AA2024–CNTs composites by milling process after T6-temper condition

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Abstract

The 2024 aluminum alloy produced from elemental powders and reinforced by the addition of carbon nanotubes were microstructural and mechanically characterized. Composites were synthesized through milling process followed by cold consolidation, sintering and hot extrusion. The effect of the nanotubes on the microstructure and mechanical properties were analyzed in the composites after T6 thermal treatment. Al4C3 formation was due to the reaction of carbon nanotubes and Al matrix. Precipitated Al–Cu phase coexists and presents interaction with crystallized carbide and dispersed nanotubes. There is a direct relationship of carbon nanotubes content and hardness answer.

Highlights

► By Milling process can be produced Al–CNT composites. ► CNTs keep their morphology after milling process, are well dispersed into Al matrix and have an important effect over mechanical properties. ► The crystallization of Al4C3 from the reaction between Al and CNTs is obtained. ► Co-existence of CNTs, Al4C3 and Al–Cu is observed after aging heat treatment.

Introduction

The continuous development of materials with improved mechanical performance for aerospace, aeronautic and automotive industries has been a constant interest by many research groups. In this respect, aluminum has been widely investigated and is presented as key material in order to face these challenges. When alloyed with other metals, its mechanical properties are vastly improved making it suitable for innumerable structural applications [1], [2], [3]. Aluminum alloys have been extensively investigated in order to improve their mechanical performance. Even though aluminum is alloyed with several metals through foundry routes, and susceptible to strengthening by the precipitation due to heat treatments [4], [5], notable results have been achieved by the dispersion of strengthening particles by using powder metallurgy technique [6], [7]. However, the dispersion process can be improved by the use of mechanical alloying (MA) [8], [9], [10], which is a technique that allows the introduction of the reinforcement phase in the matrix by the repeated cycle of welding–fracture–welding of the particles involved in the process. This process allows a homogeneous dispersion of the strengthening phase and the formation of supersaturated solid solutions [11].

Several strengthening phases have been dispersed by MA into aluminum alloys achieving composites with noticeable mechanical properties. The use of carbon nanotubes (CNTs) as one of them has attracted the attention of several research groups [9], [10], [12]. By MA it is achieved the formation of a solid solution, microstructure refining and, at the same time, the homogeneous CNTs dispersion into aluminum matrix. Nanocomposite formed during milling process will present better mechanical properties than unreinforced alloy produced by the same route [8], [10], [13], [14], [15], [16], [17].

The focus of this work is to produce the AA2024 aluminum alloy from elemental powders and the dispersion of CNTs by a milling process. The effect of heat treatment of precipitation on consolidated samples is evaluated from mechanical and microstructural point of view.

Section snippets

Experimental procedure

Al (99.5% pure, −200 mesh), Cu (99.3% pure, −325 mesh), Mg (99.8% pure, −325 mesh), Mn (99.3% pure, −325 mesh), Ti (99.3% pure, −325 mesh) and Zn (99.9% pure, −100 mesh) powders were used to fabricate AA2024-based composites. CNTs synthesized by chemical vapor deposition (CVD) were added in amounts of 0.0, 0.5, 2.0 and 5.0 wt.%. Each mixture (80 g) was milled during 5 h in a high energy mill (Simoloyer). Ar as inert atmosphere was used for all runs. Due that the production of the AA2024 is from

Results and discussion

Fig. 1 displays the Raman spectrum of the raw multi-walled CNTs used in this work and it is compared with that of the AA2024 composite reinforced with 5.0 wt.% of nanotubes. The spectrum corresponding to the CNTs displays two dominant peaks for the D band around 1325 cm−1 and the G band located at around 1574 cm−1 which are directly related with nanotubes structure. The addition of nanotubes into the AA2024 matrix by milling process causes a very clear change in intensity and a slight shift in

Conclusions

The AA2024 aluminum alloy was successfully reinforced by the dispersion of CNTs through milling process. Milled powders were cold compacted, sintered, extruded and finally heat treated by solution and artificial aging. XRD results indicated the presence of Al4C3 produced during the sintering process, of as well as Al2Cu precipitated during T6 heat treatment condition. The presence of both phases was corroborated by TEM. The observation of CNTs interacting with Al–Cu precipitates was also

Acknowledgements

This research was supported by CONACYT (106658). Thanks to Redes Temáticas de Nanotecnología y Nanociencias, Reg. 0124623. Thanks to O. Solis-Canto, C. Leyva-Porras, P. Pizá-Ruíz and C. Ornelas-Gutiérrez for their technical assistance.

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