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Title: Assessment of Microstructure Refinement and Mechanical Properties of Al alloys Processed by Accumulative Roll Bonding
Authors: Mohammed Khamis G. Alyazidi 
Supervisor: Dr. Khaled Al-Fadhalah
Keywords: Microstructure : Accumulative Roll Bonding
Issue Date: 2017
Publisher:  Kuwait university - college of graduate studies
Abstract: Accumulative Roll Bonding (ARB) has been utilized for microstructure refinement and enhancement of mechanical properties of AA1100 and AA6061 aluminum alloys. In particular, the study focused on the homogeneity of microstructure and hardness developed through thickness of the processed samples. ARB was applied using 7 cycles for AA1100 and 5 cycles for AA6061. Observations by scanning electron microscopy (SEM) show that the severe plastic deformation by ARB resulted in homogeneous microstructure through sheet thickness for both alloys. Applying 7 cycles to AA1100 resulted in grains size reduction from 20.2 to 0.46 μm, in normal direction (ND), and from 29.7 to 1.03 μm, in rolling direction (RD). For AA6061, grain size is reduced after 5 cycles from 18.8 to 0.40 μm in ND and from 33.2 to 1.18 μm in RD. In addition, there is an increase in Vickers microhardness of AA1100 occurred from 43.5 Hv for as-received sample to about 63 Hv after 7 cycles, demonstrating hardness homogeneity. In the case of AA6061, the microhardness remarkably increased from 43.5 Hv in as-received condition to about 99 Hv after processing by 5 cycles. Yet, no saturation in hardness occurred for this alloy. Also, nanoindentation measurements were done and it provided similar hardness profiles to that by Vickers microhardness. Nevertheless, AA6061 demonstrates slightly stronger indentation depth dependency compared to AA1100. ARB was also found to increase the tensile strength of AA1100 to 249 MPa, about 2 times its initial value, and to 346 MPa for AA6061 (2.5 times its initial value). On the other hand, ductility of AA1100, measured by percentage of elongation EL%, was reduced from 8.2 to 2.8 % with increasing ARB cycles. For AA6061, EL% decreased from16.7 % to 2.3 % after 1 cycle and then gradually increased to 3.3 % upon applying 5 cycles. The evolution of strength and hardness of ARB samples was shown to be significantly affected by strain hardening behavior of the two alloys. The high stacking fault energy (SFE) for AA1100 provided high rates of strain hardening and dynamic recovery, resulting in equilibrium ultra-fine grain (UFG) structure, and ultimately saturation in strength and hardness, after processing 3 cycles and more. On the other hand, the presence of alloying elements and hardening precipitates in AA6061 lowered SFE and thus lowered the rate of the dynamic recovery during ARB. Therefore, extended strain hardening at very high strains is shown to occur for this alloy, resulting in continuous increase in strength and hardness, and slight increase in ductility with increasing ARB cycles.
Appears in Programs:0630 Mechanical Engineering

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