Cracking suppression in additive manufacturing of hard-to-weld nickel-based superalloy through layer-wise ultrasonic impact peening

Fusion-based additive manufacturing (AM) techniques that use high energy beams, such as laser and electron beam, to layer-wise fuse and form materials, are quickly becoming mainstream in the fabrication of performance-critical metal parts. However, the vast majority of alloys currently used in industry are not compatible with fusion-based AM. One major reason is the occurrence of high thermal stress and solidification cracking resulting from the far-from-equilibrium thermal history. In this research, a novel hybrid AM process that combines directed energy deposition (DED) and layer-wise ultrasonic impact peening (UIP) is used to mitigate residual stress formation and to suppress the occurrence of solidification cracking in a hard-to-weld Inconel 100 superalloy. A series of materials characterization techniques, including 3D topography, optical microscopy, electron backscattered diffraction, and X-ray residual stress analysis, is carried out to investigate the effect of layer-wise UIP on the additively manufactured products. Three levels of UIP force, i.e., 25 N, 50 N, and 75 N, are investigated. The results indicate that there is an inverse relationship between the crack density and applied UIP force. At the highest level of UIP force, the crack density reduces to near-zero with significantly improved surface quality. To successfully suppress detrimental residual stress, the depth of the peening-affected-zone (PAZ) must be larger than the heat-affect-zone (HAZ) of laser melting.