Dissertation Defense - Dhyai Hassan Jawad Aljashaami (PHDME)

Dhyai Hassan Jawad Aljashaami- Ph.D. Mechanical Engineering

Assoc.Prof. Güney Güven Yapıcı - Advisor

Mechanical Performance of Layered Metallic

Composites Processed by Accumulative Roll Bonding

Date: 20.08.2019

Time: 14:00

Location: AB1 510

Thesis Committee: 

Assoc. Prof. Güney Güven Yapıcı,  Özyeğin University

Asst. Prof. Altuğ Melik Başol,  Özyeğin University

Asst. Prof. Zeynep Başaran Bundur,  Özyeğin University

Asst. Prof. Yasemin Şengül Tezel,  Sabancı University

Asst. Prof. Alpay Oral,  Yıldız Technical University

Abstract

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 ultrafine-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 into 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 include Al6061, Al2024 and interstitial free (IF) steel in various stacking sequences.

Microhardness and uniaxial tensile test were applied to analyze 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 employing the fatigue test. The high cycle fatigue (HCF) properties of laminated metal composites were investigated by tension-tension fatigue test under the stress control with R =0.1.

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 the 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 Al6061/Al2024/Al6061/Al2024 composite reached over 320 MPa 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 the further ARB process along with 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.
For the second part, necking and fracture of IF steel layers were detected in macrostructure observation after three passes ARB process. Furthermore, after five ARB passes, a multi-layer IF steel/Al composite with homogeneously distributed IF steel chips in an aluminum matrix was attained for all groups 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 groups of IF/Al2024. Microstructure and mechanical characteristics of a fourth-type layer arrangement of composites were analyzed within several amounts 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 the crack branching at the interface region when it propagated from the softest to the harder metal. Fatigue fracture surfaces and crack growth paths of the samples were observed by scanning electron microscopy (SEM). Also, fracture morphology analysis of composites demonstrated that despite the surface cracks on the outer layers, the fatigue cracks of interface layers were caused to the fracture of fatigue 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 the number of cycles, the fracture mode remained as a ductile fracture manner 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.

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 calculated. Results showed a significant agreement between the numerical simulations and the experimental part.
Finally, the high cycle fatigue was studied numerically using finite element methods constructed through the ANSYS Workbench. The results of the simulations were in good agreement with the empirical data in term of fatigue life. Also, as expected, the fatigue life for the experimental work for all conditions was lower than the simulations in relation with the microcracks and scratched on the surface of the samples.