Gas Atomization of Amorphous Aluminum: Part I. Thermal Behavior Calculations

Abstract

In this article, the thermal history and cooling rate experienced by gas-atomized Al-based amorphous powders were studied via numerical simulations. Modeling simulations were based on the assumption of Newtonian cooling with forced convection, as well as an energy balance, which involves gas dynamics, droplet dynamics, and heat transfer between gas and droplet. To render the problem tractable, phase transformations, crystal nucleation, and growth were not taken into account in the analysis of the solidification of Al droplets; instead, an energy balance approach was formulated and used. The numerical results and associated analysis were used to optimize processing parameters during gas atomization of Al-based amorphous powder. The results showed that the cooling rate of droplets increases with decreasing powder size and can reach in excess of 10 5 K/s for powder <20 lm in diameter. Gas composition has a more significant influence on cooling rate than gas pressure, and 100 pct He has the highest cooling effect. The results also showed that the cooling rate increases with increasing melt superheat temperature.

Gas Atomization of Amorphous Aluminum Powder: Part II. Experimental Investigation

“…This result is consistent with the calculation results as reported in Figure 7 of part I. [1] The effect of gas pressure on the decreasing droplet size is not apparent when gas pressure is higher 2.62 MPa because of aspiration effects.…”

“…The numerical results [1] indicate that of the influence of gas pressure on cooling rate decreases as gas pressure approaches a certain value, which is about 2.76 MPa for the conditions used in this study. An experimental comparison of C 1 and A 1 shows that an increase of gas pressure from 1.24 to 2.62 MPa leads to an increase in cooling rate. Based on the above results, gas composition seems to have more dominant effect on the cooling rate over that of gas pressure as evident from a comparison of D 1 with A 1 , B 1 , and C 1 .…”

“…Comparison of the results obtained for D 1 and A 1 indicates that 100 pct He (A 1 ) has apparently increased the cooling rate of the powders over a gas mixture of 80 pct He + 20 pct N 2 (D 1 ), which is also in agreement with the previous numerical results. [1] Although the change in gas composition was offset to some extent by pressure (5.52 MPa) and nozzle diameter (1.7 mm), gas composition still exerted a greater effect. A higher gas pressure (5.52 MPa) and a smaller nozzle diameter (1.7 mm) were used in D 1 , as compared with a gas pressure of 2.62 MPa and a nozzle diameter of 2.16 mm used in A 1 .…”

“…[1] A comparison of gas volume consumed per unit weight of powder also showed He to be superior to other types of gas or gas mixtures. [1] However, the cooling effects of He on melt viscosity also need to be considered. Comparing N 2 , Ar, and other gas mixtures, the higher cooling capacity of He gas can lead to a rapid increase in melt viscosity.…”

“…In part I, [1] we described the numerical framework used to predict the thermal profile that is present during GA Al-based amorphous powder. More specifically, the numerical simulations were based on the assumption of Newtonian cooling with forced convection.…”