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Metal Casting Technologies : September 2005
Earlier work on effect of pressure during solidification has also shown similar trends. Porosity has reduced in most of the works, especially where gassy metal has been used in the castings. However, the mode of application of pressure in that work  was in the form of compressed gas. There are both merits and demerits of this type of pressure application. The pressure becomes active not only over riser, it acts all over the surface of the casting resulting in a situation of isostatic pressing. Additionally, the change in the surface geometry of the riser in the form of concave surface, as solidification continues, would not prevent the pressurizing effect till the thickness solidified at the top surface of riser is less and it is deformable. On the other hand, pressure in the form of compressed gas over the surface of casting may penetrate to depth under certain condition of solidification pattern resulting in surface depression. The mode of pressure application in the present work would not cause such surface depression, as the pressure is applied over the riser to feed liquid metal. But the change of surface geometry of riser during solidification may discontinue the effect of pressure if the thickness of the metal solidified at the wall of riser is sufficient to support the load applied over it. From that period onwards, the effect of pressure over riser in the manner applied in the present work would become ineffective, although hot forging type of effect may still operate up to certain period. CONCLUSION Present work shows that application of pressure over riser in the form of static load is effective to reduce porosity in and to improve hardness of aluminum alloy gravity die casting. The technique is very simple to be adopted in the shop floor and involves practically no additional cost. Although the principle is proved to be effective for aluminum alloys, apparently it can be effective for magnesium, copper and other alloys. Further investigations are required to optimize the level of pressure for each type of alloy and shape and size of castings. TECHNICAL FEATURE TABLE 2 Effect of pressure over riser on hardness of aluminum gravity die casting Pressure Hardness in BHN along centerline at distance from riser end over riser (kPa) 20 mm 60 mm 100 mm 150 mm No pressure 53 40 50 57 30 69 61 61 69 100 92 86 80 100 REFERENCES 1. G. K. Sigworth and C. Wang, AFS Transactions, 1992, p.979. 2. S. Viswanathan, A. J. Duncan, A. S. Sabau, Q. Han, W. D. Porter and B. W. Riemer, AFS Transactions, 1998, p.411. 3. K. S. S. Murthy and J. O. Edwards, AFS Transactions, 1971, p.281. Dr PC Maity is a Professor in the Foundry Technology Department of the National Institute of Foundry and Forge Technology in Ranchi India. E: email@example.com W: www.mainnotech.com visit www.metals.rala.com.au www Visit the Metals website: Subscribe online Download advertising rates View the company directory listings For further information: Tel +61 2 9555 1944 Fax +61 2 9555 1496 Email firstname.lastname@example.org 52 www.metals.rala.com.au