Effect of heat treatment on Microstructure and properties of extruded TC18 titanium alloy tube
The air cylinders must have high reliability and safety, so the material strength and plasticity of the demand is higher, at 1080 ~ 1280MPa between the tensile strength, yield strength more than 1010MPa, the elongation rate is more than 10%, more than 35% section shrinkage.
TC18 titanium alloy is a high alloying titanium alloy, the nominal component of Ti-5Al-5Mo-5V-1Cr-1Fe, corresponding to the former Soviet Union brand BT22. The transition metal type alpha + beta titanium alloy, the characteristics of both two-phase titanium alloy and titanium alloy, has high strength, good plasticity deformation, excellent hardenability and good weldability and a series of advantages, has been widely used in the manufacture of aircraft landing gear, frame, beam, ribs, and joints fasteners. It is reported in the literature that the strength of TC18 titanium alloy can reach 1145MPa and reach 16% after solid solution aging heat treatment. It is a candidate material for aircraft air cylinder.
By extrusion forming of TC18 titanium alloy tube with diameter of 170mm * 30MM * L as BaoTi group to make air cylinder blank. The sample is tested by annealing and the tensile strength is between 1060 and 1080MPa. The yield strength is between 960 and 980MPa, which can not meet the requirement of mechanical properties of the air cylinder. Therefore, research on two-phase region of solid solution + aging and double solution + aging effect on two kinds of process and properties of TC18 titanium alloy tube extrusion forming organization, provide the basis for the development of TC18 titanium alloy tube heat treatment process of batch hot extrusion with the air cylinder.
Selection of high quality sponge titanium, chromium, vanadium metal aluminum intermediate alloy, aluminum wire and other raw materials, after three times of TC18 titanium alloy ingot vacuum self consuming arc melting into a diameter of 700mm, beta phase transition point measured by the metallographic method of ingot is 870 - 5 DEG C. In 3150T hydraulic press ingot forging repeatedly, processed with diameter of 210mm * 750mm billet billet; hot extrusion in 3150T hydraulic machine, extrusion temperature is 900 DEG C, 20min insulation, get the specifications for the 170mm * 30MM * L hot extrusion tube.
The metallographic and tensile specimens were cut from the TC18 titanium alloy tube by spark wire cutting machine. The test selected two kinds of heat treatment of the solid solution plus aging treatment 780 c * 1h/AC, then in 540, 560, 580, 620 DEG C 8h, air cooling; the dual solution and aging treatment of 1H 840 DEG C, furnace cooling to 750 DEG C, 1H insulation, air cooling, and then in 540 580 and 620 DEG C for 5h, air cooling. The microstructure and tensile properties were tested. The test results are as follows:
(1) by means of solid solution + aging heat treatment, the acicular phase with diffuse microstructure is obtained, and the lamellar alpha and equiaxed phases with dispersive distribution are obtained by double solution aging and aging heat treatment. With the increase of aging temperature, the primary alpha phases in the two processes were significantly reduced.
(2) the mechanical properties of TC18 titanium alloy tubes can meet the requirements of air cylinders by solid solution + aging and double solution aging heat treatment.
