Study On Tribological Behavior And Tool Life In Ti6Al4V High Performance Machining

High efficiency, high accuracy, flexibility and greenization are the development directions of modern manufacturing technologies. In the last two decades years, High Speed/Performance Machining (HSM/HPS) is successfully applied in cutting steel, cast iron and alumina alloy, due to its unique advantages. However, it is not yet prevalent in high temperature alloy machining. This dissertation focuses on Ti6A14V HPM, systematically analyzes the tribological behaviors of tool-chip interface (rake face), tool-workpiece interface (flank face), the thermo-mechanical coupling effect and its influence on tool wear and tool life in Ti6A14V machining, finally provides theoretical and practical support for tool materials selection, tool geometries and cutting parameters optimization.Tribological behaviors of Ti6A14V-tungsten carbide (WC-6%Co) were studied. In friction process, friction coefficient is approximately linear with friction temperature. Wear rate of carbide increased with sliding speed, wear mechanism was adhesion, diffusion, oxidation, flaking and fracture, several wear mechanisms is interactive. Diffusion of carbide binder Co is the key factor for carbide substrate weakening and flaking.The friction models in tool-chip (rake face) and tool-workpiece (flank face) interface was constructed, based on friction characteristics analysis in Ti6A14V orthogonal cutting tests. Adhesion is the main contact morphology at tool-chip, tool-workpiece interface, which consist of tight adhesion region and non-tight adhesion region. The principal stress and frictional stress were a negative exponential distribution in whole contact length; friction coefficient is a function of average cutting temperature. The stress and sliding speed continuity hypothesis at tool-chip, tool-workpiece interface and edge area were assumed in visco-plastic material cutting process, base on DEFORM 3D finite element simulation and experimental analysis. It is a foundation for complete research on tribological behaviors and thermo-mechanical coupling effect of tool-chip, tool-workpiece interface in Ti6A14V machining process.Thermo-mechanical coupling analytical models of tool-chip and tool-workpiece interface were constructed; their computation results showed that the contact length, stress distribution, cutting force, cutting temperature at rake face contact area is influenced by cutting parameters and tool geometries, as well as flank wear value in machining process. These models provide the theoretical foundation for the research of tool wear laws in Ti6A14V machining.Diffusive crater wear model was established, based on tool wear mechanism and the thermo-mechanical coupling analysis in Ti6A14V machining. And its computation result showed that diffusion of Co element from tool into workpiece material at elevatated cutting temperature is the key factor of tool wear. The wear rate is proportional to Co content, and these results provide the theoretical foundation for tool material selection in Ti6A14V machining. The flank wear model was also constructed, base on diffusion, adhesion and abrasion wear mechanisms in Ti6A14V machining, its computation results showed that the proportion of three wear mechanisms was affected by cutting speed and flank wear value VB, the proportion of diffusion wear severely increase with VB and cutting speed increase in total wear value.The influence of tool geometries on turning process were analyzed, influence of inclination angle on cutting temperature and cutting force is not significant, when it changes from -15°to 15°. But, the influences of lead angle is remarkable, the smaller lead angle is more favorable for tool life in Ti6A14V turning. Therefore, appropriate cutting speed, lager depth of cut, smaller lead angle are the optimal direction for Ti6A14V high efficiency turning. The Ti6A14V turning cutting experiments with different materials, such as uncoated carbide, coated carbide (TiAlN) and PCBN were conducted, to analyze the influence of cutting parameters on cutting force, cutting temperature, tool wear and tool life. Finally, cutting parameters were optimized for three tools, based on tool life-cutting efficiency relationship analysis. The cutting performances of three tools are uncoated carbide < coated carbide < PCBN. The use of PCBN tool will be capable of Ti6A14V high speed turning.DEFORM 3D milling process simulation results showed that the milling process is different from turning, in which the intermittently mechanical impact and thermal shock result in tool wear and fracture. The Ti6A14V milling test was performed for uncoated carbide and coated carbide (TiAlN), the influences of coating, cutting parameters, coolant on cutting force, cutting temperature and tool life were analyzed. The cutting performance of coated carbide is more excellent than uncoated carbide at lower cutting speed, but it has no remarkable difference in higher cutting speed. The coolant can significantly improve the tool-chip, tool-workpiece interface contact situation and prolong the tool life. Finally, cutting parameters of both tools were optimized, based on tool life-cutting efficiency relationship analysis. This work is sponsored by the National Natural Science Foundation of China (50575126) and National Basic Research Program of China (973) (2009CB724401 and 2009CB724402).