The influence of cooling rate on the β quenching microstructure of TC2 titanium alloy sheets

TC2 titanium alloy is a low-strength, high-plasticity near-α type titanium alloy, containing 4% α-stabilizing element Al and 1.5% β-stabilizing element Mn. The alloy consists of α phase and a small amount of β phase at room temperature equilibrium state, with the content of β phase generally ranging from 2% to 4%. This alloy has good process plasticity and thermal stability, and has been widely used in the aerospace industry.

When TC2 titanium alloy sheets are used as aircraft parts, cold forming processing is adopted, so there are higher requirements for their microstructure and properties to prevent uneven deformation during cold deformation. To meet the requirements of cold forming of TC2 titanium alloy aircraft components, it is required that their microstructure be uniform and fine equiaxed structure.

β quenching is an effective method to eliminate abnormal microstructure of TC2 titanium alloy sheets. However, in production practice, when quenching the sheets with large dimensions, one end of the sheet has already entered the water, while the other end will definitely delay for a certain period of time before entering the water. To investigate the influence of this delay on the quenching microstructure of the sheets, researchers studied the method of water quenching + air cooling composite, simulating the actual quenching process in production, and then studied the influence of cooling rate on the β quenching microstructure of TC2 titanium alloy sheets.

Using a vacuum self-dissolving arc furnace, a single vacuum and secondary argon filling melting process was adopted to prepare a Φ420mm TC2 titanium alloy finished ingot, with its phase transformation point being 975 ± 10℃. The ingot was forged into a 200mm thick billet on a large-tonnage fast forging machine, and then rolled into a 11mm thick TC2 titanium alloy sheet using a 2800mm hot rolling machine. Metallographic samples were taken along the rolling direction of the sheet, and then the sheet was subjected to 760℃ annealing treatment. After annealing, metallographic samples were taken along the rolling direction of the sheet, and water quenching + air cooling composite treatment samples with a size of 25mm × 25mm × 600mm were cut along the sheet's transverse direction. Then the sheet was subjected to β quenching, and subsequent processing was carried out to obtain the finished TC2 titanium alloy sheet. The first and last ends of the sheet entering the water during β quenching were selected for metallographic samples. Water quenching + air cooling composite treatment is to heat the sample to 1000℃, hold it for a period of time, and then quickly immerse one end of the sample in water while the other end is still in the air. The microstructure of the sheet at the rolling direction during hot rolling, the microstructure of the annealed state at 760℃, the corresponding parts of the microstructure of the water quenching + air cooling composite treatment samples, and the microstructure of the sheet during β quenching with the first and last ends entering the water were observed using an Olympus metallographic microscope. The test results show:
(1) The water quenching part of the water quenching + air cooling composite samples undergoes martensitic transformation, but the process is different. With the decrease of cooling rate, the amount of intermittent grain boundary α phase precipitated increases, and the grain boundaries gradually connect. Clear microstructure features cannot be observed under an optical microscope.
(2) The air cooling part of the water quenching + air cooling composite samples is all lamellar structure. With the decrease of cooling rate, the α clusters become significantly larger, and the width of the α grains increases. The α edge trimmings gradually become wider at the grain boundaries.
(3) The sheet quenched with the delayed end entering the water is not uniform in the metallographic structure of the finished sheet, showing a layered distribution; while the sheet with the faster end entering the water during quenching has a metallographic structure close to water quenching due to a faster cooling rate, and the metallographic observation shows an uniform and fine equiaxed structure.