Effect of synergistic action of ultrasonic vibration and solidification pressure on tensile properties of vacuum counter-pressure casting aluminum alloy

The effect of synergistic action of ultrasonic vibration and solidification pressure on tensile properties of vacuum counter-pressure casting ZL114 A alloys was studied systemically through testing and analyzing the tensile strength and elongation subjected to different ultrasonic powers and solidification pressures. The results indicate that the synergistic action of ultrasonic vibration and solidification pressure can result in the refinement of grains and improvement of tensile properties. Both the highest tensile strength and elongation of aluminum alloy were obtained under synergistic action of 600 W ultrasonic power and 350 kPa solidification pressure. Moreover, the tensile fracture morphology shows obvious ductile fracture characteristics. When the solidification pressure is lower than 300 kPa, the effect of ultrasonic power on tensile strength and elongation is more obvious, but when the solidification pressure is higher than 300 kPa, the effect of solidification pressure on tensile strength and elongation is greater. Meanwhile, the size and morphology of the eutectic silicon were improved significantly by the ultrasonic vibration and pressurized solidification. The strip and massive eutectic silicon phase are completely converted into small short rod-like and evenly distributed Si phases at the grain boundary of primary α-Al.     

Effect of phosphor addition on eutectic solidification and microstructure of an Al-13％Si alloy

As the refiner or modifier, the master alloys containing high concentration phosphor are widely used in preparing eutectic or hypereutectic Al-Si alloys. To study the effect of phosphor addition on the eutectic solidification and microstructure of the Al-13%Si alloy, an investigation has been undertaken by means of thermal analysis and micro/macro-structure observation. Results indicate that addition of phosphor in near eutectic Al-Si alloy promotes the nucleation of eutectic but has little refinement impact on primary Si particles as expected. Conversely, both primary Si particles and eutectic Si flakes become slightly coarser in P-rich alloys. The coarsening of eutectic Si flakes ties closely to the increased eutectic growth temperature with phosphor addition. The eutectic solidification of the alloy proceeds from the near mold zone towards the center, and it is also found that a few independent nucleation regions emerge in liquid at the solidification front due to the addition of phosphor.     

Effect of Ta on solidification characteristics and mechanical properties of DZ411 Ni-based superalloy

The effects of Ta content(2.72wt.%, 3.10wt.% and 4.00wt.%) on the solidification characteristics and mechanical properties of directionally solidified DZ411 Ni-based superalloys were investigated. It is found that the content of Mo decreases with the increase of Ta in liquid phase after directional solidification, indicating the addition of Ta can reduce the element segregation in alloys. The primary and secondary dendrite arm spacings(PDAS and SDAS) of the DZ411 alloy increase with the addition...     

Simulation of local effect of reinforcement on temperature field during solidification of aluminum metal matrix composite

The effect of reinforcement on the solidification of pure metal matrix composites （MMCs） was simulated using a two-dimensional solidification temperature field model by the finite element method. The concept of the character length was proposed to describe the size of reinforcement local heat influential zone in MMCs solidification according to the change of the morphologies of solid-liquid interface. The relationship between the character length and the geometrical conditions, the boundary condition and physical properties of the reinforcement were studied, respectively. The results show that the width of the unit and the cold boundary temperature have no effect on the character lengths but have effect on the distance between cold boundary and reinforcement （/） and the thermal parameters of the reinforcement. An experimental rule to predict the value of the character length was derived and applied.     

State Key Laboratory of Solidification Processing,Northwestern Politechnical University

The State Key Laboratory of Solidification Processing at Northwestern Polytechnical University was established in 1995, on the basis of the university＇s original foundry division, a national key discipline. The current Director of the laboratory is Professor Huang Weidong. His research teams have been pioneers in study of fundamental research of solidification, advanced solidification technology and materials preparation, which are of great importance in understanding the solidification behavior of metals and other material.     

MODELING STUDY OF CONVECTION AND CONSTITUTION VARIATION IN INGOT DURING SOLIDIFICATION

Convection and constitution variation of liquid metal during solidification has been studied us-ing NH_4Cl aqueous solution.The liquid in mushy zone attached to the side wall of the moldwill flow upward and form a low concentration region,in which no significant convection oc-curs,on the top of liquid zone.This region develops downward gradually during solidification.Addition of adequate component may suppress the formation of this region.     

Microstructures,micro-segregation and solidification path of directionally solidified Ti-45Al-5Nb alloy

To investigate the effect of solidification parameters on the solidification path and microstructure evolution of Ti-45Al-5Nb(at.%) alloy, Bridgman-type directional solidification and thermodynamics calculations were performed on the alloy. The microstructures, micro-segregation and solidification path were investigated.The results show that the β phase is the primary phase of the alloy at growth rates of 5-20 μm·s~(-1) under the temperature gradients of 15-20 K·mm~(-1), and the primary phase is transformed into an α phase at relatively higher growth rates(V 20 μm·s~(-1)). The mainly S-segregation and β-segregation can be observed in Ti-45Al-5Nb alloy at a growth rate of 10 μm·s~(-1) under a temperature gradient of 15 K·mm~(-1). The increase of temperature gradient to 20 K·mm~(-1) can eliminate β-segregation, but has no obvious effect on S-segregation. The results also show that 5 at.% Nb addition can expand the β phase region, increase the melting point of the alloy and induce the solidification path to become complicated. The equilibrium solidification path of Ti-45Al-5Nb alloy can be described as L L→β L+β L+β→αα+β_R β→ααα→γα+γα→α_2+γγ_R+(α_2+γ), in which β_R and γ_R mean the residual β and γ     

State Key Laboratory of Solidification Processing,Northwestern Polytechnical University

The State Key Laboratory of Solidification Processing at Northwestern Polytechnical University was established in 1995, on the basis of the university＇s original foundry division, a national key discipline. The current Director of the laboratory is Professor Huang Weidong. His research teams have been pioneers in study of fundamental research of solidification, advanced solidification technology and materials preparation, which are of great importance in understanding the solidification behavior of metals and other material.     

Phase-field simulation of dendritic solidification using a l＇ull threaded tree with adaptive meshing

Simulation of the microstructure evolution during solidification is greatly beneficial to the control of solidification microstructures. A phase-field method based on the full threaded tree （FTT） for the simulation of casting solidification microstructure was proposed in this paper, and the structure of the full threaded tree and the mesh refinement method was discussed. During dendritic growth in solidification, the mesh for simulation is adaptively refined at the liquid-solid interface, and coarsened in other areas. The numerical results of a three- dimension dendrite growth indicate that the phase-field method based on FTT is suitable for microstructure simulation. Most importantly, the FTT method can increase the spatial and temporal resolutions beyond the limits imposed by the available hardware compared with the conventional uniform mesh. At the simulation time of 0.03 s in this study, the computer memory used for computation is no more than 10 MB with the FTT method, while it is about 50 MB with the uniform mesh method. In addition, the proposed FTT method is more efficient in computation time when compared with the uniform mesh method. It would take about 20 h for the uniform mesh method, while only 2 h for the FTT method for computation when the solidification time is 0.17 s in this study.     

Effect of Phosphorus on Solidification Behavior of IN783 Alloy

Phosphorus plays an important role in the solidification, segregation and as-cast microstructure of superalloys. In order to control the as-cast microstructure and homogenization of IN783 alloy, the effect of phosphorus on the as-cast microstructure and the solidification sequence of IN783 alloy was investigated and analyzed. The as-cast microstructure of IN783 alloy is constituted by γ matrix mainly located in dendrite core and segregation zone, together with β phase, blocky Laves phase and P-enriched phas...     

单相合金定向凝固的特征尺度和过冷度研究

采用透明合金丁二腈（SCN）-1．48％水杨酸苯酯（Salol）（质量分数）进行恒速定向凝固实验,考察了一次间距、尖端半径和尖端温度随凝固生长速度变化的规律．通过考察平界面失稳孕育时间和失稳时界面速度的关系,获得了平界面失稳的临界速度．实验发现,随着生长速度的增大,一次间距先减小到一个极小值然后又增大到一个极大值,随后逐渐下降,呈现S形的变化趋势,并且在从极小值增大到极大值区间发生了胞／枝转变．将实验结果与经典的BHL模型以及自洽数值模型进行了比较,发现BHL模型所预言的尖端温度与实验结果存在较大偏差,而所有实验结果与自洽模型吻合较好．     

A model of solidification microstructures in nickel-based superalloys: predicting primary dendrite spacing selection

A combined cellular automaton-finite difference (CA-FD) model has been developed to simulate solute diffusion controlled solidification of binary alloys. Constitutional and curvature undercooling were both solved to determine the growth velocity of the solid/liquid interface. A modified decentered square/octahedron (in two or three dimensions) growth technique was implemented in the cellular automaton to account for the effect of crystallographic anisotropy. The resulting model is capable of simulating the growth of equiaxed and columnar dendritic grains in 2D and 3D, with the <100> directions either aligned or inclined with the grid. The algorithm used can also be used on coarser grids, with a concomitant loss in resolution, allowing simulation of sufficiently large numbers of dendrites in 3D to investigate the distribution of spacings, as well as average behavior.Simulations were performed for directional solidification with a range of withdrawal velocities and nucleation conditions, but a constant thermal gradient. The simulations capture the full microstructural development and primary spacing selection by both branching and overgrowth mechanisms. The model illustrates that there is a range of possible stable spacings, and that the final spacing is history dependent. It was also found that a minimum deviation from the steady state dendrite spacing is required before the spacing adjustment mechanisms are activated. The influence of perturbing the withdrawal velocity upon the stability of the spacing was also investigated. It was found that perturbations significantly reduce the range of stable primary dendrite spacing.     

Zn—2.75%Cu合金定向凝固组织及对流的影响

研究了Zn-2.75%Cu合金在传统Bridgeman法和ACRT－B法下定向凝固组织的差别。着重讨论了生长速度和液相强制对流对Zn-Cu包晶定向凝固组织的影响。实验发现,在Bridgeman法定向凝固过程中,生长速度的增加使得液相中的温度梯度减小,一次枝晶间距减小,一次枝晶间距与生长速度和温度梯度的关系式为：λ1=0.6064R^-0.25GL^-0.5。强制对流使得一次枝晶发生分叉和偏转。并且随着对流强度的加大,一次枝晶间距降低,二次枝晶的生长被抑制。包晶反应的存在,使得枝晶的尖端和二次枝晶熔解,造成了固相中枝晶和初生枝晶的差别。     

Solidification and segregation behaviors in 6061 aluminum alloy

The characteristics of a solid/liquid interface established during directional solidification were investigated in a 6061 aluminum alloy (Al6061 alloy) in which interface patterns, such as planar, cellular and dendritic, form in the mushy zone. Directional solidification experiments were carried out in alumina tubes with an inner diameter of 5 mm to obtain the different microstructures for varying growth rates under an imposed thermal gradient of 9.45 K/mm. As a consequence, primary arm spacing, secondary arm spacing and the length of primary arm were measured for steady-state growth conditions. Furthermore, the solute concentration profiles for the final solidification region were obtained using a SEM-EDS system in order to examine the segregation behaviors of alloying elements. Therefore, it was found that the degree of segregation in the Al6061 alloy was dependent on the growth rate under given solidification conditions, that is, the elemental segregation became more severe with increasing growth rate in the interdendritic region.     

来流对Al-Cu合金三维树枝晶生长的影响

建立了模拟二元合金树枝晶生长的三维元胞自动机模型,以Al-4%Cu(质量分数)为模型合金,模拟了合金过冷熔体中树枝晶的生长过程,研究了来流对枝晶生长的影响.结果表明,来流对合金过冷熔体中三维树枝晶生长影响显著,迎流侧枝晶尖端生长速度随来流速度的增大而增大,枝晶尖端半径随来流速度的增大而减小;随着来流速度的增大,枝晶尖端选择参数减小;在给定过冷度条件下,随界面能各向异性的增大,来流对枝晶尖端选择参数的影响增强;对于给定的合金(或界面能各向异性),来流对枝晶尖端选择参数的影响随着过冷度的增大而增强.     

Influence of crystal orientation on cellular growth of a nickel-base single crystal superalloy

The cellular pattern evolution during directional solidification of a nickel-base single crystal superalloy has been studied in different crystallographic orientations using re-oriented seed crystals. Under the same thermal gradient and solidification velocity, the microstructures of differently oriented cellular single crystals are schematically investigated. It is concluded that the cellular growth direction is less affected by the seed orientation and depends on the heat flow, and is usually along heat flow direction. Cellular interface stability and microstructure are greatly influenced by the crystallographic orientation. When increasing the misorientation, the cell spacing increases correspondingly and the cellular growth interface becomes more unstable.     

Convection Effects During Bulk Transparent Alloy Solidification in DECLIC-DSI and Phase-Field Simulations in Diffusive Conditions

To study the dynamical formation and evolution of cellular and dendritic arrays under diffusive growth conditions, three-dimensional (3D) directional solidification experiments were conducted in microgravity on a model transparent alloy onboard the International Space Station using the Directional Solidification Insert in the DEvice for the study of Critical LIquids and Crystallization. Selected experiments were repeated on Earth under gravity-driven fluid flow to evidence convection effects. Both radial and axial macrosegregation resulting from convection are observed in ground experiments, and primary spacings measured on Earth and microgravity experiments are noticeably different. The microgravity experiments provide unique benchmark data for numerical simulations of spatially extended pattern formation under diffusive growth conditions. The results of 3D phase-field simulations highlight the importance of accurately modeling thermal conditions that strongly influence the front recoil of the interface and the selection of the primary spacing. The modeling predictions are in good quantitative agreements with the microgravity experiments.     

From constrained to unconstrained growth during directional solidification

A one-dimensional solidification model has been developed to study the directional solidification of dendritic alloys. It is based on the resolution of the heat flow equation using a two-interface front tracking technique. The two interfaces are defined by imaginary limits, assumed to be macroscopically flat, which correspond to the positions of the growing dendritic and eutectic interfaces. These delimit the three regions that are considered: liquid, mushy zone and solid. Growth kinetics laws are applied to the interfaces by velocity vs temperature relationships. It was found that, if complete solidification was carried out directionally up to the top of the ingot (i.e. formation of a fully columnar structure), then the velocity of the dendrite tips first increased during the stage of the superheat loss, then decreased when no substantial thermal gradient remained in the liquid ahead of the growing dendritic interface. Applied to directional solidification experiments carried out with aluminium–silicon alloys, the model shows that this maximum velocity was reached when the top position of the mushy zone (i.e. the dendritic interface) reached about two-thirds the length of the ingots. This position being in the vicinity of the columnar-to-equiaxed transition (CET) observed in the longitudinal section of the ingots, a CET scenario is proposed based on a constrained-to-unconstrained growth transition, leading to breakdown of the columnar dendritic front.     

Morphological development of solidification structures under forced fluid flow: a Monte-Carlo simulation

A Monte-Carlo simulation of microstructural development under forced convection is presented. The model takes into account both diffusive and forced fluid flow, kinetics of atomic attachment at the solid–liquid interface and structural modification under the influence of capillary forces. It has been shown that the nature of fluid flow has a very significant influence on the morphology of the solidification structure. A laminar type flow is shown to destabilize the solid–liquid interface promoting dendritic growth for solid growing from fixed substrate. Particle rotation under streamlined flow, or a periodic change in the fluid flow direction around the growing solid is, however, shown to produce the rosette type solidification morphology. A turbulent type flow penetrating into the interdendritic region produces fine and compact solidification structures with or without liquid entrapment.     

Quantitatively comparing phase-field modeling with direct real time observation by synchrotron X-ray radiography of the initial transient during directional solidification of an Al-Cu alloy

The initial transient during directional solidification of an Al-4 wt.% Cu alloy was simulated by a quantitative phase-field model solved with the adaptive finite element method. The simulated solidification process was compared with the related analytical theory and in situ and real time observations by means of X-ray radiography at the European Synchrotron Radiation Facility. The simulated velocity of the planar interface and solute profile ahead of the solidification front in the liquid are close to the predictions of the Warren-Langer model of the initial planar solidification transient, but in fair quantitative agreement with experimental results only at early stages of planar solidification. After the accelerated flat interface lost its stability a transition to cellular solidification was initiated. The initial cell spacing predicted by the phase-field simulation agreed well with the experimental observations in the region where the cell growth direction was perpendicular to the fluid flow, whereas a discrepancy was obvious in the corners where the fluid flow was parallel to growth. An analytical relation describing the wavelength of the initial solid-liquid interface corrugations under diffusion-limited transport is screened out by comparing the phase-field simulation data with expressions based upon the Mullins-Sekerka linear stability analysis theory or derived for primary spacing. The gravity-driven natural convection in the experiment resulted in misfits between the phase-field predictions and the experimental observations in the late stage of planar solidification, onset and development of morphological instability.