Effect of quasi-isothermal hot forging and isothermal roll forming on the microstructure of a wrought nickel-based superalloy

Abstract

In the present work, the novel processing route including isothermal roll forming was applied for obtaining a disc workpiece intended for subsequent manufacturing of a disc, which could be used in gas turbine engines. A wrought nickel-based superalloy Ni-10.3(Al,Ti,Nb)-31.6(Co,Cr,Mo,W) (wt%) (Russian abbreviation ЭП962) was taken as a starting material. In the as-received condition, the superalloy had the microstructure with a γ grain size of about 50 µm. From the as-received superalloy the billet with a size of Ø200 × 315 mm was cut and put into a can made of a stainless steel. The canned billet was subjected to three-stage subsolvus forging under quasi-isothermal conditions with intermediate annealing. As a result, the pancake with a size of about Ø410 × 75 mm was produced. Microstructural examination showed that predominantly recrystallized and fine-grained structure (d<10 µm) was formed in the central part of the pancake, while coarser microstructure was preserved near the flat surfaces of the pancake. Before roll forming, the central part of the pancake with a diameter of 70 mm was cut out and used to make small cylindrical samples, which were compressed at 1100°C with switching of the strain rate to determine the strain rate sensitivity coefficient m and thus the superplastic conditions. The obtained workpiece was then subjected to isothermal subsolvus roll forming under near superplastic conditions at T = 1100 ± 15°C and έ= 5 × 10⁻⁴-5 × 10⁻³ s⁻¹. Complex-shaped disc workpiece with a size of about Ø510 × (35−70) mm was produced. Microstructural analysis of the produced disc workpiece showed that recrystallized and gradient microstructure was formed after roll forming. The average γ grain sizes changed from d≈ 9.7 µm in the bore zone to d≈ 8.8 µm in the web zone and to d≈ 20 µm in the rim zone of the disc workpiece. The advantages of using this processing to manufacture near-net shaped discs for gas turbine engines are discussed.