Microstructural characterization and properties of selective laser melted maraging steel with different build directions

A nearly fully dense grade 300 maraging steel was fabricated by selective laser melting (SLM) additive manufacturing with optimum laser parameters. Different heat treatments were elaborately applied based on the detected phase transformation temperatures. Microstructures, precipitation characteristics, residual stress and properties of the as-fabricated and heat-treated SLM parts were systematically characterized and analyzed. The observed submicron grain size (0.31 μm on average) suggests an extremely high cooling rate up to 107 K/s. Massive needle-shaped nanoprecipitates Ni3X (X = Ti, Al, Mo) are clearly present in the martensitic matrix, which accounts for the age hardening. The interfacial relations between the precipitate and matrix are revealed by electron microscopy and illustrated in detail. Strengthening mechanism is explained by Orowan bowing mechanism and coherency strain hardening. Building orientation-based mechanical anisotropy, caused by ‘layer-wise effect’, is also investigated in as-fabricated and heat-treated specimens. The findings reveal that heat treatments not only induce strengthening, but also significantly relieve the residual stress and slightly eliminate the mechanical anisotropy. In addition, comprehensive performance in terms of Charpy impact test, tribological performance, as well as corrosion resistance of the as-fabricated and heat-treated parts are characterized and systematically investigated in comparison with traditionally produced maraging steels as guidance for industry applications

Selective laser melting of high-performance pure tungsten: parameter design, densification behavior and mechanical properties

Selective laser melting (SLM) additive manufacturing of pure tungsten encounters nearly all intractable difficulties of SLM metals fields due to its intrinsic properties. The key factors, including powder characteristics, layer thickness, and laser parameters of SLM high density tungsten are elucidated and discussed in detail. The main parameters were designed from theoretical calculations prior to the SLM process and experimentally optimized. Pure tungsten products with a density of 19.01 g/cm3 (98.50% theoretical density) were produced using SLM with the optimized processing parameters. A high density microstructure is formed without significant balling or macrocracks. The formation mechanisms for pores and the densification behaviors are systematically elucidated. Electron backscattered diffraction analysis confirms that the columnar grains stretch across several layers and parallel to the maximum temperature gradient, which can ensure good bonding between the layers. The mechanical properties of the SLM-produced tungsten are comparable to that produced by the conventional fabrication methods, with hardness values exceeding 460 HV0.05 and an ultimate compressive strength of about 1 GPa. This finding offers new potential applications of refractory metals in additive manufacturing

Selective laser melting of tungsten carbide reinforced maraging steel composite

In this work, tungsten carbide (WC) reinforced maraging steel matrix composites were in-situ manufactured by selective laser melting (SLM) from powder mixture. The SLM processed samples presented high relative density (over 99%) with a homogenous distribution of WC. The as-fabricated surface quality of SLM processed samples was improved significantly by the addition of WC. Focused ion beam and transmission electron microscopy were employed to characterize the interfacial properties between tungsten carbide and steel matrix. The elemental analysis indicates that metallurgical bonding appears at interfacial region due to the diffusion. Tensile behavior of SLM processed maraging steel was different from their composite with several WC contents

Acoustic emission monitoring of rockbursts during TBM-excavated headrace tunneling at Jinping II hydropower station

AbstractTo better understand the mechanical properties of marble at Jinping II hydropower station, this paper examines the changes of brittle rocks in excavation damaged zones (EDZs) before and after excavation of tunnel with the tunnel boring machine (TBM). The paper attempts to employ the acoustic emission (AE) to study the AE characteristics and distribution of rockburst before and after TBM-excavated tunnel. It is known that the headrace tunnel #2, excavated by the drill-and-blast (D&B) method, is ahead of the headrace tunnel #3 that is excavated by TBM method. The experimental sub-tunnel #2–1, about 2000m in depth and 13m in diameter, between the two tunnels is scheduled. In the experimental sub-tunnel #2–1, a large number of experimental boreholes are arranged, and AE sensors are installed within 10m apart from the wall of the headrace tunnel #3. By tracking the microseismic signals in rocks, the location, frequency, quantity, scope and intensity of the microseismic signals are basically identified. It is observed that the AE signals mainly occur within 5m around the rock wall, basically lasting for one day before tunnel excavation and a week after excavation. Monitoring results indicate that the rockburst signals are closely related to rock stress adjustment. The rock structure has a rapid self-adjustment capacity before and after a certain period of time during tunneling. The variations of rock stresses would last for a long time before reaching a final steady state. Based on this, the site-specific support parameters for the deep tunnels can be accordingly optimized

Multi-objective optimisation of process parameters for laser-based directed energy deposition of a mixture of H13 and M2 steel powders on 4Cr5Mo2SiV1 steel

In present paper, a universal method for multi-objective parameter optimisation of additive manufacturing processes was proposed and successfully applied to laser-based directed energy deposition (DED) experiments with mixtures of H13 and M2 steel powders that were deposited on 4Cr5Mo2SiV1 hot work die steel. The DED experiments were designed and completed based on the response surface method with 13 groups of laser parameters. The microstructure of the deposited alloy steel was observed and its mechanical properties were tested. The deposited steel alloy achieved an ultimate tensile strength (UTS) of 1821 ± 30 MPa with a reasonable elongation of approximately 4.5%, and the bond strength specimens achieved a bond toughness of ∼10.66% with a moderate UTS (1329 ± 28 MPa). A multi-objective optimisation method was proposed based on response surfaces which were established according to microstructural characteristics and mechanical properties data. It provided a basis for achieving high strength or high toughness DED-fabricated steel alloys.</p

Ductilization of selective laser melted Ti6Al4V alloy by friction stir processing

The low ductility of Ti6Al4V alloy manufactured by Selective Laser Melting (SLM) adversely impacts the component performance in practical applications. A local post-treatment by Friction Stir Processing (FSP) significantly reduces the porosity and homogenizes the microstructure. This results in an increase in fracture strain from 0.21 after SLM to 0.65 following the FSP post-treatment. The porosity reduction was evidenced by 3D X-ray micro-computed tomography. A fully transformed β microstructure is formed after FSP. This microstructure involves α plates, α colonies, as well as equiaxed dynamically recrystallized α phases inside equiaxed prior-β grains. The deformed microstructure was observed during in-situ tensile test, using scanning electron microscopy, with the aim to unravel the damage mechanisms. In addition to the beneficial effect of initial porosity reduction, the transformed microstructure after FSP bears more damage before failure than the typical α’ martensite laths in the as-built SLM samples.status: Published onlin

Ductilization of selective laser melted Ti6Al4V alloy by friction stir processing

The low ductility of Ti6Al4V alloy manufactured by Selective Laser Melting (SLM) adversely impacts the component performance in practical applications. A local post-treatment by Friction Stir Processing (FSP) significantly reduces the porosity and homogenizes the microstructure. This results in an increase in fracture strain from 0.21 after SLM to 0.65 following the FSP post-treatment. The porosity reduction was evidenced by 3D X-ray micro-computed tomography. A fully transformed β microstructure is formed after FSP. This microstructure involves α plates, α colonies, as well as equiaxed dynamically recrystallized α phases inside equiaxed prior-β grains. The deformed microstructure was observed during in-situ tensile test, using scanning electron microscopy, with the aim to unravel the damage mechanisms. In addition to the beneficial effect of initial porosity reduction, the transformed microstructure after FSP bears more damage before failure than the typical α’ martensite laths in the as-built SLM samples

Mechanistic Insights into the Decoupled Desaturation and Epoxidation Catalyzed by Dioxygenase AsqJ Involved in the Biosynthesis of Quinolone Alkaloids

AsqJ from Aspergillus nidulans is a nonheme Fe^II/α-ketoglutarate-dependent dioxygenase that catalyzes the conversion of benzodiazepinedione into 4′-methoxyviridicatin, which is a key step in the biosynthesis of quinolone alkaloids. A series of recent experiments have demonstrated that AsqJ is able to perform the decoupled desaturation and epoxidation reactions. Herein, on the basis of the published crystal structures, combined quantum mechanics and molecular mechanics (QM/MM) calculations have been performed to explore both the desaturation and epoxidation processes. Our calculations reveal that the quintet state of the Fe^IV–O complex is the ground state, and the catalytic reaction occurs on the quintet-state surface. The Fe^IV–oxo species should first undergo an isomerization to initiate the reactions. In the desaturation process, the abstraction of the first hydrogen atom is suggested to follow the σ-channel mechanism. This step is calculated to be rate-limiting with an energy barrier of 19.3 kcal/mol. The abstraction of the second hydrogen atom is found to be quite easy. After the desaturation process, the regenerated Fe^IV–oxo species first attacks the CC bond of the desaturated intermediate to form a carbon-based radical intermediate, corresponding to an energy barrier of 18.1 kcal/mol, then the radical intermediate completes the ring closure with a barrier of 3.9 kcal/mol. Besides, the calculations using the substrate analogous that lacks the N4-methyl reveal that the H atom abstraction by Fe^IV–oxo is still accessible, which suggests that the absence of N4-methyl does not affect the desaturation process itself but may influence the other processes that occur prior to the desaturation