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Due to variable cross-sections and a thin-walled structure, gas turbine blades have stringent dimensional, and geometrical tolerance requirements. Single-crystal hollow blades are manufactured using the following investment casting processes: ceramic core preparation, wax injection, ceramic coating, wax removal, metal casting, and finishing. The main causes of the final casting deformation are wax pattern deformation, core deflection, and metal solidification warpage. This paper proposes a numerical simulation method to predict the deformation of the wax pattern, core deflection, and the directional solidification (DS) process of large single-crystal blades. Additionally, it investigates the displacement field and the influence of casting process parameters on dimensional accuracy. Three groups of DS process parameters were selected for experiments, and the deformation prediction was in agreement with the experimental results. The selected blade section deformation is the smallest when the pouring temperature is 1530°C and the withdrawal rate is 5 mm/min. The proposed finite element model is efficient to predict the deformation in all the investment casting processes, providing geometric guidance for the control of the dimensional accuracy of the turbine blade.

A novel method by adding inorganic heat absorption material (IHAM) into the molten steel was proposed to improve the quality of large steel ingots. The IHAM was found to melt and float up after absorbing the heat of molten steel, stirring the melt and thus accelerating the cooling process. The molten steel was also purified as the inclusions were adsorbed during the floating of IHAM. The addition of IHAM rod was conducive to increase the equiaxed grain ratio (EGR) and alleviate the central carbon segregation, while the IHAM cored wire played a more important role in reducing the inclusion index. This method holds broad appeal to solidification of large steel ingot, as it can improve the microstructure of solidified ingot.

A novel method by adding inorganic heat absorption material (IHAM) into the molten steel was proposed to improve the quality of large steel ingots. The IHAM was found to melt and float up after absorbing the heat of molten steel, stirring the melt and thus accelerating the cooling process. The molten steel was also purified as the inclusions were adsorbed during the floating of IHAM. The addition of IHAM rod was conducive to increase the equiaxed grain ratio (EGR) and alleviate the central carbon segregation, while the IHAM cored wire played a more important role in reducing the inclusion index. This method holds broad appeal to solidification of large steel ingot, as it can improve the microstructure of solidified ingot.

• Columnar NiAl dendrites were refined when a low magnetic field was applied. • CET was induced with high magnetic field during directional solidification. • Refined grains and CET were attributed to thermoelectric magnetic convection or force. • Both plasticity and tensile strength were enhanced by imposing a low magnetic field. The influence of a longitudinal static magnetic field on microstructures and mechanical properties of Ni 3 Al-based alloy during directional solidification at the growth speed of 25 µm/s and 100 µm/s has been experimentally investigated. Results reflected that the utilization of a 0.5 T magnetic field refines the NiAl dendrites at both speeds of growth. When applying a high magnetic field, the columnar-to-equiaxed transition (CET) occurred at growth speed of 25 µm/s and dendrite networks formed at growth speed of 100 µm/s. Tensile property results indicated that the refinement of dendrites enhanced both plasticity and ultimate tensile strength of Ni-Al alloy. The change of microstructures and mechanical properties should be attributed to the combined action of the thermoelectric magnetic convection (TEMC) in mushy zone together with the thermoelectric magnetic force (TEMF) acting on the solid. When applying a low magnetic field (0.5 T), the TEMF is too small to fragment the dendrites, and the refined dendrites is mainly due to the TEMC in the interdendritic regions. At a lower growth speed, the TEMF is supposed to strong enough to fragment the dendrites and induce the occurrence of CET under 2 or 4 T. When the growth speed increased to 100 µm/s, no obvious CET was observed, but a vertical secondary convection is induced by the circulation in the parallel plane, which promotes the growth of secondary and tertiary branches, leading to the formation of abnormally developed high order dendrites. The hierarchical dendritic structure was suggested to provide a channel for rapid crack propagation and thus degraded the mechanical properties. [Display omitted] [ABSTRACT FROM AUTHOR]

In this work, the microstructural evolution, micro-crack formation, and mechanical anisotropy of the selective laser melted (SLM) carbon-free Mar-M509 Co-based superalloy were systematically studied under different linear energy densities (LED). Observation shows that the SLM Mar-M509 superalloy possesses a fully dense structure, whereas some micro-cracks exist along the building direction. The electron backscatter diffraction results reveal that dominant columnar grains tend to elongate along the building direction parallel to the XZplane. Meanwhile, both a < 001 > near fiber texture and a {100} < 001 > near sheet texture are observed in different specimens. For the specimen with fiber texture, a high misorientation angle exists among different columnar grains, which aggravated the generation of micro-cracks under thermal stress. Higher LED results in higher micro-crack density in the SLM specimen due to higher thermal stress. Mar-M509 specimen fabricated under lower LED exhibits higher tensile strength due to more significant grain refinement. More prominent anisotropy of tensile performance was found in the high LED specimen, which can be attributed to the higher density of micro-cracks and crystallographic texture. Furthermore, the SLM Mar-M509 superalloy exhibits better mechanical properties than the traditional cast technique. In summary, this work can contribute to the development and the future application of SLM-fabricated Co-based superalloy.

In this paper, the diversity of complicated dendrite microstructure and its evolution behavior during solidification in different magnesium alloys under various processing conditions were illustrated using synchrotron X-ray imaging technique. A variety of dendritic morphologies and branching structures were revealed, i.e., sixfold plate-like symmetric structure in Mg–Al-based structure, 12-branch structure in Mg–Zn-based alloys and 18-branch structure in Mg–Sn- and Mg–Ca-based alloys as well as seaweed like hyper-branched structure in Mg-38wt%Zn alloy. In addition, a dendrite morphology and orientation transition with increasing addition of Zn content were also observed in Mg–Zn alloy, with dendrite growth pattern transform from anisotropy (low Zn addition) with sixfold symmetric snow-flake structure to relative isotropy (intermediate Zn addition) where seaweed morphology presented and then back to anisotropy (high Zn addition) when only 12 branches with preferred < 11 2¯1 > orientations were observed. The phase-field model representing the typical dendritic morphologies and branching structures under various conditions was also depicted and discussed. Further, the two-dimensional (2D) real-time dendrite growth dynamics in different Mg-based alloys captured using synchrotron X-ray radiography for unveiling the originate of the α-Mg dendrite was reviewed. Following this, the four-dimensional (3D + time) synchrotron X-ray tomographic in situ observation of dendritic morphology evolution indicating the formation mechanism of the diverse dendritic morphology during Mg–Sn- and Mg–Zn-based alloys was also summarized. Finally, the future study on exploring the complicated dendritic morphologies and their origination during solidification of Mg-based alloys is prospected.

Additive manufacturing (AM) is a rapid prototyping technology based on the idea of discrete accumulation which offers an advantage of economically fabricating a component with complex geometries in a rapid design-to-manufacture cycle. However, various internal defects, such as balling, cracks, residual stress and porosity, are inevitably occurred during AM due to the complexity of laser/electron beam-powder interaction, rapid melting and solidification process, and microstructure evolution. The existence of porosity defects can potentially deteriorate the mechanical properties of selective laser melting (SLM) components, such as material stiffness, hardness, tensile strength, and fatigue resistance performance. Synchrotron X-ray imaging and diffraction are important non-destructive means to elaborately characterize the internal defect characteristics and mechanical properties of AM parts. This paper presents a review on the application of synchrotron X-ray in identifying and verifying the quality and requirement of AM parts. Defects, microstructures and mechanical properties of printed components characterized by synchrotron X-ray imaging and diffraction are summarized in this review. Subsequently, this paper also elaborates on the online characterization of the evolution of the microstructure during AM using synchrotron X-ray imaging, and introduces the method for measuring AM stress by X-ray diffraction (XRD). Finally, the future application of synchrotron X-ray characterization in the AM is prospected.

[Display omitted] • Fraction of non-equilibrium eutectic decreased with the application of magnetic field. • Solute partition coefficient was increased by magnetic field during solidification. • Thermoelectric magnetic convection is supposed to homogenize the solute distribution. • The dislocation caused by thermoelectric magnetic force can help to trap solute atoms. The effect of the high magnetic field (MF) on the distribution of solute concentration during directional solidification of Al-Cu alloy under low growth speed was experimentally investigated. The amount of non-equilibrium eutectic is quantified via X-ray computed tomography (XCT) and demonstrated to reduce with the application of MF. Further, experimental results reveal that the MF alleviates the microsegregation and increases the average Cu concentration in solid solution, leading to the increases of the effective partition coefficient k e. It was also found that Cu concentration in solid solution increases continuously with the increasing intensity of MF, following the strengthening of micro-hardness. The change of k e under the MF is demonstrated to attribute to the thermoelectric magnetic convection (TEMC) in the mushy zone and the thermoelectric magnetic force (TEMF) acting on the solid. The TEMC is supposed to cause secondary convection owing to the inequality in flow velocities of circulation in different positions of dendrite stem. And the vacancies created by the proliferation and movement of dislocations induced by TEMF in the matrix is supposed to be able to capture solute atoms and thus enhance the solute concentration in the solid solution. [ABSTRACT FROM AUTHOR]

Ni3Al-based alloys are excellent candidates for the structural materials used for turbine engines due to their excellent high-temperature properties. This study aims at laser powder bed fusion and post-hot isostatic pressing (HIP) treatment of Ni3Al-based IC-221 ​M alloy with a high γ′ volume fraction. The as-built samples exhibits unavoidable solidification cracking and ductility dip cracking, and the laser parameter optimization can reduce the crack density to 1.34 ​mm/mm2. Transmission electron microscope (TEM) analysis reveals ultra-fine nanoscale γ′ phases in the as-built samples due to the high cooling rate during rapid solidification. After HIP treatment, a fully dense structure without cracking defects is achieved, which exhibits an equiaxed structure with grain size ∼120–180 ​μm and irregularly shaped γ′ precipitates ∼1–3 ​μm with a prominently high fraction of 86%. The room-temperature tensile test of as-built samples shows a high ultimate tensile strength (σUTS) of 1039.7 ​MPa and low fracture elongation of 6.4%. After HIP treatment, a significant improvement in ductility (15.7%) and a slight loss of strength (σUTSof 831.7 ​MPa) are obtained by eliminating the crack defects. Both the as-built and HIP samples exhibit retained high σUTSvalues of 589.8 ​MPa and 786.2 ​MPa, respectively, at 900 ​°C. The HIP samples exhibita slight decrease in ductility to ∼12.9%, indicating excellent high-temperature mechanical performance. Moreover, the abnormal increase in strength and decrease in ductility suggest the critical role of a high γ′ fraction in cracking formation. The intrinsic heat treatment during repeating thermal cycles can induce brittleness and trigger cracking initiation in the heat-affected zone with notable deteriorating ductility. The results indicate that the combination of LPBF and HIP can effectively reduce the crack density and enhance the mechanical properties of Ni3Al-based alloy, making it a promising material for high-temperature applications.

Using the on-the-fly machine learning force field, simulations were performed to study the atomic structure evolution of the liquid-Al/solid-TiB2interface with two different terminations, aiming to deepen the understanding of the mechanism of TiB2as nucleating particles in an aluminum alloy. We conducted simulations using MLFF for up to 100 ps, enabling us to observe the interfacial properties from a deeper and more comprehensive perspective. The nucleation potential of TiB2particles is determined by the formation of various ordered structures at the interface, which is significantly influenced by the termination of the TiB2(0001) surface. The evolution of the interface during heterogeneous nucleation processes with different terminations is described using structural information and dynamic characteristics. The Ti-terminated surface is more prone to forming quasi-solid regions compared to the B-termination. Analysis of mean square displacement and vibrational density of states indicates that the liquid layer at the Ti-terminated interface is closer in characteristics to a solid compared to the B-terminated interface. We also found that on the TiB2(0001) surface different terminations give rise to distinct ordered structures at the interfaces, which is ascribed to their different diffusion abilities.

The columnar-to-equiaxed transition (CET) or the formation of stray grains in the laser melting deposition is the least desirable for the repair of single-crystal blades. In this work, the forced water-cooling was conducted on a single-crystal René N5 substrate during the direct energy deposition (DED). The single track remelting, one-layer, two-layer, and eight-layer depositions were investigated to explore the grain growth mechanism. The solidification conditions of the DED process, including temperature field, temperature gradient, and solidification speed, were numerically analyzed by a finite element model. The single-track remelting results showed that the fraction of columnar crystal regions increases from 55.81 % in the air-cooled sample to 77.14 % in the water-cooled one. The single-track deposits of one- and two-layer have the same trend, where the proportion of columnar crystal height was higher under the forced water-cooled condition. The electron backscattered diffraction (EBSD) grain-structure maps of an eight-layer deposit show that the epitaxial growth height increases from 1 mm in the air-cooling sample to 1.5 mm in the water-cooling one. The numerical results showed that the temperature gradient in [001] direction was significantly increased by using forced water-cooling. In conclusion, the in-situ substrate cooling can become a potential method to promote epitaxial growth during DED via the influence on CET occurrence. [ABSTRACT FROM AUTHOR]

The morphological instability of solid/liquid (S/L) interface during solidification will result in different patterns of microstructure. In this study, two dimension (2D) and three dimension (3D) in-situ observation of solid/liquid interfacial morphology transition in Al-Zn alloy during directional solidification were performed via X-ray imaging. Under a condition of increasing temperature gradient (G), the interface transition from dendritic pattern to cellular pattern, and then to planar growth with perturbation was captured. The effect of solidification parameter (the ratio of temperature gradient and growth velocity (v), G / v) on morphological instabilities was investigated and the experimental results were compared to classical "constitutional supercooling" theory. The results indicate that 2D and 3D evolution process of S/L interface morphology under the same thermal condition are different. It seems that the S/L interface in 2D observation is easier to achieve planar growth than that in 3D, implying higher S/L interface stability in 2D thin plate samples. This can be explained as the restricted liquid flow under 2D solidification which is beneficial to S/L interface stability. The in-situ observation in present study can provide coherent dataset for microstructural formation investigation and related model validation during solidification. [ABSTRACT FROM AUTHOR]

Effect of high static magnetic field on the dendritic morphology and growth direction in directionally solidified Al-10 wt.%Zn alloy were studied by three-dimensional (3D) X-ray micro-computed tomography, Electron Back-scattered Diffraction (EBSD) and X-ray Diffraction (XRD). The application of high static axial magnetic field (5T) during directional solidification was found to destabilize the solid/liquid interface and cause the growth direction of dendrite deviate from thermal gradient, leading to irregular solid/liquid interfacial shape and cellular to dendritic morphology transition. The thermoelectric magnetic convection (TEMC) caused by the interaction of thermoelectric effect and magnetic field was supposed to be responsible for the transition. In addition, the EBSD and XRD results confirm that the preferred growth direction of α-Al was found to transform from the traditionally expected <100> to <110>. The dendrite orientation transition (DOT) in Al-10 wt.%Zn alloy can be attributed to the effect of applied magnetic field on the anisotropy of crystal during solidification. The result indicates the potential application of high static magnetic field in altering the morphology and preferred growth direction of dendrite during directional solidification. [ABSTRACT FROM AUTHOR]

This work focused on the microstructure and mechanical properties of nickel-based single crystal (SX) superalloy René N5 prepared by laser-directed energy deposition (L-DED) using a flat-top laser beam. A comparison was conducted with the conventional directional solidification technique. Results show that the crack-free single-crystal superalloys with a fraction of low-angle grain boundary (LAGB) higher than 98.5% were obtained by epitaxial growth under different laser powers. The flat fusion line using a flat-top laser beam can suppress the formation of stray grain and high-angle grain boundaries (LAGB) during the L-DED process. Microstructure observation shows the extremely refined dendrites in L-DED SX superalloys with primary dendrite arm spacing (PDAS) values around 20–30 µm, which is significantly lower than as-cast ones (∼350 µm). PDAS increases with the increasing laser power. Segregation of refractory elements of Re, W, and Ta increases with increasing laser power, and the SX specimens with laser power of 1100 W and 1300 W have lower elemental segregation compared with as-cast specimens. The SX specimens exhibit a uniform square γ′ phase at the dendrite core, with an average size of 79–101 nm and a volume fraction of 57–68%. Besides, the carbides with discontinuous and refined shapes show a uniform distribution in the L-DED specimens in comparison with as-cast specimens. The microhardness of as-deposited specimens increases with the increasing laser power, which is higher than that of as-cast ones. This work can provide potential guidance for the microstructure control in the laser additive manufacturing of Ni-based SX superalloys and a further improvement in high-temperature mechanical performance.

The volumetric energy density (EV) is considered as a comprehensive index to evaluate the complex parameters in laser powder bed fusion (L-PBF) of metallic parts. This work aims to investigate the influence of varying laser power and scanning velocity coupling under a specific EVvalue on the microstructure, texture, and tensile properties of L-PBF Ti-6Al-4 V (Ti64) alloy to illustrate the limitation of EV. Three sets of laser parameters under a specific EVof 58.8 J/mm3were employed to fabricate dense parts. Analyses by electron backscatter diffraction (EBSD) and X-ray diffraction (XRD) revealed that the same EVin the L-PBF process can result in distinct thickness of α′ lath, crystallographic texture and tensile behavior. The α′-lath was refined from 0.89 µm to 0.66 µm while laser power increased from 180 W to 240 W. Meanwhile, the crystallographic orientation of prior-β grains was oriented < 111 >βdirection to parallel building direction (BD) from < 001 >β//BD, which determined the orientations of α′ lath. Tensile test results exhibit that the yield strength, ultimate tensile strength and elongation to fracture all increase with the linearly increases in laser power and speed. An excellent balance between yield strength of ~1042.2 ± 37.1 MPa and total elongation of ~ 10.8 ± 1.1% was obtained after the variant optimization. The finite element analysis shows that even under a constant EVvalue, varying laser power and scanning velocity coupling will carry out distinct thermal histories, leading to the grain refinement and higher dislocation density. This work shows the limited significance of EVin the guidance of the L-PBF process with complex physics. More efforts should be made on microstructural control via the combination of specific laser parameters.

The in-situ strengthening approach has attracted increasing attention in the selective laser melting (SLM) of high-performance alloy. In the present work, the argon-nitrogen (Ar-N2) reactive atmosphere was introduced in the SLM of Ti6Al4V alloy to achieve in-situ strengthening. The microstructure evolution and mechanical performance were studied under various atmosphere conditions and laser processing parameters, respectively. The results show that the solid-solution of 871 ppm ~ 1460 ppm nitrogen was obtained in the titanium matrix through the SLM in the Ar-N2atmosphere, whereas no trace of nitride was found. Compared with the pure Ar atmosphere condition, an increasing ratio of nano-scale β phase and coarser martensites could be found after the addition of 5 vol% N2in the Ar atmosphere, and the twinning phenomenon becomes more conspicuous as well. Under the Ar-N2reactive atmosphere, the as-built Ti6Al4V alloy exhibits a significantly high yield strength of 1336 MPa and ultimate tensile strength of 1418 MPa, which are notably higher than previously reported SLM and conventionally manufactured Ti6Al4V alloys. Meanwhile, the fracture elongation of ~6% was obtained while attaining the high tensile strength. Analysis exhibits that the solid solution of the nitrogen atom is the main reason for improving strength in the nitrogen-strengthened Ti6Al4V samples. The strength of Ti6Al4V alloy was improved without excessive sacrifice of ductility, which was attributed to the multiple effects of the nitrogen solid-solution and the intrinsic heat treatment. This work adopts a practical methodology with designed low nitrogen content in the SLM process to achieve an in-situ strengthening of Ti6Al4V alloy without the formation of brittle nitrides.