Solidification Process Of Alloys Research Articles

New Analyzing Approaches for In Situ Interdiffusion Experiments to Determine Concentration-Dependent Diffusion Coefficients in Liquid Al–Au

Interdiffusion coefficients are key parameters for the solidification process of liquid alloys. However, the determination of interdiffusion coefficients in liquid metals at high temperatures is a challenging and extensive task, due to a variety of potential systematic errors. In recent years we have developed an X-ray in situ shear cell method for the measurement of interdiffusion coefficients in binary metallic melts. This technique enables the monitoring of the experiment in order to exclude fatal errors. Utilizing X-ray contrast, the method also provides a time-resolved concentration distribution. Such an in situ data set contains significantly more information than ex situ evaluated experiments. Available analyzing strategies do not fully exploit this potential yet. We present three new analyzing approaches that are able to retrieve a concentration-dependent interdiffusion coefficient from only one in situ data set. In that way, larger concentration differences become accessible for an experiment, which considerably decreases efforts. Using simulations, the approaches are checked for robustness. Furthermore, the approaches are run on real in situ data from a binary (0 to 9 at% Au-content) Al–Au alloy at 1000 °C which results in a concentration-dependent interdiffusion coefficient within the measured concentration range.

Effects of electromagnetic pulse treatment on spinodal decomposed microstructure, mechanical and corrosion properties of AlCoCrFeNi high entropy alloy

Previous application of electromagnetic pulse (EMP) treatment during alloy solidification process provided a potential tool box for controlling alloys’ properties. In this work, EMP treatment was firstly applied during the preparation process of AlCoCrFeNi high entropy alloy (HEA), and its effects as well as mechanism were discussed. The crystal structure, microstructure, mechanical properties and corrosion properties of AlCoCrFeNi in current and perpendicular directions were characterized. The results demonstrated that EMP treatment can alter spinodal decomposition process, improve the plasticity and pitting corrosion resistance of AlCoCrFeNi HEA. The wall thickness of spinodal decomposed microstructure enlarges from 293.5 nm to 446.5 nm with EMP frequency increasing. Meanwhile, the compression ratio increases by 18.0% with sacrificing only 1.1% fracture strength. Pitting potential increased from −0.24–0.14 V in electric direction, while it increased form −0.26–0.05 V in magnetizing direction, which revealed that EMP treated samples have better pitting corrosion resistance than untreated state. In addition, alloy’s original anisotropy differences were reversed and altered with increasing EMP frequency.

The effect of HCP on the formation of twin boundaries and dislocations in Ni–Co alloys

In this study, molecular dynamics (MD) was used to simulate the rapid solidification process of Ni47Co53 and Ni48Co52 alloys at a cooling rate of 1012 K/s. The effects of HCP on the formation of twin boundaries and dislocations in two Ni–Co alloys are studied. It is found that the difference of HCP clusters is the main effect that producing discrepancies on microstructure of two alloys. The number of HCP clusters accounted for 9.23% in Ni47Co53 alloy. They are regularly arranged to form the number of single-layer twin boundaries, and each twin boundary ends in a dislocation. The FCC and HCP structures coexist in the same atomic layers, which is easy to create dislocations. The relatively standard FCC crystal and only 0.32% HCP clusters are formed in Ni48Co52 alloy at 300 K. That small amount of HCP clusters are dispersed on the surface, and cause the formation of dislocation in the border with FCC clusters.

Simulation of dendrite collision behavior based on cellular automata-lattice Boltzmann model

In the alloy solidification process, the movement behavior of free dendrites in the melt is one of the key issues in studying the formation process of the alloy solidification structure. The cellular automata-lattice Boltzmann coupling model has become a main numerical model for numerical simulation of solidification microstructure in recent years. In this paper, cellular automata and lattice Boltzmann model for simulating dendrite growth are improved to simulate the movement of equiaxed grains in undercooled melt. In the improved model, the Galilean invariant momentum exchange method is used to calculate the fluid force, the motion equation of the center of mass is solved to calculate the motion displacement of the dendrite, the motion of the dendrite is realized through the dynamic mesh technology, and the rigid sphere model is used to deal with the collision of the dendrite. The settlement of a single dendrite in the undercooled melt of Al-4.7% Cu alloy, the settlement of two round particles in the Newtonian fluid, and the elastic collision of two dendrites are all simulated by this model. The simulation results show that this model can maintain the morphology of dendrites well in the process of calculating dendrite growth motion, and this model can calculate the collision process of irregular dendrites. The movement of dendrite disturbs the melt around it, resulting in a significant change in the concentration of melt around it, which affects the growth of dendrite and aggravates the asymmetry of dendrite growth.

A Monte Carlo approach to simulate dendritic microstructures during binary alloy solidification

A Monte Carlo (MC) approach to model the evolution of microstructures during the binary alloy solidification process has been presented. The evolution of these microstructures is comprehensively modelled by ensuring a local minimization of free energy taking into account the contributions of the free energy change during phase transformation, solid-liquid interfacial energy, interfacial energy anisotropy and grain boundary energy, which are some of the ignored aspects of the existing MC models. Further, the effect of imposed orientation is also represented in this model. The proposed method is able to simulate the preferential growth of cubic structures in ⟨100⟩ direction, growth of large number of grains and competition among them, and also incorporates the grain boundary energetics. The model is applied to simulate the growth of free and constrained dendrites, the transition from stable to cellular and dendritic growth under different growth conditions. Growth of multiple grains, the competition during columnar growth, columnar to equiaxed transition by seeding the undercooled melt with solid phase and the impingement of equiaxed grains has also been simulated using this model.

Cellular automaton modelling of M7C3 carbide growth during solidification of Fe-C-Cr alloy

A microscopic cellular automaton combined with macroscopic heat and solute transport was developed to simulate the mutual growth and evolution of austenite and M7C3 carbide grains during Fe-C-Cr ternary alloy solidification process. The diffusion of solute C and Cr are contributed together to the constitutional undercooling, together with curvature undercooling, for obtaining the grain growth rate of austenite and M7C3 carbide. Results show that, the absorption of solute C and Cr by M7C3 grain and rejection by austenite grain promote the two grains’ cooperative growth. Once they approach to each other, the austenite grain quickly overgrows towards the M7C3 grain till finally envelops it. The simulated solidification morphology of the Fe-4wt%C-17wt%Cr alloy, predicted averaged grain size of M7C3 carbides and C and Cr concentration in austenite grains agree with the experimental measurements and solidification path prediction. The predicted average liquid concentration curve fits with the LR, GS and PE prediction at the initial M7C3 precipitation regime and after austenite grains fully enveloping towards the M7C3 grains, returns to overlap the LR, GS and PE prediction curves.

Hereditary effect of the charge structure on Al-Si-Cu alloy density, gas content and solidification processes

The paper provides the results obtained when studying the initial charge structure effect on the density, gas content and temperature-time parameters of AK6M2 (Al-6%Si-2%Cu) alloy solidification. Coarse crystalline charge billets ( C -charge) were obtained when pouring the melt into ceramic molds with sand filling providing a cooling rate υ охл ~0.5÷ 1.0 °C/s. Finely crystalline charge billets ( F -charge) were prepared by pouring the melt to cold cast iron molds ( υ охл ~ 5÷ 10 °C/s). Charge billets obtained were separately remelted with the same temperature-time conditions, rerefined and degassed with samples taken to determine hydrogen content as well as density values in liquid and solid states. It is established that structural information inherited from the initial charge billets is stable in the solid-liquid-solid system. It was found using direct thermal analysis that the melt obtained from C -charge solidifies with a reduction in liquidus temperature by 3 °С and in the temperatures of the beginning and end of eutectic solidification by 10 °C and 3 °С, respectively, as compared to the melt obtained from C -charge. At the same time, α-Al and eutectic dendrites form in the C -charge melt 0.4 and 0.6 min faster, respectively. The results obtained at the Paraboloid-4 unit showed that the F -charge melt has higher density as compared to the C -charge alloy due to a smaller number of pulses passing through it in the studied temperature range of 750 — 450 °C. Temperature values of aluminum and eutectic dendrite formation determined by J—t temperature relationships (where J is the number of γ pulses, t is the temperature) correlate with the results of direct thermal analysis. General practical recommendations on the purposeful conservation of positive structural information in aluminum alloys are formulated in terms of the structural heredity phenomenon.

Determination of atomic-scale structure and compressive behavior of solidified AlxCrCoFeCuNi high entropy alloys

The atomic configurations play a key role in predicting the solidification process of high-entropy alloys (HEAs). The atomic scale structures of AlxCrCoFeCuNi (x = 0.5, 1.5, 3.0) HEAs that emerge during solidification with a cooling rate of 12 × 109 (K/s) are evaluated using molecular dynamics (MD) simulation. While BCC (body-centered cubic) structure is obtained for Al0.5CrCoFeCuNi and Al1.5CrCoFeCuNi where lattice distortion increases with increasing aluminum fraction from x = 0.5 to x = 1.5, for Al3.0CrCoFeCuNi, an amorphous structure is formed under the same cooling rate. The diffusion coefficient of all the elements at 2200 K and the super-heating temperature of each alloy are evaluated to explain the disordering mechanism due to aluminum addition, which affects both the aluminum mobility and diffusion of the constituent atoms in the HEA. Finally, the compression behavior of all the three HEAs was studied to show the effect of crystalline structure on the stress fluctuation. It was found that phase transformation induced plasticity occurred which led to a secondary hardening of crystalline alloys after ultimate compressive strength (UCS).

Solidification process and hot tearing behaviors of AZ series magnesium alloys

In the present work, the solidification process and hot tearing behaviors of AZ series magnesium alloys were investigated. The solidification process was analyzed by differential thermal analysis (DTA), and the hot tearing behaviors were studied by a home-made T-shaped mold. The Clyne and Davis’ model was modified to suit for the prediction of the hot tearing susceptibility (HTS) of ternary alloys. The results showed that the predicted HTS using the modified CSC’ are consistent with the experimental tested hot crack volume for AZ series alloys. The HTS of the alloys are, CSC’ (AZ31) > CSC’ (AZ61) > CSC’ (AZ91). With the increasing of Al, the content of Mg17Al12 phase increased and the temperature of coherent point decreased. The Mg17Al12 phase significantly influenced the later stage refilling during the solidification process.

Microstructure and Mechanical Properties of Ni-based Superalloy K418 Produced by the Continuous Unidirectional Solidification Process

Adopting effective strategies to control the solidification structure of Ni-based superalloys is a very interesting subject for metallurgists. Despite the achievement of developing and applying the continuous unidirectional solidification process in simple alloys, the utilization of this process for K418 alloys has been ignored. The microstructure and mechanical properties of a K418 alloy ingot produced by the continuous unidirectional solidification process were investigated. We found that the γ dendrites were typically cross-shaped in the transverse section. The orientation of surviving grains along the casting direction was during competitive growth. The secondary dendrite arm spacing was 32.3 ± 2.5 μm at a cooling rate of 1.75 ± 0.35 °C/s from the surface to the center of the K418 alloy ingot. Due to the higher cooling rate than that of the conventional casting process, a more uniform microstructure and finer γ′ precipitation were obtained in the ingot. Thus, compared with the conventional casting, the tensile strength and the elongation are increased by 8.4 and 21.3%, respectively, at 25 °C. The tensile strength and elongation increased by 15.2 and 49.3%, respectively, at 800 °C. In addition, the fracture surfaces exhibited numerous typical dimples and dendritic fracture characteristics.

Effect of Diffusion Length in Modeling of Equiaxed Dendritic Solidification under Buoyancy Flow in a Configuration of Hebditch–Hunt Experiment

Modeling of equiaxed solidification is vital for understanding the solidification process of metallic alloys. In this work, an extended literature review is given for the models currently used for equiaxed solidification simulations. Based on this analysis, we present a three-phase multiscale equiaxed solidification model in which some approximations regarding solute transport at microscopic scale are put together in a new way and incorporated into macroscopic transport equations. For the latter, a term relating to the momentum exchange between the two phases is revised, and a modification for the grain packing algorithm is proposed. A modernized model for equiaxed dendrite growth is tested using a case of solidification of Sn-5 wt pct Pb alloy in a parallelepiped cavity that mimics the Hebditch–Hunt experiment. The results obtained using two approaches to calculate diffusion length are presented and compared both with each other and with numerical results from elsewhere. It is demonstrated that diffusion length has a crucial effect on the final segregation pattern.

Enhanced Tensile Plasticity in Ultrafine Lamellar Eutectic Al-CuBased Composites with α-Al Dendrites Prepared by Progressive Solidification

In this paper, a new class of Al-Cubased composites which combine the ultrafine lamellar eutectic matrix (α-Al + θ-Al2Cu) and micron-sized primary α-Al dendrites was prepared by progressive solidification. By adjusting the alloy composition and solidification process, the formation of favorable microstructural and micromechanical features can be achieved. The ultrafine lamellar eutectic composite Al94Cu6 exhibits excellent mechanical properties with 472 MPa fracture strength and 7.4% tensile plastic strain. The plasticity of the ultrafine lamellar eutectic composite relies on the volume fraction and work hardening ability of micron-scale primary phase. The present results provide a new perspective for improving the plasticity of the ultrafine lamellar eutectic alloys.

Effect of Pressure on Microstructure and Cooling Curves of A356 Aluminum Alloy During Solidification

During solidification process of metals and alloys in die casting, the cooling rate of molten material and also freezing time are affected by the heat transfer coefficient of metal/mold interface which is not constant at whole casting time. Therefore, determination of the cooling curves and the heat transfer coefficient is a crucial step in optimization of casting process by simulation because the resultant microstructure of cast components is affected by cooling rate. In the present study, the effect of applied pressure on cooling curves and microstructures of A356 aluminum alloy was investigated using a specially designed apparatus. Results showed that the cooling curves obtained from the central axis of cylindrical sample clearly represent the solidus and liquidus temperatures which can be used for determination of freezing time. It was observed that the solidification time decreases with increasing applied pressure. Also, the side thermocouples showed a temperature drop at the moment of applying pressure in which its magnitude increases with the increase in applied pressure. Moreover, the metallographic examinations showed that the microstructure of as-cast samples is refined with the increase in pressure. Also, the heat transfer coefficient obtained from experiments was used in the simulation of casting process and the simulated cooling curves were matched carefully with the experimental curves.

Numerical Modelling of Transport Phenomena and Macrosegregation during Ternary Alloy Solidification: Solutal Undercooling Effects

In this paper, a macroscopic mathematical model is developed for simulation of transport phenomena during ternary alloy solidification processes, taking into account non-equilibrium effects due to solutal undercooling. The model is based on a fixed-grid, enthalpy-based, control volume approach. Microscopic features pertaining to non-equilibrium effects on account of solutal undercooling are incorporated through a novel formulation of a modified partition coefficient. The effective partition coefficient is numerically modeled by means of macroscopic parameters related to the solidifying domain. Numerical simulations are performed for ternary steel alloy by employing the present model and the resulting convection and macrosegregation patterns are analyzed. It is observed that the consideration of non-equilibrium solidification in the present mathematical approach is able to capture the thermo-solutal convection and leads to prediction of accurate value of macrosegregation. The results from the present model matches well with the experimental observations published in the literature.

Investigation on the intermetallic compound Co3Ta and high-temperature phase equilibria in the Co[sbnd]Ni[sbnd]Ta system

The crystal structure of Co3Ta was identified to be of the BaPb3 type having hexagonal R3¯m symmetry and Pearson symbol hR36 on the basis of X-ray diffraction data. The solidification process of the as-cast CoNiTa alloys and phase equilibria at 1473 K of the annealed CoNiTa alloys were studied by using scanning electron microscopy (SEM) with energy dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD) and differential thermal analysis (DTA) techniques. The liquidus surface projection and isothermal section at 1473 K of the CoNiTa system were first established. The liquidus surface projection consisted of 11 primary solidification regions and 9 ternary invariant reactions, and the isothermal section at 1473 K was made up of 8 three-phase regions and 17 two-phase regions. The addition of nickel can stabilize Co3Ta such that the peritectic temperature is at least 1594 K in the CoNiTa system, which is higher than the peritectoid one at 1306 K in the CoTa system. The present experimental results could be used as guidance to cobalt-based superalloys and offer potential candidates for cobalt/nickel intermetallic compounds.

Numerical model of solidification process of Fe-C alloy taking into account the phenomenon of shrinkage cavity formation

Mathematical and numerical models of Fe-C alloy solidification process based on the finite element method (FEM) are presented in the paper. The phenomenon of the defect called "shrinkage cavity" is introduced to the model. It is very important aspect of presented work which makes possible the prediction of the location and shape of mentioned defect depending on the geometry of the casting and cooling condition of the process. An original computer program using Visual C++ environment has been written in order to simulate described process. The results of computer calculations of the three-dimensional solidification problem of the casting with riser are presented and discussed in details.

The influence of the riser dimensions on the effectiveness of feeding solidifying cast elements

The mathematical model and numerical simulation results of metals alloy solidification process based on the finite element method (FEM) are presented in this paper. After completion of the mould filling process the main solidified of molten metal takes place and its shrinkage. The phenomenon of casting shrinkage cannot be avoided. However, it is possible to minimize the occurrence of its negative effects on the casting quality. The phenomenon of shrinkage defects formation was included in the numerical calculations and they were tried to reduce them. It is important aspect of this work which makes possible the prediction of the location of the casting defect depending on shape of the riser. The results of computer calculations of the solidification process of the three-dimensional casting together with the conical or cylindrical riser are discussed in detail and presented.

Examination of Dendritic Growth During Solidification of Ternary Alloys via a Novel Quantitative 3D Cellular Automaton Model

A three-dimensional (3D) quantitative cellular automaton (CA) model was developed to simulate dendritic growth during solidification processing of ternary alloys. A detailed method was proposed to solve solute diffusion and calculate the solid fraction for ternary alloys during solidification. The present model has been shown to accurately predict dendrite morphologies and solute distributions of both single equiaxed dendrite and columnar dendrites during solidification. The model was also used to study the influence of the concentration of a third component (Mg in the Al-Si-Mg system) on dendritic growth. With increasing Mg concentration, the steady-state dendrite tip growth velocity was shown to decrease, resulting in a shorter primary dendrite length. The 3D CA simulation results agree well with the prediction of the LGK theoretical model. Multi-columnar dendrite growth was simulated with different cooling rates. Primary and secondary dendrite morphology was shown to have little variation at low cooling rates (1.0 to 2.0 K/s) and a constant undercooling. The average secondary dendrite arm spacing was shown to decrease with the increase of cooling rate within this range. The 3D CA simulation results are in good agreement with the experimental directional solidification experiments of a commercial A356 (Al-7 wt pct Si-0.3 wt pct Mg).

Kesan Pembentukan Tikso terhadap Mikrostruktur dan Sifat Mekanik Aloi Al-Si-Cu-Mg

Forming of metals in their semi-solid state is one of the options for producing near net shape parts. It can produce a strong and highly reliable product. Al-Si-Cu-Mg casting alloy is commonly used in automotive engine components which is now widely used in the semi-solid processing. The main physical properties of the aluminium alloy and alloying element content such as silicon, copper and magnesium, plays an important role in determining the mechanical properties of the alloy. In this paper, the effect of the formation process of Al-Si-Cu-Mg permanent die casting alloy and the thixoforming process has been investigated towards the evolution of microstructure and mechanical properties. Compound phases, shapes and morphology of the two processes were compared. The porosity effect was examined at permanent die castings and thixoforming process. The result of image analysis showed significant change of the thixoforming process of α-aluminum solid phase structure in spherical shape. Size and morphology, as well as the uniformity of the silicon eutectic phases and intermetallic compound distributions of Al2Cu, Al5Fe Si and Al15(Mn,Fe)3Si2 changed due to the compression during thixoforming process. Thixoforming process of the Al-Si-Cu-Mg alloy exhibit the lowest percentage and microporosity area thus proved that porosity issue significantly reduced during the aluminium alloy solidification process in thixoforming process . The tensile properties of thixoformed alloy improved significantly with its ultimate tensile strength, yield strength, and elongation to fracture at 193MPa, 163MPa and 1.6%,respectively.

Experimental investigation of the effect of travelling magnetic field on the CET in Sn-10wt.%Pb alloy

In order to eliminate internal and surface defects, which appear in the ingots during solidification process of metallic alloys, homogeneous and fine structures are often required. Therefore, when the adjunction of inoculant substances is not possible, the electromagnetic stirring (EMS) process thanks to the use of travelling magnetic fields has technically shown its effectiveness. The present experiments consist to create convective movements in a Sn-10wt.%Pb liquid metal, by the action of an electromagnetic field during its solidification. A significant improvement in the columnar-equiaxed transition (CET) mechanism has been observed and consequently a fine equiaxed grain structure can then be detected. The main goal of this experimental study is to examine the effect of the EMS process on grain structures for that alloy as well as the corresponding segregations. In particular, it included the determination of the optimal experimental parameters to obtain a grain refinement, the characterization of their effect on the solidification and the understanding of the physical mechanisms driving grain refinement.