Thesis defense – Juliette Theodore ‘Accommodation of residual stresses and strains in thick multi-pass welds by introducing variability in filler metal composition’

Juliette Théodore, PhD student in the TRU MULTIX, will defend her thesis on December 1st, 2023 at 9am at Polytech Nantes (Institut des Matériaux Jean Rouxel) on the subject ‘Accommodation of residual stresses and strains in thick multi-pass welds by introducing variability in filler metal composition’.

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Composition of the jury :

– Sophie BERVEILLER, Enseignante-chercheuse (HDR) ENSAM, Metz
– Christophe DESRAYAUD, Professeur des Universités ENSM, Saint-Étienne

– Aude SIMAR, Professeure des Universités Université Catholique de Louvain
– Frédéric DESCHAUX-BEAUME, Professeur des Universités Université de Montpellier
– Pascal PAILLARD, Professeur des Universités Nantes Université

Thesis supervisor : 
– Bruno COURANT, Maître de conférences (HDR) Nantes Université

Co-advisors : 
– Laurent COUTURIER, Maître de conférences Nantes Université
– Baptiste GIRAULT, Maître de conférences Nantes Université

Abstract :

In the field of welding, assembling of thick plates requires the chamfering of the base metal and the use of a multipass welding sequence. The process generates thermal, mechanical, and metallurgical phenomena resulting in the formation of residual strains and stresses. Residual stresses are especially problematic in structural assemblies, as they add to the external applied stresses during service, often reducing the in-service lifetime and increasing the risk of failure. Strains must be avoided or at the very least limited, as they can cause misalignments during welding sequence and lead to geometrical defects. Generally, during welding operations, the filler metal composition used is close or identical to at least one of that of the base metals of the parts to be joined. In addition, a single chemical composition of filler metal is used. The objective of this study is to use the TIG process to reduce strains and stresses in the case of stainless steel deposits or welds, by adding a second filler wire with significantly different properties to the first wire. The second alloy can be added to the weld either by using alternatively the two-filler metals, cord after cord, or by mixing them in situ. In the case of welds, in situ mixing allows the proportion of one wire to the other to be varied gradually according to the thickness of the welded joint.

A model of the geometry of TIG deposits is proposed using the disc method to predict the shape of the deposits. The microstructures, strains and stresses of the parts are evaluated and compared with those of parts made with a single filler wire. Finally, finite-element numerical simulations of TIG deposits were carried out using COMSOL Multiphysics software, simulating stresses, strains and temperature fields in order to compare them with the experimental results.

Keywords : Welding, Stainless steels, Stresses, Strains, Microstructures, Simulations