Kyle Fezi

Simulation of Slag-Skin Formation in Electroslag Remelting Using a Volume-of-Fluid Method

A modified volume-of-fluid method is implemented in a fixed-grid, finite-volume model simulating transport phenomena, solidification, and electromagnetics. The VOF model agrees well with published results, and the complete model is used to investigate process variations in the electroslag remelting process, in which liquid metal is melted from a consumable electrode immersed in an electrically resistive slag. The molten metal sinks through the slag cap floating on the liquid metal pool while a slag skin freezes to the mold. Here a VOF tracks slag skin formation and its effects on melt rate with different current levels and ingot diameters.

A Metric for the Quantification of Macrosegregation During Alloy Solidification

A metric for quantifying the degree of solidification macrosegregation is proposed that statistically fits compositional data from experiments and simulations to a three-parameter Weibull distribution. The method for fitting such a distribution is described and examples are presented. The new metrics are compared to existing macrosegregation measures and the Weibull distribution is shown to be the best fit to data. The fitted three-parameter Weibull distribution is generally found to have better agreement with the composition data than a Gaussian distribution, upon which the macrosegregation number is based, because the Weibull better accounts for asymmetry in the dataset. Trends in macrosegregation results are identified using the new metrics, specifically the normalized Weibull deviation, and compared to the trends identified by the macrosegregation number. A grid dependence study is performed using both metrics as tests for convergence. The utility of the Weibull distribution is demonstrated by comparing composition data with different degrees of asymmetry due to different solidification cooling rates. The difference between the values of the two metrics is a measure of the asymmetry in the compositional distribution.

Macrosegregation modeling during direct-chill casting of aluminum alloy 7050

ABSTRACT A fully transient model of the direct-chill casting process is used to predict the macrosegregation development of aluminum alloy 7050. The ingot diameter, casting speed, superheat, secondary cooling, and thickness of pure Al at startup are varied. Predicted radial composition distributions are fit to Weibull probability density functions at each axial location, and the normalized standard deviation describes the macrosegregation level and the time when the process reaches steady state. The sump depth, steady-state height, and macrosegregation level were most affected by changes in casting speed and ingot diameter. The pure Al dilutes the alloy and delays compositional steady state.

Modeling Macrosegregation during Electroslag Remelting of Alloy 625

A numerical model of ESR is used to examine the effects of ingot diameter, process current levels, and initial composition on macrosegregation in alloy 625. Composition variations are made within the standard ranges for alloy 625 to evaluate their effect on buoyancy driven flows. Overall steady state macrosegregation, along with composition distributions are compared, to identify processing conditions that best ameliorate this defect. The composition distributions are analyzed to illustrate how macrosegregation affects the ability to meet the compositional specification. As expected, increasing the ingot size from the standard processing condition of 50 cm (20 inch) to 76.2 cm (30 inch) increases the segregation level overall and at the centerline. The segregation tends to decrease with decreasing mushy zone velocity and sump depth. Processing ingots with a low current and an initial composition in the low end of the specification range is the best choice to reduce macrosegregation.

Erratum to: Macrosegregation During Electroslag Remelting of Alloy 625

THE sentence on lines 110–115 (Another metric has been developed recently by Voller and Vušanović that calculates the volume of the solid ingot which has a macrosegregation level (Mi = Ci Co) greater than or equal to Mi, termed the Cumulative Distribution Function (CDF).) of this article should have a reference number added [23] at the end. Also, the new reference should be added at the end of the references: 23. V.R. Voller and I. Vušanović: Int. J. Heat Mass Transf., 2014, vol. 79, pp. 468–71.

Uncertainty quantification in modelling equiaxed alloy solidification

Abstract Numerical simulations can provide insight into physical phenomena during alloy solidification processes that cannot be observed experimentally. These model predictions depend on the material, process and numerical parameters which contain inherit uncertainties due to their origin in experimental measurements or model assumptions. As a step towards understanding the effect of uncertain inputs on solidification process modelling, uncertainty quantification and sensitivity analysis are performed on a transient model of transport phenomena during the solidification of grain refined Al-4.5 wt.% Cu in a rectangular cavity. The effect of microstructural model parameters, thermal boundary conditions and material property input uncertainties are examined for their effect on macrosegregation levels and solidification time. Predictions of the macrosegregation level are most sensitive to the dendrite arm spacing of the rigid mushy zone.

Influence of a wiper on transport phenomena in direct chill casting of aluminium alloy 7050

Abstract A fully transient model of the direct chill casting process is used to examine the influence of implementing a wiper to divert the free falling water from the ingot surface on transport phenomena and segregation profiles for aluminium alloy 7050. The ingot diameter and casting velocity are varied to determine the proper wiper placement as determined by the predicted composition, temperature, and fraction solid fields. The segregation level was evaluated by comparing steady state radial composition profiles. The wiper alters the dominant radial heat transfer mechanism, causing the surface of the ingot to heat up below the wiper. Placing the wiper too close to the mould causes the surface temperature to increase above the solidus, making the ingot susceptible to bleed outs. The optimal wiper location was found to be near the bottom of the sump for all process conditions examined.

Macrosegregation during Direct Chill Casting of Aluminum Alloy 7050

A fully transient numerical model of the direct chill casting process is used to examine the influence of casting parameters on the macrosegregation of alloy 7050. The casting speed and secondary cooling boundary condition were varied. The secondary cooling was altered by placing a wiper to divert the free-falling water away from the solidified metal surface. Both the casting rate and wiper placement affect the sump depth and shape, and therefore influence the level of macrosegregation, especially at the centerline. The sump depth and level of macrosegregation increased with the casting rate, while placing the wiper nearer the mold could decrease negative centerline segregation.

Quantification of Input Uncertainty Propagation Through Models of Aluminum Alloy Direct Chill Casting

Insight into transport phenomena in complex solidification processes, such as direct chill (DC) casting, that cannot be found from experimental observation can be gained from numerical simulations. These predictions depend on material, process, and numerical parameters which contain inherit uncertainties due to experimental measurements or model assumptions. A fully transient numerical model of the direct chill casting process of Al-4.5 wt pct Cu was used to examine the propagation of input uncertainty to outputs of interest. The effect of microstructural model parameters, thermal boundary conditions, and material property input uncertainties were examined. Probability density functions were calculated based on these input uncertainties for metrics that characterize the ingot macrosegregation and sump depth. The macrosegregation-level predictions depend strongly on parameters that control the formation of the rigid mushy zone and shrinkage-driven flow. The heat release and transfer in the mushy zone are the dominant factors for determining the sump depth.

Uncertainty Propagation in Numerical Modeling of Direct Chill Casting

Complex solidification processes are often simulated to gain insight into physical phenomena that cannot be experimentally observed. Validation of these models is done through direct comparison to scarce experimental data, in which agreement can be misinterpreted due to uncertainties from the model and experimental measurements. A fully transient numerical model of the direct chill casting process of Al-4.5 wt.% Cu is used to examine the propagation of input uncertainty to outputs of interest. This model solves equations for momentum, temperature, and species conservation. The effect of material property input uncertainties are examined. Probability density functions are calculated based on these input uncertainties for metrics that characterize the ingot macrosegregation and sump depth.