Final Stage Of Solidification Research Articles

Porosity forming mechanism and numerical simulation of casting process optimization of nickel-based heat-resistant alloy electrode ingot with large height to diameter ratio

Based on the simulation results of ProCAST software for the filling and solidification process, the formation mechanism of the VIM electrode ingot's pipe and porosities and the influence of the casting process on the formation of electrode shrinkage were studied in detail. The research results indicate that the critical condition for the formation of 200 kg grade C650R heat-resistant alloy is verified to be G/R≤5.0 (K·s)0.5/cm. In addition to the alloy's inherent characteristics, the substantial height to diameter ratio (H/D) of the VIM electrode ingot is the primary factor contributing to the formation of deep pipe and severe porosities. Simply adjusting the casting time or casting speed alone does not significantly improve the solidification quality of the ingot. While the use of a hot top can eliminate pipe, its effectiveness is limited, as the alloy melt at the hot top location has minimal influence on the solidification process in the lower regions of the electrode ingot, thereby failing to effectively reduce the formation of porosities. Properly reducing the casting speed and casting superheat lead to the formation of a V-shaped melt at localized positions during the final stage of solidification. This enhances the ability of the liquid phase for further feeding, ultimately significantly decreasing both the depth of pipe and the porosity rate. The optimal casting process optimization solution for C650R alloy electrode ingot with a size of Φ130 mm × 1240 mm is the casting speed of 60 kg/min and the superheat of 40 °C.

Petrology of the Skaergaard Layered Series

The Skaergaard intrusion is a layered, ferrobasaltic intrusion emplaced during the Early Eocene into the rifting volcanic margin of East Greenland. The magma chamber crystallised in response to cooling from the roof and margins upwards and inward, forming upper, marginal and bottom series, the latter referred to as the Layered Series. The phase layering in the bottom series suggests an evolved, olivine-normative tholeiitic melt saturated in plagioclase and olivine, followed by augite, and then simultaneously by ilmenite and magnetite forming primocrysts. Pigeonite appears in the lower parts and continues until the centre of the series. Apatite appears in the upper part concurrently with liquid immiscibility. Cryptic variations of the individual primocrysts record a systematic upward increase in iron and decrease in magnesium for the mafic minerals and a systematic increase in sodium and decrease in calcium for plagioclase. The appearance of pigeonite is caused by reactions and crystallisation in the trapped melt and by subsolidus adjustments without this phase reaching liquidus saturation. The high mode of olivine at the base of the upper part with the appearance of apatite is interpreted to mark the onset of liquid immiscibility. This may have led to the separation of conjugate melts with granophyre migrating upward and the basic component largely staying stationary or sinking. Petrologic and geochemical observations indicate differentiation in the lower part of the intrusion, principally controlled by crystal fractionation with the efficiency of fractionation controlled by the evolution and escape of liquid from the solidifying mush. During the final stages of solidification, the onset of liquid immiscibility and termination of melt convection impeded differentiation. Modelling by perfect Rayleigh fractionation shows that major and included trace elements conform reasonably to observations, while excluded elements deviate from model predictions. This decoupling is caused by the mobility of a granophyre component formed in the trapped melt and in the main residual magma chamber. Consequently, the sampled gabbros may not be representative of the final solid-melt mush. By restoring the gabbros to their original mush compositions, it is possible to constrain granophyre migration pathways. We suggest that the granophyre formed in the trapped melt in the lower part of the intrusion mostly migrated laterally through pressure release pathways to form lenses and pockets with only limited upward migration into the main magma reservoir. Near the end stage of differentiation, the residual magma exsolved and formed complex mixtures of ferrobasaltic and granophyric melts. Estimates predict that a substantial amount of the granophyric melt penetrated as sills into the downward crystallising, upper part of the body as well as into the host rocks. The redistribution of granophyric melts within the solidifying crystal mush complicates predictions of trapped-melt content and mass-balance calculations but helps to explain apparent decoupling of included and excluded trace elements, especially towards the end stages of evolution. Final crystallisation was controlled mostly by in situ crystallisation leaving complex mixtures of ferrodiorite and granophyre components.

Dissolution Behavior of Eutectic Carbides in Medium-Alloy Steels for Heavy Forgings.

In this study, we investigate the dissolution behavior of eutectic carbides in heavy forgings. High-temperature diffusion treatment was conducted on 35Cr3Ni3MoVW2 (MoVW2) and 35Cr2Ni3MoV (MoV) steels at 1230 °C for a duration ranging from 0 to 100 h. The dissolution of eutectic carbides and its effects on the microstructure and hardness of the steels were characterized and analyzed via SEM+EBSD, ImageJ, and Thermo-Calc. The results show that the coarse eutectic carbides in both steels gradually dissolved. The distribution and morphology tend to be uniform and spherical, respectively. For holding 50 h, the hardness of both steels significantly exhibited an increasing trend, and it was attributed to the combined effects of solid solution strengthening. Thermodynamic calculations indicated that the higher W content in MoVW2 steel promoted the precipitation of M6C eutectic carbides. Moreover, both MoVW2 and MoV steels exhibited the precipitation of M7C3 eutectic carbides in the final stage of solidification, facilitated by the enrichment of C and Cr in the liquid steels.

Cracking behaviour and its suppression mechanisms with TiB2 additions in the laser additive manufacturing of solid-solution-strengthened Ni-based alloys

This study systematically investigated the cracking mechanisms in the laser powder bed fusion (LPBF) of GH3230 solid-solution-strengthened Ni-based alloy (GH0). The results show that the micro-cracks that formed in GH0 specimens included both solidification and solid-state cracks. The initiation of solidification cracks was associated with the formation of continuous liquid films on high-angle grain boundaries at the final stage of solidification, while the solid-state cracks were found to be ductility-dip cracks, associated with a reduction in the material's plasticity within the heat-affected zone. The study also found that the residual stresses decreased with increasing LSS values, leading to reductions in crack length. The introduction of 1 wt% TiB2 particles to GH3230 (GH1-composite) was found to suppress cracking by promoting grain refinement and generating special high-angle grain boundaries, although residual thermal stresses increased. The ultimate tensile strength values of the GH0 and GH1-composite specimens at 900 °C were found to be 213 and 352 MPa, respectively. These findings provide significant insights into the LPBF of high-performance crack-free Ni-based superalloys.

Effectively mitigated macro-segregation and improved tensile properties of twin-roll cast Al–Mg–Si (6022) alloy strips via Zr addition

It is a great challenge to avoid the central macro-segregation during the solidification process of twin-roll casting (TRC), which seriously limits the mechanical properties of 6xxx series Al alloys. In this study, we find that the trace addition of 0.08 wt% Zr to Al-1.0Mg-1.2Si (6022) alloy can effectively promote the columnar-to-equiaxed transition and refine the solidification microstructure (particularly α-Al dendrites and π-AlFeMgSi phase). More importantly, the macro-segregation of the as-cast TRC strip is well mitigated with Zr addition. The refinement of α-Al dendrites and π-AlFeMgSi phase may be attributed to the formation of (Al, Si)3Zr phases when introducing Zr element, which can act as heterogeneous nucleation sites for both of them. The secondary dendrite arm spacing (SDAS) in the central region of strips is sharply decreased from ∼33 to ∼16 μm, which facilitates the incorporation of more solute elements into the Al matrix, thus increasing the partition coefficients of Mg (kMg) and Si (kSi) elements. As a result, there are fewer solute atoms in the residual liquid phase during the final stage of solidification, leading to the alleviation of central macrosegregation. The tensile properties of the as-cast, solid solution treated (T4) and paint baking treated (T6) alloys are synchronously optimized with Zr addition. In particular, the fracture elongation of the T4 alloy is increased from ∼30.9% to ∼33.4%, indicating excellent formability. The yield strength of the T6 alloys is increased from ∼227 MPa to ∼239 MPa, satisfying the more demanding service requirements. This work can provide a strategy for the effective control of macro-segregation and produce alloy strips with high performance by the TRC process.

A Study of Sliver in C-Shaped Grain Selectors during Investment Casting of Single-Crystal Superalloy

In this study, the formation mechanism of sliver defects in C-shaped 2D grain selectors during investment casting of single-crystal superalloy was investigated by analyzing the effects of the C-shaped 2D grain selectors on the solidification behavior of superalloy. The physical field properties of the sliver formation and solidification characteristics of CM247LC nickel-based superalloy were determined. The temperature and stress fields were simulated using ProCAST. The results showed that the stress fields of solidification played an essential role in the formation of sliver, indicating that the solidification interval characteristics of the alloy and stress based on the geometry of the C-shaped 2D grain selector are crucial for the formation of sliver. In addition, the origin of sliver depended upon tensile stress during solidification, relying on the constraints of dendrite boundaries. The findings suggested that the joint sections of the starter block—i.e., selector and selector-casting joint of C-shaped selector sections—are stress-sensitive areas where sliver can form readily. Furthermore, sliver is formed in the final stages of solidification and especially originates in the grain selection part where the accumulated thermal stress is high, and there is only a small quantity of liquid phase with a low melting point between the dendrites. Therefore, the solidification and stress conditions generate thermal cracks, which can also cause sliver defects.

Mineral chemistry and genesis of monazite-(Sm) and monazite-(Nd) from the Blue Beryl Dyke of the Julianna pegmatite system at Piława Górna, Lower Silesia, Poland

AbstractMonazites are one of the most interesting groups of accessory mineral components of crystalline rocks due to the information on geochemical evolution of the crystallisation environment coded in their chemical compositions, in addition to comprising one of the most valuable objects for geochronology studies. This paper presents monazite-(Sm) and monazite-(Nd) from the Blue Beryl Dyke of the Julianna system of rare-element pegmatites at Piława Górna, Lower Silesia, Poland. These monazites are unique due to their unusually high Sm and Nd contents, reaching 33.22 wt.% Sm2O3 and 34.12 wt.% Nd2O3, respectively. We consider the most significant factors of the enrichment in Sm and Nd to be the occurrence of highly fractionated pegmatite-forming melts during the final stages of solidification and associated hydrothermal fluids that were strongly enriched in rare earth element REE–Cl and REE–F complexes. Local disequilibria allowed for the rapid growth of accessory phases under supercooling conditions associated with the scavenging of selected elements, leading to their local depletion, which was not balanced by diffusion processes. As a consequence, the depletion of light rare earth elements (LREE) led to the incorporation of available middle rare earth elements (MREE, Sm–Dy) in the case of Sm and Nd, which could occupy an acceptable structural position in minerals of the monazite group.

Cracking behavior of newly-developed high strength eutectic high entropy alloy matrix composites manufactured by laser powder bed fusion

A novel AlCoCrFeNi2.1 eutectic high entropy alloy (EHEA) composite doped with SiC particles was designed and fabricated by laser powder bed fusion (LPBF). Its microstructure characteristic, tensile properties, and metallurgical defects, with an emphasis on cracking behavior, have been investigated. The results showed that the addition of SiC particles into the AlCoCrFeNi2.1 matrix enabled the development of a {100} texture and highly elongated columnar grains, which were the main contributors to mechanical behavior anisotropy. The ultimate tensile strength of 1466 ± 26 MPa and elongation of 9% ± 3% achieved in the as-deposited EHEA composite surpassed those of advanced metal alloys subjected to additive manufacturing processes. Unfortunately, severe horizontal and longitudinal cracks, as well as a few micro-cracks were observed in the as-deposited bulk samples. Micro-cracks were verified to be associated with the aggregation of carbon and oxide particles. They formed in the final stage of solidification owing to insufficient liquid feeding ability and solidification contraction. The formation of macroscopic cracking was induced by the tensile stress accumulations at sample edges, and the stress concentration areas where microcracks and pores were located were the predominant propagation location. This work provides guidelines for defect control in SiC-reinforced EHEA, assisting in the high-performance design and integrated manufacturing of EHEA composite components.

Tracking the evolution of hot tears in aluminium alloys using high-speed X-ray imaging

Hot tears are detrimental defects forming during the final stage of solidification when the remaining liquid loses the capacity to compensate for liquid to solid volume shrinkage. Although a mature semi-quantitative description of hot tearing has been developed, little is known about the dynamic evolution of hot tears as experimental studies have been conducted mostly post-solidification or in semi-static in-situ conditions. Here, we present a methodology to investigate the evolution of hot tears with high spatial and temporal resolution using synchrotron-based X-ray radiography. We develop a novel hot tear detection and tracking algorithm for quantification of hot tear density, area fraction and merging from the analysis of radiographic sequences of the solidification of thin metal samples. The methodology is demonstrated for an Al-5wt%Cu alloy and examples of the results and new insights that can be achieved are described.

Study on Solidification and Phase Transformation Behaviors in Ni-based Superalloy IN625

The solidification and phase transformation behaviors of IN625 with low C and Fe were investigated through directional solidification and quenching experiments. The primary and secondary dendrite arm spacing decreased exponentially as the solidification rate increased, demonstrating good agreement with theoretical equations. The MC carbide formed at a constant temperature gradient regardless of the solidification rate. It is reasonable to assume that the Laves phase solidified in the final stage of solidification, as it was found at the bottom of the mushy zone. The morphologies and sizes of the MC carbide and the Laves phase were closely related to the solidification rate. Increasing the solidification rate at a constant thermal gradient resulted in the development of the MC carbide from blocky to Chinese script, and eventually spotty shapes, along with decreasing size. As a result of this research, it is believed that the weldability of the alloy could be improved at the expense of mechanical properties, especially with low C and Fe contents and a relatively high solidification rate, which are able to generate the finely distributed MC and Laves phase.

Effects of the filler wire composition on the structures and mechanical performance of 2195 Al Li alloy TIG joints

The mechanical properties of AlLi alloy joints are closely related to the composition of the welding consumables. The present study aims to improve the mechanical properties of 2195 AlLi alloy joints by adjusting the filler wire composition. The 2195 AlLi alloy filler wire (same composition welding, SCW) and a self-designed filler wire (dissimilar composition welding, DCW) are used for the TIG welding of 2195 AlLi alloy sheets in this study. The results reveal that increasing Cu concentration in the filler wire leads to more eutectic formation in the DCW weld, and the possible solidification crack can be healed in time by the liquid metal in the final solidification stage; significant grain refining in the DCW weld is caused by adding Ti and Zr into the filler wire, the refined grains enable the joint to tolerate more plastic deformation and reduce the continuity of the eutectics along the grain boundaries. The positive effects of filler wire composition adjustment on the joint mechanical properties are verified by the mechanical tests. The plastic deformation is concentrated in the weld, and the weld elongation of the DCW joint is much higher than the SCW joint. In addition, the solidification crack can be observed in the SCW joint but not in the DCW joint. These findings can guide the design of subsequent filler wires matched to 2195 AlLi alloy.

The effects of chemistry variations on hot cracking susceptibility of Haynes® 282® for aerospace applications

Hot cracking susceptibility of Haynes® 282® with varying amount of C (0.05–0.09 wt%), Mn (0.03–0.12 wt%), Si (0.03–0.16), B (0.005–0.006 wt%) and Zr (0–0.01) are investigated. Synergistic role of C and B is found on solidification and heat affected zone (HAZ) liquation cracking susceptibility. High amount of C and B promote formation of eutectic constituents during final stages of solidification and promote crack healing by backfilling effect. When C and B are added in low amount the crack healing does not occur due to the absence of eutectic consituents therefore cracking extent increases. Thermodynamics simulations indicate C and B tie up to MC carbides and M3B2 borides during solidification. Scanning Electron Microscopy and Nanoscale secondary ion mass spectrometry analysis reveal C and B to be present both in solid solution and in form of precipitates to Ti-Mo rich carbides and Mo rich borides, respectively. In HAZ, these phases promote liquation cracking where cracking extent correlates to the amount carbides and borides. Lower C and B is found to reduce the liquation cracking in the HAZ. Furthermore, a high temperature homogenization heat treatment at 1190 °C excarbates the cracking by dissolving the borides and releasing B to the grain boundaries.

Improved solidification structures and mechanical properties of Al–20 wt% Sn alloys processed by an electromagnetic vibration technique

Homogenous distribution of the Sn phase in the Al–Sn alloy is beneficial for improved properties. However, the alloy has a metastable immiscibility gap that results in macrosegregation of phases in a usual casting. In the present study, the alloy with an actual composition of Al–20 wt% Sn–1 wt% Cu (referred to Al–20 wt% Sn hereafter for simplicity) was solidified in a static magnetic field when alternating current flowed through the alloy, from which electromagnetic vibration (EMV) was imposed upon the solidifying liquid. Solidified microstructures were observed and characterized in terms of the low-melting-point Sn phase. It was found that when EMV was imposed at the frequency of f = 100 Hz, the Sn phase could be refined and dispersed homogenously in the primary Al matrix, which formed a contrast with the continuous Sn network in a usual casting solidified without EMV imposition. The specimen with fine Sn dispersions had an improved elongation in tensile tests. The solidification mechanism was discussed in terms of the structure formation and solute distribution when EMV was imposed during directional solidification. The uncoupled movement between the primary Al solid solution phase and the surrounding liquid may breakdown the Sn liquid film into discrete segments and thus form isolated particles at the final solidification stage. Meanwhile, this uncoupled movement can induce forced fluid flow that can stir the melt to homogenize the solute distribution in the remaining liquid pool. The composition transition was considered when EMV was imposed. To better support our analysis on the chemical composition variation during EMV processing, optical microstructures and differential scanning calorimetry (DSC) profiles were supplemented from a continuously cast Al–7 wt% Si alloy billet. The results of the Al–20 wt% Sn and Al–7 wt% Si alloys demonstrate that the EMV technique should be quite effective and applicable in improving solidification structures and mechanical properties provided that it is proceeded under appropriate conditions.

Understanding the significant effect of boron content on the printability of IN738LC superalloy fabricated using laser powder bed fusion

This study intends to investigate the effect of boron on the cracking behavior of Inconel738LC during laser powder bed fusion, which has been rarely reported in previous literature. 2 batches of Inconel738LC powders with different contents of boron are selected to fabricate parts. More cracks are detected in the sample fabricated by the powders with a high content of boron compared to the low-content-boron powders. The formation of boride is confirmed through transmission electron microscope, atom probe tomography and Thermo-Calc calculation, leading to solidification cracking at the final stage of solidification. In addition, the cracks tend to occur at the grain boundaries with high misorientation angles. Significant enhancement in the mechanical performance is achieved with the marked decrease of the boron due to a low level of cracking in the samples.

Formation of delta-ferrite and its effect on impact toughness of GTAW weld metals in a 12 wt-% Cr steel

This research focused on the formation of the δ-phase and the corresponding effect on the impact toughness in a GTAW weld metal of a 12%Cr F/M steel. Two kinds of δ-phase were identified: one located at the prior austenitic grain boundaries (PAGBs) (called δPAGB) and the other was in the inter-dendritic region (called δinter). The δinter implied that the peritectic transition that occurred at the final stage of solidification in the DSC test, was suppressed in the welding process. A preheating treatment at 95°C decreased optimally the δ-phase fraction and thereby decrease the ductile-brittle transition temperature (DBTT). However, excessive preheating at higher temperatures enhanced the fraction of the δinter by promoting the micro-segregation and thus increased the DBTT.

Microstructural features and improved reversible hydrogen storage properties of ZrTiVFe high-entropy alloy via Cu alloying

ZrTiVFe high-entropy alloy has shown desirable hydrogen absorption and desorption properties due to its lattice distortion effect and high content of C14 phase that can store hydrogen. In this study, element Cu was used to improve the reversible hydrogen storage properties of equimolar ZrTiVFe alloy by increasing valence-electron concentration (VEC), and (ZrTiVFe)1-xCux (x = 0.05, 0.1, 0.2) alloys were prepared. After studying their microstructural features and hydrogen storage properties, the results indicate that (ZrTiVFe)0.95Cu0.05 and (ZrTiVFe)0.90Cu0.10 alloys are mainly consisted of C14 Laves phase and a small amount of α-Ti and α-Zr phases. When the Cu content increases to 20 at. %, the microstructure transforms to reticular ZrTiCu2 phase around C14 Laves phase, and the Cu8Zr3 phase is formed in final solidification stage. The fastest hydrogen absorption rate of (ZrTiVFe)0.80Cu0.20 alloy at room temperature suggests the ZrTiCu2 and Cu8Zr3 phases can provide preferential paths for hydrogen atoms diffusion. The amount of hydrogen in (ZrTiVFe)0.90Cu0.10 hydride that cannot be desorbed at 400 °C in vacuum is greatly reduced from 0.370 wt% to 0.084 wt% comparing with ZrTiVFe hydride. The addition of element Cu reduces the stability of ZrTiVFe hydride significantly, which favors the hydrogen desorption of the (ZrTiVFe)1-xCux alloys.

Probing interdendritic flow and hot tearing during solidification using real time X-ray imaging and droplet tracking

Despite the well-known importance of controlling shrinkage-induced liquid flow in alloy castings to avoid the formation of catastrophic hot tears during the final stages of solidification, there has been little direct experimental measurement of liquid metal flow and hot tear formation under practical conditions. We use synchrotron X-rays to obtain radiographic video sequences of the solidification of monotectic Al-Pb alloys in which Pb droplets form as a fine-scale emulsion. We track and measure the velocity of thousands of Pb droplets as they move through interdendritic regions due to the effect of liquid to solid shrinkage during the final stages of solidification, up to the point of hot tear formation. Based on the droplet velocities, we present an analysis to estimate the interdendritic liquid velocity as solid fraction increases, and thus the shrinkage pressure drop driving the flow. The analysis is applied for video sequences obtained for both equiaxed and columnar microstructures, each under a range of cooling rates. Our measurements of the critical shrinkage-induced pressure for hot tear formation agree well with prior model-based and theoretical suggestions. The limitations and prospects for droplet tracking measurements of liquid metal flows are discussed.

Linking Processing Parameters and Rheology to Optimize Additive Manufacturing of k-Carrageenan Gel Systems.

Additive manufacturing—in particular, three-dimensional (3D) printing—has been introduced since the late 1980s, offering a novel paradigm for engineering design and manufacturing, as it allows the fabrication of very complex structures. Additive manufacturing of hydrogels is a very popular method to produce scaffolds to be used in tissue engineering and other biomedical applications, as well as in other advanced technological areas. When printing a thermoreversible physical hydrogel, a subtle balance between thermal and rheological parameters exists. The characteristic times of the sol–gel transition, regulated by a well-defined thermal history, must be optimized with respect to the characteristic processing times. In this work, we use this thermo-rheological approach to the additive manufacturing of a physical hydrogel. A low-cost desktop 3D printer for thermoplastic polymers was suitably modified to print a 1.5 wt% solution of k-carrageenan. The thermal behavior of the printer was determined by performing experimental measurements of the temperature–time evolution during the different processing steps, from solution loading, to the extrusion of the incoming gel, to the final solidification stage. In parallel, linear viscoelastic oscillatory shear measurements were performed in a rotational rheometer under thermal histories as close as possible to those previously measured in the printing process. The comparison between the rheological results and the quality of printing under different thermal histories is presented and discussed, highlighting the main relations between rheological and processing behavior, which are helpful in the assessment and optimization of the printing conditions.

Study the failure of casted copper anode: The formation of bumps defects on the surface of the anode during casting

Copper anode (∼98%) is widely used to produce the cathode (99.995 wt%) in electrorefining process. These anodes sometimes become problematic from the production line due to the presence of bumps/holes on their surface. To find the reason of this defect, chemical composition of the three casted anodes was determined by the quantmeter analysis. Also, the microscopic and phase analysis were done by FESEM, EDS, XRD, and XRF for three types of anodes bump. The chemical analysis of anodes confirmed a high level of sulfur and oxygen, which have a key role in the formation of bumps. The results confirmed the presence of impurities including Cu2S, BaSO4, CaCO3 and MgCO3, which cause the formation of gas bubbles at the final stage of solidification. The main source of these impurities was barium sulfate as mold covering and magnesia-chrome bricks (as refractories). To prevent bumps phenomena, the control of impurities content is necessary.

Mitigating hot-cracking of laser melted CoCrFeNiMnTix high-entropy alloys

Additive manufacturing is a promising technique for fabricating high-alloyed metallic components such as CoCrFeMnNi high-entropy alloy with very fine microstructure and reduced macro-segregation. However, hot cracking nucleation at the interdendritic regions always occurs during preparation, which can hardly be controlled. In this study, the hot cracking possibility could be reduced by introducing element Ti into CoCrFeMnNi high-entropy alloy with the improved alloy microhardness (182 ± 8.7 HV to 973 ± 43.2 HV). At the final solidification stage, eutectic interaction takes place in the interdendritic regions. Body-centered-cubic (BCC) and a few σ phases form, dividing the liquid film and releasing the local stress concentration. CoCrFeNiMnTi0.6 sample without cracks is observed.