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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.

To more accurately analyze the atomization process of vacuum induction melting gas atomization (VIGA) technology at high gas pressure, a new method was proposed based on the combination of VOF and DPM theories. Numerical simulations and powder preparation tests were used to verify the applicability of this method. The results indicated that the large eddy simulation model could accurately visualize the large droplet separation process from the liquid column in the primary atomization. Additionally, the TAB model was used to predict the particle size distribution in the secondary atomization. The simulated results were consistent with the powder preparation test. 24CrNiMoY alloy steel powder prepared at 9.0 MPa had the highest flowability and apparent density. The powder is suitable for selective laser melting technology. This research is of guiding significance and serves as a methodological reference for understanding the atomization process of VIGA technology.

24CrNiMoY alloy steel samples were fabricated by selective laser melting (SLM), and a phase transformation model was established to study the alloy steel microstructural evolution. Meanwhile, microhardness and tensile properties of 24CrNiMoY alloy steel prepared by different laser energy densities (Ev) were investigated. Results indicate that the microstructural evolution of 24CrNiMoY alloy steel is consistent with the phase transformation model. The main microstructure changed from martensite to bainite with the increase in thermal cycle numbers. In addition, a suitable Ev plays an important role in refining the bainite structure and improving the alloy steel properties. When the Ev decreases from 210 to 140 J/mm3, the bainite lath width reduces from 1.7 to 0.6 μm. Simultaneously, the relative density, tensile strength and microhardness of the fabricated samples increase first and decrease later. 24CrNiMoY alloy steel sample prepared by 160 J/mm3 has fine mechanical properties: The tensile strength is 850 MPa and microhardness is 360 HV0.2.

24CrNiMoY alloy steel used for brake disc was prepared by high power selective laser melting (HP-SLM) in order to increase the productivity. In this work, the microstructure and mechanical properties of as-fabricated part were characterized. Effects of the location of as-fabricated 24CrNiMoY part (top, middle and bottom) and scanning speed on microstructure evolution were studied. The results of this work indicated that the as-fabricated part microstructure mainly consisted of granular bainite and lath bainite. Demonstrating that bainite was obtained by a large temperature gradient and sufficient holding time by experiment. With increasing scanning speed, the size d B of the bainitic ferrite lath beam spacing and the size ∅ B of the mixed structure of polygonal ferrite and quasi-polygonal ferrite were gradually refined and larger, respectively. The HP-SLM-produced parts were dominated by high-angle (>15°) grain boundaries (HAGBs) and the contents of HAGBs decreased at first while increased (62.3%, 63.5%, 50.1%) when scanning speed varied from 5 to 9mm/s. Supersaturated solid solution, dislocation, and subgrain cell structure, which results in a high microhardness and fine strength.

24CrNiMo alloy steel samples for preparing the brake disc of high-speed train were fabricated by direct laser deposition (DLD) with different laser energy area density (EAD). The effect of various energy area densities on the distribution, size and mechanism of pores, slag inclusions and cracks in the DLD 24CrNiMo alloy deposition layer were investigated. The results show that the increase of the EAD leads to the increase of the dilution index and area of the molten pool. The density of multilayer samples reached highest (99.25%) when the EAD was 100 J/mm2. The morphology of pores had irregular deformation when the EAD was low while had smooth sphere morphology when the EAD was high. The cracks ratio increased with the increase of the EAD while the content of inclusion defects decreased. A behavioural process for the re-melting of pore defects at different energy densities was discussed, and the formation of inclusion defects with low EAD was studied to illuminate the EAD effects on defects. The samples deposited at the 100 J/mm2 have no obvious defects and relatively high density, this EAD can be considered as a reference to prepare 24CrNiMo samples with low defects by DLD.

Nickel-based K417G superalloy powder is the main material used in laser additive manufacturing (LAM) of aerospace engine turbine blades. In this study, K417G powder was prepared by the vacuum induc...

24CrNiMo alloy steel used for brake disc was prepared by selective laser melting technology with the processing parameters of high power input and thicker powder layer. The Microstructure and mechanical properties of as-fabricated part were characterized. Effects of processing parameters on the production efficiency, thermal history and microstructure evolution mechanism were studied. The results of this work indicate that the as-fabricated steel microstructure mainly consist of granular bainite and meta bainite, which results in a high microhardness and fine strength. High laser power input leads to a high thermal accumulation level in a SLM process and the microstructure mainly transforms into bainite. In addition, compared to the commonly used parameters, the overall preparation efficiency in this study is enhanced by 82.9%. These presented results provide an insight on how to improve SLM efficiency, and preparation of high-performance 24CrNiMo alloy steel by SLM technology shows high potential for manufacturing of the brake disc core part.

Effect of different TA15 powder additions on high temperature oxidation kinetics of pre-laid laser melting 45CrNiMoY alloy steel was studied. Thermogravimetric method was used to determine the kinetics curve of constant temperature oxidation of alloy steel sample at 700℃. The results show that bainite and acicular ferrite are the main microstructures of alloy steel after adding TA15 powder. With the increase of TA15 powder, the microstructures showed a distinct trend of refinement. The oxidation kinetics curves of alloy steels follow the linear law. The alloy steels with 1.0 wt.% TA15 powder have the best resistance to high temperature oxidation, and the oxidation rate constant is 1.48×10-4 mg2·cm-4·h-1. Particle reinforcement phase formed in alloy steel can provide pinning effect for oxide film, while micro-cracks added in excess TA15 alloy steel sample are not conducive to the improvement of high temperature oxidation resistance.

The precise laser displacement sensor monitoring system during direct laser deposition process was established. The two-dimensional and three-dimensional topography images were acquired through data analysis and point cloud reconstruction method. The results of monitoring system show that with the increasing of scanning speed from 2 mm/s to 6 mm/s, the width of single track deposition kept stable due to the laser energy distribution, and the height of deposition decreased from 1.59 mm to 0.33 mm. The relationship between scanning speed and deposited 24CrNiMo alloy topography was established through data fitting. Deposition surface convex and concave defects can be distinguished through the feature of three dimensional images, uneven surface mainly existed in the deposition sides, which was represented by blank zone of acquired images. With the scanning speed increasing, the surface quality of deposited samples improved while the depositing efficiency decreased. The formation of surface defects is related to the solidification speed of molten pool, the phase structure of single track deposition was martensite owing to the high solidification speed, and the martensite has strain along the block boundary. The solidification rate increased with scanning speed increased, which led to the less time in forming surface defects.

Based on the principle of in-situ precipitation strengthening, TA15 powder (Ti-6.5Al-1Mo-1V-1.5Zr) was used as an additive to improve the mechanical properties of 45CrNiMoY alloy steel by pre-laid laser cladding (PLLC) technology. Effects of the content of TA15 powders (0–1.5 wt%) on the microstructure, precipitation mechanism and mechanical property of PLLC 45CrNiMoY alloy steel samples were investigated. Results indicate that TA15 alloy has a significant effect on the microstructure of 45CrNiMoY alloy steel through metallurgical reaction in the laser molten pool. With the increase of TA15 powder content, the granular bainite (GB) microstructure were remarkably refined, the mount of the acicular ferrite (AF) was increased, and in-situ precipitates such as Ti-Y2O3-Zr (TYZ) composite oxide, TiC and ZrC carbide were produced. The 45CrNiMoY alloy steel sample with 1.0 wt% TA15 alloy addition has the best combination of tensile strength (1158 ± 9.7 MPa) and elongation (9.8 ± 0.4%), its microhardness was improved by 25.6% compared with the sample without TA15 powder. The TYZ composite oxides did not only serve as the AF nucleation sites but also spheroidize the detrimental manganese sulfide phase leading to a good elongation of the 45CrNiMoY alloy steel, the TiC and ZrC carbides played a key role in the improvement of the microhardness and tensile strength. The proper addition of TA15 powder can effectively improve the microstructure and mechanical properties of the PLLC 45CrNiMoY alloy steel.

In order to effectively eliminate the crack initiation, reduce the amount of pores and improve the structure of large alloy steel parts with high-performance, the direct laser deposited (DLD) technique was used to prepare the 24CrNiMoY alloy steel samples by laser depositing power of 2000W and scanning speed of 6 mm/s at 100 °C,150 °C, 200 °C, 250 °C preheating temperatures, respectively. The structure and properties of the samples were studied using OM, SEM, XRD, TEM, EBSD, hardness tester and tensile tester. The results showed that with the increase of preheating temperature, the cracks in deposited alloy steel samples were removed, and the amount of pore was significantly reduced. The structure of the samples were mainly composed of granular bainite (GB) and lath bainite (LB), the content of LB was reduced from 95.7% to 3.5% with the increase of preheating temperature because GB played a segmentation role to LB. When the preheating temperature was 200 °C, the deposited sample with GB (34.7%) and LB (65.3%) had the most uniform microhardness distribution, the highest microhardness of 414 HV0.2, and the best match of strength and toughness with the tensile strength of 1051 MPa and the elongation of 7.4%. The proper preheating temperature of 200 °C can effectively eliminate the crack, decrease the pores and significantly improve the structure and properties of the DLD 24CrNiMoY alloy steel.

The unique thermal behavior during the multi-layer laser rapid manufacturing process brings about unique microstructure for the as-produced parts. In this paper, a comprehensive three-dimensional self-consistent transient temperature field model of 24CrNiMoY alloy steel by pre-laid laser cladding technology (PLLC) was established, and the reliability of model was verified by the experiment. The simulation results showed that as scanning speed increased (5, 7, 9 mm/s), the peak temperature, dimensions and liquid lifetime of the molten pool decreased, while the G × R increased and G/R decreased at the same position. With increasing layer, the peak temperature of molten pool and the temperature of the whole domain increased gradually until forming a dynamic balance. The contrast experimental results showed that with scanning speed increased, the average size of equiaxed grain increased (4.682, 5.012, 5.462 μm) and the average aspect ratio of columnar grain decreased (11.890, 11.411, 10.593). During the multi-layer process, grain growth modes were different along the deposition sections. The structure in parallel scanning direction mainly consisted of equiaxed grains, while in the overlapping direction were equiaxed and columnar grains. There was a clear columnar-to-equiaxed transition (CET) boundary that controls the change in grain types. The grain growth behavior of a multi-layered in vertical scanning direction section was different from that of a single track. All the above results illustrated that temperature field had a significant effect on the thermal behavior during PLLC process, which affected the grain evolution of 24CrNiMoY alloy steel.