Formation and Elimination Mechanism of Lack of Fusion and Cracks in Direct Laser Deposition 24CrNiMoY Alloy Steel

The defects of lack of fusion (LOF) and cracks produced in the process of direct laser deposition 24CrNiMoY alloy steel have a great influence on the mechanical properties of samples. The effects of laser energy area density (EAD) on the distribution and formation mechanism of the LOF and cracks are studied by TEM, EBSD, SEM, OM, laser confocal microscopy (LCM) and macroscopic stereo microscope (MSM). The results show that different EADs have an important influence on the formation and elimination of defects in direct laser deposition 24CrNiMoY alloy steel sample. When the EAD is 67 J/mm(2), the wettability is adverse, the molten pool parameters do not satisfy the complete fusion formula, and the LOF defects occur in the samples. However, as the number of deposition layers increases, the proportion of LOF defects decrease. When the EAD is increased to 78 J/mm(2), thermal stress causes the liquid film to rupture and inclusion cracking, and cracks appear in the sample. However, when the EAD is 72 J/mm(2), due to good liquid flow in the molten pool, the solid-liquid phase wettability is increased and elemental segregation is weakened. Meanwhile, the thermal stress in the sample is moderate. Therefore, the sample without LOF defects and cracks is prepared, and the sample has the best microhardness (386 HV) and tensile properties (ultimate tensile strength is 788 MPa; elongation is 9.1%), which lays a foundation for the preparation of defect-free 24CrNiMoY alloy steel samples by direct laser deposition.