AN INVESTIGATION OF MECHANICAL FRACTURE BEHAVIOR OF ULTRAFINE GRAINED 6082 AL ALLOY

Abstract

6082 Al alloy is an important aluminium alloy for structural applications. 6xxx Al alloys are wrought alloys with magnesium and silicon as the main alloying elements. These alloys are easy to machine and can be precipitation hardened. 6082 Al alloy has a FCC crystal structure at the atomic level. This leads to high strength because it is more difficult for the crystal cells to slide past one another. This alloy forms a layer of Al2O3 It is well known that useful properties of Al alloys can be further enhanced by creating fine grain structure. The ultrafine grain (UFG) structure shows superior mechanical properties as expressed by the Hall-Petch relationship. In UFG, the grain sizes are in sub-micron (100–1000 nm) or nano-crystalline (<100 nm) range. UFG materials enclose a very high density of grain boundaries in their microstructure, which can play a significant role in the development on contact with air, which prevents further corrosion. Thus, 6082 Al alloy is a medium strength alloy having excellent corrosion resistance. It has a number of useful properties, including high strength compared to other Al alloys. It possesses the highest strength of the 6xxx series alloys. Due to its high strength, it has replaced 6061 aluminium alloy in many applications. It is often used to build structures due to its high mechanical strength. It contains a large amount of manganese, which controls the grain structure and a stronger alloy is achieved. This alloy is widely used for light to medium strength applications. 6xxx series Al alloys can be easily strengthened by heat treatment (precipitation hardening) process in the presence of their main alloying elements silicon and magnesium. Heat treated 6082 Al alloy have good formability, weldability, corrosion resistance and strength after heat treatment. The most common applications of heat-treated 6082 Al alloy are in containers, foils, collapsible tubes, radiator tubes, wide jar closures, printing plates, strip for heat exchanger, boiler making, insulation foils, kitchen-ware, chemical and food industry equipment, containers, automotive trim, light reflectors, architecture, vessels and piping. of novel properties. Ultrafine/nano grained Al alloys have many attractive properties such as outstanding strength, enhanced hardness, good toughness, enhanced diffusivity, higher specific heat, and enhanced thermal expansion coefficient and superior magnetic properties. For nanostructured materials, low ductility is reported by many researchers. Using accurate thermo mechanical treatment, the ductility can be improved. However, it is seen that the precipitate may grow in size, and consequential reduction in strength may occur. In the present thesis work, 6082 Al alloy is cryorolled (CR) and room temperature rolled (RTR) for different amount of thickness reductions to improve the mechanical (hardness, yield strength, ultimate strength), fracture and fatigue behaviors of the alloy. Techniques such as optical micrographs, FESEM, EBSD, TEM and DSC, are used to characterize the bulk, CR and RTR alloy. The effect of cryorolling, room temperature rolling and heat treatment on the tensile and impact properties is investigated by performing Charpy impact and tensile tests. The microstructure of the CR and RTR alloy with different thickness reductions i.e. 40%, 70% and 90%, are characterized by FESEM to reveal the modes of failure. An improvement in tensile strength and impact toughness of the CR and RTR alloy has been clearly observed. The fracture and fatigue testing of bulk, CR and RTR alloy have been performed to evaluate the fracture and fatigue behavior. The fracture and fatigue simulations are performed in ABAQUS and ANSYS software respectively for bulk, CR and RTR alloys. The experimental testing and simulations shows that the mechanical, fracture and fatigue behaviors of Al alloy are significantly improved by cryorolling and room temperature rolling. The improvement in tensile properties is found to be more in case of CR alloy as compared to RTR alloy for the same amount of thickness reduction.