Solidification Structure Formation Research Articles

Highly complex materials processes as understood by phase-field simulations: Additive manufacturing, bainitic transformation in steel and high-temperature creep of superalloys

Abstract Recent breakthroughs resolving open questions in materials science by phase-field simulations are reported. They relate to solidification structure formation in additive manufacturing, carbon redistribution during bainitic transformation, and the onset of damage during high-temperature creep of superalloys. The first example deals with the balance between epitaxial growth and nucleation in solidification. The second relates to the controversy regarding diffusion control and dominance of massive transformation in bainite transformation. The third relates to directional coarsening (rafting) in superalloys as a diffusion-controlled phase transformation: loss of coherency of precipitates marks the onset of damage associated with rotation of the crystal lattice and topological inversion. Technical details of the phase-field method are reviewed as necessary, and limitations of the approach are discussed. Graphical abstract

The influence of composite pulse current and static magnetic fields on the solidification structure formation in Pb–Zn alloys

The influence of composite pulse current and static magnetic fields on the solidification structure formation in Pb–Zn alloys

Numerical simulation on the solidification structure of Ø600mm continuous casting round bloom

A FE (Finite Element)—CA (Cellular Automation) coupling model was developed for the simulation of solidification structure formation during the Ø600mm round bloom continuous casting process of Q345E steel. The simulation result of the temperature field was consistent with the nail-shooting experimental result, and the simulated solidification structure of the bloom was in great agreement with corrosion testing under the same casting condition. The simulation results showed that the centre equiaxed crystal ratio increased slightly with the increase of secondary cooling water rate and decreased with the increase of casting temperature and casting speed. When the secondary cooling water rate was over 0.08L/kg, it had less effect on the solidification structure. As the casting temperature increased by 1°C or the casting speed increased by 0.01m/min, the centre equiaxed crystal ratio would decrease by 0.4%∼1.2% and 3%∼0.8% respectively. According to the simulation results, the optimized continuous casting process of Ø600mm round bloom should be the secondary cooling water rate of 0.08L/kg, the casting temperature of 1532°C∼1539°C and the casting speed of 0.20m/min∼0.22m/min. It was found that the solidification structure of Ø600mm Q345E steel round bloom was much improved after the optimized continuous casting process was adopted in practical production.

Numerical Simulation of Titanium Alloy Ingot Solidification Structure during VAR Process Based on Three-Dimensional CAFE Method

Abstract A multiscale three-dimensional (3D) mathematical model has been established for simulating the temperature field, the fluid flow and the solidification structure of Ti-6Al-4V alloy ingot during vacuum arc remelting (VAR) process, which consists of the macroscopic mass, the momentum and energy conservation equations and the macroscale model of the nucleation and growth of grains. On the basis of heat transfer and fluid flow calculation, the 3D solidification structure formation of the ingot during whole VAR process has been obtained. Comparing the simulation result with the experimental observation, a reasonably qualitative agreement is achieved on grain structure and grain growth pattern. In particular, when taking the heat radiation into consideration during the calculation, the columnar grains on the ingot top are predicted well. Furthermore, a sensitivity study of the effect of natural convection on the grain structure has been carried out. The results show that the natural convection has a great influence on columnar-to-equiaxed transition (CET) and grain size, expressing as promoting the CET and refining the grain.

3D Coupled Cellular Automaton (CA)–Finite Element (FE) Modeling for Solidification Grain Structures in Gas Tungsten Arc Welding (GTAW)

A coupled Cellular Automaton (CA) – Finite Element (FE) model is proposed to predict the grain structure formation during Gas Tungsten Arc Welding (GTAW). The FE model solves the heat flow problem based on an adaptive meshing. This is done on a first FE mesh. The CA model simulates the development of the envelope of the grains in the liquid. For that purpose, a second FE mesh, referred to as CA mesh, is used. Fields can be interpolated between the adaptive FE mesh and the CA mesh. A CA grid made of a regular lattice of cubic cells is defined and superimposed onto the CA mesh. A new dynamic strategy for the allocation/deallocation of the CA grid is proposed to reduce the computation and memory costs. This CAFE model is applied to partial melting of an initial grain structure and epitaxial growth in the undercooled zone of a liquid pool, thus simulating the formation of solidification structure during the GTAW process. Examples of single linear passes simulations for various processing conditions and a multiple pass simulation are presented.

Solidification behaviors of undercooled Ni-P alloys

In order to gain an insight into the primary phase selection and solidification structure formation while undercooled eutectic alloys solidify, Ni100-xPx (x=18, 19, 19.6, 20, 21, atomic percent) alloy melts are undercooled to different temperatures below the equilibrium liquidus. The recalescence behavior associated with rapid solidification is monitored by a high-speed infrared pyrometer, and the solidification structure is analyzed systematically. When α-Ni/Ni3P regular eutectic forms as the primary phase during rapid solidification, single recalescence event takes place. In the resulting anomalous eutectic, fine granular grains of α-Ni are distributed uniformly in the Ni3P matrix. When the primary phase is one of the eutectic phases, however, there is a second recalescence event following the first one, resulting from nucleation of the other phase in the remaining liquid and the subsequent rapid eutectic growth. In this case, there are two types of granular grains whose sizes are significantly different from those in the anomalous eutectics. Finally, a coupled growth zone of eutectics is determined. At large undercoolings, α-Ni rather than Ni3P solidifies as the primary phase even in the Ni-P hyper-eutectic alloys due to its rapider growth kinetics.

Numerical Simulation of Solidification Structure Formation in High Mn Steel Casting Using Cellular Automaton Method

Numerical Simulation of Solidification Structure Formation in High Mn Steel Casting Using Cellular Automaton Method

Numerical Model of Solidification Structure Formation in Fe–C Alloy with Peritectic Transformation

A numerical model was developed for the simulation of structure formation during the solidification of an Fe–C alloy with peritectic transformation. In this model, the front tracking method was used to simulate the dendrite growth of primary δ phase and subsequent peritectic transformation. Diffusion in liquid and solid, mass conservation at the solid/liquid interface and local equilibrium at the solid/liquid interface with consideration of curvature undercooling were solved to determine the positions of the solid/liquid and δ/γ interfaces. A simulation was carried out for the growth of many primary δ dendrites to simulate the formation of initial gamma grain structure. The simulated results showed that the number of the initial gamma grain depends on the nucleation mechanism of gamma phase during peritectic reaction.

Role of size and cooling rate in quenched droplet of Sn [sbnd]Bi eutectic alloy

Employing the solvent-encapsulation remelting and quenching (SERQ) process combined with the Newtonian cooling model, the droplets were quenched at different temperatures, by which the role of size and cooling rate in droplet solidification of Sn Bi eutectic alloy can be investigated separately. Undercooling is shown to be the key factor in determining the solidified microstructure evolution from “Regular + Irregular” (RI) to “Totally Irregular” (TI). Direct influence of the size and cooling rate is not observed, but the undercooling of droplets plays a decisive role in the formation of solidification structure. Under the present experimental conditions, the undercooling increases from 34.7 K to 51.5 K as the droplet size reduces from 34 μm to 20 μm, with an increment of 16.8 K. In contrast, only a negligible increment of 0.4 K is obtained as the cooling rate is enhanced from 4.08 × 10 4 K/s to 1.23 × 10 5 K/s with respective undercoolings of 51.1 K and 51.5 K. Obviously, a significant increase in undercooling can be achieved by reducing the droplet size during rapid solidification.

Numerical Simulation of Solidification Structure Formation during Continuous Casting in Fe-0.7mass%C Alloy Using Cellular Automaton Method

A numerical model was developed for the simulation of solidification structure formation during the continuous casting process of an Fe–0.7mass%C alloy. In this model, the cellular automaton method was combined with heat transfer calculation during the continuous casting process. The effect of electromagnetic stirring (EMS) during the continuous casting process was introduced as an increase in thermal conductivity in liquid. Furthermore, the effect of fragmentation of dendrites due to fluid flow during EMS was taken into account in the model as an increase in the formation of crystals. Simulations of grain structure formation in continuously cast billets with and without EMS were carried out, and the degree of fragmentation was evaluated from a comparison between experimentally observed and simulated macrostructures of the billets.

A Monte Carlo simulation of solidification structure formation under melt shearing

Semi-solid metal (SSM) processing of alloys under applied shear has a promising future for near net-shape production of components. Although non-dendritic growth of solid is critical for semi-solid metal processing, a comprehensive understanding of morphological evolution under melt shearing is lacking. Through Monte Carlo simulation a growth-based evolution of solidification structure under different melt shearing conditions is proposed in the present work. It has been demonstrated that the nature of the diffusion boundary layer determines the growth morphology of the solid under shear. In effect, the nature of fluid flow determines the morphology of the solid by influencing the geometry of the boundary layer. A generalized explanation of morphological evolution under melt shearing is presented that qualitatively corroborates the experimental observations.

Solidification structure formation in undercooled Fe–Ni alloy

The Fe alloy melts containing 7.5, 15, 22.5 and 30 at% Ni were bulk undercooled to investigate the structure evolution. When the undercooling of the four melts is lower than the critical value 110, 125, 175 and 325 K, respectively, only the stable face-centered cubic phase crystallizes. In this case a grain refinement caused by solid superheating is observed in all the alloys, but another grain refinement induced by recrystallization can merely occur in the Fe–30 at%Ni alloy undercooled by 190–325 K. Alternate crystallization of the metastable body-centered cubic phase occurs above the critical undercooling. It is indicated that the subsequent heterogeneous nucleation of the stable phase in the metastable solid and remaining liquid coexisting system is influenced not only by the morphology and surface area of the metastable solid, but also by the effective undercooling of the remaining liquid. On the basis of the experimental results and the theoretical analyses, a structure evolution map for bulk Fe–Ni system is constructed.

18Cr-8Niステンレス鋼の双ロール鋳造における凝固組織の形成

18Cr-8Niステンレス鋼の双ロール鋳造における凝固組織の形成

Formation of Solidification Structure in Twin Roll Casting Process of 18Cr-8Ni Stainless Steel.

Formation of Solidification Structure in Twin Roll Casting Process of 18Cr-8Ni Stainless Steel.

Solidification synthesis of pressure-infiltrated Al alloy 2014-SiC platelet composites

Cast Al alloy 2014-SiC platelet composites were synthesized using counter-gravity pressure-assisted liquid metal infiltration of packed beds of SiC platelets; the platelets used were stoichiometric α (hexagonal) phase with nominal dimensions of 30–500 μm diameter and 1–25 μm thickness. The composites were synthesized for a range of material and process parameters which include platelet size, surface chemistry of platelets, pressure levels and pressurization time. The microstructures of the matrix alloy solidifying in the constricted interstices between randomly oriented platelets were characterized. A theoretical discussion of the solidification structure formation in porous beds of SiC platelets is also presented.