Introducing a Single Crystal Structure in Polycrystalline Inconel 718 Superalloy Via Laser Directed Energy Deposition

The grain boundaries have always been the accomplice in deteriorating high-temperature mechanical properties of alloys because the bonding strength of grain boundaries is lower than that of internal grains at high temperature. Reducing or even eliminating the transverse grain boundary perpendicular to the stress axis is the best choice to boost the high-temperature mechanical properties of Inconel 718 alloy without changing the alloy composition. Herein, Inconel 718 single-wall specimen (height: 20 mm) with a single crystal (SX) structure was successfully prepared by laser directed energy deposition (LDED) for the first time. Except for few stray grains (SGs) on the top region, the rest are all SX structure. Subsequently, the stress rupture life at 650 °C of sample subjected to direct aging treatment was tested and up to 180 h, far exceeding the measured value of the forging (110 h). This indicates that imparting SX structure to Inconel 718 polycrystalline alloy is beneficial to improve high-temperature mechanical properties. At last, the formation of SG during LDED process were elucidated combined numerical simulation and solidification theory, revealing that it is primarily affected by the angle ψmax between normal direction to the side edge of melt pool and [001] direction. The melt pool with shallower and flatter shape could facilitate the formation of SX. Moreover, the effect of liquid flow on the dendrite deflection was also investigated and found that employing reciprocating scanning strategy between layers could inhibit this affection.