Local Solidification Time Research Articles

Effect of current intensity on molten pool and solidification in vacuum arc remelting process of Ti60 alloy

Current intensity is a critical parameter in the vacuum arc remelting (VAR) process, significantly impacting the molten pool, solidification structure, and ultimately, the quality of the ingot. This study establishes a transient electromagnetic-fluid-thermal-solidified coupled model to examine the effects of current intensity on the shape of molten pool and the solidification process of Ti60 alloy ingots produced by VAR process. Findings indicate that with the increase in current intensity, there is a corresponding rise in the magnetic induction intensity, current density, and Lorentz force. This enhancement leads to more vigorous flow in the liquid phase, creating a vortex flow opposing to the ingot's center at the boundary, influenced by the strong current. As a consequence, the molten pool's shape tends to shift from a “V” shape to a “U” shape, and the depth of molten pool increases while the mushy zone thickness diminishes. For instance, at 17000 A, the depth of molten pool surges by 57.43%, and the mushy zone thickness contracts by 62.01% compared to 5000 A. Over time, the depth of molten pool gradually decreases, with its reduction range ( Δh) within 400 to 6000 s inversely related to current intensity. At 17000 A, Δh is 23.71% less than at 5000 A. Furthermore, elevating the current intensity results in a higher temperature gradient within the ingot and a marked reduction in local solidification time, particularly in the ingot's central area.

A Numerical Time Integration Procedure for Secondary Dendrite Arm Spacing in Hyper-Peritectic Steel Alloys

AbstractA numerical time integration procedure for the calculation of the secondary dendrite arm spacing (SDAS) in FeC hyper-peritectic alloys is presented, a preferred group of low-carbon casting steel. The procedure incorporates a three-stage thin arm dissolution model, which is solved at each time step using Newton–Raphson iteration. This is coupled to a coarsening model based on the integral forms of the dissolution model, which are solved using Gaussian quadrature, as well as a growth model for solid fraction evolution. The procedure can easily be embedded into numerical models for solidification, in which the space–time evolution of the SDAS is required for determining the dynamic permeability in the mushy zone. Higher order approximations for both growth and solute concentration evolution can easily be incorporated. Temperature dependence of thermophysical parameters is taken into account using inter-dendritic solidification empirical models, and an alloy-specific peritectic reaction constant is used to determine the isothermal peritectic holding time. The procedure is validated against experimental data presented in literature. Various cases of SDAS as a function of local solidification time, cooling rate and carbon composition are investigated. The method is compared to experimental results of SDAS obtained from test castings of a hyper-peritectic steel alloy and can be used to iteratively determine the alloy-specific peritectic reaction constant by comparing the solid fraction evolution during the peritectic reaction with that found from the experimental cooling curve.

Morphology Refinement of Eutectic Carbides Assisted by Static Magnetic Field during Laboratory Electroslag Remelting Process

A transverse static magnetic field (TSMF) is implemented in a laboratory electroslag remelting (ESR) experiment to control eutectic carbides morphology of Cr8 die steel herein. Under action of a TSMF, the temperature distribution of the molten pool becomes homogeneous, the local solidification time is shortened, and the interdendritic region space shrinks, which result in an evenly spaced distribution of solute atoms and a reduction in interdendritic segregation. Therefore, the formation and subsequent growth of eutectic carbides are further inhibited, thereby causing the generation of smaller and more uniform distributed eutectic carbides due to the application of the TSMF. Moreover, the increase of the number of Al2O3–(Ti,V)N inclusions serves to be the heterogeneous nucleus of carbides (inclusions also become finer) caused by TSMF, which also contributes to the refinements of eutectic carbides in Cr8 ingots.

Mechanism of local solidification time variations with melt rate during vacuum arc remelting process of 8Cr4Mo4V high-strength steel

Mechanism of local solidification time variations with melt rate during vacuum arc remelting process of 8Cr4Mo4V high-strength steel

Morphology Control of Metal Pool and Eutectic Carbides in Electroslag Remelted M2 HSS with an External Axial Static Magnetic Field

This study investigates the influence of a superimposed axial static magnetic field (ASMF) on the morphology of metal pool and eutectic carbides (ECs) in electroslag remelted M2 high-speed steel (HSS). The application of ASMF caused the metal pool to become shallower, and the solidified structure to expand axially, along with finer (i.e., more uniformly distributed and crystallographically oriented) ECs. Lorentz force-driven unidirectional circulation in slag pool was the primary cause of the metal pool’s morphological adjustment; this resulted in a more homogenous temperature distribution in slag pool. Thus, the heat transfer from the slag pool to the metal pool became more uniform, creating a metal pool that is shallower. Additionally, local solidification time (LST) became shorter, while the number of (Ti, V)N-Al2O3 inclusions serving as heterogeneous nuclei for EC formation increased due to ASMF, enabling finer EC with more crystallographic orientations in ESR ingots.

Cleanliness improvement and microstructure refinement of H13 die steel by laboratory magnetic-controlled electroslag remelting

The current work investigated the impact of transverse static magnetic field (TSMF) with varied magnetic flux density (MFD) on cleanliness and microstructure of laboratory scale electroslag remelted H13 die steel. The inclusion morphology and microstructure evolution of electroslag remelted ingots were examined utilizing scanning electron and optical microscopes, respectively. The number and size of inclusions in ingots were detected using the FEI Aspex Explorer. The results demonstrated that the number/size of inclusions in H13 electroslag remelted ingots were decreased as the MFD of applied TSMF increased. This resulted from the application of the TSMF, which produced the thinner liquid melting film (LMF), smaller droplets, and shallower metal molten pool, strengthening the kinetic conditions for inclusion migration to the slag-metal interface and the removal process. When the MFD of the applied TSMF was higher (95 mT and 140 mT), the LMF became thinner, the droplets became smaller, and the metal pool became shallower were more visible. Moreover, the metal pool became shallower and the local solidification time became shorter after applying TSMF with larger MFD, resulting in finer dendritic structure and carbides.

Effect of Cooling Rate and Sulfur Content on Sulfide Inclusions in Invar Alloy

Invar alloy has been widely used in modern industry for its extremely low coefficients of thermal expansion (CTE). Sulfide inclusions have a significant influence on the mechanical performance and corrosion resistance of Invar alloy. To improve the mechanical properties of this alloy, which are significantly influenced by the existing sulfide inclusions, a good understanding of the characteristics, including the morphology, size, distribution, and formation mechanism of sulfide inclusions, is required. This study outlines three different cooling modes, water cooling (43.1 K/s), air cooling (16.8 K/s), and furnace cooling (3.1 K/s), to examine the characteristics of calcium sulfide (CaS) inclusions. In addition, a variety of initial sulfur contents under air cooling conditions were investigated. Both laboratory experiments and thermodynamic calculations support the research foundation. The sulfide inclusion particles were extracted through non-aqueous solution electrolysis for further morphology analysis. From the results, we conclude that the cooling rate affects the sulfide inclusion size through the local solidification time. The faster the cooling rate, the larger the average inclusion size. The cooling rate also indirectly influences the morphology of calcium sulfide inclusions that precipitated during the solidification process. In addition, the precipitation of CaS takes precedence over MnS under any circumstances. MnS can only precipitate when there is excessive S content. The different initial S content derived separate supersaturation during CaS precipitation and growth, further influencing the inclusion morphologies.

Comparison of Dynamical and Empirical Simulation Methods of Secondary Dendrite Arm Coarsening

The physical and mechanical properties of an entirely (wrought alloys) or partly (cast alloys) dendritically solidified alloy strongly depend on the secondary dendrite arm spacing (SDAS). The casting practice and the simulation of solidification need a usable but simple method to calculate the SDAS during and at the end of solidification as a function of the cooling rate. Based on many solidification experiments, a simple equation to calculate the SDAS (empirical method) is known to use the local solidification time, which can be obtained from the measured cooling curves (equiaxed solidification), or can be calculated from the temperature gradient and front velocity (directional solidification). This equation is not usable for calculating the SDAS during solidification. Kirkwood developed a semi-empirical method based on the liquid phase’s diffusion, which contains only one geometric factor that seems constant for different alloys. This equation contains some physical parameters that depend on the temperature, so the equation cannot be integral in closed form. In the present work, first, we show the effect of the curvature of the solid/liquid interface on the equilibrium concentrations and then the different processes of SDA coarsening. In our earlier paper, we demonstrated that using the empirical method, the final SDAS can be calculated with acceptable correctness in the case of four unidirectional solidification experiments of Al-7wt%Si alloy. The present work shows that numerically integrated Kirkwood’s equations used the known cooling curve; the SDAS can be calculated at the end and during solidification in good agreement with these experimental results. Compared to the two calculation methods, we stated that the correctness of the methods is similar. Still, the results of the solidification simulation (the microsegregation) will be more correct using the dynamical method. It is also shown that with the dynamical method, the SDAS can be calculated from any type of cooling curve, and using the dynamical method, it is proved that some different SDASs could belong to the same local solidification time.

Microstructure Analysis of Al-7 wt% Si Alloy Solidified on Earth Compared to Similar Experiments in Microgravity

During ground-based solidification, buoyancy flow can develop by the density difference in the hypoeutectic type of the alloys, such as Al-7 wt% Si alloy. Buoyancy flow can affect the thermal field, solute distribution in the melt, and the position and amount of the new grains. As solidification is a very complex process, it is not very easy to separate the different effects. Under microgravity conditions, natural convection does not exist or is strongly damped due to the absence of the buoyancy force. Therefore, experiments in microgravity conditions provide unique benchmark data for pure diffusive solidification conditions. Compared to the results of the ground-based and microgravity experiments, it is possible to get information on the effect of gravity (buoyancy force). In the framework of the CETSOL project, four microgravity solidification experiments were performed on grain refined (GF) and non-grain refined Al-7 wt% Si alloy onboard the International Space Station in the Materials Science Laboratory. These experiments aimed to study the effect of the solidification parameters (solid/liquid front velocity vSL, temperature gradient GSL) on the grain structure and dendritic microstructures. The microgravity environment eliminates the melt flow, which develops on Earth due to gravity. Four ground-based (GB) experiments were performed under Earth-like conditions with the same (similar) solidification parameters in a vertical Bridgman-type furnace having four heating zones. The detailed analysis of the grain structure, amount of eutectic, and secondary dendrite arm spacing (SDAS) for different process conditions is reported and compared with the results of the microgravity experiments. GB experiments showed that the microstructure was columnar in the samples that do not contain GF material or in case the solid/liquid (vSL front velocity was slow (0.02 mm/s)). In contrast, in the sample which contained GF material, progressive columnar/equiaxed transition (PCET) was observed at vSL = 0.077 mm/s and GSL = 3.9 K/mm. The secondary (SDAS) dendrite arm spacing follows the well-known power law, SDAS=K[t0]13, where K is a constant, and t0 is the local solidification time for both GB and µg experiments.

Refinement of Eutectic Carbides in M2 High Speed Steel by Adjusting Magnetic Flux Density During Magnetic Controlled ESR Process

The current study examines the effect of magnetic flux density (MFD) on the morphology of the metal pool and the refinement of eutectic carbides in magnetic controlled electroslag remelting (MC-ESR, M2 high speed steel). When a transverse static magnetic field (TSMF) with a larger MFD was used, it was easier to obtain a shallower metal pool and finer eutectic carbides. These phenomena are explained by the smashing effect of the droplet neck (stronger at higher MFD values) induced by electromagnetic vibration, resulting in smaller/more dispersed droplets and thus a more uniform temperature distribution in the metal pool. Additionally, as MFD increased, the local solidification time (LST) decreased (0 to 140 mT) and the number of inclusions serving as heterogeneous nuclei for eutectic carbide formation increased (0 to 65 mT), resulting in a gradual size reduction of eutectic carbides. However, the refinement of eutectic carbides in MC-ESR must consider the above two factors comprehensively, and the MFD of applied TSMF need not be excessively large.

Numerical investigation on the effect of axial magnetic field on metallurgical quality of ingots during vacuum arc remelting process

The electromagnetic stirring force generated by the axial magnetic field (AMF) interacting with the process current greatly affects the solidification process and consequently the metallurgical quality of ingots, but there are few studies about the effect of AMF on the multi-physics field involving solute transport during VAR process. Based on this, a three-dimensional full-size transient model has been developed to investigate the role of AMF on the metallurgical quality evaluated by the segregation defect and solidification quality in this paper. All essential considerations of the fluid flow, heat transfer and solute transport are incorporated in the model. Both the local solidification time (LST) and secondary dendrite arm spacing (SDAS) are adopted as indexes to characterize the solidification quality. The results show that the stirring force caused by the AMF greatly promotes the downward heat transfer and thus leads to deeper molten pool. The final value of volume-averaged segregation extent is greatly reduced by 24.13% as the AMF intensity increases from 0 to 50 G, while the difference of this value between 50 G and 100 G is only 1.96%. The volume-averaged values of the LST and SDAS are all reduced with the increase of AMF intensity, in which both values change from 1088 s to 190 μm without AMF to 890 s and 182 μm under 100 G. These results indicate that the application of AMF can significantly improve the metallurgical quality of ingots, but this effect becomes more and more limited with the increase of AMF intensity.

Carbides refinement and mechanical properties improvement of H13 die steel by magnetic-controlled electroslag remelting

The effect of an axial static magnetic field (ASMF) on the carbides and mechanical properties of electroslag remelted H13 die steel was studied in current work. Optical microscope (OM) and scanning electron microscope (SEM) were used to analyze the solidification structure and carbide morphology of electroslag remelting (ESR) ingots. The FEI Aspex Explorer was utilized to detect the inclusions in H13 ESR ingots. Compared with conventional ESR, the application of ASMF in this study made the metal molten pool shallower and flatter, resulting in shorter local solidification time and uniform distribution of solute atoms, which alleviated the element segregation degree, and further inhibited the formation of primary carbides. The magnetic-controlled electroslag remelting (MC-ESR) process not only refined the dendrite structure (tended to grow parallel to the ingot axis) and the carbides, but also improved the inclusion removal efficiency and mechanical properties of H13 ESR ingots.

Comparative investigation on ingot evolution and product quality under different arc distributions during vacuum arc remelting process

The arc characteristics especially the arc distribution of the vacuum arc remelting (VAR) process are closely related to the ingot evolution and consequently the final product quality. Taking segregation defects and solidification quality as the indexes to evaluate the final product quality under different arc distributions, a three-dimensional full-scale transient model for predicting segregation defects is developed in this paper. Local solidification time (LST) and secondary dendrite arm spacing (SDAS) are used to characterize the solidification quality. This model is adopted to predict channel segregation (freckle) and macrosegregation defects during VAR process of Ni-5.8 wt.%Al-15.2 wt.%Ta alloy under several typical arc distributions. The results show that the diffuse arc has the shallowest molten pool among several arc distributions. At the early stage of solidification, the solute enrichment channels formed under the Gaussian and diffuse arc are slender and concentrated, while they are relatively sparse and uneven in thickness under the rotating arc. The value of volume-averaged segregation extent is always the smallest under the diffuse arc and the largest under the Gaussian arc, with final values of 67.53% and 69.66%, respectively. The volume-averaged values of LST and SDAS are relatively the largest under the Gaussian arc, about 1599 s and 209 μm, respectively; and they are the smallest under the diffuse arc, about 1520 s and 206 μm, respectively. In the same process conditions, the product quality of VAR ingots prepared under the diffuse arc is the best, followed by the rotating arc, and the worst under the Gaussian arc.

Morphology tailoring of metal pool and eutectic carbides in magnetic-controlled electroslag remelted M2 high-speed steel

A transverse static magnetic field (TSMF) was superimposed during electroslag remelting (ESR) process to regulate the shape of metal pool, the eutectic carbides morphology as well as the mechanical properties of electroslag remelted M2 high-speed steel (HSS) in current study. The metal pool became shallower and flatter after applying a 65 mT TSMF. Moreover, refined eutectic carbides and solidification structure (tended to grow axial) of the ESR ingot were obtained, together with uniform/improved mechanical properties (hardness, wear). Modifications of the morphology of metal pool and grain growth angle under the TSMF were attributed to a periodic electromagnetic excitation effect on droplets transition characteristics, which changed the drop path of the droplets (single to multiple drop) and homogenized the temperature distribution in metal pool. In addition, local solidification time (LST) became shorter and the number of inclusions ((Ti, V)N–Al2O3 and (Ti, V)N) which acted as heterogeneous nuclei for eutectic carbides formation increased (the inclusions also became finer) due to TSMF, resulting in the refinement of the eutectic carbides in ESR ingots.

Role of Electrode Rotation on Improvement of Metal Pool Profile in Electroslag Remelting Process

A comprehensive transient model is developed to study the effect of electrode rotation on the evolution of metal pool profiles and the solidification quality of ESR ingots. Magnetohydrodynamic flow, heat transfer, solidification, and electrode melting are considered simultaneously in the model. The growth of the ESR ingot is predicted using the dynamic layering method. The numerical results show that the productivity reaches a maximum of 15.97% at the rotating speed of 40 rpm without increasing power. With the increasing rotating speed, the maximum temperature of the melt decreases, and the temperature distribution becomes more uniform. Compared with the static one, the pool profiles are flattened by −1.19%, −8.52%, and 12.44% at the rotating speeds of 20, 40, and 60 rpm, respectively. The metal pool profile was improved remarkably, but only at the higher speed (i.e., 60 rpm). The effect of rotating speed on the metal pool profile depends on the competition between the melting rate and slag temperature. Meanwhile, the local solidification time and the secondary dendrite arm spacing are slightly improved at lower rotating speeds but are significantly worse at higher rotating speeds.

Impact of microstructural properties on the crack threshold of aluminium castings

This work focuses on the technological size effect of the cast aluminium alloy EN-AC 46200. Herewith, specimens inheriting specific local cooling rates and therefore varying local dendrite arm spacings are taken from crankcases with hot isostatic pressing (HIP) post-treatment. In order to evaluate the statistically distributed microstructural properties, the dendrite arm spacing λ2 is probabilistically assessed. Thus, the λ2 values follow the lognormal distribution, detached of local solidification times. Crack propagation tests are conducted at varying load ratios, such that the impact of the microstructure by means of λ2 is evaluated. The statistically assessed investigations propose the intrinsic crack threshold to be independent of microstructural properties, in opposite to the long crack threshold which is significantly affected by varying λ2 values due to enhanced crack closure effects. The evaluated cyclic R-curve results can be generalized into a microstructural independent manner. Moreover, the load ratio effect and its convolution with microstructural properties is evaluated based on the Newman approach. The linear extrapolation of the newly raised three-dimensional Newman approach, considering arbitrary λ2 values as technological effect, is validated by literature data and the proposed stress intensity factor threshold course is revealed to be slightly conservative by about twelve percent on average.

Micro-segregation Study of Directional Solidified Fe–Mn–C–Al TWIP Steels

Chemical variation occurring during the solidification process can cause undesirable effects on the material property, especially for alloys with high solute content. The current study investigated the distributions of solute elements in directional solidified Fe–Mn–C–Al TWIP steels and compared the micro-segregation with various pulling rates and C contents using the phase field model and the grid analysis based on the data performed by EPMA, which could provide advice for mitigating micro-segregation in high Mn steel. The results indicated that C and Mn preferentially segregated toward the liquid phase, while Al entered the solid phase. The occurrence of solid-state diffusion was observed, which led to a homogenization effect in the solid phase during solidification. The pulling speed increase made a more severe micro-segregation because the change of pulling speeds had more effect on the SDAS than on the local solidification time; thus, the dimensionless diffusion time decreased with increasing pulling rates, representing a more serious micro-segregation. For a C content increasing from 0.06 to 0.68 wt pct, the micro-segregation level of C and Al reduced first and then increased, which was caused by the change in the solidification mode. The continuous accumulation of solute elements and the slower solute element diffusion in austenite in combination led to the presence of more severe micro-segregation in the case of one-phase solidification. Besides, a C content increase could attract Mn, resulting in more serious Mn micro-segregation.

The dependency of the microhardnes on microstructure and solidification parameters of directionally solidified Al–4.5wt.%Cu in clay mold

Improvement of material properties is achieved by controlling parameters involved in the solidification process; therefore, understanding them and their implication are essential. This work investigated the dependency of solidification parameters (cooling rate (TR), growth rate (VL), local solidification time (tSL), temperature gradient (G)), microstructure parameters (primary (λ1) and secondary (λ2) dendrite arm spacing), and micro-hardness values (HV) of Al-4.5wt.%Cu in the clay mold. The samples were directionally solidified in Bridgman vertical apparatus and the temperature is recorded during the cooling. The solidification parameters were obtained from the cooling curve. The microstructures and micro-hardness were characterized using an optical microscope and micro-hardness tester. The microstructure parameters were measured and plotted as functions of solidification parameters using linear regression. The relation between HV and microstructure parameters are analyzed. The results show the λ1 and λ2 change inversely with solidification parameters except for tSL. Comparison to other works shows the exponent values of solidification parameters of the clay mold are lower than that of the carbon and stainless-steel mold. The exponent value of λ2 in the clay mold is -0.183, close to the value in the graphite mold. The clay has the potential as mold material since it characteristic close to the graphite.

In Situ Observation of the MnS Precipitation Behavior in High-Sulfur Microalloyed Steel Under Different Cooling Rates

In situ observation of the precipitation behavior of MnS in a high-sulfur microalloyed steel has been conducted by means of a confocal laser scanning microscopy (CLSM) with cooling rates increased from 50 to 400 °C/min. Differential scanning calorimetry analysis and thermodynamic calculation were carried out prior to and following the CLSM experiments using scanning electron microscopy (SEM) and X-ray energy-dispersive spectrometer. The results suggested that the initial MnS precipitate temperature decreased from 1440.0 °C to 1429.6 °C and the final temperature reduced from 1413.7 °C to 1381.6 °C as the cooling rate increased from 50 to 400 °C/min in the CLSM test. Three typical MnS morphologies of small angular, globular and larger dendrite cluster MnS were observed in the melting surface. Furthermore, with an increase in the supersaturation of MnS and a significant reduction of the local solidification time when the cooling rate increased from 50 to 400 °C/min, the large clusters MnS at interdendritic regions became thinner and more small angular MnS precipitates formed at dendrite crystals.

Effect of Melting Rate of Electroslag Rapid Remelting on the Microstructure and Carbides in a Hot Work Tool Steel

The microstructure and primary carbides in the steel billets produced by an industrial-scale electroslag rapid remelting (ESRR) at different melting rates were studied. The amount and size of primary carbides MC and M2C at the center of the remelted ingots is larger than that at the mid-radius of the remelted ingots. The amount of primary carbides and secondary dendrite arm spacing of the ingot increase with the increase in the melting rates of ESRR, caused by the increase in the local solidification time. The microsegregation of Mo in the remelted ingots after annealing is most serious among the carbide-forming elements. The microsegregation of Mo and Cr increases linearly with increasing the melting rate, and the microsegregation of V keeps nearly constant with further increasing the melting rate from 400 to 500 kg/h. Increasing the melting rate of ESRR do not change the types of primary carbides until 500 kg/h, at which another type of primary carbide M7C3 is formed. The amount of the primary carbides and the microsegregation degree of Mo, V and Cr in the ESRR billets at the melting rate of 400 kg/h is nearly same as that produced by same-scale conventional ESR.