Optimization of porosity formation in AlSi9Cu3 pressure die castings using genetic algorithm analysis

In this investigation, an effective approach based on multivariable linear regression (MVLR) and genetic algorithm (GA) methods has been developed to determine the optimum conditions leading to minimum porosity in AlSi₉Cu₃ aluminium alloy die castings. Experiments were conducted by varying holding furnace temperature, die temperature, plunger velocities in the first and second stage, and multiplied pressure in the third stage using L₂₇ orthogonal array of Taguchi method. The experimental results from the orthogonal array were used as the training data for the MVLR model to map the relationship between process parameters and porosity formation of the die cast parts. With the fitness function based on this model, genetic algorithms were used for the process conditions optimization. By comparing the predicted values with the experimental data, it was demonstrated that the proposed model is a useful and efficient method to find the optimal process conditions in pressure die casting associated with the minimum porosity percent.     

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     

REVIEW OF NEW PROCESS TECHNOLOGIES IN THE ALUMINUM DIE-CASTING INDUSTRY

There have been many challenges in aluminum die casting to establish casting processes to produce high-integrity components from aluminum alloys. Advances in new casting technology mainly have been in pressure die casting; in particular, to obtain mold filling at low speed. This can be achieved by using innovative filling processes with aluminum alloys in the liquid or semisolid state. Different techniques such as high-pressure die casting (HPDC), Cosworth process, low-pressure systems, squeeze casting, indirect squeeze casting, metal compression forming (MCF), and semisolid metal (SSM) processing have been developed. Semisolid forming includes thixoforming and rheoforming. During the semisolid casting process, preheating by induction is needed to obtain the same temperature and the same liquid fraction through the billet in a short time. Thixocasting in the semisolid state helps to avoid turbulence during mold filling.     

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.     

Numerical Modeling of Grain Structure in Continuous Casting of Steel

A numerical model is developed for the simulation of solidification grain structure formation (equiaxed to columnar and columnar to equiaxed transitions) during the continuous casting process of steel billets. The cellular automata microstructure model is combined with the macroscopic heat transfer model. The cellular automata method is based on the Nastac's definition of neighborhood, Gaussian nucleation rule, and KGT growth model. The heat transfer model is solved by the meshless technique by using local collocation with radial basis functions. The microscopic model parameters have been adjusted with respect to the experimental data for steel 51CrMoV4. Simulations have been carried out for nominal casting conditions, reduced casting temperature, and reduced casting speed. Proper response of the multiscale model with respect to the observed grain structures has been proved.     

Heat transfer study of submicro-encapsulated PCM plate for food packaging application

In recent studies, encapsulated phase change materials (PCM) were developed as novel materials for food packaging because of their improved thermal insulation capacity. The PCMs (often liquid in room temperature) are encapsulated in a shell material so as they can be practically handled. In this work, the thermal behaviour of an encapsulated PCM material (Rubitherm RT5 encapsulated in polycaprolactone PCL) with two different PCM mass fractions was studied. The model was validated by experimental cooling and heating processes, under controlled air temperature conditions. The numerical result demonstrated a better thermal buffering capacity of the encapsulated PCM material compared to a standard one (cardboard).     

Study on Numerical Simulation of Mold Filling and Solidification Processes under Pressure Conditions

The mold filling and solidification simulation for the high pressure die casting (HPDC) and low pressure die casting (LPDC) processes were studied.A mathematical model considering the turbulent flow and heat transfer phenomenon during the HPDC process has been established and paralled computation technique was used for the mold filling simulation of the process.The laminar flow characteristics of the LPDC process were studied and a simplified model for the mold filling process of wheel castings has been developed.For the solidification simulation under pressure conditions,the cyclic characteristics and the complicated boundary conditions were considered and techniques to improve the computational efficiency are discussed.A new criterion for predicting shrinkage porosity of Al alloy under low pressure condition has been developed in the solidification simulation process.     

Theoretical modeling of optimal focusing conditions using laser-induced breakdown spectroscopy in liquid jets

Optimal conditions are determined for laser-induced breakdown spectroscopy in liquid jets by investigating laser de-focusing and laser energy variation in aqueous liquid jets containing dilute levels of calcium chloride. It has been found that the atomic emission shows a strong correlation with both laser pulse energy and focal position. The data cannot be rationalized on the basis of electron density or ionization temperature changes alone, but rather it requires the additional consideration of the volume of the liquid sample interacting with the laser and that portion of the volume which is above the threshold energy for plasma formation. A moving breakdown model has been applied to the plasma formation in the jet to calculate the amount of sample ablated with sufficient energy for plasma formation, which models well the observed results and allows prediction of optimal focusing conditions for a given laser energy.     

Kinetics and dilatometric behaviour of non-isothermal ferrite–austenite transformation

Abstract

A model that describes the ferrite–austenite transformation during continuous heating in Armco iron and three very low carbon, low manganese steels with a fully ferritic initial microstructure is presented. This model allows calculation of the volume fractions of austenite and ferrite during transformation as a function of temperature, and hence knowledge of the austenite formation kinetics under non-isothermal conditions in fully ferritic steels. Moreover, since dilatometric analysis is a technique very often used to study phase transformations in steels, a second model, which describes the dilatometric behaviour of the material and calculates the relative change in length that occurs during the ferrite–austenite transformation, has also been developed. Both kinetics and dilatometric models have been validated by comparison of theoretical and experimental dilatometric heating curves. Predicted and experimental results are in satisfactory agreement.     

Influence of Process Parameters and Sr Addition on the Microstructure and Casting Defects of LPDC A356 Alloy for Engine Blocks

The effects of Sr addition and pressure increase on the microstructure and casting defects of a low-pressure die cast(LPDC)Al Si7Mg0.3 alloy have been studied.Metallographic and image analysis techniques have been used to quantitatively examine the microstructural changes and the amount of porosity occurring at different Sr levels and pressure parameters.The results indicate that an increase in the filling pressure induces lower heat dissipation of the liquid close to the die/core surfaces,with the formation of slightly greater dendrite arms and coarser eutectic Si particles.On the other hand,the increase in the Sr level leads to finer microstructural scale and eutectic Si.The analysed variables,within the experimental conditions,do not affect the morphology of eutectic Si particles.Higher applied pressure and Sr content generate castings with lower amount of porosity.However,as the filling pressure increases the flow of metal inside the die cavity is more turbulent,leading to the formation of oxide films and cold shots.In the analysed range of experimental conditions,the design of experiment methodology and the analysis of variance have been used to develop statistical models that accurately predict the average size of secondary dendrite arm spacing and the amount of porosity in the low-pressure die cast Al Si7Mg0.3 alloy.     

Study on Numerical Simulation of Mold Filling and HeatTransfer in Die Casting Process

1. IntroductionDie casting is a very complicated process which involves the high speed flow of molten metal filling into3-dimensional complex geometry. The molten metalflow pattern and the temperature distribution of thedie casting and the dies, have a critical influence onthe quality of the cast products, the production rateand the die life.The application of CAE (computer aided engineering) analysis in die design gradually becomes possibleand popular. The main idea is to analyze or predict…     

Scheduling of die casting operations including high-mix low-volume and line-type production

This article addresses the scheduling problem of a real die casting shop. The problem is of practical importance and yet complicated, especially for a modern casting environment where a variety of cast products made of different alloys are simultaneously manufactured in relatively small lot sizes. As a simple and robust scheduling methodology, a Linear Programming (LP) model is proposed so as to determine the quantity of each product in a casting shift. The solution of the LP model maximises the average efficiency of melting furnaces, i.e., the percentage use of molten alloys throughout the shifts. Our model can represent a most general casting environment to the extent that some die casting machines carry out frequent in-process die exchanges for flexible manufacturing. At the same time, we employ line-type casting as well as a combination die with multi-cavities which can cast dissimilar shapes concurrently. In the high-mix low-volume manufacturing world, the proposed LP model can assist the die casting industry to strengthen its competence by providing an optimal schedule that satisfies practical constraints on casting processes.     

低压铸铝件缩孔缺陷数值模拟与工艺改进

目的 为了避免铝合金盖板低压铸造过程产生的缩孔缺陷,对盖板结构进行分析。方法 采用数值分析技术,对盖板低压铸造过程进行数值模拟,将模拟结果与实际铸件缺陷进行对比,分析缩孔缺陷的产生原因。为消除铸件缩孔缺陷,在模具内增设冷却水道,并对该工艺进行模拟分析。结果 铸件凝固过程中,当合金固相率达到75%时,铸件特征位置处的补缩通道开始陆续关闭,形成了孤立液相区,导致凝固结束时形成缩孔缺陷。当模具内增设冷却水道后,铸件的缩孔体积分数由3.5%下降到0.8%。结论 通过在模具内增设合理的冷却水道,能加快铸件厚大部位的凝固速度,有利于实现顺序凝固,从而避免缩孔缺陷,提高铸件质量。     

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.     

Microstructure of Zn–Pb immiscible alloys obtained by a rheomixing process

Abstract

A rheomixing process has been developed for processing immiscible metallic liquids. In this paper, a binary Zn–Pb system is used to demonstrate the rheomixing process. During the rheomixing process, liquid Zn and Pb are premixed in the miscibility gap using a propeller mixer to achieve coarse dispersion of Pb liquid droplets in Zn liquid matrix. The coarse mixture is then transferred into a twin-screw rheomixer, where it is continuously cooled down to a temperature below the monotectic temperature to form a semi-solid slurry under the intensive shear mixing action of the twin-screw rheomixer. The semi-solid slurry is finally extruded through a cylindrical extrusion die to form a continuous bar. The microstructure of immiscible alloys produced by this method is characterised by a fine dispersion of Pb particles distributed uniformly throughout a Zn matrix. The effects of rheomixing temperature and alloy composition on the resultant microstructure have been investigated. It has been found that the average size of Pb particles increases linearly with increasing rheomixing temperature and with increasing Pb concentration in the Zn–Pb alloys.     

Effect of melt temperature on the formation of air pockets during splat quenching

The effect of melt temperature on the formation of air pockets at the contact surface of the ribbons was studied. An alloy with a melting point slightly higher than room temperature (320 K) was used and was heated to 353, 573 and 773 K, respectively, to investigate the thermal effect of the melt temperature on the gas. Under high casting velocity (20 ms^–1) and extremely low melting temperature (353 K), the air pockets were slender and parallel to the casting direction. The liquid melts with a higher casting temperature (773 K) induced coarse air pockets at various casting velocities, and the air pockets were coarser than those induced by the pressure of the gas boundary layer only. The heat flow analysis showed that the local pressure of the entrapped gas, built up by the thermal effect and assuming constant volume, is far greater than the pressure by the gas-boundary layer effect. Thus the thermal effect of the liquid melt should be taken into account in the formation of air pockets during splat quenching.     

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     

Piezoelectric and mechanical properties of novel composites of PZT and a liquid crystalline thermosetting resin

A novel series of lead zirconate titanate (PZT) ceramic–polymer composites has been developed and characterized. The matrix polymer is a liquid crystalline thermosetting resin (LCR) based on a HBA-HNA backbone and phenylethynyl end-groups. The composites show excellent high temperature processability. The dielectric properties were studied as a function of PZT volume fraction and processing conditions. Piezoelectric behaviour was compared to Yamada et al. model for 0–3 composites. For a moderate PZT volume fraction a high value for the piezoelectric stress constant of g ₃₃ = 48 mV m/N was measured, which, in combination with a good chemical and thermal resistance of the polymer matrix, makes the material a good candidate for sensor applications at elevated temperatures. The liquid crystalline thermosetting character of the polymer imparts interesting high temperature post-formability.     

Oxide bifilms in aluminium alloy castings – a review

Abstract

Aluminium alloy castings are most widely used in automobile industry because of their light weight, better castability and improved properties. The liquid aluminium surface easily oxidises during melting, transferring and pouring operation which may entrain oxide films into the casting. Research work has shown that the entrainment of this surface film and formation of bifilms in castings appear to be the source of most of the casting defects leading to a significant reduction in the mechanical properties of aluminium alloy castings. In this paper, the phenomenon of formation of oxide bifilms in aluminium alloy castings, effect of these bifilms on casting properties and their assessment techniques are discussed. For enhancing the quality of casting, research should focus towards development of process techniques for healing of bifilms in liquid metal during solidification.     

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.