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Browsing by Author "Aljashaami, Dhyai Hassan Jawad"

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    Mechanical performance of layered metallic composites processed by accumulative roll bonding
    (2019-08-20) Aljashaami, Dhyai Hassan Jawad; Yapıcı, Güney Güven; Yapıcı, Güney Güven; Başol, Altuğ; Bundur, Zeynep Başaran; Oral, A.; Tezel, Y. Ş.; Department of Mechanical Engineering; Aljashaami, Dhyai Hassan Jawad
    Mechanical Performance of Layered Metallic Composites Processed by Accumulative Roll Bonding.--Multi-layered metal composites have received considerable attention due to their advanced mechanical and physical properties. The current work is an experimental study to fabricate the ultra-fine grained combination of similar and dissimilar composites utilizing accumulative roll bonding (ARB) process as a severe plastic deformation (SPD) technique. The experimental work was organized in two parts. The first part describes the combination of Al2024 and Al6061 in similar and dissimilar aluminum composites, while the second part has different Al/IF steel composites including Al6061, Al2024 and interstitial free (IF) steel in various stacking sequences. Microhardness and uniaxial tensile tests were applied to analyse the surface and bulk mechanical properties of processed materials, respectively. This study not only investigates the monotonic mechanical behavior of multi-layered metal composites but also inspects the cyclic behavior of the prepared composites by employing the fatigue test. The high cycle fatigue (HCF) properties of layered metallic composites were investigated by cyclic testing under stress control with positive mean stress. For the first part, the processed structure after four passes ARB contained the various layer combinations of Al2024 and Al6061. Remarkable enhancement was observed in the hardness level of the samples with increasing number of ARB passes. Accordingly, improvement levels, up to 1.5 and 2 times, were recorded for Al2024 and Al6061 layers, respectively. The tensile strength of the composite with an interchanging layer architecture reached over 320MPa after two cycles, coinciding with more than two-fold of the as-received Al6061. The fatigue life was also improved, especially at the high stress amplitude. Microstructural observations revealed a significant grain refinement in further ARB processing along with the explanation of possible fracture mechanisms under tensile straining. Additionally, the mechanical properties of processed materials were evaluated using shear punch testing (SPT). The correlation between the results of tension experiments and shear strengths was calculated. Experimental results demonstrated that the shear strength enhanced by increasing the number of ARB passes. However, the shear elongation exhibited a notable reduction when the number of ARB passes increased. Inspection of the tensile and SPT results revealed that they follow a similar trend for both strength and ductility. Therefore, it can be asserted that the shear punch test represents a useful and complementary tool in the mechanical analysis of the ARBed samples. According to the SEM micrographs, in multi-passes ARB process, the interface of the previous pass bonds strongly during the next cycle, due to the improvement of the atomic diffusion and high pressure with further passes. The first ARB pass imposed a moderate strain and materials showed a ductile fracture with microvoids and dimples. With increasing cycles, the fracture mode remained as ductile with the existence of shear rupture and dimples. Nevertheless, these dimples were shallow and elongated, especially for the Al2024 layers as compared to those observed in Al6061. For the second part, necking and fracture of IF steel layers were detected in the macrostructural observation after three passes of ARB process. Furthermore, after five ARB passes, a multi-layer IF steel/Al composite with homogeneously distributed IF steel lumps in aluminum matrix was attained for all stackings of IF/Al6061. However, the low difference between the hardness of the Al2024 and IF steel prevents the occurrence of the same phenomena in Al2024/IF steel composites. Thus, the continuity of the layers after the third and fourth passes has remained for all stackings of IF/Al2024. Microstructure and mechanical characteristics of a fourth layer architecture were analyzed within a number of ARB passes. The results revealed that the monotonic and cyclic behavior of all dissimilar composites were significantly increased compared to the base aluminum alloys, while the composites with the outer aluminum layers exhibited the highest fatigue life, due to crack branching at the interface region when it propagated from the softer to the harder layer. Fatigue fracture surfaces and crack propagation paths of the samples were observed by scanning electron microscopy (SEM). Also, fracture morphology analysis demonstrated that despite the surface cracks on the outer layers, indeed the fatigue cracks of interface layers were caused by the fracture of samples. The ARB process was simulated utilizing finite element analysis. The effective stress and the distributions of equivalent strain along the thickness of ARBed sheets were determined. Results showed a significant agreement between the numerical simulations and the experimental findings. Finally, high cycle fatigue analysis was carried out and the results of the simulations were in decent agreement with the empirical data in terms of fatigue life. Also, as expected, the experimental fatigue life values for all conditions were lower than the simulations in relation with the existence of microcracks and scratches on the sample surface.