Formation and Elimination Mechanism of Lack of Fusion and Cracks in Direct Laser Deposition 24CrNiMoY Alloy Steel
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Formation and Elimination Mechanism of Lack of Fusion and Cracks in Direct Laser Deposition 24CrNiMoY Alloy Steel Qian Guo, Suiyuan Chen, Mingwei Wei, Jing Liang, Changsheng Liu, and Mei Wang (Submitted November 20, 2019; in revised form August 26, 2020; Accepted: 14 September 2020) 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/mm2, 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/mm2, thermal stress causes the liquid film to rupture and inclusion cracking, and cracks appear in the sample. However, when the EAD is 72 J/mm2, 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. Keywords
24CrNiMoY alloy steel, cracks, direct laser deposition (DLD), energy density, formation mechanism, lack of fusion (LOF)
1. Introduction In recent years, increasing attention has been paid to laser additive manufacturing technology in industrial production, which has developed from a single variety of small-scale production to large-scale mass production. Moreover, laser additive manufactured parts have innovative shapes, complex features and lightweight construction (Ref 1-3). Currently, laser additive manufacturing technology is widely used in metallic materials such as Co-Cr-W alloy, pure Ti, AlSi10Mg and stainless steel (Ref 4-7). Laser additive manufacturing technology is divided into selective laser melting (SLM) (Ref 8, 9) and direct laser deposition (DLD), and DLD process is realized by simultaneously transporting metal powder and focusing laser energy (Ref 10-12). Since DLD process is a multi-physics coupling process, there are various unstable factors in the forming process. Qian Guo, Suiyuan Chen, Mingwei Wei, Jing Liang, and Changsheng Liu, Key Laboratory for Anisotropy and Texture of Materials, Ministry of Education, Key Laboratory for Laser Application Technology and Equipment of Liaoning Pro
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