| Reducing reflecting layer, transparent electrode, emitter and cathode in the LCD manufacturing technology are all carried out using sputtering method. Sputtering target is usually made of pure molybdenum sheet. In recent years, with the growing size of LCD screen, the volume of sputtering shape molybdenum target increases correspondingly. If molybdenum tube is used as target, the length of screen is depended on the length of molybdenum tube while the width is unlimited. To improve the uniformity of sputtered coating, the density of sputtering target must be over 99.7%. However, the common molybdenum product technology can not meet the density demand for targets. Therefore, hot working process is added to raise the density of molybdenum products. In this paper, the hot extrusion process is adopted in this work.In view of low density of molybdenum products prepared by hot extrusion process of powder metallurgical molybdenum tube, methods of high-temperature uniaxial compression test combined with finite element numerical simulation are used to investigate the hot extrusion process of powder metallurgical molybdenum tube. The research include high-temperature plastic deformation behavior of powder metallurgical molybdenum bars as well as optimization of molybdenum tube hot extrusion process and extrusion die structure.Uniaxial compression test was carried out on Gleeble-3800 thermal simulator at 1100, 1150, 1200 and 1250℃and at strain rates of 11.5, 17.3 and 23s-1. The size of powder metallurgical molybdenum specimen isφ8×12. The high-temperature steady flow stress constitutive equation for powder metallurgical molybdenum bars was set up based on the experimental data and the flow stress mechanical behavior characteristic in deformation process is studied.Results show that: powder metallurgical molybdenum shows obvious characteristics of dynamic recovery and recrystallization in the high temperature deformation. Molybdenum is strain rate sensitive material, that is, flow stress increases with increasing strain rate. The high-temperature plastic deformation process of powder metallurgical molybdenum is thermally activated process controlled by dislocation movements and can be described by using Zener-Hollomon parameters including Arrhenius.The density variations of deformed powder metallurgical molybdenum bars were tested by using suspending weight method. Experiment results show that powder metallurgical molybdenum bars can meet the density demand of target when the deformation of powder metallurgical molybdenum bars is over 70%.Effect of heat-treatment temperatures (750,850和950℃) after thermal deformation on the microstructure was investigated by microstructure observation. Microstructure analysis shows that: The optimal process is annealing at 850℃for 60min when the relative deformation of powder metallurgical ingots is 70%. At this time, most grains overlapped accompanied by formation of fine equiaxed subgrains. Most banded structures are retained and residual stress is eliminated, so the performance is the best.Simulation analysis by finite element software DEFORM was done to optimize the extrusion die structure for manufacturing powder metallurgical molybdenum tubes. The extrusion mold with die angles of 55°,60°,65°,70°, straight die length of 8%D,10%D,11%D and 12%D, extrusion speed of 225mm/s and extrusion temperature of 1250℃were simulated. Results show that: taking extrusion rate, tube rolling yield, all-in cost saving ratio and stress and strain status as standard, the optimal die structure is obtained when straight die length of flat taper mold is 11%D and die angle is 60°. |
