December 20, 2022
10:00
Palaiseau
Ecole Polytechnique
For the degree of Doctor of Engineering Sciences and Technology
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 a tool mounted on a CNC machine and applying friction and plastic deformation. Friction Stir Welding allows homogeneous or bi-material joints with a limited decrease in mechanical properties (compared to fusion welding processes) as well as an excellent repeatability as it is highly automated.
Initially applied to aluminum alloys, this process has been extended 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 aluminum alloys). The thesis industrial partner, Thales Group, desired to better understand the specificities of this process and thus established a collaboration with UCLouvain and Ecole Polytechnique de Paris.
The thesis is centered on the dissimilar welding of AA6061 to AA7075 aluminum alloys. Particular attention is devoted to the impact of process parameters on previously described properties. A methodology is proposed to establish an optimized process
window for AA6061/AA7075 with an emphasis on welds quality and thermal control. Selected similar and dissimilar joints are then characterized 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. An innovative tensile test carried out inside a scanning electron microscope 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 selected welding conditions and relationships are established between thermal input during welding as well as stir zone morphology and fatigue life properties. Exploratory aluminum to titanium friction stir welds are presented and characterized in the appendix.
Jury members :
- Prof. A. Simar (UCLouvain, Belgium), supervisor
- Prof. E. Charkaluk (Ecole Polytechnique, France)
- Prof. H. Jeanmart (UCLouvain, Belgium), chairperson
- Dr. F. Hannard (UCLouvain, Belgium)
- Prof. J.-Y. Buffiere (INSA Lyon, France)
- Dr. M.-N. Avettand-Fenoel (Polytech Lille, France)
- Dr. T. Sapanathan (Curtin University, Australie)
- Dr. D. Weisz-Patrault (Ecole Polytechnique, France)