Microstructure evolution and machinability of electropulsing treated titanium alloys in ultra-precision machining

Abstract

The titanium and its alloys with high strength and low weight have increasingly been applied in automotive, aerial and biomedical industries for their superior mechanical properties as well as exceptional biocompatibility. However, the inherent nature of inferior heat conductivity, chemical reaction at elevated temperature, and low modulus of elasticity brings great challenges in machining this type of material due to the poor machinability, including severe tool wear, high vibration, and low material removal rates, to mention a few. Though many studies have been carried out on investigating the machinability of titanium alloys by optimizing cutting parameters, improving cooling conditions and adopting hybrid machining, investigation about machinability of the alloy is still rare with respect to material microstructures which ultimately determine the mechanical properties of titanium alloys. Besides, electropulsing treatment (EPT) is a technique to rapidly alter the microstructures of conductive metals and alloys. Study about microstructure evolution induced by EPT is also needed to pay more efforts and further exploration during treating titanium alloys. In this thesis, microstructure evolution induced by various EPT conditions and its effects on wear and corrosion resistance of Ti6Al4V alloys are presented in the first part. A homogeneous grain distribution with equiaxed grains is obtained after performing EPT on Ti6Al4V alloys at temperature below β phase transformation temperature. The phase transformation from (α + β) phase to single β phase occur in Ti6Al4V alloys at temperature above β phase transformation, while the inverse transformation happens after cooling from single β phase region. The final thickness and length of the lamellar α phase highly are closely related to cooling rates. Increase in cooling rates would result in a thin lamellar martensitic distribution. In addition, β grains coarsen dramatically at a high temperature which is attributed to the high atom diffusion and large driving force induced by the thermal and athermal functions of EPT. The corrosion and wear resistance of the lamellar Ti6Al4V alloy are both higher compared with the as received alloy. In the second part, three typical microstructures (equiaxial, bimodal and fully lamellar structures) are firstly obtained by high efficient EPT. The temperature evolution induced by the thermal effect of EPT is theoretically simulated and experimentally verified, and the dislocation and vacancy motion accelerated by the athermal effect of EPT was discussed by considering electropulsing voltages. The phase composition, micro-hardness, cutting forces and machining surface morphologies are also presented in detail with respect to various microstructures. The surface roughness and peak to valley value can reach about 9 nm and 31 nm after diamond turning, which is smaller than those with equiaxial and bimodal microstructures. The various turning surface profiles are mainly attributed to the different swelling and recovery of the Ti6Al4V alloys. As several orientations of acicular martensites would be nucleated in a single β grain, martensitic Ti6Al4V alloy could exhibit a local anisotropic behaviour with respect to martensitic orientations. Hence, the third part is dedicated to investigating the influences of martensitic orientations on deformation mechanisms and cutting force variation in low speed ultra-precision rotary cutting. A geometrical and physical model is proposed for simulation of resolved shear stresses in different slipping or twinning systems, and the results demonstrated that activation of deformation modes varies with martensitic orientations. Besides, the average cutting forces are not only affected by α phase sizes but also influenced by the coordination and competition of diverse slipping directions. Finally, the effect of grains and twins on the deformation of commercial pure titanium (CP Ti) in ultra-precision diamond turning (UPDT) is presented in the fourth part. CP Ti workpiece with gradient distribution of grains and deformation twins is obtained via compressive test followed by EPT of 30 s for exploring the grain/twin size effects on surface generation. The results show that microcracks primarily nucleate in large grains and thick twins rather than small grains or twins. Furthermore, critical resolved shear stress (CRSS) and barrier stresses induced by grain or twin boundaries and dislocations are introduced in the proposed model, which not only compares the deformation difference between grains and extension twin of {1012} <1011> type, but also simulates the relationship between resolved shear stresses, cutting shear angles strain rates and temperature for the first time in ultraprecision diamond turning. This research contributes a better understanding of EPT induced microstructure alteration of the Ti6Al4V alloy and CP Ti and reveals the deformation mechanisms in ultra-precision machining (UPM) of the two types of materials through theoretical simulation and experimental validation, which is helpful for academic investigation and industrial application of titanium alloys.