Study Of Tensile Properties And Columnar Crystal Structure Of Rapidly Solidified Titanium Alloy

TC18 alloy is a kind of typical precipitation hardened transition metastableα+βalloy. Ti-5Al-5Mo-5V-1Cr-1Fe(TC18)is equivalents to the Soviet BT22 alloy. It has the higher contents of Mo and contains Cr and Fe (each about 1%,in weight) so that it is equivalents to Ti-1023 alloy in the ways of heat treatment strengthening effect and high hardenability and also,this alloy is suitable for manufacturing large air force bearing structure. The rapid prototyping technology of the metal parts is new material forming processing technology developed in recent ten years. The technology can be driven by three–dimensional digital model of the parts and can be used in directly manufacturing near net forming structure without any mold. The rapid prototyping technology has the advantages of short processing period, low cost, high flexibility in comparison with traditional forging method. Domestic and foreign research work almost began at the same time in the medium of last century. The relative application in the field of aviation was reported. Electron beam deposition manufacturing is a kind of forming technology in which the metal material is molten by an electron beam and then is accumulated into certain shape. As it is a combination of material preparation and forming composite processing, the fundamental research on electron beam deposition manufacturing is still necessary.The columnar crystal structure of TC18 titanium alloy was obtained by electron beam deposit. The effects of heat treatment conditions on the tensile properties and microstructure of TC18 are investigated.The tensile and yield strength of the alloy in the direction parallel to the length of the columnar grains increases, at the expense of ductility, with increasing solution temperature from 740 to 830℃followed by the aging treatment of 550℃×4 h. Meanwhile, the amount of primaryαphase and the ratio of length /width decreasesThe tensile and yield strength of the alloy in the direction parallel to the columnar grain length decreases but the ductility increases with increasing ageing treatment temperature from 550 to 650℃in the process of solution treatment at 830℃followed by ageing treatment. The tensile properties are influenced mainly by the secondaryαcaused by heat treatment.Fracture surface investigation is also conducted and it is found that two failure modes, ductile and intergranular + quasi-cleavage, may exist under the same heat conditions, which may account for the dispersion of tensile ductility data under certain heat treatment conditions. And also, the formation reasons of the two fractures are analyzed.