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Minimizing Porosity in Cast Junctions


Casting modeling helped to create general design guidelines to improve the manufacturability of junctions in metal castings.


D. Joshi, SGS Institute of Technology and Science, Indore, India B. Ravi, Indian Institute of Technology, Bombay, India


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any casting defects that cannot be eliminated by changes to tooling and process param- eters can be attributed


to poor part design with respect to manufacturability. One common defect is shrinkage porosity occurring at casting junctions. Most castings can be viewed as an


assembly of junctions. A casting junction is an abrupt increase in local thickness caused by two or more elements (walls) meeting together, resulting in regions of high thermal concentration. Molten metal cools slowly at junctions, leading to shrinkage porosity defects. The size and extent of the defect regions depend on the thickness and number of elements


involved and the angle between them, all of which affect the rate of heat transfer from the casting. The defects can be avoided by casting


process design changes, such as placing a riser above the junction or a chill below it, but these add to the cost of produc- tion and may not always be feasible. An alternative is to modify the junctions to minimize such problems before solidifi - cation using 3-D modeling and analysis. General design for manufacturabilty


guidelines have been established to re- duce porosity defects in L, V, T, Y and X junctions through minor changes in the design, such as adding fi llets, reducing thicknesses or adding a central cored hole to reduce stress concentrations (Fig. 1). These guidelines apply to castings with


simple junctions, but castings with com- plex shapes require numerical simulation of solidifi cation to predict the location and extent of shrinkage porosity defects.


Predicting Hot Spot Areas One approach to improving junction


designs in complex castings involves the vector element method (VEM), which traces feed metal paths in reverse to pinpoint the location of hot spots. It is based on the principle that the direction of the highest temperature gradient at any point inside a casting is given by the vector sum of individual thermal fl ux vectors in all directions around the point. Multiple hot spots are detected by starting from several seed points. Junctions can be classifi ed by their


Fig. 1. Porosity defects in standard junctions are shown with guidelines for minimizing the defect. Table 1. Effectiveness of Design Changes


Sample Inner Effectivness Outer Effectiveness Thickness Effectiveness Angle Effectiveness Length Effectiveness radius (mm)


1 2 3 4 5 6


0


15 30 45 60 -


0


173.08 158.93 134.25 107.76 -


radius (mm) 0


15 30 45 60 -


34 Metal Casting Design anD PurChasing 0


83.21 39.94 25.15 17.47 -


(mm) 30 36 42 48 54 60


0


2.93 3.32 3.66 4.78 6.05


30 60 90


120 150 -


0


3.38 3.31 3.28 3.25 -


(mm) 5


10 20 30 60 90


0


75.13 42.47 32.01 18.71 12.87


January/February 2010


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