Abstract
In order to produce structural products, Al-Zn-Mg alloys undergo various forming processes. Problems that are usually found in the forming process include peripheral coarse grain (PCG) and hot tearing which decrease mechanical properties and corrosion resistance of the alloys. Addition of microalloying element such as chromium (Cr) is an alternative to overcome these problems. The presence of Cr in Al-Zn-Mg alloys supresses the grain growth by preventing excess recrystallization. In this research 0.9 wt. % Cr was added to Al-4.5Zn-1.5Mg alloy and the deformation behaviour as well as subsequent recrystallization was observed. The alloy was fabricated by squeeze casting followed by homogenization at 400 °C for 4 h. The samples were cold rolled for 5, 10, and 20 %. The 20 % deformed samples were then annealed at 300, 400, and 500 °C for 2 h. Material characterization consisted of microstructure analysis using optical microscope and Scanning Electron Microscope (SEM) – Energy Dispersive Spectroscopy (EDS), hardness testing using Micro Vicker methods. The results showed that the deformed grain ratio was 1.6, 2.84, and 2.99 in the 5, 10, and 20 % deformed samples, respectively. The elongated dendrites were effective to increase the hardness of the alloy. Recrystallization was not detected during annealing at 300 and 400 °C, but was observed at 500 °C. Whereas, for the samples without Cr addition, recrystallization occurred at 400 °C. It means that the addition of Cr increased the recrystallization temperature of the alloy. It occured because (Al, Zn)7Cr dispersoids with size less than 1 μm impeded the dislocation motion during annealing, so that recrystallization was retarded. On the other hand (Al, Zn)7Cr dispersoids with size more than 1 μm promoted the formation of new grains around them by Particle Stimulated Nucleation (PSN) mechanism. In this case, the fine (Al, Zn)7Cr dominated so that recrystallization was slower.
Highlights
Aluminium 7XXX series are alloys with zinc and magnesium as alloying elements
Problems which usually occur during deformation are Peripheral Coarse Grain (PCG) and hot tearing [4]
The results showed that higher deformation led to the increase in nucleation rate during recrystallization so that finer grains were produced and higher strength and hardness were obtained
Summary
Aluminium 7XXX series are alloys with zinc and magnesium as alloying elements. With 83.1 wt. % solubility in aluminium, addition of zinc enhances the strength of the alloy through solid solution strengthening [1]. While addition of magnesium promotes the MgZn2 precipitate that increases the strength and hardness of the alloy after ageing processes [2]. These alloys have good mechanical properties and are commonly used in the aircraft industry as small and large components. To overcome the problems, alloying elements such as Cr are added to the alloys to promote the formation of CrAl7 intermetallic phase [5]. These Cr intermetallic phases were found to inhibit grain growth and to retard recrystallization during annealing process
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