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

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*Corresponding author: Tel: +86-27-8755-6544. Fax: +86-27-8754-3778.

E-mail address: szcai@mail.hust.edu.cn (Shuizhou Cai)