Microstructure And Properties Of TI6AL4V Alloy Prepared By Magnetic Pulse Compaction And Sintering Of Hydrogenated Powder

Titanium and titanium alloys have been widely used in aviation, biomedical and chemical industries due to their outstanding properties, such as good comprehensive mechanical properties, low specific weight, high specific strength, high temperature strength, excellent anti-corrosion performance and high biocompatibility. Powder metallurgy (PM) is an ideal approach for fabrication of high–performance and low–cost titanium alloys. The combination of PM and thermohydrogen processing (THP) can reduce the temperature, time of consolidation and the porosity of sintered body during the sintering process, as well as improving its mechanical properties. Ti6Al4V, a typicalα+βtitanium alloy, is most widely used and investigated because of its excellent comprehensive mechanical properties. Magnetic pulse compaction (MPC) allows the powder compaction to carry out successfully under conditions of heating, vacuum or protective atmosphere. This method can achieve the automated mass production and fully dense (or near fully dense) powder compact by using high strength impact loading, which has been widely investigated in the compaction of ferrous metals, nonferrous metals, intermetallic compounds, ceramic and composite materials.In this dissertation, the deformation behavior and mechanism of MPC of Ti6Al4V alloy powder are systematically investigated by theoretical analysis, numerical simulation, experiments and microanalysis. The microstructure and properties of sintered bodies are investigated by the combination of experiments and microanalysis. Thermal oxidation (TO) process and TO + oxygen diffusion (OD) process have been investigated to improve the wear-resistance of sintered samples. Then the sliding frictional properties of the treated samples are investigated.The compaction pressure of MPC is measured by an electric pressure sensor, and the general formula of it and the density of compact is established. The correlation of compaction energy density and the porosity of compact are studied. The MPC process is analysed by a loose coupling numerical scheme. The discharge circuit is simplified as a RLC circuit in ANSYS. As the boundary condition of simulated current, the electromagnetic field model of current incentive is established by ANSYS/Multiphysics module. As the boundary condition of electromagnetic field simulation result, the powder compaction analysis model is established by Shima yield criteria and the powder module in MSC.MARC. As the boundary condition of simulated compaction velocity, the micro deformation model of powder is constructed based on the Johnson-Cook relation by using MSC.MARC. The simulation results show that the driver plate is mainly subjected to the axial magnetic force and its distribution is uneven along with the radial direction of the drive plate. The magnetic force along with the driver plate thickness is a gradient distribution. The average relative density of compact and the distribution of relative density are apparently affected by discharge voltage, compaction temperature and friction coefficient. The discharge voltage has a large effect on the compaction velocity. The deformation and temperature rise of powder particles manily occur in the surface layer, and they enhance with increasing discharge voltage and compaction velocity.The relative density and hardness of green body of hydrogenated Ti6Al4V powder compacted by MPC increase as compaction temperature rises. The relative density of compact pressed by MPC is about 12 percent higher than that prepared by static compaction under the same compaction pressure. The relative density, hardness and compressive strength of vacuum annealed samples can be increased by increasing discharge voltage, and they basically reach their maximum values at 200℃. As hydrogen content rises, the compressive strength of vacuum annealed samples first increases and then decreases. The microstructure of vacuum annealed samples consists of primaryαphase andβphase. The number of equiaxed grains increases with the augment of hydrogen content.The coating of samples processed by TO is dense and insulated, and its thickness increases as the temperature and time increase, but the coating may break off when it is too thick. The primary substance of the coating is Al₂O₃ which is on the surface of the coating and TiO₂ inside. On the inner side of the coating, there is a hardened layer with thickness of about 140μm. The TiO₂ of coating of samples treated by TO + OD decomposes to TiO. The thickness of hardened layer reaches approximately 210μm. The wear rate of samples treated by the two methods is about two orders of magnitude lower than that of untreated sample. The TO + OD treatment appears to be more efficient in surface hardening and wear resistance than the TO process.