Date of Award

12-2018

Document Type

Thesis

Degree Name

Master of Science (MS)

Department

Mechanical Engineering

Committee Member

Dr. Xin Zhao, Committee Chair

Committee Member

Dr. Huijuan Zhao

Committee Member

Dr. Laine Mears

Abstract

With the objective of minimizing carbon footprint of vehicles, different organizations across the world are increasingly enforcing higher fuel efficiency targets for the automobile manufacturers. To improve the fuel economy while retaining or further improving the structural integrity, the automobile industry is vigorously shifting towards substituting conventional heavy materials like cast iron with new age materials such as aluminum alloys, steel alloys, etc. which are not only much lighter but also offer superior strength-to-weight ratio. Engineers use a mix of these new age materials with the aim of maximizing the benefits from each material. However, the utilization of such materials is currently limited in the industry as welding them using conventional methods such as resistance spot welding or fusion welding process, is plagued with inherent difficulties such as formation of brittle inter-metallic compounds, irreversible and adverse changes in the thermal and mechanical properties of the materials. Dissimilar material joining is of critical importance in aiding the manufacturers realize the crucial objective of a safer and more fuel efficient vehicle. Friction element welding (FEW), a friction based joining process, has been proposed for joining highly dissimilar materials in minimal time and with low input energy. FEW process can join a variety of materials which differ significantly in their mechanical, thermal, and metallurgical properties without inducing any of the defects associated with conventional welding methods. The fundamental governing mechanisms that characterize the FEW process needs to be investigated to help optimize the process for specific applications. Conducting experimental investigation is undesirable and infeasible due to the highly complex thermal-mechanical procedures occurring simultaneously in a very short period of time of about one second. As such, the utilization of a finite element model to simulate and analyze the FEW process is warranted which would help understand the underlying mechanisms of the process in detail and provide an efficient yet effective tool to observe the effect of different process parameters on the weld quality. A coupled thermal-mechanical finite element model (FEM) is developed in this work to simulate the FEW process and gain an understanding of the physical mechanisms involved in the process and help predict the influence of variation of process parameters on the evolution of temperature, material flow, and their effect on weld quality. The primary difficulty in simulating a highly transient process like FEW, wherein not only the workpiece is subjected to deformation but also the auxiliary joining element i.e. friction element undergoes extensive deformation, is that the mesh elements are prone to distortion failure while trying to capture such high amount of deformation. The presence and importance of temperature effect on material properties further complicate the FEM. To help eliminate the distortion issue while simultaneously achieving an accurate simulation of the FEW process, the coupled Eulerian-Lagrangian (CEL) approach is adopted. The novelty of the current approach employed lies in using a Eulerian definition for the tool as against the more traditional convention of adopting a purely Lagrangian definition. The Eulerian definition enables to simulate the extreme deformation of friction element and capture the material flow without any computational issues. To inspect for the accuracy of the FEM results, mechanical deformation for different parts observed in the FEM is compared against the experimental results. To further validate the FEM, experimental measurements of temperature at different locations at the interface of two layers of workpiece are compared against the FEM results at same locations in the model. With respect to, both, thermal and mechanical measurements comparisons good agreement is shown between the simulation results and the experimental data. The simulation results for sets with varying process parameters show that the rotational speed of the friction element has the highest influence on the amount of frictional heat generated followed by the time period for different steps. Higher amount of heat is generated and conducted into the top aluminum layer for longer Penetration time, whereas for more heat concentration into the friction element to achieve the required deformation, longer Welding step with higher rotational speed is desired.

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