Date of Award

6-30-2016

Document Type

Open Access Dissertation

Department

Mechanical Engineering

First Advisor

Xiaomin Deng

Abstract

Friction extrusion process is a novel manufacturing process that converts low-cost metal precursors (e.g. powders and machining chips) into high-value wires with potential applications in 3D printing of metallic products. However, there is little existing scientific literature involving friction extrusion process until recently. The present work is to study the heat transfer and material flow phenomena during the friction extrusion process on aluminum alloy 6061 through numerical models validated by experimental measurements.

The first part is a study of a simplified process in which flow of a transparent Newtonian fluid in a cylindrical chamber caused by frictional contact with a rotating die is considered but extrusion is omitted. This simplified process facilitates an analytical solution of the flow field and experimental visualization and measurements of the flow field in the chamber. The fluid choice and rotation speed are chosen so that the Reynolds number of the flow is approximately the same as that in the friction extrusion process of an aluminum alloy and that the resulting fluid flow is a laminar flow. An analytical solution for the velocity field of the fluid flow has been obtained, and the process has also been simulated using fluid dynamics. The analytical solution, the numerical predictions, and experimental measurements have been compared and good agreements can be observed.

Second, a pure thermal model has been developed to investigate the heat transfer during the friction extrusion process. A volume heating model is proposed to approximate the heat generation. A layer under the interface between the die and the aluminum alloy sample is chosen as the heat source zone. The distribution of the heat generation rate is assumed linear along both vertical and radial directions. The total power input into the system is related to the mechanical power recorded in the friction extrusion experiment. Only heat transfer is considered in this model and the material flow is neglected. The temperature predictions have a good agreement with experimental measurements, indicating that the proposed model can capture the heat transfer phenomenon.

A three-dimensional thermo-fluid model also has been developed to provide a comprehensive understanding of the friction extrusion process. Both heat transfer and material flow are simulated. The volume heat model in the pure thermal model is utilized for the heat generation. The predicted temperature results show that the material flow has limited influence on heat transfer. The sample is treated as a non-Newtonian fluid whose viscosity is temperature and strain rate dependent. Massless solid particles are used in the fluid as tracers to study the material flow patterns. The predicted distribution of the particles on extruded wire cross sections compare qualitatively with experimental measurements, suggesting that the material flow can be captured by the thermo-fluid model. The model provides predictions, such as the fields of velocity, strain rate, and viscosity which are not available from experimental measurements. The particle path lines also show how the material flows to form the wire.

Rights

© 2016, Hongsheng Zhang

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