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Metal Casting Technologies : September 2005
riser end. Similarly, Brinell hardness was also measured at different locations along centerline of the casting to assess their soundness and to correlate it with the porosity distribution. POROSITY IN CASTINGS The distribution of porosity in different locations of the castings is shown in Table 1. Sections I to IV as shown in the Table are four sections of the castings along its length from the riser end, each one is of 50 mm. The no. of the pores are more and the size of the pores are smaller in general near the riser and these are gradually reduced towards other end due to the nature of directional solidification. The solidification starts from the end of the castings opposite to riser and proceeds towards the riser. Hence the end sections are sound as the feeding is effective at the early stage of solidification. As solidification progresses, the feeding of liquid metal at the centerline becomes difficult due to increased viscosity of liquid metal, narrow channel for feeding and projected grains of progressive solidification. Such difficult feeding at centerline results in well established centerline shrinkage porosity. The application of 30 kPa pressure over riser reduces both no. and size of the pores and the effect is more pronounced with the application 100 kPa pressure over riser. It may be noted that the subsurface porosity in the casting without pressurized riser (Fig.2, Casting no.1) is completely eliminated even at 30 kPa pressure over riser (Fig.2, Casting no.2). Subsurface porosity are generally revealed during machining of castings and sometimes this defect leads to the rejection of the castings in case of applications where the surface has to be free from porosity defect or look of the casting is important. 100 kPa pressure over riser is still more effective to minimize porosity in the castings. Practically the castings are free from porosity under this condition of application of pressure. A few pores are present only in the middle zone of the casting (Fig.2, Casting no.3). The application of pressure over the riser acts in two ways to minimize the porosity in castings. First, the feeding of the castings, either by liquid feeding or by mass feeding operates to compensate for the shrinkage during solidification. Moreover, the pressure makes nucleation of gas porosity more difficult. As discussed earlier, porosity generally arises from the combined effect of shrinkage and dissolved gases, additional pressure during solidification suppresses the porosity supposed to be formed by both mechanisms. HARDNESS The hardness of the castings along centerline at different locations improved considerably on application of pressure over riser, as shown in Table 2. Hardness values conforms to the distribution of porosity in the castings, i.e., where the no. of pores are more, the hardness value is less in that location and vice versa. The presence of porosity, whether it is due to gas or shrinkage, is supposed to make compressive deformation easier, as the pores allow space for the compressive flow of metal. Therefore the hardness is reduced in presence of pores. Photographs of the castings sectioned at center: (1) Casting without pressure over riser (2) Casting with 30 kPa pressure over riser (3) Casting with 100 kPa pressure over riser. 2 TABLE 1 Effect of pressure over riser on porosity in aluminum gravity die casting Casting condition Section I Section II Section III Section IV PnP s(mm) Pn P s(mm) Pn P s(mm) Pn P s (mm) No pressure 620.6600.8330.4210.3 over riser 30 kPa pressure 38 0.4 37 0.5 23 0.3 9 0.2 over riser 100 kPa pressure 3 0.3 17 0.3 18 0.3 5 0.1 over riser Note: Pn and Ps are no. and size of pores respectively. Section I to IV are of 50 mm each from riser end consecutively. Who's Who of Metals 2005/6 51