In-situ ultrasonic impact treatment (UIT) to improve the fatigue properties of laser powder bed fusion (LPBF) manufactured inconel 718

Additive manufacturing (AM) has gained wide popularity for manufacturing geometrically complex metal parts. However, the mechanical properties of these materials under as-built conditions, and many times, after postbuild heat treatments, have been found inferior to their conventionally manufactured counterparts. Combining Ultrasonic Impact Treatment (UIT) with AM processes has been reported recently as a promising technique to enhance quasi-static mechanical properties. In the present study, the effectiveness of UIT to enhance the low cycle fatigue properties of nickel-based superalloys such as Inconel 718 (IN718) is investigated. The results show that the application of UIT enhances low cycle fatigue (LCF) life substantially by about 110 %. The stress-strain and nanoindentation behaviors also show an overall enhancement in strength and toughness. The mechanism responsible for the dramatic increase in LCF performance and strengthening is determined as an increased resistance to dislocation gliding and fatigue crack propagation due to UIT induced lattice distortion. The fracture surface analysis of the failed specimens shows a reduction in fatigue crack propagation rate confirming the enhancement in fatigue crack resistance.