The Study On The Law Of Isothermal Foaming To Fabricate Ti-6Al-4V Foams By Gas Entrapment And Their Performance

The pore wall is compact and mechanical properties are stable for using the technology of hot isostatic pressing during the process of fabricating titanium and its alloys foams by gas entrapment. Meanwhile, there is no any other pollutant in the whole process which ensuring a requirement of biomedical materials purity. So they are of potential interest for applications such as aviation, aerospace, medical equipment and so on. The process of titanium and its alloys foams fabricated by gas entrapment through isothermal foaming is simple, the structure of the pores is regular and extension in the existing industrial foundation is possible. However the main problems are that the porosity is usually low and the foaming temperature is too high.Therefore, the effects and mechanism of filling process, consolidation process and isothermal foaming process on the Ti-6A1-4V billets foaming behavior were studied in this work. On the basis of the results, the main factors which affecting the isothermal foaming ability of the billets ware revealed, and the Ti-6A1-4V foams with the porosity of 40% were successfully fabricated. A new method of hydrogen assisted isothermal foaming was presented which reduced the isothermal foaming temperature by 60℃ and improved the isothermal foaming results of the billets below 900℃. The compression performance of Ti-6A1-4V foams was also detected and analyzed, and the mechanism of compression was determined. The main results are as follows:Many effects of billets and their isothermal foaming were studied by changing the filling process parameters, for example, argon pressure and powder size. The results showed that:increasing the filling argon pressure appropriately could increase the diameter and number of pores after foaming, but the over high pressure was not conducive to isothermal foaming for prematurely which caused the pores communication with each other along the edge of the original powder. Moreover the powder size was larger, the more was unfavorable to the isothermal foaming of the billets. The relatively appropriate filling process parameters were the argon pressure of 0.4 MPa and the powder size of 75-150μm.Based on the results of the effects of argon pressure on the billets and their isothermal foaming behavior, the mechanism of the densification process was studied in order to improve the ability of the billets isothermal foaming. The results showed that:changing the distribution of pores and micro defects around the original powder could significantly improve the ability of the billets isothermal foaming, but the reticular connected pores were not conductive to isothermal foaming, which formed in the rolling insulation stage caused by high filling argon pressure and were difficult to eliminate by rolling. So a new idea for improving the billets isothermal foaming ability was proposed by controlling the filling argon pressure and the 90° cyclic cross rolling reduction. Ti-6A1-4V foams containing up to porosity of 40% were successfully produced after 950℃/20 h isothermal foaming by using the billets which were made from the filling argon pressure of 0.4 MPa and the powder size of 75-150μm after the 900℃-90℃cyclic cross rolling with 60% reduction.Moreover, the process of isothermal foaming was studied, and the results showed:increasing the foaming temperature could improve the argon pressure of pores and the ratio of the lower flow stress β phase, so the porosity of Ti-6A1-4V foams was improved by increasing the diameter of the pores and inhibited pores connectivity. However when the foaming temperature was higher than 950℃, it was not conducive to the foaming for large size β phase. At the same time, the shape of the pores was changed from sphere to irregular polyhedron. The increase of the isothermal foaming time could improve the porosity of Ti-6A1-4V foams by promoting pores growth and the number of spherical pores, but too long foaming time did not cause more change.Based on the results of isothermal foaming process, a new method for isothermal foaming assisted by hydrogen was presented. The results showed that hydrogen, by decreasing (a+(3)/(3 phase transition temperature, improved ductile β phase ratio and could soften the a phase in some degree and reduced high temperature flow stress of billets, and further decreased the optimum isothermal foaming temperature and improved the foaming ability of the billets below 900 ℃. The hydrogen of mass fraction 0.15% could reduce the Ti-6A1-4V billets optimum isothermal temperature by 60℃, which the pore morphology was sphere and the pore distribution was dispersed evenly after isothermal foaming at 890℃.Quasi static and dynamic compression properties of Ti-6A1-4V foam fabricated by isothermal foaming were studied and the results showed that the compression deformation mechanism was the macro deformation was coordinated by extension and bending for the pores wall had good elastic and plastic deformation ability. The crack expansion perpendicular to the direction of pressure would also coordinate the deformation to a certain extent with increasing compressive strain. So the Ti-6A1-4V foams had high compression stability, even though the strain rate reached 4300s-1. pores devastating collapse and pores wall partial adiabatic shear band did not appeared. Pores also inhibited crack propagation along the direction of maximum shear stress during compressing. When the compress stress reached 1700MPa upper its compressive strength, transient instability of stress was not appeared.