Study On Microstructures And Mechanical Properties Of High-strength BT22 Cast Titanium Alloys

The high strength titanium alloys are of advantages such as high strength, highspecific strength and excellent corrosion resistant characteristics, and have beenwidely applied in aerospace, etc. However, the sorts of high-strength titanium alloysare few at home and the study on microstructures and mechanical properties ofhigh-strength cast titanium alloys is not systematic, too. Therefore, author studied onmicrostructure and mechanical properties of BT22 titanium alloy as cast and furtherstudied the influence of the solution heat treatment temperature, the cooling rate, theaging temperature and the soaking time on the microstructures and the mechanicalproperties of BT22 cast titanium alloy, and determined the heat treatment processparameters for the BT22 high-strength cast titanium alloy. The work depicted aboveprovided theoretical basis for putting the high-strength cast titanium alloys intopractice.The study results proved that the transition point of a phase toβphase of BT22cast alloy is 885℃, and the heat treatment process parameters have great effect on themicrostructures and properties of the alloy. Under the condition of less 885℃solutionheat treatment, the needle-likeαphases will separate out in the originalβphase grainsfor the alloy, under the condition of over 885℃solution heat treatment and thencooling by water, singleβphase microstructure will be obtained, and the alloy will beof good plastic property, under the condition of 810℃and 760℃two-stage solutionheat treatment and then cooling in air, the microstructures of the alloy will behomogeneous and the synthetic mechanical properties of the alloy will be better.During the aging treatment of BT22 titanium alloy, theαphases in the solutionmicrostructures will grow firstly, then dissolve into the base ofβphases, while the primaryαphases begin to separate out, at the same time, the analysis of SEM provethat the small secondary needle-like a phases will separate out between the primaryαphases and the base ofβphases. The microstructures of the alloy after aging are stilltheα+βphases. With the increase of the aging temperature, the strength and theplasticity of the alloy will increase firstly then decrease. With the extension of thesoaking time, the strength of the alloy will decrease; while the plasticity will increasefirstly then decrease.The solution and aging heat treatment processes can improve the microstructuremorphology and distribution ofαphases of the alloy, and this is also an importantmeans to improve the strength and plasticity of the alloy. Under the condition ofsolution heat treatment at 810℃holding 1 hour, next, cooling to 760℃in furnaceholding 1 hour, then; cooling in air, finally, aging heat treatment at 620℃holding 8hours, the BT22 cast titanium alloy will be of excellent synthetic mechanicalproperties: tensile strength Rm≥1150 MPa, yield strength R_p0.2≥1100MPa, elongationA≥5% and necking down ratio Z≥15%.