Directional recrystallization of an additively manufactured Ni-base superalloy

Metal additive manufacturing processes can create intricate components that are difficult to form with con-ventional 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 gamma ' solvus of 1183 degrees C. Varying the draw rate from 1 to 100 mm/hr while maintaining a fixed peak temperature of 1235 degrees C and a thermal gradient of order 105 degrees 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 mu m. Specimens pro-cessed 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 gamma ' 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.

Automated summary

Metal additive manufacturing processes often result in fine grain structures, which affect high-temperature creep properties. This study demonstrates how directional recrystallization can create large columnar grains, manipulate crystallographic texture to minimize thermal stresses, and selectively enhance fatigue or creep performance of additively manufactured Ni-base superalloys.