Date of Award

2016

Document Type

Open Access Dissertation

Department

Mechanical Engineering

Sub-Department

College of Engineering and Computing

First Advisor

Anthony P. Reynolds

Abstract

Conventional shoulder friction stir welding (CSFSW) produces uneven heat input through welded material thickness: higher close to the top and lower close to the bottom. When CSFSW is applied on certain aluminum alloys, such as 7xxx and 2xxx series high strength aluminum alloys which contain low melting point intermetallic, overheating and local melting may happen close to weld crown. Stationary shoulder friction stir welding (SSFSW) may generate much more uniform heat input through plate thickness than CSFSW due to the non-rotating shoulder and rotating pin. Therefore, overheating and local melting are expected to be avoided in SSFSW. Furthermore, local properties of joint made by SSFSW should be more uniform through its thickness than those of joint made by CSFSW.

In this study, thermal management was mainly approached by applying a rotating shoulder tool (CSFSW) and a stationary shoulder tool (SSFSW) in FSW. Beside the thermal management implemented by the shoulder, single pass (SP) FSW, dual-pass (DP) FSW, various pin features such as flats and flutes, have also been introduced in this investigation to achieve different thermal distribution.

A series of 24.9 mm and 25.4 mm thick AA7099-T7651, 32 mm thick AA7050-T7451 and 25.4 mm thick AA6061-T651 aluminum alloy plates have been friction stir welded using four different process variants. The process variants used are: stationary shoulder single pass (SSSP), conventional shoulder single pass (CSSP), stationary shoulder dual pass (SSDP), and conventional shoulder dual pass (CSDP). FSW parameters, such as speeds, forces, temperatures, torques, powers and grain size, have been recorded, calculated and analyzed. Welding quality, material flow and deformation, as well as microstructure have been examined by various metallographic means. Mechanical examinations have been adopted to test mechanical properties of joints made with CSFSW and SSFSW. The TPM model implemented in COMSOL MULTIPHYSICS 4.0/4.4 has also been adopted in this research to simulate thermal distributions in FSW process when different process variants are applied.

Goals of this study include further understanding CSFSW and SSFSW mechanical, thermal and metallurgical processes, producing high quality thick plate SSFSW joint on 7xxx aluminum alloys, as well as investigating the influences of thermal management, pin features, process control parameters and different process variants in process response parameters, achievable welding speeds, thermal distribution and history in welded joint metallurgical and mechanical properties.

Rights

© 2016, Xiaomin Huang

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