On cavities in thermally spheroidized powder particles

A melted spherical particle begins to solidify on cooling, and the radius of the first thin solid layer is approximately equal to the radius of the melted particle, Since the density of the solid phase is higher than that of the liquid one, porosity forms inside the sphere during further solidification, The pressure in the remaining melted material may decrease considerably, The requirement for a pressure balance implies that a relationship for the pore radius can be derived, The pore arises as a bubble in a boiling liquid, The bubble stability and its minimum radius are derived, as well as conditions for its formation, It is shown that, at most, one bubble can occur in the particle. Analytical results are applied to the case of alumina particles, and the growing process of the cavity is simulated. Craters found on some spheroidized particles stem from asymmetry of the solid shell formation and the simultaneous action of the atmospheric pressure, The practical importance of this effect is that additional porosity may be formed in thermal spray products.