On the removal of pores from castings by sintering

The causes, sizes, and distribution of porosity in castings have been reviewed and quantitatively evaluated for several important modes of alloy solidification. In general, gas exsolution is found to be the most probable cause of porosity in castings which solidify in either a cellular or dendritic fashion. On the other hand, solidification alone may cause porosity creation if the interdendritic liquid metal cannot feed the solidification shrinkage. This effect may be enhanced by gas exsolution. Removal of porosity by “sintering” after solidification requires that the grain size be of the order of, or smaller than, the pore spacing, and that the pores be small (>1 µ) for removal within reasonable times (tens of hours). When gas exsolution is the cause of pore creation, the gas must be diffused out of the sample to permit pore shrinkage. Small ingot sizes (>10 cm) and rapidly diffusible gases (H₂) are required for pore elimination within reasonable times (tens of hours). The application of low pressure (>20 atm) during sintering increases the rate, or the size (to >10 mµ) of the pores which can be eliminated within >20 hr.