Date of Award

January 2018

Document Type

Thesis

Degree Name

Master of Science (MS)

Department

Mechanical Engineering

First Advisor

Meysam Haghshenas

Abstract

Friction stir welding, as a solid-state joining method, seems a viable joining/assembly method for difficult-to-weld cast magnesium and cast aluminum alloys. Due to the high flammability of Mg alloys, and the brittle joints of welded Al alloys, fusion based welding techniques may not be successful to join Mg or Al made component. Therefore, a solid-state operation, in which applied heat input and induced temperature within the work piece are below the melting temperature of the base alloys, can be considered as a viable method to join so-called unweldable Mg and Al alloys. To this end, friction stir welding, as one of the most common solid-state joining and processing methods, has been extensively studied for similar/dissimilar welding of different grades of magnesium alloys.

In this thesis, the effects of FSW parameters (i.e. tool traverse speed and tool rotational speed) have been examined on two different cast alloys with particular focus on microstructure, hardness distribution and fatigue life:

i) AM60 as-cast magnesium alloy

ii) Al-10%Si as-cast hypoeutectic aluminum alloy

The influence of different processing parameters has been explored at a vital level. Hardness and fatigue testing were performed to determine how the changes in parameters influence the mechanical properties. Fractography was carried out on the fatigue fracture surface to determine the means of failure and crack initiation points; optical microscopyand scanning electron microscopy were performed on the FSW joint to observe the microstructural changes with varying weld parameters.

Upon the FSW process, the microstructure of both alloys was greatly refined due to the severe plastic deformation imposed by the rotating tool. The hardness for the AM60 Mg alloy was higher in the SZ, compared with the base metal, while for the Al-10%Si alloy the hardness value was lower in the SZ than the parent materials with the lowest value being seen in the HAZ and TMAZ.

The resulting stress amplitude vs. cycles to failure (S-N) curves show an increase in fatigue life with a decrease in stress amplitude. SEM fracture surface observations show crack initiation occurring from the imperfections on the surface and internally. The fracture for AM60 was a combination of both brittle and ductile while the Al-Si10% alloy showed pure ductile fracture (collection of dimples on the fracture surface). The fracture surfaces also exhibited features unique to fatigue failure, including fatigue striations and beach marks

In Chapter V a summary of the present work along with recommended future research have been provided. Also, various properties of the FSWed AM60 have been compared against those of FSWed Al-10%Si alloy and the differences have been explained.

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