Experimental Research On High-speed Grinding Of Titanium Matrix Composites With Single-layer CBN Wheels

Ti Cp/Ti-6Al-4V titanium matrix composites have broad application prospect in the aerospace industry because of their excellent physical and mechanical properties, such as high specific strength,high temperature durability and good wear resistance. However, two unfavorable issues are always encountered during the conventional grinding operations due to high toughness of the Ti-6Al-4V bulk materials and high hardness of the Ti C reinforcing particles. One is the surface defects originated from fracture or pull-out of the particulate reinforcements, while the other is workpiece burn related to the elevated grinding temperature, which deteriorates the surface quality and hinders the machining efficiency significantly. Therefore, in order to realize the high-efficiency and precision grinding, it is put forward to employ the single-layer cubic boron nitride(CBN) wheels and high speed grinding technique to machine the Ti Cp/Ti-6Al-4V titanium matrix composites.In the present investigation, as part of the whole work, high speed grinding experiments have been conducted using single-layer electroplated CBN wheels and single-layer brazed CBN wheels,respectively. Some critical issues, i.e., assessment of grindability, characterization and prediction of workpiece burn, characterization and quantitative evaluation of surface defects, are analyzed. The main research work and results are summarized as follows:(1) The forces and temperature of Ti Cp/Ti-6Al-4V titanium matrix composites in high speed grinding are investigated, which is utilized to evaluate the grindability. Generally, the grinding forces and temperature of Ti Cp/Ti-6Al-4V titanium matrix composites are larger than that of Ti-6Al-4V titanium alloy, which is due to existence of the reinforcements. The grinding forces increase with decreasing wheel speed and increasing workpiece speed and depth of cut, while the grinding temperature increases with increasing wheel speed, workpiece speed and depth of cut. The wheel speed has a most significant influence on the grinding temperature, followed by depth of cut and workpiece speed. Grinding forces and temperature obtained by using the brazed CBN wheel are larger than that obtained when using the electroplated CBN wheel. Meanwhile, the maximal material remove rate to generate burn-free surface with the brazed CBN wheel is increased significantly compared to that with the electroplated CBN wheel.(2) The macro- and micro-topography of burn-out and burn-free ground surface during high speed grinding of Ti Cp/Ti-6Al-4V titanium matrix composites are detected. The surface color of burned workpiece shows a good relationship with the grinding temperature. With increasing burn degree,microstructural alterations of subsurface material become severer and may present thermal softeningin the immediate sub-surface of the workpiece. Furthermore, compared to Ti-6Al-4V titanium alloy,Ti Cp/Ti-6Al-4V materials produce a thicker metamorphic layer in high speed grinding. A prediction model is established based on specific grinding energy to forecast workpiece burn during high speed grinding of titanium matrix composites.(3) The morphological details of ground surface in high speed grinding of Ti Cp/Ti-6Al-4V titanium matrix composites are detected. Generally, the ground surface consists of the grinding striations, the pores, the smooth areas, and the irregular areas. An increase in workpiece infeed speed and depth of cut or a decrease in grinding speed resulted in a brittle-to-ductile transition for material removal mode of the reinforcements. Meanwhile, in comparison to an electroplated CBN wheel, a brazed CBN wheel is more favorable in terms of reducing surface defect of Ti Cp/Ti-6Al-4V materials.Results obtained in this investigation could enhance the understanding of high speed grinding Ti Cp/Ti-6Al-4V titanium matrix composites with the single-layer CBN wheels. A theoretical foundation could be accordingly established for further investigation on controlling surface defect and eliminating workpiece burn-out during grinding of Ti Cp/Ti-6Al-4V titanium matrix composites.