Investigation of Mechanical, Corrosion and Microstructural Behaviour of Heat Treated Cr-Modified Al-Mg-Si Alloy

Abstract

Despite the choice of Al-Mg-Si alloy as a material for innumerable industrial and structural applications, challenges such as undesired scratch resistance, formability and mechanical properties deterioration in saline environment hinders the extent of its application for automotive and aerospace components. Nevertheless, with the growing interest in the application of Al-Mg-Si alloy in automotive and aerospace industries, there is need for cautious control of thermal treatments and inclusion of alloying elements with requisite potentials for enhancing the microstructure and mechanical properties of the alloy. Chromium is known to improve strength and corrosion resistance in several applications. Therefore, this study focuses on the investigation of the effect of Cr particles inclusion in Al-Mg-Si alloy. The effect of ageing heat treatment on selected properties of Al-Mg-Si-Cr alloy was also studied in this work. The Al-Mg-Si and Al-Mg-Si-Cr alloys were developed using a two-step stir casting technique. Chromium was added to Al-Mg-Si alloy at varying percentages of 0, 0.5, 1.0, 1.5, 2.0 and 2.5. All the samples were solution treated in an electric furnace at 500 oC for 30 minutes and water quenched. Then the samples were artificially aged at 210 oC for 3 hours and quenched in natural air. The hardness test revealed that the inclusion of Cr particles in Al-Mg-Si alloy samples increased hardness from 35.03 Kgf/mm2 (hardness of Al-Mg-Si-0%Cr alloy sample) to a maximum value of 126.54 Kgf/mm2 (hardness of Al-Mg-Si-1.5%Cr alloy sample). After heat treatment, the hardness of Al-Mg-Si-0%Cr alloy sample increased to 80.84 Kgf/mm2, while that of Al-Mg-Si-1.5%Cr alloy sample decreased slightly to 120.88 Kgf/mm2. The impact strength test also showed that the inclusion of Cr in Al-Mg-Si alloy increased impact strength from 9.52 J/mm2 (impact strength of Al-Mg-Si-0%Cr alloy sample) to a maximum value of 19.04 J/mm2 (impact strength of Al-Mg-Si-2.0%Cr alloy sample). After heat treatment, the impact strength of Al-Mg-Si-0%Cr alloy sample increased marginally to 10.09 J/mm2, while that of Al-Mg-Si-2.0%Cr alloy sample decreased slightly to 17.57 J/mm2. The tensile and electrochemical tests revealed that the heat-treated Al-Mg-Si-1.0%Cr alloy sample exhibited the highest tensile strength and lowest corrosion rate of 152 MPa and 0.0014 mm/year, respectively. The microstructural examination further revealed that the inclusion of Cr particles in Al-Mg-Si alloy improved its surface morphology. Al-Mg-Si-1.0%Cr alloy sample was adjudged to possess the best microstructural properties. Therefore, this sample is recommended as a potential material for machine tools and other structural or advanced manufacturing applications.