Water-cooled Copper Mold Research Articles

Effect of homogenization treatment on microstructure and properties of novel Al-Mg-Zn-Sc-Zr alloys with different Mg/Zn ratios

This study investigated the effect of homogenization treatment on microstructure and properties of Al-Mg-Zn-Sc-Zr alloys with different Mg/Zn ratios using optical microscope (OM), scanning electron microscope (SEM), X-ray diffraction (XRD), differential scanning calorimetry (DSC), electron backscatter diffraction (EBSD), transmission electron microscope (TEM), hardness, and conductivity tests. The results show severe dendrite segregation in Al-Mg-Zn-Sc-Zr ingots produced by water-cooled copper mold casting, with the as-cast microstructure mainly composed of an α-Al matrix, non-equilibrium T-Mg32(Al,Zn)49 phase, and a small amount of coarse primary Al3(Sc, Zr) phase. As Mg/Zn ratio decreases, the volume fraction of T-phase increases from 4.92 % to 5.41 %, and the grain size increases from 15.41 μm to 20.43 μm. DSC curves show the first endothermic peak at 468–475 °C, corresponding to the melting temperature of non-equilibrium second phase, setting the homogenization temperature to 460 °C. As homogenization time increases, non-equilibrium phase gradually dissolves. When Mg/Zn ratio is 1.71, the over-burning phenomenon occurs at 24 h; With the reduction of Mg/Zn ratio to 1.33, the over-burning occurred at 16 h. Two-stage homogenization significantly increased the amount of the second phase Al3(Sc, Zr) and refined grain size. After two-stage homogenization, the hardness and conductivity of the studied alloys with Mg/Zn ratio of 1.77 and 1.31 reach maximum values of 135.53HV and 144.22HV, and the conductivity is 30.10 % IACS and 28.71 % IACS, respectively.

Stress state of billet – mandrel system during production of hollow steel billet in a unit of continuous casting and deformation. Part 2

The paper presents the results of theoretical research of stress-strain state of a billet – mandrel system when producing steel hollow billets in a unit of combined continuous casting and deformation, in which the working surface of the calibrated anvils are made with a variable radius. The necessity of making the working surface of the calibrated anvils with a variable radius is substantiated and initial data for the calculations is given. The calculation results are considered along the lines of the volumetric model passing through the characteristic points of deformation centers. The authors determined the forces when the anvils compress the wall of a hollow billet and the force of pulling the hollow billet from the mold. The laws of metal axial displacements and stresses in the deformation centers during compressing the wall of a hollow billet was established at the combined process of continuous casting and deformation in the unit. Nature of the stressed state of the metal wall of a hollow billet is considered from the perspective of improving its quality. The technique studied allows to determine the stress-strain state of a mandrel when producing a hollow steel billet using such a unit. The authors provided the recommendations for reliable gripping and compression with calibrated anvils of a hollow steel billet coming from a water-cooled copper mold of the unit of combined continuous casting and deformation.

Effects of the asymmetric and oscillating turbulent melt flow on the heat transfer and solidification inside the thin slab continuous casting (TSC) mold under the applied electromagnetic brake (EMBr)

The thin slab casting (TSC) is a breakthrough near-net-shape technique for flat products accompanied by rapid casting and solidification rates. The TSC quality hinges on the turbulence, super-heat flow and growth of the solidified shell. The electromagnetic brake (EMBr) is commonly applied to control the fresh melt flow after feeding through a submerged entry nozzle (SEN). Numerical modelling is a perfect tool to investigate the multiphase phenomena in the continuous casting (CC). The presented study considers the heat transfer through the solid shell and water-cooled copper mold including the averaged thermal resistance of the slag skin and the air gap coupled with the turbulent flow and magnetohydrodynamics (MHD) model using an in-house code developed inside the open-source computational fluid dynamics (CFD) package OpenFOAM®. The model is applied to investigate different undesired asymmetric melt flow issues: (i) with the misaligned or (ii) partially blocked SEN; (iii) caused by the mean flow fluctuations with the natural frequencies; (iv) related to the oscillations of the fresh melt jets for the specific SEN designs and casting regimes. The variation of the flow pattern and superheat distribution is studied and presented for different scenarios both with and without applied EMBr.

Precipitation Kinetics of Water-Cooled Copper Mold Al-Mg-Si(-Mn, Zr) Alloy during Aging

The aging precipitation behavior of 6061 aluminum alloy that underwent iron casting and water-cooled copper casting and 6061 aluminum with Mn and Zr elements added was studied. Firstly, the hardness curves, tensile properties, and fracture morphology of four aging alloys—6061 (iron mold casting), 6061 (water-cooled copper mold casting), 6061-0.15Mn-0.05Zr (iron mold casting), and 6061-0.15Mn-0.05Zr (water-cooled copper mold casting)—were studied. The results of the aging hardness curve show that the aging precipitated phase of the 6061 alloy cast with a water-cooled copper mold is dispersed. The addition of Mn increases the amount of coarse inclusion α-(AlMnFeSi) in the alloy, resulting in a decrease in the age hardening property. The addition of Zr is related to the nucleation and growth of the G.P. region in the early aging period, mainly changing the formation rate and quantity of the G.P. region, leading to the advancement of peak aging and an increase in hardness. After the G.P. region gradually transforms into the β phase, the hardness of the alloy increases with the increase in the volume fraction of the β phase. When the β″ phase is coarsened to the point where the fault line can be bypassed, the transitional metastable β′ phase begins to precipitate, and the coherent distortion around it weakens, indicating over-aging. Finally, the equilibrium phase Mg2Si is formed. The results of the tensile tests indicate that the tensile strength and yield strength of the 6061-0.15Mn-0.05Zr alloy produced by water-cooled copper casting after aging are 356 Mpa and 230 Mpa, respectively. These values are 80 MPa and 75 MPa higher, respectively, than those of the 6061 aluminum alloy produced via iron casting. However, the elongation is by 5%. The fracture morphology of the tensile sample of the aging alloy shows that dislocation slip in the alloy results in dislocation plugging, stress concentration, and the initiation of crack cleavage on the surface. The fracture of the water-cooled copper mold-casting alloy is a ductile fracture of the microporous aggregation type, and the macroscopic fracture exhibits an obvious “neck shrinkage” phenomenon. The fracture analysis is consistent with the mechanical properties. The DSC curve shows that there is no enrichment process of solute atoms during the heating process, and the aging precipitation process after homogenization is as follows: G.P. zone → β″ phase → β′ phase. The aging precipitation process of the water-cooled copper casting alloy after homogenization treatment is as follows: β″ phase → β′ phase (no precipitation in the G.P. zone was observed). The results of the differential scanning calorimetry (DSC) analysis show that the main strengthening phase in the experimental alloy system is the β″ phase. The activation energies for the β″ phase precipitation were calculated and found to be 147 KJ/mol, 217 KJ/mol, 185 KJ/mol, and 235 KJ/mol, respectively. Additionally, a kinetic equation for the β″ phase precipitation during alloy aging was fitted.

Effects of Cu and Ag Elements on Corrosion Resistance of Dual-Phase Fe-Based Medium-Entropy Alloys.

The effect of adding elements to promote phase separation on the functional properties of medium-entropy alloys has rarely been reported. In this paper, medium-entropy alloys with dual FCC phases were prepared by adding Cu and Ag elements, which exhibited a positive mixing enthalpy with Fe. Dual-phase Fe-based medium-entropy alloys were fabricated via water-cooled copper crucible magnetic levitation melting and copper mold suction casting. The effects of Cu and Ag elements microalloying on the microstructure and corrosion resistance of a medium-entropy alloy were studied, and an optimal composition was defined. The results show that Cu and Ag elements were enriched between the dendrites and precipitated an FCC2 phase on the FCC1 matrix. During electrochemical corrosion under PBS solutions, Cu and Ag elements formed an oxide layer on the alloy's surface, which prevented the matrix atoms from diffusing. With an increase in Cu and Ag content, the corrosion potential and the arc radius of capacitive resistance increased, while the corrosion current density decreased, indicating that corrosion resistance improved. The corrosion current density of (Fe63.3Mn14Si9.1Cr9.8C3.8)94Cu3Ag3 in PBS solution was as high as 1.357 × 10-8 A·cm-2.

Observations and simulations for phase separation process of immiscible Fe50Sn50 alloy droplets placed on a chilling surface

Liquid phase separation and microstructure evolution mechanisms of immiscible Fe50Sn50 alloy droplets placed on a chilling surface have been investigated by both arc melting experiments and lattice-Boltzmann simulations. The pattern selection of phase-separated morphologies strongly depended on the sample diameter. With the reduction in sample size, phase separation time was shortened, two-layer macrosegregation morphologies first transformed into intermediate phase-separated morphologies and finally changed into a homogeneously dispersed microstructure. The farther away from the sample bottom the longer the phase separation time and the larger the Sn-rich phase. A heat transfer equation was proposed to analyze the temperature field characteristics of Fe50Sn50 alloy droplets placed on the surface of a water-cooled copper mold. Theoretical analyses revealed that the sample gradually cooled from the bottom to the top of the sample. There existed a relatively high temperature gradient of several hundred Kelvins per millimeter between the top and the bottom of the arc melted sample, which induced the occurrence of an extremely intense Marangoni convection. The smaller the sample size the larger the temperature gradient and the greater deviation-degree of Fe-rich zone in two-layer macrosegregation structure from the bottom of the sample. The simulated two-layer phase-separated morphology agreed well with the experimental observations. Numerical simulations demonstrated that the effect of Marangoni convection on the phase separation was significantly stronger than the Stokes motion, and Fe-rich globules with a larger density tended to move upward to form a Fe-rich zone deviating from the bottom of the sample.

On modelling conjugated heat transfer in the thin slab CC mold and solid shell formation under the applied EMBr

Continuous casting (CC) became one of the dominant steel production technologies throughout last decades. Better quality, energy savings and high production rates are the main aims of the research especially in the field of the thin slab casting (TSC). The electromagnetic brake (EMBr) is applied to control the highly turbulent flow after the fresh melt is fed through the ports of a submerged entry nozzle (SEN). The numerical modelling is a perfect tool to investigate the multiphase phenomena of the turbulent flow in the CC mold, heat transfer and solidification coupled with the effects of the magnetohydrodynamics (MHD). Traditionally the heat transfer in the CC mold during the numerical simulations is predefined by the heat flux profile which could be taken from the plant measurements, published data, or is described by the semi-empirical formulas. In all these cases the heat extraction in the CC mold cavity is strictly predefined and is not significantly influenced by the transient flow behavior. Moreover, the heat flux, used in a simulation, is frequently measured for the different flow pattern inside the mold. That is especially important when the EMBr effects on the solid shell formation are investigated. Thereby, the presented study considers the coupled heat transfer in the water-cooled copper mold, including the averaged thermal resistance between the slab and mold, implemented using OpenFOAM® open-source CFD software. The melt flow, the temperature field, and the induced electric current density are compared between the traditional approach (the applied heat flux) and the modelled heat transfer in the TSC mold. Different scenarios are studied without and with the applied magnetic field.

Study on the morphology and distribution of Fe phase in solidification structure of Cu-15Fe alloy ingots

The realization of high strength of copper-iron (Cu-Fe) alloy is related to its solidification structure. The morphology and distribution characteristics of Fe phase in the solidification structure of Cu-15Fe alloy ingot were analyzed, and the deformation strength of the alloy was compared. The results show that the cooling conditions can affect the size, morphology and distribution of Fe phase in the solidified structure. The average distribution uniformity in the water-cooled copper mold ingot (W-ingot) is 0.45% higher than that in the quartz mold ingot (Q-ingot). The distribution quality of Fe phase in solidification structure can be evaluated by fractal dimension and average Fe phase area. The larger the fractal dimension is, the smaller the average Fe phase area is, where Fe phase is smaller and more uniform in the corresponding region. In the experiment, the strength of the strip increased from 510 to 547 Mpa corresponds to the Q-ingot and the W-ingot.

The effect of Ta addition on mechanical properties of Zr-based bulk metallic glasses

Bulk metallic glasses (BMGs) exhibit poor room temperature plasticity, so improving their room temperature plasticity is important for amorphous alloys in engineering applications. In this paper, Zr60Cu(20-x)Al10Ni10Tax (x = 0, 1, 3, 5) bulk metallic glasses with 3 mm diameter rods were prepared by water-cooled copper mold suction casting, and investigated the effect of minor additions of Ta on the properties of Zr–Cu–Ni–Al alloys, especially on plasticity. The results show that the Zr-based alloys with a 3 mm diameter are amorphous. With the addition of Ta elements, the glass transition temperature hardly changed, the crystallization onset temperature increased from 747 K to 767 K, the width of the supercooled liquid phase zone increased from 49 K to 68 K, and the thermal stability of the alloy was improved. The room temperature uniaxial compression test found that with the increase of Ta content, the strength and plasticity of alloys showed a trend of first increasing and then decreasing, when the Ta content was 1 at.%, more interactive shear bands were observed on the side surface of the alloy after compression, while the fracture surface exhibited a complete vein pattern, and fracture strength and plasticity of samples reached to 1920 MPa and 13.8%, respectively. The results of nanoindentation tests indicated that when the Ta content was 1 at.%, the alloy exhibited the maximum indentation hardness and elastic modulus of 8.03 GPa and 101.48 GPa, respectively. It can be concluded that the right amount of Ta in Zr-based BMGs improves the mechanical properties, especially, the fracture strength, the plasticity, the indentation hardness, and the elastic modulus.

Corrosion behavior of bulk amorphous steels with low carbon content

The Fe-Cr-Mo-(Y, Ga)-C-B-Si alloy system was chosen to synthesize bulk amorphous steels by water-cooled copper mold casting, in form of discs with 10 mm diameter. The structure of the obtained samples was evaluated using X-ray diffraction and differential scanning calorimetry and the corrosion behavior was assessed by electrochemical polarization tests and electrochemical impedance spectroscopy in 0.5 M H2SO4 electrolyte, as well as immersion tests in 3.5 wt% NaCl solution. It was found a better corrosion performance of the elaborated alloys compared to a reference 316 L stainless steel, the corrosion rate being strongly influenced by the content of Cr, Ga, and Y.

The effect of cooling rate and degassing on microstructure and mechanical properties of cast AZ80 magnesium alloy

The effect of casting cooling rate and degassing on the mechanical properties of cast AZ80 alloy is investigated. A water-cooled copper mould with a wedge geometry is chosen for the casting, which provides different cooling rates at different sections. The casting is performed with and without the addition of hexachloroethane degassing agent. Macrohardness, hot compression, quasi-static tensile, and stress-controlled cyclic tests are performed to characterize the mechanical properties of the cast samples. The microstructure of the material is studied using optical microscopy and X-ray computed tomography methods. The connection between the microstructure and mechanical test results shows that the morphology of primary α-Mg and porosity defects affect the mechanical properties of cast AZ80. The casting cooling rate controls both of these microstructural features. Higher cooling rates result in a finer dendritic morphology of α-Mg that improves mechanical properties and creates a finer and more discrete morphology of β intermetallic particles. The fine dendritic morphology results in smaller interdendritic regions, which in turn prevents pore coalescence and negates the detrimental effect of porosity on the quasi-static tensile and cyclic properties. In lower cooling rates, where the coarse grain structure of the cast material cannot prevent pore coalescence, degassing benefits the mechanical properties, especially fatigue resistance, by reducing the volume fraction of porosity defects. Moreover, the hot deformation behavior of cast samples shows that the addition of the degassing agent and casting with a higher cooling rate improves the range of deformation conditions where cracking will not occur.

Effect of thermomechanical treatment on the microstructure evolution and mechanical properties of lightweight Ti65(AlCrNb)35 medium-entropy alloy

Lightweight Ti65(AlCrNb)35 medium-entropy alloy (Ti-65) ingots were produced through arc melting and drop casting in a water-cooled copper mold. These alloy ingots were then treated through a sequence of homogenization, hot rolling, cold rolling, and recrystallization. The effect of this thermomechanical treatment (TMT) on their microstructures and mechanical properties was investigated through X-ray diffraction (XRD), electron backscatter diffraction analysis through scanning electron microscopy, and mechanical testing. The XRD results demonstrated that the Ti-65 alloy maintained a body-centered cubic structure after 50% hot rolling, 70% cold rolling, and recrystallization at 900 °C, 1000 °C, and 1100 °C, respectively. The fully recrystallized sample had an 80% smaller grain size than the as-cast sample. The cold-rolled Ti-65 alloy specimen exhibited a high tensile strength of 1620 MPa. In comparison with the tensile strength of the as-cast Ti-65 sample (1100 MPa), that of the Ti-65 alloy following annealing and subsequent partial recrystallization was significantly increased (1380 MPa). This tensile strength enhancement is attributable to its heterostructure composition of deformed bands and smaller recrystallized grains. This study demonstrated that an effective strength–ductility synergy of the Ti-65 alloy can be achieved through various TMTs.

Electrochemical characterization of rapidly solidified Al-(Cr,Cu,Ni,Y,Zr)-Fe alloys

The aim of this work was to investigate the multiphase structure of new Al71(Cr, Cu, Ni, Y, Zr)24Fe5 and Al65(Cr, Cu, Ni, Y, Zr)20Fe15 alloys, and the influence of chemical composition on their corrosion behavior. Samples were prepared using two different cooling rates: slowly cooled (master alloys), and rapidly solidified by high-pressure die casting into a water-cooled copper mold (plates). The structure of the studied alloys were investigated by X-ray diffraction (XRD), light microscopy, and Mössbauer spectroscopy. The crystallization mechanisms are described based on differential scanning calorimetry (DSC) heating and cooling curves. Electrochemical potentiodynamic measurements were conducted in 3.5% NaCl solution at 25 °C, and electrochemical impedance spectroscopy (EIS) tests were also carried out. The alloys were characterized by a multiphase crystalline structure, except Al65Cu20Fe15 and the plate form of Al71Ni24Fe5 alloy, for which a quasicrystalline phases were identified. Moreover, the complex metallic alloy (CMA) phase was detected for Al-Cr-Fe alloys. For samples in the form of plates, structure fragmentation was found. On the basis of the experimental polarization curves, extrapolation of Tafel curves and EIS results, the best corrosion properties were found for alloys with zirconium additions, while the alloys with addition of yttrium showed the least resistance.

Effect of cooling rate on the size fluctuation of V-containing phases in Al-V master alloys

To investigate the relationship between the solidification cooling rate and the size fluctuation of different V-containing phases, Al-4 wt. % V master alloys with various V-containing constituents were prepared at different cooling rates carried out by a wedge-shaped water-cooled copper mold, a graphite mold and a refractory mold at 1050°C, respectively. The variance of lognormal density function was used to characterize the size fluctuation of V-containing phases quantitatively. The results show that the largest size difference of both Al10V phases and Al3V phases is simultaneously present in the ingot prepared by the average solidification cooling rate of 2°C·s-1, while the smallest difference of Al10V phases in size is present in the ingot prepared by the average solidification cooling rate of 271°C·s-1. The size fluctuation of the Al3V phases first increases slightly and then decreases with increasing average solidification cooling rate in the range of 30~195°C·s-1.

Effect of Al content on the mechanical properties and toughening mechanism of Zr-Co-Al alloys prepared by rapid solidification

Zr50-x/2Co50-x/2Alx (x = 0, 4, 7) alloys fabricated by water-cooled copper mold casting yields excellent mechanical properties like high strength and large plasticity. Al can reduce the stacking fault energy and simultaneously improve strength and yield strength of the alloys. With increasing Al content, the effect of grain refinement strengthening, solid solution strengthening and second phase precipitation strengthening are enhanced to a certain extent. The good plasticity of these alloys primarily originates from the dislocation entanglement at the end of dislocation pileup and the transformation-induced plasticity associated with B2-to-B33 deformation-induced martensitic transformation. Zr46.5Co46.5Al7 alloy shows the highest ultimate compressive strength of 1787 MPa, and the largest ultimate failure strain of more than 25%.

Metal/mold thermal conductance affecting ultrafine scale microstructures in aluminum eutectic alloys

Ultra-thin microstructures of eutectic aluminum alloys have attracted attention due to their excellent mechanical properties. In this context, it is known that the massive production of industrial components may require adaptations in the processing routes with severe thermal control. Considering the benefits of molding applications, knowledge of proper conditions of the metal/mold interface is essential. The interfacial heat transfer efficiency controls the solidification kinetics and so the microstructure evolution. Furthermore, much of the existing work in this field involves the use of either water-cooled or massive copper molds. The goal is achieving moderate or fast cooling conditions and produce refined structures. In this sense, the association of the interfacial heat transfer coefficient, h, with desired microstructures can expand the application to other types of mold and process conditions. In this respect, the present research work applies a numerical mathematical model based on an inverse heat conduction problem (IHCP) for the solidification of relevant binary eutectic alloys, considered as priority for the production of ultrafine eutectics. It was demonstrated the compromise existing between the overall interfacial coefficient hg and the eutectic spacing for the Al-6.3 wt% Ni, Al-33 wt% Cu, Al-wt.12% Si and Al-1.0 wt% Co eutectic alloys. Expressions relating hg as a function of time (t) and hg versus the representative eutectic microstructural spacing (λ) of each alloy are proposed. The hg vs λ expressions allow inferring hg values necessary to induce the formation of ultrafine λ values.

Isothermal and Non-isothermal Crystallization Kinetics of Mold Fluxes used in Continuous Casting of Steel: A Review

Casting powders or mold fluxes, as they are more commonly known, are used in the continuous casting of steel to prevent the steel shell from sticking to the copper mold. The powders first melt and create a pool of liquid flux above the liquid steel in the mold, and then the liquid mold fluxes penetrate into the gap between water-cooled copper mold and steel shell, where crystallization of solid phases takes place as the temperatures gradually drop. It is important to understand the crystallization behavior of these mold fluxes used in the continuous casting of steel because the crystalline phase fraction in the slag films plays a crucial role in determining the horizontal heat flux during the casting process. In this work, the existing literature on the crystallization kinetics of conventional and fluoride-free mold fluxes used in the continuous casting of steel has been reviewed. The review has been divided into two main sections viz. the isothermal crystallization kinetics and non-isothermal crystallization kinetics. Under each of these sections, three of the most widely used techniques for studying the crystallization kinetics have been included viz. thermoanalytical techniques such as differential scanning calorimetry/differential thermal analysis (DSC/DTA), the single and double hot thermocouple technique (SHTT and DHTT), and the confocal scanning laser microscopy (CSLM). For each of these techniques, the available literature related to the crystallization kinetics of mold fluxes has been summarized thereby encompassing a wide range of investigations comprising of both conventional and fluoride-free fluxes. Summaries have been included after each section with critical comments and insights by the authors. Finally, the relative merits and demerits of these methods vis-à-vis their application in studying the crystallization kinetics of mold fluxes have been discussed.

Effect of Heat Treatment on the Properties of (Fe<sub>0.6</sub>Co<sub>0.4</sub>)<sub>71</sub>Si<sub>5</sub>Nb<sub>4</sub>B<sub>20</sub> Bulk Metallic Glass

Bulk metallic glasses (BMGs) are an important class of materials with unique set of properties. A bulk metallic glass with composition of (Fe0.6Co0.4)71Nb4Si5B20 was cast in the form of a 1 mm thick strip in a water cooled copper mold. The BMG produced was characterized for structure, thermal and mechanical properties. The X-ray diffraction performed on the as cast alloy has shown completely amorphous structure. The glass transition and crystallization peak temperatures obtained through differential scanning calorimetry scan were 542 °C and 588.4 °C, respectively. Some cast amorphous alloy sample was annealed below glass transition (450 °C for 30 mi93nutes) and others above glass transition (580 °C for 5 minutes) temperatures. Nano- indentation hardness of 13.3 GPa was obtained for as cast alloy while a hardness values of 12.8 and 15.84 GPa were measured for heat treated alloys at temperature of 450 °C and 580 °C, respectively. Increase in hardness was attributed to formation of crystals in an amorphous matrix whereas decrease in hardness was due to relaxation of quenching residual stresses. The maximum value of elastic modulus obtained through indentation was 255 GPa for 580 °C heat treated sample.

Rapid cooling effect during solidification on macro- and micro-segregation of as-cast Mg–Gd alloy

The high performance of as-cast Mg-RE alloys is always related to their high RE additions. However, RE elements can be readily segregated in Mg alloys and the segregation becomes more significant with the increasing RE content. In this research, the effect of cooling rate on the macro- and micro-segregation in the as-cast Mg–8Gd alloy was studied. The Gd content at the bottom of the fabricated ingot with the cooling rate of 4.6–6.9 ​°C/s was ~1.7 times of that at the top and coarse eutectics as well as some non-equilibrium phases of α-Gd, MgGd, Mg2Gd were distributed along the grain boundaries. The formation mechanisms of the specific gravity segregation and grain boundary segregation were also proposed. Upon the application of the water-cooled copper mold with the cooling rate of 27–200 ​°C/s, only fine Mg5Gd and some nanoscale metastable β1 and β′ phases were found to disperse uniformly in the grain interior, thus the homogeneity of the composition, microstructure, and performance within the whole ingot was considerably improved. It is expected that these results will facilitate the processing design for the fabrication of the highly-homogenized Mg-RE alloy castings.

High-throughput laser fabrication of Ti-6Al-4V alloy: Part I. Numerical investigation of dynamic behavior in molten Pool

In the present work, an innovative high-throughput method was developed to fabricate a Ti-6Al-4V alloy via pulsed laser melting with water-cooled copper mold. Moreover, a three-dimensional transient model was established via numerical simulation to reveal the dynamic mechanism of heat transfer and fluid flow of the molten pool during high-throughput laser fabrication processes. Different laser powers, laser pulse durations and Marangoni coefficients were adopted to perform the numerical simulations, revealing the underlying physics of the heat transfer and fluid flow during the melting and solidification process. Special attention was paid to the cooling rate of the molten pool affected by cooling water velocity. Numerical results indicated that a “button” sample can be prepared in a short time of 200 ms by adjusting the processing parameters in this high-throughput preparation process. The maximum temperature of the molten pool was attained at the center underneath the laser beam and decreased radially outward. There was vigorous fluid flow mainly driven by thermal-capillary force in the molten pool. With increases in laser power and laser pulse duration, both the peak temperature and velocity of the molten pool increased. Accordingly, a high cooling rate was obtained under high cooling water velocity. Furthermore, the Marangoni effect fully changed the flow direction of the molten pool fluid, and the Marangoni convection flow influenced the temperature distribution of the molten pool. A new principle of the metallurgical dynamic behavior for the high-throughput laser fabrication process was established in this study and can provide technical guidance for high-throughput preparation experiments, such as laser parameter selection, structural control, and performance optimization.