Research On Effects Of Traditional Sintering And New Hydrogenation-thermal Cycling Sintering On Microstructure And Properties Of Titanium Alloys

Titanium alloys are widely used in aviation and navigation fields due to their high specific strength,corrosion resistance,and high-temperature mechanical properties.However,because of the high cost of traditional processing,as an economical method,powder metallurgy process has attracted widespread attention.In present investigation Ti80 alloy(Ti-6Al-3Nb-2Zr-1Mo)is firstly used to explore the influence of temperature and time on titanium alloy in the traditional blended elemental powder metallurgy(BEPM)sintering process.Basing on the characteristics of the traditional sintering process,TC4(Ti-6Al-4V)is used to explore a new type of hydrogenation and thermal cycle sintering,propose to improve and optimize the sintering process.Usually,with the increase of sintering temperature and time,the density of titanium alloy will increase either,but at the same time it will result in the grain grow coarsly,while the relatively low sintering temperature and shorter time can maintain the fine grain,but will lead to insufficient densification,which is not conducive to the balance of mechanical properties,so it is necessary to explore the best combination of temperature-time parameters.Using Ti H₂as the starting powder,the presence of hydrogen improves the diffusion activity,so that a uniform structure is obtained in the presence of Mo and Nb hard-to-diffusion elements in Ti80.The experimental results show that when the sintering at 1200?for 4 hours,Ti80 material has obtained low porosity(1.6%)and relatively small?grains(108?m),material obtain a relatively balanced mechanical properties(tensile strength of 948±12MPa,elongation of14.0±0.8%).The Ti80 alloy can be further thermally processed after sintering to obtain high density and finely broken(?+?)structure.Highly densification,fine grains,small and regular pores are usually the characteristics pursued by powder metallurgy materials.However,due to its own limitations,the traditional sintering process is difficult to form the mentioned structure without subsequent processing.Therefore,it is very important for powder metallurgy to explore a new sintering process making materials own a structure with high density,fine grains,small and regular pores.The author proposes the thermal cycle sintering and hydrogenation cycle sintering processes,using TC4 to explore the influence of the above processes on the material structure.Experiments have proved that by performing thermal cycling sintering and hydrogenation cycling near the phase transition temperature,the continuous?(?)?_Hphase transition of the material can be used to generate a large number of dislocations and other defects to accelerate the diffusion of elements,and at the same time helping repair the pore structure.After pre-sintering at 1100?for a certain period of time,and then performing 5 times thermal cycles between 1100?and 850?or 2 hydrogenation cycles at 850?(hydrogenation for 45 min,dehydrogenation for 30 min),finally raised the temperature to 1100?to complete the total 3 hours sintering time,the titanium alloy structure shows a good improvement compared with sintering at 1100?.The thermal cycle and the hydrogenation cycle are simultaneously applied in the same sintering process.After optimizing the parameters,the hydrogenation thermal cycle sintering process is obtained,that is,pre-sintering at 1100?for 30 minutes,furnace cooling to 850?for two hydrogenation cycles,and heating to 1100?for 30minutes after sintering,Carry out 5 thermal cycles at 1100?and 850?,and finally keep the temperature at 1100?for 3 hours.At this time,Ti-6Al-4V obviously eliminates the typical irregular and coarse pores in traditional constant temperature sintering,so that it can be repaired to a small circle to a great extent.Using the same powders,comparing with the material obtained by 1250?-4h sintering,the average?grain size is reduced by 40%,the density is increased by 0.9%,and the average pore roundness and average pore length are reduced by 1.67%and 2.66%.When using the fine mixed powder,relative to the 1250?-4h group,the average?grain size is reduced by 55%,the density is increased by 1.24%,and the average pore roundness and average pore length are reduced by 2.85%and 65.99%.At this time,Ti-6Al-4V obtained fine?grain size(45-60?m),lower oxygen content(0.16-0.21%)and higher relative density(97.7-98.0%),showing strength and elongation The good balance between the properties,the tensile strength can be as high as 880±20MPa at an elongation of 15±2%,which provides an excellent organizational basis for the subsequent improvement of other properties.