The mechanism of porous column formation during spray forming

It is the objective of this article to provide preliminary results on the mechanism of porous column formation during spray forming and to help find methods to avoid it. Tool steel (grade A2) was sprayed in a shaped ceramic mold. The formation of porous columns was observed. The porous columns develop when the deposition angle decreases below a certain small value, which reduces the spray density per unit surface area to the degree of a dry or cold spray. In addition to this, there is also a shadowing effect; i.e., there is a narrow shadow just below a column, where no droplets impinge. The porous column formation can be prevented by avoiding a mold or deposit surface, which lies nearly parallel to the spray axis, or by directing the spray axis perpendicular to the mold or deposit surface.