on the work hardening of Ti-6Al-4V machined surface are revealed. First, transmission electron microscope (TEM) and electron backscattered diffraction (EBSD) experiments are conducted to observe the twinning behaviors of machined surface with different cutting speeds (50-600 m/min). Then, the distributions of deformation variables including stress, strain rate and temperature in Ti-6Al-4V machined surface are analyzed to disclose the factors on the formation of deformation twinning under different cutting speeds. A simple model is established to illustrate the formation mechanism of {101¯1} twin of Ti-6Al-4V. Finally, nanoindentation technique in conjunction with x-ray diffraction and light microscopy are used to elucidate the effect of deformation twinning on the hardness evolution of Ti-6Al-4V machined surface. The TEM experimental results show that {101¯1} nano-scale twins of b1 mode generate on the machined surface under the cutting speed higher than 200 m/min. High stress and strain rate are the main factors in promoting the formation of {101¯1} twin of Ti-6Al-4V machined surface. Nanoindentation experiments show that the appearance of nano-twins can enhance the hardness of machined surface. This paper proves that HSM can be applied as an efficient approach to prepare nano-twinned material by selecting appropriate cutting speed to control the emergence of deformation modes. It also reveals that nano-twins make a significant contribution to the work hardening of Ti-6Al-4V machined surface at higher cutting speed.