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Friction stir welding and heat treatment of aluminium alloys

Tonkin, Mitchell (2016). Friction stir welding and heat treatment of aluminium alloys. Bachelor of Engineering (Honours) Thesis, Charles Darwin University.

Document type: Thesis
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Author Tonkin, Mitchell
Title Friction stir welding and heat treatment of aluminium alloys
Institution Charles Darwin University
Publication Date 2016-10
Thesis Type Bachelor of Engineering (Honours)
Supervisor Kannoorpatti, Krishnan
Thennadil, Suresh
Abstract Friction stir welding (FSW) is a form of solid state welding which has generated interest in the manufacturing sector since its inception in 1991. This technology has particular benefits when working with heat treatable aluminium alloys which are vulnerable to weld defects when joined using conventional fusion welding techniques including weld cracking and softening across the joint. Typically welding is conducted with material in an age hardened condition, however despite the lower temperatures involved in friction stir welding it has been found that mechanical properties are degraded when joining aluminium alloys in the 2XXX, 6XXX and 7XXX alloy series. It is highly desirable that these alloys be joined in a manner which achieves the peak mechanical properties of the material across the weld joint.

It has been suggested that welding 2XXX, 6XXX and 7XXX series aluminium alloys while in the soft, annealed condition can be beneficial in terms of manufacturing and help to reduce the requirements for friction stir welding. Applying heat treatments to the joint post welding can then be used to restore the mechanical properties. However it has been found that significant grain coarsening occurs in the weld joint after post weld heat treatment which has negative consequences for the joint’s performance.

This investigation explored the use of pre weld heat treatment of FSW aluminium alloys by welding AA6061 in a solution treated condition and subjecting the specimens to artificial ageing. The hardness and microstructure of the joints was then compared with specimens that were produced in AA6061 in the annealed condition which had been subjected to a standard post weld heat treatment in order to compare the difference and similarities between the two welding procedures.

The results of this work suggest that it is more beneficial to weld in the annealed condition with respect to hardness. After ageing the specimens welded in the solution treated condition showed signs of over-ageing, while the specimens welded in the annealed condition exhibited high, uniform hardness across the weld joint. However in terms of the grain structure it was found that welding specimens in the solution treated condition and then ageing was beneficial. It was found that the fine grain structure that formed in the nugget zone was retained after ageing, opposed to the significant grain coarsening seen in the specimens that were welded in the annealed condition and post weld heat treated.
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Created: Thu, 02 Feb 2017, 11:05:46 CST by Jessie Ng