Evaluation of Porosity in Gas-atomized Powder by Synchrotron X-ray CT and Investigation of the Effect of Gas Species

Abstract

The presence of pores in gas-atomized alloy powders induces a significant deterioration in the properties of the final product. However, there is no established technique to quantitatively analyze the porosity of gas-atomized powders. In this study, the pores in gas-atomized amorphous Fe₇₆Si₉B₁₀P₅ powder particles prepared under different atomization conditions were analyzed in detail using synchrotron radiation X-ray computed tomography. This technique allowed the detection of small pores with diameters below 10 µm. It also enabled the quantification of the porosity; thus, the pore diameter and volume ratio under different atomization conditions were determined. The volume ratio of the pores with the use of low-pressure Ar as the atomization gas was lower than that with the use of high-pressure Ar. The use of a low-pressure gas during spraying induced an increase in the diameter of the powder particles, thereby resulting in the presence of numerous irregular-shaped particles. The results of X-ray diffraction confirmed the partial precipitation of a crystalline phase with a decrease in the cooling rate. The use of 3 or 7% Ar–H₂ mixtures as the atomization gas induced a decrease in the number and volume of pores, without affecting the particle size and cooling rate. The presence of H₂ as a reducing gas suppressed the surface oxidation of the droplet during the atomization of the molten-metal stream, which allowed trapped gas bubbles to be efficiently removed before solidification.