International Journal of Materials Engineering 2015, 5(1): 5-9 DOI: 10.5923/j.ijme.20150501.02 Effect of As-Cast Cooling on the Microstructure and Mechanical Properties of Age-Hardened 7000 Series Aluminium Alloy Isadare D. A. 1,2 , Adeoye M. O. 2 , Adetunji A. R. 1,2 , Oluwasegun K. M. 2 , Rominiyi A. L. 1 , Akinluwade K. J. 1,2,* 1 Department of Engineering, Prototype Engineering Development Institute Ilesa, [National Agency for Science and Engineering Infrastructures, NASENI Abuja] 2 Department of Materials Science and Engineering, Obafemi Awolowo University, Ile-Ife Abstract This study investigates the influence of as-cast cooling on the microstructure and mechanical properties of age-hardened Al-Zn-Mg-Cu alloy. The material was cast in the form of cylindrical rods in green sand mould where some samples were rapidly cooled and others gradually cooled to room temperature. From the samples that were gradually cooled, some were annealed while others were T6 tempered. Both the as-cast and heat treated samples were subjected to tensile and hardness tests and the morphology of the resulting microstructures were characterised by optical and scanning electron microscopy. The results revealed formation of microsegregations of MgZn 2 during gradual solidification which was not present during rapid cooling. It was also found that age hardening and annealing heat treatment operations eliminated microsegregations thus improving mechanical properties of Al-Zn-Mg-Cu alloy. Keywords Al-Zn-Mg-Cu alloy, Microsegregation, Age hardening, Annealing, Strength 1. Introduction The demand for aluminium grows rapidly because of its unique combination of properties which makes it one of the most versatile engineering and structural materials [1-3]. The optimum properties of aluminium are achieved by alloying and heat treatments. These promote the formation of coherent precipitates which interfere with the movement of dislocations and improve its mechanical properties [4-7]. One of the most commonly used aluminium alloy for structural applications is the 7075 Al alloy due to its attractive properties such as low density, high strength, ductility, toughness and resistance to fatigue [8-11]. It has been extensively utilized in aircraft structural parts and other highly stressed parts [12-16]. Nevertheless, aluminium-zinc alloys are susceptible to embrittlement because of microsegregation of magnesium zinc (MgZn 2 ) precipitates which may lead to failure of components produced from them [17, 18]. These alloys are also susceptibility to stress corrosion cracking [19, 20] due to inhomogeneity and inherent residual stresses associated with their fabrication methods [21]. These microsegregations and * Corresponding author: jakinluwade@yahoo.com (Akinluwade K. J.) Published online at http://journal.sapub.org/ijme Copyright © 2015 Scientific & Academic Publishing. All Rights Reserved inherent residual stresses have serious deleterious effects on mechanical properties [18]. Hence, this study aims at resolving the problems of microsegregations and inherent residual stresses that are associated with aluminium-zinc alloys, for improved performance in service. The aim of this work is to investigate the influence of as-cast cooling on the structure and mechanical properties of T6 tempered 7000 series aluminium alloy via annealing and age hardening heat treatment processes. 2. Materials and Method The elemental composition of the 7000 series Al alloy used for this study is shown in Table 1. The material was cast in the form of cylindrical rods. Some of the cast rods were rapidly cooled to room temperature by knocking them out 5 minutes after casting while others were cooled gradually inside the mould. Tensile samples were machined from these categories of rods according to British Standard BSEN 10002-1:1990 [22]. Samples were also sectioned for metallographic and micro hardness tests. Age hardening and annealing heat treatments were carried out on samples machined from slowly cooled castings. Annealing was carried out on samples prepared for metallographic analysis and hardness test. These samples were heated to 470 °C, soaked at this temperature for 4 hours