Fullerene/A5083 composites fabricated by material flow during friction stir processing

https://doi.org/10.1016/j.compositesa.2007.07.004Get rights and content

Abstract

Fullerene was successfully dispersed into A5083 by friction stir processing (FSP). Dispersion of the fullerene enhanced the grain refinement by recrystallization during the FSP and the grain size reached ∼200 nm. The hardness was also remarkably increased by both the grain refinement and the dispersion of the fullerene molecules.

In this study, material flow during the FSP was investigated with respect to the dispersion of the fullerene. It was revealed that the formation mechanism of the “onion ring” was closely related to the convectional flow induced by the shoulder of the rotating tool. This material flow by the shoulder is very important when fabricating the surface composites by the FSP in order to provide a uniform dispersion of the reinforcement.

Introduction

Much attention has been paid to friction stir processing (FSP) which is well known as a surface modification technique based on friction stir welding (FSW) [1], [2], [3], [4], [5]. It has been basically developed for deleting various defects in cast products or improving the mechanical properties of light metals by the grain refinement related to the Hall–Petch relation. Recently, the FSP has been studied as a new process for fabricating surface composites. In this case, the surface of metal plates was modified by the dispersion of various ceramic particles with a low energy consumption [6]. The author’s group reported that magnesium alloy based composites reinforced by SiC particles, MWCNTs, or fullerene could be fabricated by the FSP [7], [8], [9]. The dispersion of the reinforcement enhanced the mechanical properties of the processed area. Specifically, the fullerene dispersed AZ31 showed three times higher hardness compared to the as-received AZ31. This indicated that the FSP seems to be the best way to use the fullerene molecule, which is the hardest and smallest particle, as a nanometer-sized reinforcement. The fullerene molecule is unstable in the matrix at high temperature during the fabrication process such as a sintering [10]. The FSP which is well known as a solid-state process should be appropriate process to disperse the fullerene into the metallic materials, because of its low process temperature. However, the dispersion mechanism by the FSP is still unclear. Investigation of the basic dispersion mechanism is indispensable for further development of the FSP. The aim of this study is the establishment of a fabricating process of the surface composites by the FSP using the fullerene dispersion.

Understanding of the material flow in the stir zone is inevitable for the FSW to obtain an excellent joining [11], [12], [13]. Although many researchers have mentioned the “onion ring” formed in the stir zone [14], [15], [16], its formation mechanism has not been explained in detail. Therefore, the other aim of this study is to reveal the formation mechanism of the onion ring using the visualized material flow attained by the fullerene dispersion.

Section snippets

Experimental procedure

Commercially available fullerene powder (95% pure, C60: 80 vol%, C70: 15 vol%, Honjo Chem. Co., Japan) was used. The fullerene powder was filled into a groove (1 × 2 mm) on an A5083 plate (mean grain size: 25.4 μm) before the FSP was carried out. This process was explained in detail elsewhere [7]. The FSP tool made of SKD61 has a columnar shape (φ12 mm) with a probe (φ4 mm, length: 1.8 mm). The probe was inserted into the groove filled with the fullerene powder. The constant travel speed was 50 mm/min

Material flow in stir zone

Cross sections of the FSPed samples are shown in Fig. 1. The material flow could be easily observed by the fullerene dispersion. For the sample FSPed at 500 rpm, fullerene was gathered in a specific area of the stir zone. On the other side, the fullerene was dispersed so as to form the onion ring in the stir zone for the sample FSPed at 1000–2000 rpm. The shape of the stir zone was changed from triangular to trapezoidal by the increase in the rotating speed. The onion ring could be clearly

Conclusions

The fullerene/A5083 composites were successfully fabricated by the FSP. The material flow in the stir zone during the FSP was studied by visualization using the fullerene dispersion. Additionally, the microstructure and microhardness were evaluated by observing the grain size and the dispersion of the fullerene. The obtained results can be summarized as follows.

  • (1)

    The fullerene molecules can be dispersed into the A5083 matrix using the FSP.

  • (2)

    The onion ring is formed by the convectional flow due to

Acknowledgements

This research was partially supported by the Ministry of Education, Science, Sports and Culture, Grant-in-Aid for Young Scientists Start-Up, 18860092, 2006.

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