Effect of Y2O3 on microstructure and mechanical properties of 34CrNiMo6 steel fabricated by laser-directed energy deposition

Due to the extremely unbalanced temperature gradient at different heights along the deposition direction during additive manufacturing, the microstructure of different areas of the formed components is quite different, which limits the part's service life to a large extent. In this paper, microstructure of the laser-directed energy deposition (LDED) 34CrNiMo6 steel is homogenized by adding Y2O3, and excellent mechanical properties are obtained. The results showed that the homogenization of microstructure at different heights of the as-deposited samples was realized due to addition of Y2O3. The microstructure of the as-deposited samples at different heights contains lath martensite and lower bainite, only retained austenite is present at the top. With the increase of Y2O3 content, the size of the austenite grain of the as-deposited LDED 34CrNiMo6 steel showed a tendency of decreasing and then increasing. The austenite grain size and martensite domain size were minimized when the Y2O3 content was 2 wt%. The average microhardness showed an increasing and then decreasing trend with the increase of Y2O3 content. When the content of Y2O3 was 2 wt%, it had the highest average microhardness, which was 597 HV and 640 HV, respectively. The elongation and percentage reduction of area showed a trend of decreasing and then increasing, and tensile strength showed a trend of increasing and then decreasing. When the Y2O3 content was 2 wt%, reached the maximum tensile strength of 1501 MPa, an increase of about 56 %, and its elongation and percentage reduction of area was the lowest in the rare-earth-containing samples, and the fracture mechanism is quasi-dissociative fracture.