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School of Engineering

30/05/2016

The Degree of Doctor of Philosophy in Engineering (Mechanical): Dreyer Bernard

Previous qualifications: 2008 NDip (Mechanical Engineering) (Cum Laude) Nelson Mandela Metropolitan University 2009 BTech (Mechanical Engineering) (Cum Laude) Nelson Mandela Metropolitan University 2013 MTech (Mechanical Engineering) (Cum Laude) Nelson Mandela Metropolitan University

Thesis: INFLUENCE OF INCREASED PROCESSING SPEED ON MICROSTRUCTURE EVOLUTION AND MECHANICAL PROPERTY RELATIONSHIP IN FRICTION STIR WELDING OF AA5182-H111 (T500)

Over the past two decades, friction-stir welding has been proven to produce joints in aluminium alloys with superior static and dynamic properties when compared with conventional fusion welding. However, friction-stir welding of these materials is generally associated with production speeds well below 500mm/min; while fusion welding could achieve travel speeds exceeding 1000mm/min. This has presented researchers globally with a challenge to improve their understanding of the influence of increased travel speed on microstructural evolution, and the linked relationship to mechanical properties. One of the most important contributions made by the candidate, achieved through complex thermal and microstructural characterisation, was his finding that at high tool travel speeds; the tool position could exceed that of the heat-transfer zone. This finding contradicts the current state of understanding; and it has formed the basis for a knowledge contribution.

By applying complex experimental and advanced metallographic analytical techniques, the microstructural evolution achieved during high speed welds could best be described as a low temperature-high strain-rate deformation and recrystallization phenomenon. The increased plastic strain energy has created the necessary driving force for dynamic recrystallization to occur, in the absence of heat, ahead of the heat transfer zone. Compared to the base alloy, the weld material displayed a 25% increase in the Yield Strength, while experiencing only a marginal reduction in elongation.

This work has made a significant contribution to the creation of a better understanding of why high-integrity joints can be made at higher travel speeds during solid-state welding.