Achieving high strength 316L stainless steel by laser directed energy deposition-ultrasonic rolling hybrid process

Laser directed energy deposition (LDED) has attracted as a progressive additive manufacturing technique in fabricating the stainless steel components. The rapid melting and solidification velocity under LDED process induces the appearance of pores and the columnar dendritic, which degrades the strength of alloys. Here, a self-developed LDED-UR (ultrasonic rolling) synchronous manufacturing equipment is developed to fabricate the 316L stainless steel, and the characterization of microstructure and mechanical properties are studied. Results show that the size and fraction of pores are significantly reduced by LDED-UR processing, and fine equiaxed grains are obtained under the combined effects of ultrasonic vibration and roller pressure. With the severe plastic deformation, the average grain size of LDED-UR sample is decreased from 76.10 μm to 26.23 μm (LDED sample), and high angle grain boundaries transforms to low angle grain boundaries. Meanwhile, the dislocation density in LDED-UR sample is increased compared to the LDED sample. Owing to the grain refinement and dislocation strengthening, the average microhardness, yield strength, ultimate tensile strength and elongation of the LDED-UR sample reach 268.18 ± 13.90 HV0.2, 442 MPa, 771.2 MPa, and 49.8 %, respectively, which are larger than those of LDED sample (214.13 ± 13.76 HV0.2, 396 MPa, 682 MPa, and 45.5 %). This implies that the superior mechanical properties are obtained by employing in-situ ultrasonic rolling under LDED process. The insight of the influence of microstructure on mechanical properties extends a new approach for enhancing the mechanical properties of LDED alloys.