Kent D. Carlson

Determination of solid fraction–temperature relation and latent heat using full scale casting experiments: application to corrosion resistant steels and nickel based alloys

Abstract Casting simulation results are only useful to a foundry if they reflect reality, which requires accurate material datasets for the alloys being simulated. Material datasets include property data such as density, specific heat and thermal conductivity as functions of temperature, as well as latent heat of solidification and a solid fraction–temperature relation. Unfortunately, there are a significant number of commonly used metal alloys for which no reliable material data are available. The present study focuses on five such corrosion resistant alloys: superaustenitic stainless steel CN3MN, duplex stainless steels CD3MN and CD4MCuN and nickel based alloys CW6MC and N3M. Initial alloy material datasets are generated using thermodynamic simulation software. Comparisons of temperatures measured in full scale sand castings made from these alloys with temperatures predicted in computer simulations revealed that these initial datasets are inadequate. Therefore, an iterative method is developed to adjust the datasets (in particular the solid fraction–temperature relation and latent heat) in order to match measured and predicted temperatures and cooling rates. Uncertainties in the simulation are effectively eliminated through parametric studies. Although more tedious, the present iterative method to determine the solid fraction–temperature relation and latent heat is believed to be more accurate than traditional cooling curve analysis using small experimental castings.

Analysis of ASTM X-ray shrinkage rating for steel castings

This paper presents the results of two different studies that examined the ASTM x-ray shrinkage rating system for radiographs of steel castings. The first study evaluated the repeatability and reproducibility of x-ray shrinkage ratings through a statistical study of 128 x-rays that were each rated seven different times. It was found that the seven ratings for each x-ray were in unanimous agreement on both shrinkage type and level for 12.5% of the x-rays. All of the x-rays that had unanimous agreement for both type and level were either completely sound or very unsound (Level 5). The largest variance was found to occur in Level 2 and 3 x-rays, which had 95% confidence intervals of about ±2 x-ray levels. The average 95% confidence interval for all 128 x-rays was ±1.4. The second study involved an effort to determine the shrinkage severity level of x-rays through digital analysis of scanned radiographs. It was found that defect area and circumference correlated reasonably well with x-ray level, but only if the shrinkage type was correctly determined first.

Modeling of Porosity Formation in Aluminium Alloys

A new approach based on microsegregation of gas dissolved in the melt is used to model pore formation during the solidification of aluminum alloys. The model predicts the amount and size of the porosity in a solidified casting. Computation of the micro-/macro-scale gas species transport in the melt is coupled with the simulation of the feeding flow and calculation of the pressure field. The rate of pore growth is calculated based on the local level of gas supersaturation in the melt. The effect of the microstructure on pore formation is also taken into account. Parametric studies for one-dimensional solidification under an imposed temperature gradient and cooling rate illustrate that the model captures important phenomena observed in porosity formation in aluminum alloys. Comparisons between predicted porosity percentages and previous experimental measurements show good correspondence.

Modelling of reoxidation inclusion formation in steel sand casting

Abstract A model is developed that predicts the growth and motion of oxide inclusions during pouring, as well as their final locations on the surface of steel sand castings. Inclusions originate on the melt free surface, and their subsequent growth is controlled by oxygen transfer from the atmosphere. Inclusion motion is modelled in a Lagrangian sense, taking into account drag and buoyancy forces. The inclusion model is implemented in a general-purpose casting simulation code. The model is validated by comparing the simulation results to measurements made on production steel sand castings. Good overall agreement is obtained. In addition, parametric studies are performed to investigate the sensitivity of the predictions to various model parameters.

Microporosity Prediction and Validation for Ni-based Superalloy Castings

Microporosityin high performance aerospace castings can reduce mechanical properties and consequently degrade both component life and durability. Therefore, casting engineers must be able to both predict and reduce casting microporosity. A dimensionless Niyama model has been developed [1] that predicts local microporosity by accounting for local thermal conditions during casting as well as the properties and solidification characteristics of the cast alloy. Unlike the well-known Niyama criterion, application of the dimensionless Niyama model avoids the need to find a threshold Niyama criterion below which shrinkage porosity forms - a criterion which can be determined only via extensive alloy dependent experimentation. In the present study, the dimensionless Niyama model is integrated with a commercial finite element casting simulation software, which can now more accurately predict the location-specific shrinkage porosity volume fraction during solidification of superalloy castings. These microporosity predictions are validated by comparing modelled results against radiographically and metallographically measured porosity for several Ni-based superalloy equiaxed castings that vary in alloy chemistry with a focus on plates of changing draft angle and thickness. The simulation results agree well with experimental measurements. The simulation results also show that the dimensionless Niyama model can not only identify the location but also the average volume fraction of microporosity distribution in these equiaxed investment cast Ni-based superalloy experiments of relatively simple geometry.

A Pore-Centric Model for Combined Shrinkage and Gas Porosity in Alloy Solidification

A unified model has been developed for combined gas- and shrinkage-induced pore formation during solidification of metal alloys. The model is based on a pore-centric approach, in which the temporal evolution of the pore radius is calculated as a function of cooling rate, thermal gradient, gas diffusion, and shrinkage. It accounts for the effect of porosity formation on the liquid velocity within the mushy zone. Simulations for an aluminum alloy show that the porosity transitions smoothly from shrinkage-induced to gas-induced as the Niyama value is increased. A Blake (cavitation) instability is observed to occur when the porosity is both gas- and shrinkage-driven. A revised dimensionless Niyama curve for pure shrinkage is presented. The experimentally observed gas porosity trend that the pore volume decreases with increasing cooling rate is well predicted. The pore-centric formulation allows the present model to be solved locally, at any point in a casting, during a regular casting simulation.

SIMULATION OF POROSITY AND HOT TEARS IN A SQUEEZE CAST MAGNESIUM CONTROL ARM

Simulations are performed for the squeeze casting of AM60 and AZ91 automotive control arms. Advanced feeding flow and stress models are used within commercial casting simulation software to predict shrinkage porosity and hot tears. The simulations are validated through comparisons with observations made on experimental castings. Generally good agreement is obtained between the measured and predicted defect locations and extents. Design and process changes are introduced to mitigate the shrinkage and hot tear problems in these castings. The comparisons in the present study establish considerable confidence in the ability of casting simulation to predict shrinkage and hot tears in squeeze casting of magnesium alloys.