The volume shrinkage of semi-solid melt in a rheological forming process arises from a sum of such three parts as liquid

solid and solidification contract

which is controlled by the contract coefficient of every parts and solid fraction in semi-solid melt.The formation mechanism of shrinkage porosity in semi-solid melt is that contract can not be feed due to completely closing the liquid by solid phase to form discretely shrinkage porosity rather than centralized ones.The mechanism of feeding of the discrete porosity is solid rheological deformation

in which the feeding is carried out by wholly compression of the semi-solid alloy in directional pressing rheological forming process

while the feeding is carried out by the inflation of the alloy melt in an indirectional pressing forming process.In order to prevent the shrinkage porosity

three basic conditions are necessary and shown in mathematical equations: the first is that the pressure acted on the melt is larger than that by the deformation stress resistant

and the second is that holding time of the pressure is larger than that by the solidification

and the third is that pressing velocity is larger than the volume contraction velocity.