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Metal Casting Technologies : September 2005
ispersed porosity in aluminum castings is detrimental to its mechanical properties. The presence of dispersed porosity reduces the hardness, tensile strength, ductility and fatigue strength of the castings. Generally control of gross defects such as blow hole, shrinkage cavity, hot tear, inclusions etc. are given more importance due to their larger size, whereas fine porosity in castings are overlooked as it is difficult to trace such defects by conventional non- destructive testing methods. However this defect deteriorates the quality of the castings in terms of the life of the component as well as reliability in service. It should be mentioned here that dispersed porosity is found not only in aluminum castings, it is common in other alloy casting as well and affects the mechanical properties in a similar way. Porosity may arise in castings in two ways. First, gases dissolved in molten metal precipitate during its solidification. The solubility of a gas in liquid metal rapidly drops at the temperature of solidification. Hence during cooling of a casting, fine gas bubbles are supposed to precipitate at the liquid-solid interface. These precipitates of gas appear as gas porosity in a casting . Hydrogen in aluminum, copper and steel castings, and nitrogen in iron castings are the sources of such gas porosity. The solution to the gas porosity in a casting is to degas the liquid metal before pouring. Vacuum degassing in case of steel and use of gases such as nitrogen, chlorine, argon or solid chlorine bearing compounds in case of aluminum and copper base alloys are common practice in foundries to overcome the gas porosity in castings. On the other hand, when shrinkage takes place in a long freezing range alloy, due to mushy state of solidification and feeding difficulty through the mushy zone, dispersed shrinkage porosity forms in a casting in the regions of last solidifying liquid in interdendritic channels . For short freezing range alloy, shrinkage cavity rather than shrinkage porosity is likely to occur. The remedy to avoid shrinkage porosity is to have steep temperature gradient in casting sections during solidification by use of chill or to increase feeding pressure by increasing the height of risers. Practically, porosity in casting arises from both gases dissolved in liquid metal and mushy state of solidification, since complete removal of gases from liquid metal is difficult and many cast alloys are of moderately long freezing range. Light alloy castings in aluminum and magnesium are more prone to porosity defect due to their low density and hence less metallostatic pressure acting during solidification. Hence increasing pressure in riser by any method should be effective to reduce the tendency to porosity formation. Earlier work on applying external pressure over riser for aluminum alloy gravity die casting using compressed air has been proved to be effective in reducing porosity and improving mechanical properties of castings . However, providing facilities in the shop floor to apply pressure over riser of gravity die castings by compressed air is not only cumbersome, there also may be considerable uncertain pressure drop due to leakage at parting plane. Hence an alternative approach of applying external pressure over riser of aluminum alloy gravity die casting has been attempted in the present work by use of static load over riser with a view to reduce porosity. The effect of two levels of static load has been used in the experiments and its effect on porosity and hardness of the castings has been studied. EXPERIMENTS The objective of the present work is to study the effect of external pressure over riser on the feeding characteristics of aluminum gravity die casting to improve its soundness. Production of plate type casting in horizontal position was selected for the experiments. One set of gray cast iron die blocks with vertical parting were prepared for this purpose. The schematic view of the die blocks is shown in casting position in Fig.1. The size of riser for the plate casting of200mmX100mmX20mmsizewas calculated by Modulus Method taking modulus (V/A) of riser as 1.2 times that of casting. A cylindrical plunger of required dimension was also prepared in cast iron for application of pressure over riser in the form of static load. To reduce heat loss from liquid metal in riser to the plunger, an insulating brick was provided in the riser as shown in Fig.1. Aluminum alloy (LM6) was melted in an oil-fired furnace of 30 kg capacity. Calculated loads were placed immediately after pouring by inserting insulating brick and the plunger. Castings were produced with 30 kPa and 100 kPa pressure. One casting without pressure was also produced for comparison. The plate castings were sectioned vertically along the center to observe the nature of porosity and to measure hardness. After polishing, the size and distribution of porosity was measured at different locations of the castings from TECHNICAL FEATURE Dr P.C. Maity Reduction of porosity in aluminum gravity die castings by application of static pressure Schematic diagram of gravity die casting set up showing arrangement for pressure application by static loading. 1 D 50 www.metals.rala.com.au