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November 23, 2015

Defence thesis of Qingdong YIN

of Centre des Matériaux of MINES ParisTech

Defence thesis of Qingdong YIN

Qingdong YIN will defend her thesis which called

"Influence of chemical composition on the microstructural development and on the mechanical behaviour of welds made of hot stamping steel grades"

on november, 23 at 2:00 pm

at Ecole des Mines de Paris, 60 Bld Saint-Michel, Paris 6ème, in room L109

 

This thesis is focused on the study of welded steel grades deformed by hot stamping.

Typically, a laser welded blank is formed by butt joining two steel sheets, which can be identical or differ in chemical composition and/or sheet thickness. Such laser welded blank solution offers excellent crash performance, significant CO2 emission reduction and passive car safety thus it is of great interest for the automotive industry. The laser welding process may induce chemical and metallurgical heterogeneities arising from an incomplete mixture of the two base metals and from the thermal cycle conditions during welding and the subsequent heat treatment. The mechanical behaviour of the weld is therefore determined by the mixing quality, the chemical composition and the thermal cycle. The first part of the study aims at the quantification of the weld heterogeneity and the characterisation of the resulting microstructure.

In this work, a new method is proposed to study the metallurgical and mechanical behaviour of those heterogeneous welds, by elaborating synthetic casted alloys representative of the different chemical compositions that can be locally found in the weld. The behaviour of the weld is then reconstructed from the results obtained from the characterisation of those synthetic casted alloys. The second part of the study concerns the understanding of the microstructure formation in the molten zone before and after a thermo-mechanical treatment which replicates the hot stamping conditions. Quantitative models have been established to calculate the phase transformation temperatures, the proportion of the principal constituents in the microstructure, and hardness of the weld as a function of chemical composition and thermal cycle parameters. The last part of the study is dedicated to the evaluation of the mechanical behaviour of the synthetic alloys after heat treatment. The constitutive material behaviour laws for each alloy have been identified. The sensitivity to brittle cleavage fracture has been assessed by tensile tests on severely notched bars. The constitutive material models are then applied to determine the sensitivity of the weld to geometrical defects under uniaxial loading

Defence thesis of Qingdong YIN - MINES ParisTech

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