Melt Superheat Research Articles

Effect of Si Content of Al-Si Matrix on Roll Cast Wire Inserted Strip

Effect of Si content of Al-Si matrix on roll cast wire inserted strip was investigated in this paper. 99.5%Al, Al-4%Si , Al-12%Si and Al-18%Si were used as matrix. Slit depth of the nozzle, superheat of melt, wire-diameter and roll speed was chosen as the casting parameters, too. Addition of the Si was useful to prevent the reaction between the matrix and the wire. A downward melt drag twin roll caster could cast the wire inserted strip of the Al-Si matrix of which Si content was ranging up to18%. Composite strip, which matrix was hyper eutectic Al-18%Si, could be cast at the speeds up to 20m/min. The gap between the wire and matrix became thin when the superheat was high. When the slit depth of the nozzle was too shallow, the wire could not be inserted in the strip. The thickness of the wire inserted strip became thinner as the roll speed became higher.

Roll Casting of a Wire Inserted Strip

Casting of a wire-inserted strip was tried by a downward melt drag twin roll caster. Some numbers of wires of 0.3 mm diameter were inserted. Effects of the wire-interval, matrix alloy and melt temperature on the ability of the insertion of the wire were investigated. Minimum interval of the wire, with which the insertion of the wires was able, was 1.6 mm. Insertion of the wire became easy as the roll speed becomes higher. Superheat of the melt was not important factor for the insertion of the wire.

Computational Fluid Dynamics Modeling of Macrosegregation and Shrinkage in Large-Diameter Steel Roll Castings

Minimizing macrosegregation and shrinkage in large cast steel mill rolls challenges the limits of commercial foundry technology. Processing improvements have been achieved by balancing the total heat input of casting with the rate of heat extraction from the surface of the roll in the mold. A submerged entry nozzle (SEN) technique that injects a dilute alloy addition through a nozzle into the partially solidified net-shaped roll ingot can mitigate both centerline segregation and midradius channel segregate conditions. The objective of this study is to optimize the melt chemistry, solidification, and SEN conditions to minimize centerline and midradius segregation, and then to improve the quality of the transition region between the outer shell and the diluted interior region. To accomplish this objective, a multiphase, multicomponent computational fluid dynamics (CFD) code was developed for studying the macrosegregation and shrinkage under various casting conditions for a 65-ton, 1.6-m-diameter steel roll. The developed CFD framework consists of solving for the volume fraction of phases (air and steel mixture), temperature, flow, and solute balance in multicomponent alloy systems. Thermal boundary conditions were determined by measuring the temperature in the mold at several radial depths and height locations. The thermophysical properties including viscosity of steel alloy used in the simulations are functions of temperature. The steel mixture in the species-transfer model consists of the following elements: Fe, Mn, Si, S, P, C, Cr, Mo, and V. Density and liquidus temperature of the steel mixture are locally affected by the segregation of these elements. The model predictions were validated against macrosegregation measured from pieces cut from the 65-ton roll. The effect of key processing parameters such as melt composition and superheat of both the shell and the dilute interior alloy are addressed. The influence of mold type and thickness on macrosegregation and shrinkage also are discussed.

Solidification structure and crystallographic texture of strip casting 3wt.% Si non-oriented silicon steel

► Three as-cast strips with different initial solidification structures were produced by twin-roll strip casting process. ► The equiaxed grained strip displays weak and nearly random texture, but the columnar grained strip reveals pronounced {001} fiber texture. ► The deviation degree of the columnar grain from the normal direction decreases with the increase of melt superheat.

Effect of melt superheat on structure of unsteady state unidirectionally solidified duplex stainless steel

An experimental study with respect to the effect of melt superheat on the unidirectional solidified structure of duplex stainless steel (DSS) is reported. The results show that the melt superheat causes distinct modifications of both macrostructure and microstructure of DSS. For macrostructure of DSS, the columnar to equiaxed transition is observed at different melt superheats. The length of equiaxed zone increases and the equiaxed grain size decrease with melt superheat decreasing. For microstructure of DSS, there are significant differences in the morphology of the austenite at intragranular and boundary. In the interior of ferrite grains, the austenite appears primarily as acicular morphology. Nevertheless, the austenite takes on a widmänstatten side plate appearance at the ferrite grain boundaries. In addition, the austenite content increases with melt superheat decreasing.

Controlled Diffusion Solidification of 2024, 6082 and 7075 Al Alloys via Tilt-Pour Casting Process

Controlled diffusion solidification (CDS) is an innovative casting route for Al alloys to obtain a cast part with a non-dendritic morphology of the primary Al phase during solidification, similar to those obtained in the semi-solid rheocasting process. The process involves mixing two alloy melts with specific individual compositions and temperatures to produce the desired final alloy by mixing and immediately casting in a mould. CDS enables sound shaped casting of Al based wrought and cast alloys with a non-dendritic microstructure. The two pre-cursor alloys will have to be chosen from a critical study of the various isopleths of the multi-component phase diagrams for the respective alloys along with laboratory experiments. Further, the favorable melt superheat temperatures of the two pre-cursor alloys were evaluated from laboratory experiments as well. A tilt-pour mould designed and validated to cast tensile test bars was used for the study. The aim of the study was to demonstrate the feasibility of manufacturing sound shaped castings via the CDS route for alloys which have been impossible to shape cast in a permanent mould process due to the detrimental hot-tearing problem during solidification. The present work defines process conditions to enable shaped casting of the Al wrought alloys 2024, 6082 and 7075 by the tilt-pour casting technique. Sound casting and a favorably non-dendritic microstructure were obtained for these alloys during solidification in a ceramic crucible as well as in a metallic mould via the tilt-pour casting process. Further work is underway to optimize various process conditions and develop new heat treatment cycles for parts cast by CDS to maximize mechanical properties for these alloys.

Sensitivity of Thermophysical Material Properties on Solidification Simulation of Al-Si Binary Alloys

The challenges in the numerical simulation of the solidification of binary alloys are not only in the complexity of the algorithms themselves, but also in the validity of the data used to define the material properties of the various phases to obtain a valid simulation. The effect of material properties on the numerical simulations was investigated in the present study wherein the Al-3 wt pct Si hypoeutectic binary alloy was solidified such that the solidification front traveled against the gravity vector (upward solidification). Numerical simulations were carried out with a new algorithm that was developed to include the effect of undercooling of the liquid temperature prior to the solidification event. The effect of specific heat of solid, density of solid, solute diffusivity coefficient of liquid, and thermal conductivity of solid on transient temperature distribution and solidification start time at mushy zone/liquid interface was investigated. It was found that specific heat and thermal conductivity of the solid could not be assumed as constants, whereas most properties in the liquid phase could be assumed as constants for the temperature range used in the study and the experiments used for validation (low initial melt superheat temperature). These properties were enumerated and quantified. The results of the numerical simulations using the optimum set of material properties were validated by experiments.

Effects of Melt Treatment on Dendrite Coherency of A357 Alloy

The Dendrite Coherency Point (DCP) of A357 alloy was determined after different melt treatments by double thermocouples, and the coherency solid fraction (fscoh) was calculated by thermal analysis. The results of dendrite coherency properties show that fscoh values increase with increased cooling rate for A357 alloy. For A357 alloys, fscoh values increase after grain refined and melt superheat treatment. The coherency point was found to be dependent on not only the morphology of the dendrites but also the dendrite growth rate.

Determination of interfacial heat flux of stainless steel solidification on copper substrate during the first 0.2 s

Interfacial heat transfer is a key issue in many solidification processes. In the paper, a novel experimental apparatus has been designed and on this basis, the instantaneous interfacial heat transfer between molten steel or solidified shell and copper substrate during the first 0.2 s has been studied. The investigated parameters include melt superheat, substrate temperature and surface roughness. The results show that the peak value of the interfacial heat flux in the first stage of liquid/solid contact increases with melt superheat and changes slightly with substrate temperature and surface roughness. The interfacial heat flux in the stage of solid/solid contact has a similar trend of slow decrease in most conditions.

Modeling of macrosegregation in steel ingot: Influence of mold shape and melt superheat

A continuum model for the transport phenomena in solidification systems is used to investigate the formation mechanism of macrosegregation in a 3.3 t steel ingot. Numerical scheme with explicit time stepping in solidification problems is developed for solving coupled temperature and concentration fields, and equations of momentum. Experimental measurements and numerical results in the literature are used as indications of the validity of present prediction. Influences of the mold shape and the melt superheat upon macrosegregation are investigated. Results show that for ingots with the same weight, reducing the size of hot top favors a pronounced positive-negative-positive concentration distribution along the centerline. A-segregates and positive-negative-positive concentration distribution are not found in the ingot with a modified hot top that has a more uniform section area. Higher superheat reduces the height of bottom negative segregation cone. For cases with superheat larger than zero, positively segregated patches are observed at the ingot bottom.

Numerical Simulation of Fluid Flow and Thermal Characteristics of Thin Slab in the Funnel-Type Molds of Two Casters

A mathematical model of three-dimensional fluid flow, heat transfer and solidification in thin slab casting is developed based on the enthalpy-porosity approach in a single-phase framework and used to analyze and compare the fluid flow and thermal behaviors in the funnel-type molds of Compact Strip Production (CSP®) and Flexible Thin Slab Casting (FTSC®) caster. An Electromagnetic Brake (EMBR) model is also incorporated into this coupled model to see the effects of EMBR on fluid flow and solidification in a CSP caster with EMBR system. The numerical treatments specific to the complex funnel-shaped mold boundaries are discussed. The obtained numerical results are partly validated by those available in literature and theoretical analyses. The fluid flow and thermal features of thin slab in CSP mold and FTSC mold are analyzed and their comparisons between them are made. It is found that the flow pattern, level fluctuation, superheat dissipation, temperature distribution and shell growth in both molds present different features. The structure of SEN strongly affects the flow pattern of the impinging jet from SEN ports, and then the dissipation mode of the melt superheat and the uniform growth of solidification shell. A four-port SEN® with two small upper ports in FTSC mold is helpful to activate the meniscus flow and increase the meniscus temperature, but high level fluctuation may occur. The EMBR in a CSP mold can both suppress the meniscus flow and increase the meniscus temperature. A complete flow-control system including the design of SEN, EMBR and operational conditions is essential for thin slab production considering their individual flow and thermal features in the funnel-type molds of CSP and FTSC casters.

Numerical simulation of solidification of liquid aluminum alloy flowing on cooling slope

Preparation of semisolid slurry using a cooling slope is increasingly becoming popular, primarily because of the simplicity in design and ease control of the process. In this process, liquid alloy is poured down an inclined surface which is cooled from underneath. The cooling enables partial solidification and the incline provides the necessary shear for producing semisolid slurry. However, the final microstructure of the ingot depends on several process parameters such as cooling rate, incline angle of the cooling slope, length of the slope and initial melt superheat. In this work, a CFD model using volume of fluid (VOF) method for simulating flow along the cooling slope was presented. Equations for conservation of mass, momentum, energy and species were solved to predict hydrodynamic and thermal behavior, in addition to predicting solid fraction distribution and macrosegregation. Solidification was modeled using an enthalpy approach and a volume averaged technique for the different phases. The mushy region was modeled as a multi-layered porous medium consisting of fixed columnar dendrites and mobile equiaxed/fragmented grains. The alloy chosen for the study was aluminum alloy A356, for which adequate experimental data were available in the literature. The effects of two key process parameters, namely the slope angle and the pouring temperature, on temperature distribution, velocity distribution and macrosegregation were also studied.

Microstructure of an experimental Ni base superalloy under various casting conditions

The effect of casting parameters such as melt superheat and solidification rate on the microstructure of an experimental Ni-based superalloy made by vacuum melting and investment casting methods was investigated. The results show that the as cast microstructure of this alloy consists of dendritic γ matrix, interdendritic γ/γ′, γ′ phase, MC carbides and minor phases in interdendritic regions. DSC analyses in addition to SEM and EDS proved that the nodular phase found in minor phases is σ phase, while the η phase was found to solidify in two forms; as plate-like and blocky shape. The volume fraction ( V f) of σ and η phases increases as the melt superheat increases and the solidification cooling rate decreases. Additionally, the V f of interdendritic γ/γ′ increases with higher solidification rate and lower melt superheat. It was found that higher melt superheating temperature enlarges the size of the primary γ dendrites; however the higher solidification rate diminishes it.

Rotational rheometry of liquid metal systems: Measurement geometry selection and flow curve analysis

In the present study, a rotational measurement technique was used to evaluate viscosities of liquid metals and metallic alloys. Three types of measurement geometries in a high temperature rotational rheometer were evaluated: cone and plate, DIN coaxial, and double concentric cylinder (DCC). The DCC geometry proved to be the most effective. An analytical solution has been presented to evaluate the viscosity as a function of shear rate for DCC geometry. The flow curves and shear viscosities of pure Al, pure Zn and Sn 95.8Ag 3.28Cu 0.92 solder alloy have been evaluated as a function of shear rate and melt superheat temperature. It is proposed that liquid metal systems are non-Newtonian and strongly shear thinning in flow behavior.

Microstructure Formation in Al-Bi-Co Immiscible Alloys Directionally Solidified with Different Melt Superheat Temperatures

The directional solidification has been carried out for the Al-4Bi-2.5Co (wt pct) alloys with different melt superheat temperatures. The microstructure characterization and the quantitative metallographic analysis have been performed. The results indicated that the Bi-rich sphere size and cellular spacing decrease with increasing melt superheat temperature. The interaction between the advancing solidification interface and the Bi-rich spheres with different sizes was analyzed. The effect of the melt superheat treatment on microstructure evolution was discussed for the immiscible alloys. The microstructure development in ternary Al-Bi-Co alloys directionally solidified with different melt superheat temperatures was clarified.

Gas Atomization of Amorphous Aluminum Powder: Part II. Experimental Investigation

The optimal processing parameters that are required to atomize amorphous Al were established on the basis of numerical simulations in part I of this study. In this part II, the characterization of cooling rate experienced by gas-atomized, Al-based amorphous powders was studied via experiments. An experimental investigation was implemented to validate the numerical predictions reported in part I of this study. The cooling rate experienced by the powders, for example, was experimentally determined on the basis of dendrite arm spacing correlations, and the results were compared with the numerical predictions. The experimental studies were completed using commercial Al 2024 as a baseline material and Al90Gd7Ni2Fe1 metallic glass (MG). The results showed that the cooling rate of droplets increases with decreasing particle size, with an increasing proportion of helium in the atomization gas and with increasing melt superheat. The experimental results reported in this article suggest good agreement between experiments and numerical simulations.

Gas Atomization of Amorphous Aluminum: Part I. Thermal Behavior Calculations

In this article, the thermal history and cooling rate experienced by gas-atomized Al-based amorphous powders were studied via numerical simulations. Modeling simulations were based on the assumption of Newtonian cooling with forced convection, as well as an energy balance, which involves gas dynamics, droplet dynamics, and heat transfer between gas and droplet. To render the problem tractable, phase transformations, crystal nucleation, and growth were not taken into account in the analysis of the solidification of Al droplets; instead, an energy balance approach was formulated and used. The numerical results and associated analysis were used to optimize processing parameters during gas atomization of Al-based amorphous powder. The results showed that the cooling rate of droplets increases with decreasing powder size and can reach in excess of 105 K/s for powder <20 μm in diameter. Gas composition has a more significant influence on cooling rate than gas pressure, and 100 pct He has the highest cooling effect. The results also showed that the cooling rate increases with increasing melt superheat temperature.

Modeling the Effect of Active Fiber Cooling on the Microstructure of Fiber-Reinforced Metal Matrix Composites

A modified pressure infiltration process was recently developed to synthesize carbon-fiber-reinforced aluminum matrix composites. In the modified process, the ends of carbon fibers are extended out of the crucible to induce selective cooling. The process is found to be effective in improving the quality of composites. The present work is focused on determining the effect of the induced conductive heat transfer on the composite system through numerical methods. Due to the axisymmetry of the system, a two-dimensional (2-D) model is studied that can be expanded into three dimensions. The variables in this transient analysis include the fiber radius, fiber length, and melt superheat temperature. The results show that the composite system can be tailored to have a temperature on the fiber surface that is lower than the melt, to promote nucleation on the fiber surface. It is also observed that there is a point of inflection in the temperature profile along the particle/melt interface at which there is no temperature gradient in the radial direction. The information about the inflection point can be used to control the diffusion of solute atoms in the system. The result can be used in determining the optimum fiber volume fraction in metal matrix composite (MMC) materials to obtain the desired microstructure.

Aspects of porosity formation in spray deposited thin aluminium strip

A characteristic feature of the spray deposited metal strip is that it contains porosity. The present paper has reported and explained the effect of various process parameters such as atomising pressure, deposition distance, melt superheat and melt delivery tube diameter on porosity of the spray deposited thin aluminium strip. Porosity of the spray deposited aluminium strip greatly depends not only on the liquid fraction present on the substrate, but also on the thermal condition through the substrate. It has been shown that there are several types of porosity present in the strip, namely interstitial, mechanically entrapped gas, intersplat boundary, solidification shrinkage and porosity due to hydrogen evolution. The mechanism of their formation has been discussed. An increase in the initial temperature of the substrate lowers the porosity content in the spray deposited aluminium strip.

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

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.