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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 Y2O3content, 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 Y2O3content was 2 wt%. The average microhardness showed an increasing and then decreasing trend with the increase of Y2O3content. When the content of Y2O3was 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 Y2O3content 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.

Laser cladding repair technology is valued for its ability to repair of damaged parts with high performance. However, there are so many parameters affect the repair quality that it takes a lot of work to determine the optimal parameters. In this article, a variety of parameters were combined as dimensionless heat input (Q*), and the effect of Q* on the microstructure and mechanical properties of 300M steel repaired samples were studied. Results showed that with the increase of Q*, microstructure of heat affected zone transformed from tempered martensite to tempered martensite and low bainite. The microstructure of the repaired zone transformed from untempered martensite to untempered martensite and a small amount of retained austenite. With increasing of Q*, residence time above the austenitizing temperature was extended, and more uniform microstructure were obtained in the repaired zone. What’s more, at Q* = 5.1, the repair efficiency increases twice than Q* = 1. The tensile strength of the repaired samples reached 1017.6 MPa and the elongation reached 6.9 %, which were far lower than that of forged samples. The heat treatment was carried out on the repaired parts, which eliminated the microstructure difference due to different Q*. Compared with the repaired samples without heat treatment, the tensile strength increased by 75 % to 1783.8 MPa and the elongation increased by 86 % to 10.4 %.