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As the demand for higher-quality components increases, a variety of casting defects that compromise final-product integrity continues to challenge both scientists and engineers. Understanding the defect-formation mechanisms is challenging because casting processes involve complex interactions between a multitude of transient physical phenomena, such as heat transfer, mass transport, fluid flow, solidification, microstructural evolution, and thermal distortion.

State-of-the-art methods to study the formation of casting defects include in situ observation and computer simulations. Visualizing the results using video animations is a powerful tool for understanding multi-dimensional, transient phenomena. The JOM-e articles introduced in this commentary feature animations of the results from ongoing efforts around the world to gain new insights into the formation of a variety of defects in steel and aluminum castings.

ENTRAPPED INCLUSIONS

Exposure of molten metal to air causes reoxidation that generates inclusions, degrading both mechanical properties and surface appearance. For example, the entrapment of oxide dross greatly lowers the quality of aluminum ingots poured on a wheel caster. The article by M. Prakash et al. discusses the application of a new grid-free computer simulation method called smoothed-particle hydrodynamics to predict and visualize the evolution of transient fluid flow and oxide content during the pouring and mold filling of this complex process. (Abstracts and web addresses for all articles in this issue of JOM-e are presented in the sidebars.) The animations show striking realism. Moreover, the authors provide practical evaluations of different configurations, which resulted in optimized wheel and nozzle designs.

In the continuous casting of steel, inclusions may become entrapped in the final product, from particles entering the mold from upstream or from the entrainment of surface slag. The article by B. Thomas et al. uses large-eddy simulations of inclusion transport in turbulent flow to show how the transient fluid flow pattern in the nozzle and mold controls these detrimental phenomena.

SURFACE DEFECTS

Initial solidification at the meniscus, where the free surface of the molten metal touches the mold wall, creates the surface of the final cast product. Complex interacting phenomena at this critical location often cause surface defects that become apparent only after many expensive downstream processes. The article by J. Sengupta and B. Thomas presents animations that clearly visualize how sub-surface microstructural defects called “hooks” and surface grooves called “oscillation marks” arise at the meniscus during mold oscillation in the continuous casting of steel. Deep hooks entrap inclusions and transverse surface cracks often initiate at the roots of deep oscillation marks.

Surface defects are also affected by flow conditions in the mold. Insufficient superheat transported to the meniscus aggravates hook formation. Excessive turbulence and level fluctuations at the surface lead to longitudinal facial cracks, slivers, and breakouts. The article by B. Thomas et al. also extends the large-eddy simulations to gain insight into some of these phenomena.

Segregation leads to severe internal defects in alloy castings. At the microscale, internal hot-tear cracks become permanent defects if they fill with enriched interdendritic liquid. At the macro-scale, segregated liquid frequently concentrates near the casting center line. The article by M. Wu and A. Ludwig applies a multiphase model of thermal-fluid flow, solidification, and grain sedimentation to visualize macrosegregation, columnar-equiaxed transition, and grain size distribution in ingot castings. The model includes the effects of thermal, solutal, surface-tension, and phase-dependent forces on the convection and composition distribution.

Porosity can downgrade the integrity of a casting by providing initiation sites for hot-tear cracks during solidification and fatigue crack propagation during service. The article by P. Lee, J. Wang, and R. Atwood presents animations of in-situ measurements and advanced computations to clearly visualize the tortuous three-dimensional shapes of interdendritic microporosity in aluminum-alloy castings. Meso-scale simulations including heat transfer, dendritic solidification, grain structure, gas distribution, pore nucleation, and growth match well with the measurements. These simulations further reveal the relative importance of gas supersaturation and shrinkage effects on microporosity size and shape.

These five articles in this JOM-e topic exploit a range of modeling and experimental techniques to study casting defects. All of them apply computational models that have been validated with experimental measurements. At their best, such modeling tools can now serve as a virtual laboratory for developing casting processes, with advantages over a real laboratory. Computer animations enable researchers to visualize transient phenomena that are difficult to observe and quantify during the actual casting process, such as reoxidation (Prakash), superheat and inclusion transport (Thomas), segregation (Wu), grain sedimentation (Wu), dendritic solidification (Wu and Lee), and porosity formation (Lee).

Turbulent fluid flow in a full-scale water model behaves similarly to moltenmetal flow and is used to validate the computational flow models (Thomas). Micrographs of etched ultra-low carbon steel samples and other experimental observations are combined with computer model results to develop the animation of oscillation mark and hook formation (Sengupta). Real-time video (Lee) of internal microstructure evolution during laboratory solidification experiments shows in-situ x-ray temperature gradient stage and x-ray microtomography techniques in real metals.

These research articles showcase more than 25 animations, which are published by TMS in the electronic portion of this journal, JOM-e. The on-line journal provides an important archival medium to convey research results through animations, as the human brain processes moving visual information better than any other form of communication. Animation technology itself is a powerful tool to study the subtle complexities of casting defect formation.

In conclusion, the progressive development of both sophisticated mathematical models and advanced experimental techniques has allowed a better understanding of specific physical phenomena responsible for the formation of casting defects. Researchers and engineers continue to translate this new scientific information into real industrial processes and product quality improvements. Computer-aided visualization has emerged as one of many powerful tools available to today’s process engineer, and the articles and animations presented in this issue of JOM-e are just the tip of the iceberg representing worldwide research in the field of casting technology.

Brian G. Thomas is with the Department of Mechanical Science and Engineering at the University of Illinois at Urbana-Champaign. Joydeep Sengupta is with Dofasco Research & Development in Hamilton, Ontario, Canada.

For more information, contact B.G. Thomas, University of Illinois at Urbana-Champaign, Department of Mechanical Science & Engineering, 140 Mechanical Engineering Building, 1206 West Green Street, Urbana, IL 61801; (217) 333-6919; fax (217) 244-6534; e-mail bgthomas@uiuc.edu.