Columnar Solidification Research Articles

Selective laser melting of Hastelloy-X alloy and cerium oxide reinforced Hastelloy-X composite: Microstructural examination and corrosion behavior

The study investigated the microstructures and corrosion behaviors of Hastelloy X (HX) and HX reinforced by cerium oxide particles (HX-C) produced by selective laser melting (SLM) and compared to the wrought equivalent (HX-W). The aim was to eliminate hot cracking and investigate the HX's corrosion behavior in the presence of cerium oxide particles. The HX samples showed fine cellular and columnar solidification structures with uniform-size sub-cells and severe microcracking. Incorporating cerium oxide particles (1 wt%) resulted in finer and relatively equiaxed grains without microcracking. The HX-W alloy contained a micron-sized carbide (Mo-rich M6C type) within the matrix. The HX-C sample displayed higher porosities compared to the HX sample (1.8 % against 0.3 %). Potentiodynamic polarization and electrochemical impedance spectroscopy tests were conducted in a standard NaCl solution at temperatures of 25, 50, and 70 °C to assess the corrosion behavior of Hastelloy samples. The HX-C matrix showed superior corrosion resistance compared to the HX-W and HX samples due to its finer grain structure and stable passive layer formation. Cerium oxide particles were found to be efficient in decreasing hot cracking and improving corrosion resistance.

CMT-WAAM deposition strategies for thin overhanging disc of SS308LSi and investigation of mechanical and microstructural characteristics

This work investigates the deposition strategies with microstructural evolution and mechanical properties of SS308LSi overhanging disc produced by CMT-WAAM in horizontal position. The disc was deposited on a rotating cylindrical substrate where three primary geometrical abnormalities named sagging, start and end defect and humping were observed. The causes and remedial strategies were investigated to eliminate these abnormalities. Microstructure analysis of the layers cross section revealed a defect free layer depositions and columnar solidification with epitaxial grain growth. A very slightly coarser grains structure is observed in the middle layers of the disc compared to that of bottom and top regions. The microhardness at bottom, mid and top regions are in the range of 236 ± 9.2 HV, 227 ± 11.3 HV and 232 ± 6 HV, respectively. The microhardness values are consistent with the calculated SDAS along the build direction. Tensile tests were performed at room temperature and the samples failed with an average yield strength, ultimate tensile strength, and percentage of elongation of 391.4 ± 21 MPa, 616.8 ± 19.5 MPa and 45.6 ± 0.8% respectively. This work reveals that CMT-WAAM can successfully produce overhanging disc components of high-quality without using an industrial robot at a relatively low cost.

The Effects of the Variable-Pressure Rolling of a Wire Arc Additively Manufactured Inconel625-SS308L Bimetallic Structure

The superior deposition rates of Wire Arc Additive Manufacturing (WAAM) allow for the production of a substantial number of parts while reducing the time and cost associated with machining, and have attracted significant attention. This study focused the microstructural and mechanical properties of a bimetallic thin-wall composed of SS308L and Inconel625, manufactured using variable-pressure rolling WAAM. A detailed analysis of the cross-sectional microstructure of the bi-metallic wall of non-pressure cases revealed a columnar solidification structure. In contrast, the pressured case was dominated by equiaxed structures. The microhardness tests conducted on the bimetallic interface revealed a hardness range of 139–188 HV within the SS308L region, and a 187–222 HV hardness spectrum within the Inconel625 region. The bimetallic interface displayed no unexpected hardness variations. Tensile tests carried out in the vertical direction across the bi-metallic interface resulted in a localized failure at the 308L region. The range of ultimate tensile strength was between 539 and 585 MPa, the yield strength varied from 344 to 377 MPa, and the elongation measurements were found to be within the range of 37 to 44%. All test specimens showed a consistent pattern of ductile rupture upon fractographic inspection, with fracture occurring in the SS308L region.

Performance evaluation of interfacial characteristics between cold metal transfer-wire arc additive manufactured SS308LSi and wrought SS304L with controlled and uncontrolled interlayer temperature

The present work investigates the mechanical behavior and microstructure evolution of the interface between the cold metal transfer (CMT)-wire arc additive manufactured (WAAM) SS308LSi and wrought SS304L via controlled interlayer temperature (CIT) of 100–120°C and uncontrolled interlayer temperature (UIT). The thermal cycles on the substrate were recorded using K-type thermocouples and correlated with the mechanical and microstructure properties of deposited walls. Microstructure analysis of the interface cross-section revealed a defect-free interfacial bond with a columnar solidification structure with slightly coarser grains under UIT deposition compared to CIT deposition. The microhardness of wrought SS304L is in the range of 196 ± 9 HV, while that of UIT and CIT deposited 308LSi is 178.4 ± 7.02 HV and 183.4 ± 6 HV, respectively, with no aberrant hardness values at the interface. Tensile tests were performed on the wrought SS304L and WAAM SS308LSi transverse to their interface. The results revealed that interface strength is higher than that of WAAM deposits and failed with average ultimate tensile strengths of 589.1 and 580.9 MPa under CIT and UIT depositions, respectively. This work reveals that CMT-WAAM can produce high-quality interfacial bonding between WAAM SS308LSi and wrought SS304L.

Verification of a new cellular automata model of solidification using a case study on the columnar to equiaxed transition previously simulated using front tracking

Developing computational tools appropriate for modelling physical phenomena, such as alloy solidification, requires making optimisation decisions on the time and spatial scales that can be resolved with finite computational resources. Here we report on development of a model of grain nucleation and growth, based on a Cellular Automata (CA) approach, that can directly simulate columnar and equiaxed solidification, and their competition to trigger a columnar-equiaxed transition (CET), at the scale of a casting. Previous work has been to develop a Front Tracking (FT) model of columnar solidification and an equiaxed index capable of predicting the relative likelihood of CET formation in a casting case study. This FT model has been validated by extensive experimental studies. Here we apply the new CA model and compare the predictions to those of the FT model. The close agreement between the two models serves to verify the new CA model. The study also presents insights into the thermophysical phenomena affecting grain structure evolution in alloy solidification and confirms the validity and utility of the equiaxed index.

High power laser powder bed fusion of AlSi10Mg alloy: Effect of laser beam mode

High power laser powder bed fusion (HP-LPBF) is a burgeoning additive manufacturing technology for the high-efficiency and high-accuracy build of metallic parts. However, studies about the effect of laser beam mode on build quality is still insufficient. In this paper, both a 2 kW multi-mode fiber laser and a 2 kW Gaussian mode fiber laser are used for the HP-LPBF of AlSi10Mg alloy. Firstly, the relative density (RD) of the HP-LPBF samples has been studied for parameter optimization. The RD of the samples built with the multi-mode laser beam (hereafter called as Multi-mode sample) first increases and then remains stable with the increase of laser energy density Ev. When Ev is no less than 50 J/mm3, the RD of the Multi-mode sample is higher than 99.5%. In contrast, the RD of the samples built with the Gaussian mode laser beam (hereafter called as Gaussian sample) first increases and then decreases with the increase of Ev, and the highest RD is just 95.87 ± 0.2% at the Ev of 50 J/mm3. Based on this, the Multi-mode sample and Gaussian sample built with the same parameters (Ev=50 J/mm3) are chosen as the typical contrast samples for further studies of surface quality, microstructure and tensile property. The results show that the typical Multi-mode sample presents a flatter surface and a lower surface roughness compared with the Gaussian sample. The phase composition of both the two kinds of samples consists of α-Al matrix and eutectic Si. However, the solidification microstructure varies with laser beam mode. The Multi-mode sample presents a columnar solidification microstructure with a strong< 100 > texture along the build direction. In contrast, the solidification microstructure of the Gaussian sample is a mixture of equiaxed grains and short columnar grains. The crystallographic orientations of the two kinds of grains are multifarious, leading to a much weaker texture. The tensile strengths and elongation of the Multi-mode sample are significantly better than those of the Gaussian sample. The effect mechanisms of the laser beam mode on relative density, surface quality, microstructure and tensile property have also been revealed.

Simulation of binary alloy columnar solidification in the presence of EM forced convection

The article describes a numerical model of solidification of the SnPb alloy with electromagnetic stirring. The model is based on two open access computational codes. Comparison of the results of the calculation of directional horizontal solidification with natural and electromagnetically forced convection forces is presented. It is shown that the electromagnetic driven flow can affect the final segregation map only at the first stage of the solidification process. Tables 1, Figs 8, Refs 30.

Microstructure and mechanical property of high power laser powder bed fusion AlSi10Mg alloy before and after T6 heat treatment

ABSTRACT This paper focuses on the microstructure and mechanical property of the high power laser powder bed fusion AlSi10Mg alloy before and after T6 heat treatment. The results demonstrate that the as-printed sample presents a columnar grain structure along the build direction and a strong <100> texture. Inside the columnar α-Al grains, there are cellular dendrites decorated with network eutectic Si. Both the α-Al matrix and eutectic Si have high-density dislocations. Besides, nano-twins and stacking faults are observed in eutectic Si. After T6 treatment, although the α-Al matrix still exhibits a columnar solidification feature, the cellular dendrites disappear and the proportion of equiaxed grains increase. And the eutectic Si presents as separate plates or nanoscale particles, in which nano-twins and stacking faults are not found. The tensile property anisotropy decreases and the strength-ductility balance improves after T6 treatment. The evolution mechanisms of the microstructure and tensile property are revealed.

Effect of laser remelting on the microstructure and mechanical properties of AerMet100 steel fabricated by laser cladding

AerMet100 ultrahigh-strength steel is used in safety-critical applications with limited repair methods. A repair technology using laser cladding (LC) has recently been developed, but it also has limitations due to mechanical anisotropy of the columnar crystal in the cladding layer. Herein, laser remelting (LR) is a process in which an LR layer with lower power density but without powder delivery has been applied after each LC layer is deposited to inhibit the epitaxial growth of columnar crystals and relieve the mechanical anisotropy. It was found that the LR process melted columnar crystal tips and rod-shaped carbides of the LC layer of AerMet100 steel into fragments and smaller sized granular carbides, respectively. In the LR melt pool, the fragments blocked the columnar solidification front to hinder its growth, or nucleated at the bottom of the melt pool to form short-thick columnar and equiaxed crystals to inhibit the epitaxial nucleation of the columnar crystals. The undissolved granular carbides clustered at the tips of the columnar dendrites and stopped the growth of columnar crystals. Moreover, the undissolved carbides also served as heterogeneous nucleation sites in the LR melt pool, extending the equiaxed crystal solidification zone to higher thermal gradient and lower solidification rate, which resulted in the formation of the LR layer whose microstructure morphology was mainly equiaxed grains. The AerMet100 forged steel repaired by the LC plus LR process minimised the impact of the crystal texture of the LC layers on the mechanical anisotropy due to the slowdown of crack propagation in the equiaxed crystal zone. It exhibited superior mechanical properties to those of AerMet100 steel repaired by LC alone.

Effect of vertical electromagnetic stirring on solute distribution in billet continuous casting process

A volume averaged columnar solidification model, which couples the flow, temperature and solute concentration fields, is applied to simulate experimental continuous casting cases with and without vertical electromagnetic stirring (V-EMS). The calculated distribution of magnetic induction intensity and final macrosegregation maps are consistent with the experimental results. Calculation results reveal that the V-EMS promotes longitudinal melt flow, accelerates heat dissipation and solidification and finally reduces the central segregation of carbon. However, when V-EMS is applied, the solute distribution becomes asymmetric because the melt flow shows opposite directions between the near and far sides from stirrer. An obvious positive segregation band is observed at about 1/4 width of the billet near the stirrer in both calculated and experimental results. The position and degree of such positive segregation could be affected by installation height of stirrer, as demonstrated by additional simulation cases.

Microstructural characterization of 15-5PH stainless steel processed by laser powder-bed fusion

The 15-5PH (UNS S 15500) stainless steel combines high mechanical strength, ductility, and good corrosion resistance for aircraft and aerospace applications. This set of properties and its high added-value applications make it an excellent choice for additive manufacturing processes such as laser powder-bed fusion (LPBF). However, there is a need to understand the complex microstructure developed in LPBF-processed parts, which may show particularities such as columnar solidification, preferential orientation, different kinds of porosities, chemical segregation along the melt pools, metastable phases, and oxide nanoinclusions. We report the microstructural characterization of LPBF-processed 15-5PH stainless steel in the as-built condition and after aging. Low porosity (<1%), high hardness (420 HV0.1), and a large amount of retained austenite (15%) were found in the as-built samples. The microstructure shows the usual “fish scale”-like morphology. Grain size and hardness vary depending on the location within the melt pools. Reconstruction of the parent grains from EBSD maps indicates a microstructural refinement due to in situ reaustenitization of parts of the previous consolidated layers, where these heat-affected zones are harder and present larger amounts of retained austenite. After aging, as-built samples were harder and more resistant to overaging than annealed or wrought counterparts. Transmission electron microscopy reveals a large amount of nanometric crystalline silicon-oxide inclusions, indexed as cristobalite. The combination of fine-grained martensite, coherent Cu-rich clusters, nanometric oxide particles, and retained austenite makes LPBF-processed 15-5PH stainless steel a very promising material for high-end structural applications.

A new model of competitive grain growth dominated by the solute field of the Nickel-based superalloys during directional solidification

To accurately predict the microstructure evolution of competitive columnar grain growth, the influence of the potential dominant factors of competition growth such as thermal field, solute field and flow field on the overgrowth behavior of trinary-crystal samples during directional solidification was systematically investigated, with the preferred orientation of the experimental grains orienting parallel and at a limited misorientation angle with respect to the temperature gradient direction. It was found that the grain overgrowth rate was weakly dependent on the temperature gradient, which was inconsistent with the classical theoretical assumption that the grain overgrowth rate was determined by the difference of the tip undercooling between the competing grains. In contrast, the grain overgrowth rate was sensitive to the solute field around the dendrite tips. Additionally, with increasing the natural convection, the grain overgrowth rate tended to be promoted at low withdraw speed. These phenomena were attributed to the mechanisms of solute interaction in the converging case and sidebranching events in the diverging case, while the solute field was the dominant factor to govern the overgrowth behavior of the competing grains. Moreover, a new model of competitive grain growth based on the solute field was proposed to predict the microstructure evolution of the Nickel-based superalloys during the directional solidification process. In this new model, the primary spacing of the well-oriented grain in the converging case was smaller than that in the diverging case due to the sidebranching events and dendrite lateral motion, which highlighted a new mechanism of primary spacing evolution in the directional columnar solidification structure.

Anisotropic Response of CoCrFeMnNi High-Entropy Alloy Fabricated by Selective Laser Melting.

This study investigated the anisotropic characteristics of the microstructural, mechanical and corrosion properties of CoCrFeMnNi high-entropy alloy produced by selective laser melting (SLM) additive manufacturing (AM). Under the extremely high thermal gradient during the SLM process, a columnar solidification structure with a single face-centered cubic (FCC) phase structure was formed. The crystal structure exhibited a regular checkerboard structure in the XOY plane (perpendicular to the building direction), which was composed of {110} direction and a small amount of {100} fiber texture. The cellular-dendritic sub-structures formed in the columnar crystal structure with sizes of about 500 nm in diameter. As for the mechanical properties, the XOY plane exhibited higher ultimate tensile strength and yield strength (σ0.2) but lower elongation to failure compared to the XOZ plane (parallel to building direction), which reflected the anisotropy of the microstructure. The electrochemical test results of the different planes showed that the XOZ plane exhibited better corrosion resistance in comparison with the XOY plane in the 3.5 wt % NaCl solution, which was on account of the selective attack at the Mn-rich inter-cellular regions and the different structures of the cellular-dendritic sub-structures on different planes.

Temperature Gradients at the Solidification Front of Deep Hybrid Laser Welds

Abstract Grain refinement and the avoidance of columnar solidification is a great challenge in the deep penetration laser welds of thick-section steels. Further knowledge about the heat distribution and the temperature gradients in laser welds is vital for future attempts on the grain refinement of such welds. In this study, a comparative investigation was carried out for full and partial penetration hybrid laser welding of structural steel. The transient temperature distribution and temperature gradients were calculated for the experiments using a simplified three-dimensional finite element analysis. A comparative analysis was presented to investigate the influence of penetration mode on the temperature gradient in the liquid at different weld depths. The results of the numerical analysis suggested that, for given welding process parameters, full penetration welds have lower temperature gradients at the solidification front, meaning that they potentially have a higher chance of grain refinement, compared with partial penetration welds.

Channel segregation during columnar solidification: Relation between mushy zone instability and mush permeability

A model of mushy zone instability is developed for characterization and prediction of channel segregation in castings. The model highlights the connection of the mushy zone instability with the mush permeability. A new criterion for amplification of the mush instability is derived, which depends on the interdendritic velocity, the isotherm velocity, the temperature gradient, and explicitly on the mush permeability. The capability of the instability criterion in the characterization of channel segregates is illustrated by comparing the estimated possible channel locations with that of numerically simulated channel segregations in a benchmark test case of solidification of Sn-Pb alloy. An Opensource CFD software OpenFOAM is used to simulate the solidification of Sn-Pb alloy in a side-cooled rectangular cavity. The new instability criterion very well characterizes the channel segregates shown in the numerical simulations. The near-liquidus part of the mushy region is more prone to the initiation of instability, which can initiate the formation of channel segregates in castings. Permeability and its derivative with respect to the liquid fraction in the outer part of mushy zone (liquid fraction more than 0.9) plays a key role in the amplification of this instability that can be responsible for the development and the formation of channel segregates. The locations of channel segregates estimated by the new instability model are also compared with those with a remelting criterion, and it has been noticed that the remelting criterion severely under characterizes these defects (e.g., their number and length). We noticed that local remelting might not be necessary for the initiation of a channel; some destabilization of the mushy zone due to local instability could be sufficient.

Mesoscopic modeling of equiaxed and columnar solidification microstructures under forced flow and buoyancy-driven flow in hypergravity: Envelope versus phase-field model

Quantitative modeling of solidification microstructures growing under the influence of convection is a challenging multiscale problem. It is of particular interest in processes where strong flow is present, such as centrifugal casting of Ti–Al alloys, where hypergravity strongly reinforces the buoyancy-driven flow. We present the coupling of the mesoscopic envelope model for dendritic solidification with fluid flow. We use the model to investigate columnar and equiaxed dendritic growth of the β-solidifying Ti–45 at.%Al under the influence of flow. The calculations are compared to phase-field results in 2D. For equiaxed growth the case of forced flow is treated. For columnar growth, the influence of buoyancy-driven flow on the growing structure and on the primary dendrite arm spacing is characterized for gravity levels ranging from 0 to ± 15 g. The computational cost of the mesoscopic simulations is around two orders of magnitude lower than that of phase field. We show that the mesoscopic model can accurately reproduce the microstructure characteristics, such as grain shape and primary arm spacing (PDAS), as long as the dendrite tip remains parabolic. When flow effects in columnar growth are strong enough to change the tip shape and induce tip splitting events, the mesoscopic envelope model does not reproduce the resulting branched microstructures. It does however predict the corresponding PDAS reduction at the correct gravity level.

Numerical Modeling of Melting and Columnar Solidification with Convection in a Gradient Zone Furnace in a Centrifuge

A numerical model is built for the solidification of a binary alloy with thermosolutal convection under centrifugal condition. The model is based on the macroscopic transport equations derived using the volume-averaging method. New formulations are derived for a non-linear phase diagram and temperature-dependent thermophysical properties. Simulations are performed to substantiate experiments with TiAl alloys carried out on ESA’s Large Diameter Centrifuge (LDC) using different angular velocities, ω. These experiments comprise a region of transient columnar growth of β-Ti dendrites under decreasing temperature gradient and increasing growth velocity followed by the columnar-to-equiaxed transition (CET). The simulation results show that CET is triggered by a sudden drop of the temperature gradient ahead of the advancing solidification front being a consequence of a sudden change of the flow pattern. We show that the flow pattern driven by density inversion changes abruptly once the permeability of the columnar structure increases above some critical value. This promotes the Rayleigh–Benard instability and the development of local convection cells along with an extended region of very low temperature gradient. The intensity and asymmetry of the flow in the local convection cells increase with increasing ω. The results show that a mushy zone developing in transient conditions and with gradually increasing permeability can significantly alter the flow pattern even in centrifugal conditions with a dominant contribution from Coriolis forces.

Permeability tensor for various columnar dendrite structures

Permeability is a very important parameter determining fluid dynamics in macroscale casting simulations. For a columnar solidification structure, the permeability has anisotropy and becomes a tensor quantity. In our previous study, we proposed a permeability tensor and confirmed its validity. In this study, the applicability of the permeability tensor is shown through a series of permeability computations that are conducted for some columnar dendritic structures with different morphologies.

Growth competition during columnar solidification of seaweed microstructures : Insights from 3-D phase-field simulations.

The mechanisms by which interfacial instabilities instigate the growth of solidification patterns is a topic of longstanding interest. In columnar solidification of metallic melts, where the solid-liquid interfacial energy is anisotropic, evolving dendritic patterns compete depending on their relative misorientation. By contrast, organic "plastic crystals", such as alloys based on succinonitrile, where the anisotropy in their solid-liquid interfacial energy is extremely weak, solidify forming seaweed patterns that typically exhibit little, if any, growth competition. We explore in this study mechanisms by which columnar solidification microstructures of binary alloys with low crystalline anisotropy compete. We adopt toward this end a validated Navier-Stokes multiphase-field approach to characterize the influence of grain misorientation, seed morphology, and melt advection on the growth competition. Simulated seaweed patterns indicate profound influences of all three factors, although characteristic solidification morphologies are observed to evolve depending on the melt flow intensity.

Numerical study of the role of mush permeability in the solidifying mushy zone under forced convection

Permeability of the mushy zone during alloy solidification is an important modeling parameter for many phenomena which accompany solidification, such as the formation of macrosegregation, shrinkage porosity, hot tearing, etc. The current study incorporates a two-phase, volume-average based columnar solidification model to study the role of mush permeability in the solidifying mushy zone under forced convection conditions. A unidirectional solidification of Al-7.0 wt %Si alloy sample (cylindrical with a diameter of 8 mm), referring to the experiments of MICAST project [Ratke, et al., MICAST research report – Phase IV, ESA-MAP AO-99-031, 2013], is considered. The forced convection is applied with a rotating magnetic field (20 m T). The basic permeability law for the numerical model is derived from Carman-Kozeny, but it has been modified by an artificial factor for the parameter study. The modeling results show that (1) the thickness of the mushy zone decreases and (2) the global macrosegregation intensity increases monotonically with growing permeability. A special macrosegregation profile resembling a “Christmas tree”, as typically observed experimentally, could also be numerically predicted. However, the formation of the “Christmas tree” segregation profile depends strongly on the permeability of the material. The formation mechanism of “Christmas tree” segregation can be analyzed according to the flow-solidification interaction in the mushy zone. A quantitative determination of the permeability, depending on the dendritic structure of the mushy zone, is beyond the scope of the current study, nevertheless it also includes a brief discussion (literature review) on this topic.