Local Solidification Research Articles

Microsegregation during Solidification of Graphitic Fiber-Reinforced Aluminum Alloys under External Heat Sinks

Squeeze casting and melt infiltration were employed in processing continuous graphitic fiber-reinforced aluminum matrix composites. The fiber reinforcements were (1) uncoated carbon fiber (UNC-CF), (2) Ni-coated carbon fiber (NiC-CF), and (3) bare graphite fibers (GRFs), and they were externally cooled to enhance the local solidification of the matrix alloy. The solidified microstructures and their composition profiles were examined using optical microscopy, scanning electron microscopy–energy-dispersive X-ray, and electron probe microanalysis–wavelengh-dispersive X-ray. The resultant microstructures in the UNC-CF and NiC-CF–reinforced composites exhibited significant differences from those found in the GRF-reinforced composite, in terms of solidified morphologies and compositions. It was found that coarse columnar dendrites developed in the fiber-free matrix, fine equiaxed dendrites in the chilled matrix, and columnar-like arms in the fiber-reinforced matrices. In contrast, in bare GRF-reinforced composites, two distinct regions were clearly distinguished: (1) a region consisting of coarse equiaxed dendrites in the fiber-free matrix and (2) a featureless morphology within the fiber reinforcement regions. These distinct microstructures were attributed to preferential heat extraction through the GRFs, which possess a relatively high thermal conductivity. Apparently, heat extraction through the GRFs led to the formation of single α-Al envelopes on the fiber surfaces. In addition, the extent of solute segregation found in the GRF-reinforced alloy composite was relatively small when compared with the CF-reinforced alloy composites.

Characterization of Al–Mn particles in AZ91D investment castings

Manganese is currently added to Mg–Al alloys in order to improve the corrosion behavior of cast components. A part of this manganese is dissolved in the magnesium matrix and the balance is found as fine Al(Mn,Fe) particles dispersed within castings. For AZ91D specimens prepared using the plaster mould investment casting process, these particles were observed in very large quantity at the surface of castings. These particles were characterized by scanning electron microscopy and electron probe microanalysis. It was found that they consist of Al 8Mn 5 phase and that their morphology and size depend on local solidification conditions. Their presence at the surface of the castings is related to low solidification rates and reduced thermal gradients at the mould/metal interface.

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.

Formation of gas induced shrinkage porosity in Mg-alloy high-pressure die-castings

It is shown that gas pores can facilitate formation of shrinkage porosity in high-pressure die-castings. Air in the gas pores is an efficient heat-insulating medium that retards heat transfer in the melt as compared to regions without porosity, leading to lower local solidification rate, which can induce shrinkage porosity formation.

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.

Residual Stresses, Thermomechanical Behavior and Interfaces in the Weld Joint of Ni‐based Superalloys

The paper relates aspects of interfaces, residual stresses and dislocations with joint behaviour in the Ni–based single crystals. It is focused on the understanding the fundamental driving forces and response of the materials which leads to the formation of dislocation structure during local melting and solidification. Oscillations in the dislocation structure are observed at both macro and micro scales. The underlying mechanism of these processes is essential to real world applications.

Marangoni Convection and Fragmentation in LASER Treatment

Epitaxial Laser Metal Forming (E-LMF) consists in impinging a jet of metallic powder onto a molten pool formed by controlled laser heating and thereby, generating epitaxially a single crystal deposit onto a single crystal substrate. It is a near net-shape process for rapid prototyping or repair engineering of single crystal high pressure/high temperature gas turbines blades. Single crystal repair using E-LMF requires controlled solidification conditions in order to prevent the nucleation and growth of crystals ahead of the columnar dendritic front, i.e., to ensure epitaxial growth and to avoid the columnar to equiaxed transition. A major limitation to the process lies in the formation of stray grains which can originate either from heterogeneous nucleation ahead of the solidification front or from remelting of dendrite arms due to local solute enriched liquid flow, .i.e fragmentation. To study this last aspect, heat and fluid flow modelling is required to establish the relationship between process parameters such as laser power, beam diameter and scanning speed, and the local solidification conditions plus the fluid flow in the vicinity of the mushy zone. Surface tension driven convection known as the Marangoni effect needs to be included in the model owing to its large influence on the development of eddies and on the shape of the liquid pool. The 3D model implemented in the FE software calcosoft® is used to compute the fluid convection within the liquid pool and to assess the risk of fragmentation using a criterion based on the local velocity field and thermal gradient. The computed results are compared with EBSD maps of laser traces carried out at EPF-Lausanne in re-melting experiments.

A dual-scale coupled micro/macro segregation model for dendritic alloy solidification

The present article reports on the formulation, numerical implementation, and application of a single-domain coupled micro/macroscopic model for simulation of dendritic alloy solidification. Microscopic solutal non-equilibrium effects have been included in the macroscopic modeling of solidification by using a fixed grid dual scale numerical approach. Salient features of the present approach include a continuum model for conservation of mass, momentum, energy and species on the macroscopic scale, a microscopic solute redistribution model, and the solution procedure and auxiliary equations necessary for coupling between the two models. The coupling between macro and micro scale models is practically made possible by introducing an iterative micro/macro time step scheme. The local solidification rate is calculated by implicit iterations of macroscopic conservation equations and the microscopic solute redistribution model. The present model is capable to simulate eutectic reaction, local re-melting, and account for the inter-linkage between micro and macroscopic solute redistribution (micro and macrosegregation).

Segregation substructures in dilute Al–Cu alloys directionally solidified

Directional growth of dilute Al–0.2 wt.% Cu alloy was performed under conditions of controlled solidification. Proper metallographic analysis of the segregation patterns behind the solid–liquid (S–L) interface gives information about the effect of the local solidification conditions that determines the microsegregation. In order to explain the apparition of different solidification substructure as a function of the parameter ( G L / V C 0 ) ( G L , thermal gradient in the liquid in front of the S–L interface; V, solidification rate and C 0 , nominal composition of the alloy), as well as the small segregated instabilities detected at the cellular walls, a mechanism based on the morphological stability theory is proposed.

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.

Solidification path in the Ni-base superalloy, IN713LC—quantitative correlation of last stage solidification

An enthalpy-based method coupled with microstructural characterisation has been used to identify the solidification path and to calculate local solidification kinetics for two chemistries corresponding to IN713LC. Enhanced micro-segregation accompanying carbide precipitation during last stage solidification was associated with a retarded rate of solid evolution.

Laser developed Al–Mo surface alloys: Microstructure, mechanical and wear behaviour

Molybdenum forms a significant proportion of hard intermetallic compounds with Al, which greatly increase the strength of the material. Hence, Al–Mo alloys are potential candidates for coating materials to improve the surface hardness and wear resistance of Al-based light alloys. Surface alloys with composition ranging from 14.8 to 19.1 wt.% Mo were produced by laser surface alloying, injecting Mo powders into the melt pool created in Al substrates using a CO 2 laser. Their microstructure consists of intermetallic compound particles dispersed in a matrix of α-Al solid solution. The intermetallic particles present acicular morphology at low scanning speeds and an equiaxial, flower-like shape at high scanning speeds. TEM characterisation shows that both types of particles correspond to the rhombohedral intermetallic compound Al 5Mo(r), suggesting that the different morphologies result from changes in the local solidification conditions. The hardness of the Al–Mo alloys varies in the range 85–160 HV, while their Young's modulus, assessed by ultramicroindentation tests, varies from 84 to 92 GPa. Wear resistance was measured in dry sliding conditions against a hardened steel counterbody. The wear coefficient decreases with increasing load due to the protective effect of stable mechanically mixed layers that form on the surfaces of the samples and the counterbody during sliding. The wear mechanism in Al–Mo alloys is predominantly adhesion and material transfer and oxidation, with a minor contribution of abrasion.

Primary cellular/dendritic spacing selection in rapidly solidified peritectic alloy

Primary cellular/dendritic spacing plays an important role in the microstructure control under rapid solidification conditions, here laser resolidification experiments have been performed with Ti–Al alloys to clarify the growth mechanism of peritectic reaction. By taking both transverse and longitudinal sections of the directional growth cells/dendrites in laser traces, the primary phase separation, primary cellular/dendritic spacing, as well as the local solidification velocity are quantitatively determined by means of SEM and TEM. With the solidification velocity increasing, the primary phase evolves from phase α, two-phase ( α + γ), to single-phase γ, and the corresponding primary cellular/dendritic spacing decreases gradually. The experimental observation of primary spacing evolution exhibits a good agreement to the prediction of rapid dendritic growth model in all single-phase regions.

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.

Modelling the Marangoni convection in laser heat treatment

Epitaxial Laser Metal Forming (E-LMF) consists in impinging a jet of metallic powder onto a molten pool formed by controlled laser heating and thereby, generating epitaxially a single crystal deposit onto the damaged component. This new technique aims to be used for the repair and reshape single crystal gas turbine components. Because of the very localised melting pool, the high temperature gradients produced during the process must be carefully controlled in order to avoid both the columnar-to-equiaxed transition (CET) and the appearance of hot tears. To this end, heat flow modelling is required to establish the relationship between process parameters such as laser power, beam diameter and scanning speed, and the local solidification conditions. When modelling the heat transfer within the sample, it is necessary to include the liquid flow pattern generated by the surface tension driven convection known as the Marangoni effect. Indeed, the fluid flow in the liquid pool dictates the shape of the traces as shown by the measurements carried out at EPF-Lausanne in re-melting experiments. A three dimensional (3D) model is implemented in the finite element software calcosoft$^\text{\textregistered}$ in order to model the development of the fluid convection within the liquid pool. It is shown that the velocities due to natural convection are of the order of 1 mm/sec whereas Marangoni convection produces velocities of the order of 1 m/sec. Moreover, at low scanning speeds, the liquid pool becomes larger than the beam diameter and the development of Marangoni eddies leads to a widening and deepening of the pool. The local solidification conditions such as the thermal gradient and the solidification speed can be extracted at both the solidus and liquidus temperatures to assess the risk of CET and hot cracking.

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.

Stepped Polymer Morphology Induced by a Carbon Nanotube Tip

Stepped conical structures have been produced at the surface of poly(ethylene glycol) by contacting a single, relatively short carbon nanotube attached to an AFM tip with the molten polymer surface, followed by polymer cooling. Cooling of the polymer melt in the nanotube vicinity is the most likely mechanism for these ziggurat-like structural formations. Simple heat transfer calculations confirm the effect of the nanotube length on the propensity for local solidification of the polymer.