Contents lists available at ScienceDirect Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea Solid-state joining of powder metallurgy Al-Al 2 O 3 nanocomposites via friction-stir welding: Effects of powder particle size on the weldability, microstructure, and mechanical property M. Nosko a,* , M. Štepánek a , P. Zifčák b , L. Orovčík a , Š. Nagy a , T. Dvorák a , P. Oslanec a , F. Khodabakhshi c,** , A.P. Gerlich d a Institute of Materials and Machine Mechanics, Slovak Academy of Sciences, Dúbravská cesta 9/6319, 845 13, Bratislava, Slovak Republic b Welding Research Institute - Industrial institute of Slovak Republic, Račianska 1523/71, 831 02, Bratislava, Slovakia c School of Metallurgical and Materials Engineering, College of Engineering, University of Tehran, P.O. Box: 11155-4563, Tehran, Iran d Department of Mechanical and Mechatronics Engineering, University of Waterloo, Waterloo, ON, Canada ARTICLE INFO Keywords: Powder metallurgy (PM) Particle size Nanocomposite Friction-stir welding (FSW) Microstructure Mechanical property ABSTRACT Solid-state butt-joining of powder metallurgy (PM) fabricated Al-Al 2 O 3 nanocomposites was assessed using friction-stir welding (FSW), in which the PM materials were prepared from aluminum powder with different particle size distributions of < 9 μm, < 63 μm, and < 250 μm. After establishing a suitable working window in terms of rotational speed (w) and traverse velocity (v), the effects of initial powder particle size on weldability, microstructure and mechanical properties joints were studied. The joint quality was assessed by macro cross- sectional examinations, where deterioration occurs due to defects at the weld root with increasing traversal speed. A defect-free friction stir weld with a more homogenous cross-section is produced for all powder sizes using FSW parameters of w = 1200 rpm and v = 300 mm/min. The native amorphous aluminium oxide (am- Al 2 O 3 ) layer on the powder is redistributed and partially transformed to crystalline γ-Al 2 O 3 nanoparticles de- pending on FSW parameters which is revealed by transmission electron microscopy (TEM). Detailed electron back-scattered diffraction (EBSD) analysis indicates the formation of a fine equiaxed grain structure in the range of 1.5–3.7 μm as a function of initial aluminum particle sizes and FSW parameters with a mixture of ideal random and shear components as the dominant texture. Transverse tensile and indentation hardness testing revealed no significant changes in mechanical properties for the composite weld, with ductile fracture occurring in the base metal with hardening only occurring in the stir zone for the powder particle size. 1. Introduction To improve the energy efficiency and to reduce the greenhouse gas emission while concurrently striving for improved mechanical proper- ties, metal matrix nanocomposites (MMNCs) have been developed as a lightweight structural material for application in electronic packaging and sporting goods as well as the aerospace, automotive, military, and marine industries [1,2]. These MMNCs can be processed via various traditional routes such as, spray deposition [3–5] and stir-casting [6], as well as the new recent methods mainly based on the concept of severe plastic deformation (SPD) including powder metallurgy (PM)/me- chanical alloying (MA) [1,7,8], accumulative roll bonding (ARB) [9], high-pressure torsion (HPT) [10], friction-stir processing (FSP) [11,12], and accumulative fold-forging (AFF) [13,14]. Aluminum and its alloys possess high formability, good weldability, and favorable corrosion resistance in many environments [15] and therefore they are good candidates for the matrix when producing metal-matrix nanocompo- sites. These properties along with high specific strength, thermal sta- bility, and damping capacity [16] make it suitable for many structural and transportation applications. The size and volume fraction of reinforcement is crucial to the properties of Al-matrix nanocomposites, and there is a further ad- vantage to strengthening when an incoherent particle/matrix interface is achieved together with a fine and uniform distribution of thermally stable and insoluble particles [15,17,18]. This incoherency of strength can benefit the strength because of more effective load bearing occur- rence in the structure of composite between metal-matrix and reinfor- cing agents [19,20]. The nano-sized reinforcing particles can be https://doi.org/10.1016/j.msea.2019.03.074 Received 12 January 2019; Received in revised form 11 March 2019; Accepted 15 March 2019 * Corresponding author. ** Corresponding author. E-mail addresses: ummsnoso@savba.sk (M. Nosko), fkhodabakhshi@ut.ac.ir (F. Khodabakhshi). Materials Science & Engineering A 754 (2019) 190–204 Available online 21 March 2019 0921-5093/ © 2019 Elsevier B.V. All rights reserved. T