Public Thesis Defense - Nicolas Dimov

Title:

Dissimilar Al6061 to Al7075 friction stir welds: processing, static and fatigue properties

Abstract:

Friction Stir Welding (FSW) is a solid state (without fusion) welding technique involving no filler material. Patented for the first time in 1991 by The Welding Institute (Cambridge, UK), it consists in the mixing of the material of two parts to be assembled by plastic deformation. Friction Stir Welding allows homogeneous or bi-material joints with a small decrease in the mechanical properties (compared to conventional assembly processes) as well as an excellent repeatability. This technology does not use consumables and is adaptable to conventional CNC machines. Initialy conceived for aluminium alloys, this process is applicable to other alloys with low melting temperatures, such as copper or magnesium. Numerous industrial examples using FSW on these alloys exist. Nevertheless, questions still remain regarding some mechanical properties of aluminium assemblies, particularly in the case of fatigue of so called "dissimilar" joints (Welds containing multiple aluminium alloys).

After a brief  introduction on the process in and of itself and a wide range of industrial applications, majors results from the literature regarding mechanical and microsturctural properties of aluminium FSW joints are presented. A particular attention is devoted to the impact of process parameters on previously described properties. A chapter first describes the methodology used to establish a process window for AA6061/AA7075: welds quality control and thermal aspects are described as well. It is then followed by the characterization of said joints (similar and dissimilar) by various means: optical microscopy, Vicker hardness measurements and tensile testing using Digital Image Correlation (DIC). Links between these properties and process parameters used in the manufacturing of the joints are established. A tensile test carried out inside a scanning electron microscopie is presented in order to analyze local plastic deformation distribution inside the stir zone of a dissimilar joint. Finally, fatigue testing is carried out on some samples and relationships are established between thermal input during welding as wall as stir zone morphology and fatigue life properties. Exploratory aluminium and titanium friction stir welds are presented and characterized in the appendix.

Jury members:

Prof. Aude Simar (UCLouvain), supervisor
Prof. Eric Charkaluk (Ecole Polytechnique, France), supervisor
Dr. Julien Benoist (Thales Global Services), supervisor
Prof. Hervé Jeanmart (UCLouvain), chairperson
Dr. Florent Hannart (UCLouvain), secretary
Prof. Jean-Yves Buffière (INSA Lyon, France)
Dr. Marie-Noëlle Avettand-Fenoel (Polytech Lille, France)
Dr. Thaneshan Sapanathan (Curtin University, Australia)
Dr. Daniel Weisz-Patrault (Ecole Polytechnique, France)
Prof. Habibou Maitournam (ENSTA, France)

Videoconference