Date of Award

1-1-2013

Document Type

Open Access Thesis

Department

Mechanical Engineering

First Advisor

Anthony P Reynolds

Abstract

Friction Stir Welding (FSW), first invented by The Welding Institute of UK (TWI) in 1991, is a solid state welding process which was initially applied to welding Aluminum Alloy. FSW has wide application in industrial sectors. Stationary shoulder friction stir welding (SSFSW) was first developed to weld low thermal conductivity Ti-based alloys, which are hard to weld using conventional friction stir welding. Previous literatures showed SSFSW can produce uniform temperature distribution through thickness during the welding process. Since SSFSW is still under study phase, its advantages and disadvantages are not yet well defined. It is important to study the characteristics of SSFSW for its further application.

The object of this thesis is to develop low distortion and high mechanical properties for AA7075-T6 parallel double-pass lap joints using SSFSW. The effect of welding control parameters and tool design on process response was investigated. Then the microstructure, distortion, and mechanical properties of AA7075 after FSW were studied. The effect of post welding heat treatment (PWHT) on distortion and mechanical properties was also investigated.

The study found that rotation speed affects torque and total power in SSFSW. Z-force was the main factor affecting X-force. Tool design didn't affect process response much. Saddle shape was observed for distortion distribution of SSFSW. PWHT helped to reduce distortion and regain mechanical property. Fine and equiaxed grains were observed in the weld nuggets. Grain size increased with power input for a given welding speed. Microhardness tests revealed higher hardness present at weld nugget zone (WNZ) and base material, and lower hardness present at HAZ. Tensile test showed the maximum ultimate stress (UTS) was 537.37MPa. Most tensile tests failed at the cavity defect area or at HAZ.

Rights

© 2013, Hejun Yu

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