6533b7d5fe1ef96bd1264ee4

RESEARCH PRODUCT

Influence of the metallurgy and the microstructure on the strength of titanium-steel laser welded assemblies

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The dissimilar laser welding of titanium alloys with stainless steel is of high interest for different industrial applications. However, the joining of these materials by direct fusion is hardly achievable because of the presence of brittle intermetallic phases in the Ti-Fe system that produce spontaneous cracking of the welds. The scientific aim of this thesis is to identify a reliable criterion that allows identifying the possible conditions allowing obtaining a durable dissimilar titanium/steel joints, in terms of mechanical strength in service conditions. The determination of this criterion is necessary for the comprehension of the relation between the microstructure of the melted zone and the mechanical strength of the joint. Another aim of this thesis is to prove the feasibility of dissimilar laser titanium/steel joints in the context of small series industrial production.To meet this challenge, in the first place, the feasibility of direct joining, without intermediate material, was investigated. The application of an extreme laser beam offsets on titanium allowed reducing the development of brittle intermetallics to the µ-metric reactive interface situated at the border of the melted zone, and in this way obtaining defect-free joints. However, these welds showed a brittle tensile behavior associated with low joint coefficients and the probabilistic fracture propagation.The use of intermediate materials having an ideal metallurgical compatibility with titanium (vanadium and niobium) in two-pass welding with two separate melted zones allowed to overcome the embrittlement related to the formation of brittle titanium/steel intermetallics. The joints with vanadium interlayer showed a high joint coefficient and ductile tensile behavior within a sufficiently wide range of operating parameters. Joints fabricated with niobium interlayer showed similar performances in comparison with direct joining due to the creation of brittle intermetallics in the niobium/steel melted zone.Finally, a multimaterial approach with three-pass welding of titanium/niobium/copper/steel assembly was investigated. The absence of intermetallic phases in the individual binary systems allowed achieving a ductile tensile behavior of the assembly with tensile strength limited by the mechanical properties of copper insert.This study allowed identifying concrete solutions to meet industrial needs for the production of dissimilar titanium-stainless steel assemblies in small series and shed light on the production difficulties linked to the reproducibility of the microstructures and the quality of the preparation of the welded parts.