Directional recrystallization of an additively manufactured Ni-base superalloy

Abstract

Metal additive manufacturing processes can create intricate components that are difficult to form with conventional processing methods; however, the as-printed materials often have fine grain structures that result in poor high-temperature creep properties, especially compared to directionally solidified materials. Here, we address this limitation in an exemplary additively manufactured Ni-base superalloy, AM IN738LC, by converting the fine as-printed grain structure to a coarse columnar one via directional recrystallization. The directional recrystallization behaviors of AM IN738LC were characterized through a parameter study in which the peak temperature and draw rate were each independently varied. Recrystallization began when the peak temperature was higher than the γ′ solvus of 1183 °C. Varying the draw rate from 1 to 100 mm/hr while maintaining a fixed peak temperature of 1235 °C and a thermal gradient of order 10 5 °C/m ahead of the hot zone showed that a draw rate of 2.5 mm/hr maximized the grain size, giving a mean longitudinal grain size of 650 µm. Specimens processed under these optimal conditions also inherited the 〈100〉 fiber texture of the as-printed material. Close inspection of a quenched specimen revealed Zener pinning of the longitudinal grain boundaries by MC carbides and a discrete primary recrystallization front whose position followed the γ′ solvus isotherm. The present results demonstrate for the first time how directional recrystallization of additively manufactured Ni-base superalloys can achieve large columnar grains, manipulate crystallographic texture to minimize thermal stresses expected in service, and functionally grade the grain structure to selectively enhance fatigue or creep performance.