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Basic study on preparation of high-temperature titanium alloy by titanium hydride powder metallurgy

wallpapers News 2021-11-24
High-temperature titanium alloy is widely used in aero engines because of its high specific strength and high-temperature oxidation resistance. Its commonly used preparation method is the smelting method, which is easy to cause uneven structure and segregation phenomenon, thus affecting the properties of the alloy. The powder metallurgy method can reduce component segregation and can be nearly net forming. In this paper, high-temperature titanium alloy was prepared using cold isostatic pressing and vacuum sintering with titanium hydride as the main raw material by powder metallurgy. In this paper, the composition of high-temperature titanium alloy was designed theoretically, the dehydrogenation and deoxygenation mechanism of titanium hydride and the influence mechanism of titanium hydride on the properties and phase transition temperature of the alloy were studied, which provided a theoretical basis for subsequent experiments. The effects of titanium hydride content, sintering temperature, β -phase-stable elements W and Nb on phase composition, chemical composition, microstructure, and mechanical properties of high-temperature titanium alloy were investigated. The main conclusions are as follows :(1) the alloy composition of ti-6.5al-2.7sn-4zr-0.4mo-0.4si was designed by Al equivalent and Mo equivalent formulas. The initial phase transition temperature and phase transition point of the alloy were determined as 891℃ and 1273℃ by differential thermal analysis. (2) The sintering temperature is 1150℃ and the sintering time is 4h. With the increase of titanium hydride content, the metallic luster of the alloy increases obviously, and the oxygen content of the alloy decreases, indicating that titanium hydride can promote sintering and reduce oxygen content at the same time; The removal effect of hydrogen in titanium hydride is good, and there is only trace hydrogen element in the alloy, and it has little effect on the composition of the alloy. With the increase of titanium hydride content, the microstructure changes from equiaxed microstructure to basket microstructure, the porosity decreases, the relative density reaches 95.45%, the mechanical properties increase, the tensile strength reaches 460.1MPa at room temperature, and the hardness reaches 298.4HV. (3) All titanium sources are titanium hydrides, and the sintering time is 4h. With the increase of sintering temperature, the grains grow up gradually, and the relative density is further improved, reaching 95.84%. The mechanical properties increase first and then decrease. When the sintering temperature is 1150℃, the maximum bending strength at room temperature is 556.2MPa.Compared with the theoretical phase diagram of the alloy, Ti3Al and Sn3Zr5 did not appear in the experimental alloy. (4) When all-titanium sources are titanium hydride, sintering time is 4h, the sintering temperature is 1150℃,0.5% W can improve mechanical properties slightly;1%W can increase the bending strength by 52.4MPa, increase the hardness by 31.4hV and decrease the tensile strength by 122.5MPa.W has a great influence on the compressive strength of the alloy at high temperature. When W content is 0.5%, the flow stress of the alloy can be increased, and the maximum flow stress is 505MPa at 750℃. When the content of W is 1%, the high-temperature strength of the alloy is favorable at the low strain rate (0.1s-1) and deformation temperature (750℃800℃), but when the strain rate and deformation temperature are increased, the high-temperature strength of the alloy is unfavorable. Therefore, when the content of W is 0.5%, the alloy with good temperature and room temperature properties can be obtained. (5) When all-titanium sources are titanium hydride, sintering time is 4h and sintering temperature is 1150℃, ZrSi and Mo5Si3 phases are precipitated in the alloy with the increase of Nb, and the microstructure is gradually refined. The addition of Nb reduces the mechanical properties of the alloy at room temperature. Nb has a certain effect on the high-temperature properties of the alloy when the amount of Nb is 1%, the high-temperature strength of the alloy can be improved, the maximum flow stress is 490.1MPa at 750℃ when the amount of Nb is 2%, the high-temperature strength of the alloy will be reduced, compared with the two β phase-stable elements, W has a great influence on the high-temperature properties of the alloy.

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