Effect of laser/ultrasonic shock peening on the microstructure and mechanical properties of nickel-based superalloys prepared by Powder Bed Fusion Laser Beam (PBF-LB)

Powder Bed Fusion Laser Beam (PBF-LB) is an attractive additive manufacturing technique for fabricating K447A superalloys components. Unfortunately, these as-built parts generally exhibit unsatisfied mechanical properties because they are highly susceptible to internal defects. Herein, we propose different post-treatments, including laser shock peening (LSP), ultrasonic shock peening (USP), laser after ultrasonic shock peening (ULSP), and ultrasonic after laser shock peening (LUSP) for tuning the microstructures and performances of a PBF-LB fabricated K447A superalloys. The microstructural response, residual stress and microhardness redistribution, as well as tensile properties of the K447A superalloys treated by different post-treatments are systematically studied. Results indicate that a gradient heterogeneous structure, which is primarily characterized with gradient variations in grain size, precipitation phase size and dislocation density, is formed on the K447A specimen surface under LUSP treatment. The surface residual stress is completely transformed into compressive state with a maximum amplitude value of-185 MPa, and the peak value of microhardness is improved to 568 HV. The various strengthening mechanisms of gradient heterogeneous structures, as well as dislocation multiplication, grain refinement and precipitated phase strengthening, are responsible for achieving strength-ductility synergy. This work provides promising route and valuable theoretical guidance for optimizing mechanical properties of additively manufactured metastable high gamma ' phase fraction superalloys via tuning microstructures.