Annealing process for Gr38 titanium alloy

Titanium alloy, as a lightweight structural material, has excellent comprehensive performance, low density, high specific strength, good fatigue strength and resistance to crack extension, excellent corrosion resistance, good welding performance, etc., so it has an increasingly broad application prospect in the aviation, aerospace, automotive, shipbuilding, energy and other industries.Gr.38 titanium alloy is a new titanium alloy developed by ATI Technologies in the U.S.A., which can be used to replace the most common medium- The nominal composition of Gr.38 titanium alloy is Ti-4Al-2.5V-1.5Fe-0.25O, which is a kind of α+β-type high-strength titanium alloy. Compared with TC4 alloy, Gr.38 alloy utilizes iron instead of the more costly vanadium as the β-stabilizing element, and its strength is comparable to that of TC4 alloy, and its elongation is comparable or slightly higher, but unlike it, it is capable of hot as well as cold working, and can be made into thin sheets, coils, strips, precision hot-drawn strips, thick plates, seamless tubes, as well as castings and engineered products. In view of Gr.38 titanium alloy has excellent superplastic forming and open hole fatigue performance, but also can be friction stir welding, its use is very wide, quite suitable for replacing steel, aluminum, composite materials, pure titanium and other titanium alloys, especially in the aerospace and military defense system has a very broad application prospects. At present, there are very few research reports on this alloy, therefore, the researchers studied the effect of different annealing regimes of Gr.38 titanium alloy small bars on microstructure, mechanical properties and tensile fracture morphology.

The main raw materials used in the preparation of Gr.38 titanium alloy are titanium sponge and added alloying elements, and the added alloying elements are aluminum-vanadium alloy, aluminum beans, iron nails and titanium dioxide. After the process of mixing and electrode preparation, finally, the ingot of Φ440mm was prepared by two vacuum melting using vacuum self-consumption electric arc furnace. The phase transition point of Gr.38 titanium alloy was measured to be 970±5°C using elevated temperature metallography. Φ440mm ingot was forged for 8 fire times and finally hot rolled to Φ20mm bar in rolled condition. The annealing system is furnace cooling, water cooling and air cooling after holding at 830, 930, 950 and 1000℃ for 1h respectively.
A 75mm long test bar was cut from the finished bar as the mechanical property specimen and a 20mm long test bar was cut as the metallographic specimen to complete the test content after annealing treatment. The test content is mainly to test the microstructure, room temperature tensile properties and tensile fracture morphology under different annealing regimes. The test results showed that:
(1) After annealing at 930~950℃ with 1h insulation and then air cooling (or water cooling), Gr.38 alloy can obtain high strength and good plasticity, and the comprehensive mechanical properties are good.
(2) Gr.38 alloy with 830 ℃ heat preservation 1h after air-cooled annealing, yield strength is low, is conducive to the subsequent processing of materials
(3) Gr.38 alloy material room temperature tensile fracture morphology are honeycomb toughness fracture characteristics, 1000 ℃ heat preservation 1h after annealing, its fracture on the toughness of the nest is relatively small and shallow, it is relatively poor plasticity.