Local Solidification Time Research Articles

Application of Taguchi’s methods to investigate some factors affecting microporosity formation in A360 aluminium alloy casting

A Taguchi design investigation has been made into the relationship between the microporosity and process variables in a sand cast A360 aluminium alloy. The effects of various casting parameters including mold filling, liquid filtering, solidification time, the dissolved hydrogen level of the liquid alloy and liquid treatments of molten alloy with Sr addition were investigated. Results showed that within these five casting parameters investigated, the local solidification time and the dissolved hydrogen level of the melt were significant on the microporosity. Accordingly, the other casting parameters have been found to be less influential on the final pore size and their distribution.

Mechanical properties as a function of microstructure and solidification thermal variables of Al–Si castings

The aim of the present study was to investigate the influence of solidification thermal variables on the as-cast microstructure of hypoeutectic Al–Si alloys and to establish correlations with the casting mechanical properties. Experimental results include transient metal/mold heat transfer coefficients, tip growth rate, local solidification time, secondary dendrite arm spacing, ultimate tensile strength and yield strength as a function of solidification conditions imposed by the metal/mold system. It was found that the ultimate tensile strength increases with increasing alloy solute content and with decreasing secondary dendrite arm spacing. Yield strength seems to be independent of both alloy composition and dendritic arrangement. Such results have permitted general expressions correlating dendrite spacing with transient solidification processing variables to be established. The correlation of such expressions with experimental equations relating the ultimate tensile strength and dendrite spacing provides an insight in the preprogramming of solidification in terms of mechanical strength of Al–Si castings. Predictive theoretical and experimental approaches for dendritic growth have been compared with the present experimental observations.

Thermosolutal convective effects on dendritic array spacings in downward transient directional solidification of Al–Si alloys

Solidification thermal variables, such as tip growth rate, tip cooling rate, local solidification time and dendrite arm spacings, have been measured in Al–Si hypoeutectic alloys directionally solidified under downward transient heat flow conditions. It is a well-known fact that the dendritic spacings can affect not only microsegregation profiles but also the formation of secondary phases within interdendritic regions, which influences mechanical properties of cast structures. A reduced number of studies have been carried out in order to analyze the effects of melt convection within the interdendritic region or to verify the influence of growth direction on dendritic arm spacings. In this work, an experimental approach is used to quantitatively determine the solidification thermal variables. The work also focuses on the dependence of dendrite arm spacings on these solidification thermal variables. The experimental data concerning the solidification of Al 5, 7 and 9 wt.% Si alloys are compared with the main predictive dendritic models from the literature. A comparison between upward and downward unsteady-state results for dendritic spacings has also been conducted.

Grain refinement of superalloy K4169 by addition of refiners: cast structure and refinement mechanisms

Grain size and microstructural features of cast superalloy K4169 were investigated under various melting and casting conditions with and without the addition of grain refiners. It is found that lowering pouring temperature and adding refiners to the melt can lead to grain refinement of γ matrix and improve the proportion of equiaxed grains. At a conventional pouring temperature of 1400 °C, the average size of equiaxed grains could be refined to the order of ASTM 3.2, the proportion of equiaxed grains at transverse cross-section could be improved from 56 to 99%. The results also indicate that the average length of primary dendrite axis is shortened with the addition of refiners, but the secondary dendrite arm spacing keeps almost unchanged because local solidification time remained constant. In addition, the microsegregation of main elements such as Fe, Cr, Nb, Mo and Ti is alleviated with the decrease in grain size, and the grain morphology have transformed from dendrite in coarse- to granulate in fine-grained castings. At higher melt pouring temperature, the amount of microporosity in samples with the addition of refiners can be greatly reduced. The mechanisms of grain refinement and increase in equiaxed grain proportion were proposed.

Modelling Shape Evolution and Heat Flow of Spray-Formed Ring Preforms

A finite element model has been developed to simulate the coupled effects of deposit shape evolution and heat flow inside spray formed ring-shaped deposits. The shape model was developed using Matlab and included the features of: (1) atomiser scanning; (2) substrate movement relative to the atomiser; and (3) sticking efficiency. Atomiser scan and various substrate horizontal travel speeds were studied to optimise the ring shape in terms of useful materials suitable for downstream processing. The heat flow model was developed using the commercial finite element code Femlab. A data mapping technique was developed to transfer thermal data between different domains when the computational domains are subject to changing geometry and therefore the coupled effects of shape evolution and heat flow were addressed. Spray forming of ring deposits was performed on the large spray forming unit at Oxford University. In-situ temperature measurements were carried put for acquisition of boundary conditions and validation of the heat flow model. Heat flow modelling revealed that edge effects had a strong influence on the ring thermal history and the porosity distribution inside the deposits is closely related to the local solidification time.

Coarsening in Solidification Processing

Coarsening manifests itself in solidification of metal alloys as 1) growth of larger particles or dendrite arms with simultaneous dissolution of smaller particles or arms (“ripening”), 2) filling in of spaces between particles or dendrite arms (“coalescence”) and 3) breakup of dendrites (“dendrite multiplication”). Ripening of dendrites during solidification results in the well known dependency of dendrite arm spacing on cooling rate or local solidification time. If initial dendritic grain size is sufficiently small, the grains may spheroidize and then grow non-dendritically during isothermal holding in the liquid-solid zone. Dendrite multiplication results from convection combined with rapid cooling during initial stages of dendritic solidification. If the new grains thus formed are sufficiently high in number density, subsequent growth of these grains is non-dendritic. There is engineering interest today in finding the most reliable and economic ways of achieving such a high initial grain density, either by thermo-mechanical means or by grain refinement.

Influence of melt convection on dendritic spacings of downward unsteady-state directionally solidified Al–Cu alloys

The dendritic spacings are important microstructural parameters involved in solidification processes. They can affect not only microsegregation profiles but also the formation of secondary phases within interdendritic regions, which influences mechanical properties of cast structures. A small number of studies have been carried out in order to analyze the effects of melt convection within the interdendritic region or to verify the influence of growth direction in the dendritic arm spacings. In this work, a combined theoretical and experimental approach is developed to quantitatively determine solidification thermal variables such as transient metal/mold heat transfer coefficients, tip growth rates, thermal gradients, tip cooling rates and local solidification time during downward unsteady-state solidification of hypoeutectic Al–Cu alloys. These solidification thermal variables are correlated with dendritic spacings, which have been measured along cross and longitudinal sections of ingots solidified under downward unsteady-state heat-flow conditions. Predictive theoretical models for dendritic spacings have been compared with experimental observations. A comparison between upward and downward unsteady-state results for dendritic spacings has also been conducted.

Influence of melt convection on dendritic spacings of downward unsteady-state directionally solidified Al–Cu alloys

Abstract The dendritic spacings are important microstructural parameters involved in solidification processes. They can affect not only microsegregation profiles but also the formation of secondary phases within interdendritic regions, which influences mechanical properties of cast structures. A small number of studies have been carried out in order to analyze the effects of melt convection within the interdendritic region or to verify the influence of growth direction in the dendritic arm spacings. In this work, a combined theoretical and experimental approach is developed to quantitatively determine solidification thermal variables such as transient metal/mold heat transfer coefficients, tip growth rates, thermal gradients, tip cooling rates and local solidification time during downward unsteady-state solidification of hypoeutectic Al–Cu alloys. These solidification thermal variables are correlated with dendritic spacings, which have been measured along cross and longitudinal sections of ingots solidified under downward unsteady-state heat-flow conditions. Predictive theoretical models for dendritic spacings have been compared with experimental observations. A comparison between upward and downward unsteady-state results for dendritic spacings has also been conducted.

Macrostructural and microstructural development in Al–Si alloys directionally solidified under unsteady-state conditions

Experiments were conducted to analyze the upward unsteady-state directional solidification of Al–Si hypoeutectic alloys. A combined theoretical and experimental approach is developed to quantitatively determine solidification thermal variables such as transient metal/mold heat transfer coefficients, tip growth rates, thermal gradients, tip cooling rates and local solidification time. The observation of the macrostructures has shown that the columnar to equiaxed transition (CET) occurred essentially at the same position from the casting surface for any alloy experimentally examined. The solidification thermal variables were very similar at the CET and it was not possible to identify a structural transition criterion based on a particular variable. The article also focuses on the dependence of dendrite arm spacings on the solidification thermal variables and alloy solute content. The experimental data concerning the solidification of Al with 3, 5, 7 and 9 wt.% Si alloys are compared with the main predictive dendritic models from the literature.

Prediction of microporosity in Al–Si castings in low pressure permanent mould casting using criteria functions

Microporosity, a serious defect in Al–Si based castings, severely prevents their widespread applications in many critical conditions. The use of criteria functions to predict quantitatively microporosity level holds promise. To date, an ideal criteria function has yet to be obtained. In the present work, microporosity distribution in three prominently used hypoeutectic Al–Si alloys (319, 356 and 332) was investigated. The prediction effectiveness of single solidification parameter and existing criteria functions was evaluated by correlating thermal data of simulation studies to experimentally obtained microporosity values. Two new criteria functions are proposed based on experimental observation and multivariable regression analysis. The results indicate that thermal parameter-based criteria functions may be used to predict the microporosity in Al–Si castings but have their limitations. A general criteria function t1.18fV1.13s(tf, local solidification time; Vs, solidification velocity) can be applied to predict microporosity for the family of hypoeutectic Al–Si casting alloys within a certain error.

Multiscale modelling of solidification microstructures, including microsegregation and microporosity, in an Al–Si–Cu alloy

Phase transition phenomena in metallic alloys involve complex physical processes occurring over a wide range of temporal, spatial and energy scales. Multiscale modelling is a powerful methodology for understanding these complex systems. In this paper, a multiscale model of grain and pore formation is presented during solidification. At the microscale, a combined stochastic-deterministic approach based on the cellular automata method is used to solve multicomponent diffusion in a three-phase system (liquid, solid and gas), simulating the nucleation and growth of both grains and pores. The impingement of the growing pores upon the developing solid is also solved to predict the tortuous shape of the porosity, a critical factor for fatigue properties. The micromodel is coupled with a finite element method (FEM) solution of the macroscale heat transfer and fluid flow in industrial castings through the temperature and pressure fields. The result model was used to investigate the influence of local solidification time, hydrogen content, local metallostatic pressure and alloy composition upon the predicted grain structure and pore morphology. Comparison of the model predictions to both laboratory and industrial scale castings are presented.

The effects of viscosity and cooling conditions on the foamability of aluminum alloy

The effect of viscosity and cooling conditions on the foamability of the molten Al–Si–Mg alloy was investigated using the unidirectional solidification method. The mechanism of pore formation in the Al–Si–Mg alloy has been observed and analyzed theoretically. The results obtained are given as follows: 1. The optimum torque of the melt for foaming is approximately 0.29 N m and is about 1.7 times that of the original melt. 2. The existence of alumina particles in the melt is necessary for nucleation of the bubbles and the increase in viscosity. 3. With the increase in the solidification time, the time for bubble growth increases during the unidirectional solidification. 4. The pore size and the total volume of the porosity increase with the diffusion of hydrogen and is controlled by the local solidification time during cooling of the hydrogen saturated melt. 5. Properly controlled viscosity and solidification of the melt are good for promoting uniformity of the pore structure of the aluminum foams.

The effect of grain refining on macrosegregation and dendrite arm spacing of direct chill cast AA5182

The effect of titanium and titanium diboride inoculation on the spatial variation of local solidification time for direct chill (DC) cast ingots of aluminum alloy 5182 (AA5182) was studied. The results have been compared to those of an ingot cast without grain refining. To accomplish this, the effect of grain refining on a number of ingot characteristics such as grain size, macrosegregation, spatial variation of dendrite arm spacing, and thermal conductivity was investigated. Furthermore, the effect of grain refining on the well-known relationship between dendrite arm spacing and local solidification time had to be established for AA5182. The results indicated that the spatial variation of dendrite arm spacing in the industrial ingots was independent of grain refining, although the nonrefined ingot produced significantly finer dendrite arm spacings in its center. This was attributed to the influence of showering crystals in the nonrefined ingot. The relationship between dendrite arm spacing and local solidification time was also found to be independent of grain refining.

Quantification of the SiC p content in molten Al–Si/SiC p composites by computer aided thermal analysis

The aim of this work is to explore the feasibility of the computer aided thermal analysis as a tool for SiC p content quantification in molten Al–Si/SiC p composites. This technique was applied to three cases of interest: (1) the metal matrix alloy A356, without SiC p particle reinforcement, (2) A356/8% SiC p MMC and (3) A356/18% SiC p MMC. Tests were performed in order to identify, within the commonly used thermal analysis parameters, the parameters that could be used for SiC p content quantification. In this regard, the parameters: eutectic temperature, T EG, and local solidification time, t S, show clear trends and low dispersion in their changes as functions of SiC p content. It is found that an increase in SiC p content produces a statistically significant increase of T EG and a decrease of t S. The behavior of t S may be explained as a result of the decrease of the latent heat released during solidification as the SiC p content is increased, such as was indicated by the latent heat data obtained from Newton thermal analysis (NTA) applied to the experimental cooling curves. A simplified analysis of the experimental results indicates that, for the experimental conditions used in this work, the best resolution that could be expected from this method is ±1.3 vol.% of SiC p content, if the local solidification time is the parameter used to quantify the particle content.

Quantitative evaluation of shrinkage porosity with Iso-acritical-Fraction-Solid Method

A new numerical technique has been developed to quantitatively evaluate the shrinkage porosity defect. In this technique, the position of the shrinkage porosity defect is estimated from the solidification time till the critical fraction solid above that the melt hardly flow. Namely, it is assumed to be at hot spots having a local maximum solidification time which is calculated with conventional 3-D solidification simulation code. The volume of shrinkage porosity is estimated from the shrinkage volume of the melt in the Iso-critical-Fraction-Solid Loop which is again estimated from the solidification time data. This method has been applied to some Aluminum alloy automotive castings (high pressure die-casting) and the results agreed well with the measured one through an X-ray CT.

Effects of Solute Elements on Primary and Secondary Dendrite Arm Spacings in Fe‐based Alloys

There have been many experiments and theoretical studies of primary dendrites, and most of which have focused on the effects of extrinsic factors such as solidification rate and temperature gradient. However, intrinsic factors such as the kind of solute element and initial solute concentration are also considered to be important for controlling the spacing of primary dendrites.In this study, the effects of intrinsic factors on primary and secondary arm spacings of dendrites were investigated, and changes in a microsegregation index with changes in primary arm spacings and equilibrium distribution coefficient of each solute element were also examined. Fe‐Cr‐C, Fe‐C‐Cr, Fe‐C‐Cr‐Mo, Fe‐C‐Cr‐Mo‐V and Fe‐C‐Cr‐Mo‐V‐W alloys were solidified unidirectionally and water‐quenched. The primary arm spacing increased with increase in initial solute concentration in all alloys because the range of solidification temperature increased with increase in solute concentration. On the other hand, the secondary arm spacing in all alloys decreased with increase in initial solute concentration, and it was proportional to about the one‐third root of local solidification time. The microsegregation index increased with increasing primary arm spacing and with decreasing equilibrium distribution coefficient.

Solidification-Path and Microstructure of Al-Ti-Cr Alloy Analyzed by Progressive-Type Solidification Equation

There have been many analytical and numerical studies on microsegregation and prediction of second phase in solidification structures of alloys, and they have both advantages and disadvantages especially on the applicability of variable partition ratio (k) of solute and diffusion coefficient (D) in the solid. In this paper, we propose and apply a new progressive-type solidification equation: C Li = C Li-1 .[[1-(1-B.k i-1 )fs i }/{1-(1-B.k i-1 )fs i-1 )] (k i-1 -1)/(1-B-k i-1 ) , which has a parameter (B) including variable partition ratio (k) and D. A specific parameter B is given in each progressive solidification model: B = 2a (Flemings model), B = 2α(1 - exp(-1/α))-exp(-1/2α) (Clyne-Kurz model), or B = 2α/(1 + 2a) (Ohnaka model), where a = D.θ f /L 2 , L: effective length of volume element (= s.d 2 /2, s: structure factor (= 0.5 ∼ 1), d 2 : dendrite arm spacing), θ f : local solidification time. The difference in B-values among the above models are small when D-value is relatively small. The solidification-paths of various Al-Ti-Cr alloys, which crystallize L1 2 -type (Al,Cr) 3 Ti as the primary phase, are analyzed by the progressive-type solidification equation by using the above B values and the data of Al-Ti-Cr phase diagram including k-values (: functions of composition) and experimentally determined diffusion coefficient. The calculated results agree well with the solidification microstructures: the species and the amounts of non-equilibrium eutectic phases and the composition of primary phases.

Study on particle pushing in front of growing interface during soldification

For in-situ composite materials, the interaction between the second-phase particles and the solidification interface attracts more attentions of people, for concerning the final distribution of the particles on the matrix. With the conception of the local solidification time, a kinematics mechanism of particle pushing into the crystal boundary during the solidification process was assumed. Through the analysis of forces acted on the particle in front of the solid/liquid interface the critical velocity criterion for the particles pushing was given. The calculation results show when the growth rate of the interface V is less than or equals to the critical velocity V c , the particles on the solid/liquid interface is pushed into the boundary region of cellular crystals, where they are distributed as a chain-like straight, which forms a particle-strengthened in-situ composite materials.

An analysis on porosity formation in A356 aluminium alloy castings

The work on porosity formation and kinetic hydrogen redistribution in A356 aluminium alloy castings by Chiesa et al. was re-examined and analysed in the present study on a semi-empirical basis. The calculated volume percent of porosity in A356 aluminium castings from this study, Vp(%) = K'C0{1—exp[—(tf/tτ) n]}/Pg, can satisfactorily explain the experimental work by Chiesa et al. In the equation, K’,n are constant; C0 is the initial hydrogen content of melt; tf, ttau; are the local solidification time and relaxation time, respectively; and Pg is the gas pressure in the pore.

Modeling dendritic structure and mechanical properties of Zn–Al alloys as a function of solidification conditions

In the present article, some important trends have been shown regarding the relationship between mechanical properties, microstructure and solidification variables of Zn–Al alloys castings. Some theoretical dendritic growth models, expressing secondary spacings as function of tip growth rate or local solidification time, have been tested against experimental data obtained during unsteady-state solidification. Based on these dendritic models, on analytical expressions describing the position of solidus and liquidus isotherms in the unidirectional solidification of binary alloys and on experimental results concerning tensile testing of casting samples, expressions have been developed permitting a correlation between ultimate and yield strength, dendrite secondary spacings and solidification processing variables.