Metastable phase formation in undercooled Fe-Co melts under terrestrial and parabolic flight conditions

Soft magnetic Fe-Co alloys display primary fcc phase solidification for>19,5 at% Co in conventional near-equilibrium solidification processes. Undercooled Fe-Co melt drops within the composition range of 30 to 50 at% Co have been investigated with the electromagnetic levitation technique. The solidification kinetics was measured in situ using a high-resolution Siphotodiode. Melt drops were undercooled up to 263 K below the liquidus temperature and subsequently quenched onto a chill substrate in order to characterize the solidification sequence and microstructure. The transition from stable fcc phase to metastable bcc primary phase solidification has been observed after reaching a critical undercooling level. The critical undercooling increases with rising Co content. The growth velocity drops obviously after transition to metastable bcc phase formation. Parabolic flight experiments were performed in order to study the phase selection under reduced gravity conditions. Under microgravity conditions, a much smaller critical undercooling and an increased life time of the metastable bcc phase were obtained. This result was validated with TEM investigations. The appearance of Fe-O particles gives an indirect hint for an intermediate fcc phase formation from the metastable bcc phase at elevated temperature.     

Numerical studies on columnar-to-equiaxed transition in directional solidification of binary alloys

A numerical study on columnar-to-equiaxed transition (CET) during directional solidification of binary alloys is presented using a macroscopic solidification model. The position of CET is predicted numerically using a critical cooling rate criterion reported in literature. The macroscopic solidification model takes into account movement of solid phase due to buoyancy, and drag effect on the moving solid phase because of fluid motion. The model is applied to simulate the solidification process for binary alloys (Sn–Pb) and to estimate solidification parameters such as position of the liquidus, velocity of the liquidus isotherm, temperature gradient ahead of the liquidus, and cooling rate at the liquidus. Solidification phenomena under two cooling configurations are studied: one without melt convection and the other involving thermosolutal convection. The numerically predicted positions of CET compare well with those of experiments reported in literature. Melt convection results in higher cooling rate, higher liquidus isotherm velocities, and stimulation of occurrence of CET in comparison to the nonconvecting case. The movement of solid phase aids further the process of CET. With a fixed solid phase, the occurrence of CET based on the same critical cooling rate is delayed and it occurs at a greater distance from the chill.     

Droplet cooling in atomization sprays

Transport between droplets/particles and a gas phase plays an important role in numerous material processing operations. These include rapid solidification operations such as gas atomization and spray forming, as well as chemical systems such as flash furnaces. Chemical reaction rates and solidification are dependent on the rate of gas-particle or gas-droplet transport mechanisms. These gas-based processes are difficult to analyze due to their complexity which include particle and droplet distribution and the flow in a gas field having variations in temperature and velocity both in the jet cross-section and in the axial distance away from the jet source. Thus to study and properly identify the important variables in transport, these gas and droplet variations must be eliminated or controlled. This is done in this work using models based on a single fluid atomization system. Using a heat transport model (referred to as thermal model) validated using single fluid atomization of molten droplets and a microsegregation model, the effect of process variables on heat losses from droplets was examined. In this work, the effect of type of gas, droplet size, gas temperature, gas-droplet relative velocity on the heat transport from AA6061 droplets was examined. It is shown that for a given gas type, the most critical process variable is the gas temperature particularly as affected by two-way thermal coupling and the droplet size. The results are generalized and applied to explain the difference in droplet cooling rate from different atomization processes.     

Experimental and numerical study on the flight and penetration properties of explosively-formed projectile

The whole process of formation, flying and penetration of explosively-formed projectile (EFP) is simulated by a 3D coupled hydrocode of Ls_dyna. The caliber of the shaped charge is 60 mm and EFP is a kind of overturned shaped charge. The Arbitrary Lagrangian–Eulerian (ALE) method is adopted to consider the fluid–solid coupling problem. The velocity attenuation equation is fitted to forecast the flight distance of EFP. The penetration property of EFP to the armor plate is studied by similarity theory and numerical simulation. For validating the equation, a test is designed to study the residual velocity after penetrating a 25 mm thick steel plate from a distance of 48 m. Therefore, some important solutions are obtained from the comparison of the simulation and experiment. The solutions are optimized charge structure of EFP, the ideal shape of projectile, the attenuation rule of flight process and the penetration property after 48 m flight. The numerical solution fits the experimental data well and the study results provide important reference to the design of EFP in engineering.     

Modeling and Simulation of the Microstructure Evolution of the Gas-atomized Alloy Droplets during Spray Forming

In order to understand the solidification process of an atomized droplet and predict the fraction solidification of droplets with flight distance during spray forming, a numerical model based on thepopulation dynamics approach is developed to describe the microstructure evolution under the common action of the nucleation and growth of grains.The model is coupled with droplets heat transfer controlling equations and solved for Al-4.5 wt pct Cu alloy. It is demonstrated that the numerical results describe the solidification process well.     

Interaction rules underlying group decisions in homing pigeons

Travelling in groups gives animals opportunities to share route information by following cues from each other''s movement. The outcome of group navigation will depend on how individuals respond to each other within a flock, school, swarm or herd. Despite the abundance of modelling studies, only recently have researchers developed techniques to determine the interaction rules among real animals. Here, we use high-resolution GPS (global positioning system) tracking to study these interactions in pairs of pigeons flying home from a familiar site. Momentary changes in velocity indicate alignment with the neighbour''s direction, as well as attraction or avoidance depending on distance. Responses were stronger when the neighbour was in front. From the flocking behaviour, we develop a model to predict features of group navigation. Specifically, we show that the interactions between pigeons stabilize a side-by-side configuration, promoting bidirectional information transfer and reducing the risk of separation. However, if one bird gets in front it will lead directional choices. Our model further predicts, and observations confirm, that a faster bird (as measured from solo flights) will fly slightly in front and thus dominate the choice of homing route. Our results explain how group decisions emerge from individual differences in homing flight behaviour.     

Semi-solid deformation in multi-component nickel aluminide

A systematic study was carried out to determine the solidification and the tensile behaviour of semi-solid multicomponent nickel aluminide. Directionally solidified samples were tested at various temperatures in the mushy (semi-solid) region. A special Gleeble testing procedure was developed where transverse and longitudinal (5 mm) samples were quickly raised to a predetermined temperature in the semi-solid zone and fractured. The fracture stresses were found to decrease monotonically with temperature. The strain to fracture exhibited a ductility minimum at an intermediate temperature in the semi-solid zone. The effect of the solidification process variables, namely, the temperature gradient and velocity, on the fracture stress in the transverse direction was to increase the fracture stress at a given temperature. In the longitudinal direction, the fracture stress decreases with the temperature gradient and was relatively independent of velocity. At the temperature corresponding to the strain minimum, residual microcracks were detected on the fracture surface. The upper hot tearing temperature was noted to be a function of the solidification variables. The amount of strain accommodation and the hot tearing resistance was found to be influenced by the solidification microstructure. Fracture maps which include the transverse fracture stress, temperature, and temperature gradient during solidification (_T-T-G) for the directionally solidified microstructures are presented. A castability map is created from the fracture data.     

Simulation study on horizontal continuous casting process of copper hollow billet under rotating electromagnetic stirring Part 2—effects of electromagnetic and casting parameters on solidification process

The comprehensive three-dimensional mathematical model proposed in Part 1 is used to investigate the effect of rotating electromagnetic stirring on the solidification process of copper hollow billet during horizontal continuous casting. In this part, the model is used to investigate the effects of electromagnetic parameters and casting speed on the electromagnetic field, temperature field, fluid flow and solidification during the horizontal continuous casting with rotating electromagnetic stirring. The results show that electromagnetic frequency, current intensity and casting speed have significant influence on the tangential velocity, temperature gradient, liquid fraction and sump depth.     

A study of the heat parameters during bidirectional solidification

The temperature field and heat parameters are important in controlling metal liquid crystallinity in unidirectional and bidirectional solidification. The temperature field can be divided into three cases: a liquid temperature field; solid temperature field; and a temperature field on the solid-liquid (S–L) interface. Heat parameters can be divided into two cases: technical heat parameters; and solidification heat parameters. The temperature field on the S–L interface and solidification heat parameters are the most important for the structures and properties of materials. The temperature field on the S–L interface is determined by the alloy system, and solidification heat parameters are related to the temperature ield of the environment and technical heat parameters. The temperature ield on the S–L interface is closely related to the solidiication heat parameters.

A theoretical model describing precisely the temperature field on the S–L interface during bidirectional solidification was proposed. A series of heat parameters, including temperature gradients G, solidification rate R, cooling velocity V and characteristic temperature T_c have been derived from this model. A superalloy has been chosen as the experimental object in order to verify the theoretical model. The theoretical calculations are found to be in agreement with the experimental results.     

A study of the heat parameters during bidirectional solidification

The temperature field and heat parameters are important in controlling metal liquid crystallinity in unidirectional and bidirectional solidification. The temperature field can be divided into three cases: a liquid temperature field; solid temperature field; and a temperature field on the solid–liquid (S–L) interface. Heat parameters can be divided into two cases: technical heat parameters; and solidification heat parameters. The temperature field on the S–L interface and solidification heat parameters are the most important for the structures and properties of materials. The temperature field on the S–L interface is determined by the alloy system, and solidification heat parameters are related to the temperature field of the environment and technical heat parameters. The temperature field on the S–L interface is closely related to the solidification heat parameters.A theoretical model describing precisely the temperature field on the S–L interface during bidirectional solidification was proposed. A series of heat parameters, including temperature gradients G, solidification rate R, cooling velocity V and characteristic temperature T_c have been derived from this model. A superalloy has been chosen as the experimental object in order to verify the theoretical model. The theoretical calculations are found to be in agreement with the experimental results.     

Parameters controlling solidification of molten wax droplets falling on a solid surface

An experimental study was done to identify parameters that determine the shape of splats formed by droplets of paraffin wax impacting and freezing on a polished aluminum surface. Impact velocity was varied from 0.5 to 2.7 m/s and surface temperature from 23 to 73 °C. Droplet impact was photographed, and the splat diameter and liquid-solid contact angle measured from photographs. A simple energy conservation model was used to predict the maximum extent of droplet spread and the rate of droplet solidification. The extent of droplet solidification was found to be too small to affect droplet impact dynamics. Photographs showed liquid recoiling in the droplet center following impact on a cold surface (23 °C); the height of recoil diminished if either substrate temperature or impact velocity was increased. Droplet recoil was attributed to surface tension pulling back the periphery of the splat. Reducing the surface temperature increased surface tension, promoting recoil. At sufficiently large impact velocities droplets fragmented. A model based on the Rayleigh-Taylor instability was used to predict the number of satellite droplets that broke loose after impact.     

Analysis of Solidiflcation in Spray Atomized and Codeposited Metal-matrix Composites Part Ⅱ: Reinforcement Injection and Deposition

The influence of the injection of reinforcing particles (for the production of metal matrix composites and of the droplets-to-substrate heat transfer on the resulting microstructural uniformity of spray atomized and codeposited composite material is analyzed. The reinforcement particles injection velocity has to be limited between an upper and a lower critical values. in order to ensure entrapment into the matrix droplets in flight. The thermal history of the injected droplets during the deposition stage is calculated with the assumption that the in-flight solidifying droplets reach the substrate while containing still at least 20% liquid volume fraction, in order to avoid porosity of the deposited material. The substrate to pouring-tube orifice distance where that condition is achieved depends strongly on the atomization pressure and the convective heat transfer coefficient of the substrate. It is demonstrated that "tailoring" the microstructures and the reinforcement volume percent in the deposited material is feasible. The critical process parameters : the atomization pressure, the melt flow rate. the substrate to pouring-tube orifice distance, the reinforcement particles injection location and rate can all be adequately chosen in order to obtain any desired microstructure, grain size, reinforcement volume percent, with the additional benefit, if wanted, of rapid solidification processing     

Investigation of Failure of Fiberglass Plastic under Conditions of Flight in Dusty Atmosphere

The results of experimental investigations of thermal-erosion resistance of fiberglass plastic in a supersonic heterogeneous flow, obtained in a ground high-temperature gasdynamic stand, are used to study the resistance of the frontal surface of a hypersonic flying vehicle under conditions of flight in a dusty atmosphere with a velocity of M = 8.     

Modeling transport phenomena of ice slurry in an ice forming unit

In this article, multiphase convective and solidification transport phenomena of ice slurry is investigated by developing a one-domain macroscopic model to simulate its formation in a rectangular ice forming unit. Convection, sedimentation, interfacial drag, permeability, remelting and viscosity variations are incorporated into this model through the appropriate governing multiphase transport equations. Validation studies with literature data are performed to determine the most suitable drag law by comparing the position of the interface between the coherent and non-coherent zones. After establishing modified Stokes' Law as the best-suited drag model, solidification study of an aqueous ammonium chloride solution is performed. The results for the evolution of ice fraction, species distribution, temperature profile and multiphase velocity field are presented. Solid fraction gradient, due to the generation of ice particles, has a major influence on the density gradient leading to a counter-clockwise flow current resulting in the homogenization of temperature and species in the generated ice slurry.     

Modified stefan problem

In analytical studies of solidification, one usually prescribes the shape of the growing solid and aims to determine the velocity of growth as a function of the various pertinent parameters. The present study assumes the velocity of growth and aims to determine the temperature of the growing surface, for the case of simple geometry. The former class of problems is appropriate for study of nucleus growth, and the latter for study of dendrite growth.Originally published as General Electric Research Laboratory Report No. 64-RL-3733M.     

Numerical study of heat transfer and solidification behavior of gas-atomized Fe-6.5%Si (mass fraction) droplets

During spray atomization process, the heat transfer and solidification of droplets play very important roles for the deposition quality. Due to the difficulties of experimental approach, a numerical model is developed, which integrates liquid undercooling, nucleation recalescence and post-recalescence growth to present the full solidification process of Fe-6.5%Si (mass fraction) droplet. The droplet velocity, temperature, cooling rate as well as solid fraction profiles are simulated for droplets with different sizes to demonstrate the critical role of the size effect during the solidification process of droplets. The relationship between the simulated cooling rate and the experimentally obtained secondary dendrite arm spacing is in excellent agreement with the well-established formula. The pre-constant and exponent values lie in the range of various rapid solidified Fe-based alloys reported, which indicates the validity of the numerical model.     

Solidification contact angles of molten droplets deposited on solid surfaces

Droplet impact and equilibrium contact angle have been extensively studied. However, solidification contact angle, which is the final contact angle formed by molten droplets impacting on cold surfaces, has never been a study focus. The formation of this type of contact angle was investigated by experimentally studying the deposition of micro-size droplets (∼39 μm in diameter) of molten wax ink on cold solid surfaces. Scanning Electron Microscope (SEM) was used to visualize dots formed by droplets impacted under various impact conditions, and parameters varied included droplet initial temperature, substrate temperature, flight distance of droplet, and type of substrate surface. It was found that the solidification contact angle was not single-valued for given droplet and substrate materials and substrate temperature, but was strongly dependent on the impact history of droplet. The angle decreased with increasing substrate and droplet temperatures. Smaller angles were formed on the surface with high wettability, and this wetting effect increased with increasing substrate temperature. Applying oil lubricant to solid surfaces could change solidification contact angle by affecting the local fluid dynamics near the contact line of spreading droplets. Assuming final shape as hemispheres did not give correct data of contact angles, since the final shape of deposited droplets significantly differs from a hemispherical shape.     

Phase-field simulation for the evolution of solid/liquid interface front in directional solidification process

In this study, the phase field method was used to study the multi-controlling factors of dendrite growth in directional solidification. The effects of temperature gradient, propelling velocity, thermal disturbance and growth orientation angle on the growth morphology of the dendritic growth in the solid/liquid interface were discussed. It is found that the redistribution of solute leads to multilevel cavity and multilevel fusion to form multistage solute segregation, and the increase of temperature gradient and propelling velocity can accelerate the dendrite growth of directional solidification, and also make the second dendrites more developed, which reduces the primary distance and the solute segregation. When the temperature gradient is large, the solid-liquid interface will move forward in a flat interface mode, and the thermal disturbance does not affect the steady state behavior of the directionally solidified dendrite tip. It only promotes the generation and growth of the second dendrites and forms the asymmetric dendrite. Meanwhile, it is found that the inclined dendrite is at a disadvantage in the competitive growth compared to the normal dendrite, and generally it will disappear. When the inclination angle is large, the initial primary dendrite may be eliminated by its secondary or third dendrite.     

A lattice Boltzmann model for solidification of water droplet on cold flat plate

Since the solidification of water droplet is the initial and essential process in the whole process of frosting, a model is developed by the lattice Boltzmann method (LBM) that applies the velocity and temperature distribution functions to investigate the solidification process of water droplet on cold flat plate. The thermal transport and liquid–solid phase transition in the present model are both based on the pseudo-potential model combined with the enthalpy formation. By this LB model, the solidification process is simulated in form of temperature and solid phase variations in water droplet on cold flat plate, and the shape of solid phase in freezing can also be predicted. In addition, we apply the present LB model to preliminarily study the frost formation process. Numerical results agree well with our experimental data.     

铜钢爆炸焊接动态参数的研究

研究爆炸焊接动态参数对复合质量至关重要。本文对铜复板的动态弯折角、碰撞速度等动态参数进行了研究 ,得到了铜复板飞行姿态、超声速复合中临界碰撞角和铜复板飞行速度上限 ,对试验与生产有一定的实用价值。