Development and Characteristics of Ultrafine Grained Mg-Zn/Al Multilayer Composites by Accumulative Roll Bonding Process

Abstract

Considerable demands for weight reduction in engineering applications led to an interest in light weight composites. A multilayered laminate composite fabricated via accumulative roll bonding (ARB) with two different alloys has been reported, which combine the advantage of both the alloys. Magnesium based alloys have been attracting much attention as light weight materials due to their high specific strength, good castability, good machinability, high damping capacity, and its availability as a natural mineral. However, major limiting factors in using magnesium include its low ductility, poor creep and corrosion resistance. The primary focus of this work is to develop ultra fine grained multilayered composite materials capable of exhibiting good combination of mechanical and corrosion properties. This is necessary in order to fulfill the requirements in applications such as in aerospace, automobile and electronics sectors. In this work, Mg-Zn/Al based multilayered composites were developed to address these issues. In this study, accumulative roll bonding was chosen as the SPD method to develop Mg-Zn/Al based multilayered composites in order to study the relationship between the resultant microstructure and mechanical and corrosion properties. A thorough microstructural characterization was performed on the composites by scanning electron microscope (SEM), electron backscatter diffraction (EBSD), transmission electron microscope (TEM) and phase analysis by x-ray diffraction. In addition, mechanical properties were evaluated by microhardness and tensile tests. Corrosion behavior of the multilayered composite was examined using electrochemical polarization and immersion test. Mg-(2-6%)Zn/Al and Mg-(2-6%)Zn/anodized Al multilayered composite were developed by accumulative roll bonding process at 300 ◦C up to five passes. Electron back scattered diffraction and transmission electron microscopy results revealed ultrafine grain with fraction of high angle grain boundaries. Grain size of ARB processed composite is in the range of 0.6 μm to 1.9 μm. Enhancement of microhardness, yield strength and ultimate tensile strength of the multilayered composites as compared to rolled Mg-(2-6%)Zn alloy was attributed to strain hardening and grain refinement (grain boundary strengthening mechanism) and presence of Al12Mg17, AlMg4Zn11 andiv Al2O3. Mg-4%Zn/Al and Mg-4%Zn/anodized Al composites exhibited better strength as compared to other two systems of multilayered composites. Potentiodynamic polarisation and immersion corrosion test results revealed that Mg-6%Zn/Al and Mg- 6%Zn/anodized Al multilayered composite exhibited better corrosion resistance as compared to lower Zn alloying composites of Mg-(2-4%)Zn/Al and Mg-(2- 4%)Zn/anodized Al. Presence of higher content of Zn in the composites results in the formation of large amount of ZnO, which can enhance the protectiveness of the passive film and kinetics of passivation there by inhibiting the corrosion. Mg-(2-6%)Zn/Al-7075 and Mg-(2-6%)Zn/anodized Al-7075 multilayered composite were developed by accumulative roll bonding using Mg-(2-6%)Zn with Al and anodized Al at 350 ◦C up to four passes successfully. EBSD and TEM results revealed ultra-fine grain with fraction of high angle grain boundaries. Grain size of the ARB processed composite is in the range of 0.6 μm to 1.3 μm. Microhardness, yield strength and ultimate tensile strength of the multilayered composites increase with increase in number of ARB passes. Mg-4%Zn/Al-7075 and Mg-4%Zn/anodized Al- 7075 composite gives better mechanical properties as compared to Mg-2%Zn/Al- 7075 and Mg-2%Zn/anodized Al-7075 and Mg-6%Zn/Al-7075 and Mg- 6%Zn/anodized Al-7075. Higher corrosion resistance has been observed in Mg- 6%Zn/Al-7075 and Mg-6%Zn/anodized Al-7075 as compared to composites containing lower Zn content. Higher content of Zn in the multilayered composites results in the formation of large amount of ZnO, which can enhance the protectiveness of the passive film there by inhibiting the corrosion. Mg-(2-6%)Zn/Ce/Al multilayered hybrid composites were developed by reinforcing Ce powder between Mg-(2-6%) and Al subjected to ARB process at 300 ◦C up to five passes successfully. EBSD and TEM analysis showed the UFG with high angle misorientation in the microstructure. Hybrid composite exhibits 1.2 times lighter weight as compared to Al. The intermetallics of Al17Mg12, AlMg4Zn11 and Al11Ce3 were identified through the XRD analysis. It is observed that, tensile strength and microhardness of the composites increases with in ARB passes. Increase in strength was due to strain hardening, grain refinement and cerium particle reinforcement. Mg- 4%Zn/Ce/Al hybrid composite exhibited higher strength as compared to Mg- 2%Zn/Ce/Al and Mg-6%Zn/Ce/Al hybrid composites. The percentage elongationv decreases with increases in the number of ARB passes. Potentiodynamic polarization and immersion study revealed that Mg-6%Zn/Ce/Al hybrid composites exhibited better corrosion resistance as compared to lower Zn alloying composites of Mg- 2%Zn/Ce/Al and Mg-4%Zn/Ce/Al. The developed composites with UFG structure exhibited lower density, good mechanical properties and higher corrosion resistance and can find potential applications in automobile, aerospace and electronic sectors.