Effect of rotational speed and probe profile on microstructure and hardness of AZ31/Al 2 O 3 nanocomposites fabricated by friction stir processing M. Azizieh ⇑ , A.H. Kokabi, P. Abachi Department of Materials Science and Engineering, Sharif University of Technology, Azadi Ave., P.O. Box 11155-9466, Tehran, Iran article info Article history: Received 31 July 2010 Accepted 24 November 2010 Available online xxxx Keywords: A. Metal matrix nanocomposites C. Friction stir processing G. Microstructure abstract Friction stir processing (FSP) was used to fabricate AZ31/Al 2 O 3 nanocomposites for surface applications. The effects of probe profile, rotational speed and the number of FSP passes on nanoparticle distribution and matrix microstructure were studied. The grain refinement of matrix and improved distribution of nanoparticles were obtained after each FSP pass. By increasing the rotational speed, as a result of greater heat input, grain size of the base alloy increased and simultaneously more shattering effect of rotation, cause a better nanoparticle distribution. The average grain size of matrix of the composites was in the range of 1–5 lm and the microhardness of them was 85–92 Hv. Ó 2010 Elsevier Ltd. All rights reserved. 1. Introduction Friction stir welding (FSW), initially developed by The Welding Institute (TWI) [1], is widely used in welding of magnesium and aluminum alloys. Without being limited to welding, friction stir processing (FSP) has found various applications: grain refinement of wrought and casting parts [2,3], superplasticity improvement [4], formation of intermetallics [5] and composite fabrication [6]. FSP offers a low energy consumption route to introduce rein- forcing phases into the metal matrix, and to form bulk composites. Grain refinement is also enhanced via dynamic recrystallization and grain boundary pinning during FSP [7]. In recent years, many researches were carried out on surface composite fabrication with FSP. Mishra et al. [8] fabricated 5083Al based SiCp reinforced sur- face. Morisada et al. [9] produced fullerene/A5083 surface compos- ites with FSP. Several other studies were also done on fabrication of Mg and Al based nanocomposite by deep groove processing, and grain size in the range of 100 nm to 4 lm has been obtained [10,11]. According to above researches, surface composites were made with various base materials and reinforcement. However, the effect of rotation speed, pin profile and particle size on surface composites behavior was never studied. In the cast aluminum al- loys, as a similar case, higher tool rotation speed and further FSP passes, were the most effective parameters to refine coarse parti- cles, heal the casting porosity, and consequently increase strength [12]. Also, Krishnan [13] found that, the spacing of the onion rings is equal to the forward movement of the tool in one rotation. Therefore, with increase of rotation speed, the onion ring spacing reduces and the homogeneity of stir zone is improved. The effect of tool geometry on mechanical properties of FSPed cast alloys is complicated, and depends on the FSP parameters. However, Ma et al. [12] found that, the use of tri-flute pin, due to higher material flow, produced a better particles distribution and optimum combination of strength and ductility in A356 cast alumi- num alloy. The aims of this research were to fabricate AZ31/Al 2 O 3 nano- composites by FSP and to propose a new route producing a satis- factory distribution of nanoparticles accompanying fine-grain structure. We also investigated the effect of rotational speed, probe profile and the number of FSP passes on material flow, particle dis- tribution, grain refinement and hardness values. 2. Experimental procedure In this study, the AZ31 billets with a chemical composition of Mg–3.5 wt%Al–0.86 wt%Zn–0.56 wt%Mn and three kinds of Al 2 O 3 particles with mean diameters of 35 nm (Fig. 1a), 350 nm (Fig. 1b) and 1000 nm (Fig. 1c), respectively, were used. The billets had a nearly equiaxed grains with a size of approximately 70 lm. It was cut in a rectangular shape with 60 mm in width, 100 mm in length and 10 mm in thickness. The Al 2 O 3 powder was fed into a groove with a width and depth of 1.2 mm and 5 mm respectively, which was located on the AZ31 plate before friction stir processing. The tool profiles are shown in Fig. 2. A tool with a columnar probe without threads (Fig. 2a), a tool with a columnar probe with threads (Fig. 2b) and a tool with columnar probe with threads and three flutes (Fig. 2c). The tools were machined out of H13, and heat-treated to have 53HRC hardness. They consisted of a shoulder 0261-3069/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2010.11.055 ⇑ Corresponding author. Tel.: +98 9123798088; fax: +98 2188840750. E-mail address: azizieh@mehr.sharif.edu (M. Azizieh). Materials and Design xxx (2010) xxx–xxx Contents lists available at ScienceDirect Materials and Design journal homepage: www.elsevier.com/locate/matdes Please cite this article in press as: Azizieh M et al. Effect of rotational speed and probe profile on microstructure and hardness of AZ31/Al 2 O 3 nanocom- posites fabricated by friction stir processing. J Mater Design (2010), doi:10.1016/j.matdes.2010.11.055