Abstract: Aluminum alloys are the main structural materials used in launch vehicles. The rocket tank dome is subjected to complex stresses during service, which is known as the "crown" in the manufacturing of rocket structures. Wrinkling and splitting occur simultaneously in the integrated forming of aluminum alloy ultra-thin domes, which is an international problem that has not been substantively solved. Therefore, the integral tank dome with a diameter of 2250 mm was selected as the research subject, and its evolution of stress and strain in the integrated forming process was analyzed through simulation. To solve the challenges of traditional forming technologies such as splitting, wrinkling, and difficulty in controlling microstructure and properties, the principle of cryogenic forming for the aluminum alloy ultra-thin dome was proposed based on the “dual enhancement effect” of W-temper aluminum alloys, which refers to the simultaneous enhancement of ductility and strain hardening ability at cryogenic temperatures. The stress distribution is controlled through cryogenic temperature gradient cooling, reducing the splitting risk in unsupported region and improving densities of the dislocation and substructures, leading to the better deformation uniformity and performance distribution. The world’s first ultra-low temperature forming press was developed with 22 MN in drawing force, 3 m in platform dimension, and -190 ℃ in lowest temperature. This equipment has the full-scale research and forming capabilities for the components up to 2 m in diameter. The research on cryogenic forming process was conducted for the integral dome of the new generation launch vehicle. An integral dome with a diameter of 2250 mm was directly formed successfully through a thin sheet with an initial wall thickness of 4.0 mm. The thickness-to-diameter ratio was further improved to 0.14%. And its thickness distribution was very uniform, with an average deviation rate of only 6.5%. The mechanical properties of cryogenic forming integral ultra-thin dome could achieve the strengthening level of 2219 aluminum alloy in T8-temper. The high dimensional consistency of cryogenic formed domes could be also obtained and the ellipticity could be less than 0.05%. The integral ultra-thin dome formed through cryogenic forming can be used directly without post-forming machining, which was the first time to achieve the net-shape forming of the launch vehicle dome. Cryogenic forming offers a new approach for the integrated forming of large-size ultra-thin components made from aluminum alloys.

Key words: aluminum alloy, integral ultra-thin dome, dual enhancement effect, cryogenic forming, net-shape forming