Titanium alloys have excellent comprehensive mechanical properties: high specific strength, corrosion resistance, high strength at high temperatures and so on. Therefore, titanium alloys have been widely used in industries such as aerospace, shipbuilding, and motor racing  . However, titanium alloys are typical difficult-to-cut materials, which have high requirements for the processing environment. Conventional processing methods are inefficient and the surface quality is uncontrollable. It is often necessary to use cutting fluids . Whereas, cutting fluids is generally not an environment-friendly coolant and lubricant, as is known to all.
At present, cryogenic manufacturing process has been a hot research topic in the field of mechanical manufacturing, and it has always been regarded as a kind of clean manufacturing technology. Researchers have been investigating impact of cryogenic machining with liquid nitrogen (LN2) on machining performance in both academia and industry. Ramli et al.  and Sartori et al.  used nitrogen gas and LN2 as coolants to machine titanium and its alloys, then they compared it with dry machining. The authors reported that machining with LN2 had excellent advantages for tool wear and machined surfaces quality. Shokrani et al.  performed milling tests on the Ti-6Al-4V titanium alloy. From comparison with dry and wet conditions, it was found that cryogenic machining not only reduced the surface roughness of the machined parts but also prolonged the tool life. Similarly, Jawahir et al.  also conducted experiments on cryogenic machining. It was found that cryogenic machining can obtain better surface quality while reducing or eliminating damage caused by heat generated by the process. Biermann et al.  conducted the experiments of two kinds of titanium alloys. In their study, when machining Ti-6Al-4V titanium alloy with low temperature CO2 as coolant, the tool life could be extended compared with emulsion. When machining high strength titanium alloy Ti-6Al-2Sn-4Zr-6Mo, an additional lubrication mechanism was required to get better processing quality. Additionally, Trabelsi et al.  focused on the effect of cryogenic assistance on tool life when machining Ti17 titanium alloy, and found tool life was prolonged but temperature had little effect on cutting force. A recent study done by Zhao et al.   also reported that the cryogenic cooling (LN2 cooling) has a significant influence on the chip formation, cutting force, and surface integrity of Ti-6Al-4V titanium alloy. However, it was found from the above literatures that the analysis on the cryogenic cutting mechanism seems still insufficient enough due to the less of measurement data on the material properties at low temperature.
Analyzing the cryogenic properties of materials is very important to the study of cryogenic machining. Many researchers have studied the mechanical properties of titanium alloy materials at low temperature. As early as 1974, Campbell  found that the effect of temperature on the toughness of alloy materials depended on the alloy matrix. As the temperature decreases, titanium alloy generally had lower toughness, but it was affected by alloy content and the way of heat treatment. Some titanium alloys still maintained good toughness at low temperature. Subsequently, Moskalenko et al.  systematically studied the deformation mechanism of pure titanium and its alloys at low temperature, and found that the plasticity variation of pure titanium and its alloys at 40 ~ 120 K was due to combined action of two deformation mechanisms, i.e. slip and lower temperature activated twinning. Hong et al.  studied the low-temperature mechanical properties of Ti-6Al-4V titanium alloy and found that its strength and hardness increased with the decreases of temperature, but the plastic toughness did not change significantly with the decrease of temperature. Sun et al.  conducted tensile tests and low-cycle fatigue tests at 293K and 77K for Ti-2.5Cu titanium alloy, and found that titanium alloy had higher ductility and longer low- cycle fatigue life at 77K than that at 293K. Ono et al.  studied the performance of Ti-5%Al-2.5%Sn at low temperature and found that the ultimate tensile strength decreased with decreasing temperature, and the temperature had little effect on the crack initiation of the specimen. Bertolini et al.  compared the changes in material properties of titanium alloy under conventional parameter processing and low-temperature processing conditions. Furthermore, the morphology of fractures at low temperature was also analyzed. The results suggested that temperature and strain had a significant influence on material properties. In addition, Semenova et al.  conducted tensile tests and Charpy tests to study the influence of temperature on ultrafine-grained Ti-6Al-4V titanium alloy, and investigated that the ultimate tensile strength and yield stress enhanced in a wide range of temperature, and the absorbed energy decreased when the temperature reduced.
As evident from the above literatures review, the impact resistance is an important performance index of the material. In consideration of the high-frequency impact characteristics in the process of cutting, the impact test does help to investigate the cutting mechanism, especially to analyze the influence of cryogenic cooling on cutting process. At present, there are already studies on the impact properties of titanium alloys under low temperatures. However, the analysis of impact properties for Ti-6Al-4V titanium alloy under a relatively dense distribution of low temperatures (20 °C-196 °C) is rarely reported in the available literature. Therefore, the present work aims to analyze the impact properties of Ti-6Al-4V titanium alloy at low temperatures by investigating Charpy absorbed energy and fracture morphology at different temperatures. The investigations are focused on the effect of low temperature on the impact properties of Ti-6Al-4V titanium alloy, and expected to provide an positive guidance for investigation on the cryogenic cutting of titanium alloy and its cutting mechanism.