(753e) Comparative Study of Li and Mg Addition on the Thermal Behavior of Al-Based Alloy Powders

Comparative
study of Li and Mg addition on the thermal behavior of Al-based alloy powders

Hui Zou^a , Linfu Li^a,
Fu Hao^a, Shuizhou
Cai^a,b*

^a State Key Laboratory of Material Processing and Die & Mould Technology,

^b State Key Laboratory of Digital Manufacturing and Equipment
Technology,

Huazhong University of Science and Technology, Wuhan 430074, PR China

Abstract

Al-3Li
and Al-6Mg alloy powders were prepared through close-coupled gas atomization
(CCGA), and their thermal reactivities were
investigated. The powder morphologies, oxidation products morphologies and
particle size distributions of the two alloys were analyzed by SEM/EDS and laser particle size analyzer, respectively. The
phase compositions of the two alloy powders oxidized at different temperatures
were characterized by X-ray powder diffraction (XRD). The thermal
oxidation behaviors of the two alloy powders were examined using the
simultaneous TG-DTA technique in O₂ up to 1300°æ.
The results showed that the prepared powders had good sphericity.
The average size of Al-3Li particles is
17.5µm and the average size of Al-6Mg particles is 21.8µm. The TG-DTA results
which were shown in Fig.1 indicated that oxidation behaviors of the two alloys
were significantly different. For Al-3Li powder, the oxidation proceeded in
only one step and was complete, while the oxidation of Al-6Mg powder was
stepwise and incomplete. Large amounts of aluminum oxide (Al₂O₃)
and lithium aluminum oxide (LiAlO₂ and LiAl₅O₈)
were detected in the oxidation products of Al-3Li powder after the unique
oxidation at 1066°æ,
almost no Al phase was observed, demonstrating that the fierce oxidation of
Al-3Li powder was almost complete. However, for Al-6Mg powder, after the second
oxidation event, the major phases found in the products were Al, Al₂O₃
and MgAl₂O₄, suggesting that the oxidation reactions of
Al-6Mg powder proceeded to incompletion, which was in agreement with the TG-DTA
results. As shown in Fig.2, the kinetics of the different oxidation events of
the two alloys were investigated using Kissinger method, the apparent activation energy for Al-3Li
powder oxidation was 339.7 kJ°¤mol^-1, while the apparent
activation energies for the first and second oxidation of Al-6Mg alloy were
183.7 kJ°¤mol^-1 and 221.2 kJ°¤mol^-1, respectively. The
frequency factor value for the single oxidation of Al-3Li powder was 9.04°Á10¹²
s^-1, which was significantly larger than any that of Al-6Mg
oxidation events (1.35°Á10⁸ s^-1 for the first oxidation
event and 4.26°Á10⁷ s^-1 for the second oxidation event), suggesting
that the oxidation reaction rate of Al-3Li powder can be much higher than that
of Al-6Mg powder. Compared with Al-6Mg powder, Al-3Li powder presents a sharper
exothermic peak, a lower violent reaction temperature, a larger oxidation
reaction rate and a more complete reaction, and has moderate activation energy
for oxidation. Therefore, alloying Li by gas atomization can be more beneficial
than Mg to improve the thermal reactivity of coarse Al particles.

Keywords:
Aluminum alloys; Thermal reactivity; Oxidation; TG-DTA; Activation energy.

Fig.
1 TG-DTA traces of Al-3Li (a) and Al-6Mg (b) heated in O₂ at heating
rate of 20 K/min

Fig.2
Kinetics analysis for the different oxidation events of Al-3Li and Al-6Mg alloy
powders