Effect and mechanism of inter-layer ultrasonic impact strengthening on the anisotropy of low carbon steel components fabricated by wire and arc additive manufacturing

The problem of anisotropy in the microstructure and mechanical property of the fabrications limits the application and development of wire and arc additive manufacturing (WAAM). By introducing inter-layer ultrasonic impact (UI) strengthening in the low carbon steel WAAM process, the anisotropy can be effectively improved. After being treated by UI, the typical columnar microstructure with obvious direction is transformed into a uniform and fine equiaxed microstructure. Digital image correlation results show that the stress concentration in the tensile process is significantly improved. The anisotropy of tensile strength and yield strength is reduced from 4.2% to 1.6% and 10.1%-2.3%, respectively. Scanning electron microscope (SEM) and electron backscattering diffraction (EBSD) are applied to investigate this phenomenon. The results indicate that UI can break the constraint for dislocations to move, promoting dislocation merging and annihilating. And then, numerous substructures are formed, partially recrystallized by the subsequently deposited layers' thermal effect. The transformation can block the columnar microstructure growth and divide the columnar microstructure into the cellular or equiaxed microstructure. In addition, the results show that inter-layer ultrasonic impact strengthening can result in local misorientation reduction, grain refinement (average grain size reduction from 2.18 mu m to 1.65 mu m), and texture orientation strength weakening.