Based on the article of S. N. AQIDA etal. (page 20), the porosity content increases with increasing particle size, aspect ratio, and volume fraction of reinforcement material, whereas, increasing the holding time will decrease the porosity level.

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https://www.researchgate.net/publication/253132844_Effects_Of_Porosity_On_Mechanical_Properties_Of_Metal_Matrix_Composite_An_Overview

Article Effects Of Porosity On Mechanical Properties Of Metal Matrix...

It depends on the final use: electric conductivity (microelectronics packaging), heat conductivity (heat dissipation), wear.

Porosity, shape and size, may be intentially added. It depends on alloying elements and process type that apply press or heat, as hipping or dimox.  Kinetic of interaction induced porosity intergrainular or transgrainular.

Monolithic materials are known to have a density of crystalline structure. With an increase in impurities in the materials (), the crystal structure deforms and leads to worsening of the density. This leads to a deterioration in corrosion resistance, resistance to cracking and worsening plasticity, and increased incidence of porosity. This effect in some materials is sought. This depends on the quantity of supplies and what is sought from the resulting composite materials.

Porosity occurs due to the manufacturing methods employed to produce metal matrix composite. If stir casting is used, the absorption of air at molten state causes porosity. If powder metallurgy is used, the size and shape difference between matrix particle and reinforcement particle will cause porosity. In any case, porosity in metal matrix composites can not be avoided. Usually, a secondary processing such as rolling or extrusion is required after fabrication to reduce or alleviate porosity.    

Porosity is dominated by alloying elements, swiril action and solidification tome.  It may be followed by semisolid processing either to drive porosity out  or to distribute it locally to make composite foam.