Porosity in metal additive manufacturing and where it comes from
Almost every defect conversation in metal printing returns to porosity. The pores are not all the same, they do not all have the same cause, and treating them as one problem is the first mistake.
Porosity is the most common and most consequential class of defect in laser powder bed fusion, and it matters because pores act as crack-initiation sites that cut fatigue life and reduce ductility long before they show up as a drop in density. Understanding it begins with separating the three kinds.
Gas porosity is the formation of small, roughly spherical pores from gas that was already present, either dissolved in the powder from atomization or introduced into the melt pool. It is common in aluminium alloys such as AlSi10Mg and is largely a question of powder quality and handling.
Lack-of-fusion porosity is irregular, often contains unmelted powder, and forms between tracks and layers when the energy delivered is too low to fully melt and consolidate the material. It is a sign of operating below the process window.
Keyhole porosity sits at the opposite extreme. When the energy density is too high, the melt pool develops a deep vapour depression that can collapse and trap gas at its base. Keyholing is a sign of operating above the window, and it is the reason more energy is not simply better.
Porosity control is therefore the search for a process window bounded below by lack of fusion and above by keyholing, supported by clean dry powder and a controlled inert atmosphere. Where internal pores remain, hot isostatic pressing can close those not connected to the surface, which is why it is routine for fatigue-critical titanium and nickel parts.