Microstructure and Mechanical Properties of Cast Aluminium-Zinc-magnesium Alloys Processed by Equal Channel Angular Pressing

Abstract

Equal channel angular pressing (ECAP) is one of the severe plastic deformation (SPD) techniques used to develop ultrafine-grained (UFG) materials. In this technique, large amount of shear strain is introduced in the material, without any change in the cross sectional dimensions. Al-Zn-Mg alloys are promising light weight high strength materials, wherein precipitation strengthening will be possible. In this investigation ECAP is used to enhance the strengthening in the Al-Zn-Mg alloys. The alloys studied in the present work were prepared by gravity casting method. ECAP processing was carried out in a die having an internal angle between two channels (Φ) of 120º and outer arc curvature (Ψ) of 30º. The processing was attempted at lowest possible temperature in route BC. Techniques, like optical microscopy, scanning electron microscopy, transmission electron microscopy and X-ray diffractometer were used to characterize and analyse the microstructures before and after ECAP processing. To assess the mechanical properties, microhardness measurement and tensile tests were conducted. Fracture mode and fracture surface morphologies of the tensile test samples of the processed and unprocessed materials were studied. Wear properties were evaluated before and after ECAP processing. Wear mechanisms involved in the samples were studied. Microstructural study reveals that, in as-cast condition, alloys were composed of dendritic structure. Also, with increase in the zinc content in the alloy, volume of precipitates was increased. After ECAP processing, considerable decrease in the grain size of the alloys was noted. Also, after ECAP processing, high density dislocation structures with high fraction of high angle grain boundaries were observed. It was also noticed that, ECAP processing leads to enhance the precipitation kinetics of the alloy. In all three alloys, after ECAP processing, fine size spherical shaped precipitates were noticed and these precipitates were uniformly distributed in the alloy. After ECAP processing, significant improvement in the mechanical properties of the alloys was perceived. Microhardness and strength were increased with increase in the zinc content in the alloy. At the same time, elongation to failure of the alloy decreased with increase in the zinc content. Optimum mechanical properties were perceivedwhen the alloys were processed at lowest possible temperature. Compared to as-cast alloys, microhardness increases by 109% for Al-5Zn-2Mg alloy, 67% for Al-10Zn-2Mg alloy and 58% for Al-15Zn-2Mg alloy, processed at 200 °C. Compared to as-cast alloys, ultimate tensile strength (UTS) increases by 122% for Al-5Zn-2Mg alloy, 153% for Al-10Zn-2Mg alloy and 139% for Al-15Zn-2Mg alloy, processed at 200 °C. Brittle fracture mode was observed during tensile test of as-cast and homogenized samples. The fracture mode was changed to shear fracture after ECAP processing. Large sized dendrites were observed in the fracture surfaces of the as-cast condition tensile test samples. While, narrow and shallow dimples were noticed in the fracture surfaces of the ECAP processed samples. ECAP processing leads to considerable improvement in the wear resistance of the alloys. Wear resistance of the alloys increased with increase in the zinc content. Coefficient of friction of the alloy decreased after ECAP processing. Also, coefficient of friction of the alloy decreased with increase in the zinc content. Irrespective of the applied load, abrasive wear mechanism was reported in the as-cast and homogenized condition samples. In the ECAP processed samples, wear mechanism shifts from adhesive to abrasive wear with increase in the applied load. Also, in the ECAP processed samples, at lower load; transfer of iron particles from the disc surface to the sample surface was noticed. Compared to the Al-5Zn-2Mg and Al-10Zn-2Mg alloys better wear properties were observed in Al-15Zn-2Mg alloy.