metals Review AA7075-ZrO 2 Nanocomposites Produced by the Consecutive Solid-State Process: A Review of Characterisation and Potential Applications Huda M. Sabbar 1, *, Zulkiflle Leman 1,2, *, Shazarel B. Shamsudin 3 , Suraya Mohd Tahir 1 , Che N. Aiza Jaafar 1 , Mohamed A. Azmah Hanim 1 , Zahari N. Ismsrrubie 1 and Sami Al-Alimi 3   Citation: Sabbar, H.M.; Leman, Z.; Shamsudin, S.B.; Tahir, S.M.; Aiza Jaafar, C.N.; Hanim, M.A.A.; Ismsrrubie, Z.N.; Al-Alimi, S. AA7075-ZrO 2 Nanocomposites Produced by the Consecutive Solid-State Process: A Review of Characterisation and Potential Applications. Metals 2021, 11, 805. https://doi.org/10.3390/met11050805 Academic Editors: Asit Kumar Gain and Manoj Gupta Received: 15 April 2021 Accepted: 7 May 2021 Published: 15 May 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 Department of Mechanical and Manufacturing Engineering, Faculty of Engineering, Universiti Putra Malaysia, Serdang 43400, Malaysia; su_mtahir@upm.edu.my (S.M.T.); cnaiza@upm.edu.my (C.N.A.J.); azmah@upm.edu.my (M.A.A.H.); rubie@upm.edu.my (Z.N.I.) 2 Advanced Engineering Materials and Composites Research Centre, Faculty of Engineering, Universiti Putra Malaysia, Serdang 43400, Malaysia 3 Sustainable Manufacturing and Recycling Technology, Advanced Manufacturing and Materials Center (SMART-AMMC), Universiti Tun Hussein Onn Malaysia, Parit Raja, Batu Pahat 86400, Malaysia; shazarel@uthm.edu.my (S.B.S.); samialalimi@gmail.com (S.A.-A.) * Correspondence: hudasabbar86@gmail.com (H.M.S.); zleman@upm.edu.my (Z.L.) Abstract: Solid-state recycling is a direct conversion method for producing metal chips, whereas the materials are plastically deformed into the final product without melting, offering lower energy con- sumption and metal waste. This technique was reported for fabricating aluminium-zirconium oxide (Al-ZrO 2 ) composite and it was widely used to avoid metal chips bounding at high temperatures during the extrusion process. Aluminium alloy (AA7075) is known for its high yield strength of more than 500 MPa under optimum ageing conditions. However, AA7075 can be further reinforced by zirconium oxide nanoparticles when needed for high-performance applications. Hot extrusion is used to obtain better mechanical properties of composite materials. The equal channel angular pressing (ECAP), a severe plastic deformation technique, was recently used to produce bulk and light recycled metal chips, such as porosity-free and ultra-fine-grained aluminium nanocomposites (ANCs). Heat treatments (HT) and ECAP post hot extrusion are mostly incorporated to improve tribological and mechanical properties and aluminium nanocomposite bonding efficiency. In this review, ANCs’ fabrication by the hot extrusion technique and the effects of ZrO 2 nanoparticle are duly summarised and discussed. Furthermore, this review emphasises the importance of using HT and ECAP techniques to acquire better metal alloy incorporation, such as AA7075-ZrO 2 . In- terestingly, owing to the lightweight properties and superior performance of AA7075-ZrO 2 , it was reported to be suitable for fabricating many drones’ parts, military equipment, and some other promising applications. Keywords: AA7075; ECAP; hot extrusion; solid state recycling; zirconium oxide 1. Introduction Aluminium alloys constitute the bulk of modern structural materials that are deployed in a variety of engineering applications, including aerospace, automotive, marine, and military [1], primarily because of their low density and favourable mechanical properties [2]. The promising competitive advantages of aluminium in industrial applications and the excellent machinability are part of this review’s motivation. Earlier studies forecast a global average annual increase of 20% in demand for aluminium alloys, being justified by the vast areas of aluminium alloy applications (as presented in Table 1). Aluminium alloys are broadly categorised into two major classes: cast alloys and wrought alloys. In the classification of heat treatment, these alloys are further divided into heat treatable and non-heat-treatable alloys [3]. Metals 2021, 11, 805. https://doi.org/10.3390/met11050805 https://www.mdpi.com/journal/metals