Stochastic Modeling of Dendritic Grain Structures

A three‐dimensional cellular automaton‐finite element model has been developed over the past five years for the prediction of macrostructures formed in casting. The present article briefly summarizes the growth algorithm of the model. Applications are then given for the prediction of the grain structure formed in a directionally solidified turbine blade produced by investment casting, a continuously cast cylinder and a near‐net shape steel sheet obtained by twin‐roll casting.     

Modelling of dendritic solidification using finite element method

Abstract

Computer based numerical modelling of solidification is being increasingly used in an effort to develop and improve casting processes, a long term goal of this work being the prediction of microstructural features such as grain shape, size, and dendrite arm spacings throughout a casting. In a numerical heat flow model this can be achieved only through the inclusion of the kinetics of nucleation and dendrite growth. In the present paper strategies for including columnar and equiaxed solidification kinetics into a finite element model are reviewed. A detailed model for columnar solidification is then presented together with results obtained from calculations on an Al–5 wt-%Cu alloy and a multicomponent nickel based superalloy. It is shown that the inclusion of a dendrite tip undercooling is important, particularly in systems having a low Stefan number. Furthermore, the thermal histories in the superalloy can only be accurately calculated if an experimentally determined solid fraction versus temperature relationship is employed. A model for equiaxed solidification is also discussed and results obtained for an Al–5 wt-%Cu alloy outlined. In particular, the effects of heat transfer coefficient, heterogeneous nucleation site density, and nucleation undercooling on the grain size variation along a one dimensional casting have been examined. Finally, it is shown that grain sizes predicted by the present finite element model agree reasonably well with those of a previous numerical model for an Al–7 wt-%Si alloy.

MST/1468     

Investigation on thickness of short time oxide films in Al–1Mg and Al–2Mg alloys

Abstract

The melts of aluminium alloys are very sensitive to oxidation during casting, and the surface oxide film formed during casting can be folded and entrained into the melt due to melt surface turbulence. In this research, sandwiches of oxide–metal–oxide (OMO) formed in a very short time within the cast during solidification were investigated in order to see the effect of magnesium content (i.e. 1 and 2 wt-%) on the oxide film thickness. To form OMO sandwiches within the cast, a certain amount of air was blown into the melt every 0·5 s during casting time by means of a compressor at 0·5 atm pressure. Where bubbles of air collided, they formed a sandwich which later was used for investigating purpose. Both the thickness and the surface of oxide films were studied via SEM. The results showed that the thickness of the short time oxide film varies in the range of 150–250 and 200–300 nm for Al–1Mg and Al–2Mg alloys respectively.     

Simulation and application of dynamic heat transfer model for improvement of continuous casting process

Abstract

Numerical simulation is being increasingly used to improve the existing cooling systems. In order to attain highest quality strand, a two-dimensional dynamic mathematical heat transfer model of billet continuous casting of low carbon steel has been presented. This model can be used to compute the billet temperature distribution and shell thickness, especially it can be used to simulate the solidification process which is caused by frequently variational casting conditions. The fluctuation of measured temperature has been reduced to <10°C with thermal imaging system. The online model can monitor surface temperature and shell thickness in the casting process. So it provides the possibility for the online process control. For the validation of the dynamic model, a lot of billet surface temperature and shell thickness measurement were carried out on an actual casting machine. Finally, the dynamic model has been used for adjusting the operating parameters to improve the casting speed.     

Analysis of zonal shrinkage in alloy steels

Abstract

Thermal behaviour of the solidifying steel structure is important for understanding of the defects during ingot solidification. During solidification and cooling, most metals shrink. As a consequence in upper part of solid ingot, pores and pipes of typical shapes and size are formed. Forming of pipes is closely related to the casting and solidification processing parameters. In the present paper the influence of liquid temperature, chemical composition and temperature gradient on the shrinkage intensity are investigated. The ratio of the pipe depth to total ingot height as a criterion of the pipe size is used. The values of the temperature gradients on the base of the numerical model solidification are obtained. The experimental measurements of temperature change have been carried out on laboratory steel ingot. The results by numerical model are compared with the experimental ones and showed a good agreement.     

Microstructure and compressive properties of NiAl–Cr(Mo)–Dy near eutectic alloy prepared by suction casting

Abstract

The NiAl–Cr(Mo) eutectic alloy doped with Dy was prepared by suction casting technique and its microstructure and mechanical properties were investigated. It is found that with the addition of Dy, the Ni₅Dy phase is formed along the NiAl/Cr(Mo) phase boundary in the intercellular region. By the suction casting method, the microstructure of the alloy get well optimisation which can be characterised by the fine interlamellar spacing, high proportion of eutectic cell area and fine homogeneous distributed Ni₅Dy phase. The compression test results reveal that the room temperature and high temperature mechanical properties of the suction cast alloy improve significantly, compared with the conventionally cast alloy.     

Coupled heat and fluid flow model and experimental verification of aluminium plate die casting

Abstract

The present work aims to forecast mould filling, void shape, location and size as well as columnar to equiaxed transition (CET) in commercial pure aluminium casting. A model coupling the momentum equations of the fluid flow and heat transfer equations is presented, in which metallostatic pressure, air gap and oxide layer are considered. Different casting parameters were investigated such as casting configuration by varying the plate thickness from 5 to 20 mm, melt superheat from 40 to 120°C, mould preheat up to 200°C and different pouring heads ranging from 0·3 to 0·6 m. Regarding the microstructure and void formation, the approach based on the Niyama criterion, was considered. The experimental verification of the model was achieved by gravity die casting in the form of a rectangular cavity. Voids inside aluminium plate were investigated by X-ray imaging. Microstructure and CET was investigated microscopically. The supposed model proves its validity for mould filling and in detecting the void features and CET.     

Comparison of regression models,grey models,and supervised learning models for forecasting flood stage caused by typhoon events

Abstract

In this paper, four simple dynamic prediction methods and two supervised learning techniques including a linear regression model, a quadratic regression model, an original grey prediction model, a modified grey prediction model, a back‐propagation neural network model, and an epsilon‐SVM regression model were investigated for the forecasting of flood stage one hour ahead for early warning of flooding hazards. Quantitative evaluations were carried out between the predicted values by using the six forecasting models and the measured values obtained in the field. The comparisons confirm the ability of the simple grey prediction model to forecast flood stage by using only three observations of water stage with reasonable accuracy for the study cases, especially for study areas with scanty hydrological data.     

Modelled heat transfer coefficients for Al–7 wt-%Si alloy castings unidirectionally solidified horizontally and vertically downwards

Abstract

A model is described for the calculation of the interfacial heat transfer coefficient during the unidirectional solidification of Al–7 wt-%Si alloy castings against a water cooled copper chill. The model includes the deformation of the initial solidified skin of the casting into a convex shape within the first seconds of solidification. Thereafter, heat transfer from the casting to the chill takes place through a central contact area and an outer annulus where local separation has occurred. Modelled heat transfer coefficients for solidification horizontally and vertically downwards are compared with experimentally determined values and show broad agreement. Some limitations of the model which prevent better agreement with the experimental values are discussed.     

Development and characterization of a mucoadhesive sublingual formulation for pain control: extemporaneous oxycodone films in personalized therapy

Objective: The aim of this work was the development of mucoadhesive sublingual films, prepared using a casting method, for the administration of oxycodone.

Materials and methods: A solvent casting method was employed to prepare the mucoadhesive films. A calibrated pipette was used to deposit single aliquots of different polymeric solutions on a polystyrene plate lid. Among the various tested polymers, hydroxypropylcellulose at low and medium molecular weight (HPC) and pectin at two different degrees of esterification (PC) were chosen for preparing solutions with good casting properties, capable of producing films suitable for mucosal application.

Results and discussion: The obtained films showed excellent drug content uniformity and stability and rapid drug release, which, at 8?min, ranged from 60% to 80%. All films presented satisfactory mucoadhesive and mechanical properties, also confirmed by a test on healthy volunteers, who did not experience irritation or mucosa damages. Pectin films based on pectin at lower degrees of esterification have been further evaluated to study the influence of two different amounts of drug on the physicochemical properties of the formulation. A slight reduction in elasticity has been observed in films containing a higher drug dose; nevertheless, the formulation maintained satisfactory flexibility and resistance to elongation.

Conclusions: HPC and PC sublingual films, obtained by a simple casting method, could be proposed to realize personalized hospital pharmacy preparations on a small scale.     

CONTINUOUS COOLING OF WET FOUNDRY SAND USING A FLUIDIZED BED

Abstract

In order to establish the optimum cooling system for hot returned sand in metal casting processes, a mathematical model including simultaneous heat and mass transfer has been constructed. The present study is especially directed towards clarifying the dynamical behavior of sand cooling in a one-pass fluidized bed. A series of continuous cooling experiments has been made for various conditions. Characteristics of both transient and steady-state bed temperature are explained reasonably by the present model. Furthermore, a possible proposal has been given on the optimal operation, which raises heat exchange efficiency in fluidization cooling of hot molding sand.     

Computer model of unidirectional solidification of single crystals of high temperature alloys

Abstract

I t has been common practice to use mould withdrawal unidirectional solidification to produce single crystal castings. To grow single crystals successfully, it is important to control several solidification parameters, such as the morphology of the solidification front (solid/liquid interface), thermal gradient, and growth rate during solidification. It is the aim of this study to develop a solidification model that can predict such solidification parameters for various design and operating conditions. The solidification phenomena in the process modelled are basically controlled by two heat transfer mechanisms: conduction and radiation. A set of heat transfer equations and boundary conditions were employed to describe mathematically the heat transfer phenomena. Then the finite difference method was used numerically to solve these equations for specified boundary conditions to obtain the temperature distribution and temperature variation in the casting. The solidification parameters can subsequently be deduced from these temperature data. Several thin plate castings were tested using the model developed. The following design and operating conditions were evaluated: susceptor temperature (power input), withdrawal speed, changes of cross-sectional area in the casting, and geometrical arrangement of the casting tree.

MST/1422     

Casting methods for the production of rotationally symmetric copper bimetals

ABSTRACT

Multiple-material products are characterised by a complex property profile which is achieved by combining the particular advantages of at least two different materials. Bimetal casting is an energy- and material-efficient technology for the production of multi-metallic objects. This paper describes the development of a semi-continuous casting process for the formation of a rotationally symmetric bimetal with a cohesive bonding character at the interface of a copper–tin alloy (CuSn6) and pure copper (Cu99.5). Initial experiments are conducted by static casting to evaluate the thermal process window. Based on the results of the initial experiments, a vertical semi-continuous compound casting process is developed. A stable cohesive bond between the joining partners is accomplished by forming a solid solution at the interface.

This paper is part of a Thematic Issue on Copper and its Alloys.     

Morphological variations produced by cellular automaton model of non-isothermal ‘free’ dendritic growth

Abstract

A cellular automaton model of ‘free’ dendritic growth has been used to produce a variety of simulated dendrite morphologies. Different primary and secondary dendrite arm spacings were produced by altering two key parameters in the model; the initial supercooling in the liquid and γ, which represents the solid–liquid interfacial energy in the model. An attempt to quantify the morphological variations has been made using computed hydraulic radii. For comparison hydraulic radii and secondary dendrite arm spacings are included, as measuredfrom an aluminium alloy casting.

MST/3034     

Determination of optimal location to monitor temperature in low pressure die casting process

Abstract

Controlling and eliminating defects, such as macroporosity, in castings is a continuing challenge that manufacturers must continually address. Since the encapsulation of liquid regions by a solid shell and subsequent formation of macroporosity cannot be detected during casting, the die temperature, which is routinely measured, has been used as an indirect indicator of this defect. A finite element model has been developed to predict the evolution of temperature as well as the volume of encapsulated liquid in a casting with a high propensity to form macroporosity. The boundary conditions in the model were iteratively adjusted until the temperature predictions matched the experimental data for a variety of operational conditions. A model based methodology has been developed to analyse the correlation between the die temperature and the encapsulated liquid volume. This methodology is employed to assess the suitability of different in-cycle die temperatures for use as indicators of macroporosity formation, and to help determine the optimal location to monitor temperature for the purpose of minimising macroporosity.     

Estimation of air gap and heat transfer coefficient at different faces of Al and Al–Si castings solidifying in permanent mould

Abstract

In the casting processes, the heat transfer coefficient at the metal/mould interface is an important controlling factor for the solidification rate and the resulting structure and mechanical properties. Several factors interact to determine its value, among which are the type of metal/alloy, the mould material and surface conditions, the mould and pouring temperatures, casting configuration, and the type of gases at the interfacial air gap formed. It is also time dependent. In this work, the air gap formation was computed using a numerical model of solidification, taking into consideration the shrinkage and expansion of the metal and mould, gas film formation, and the metallostatic pressure. The variation of the air gap formation and heat transfer coefficient at the metal mould interface are studied at the top, bottom, and side surfaces of Al and Al–Si castings in a permanent mould in the form of a simple rectangular parallelepiped. The results show that the air gap formation and the heat transfer coefficient are different for the different casting surfaces. The bottom surface where the metallostatic pressure makes for good contact between the metal and the mould exhibits the highest heat transfer coefficient. For the sidewalls, the air gap was found to depend on the casting thickness as the larger the thickness the larger the air gap. The air gap and heat transfer coefficient also depend on the surface roughness of the mould, the alloy type, and the melt superheat. The air gap is relatively large for low values of melt superheat. The better the surface finish, the higher the heat transfer coefficient in the first few seconds after pouring. For Al–Si alloys, the heat transfer coefficient increases with increasing Si content.     

Forced convection flow channel defects in castings

Abstract

Slow filling of castings may lead to remelting of the partially solidified casting thereby forming channels through the structure. The channels are analogous to the channel defects formed by the segregation of solute in the pasty zone leading to the A and V segregates in ingots and large castings. Flow convection channels, however, are characterised externally by a rougher surface finish on the casting and internally by almost zero segregation. The internal structure comprises an edge region of crushed dendrites (leading to the only significant segregation); an intermediate region of refined equiaxed grain structure, coarse dendrite arm spacing, and coarse porosity; and a central region of large grains and large dendrite arm spacing. Some gas pores tend to float to the top of the channel, where they decorate its upper boundary. It is concluded that the channels are not severe defects per se, but are undesirable in castings where good control over quality is required. In particular, because of the delayed solidification, they allow the growth and alignment of porosity in sections which would otherwise freeze rapidly and be sound, or at worst, have fine distributed porosity.

MST/1655     

Microstructure and tensile properties of squeeze cast Al–Zn–Mg–Cu alloy

Abstract

It is well known that wrought aluminium alloys have tensile properties superior to those of the cast products. Wrought grade alloys cannot usually be produced by conventional casting processes to attain the same level of tensile properties. However, progress in casting methods in recent years has made it possible to produce wrought alloys by means of squeeze casting techniques. In the present study an Al–Zn–Mg–Cu alloy has been produced by squeeze casting. Tensile properties close to those of wrought products have been achieved by controlling the microstructure, pressure, and other processing parameters.     

Aqueous slip casting of MgAl2O4 spinel powder

A stoichiometric MgAl ₂ O ₄ spinel (MAS) powder was synthesized by calcining a compacted mixture of a\boldsymbol{\alpha} -Al ₂ O ₃ and calcined caustic MgO at 1400°C for 1 h and was surface treated against hydrolysis using an ethanol solution of H ₃ PO ₄ and Al(H ₂ PO ₄ ) ₃ after fine grinding. Aqueous suspensions with 41–45 vol.% treated powder were prepared using tetra methyl ammonium hydroxide (TMAH) and an ammonium salt of polyacrylic acid (Duramax D-3005) as dispersing agents. These stable suspensions were consolidated in plaster moulds by slip casting (SC) route for the first time. For comparison purposes, the treated powder was also compacted by die-pressing technique after converting into freeze-dried granules and sintered along with slip cast samples at 1550–1650°C for 1–2 h. The MAS ceramics fabricated by slip casting and die-pressing exhibited comparable properties.     

Role of grain refining in hot cracking and macrosegregation in direct chill cast AA 7075 billets

Abstract

Direct chill (DC) casting experimental results are presented for an Al–Zn–Mg–Cu alloy with and without grain refining by attempting the inoculant addition either in the furnace or in the launder at two different casting speeds. Despite considerable structural refinement macro segregation remains unchanged with grain refining at a lower casting speed. Hot cracking, on the other hand, is totally prevented at this casting speed. Depending on the grain refining practice, either hot tears do not appear at all or prior cracks existing in the non-grain refined billet are completely healed. At a higher casting speed, the severity of segregation increases. The hot cracking tendency, however, has shown a distinct difference with grain refining method, with furnace-refined alloy resulting in a crack-free billet. But the billet grain refined in the launder exhibited hot cracking. Overall, these experimental results confirm the beneficiary effect of grain refining on hot cracking.