Étude des conditions limitant la croissance anormale des grains dans les joints soudés par friction-malaxage lors du traitement thermique de l'alliage d'aluminium AA5083

Authors: Nadeau, François
Advisor: Larouche, Daniel
Abstract: The issue regarding abnormal grain growth (AGG) that appears in aluminum alloys after heat treatment in friction stir welds (FSW) is well established. Several authors have already observed abnormal growing grains and some associated mechanisms have been identified through the friction stir welding community. In spite of this understanding, only a few papers describe methods to limit or eliminate this behavior. This study focuses on the effect of varying the heat input during welding of AA5083 aluminum alloys in relation with abnormal grain growth. The main process parameters and an external heat supply are modified in order to cover a wide range of global heat inputs. It is possible to eliminate completely abnormal grain growth by using an adequate combination of these two process variables. However, a narrow welding envelope is found to obtain such a result because internal cavities are revealed if the welding temperature reaches a critical value. Temperature measurements taken with a thermal camera have identified this factor as the most critical on the onset and quantity of abnormal grain growth observed in the samples. Two models applied to abnormal grain growth are analyzed: a first one based on the effect of second-phase particles and a second based on non-uniform subgrain boundary mobilities. The second model gives a better correlation with our findings mainly when external heating is employed. A microstructural analysis using optical microscopy, secondary electron microscopy (SEM) and electron back-scattered diffraction (EBSD) has enabled the quantification of the size and the distribution of grains in the weld nugget, the misorientation of grain boundaries and the second-phase particles. These results have linked our observations with the studied models. An external heat supply reduces the grain size distribution and the subgrain boundary misorientation. These changes stabilize the microstructure at elevated temperature and inhibit abnormal grain growth.
Document Type: Mémoire de maîtrise
Issue Date: 2013
Open Access Date: 19 April 2018
Permalink: http://hdl.handle.net/20.500.11794/24337
Grantor: Université Laval
Collection:Thèses et mémoires

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