Influence of Local Chemistry and Heat Treatment on the Microstructural and Mechanical Behaviour of High Strength Steel Based Laser Welds
Qingdong Yin a,b, Anne-Françoise Gourgues-Lorenzon a, Esteban P. Busso a, Francis Schmit b
a Centre des Matériaux, MINES ParisTech, UMR CNRS 7633, BP 87, F-91003 Evry Cedex, France
b ArcelorMittal Global R&D Automotive Applications, BP 30109, F-60761 Montataire Cedex, France
Presenting author:
Laser welding of high strength steel grades differing in chemical composition and sheet thickness 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 study aims at a quantitative characterisation of the effect of these parameters on the microstructural evolution and local mechanical properties of the seam welds.
The heat treatment consisted of an austenitisation of the welded parts and then cooling them between flat tools so as to decompose the fine-grained austenite into martensite and bainite. The microstructural heterogeneities were identified by means of chemical and metallurgical analyses of the welded joints. Specially designed alloys, with compositions containing different mixture ratios of the two parent steel chemistries, were casted to characterise the behaviour of the different heterogeneous regions of the weld. They were then heat treated using cooling rates typical of those encountered during the heat treatment of the actual weld. Their microstructural evolution and mechanical behaviour were finally determined as a function of chemical composition.
A suitable internal variable-based constitutive material formulation was then developed to describe the microstructural evolution and mechanical behaviour of the heterogeneous weld microstructure. To that purpose, the relevant model material parameters were made to depend on the local chemical composition and cooling conditions of each material point within the weld. The constitutive formulation was then implemented numerically into the finite element method and used to predict the stress and strain distribution in actual welded joints subjected to in-plane tension.
Symposium: C2.III: Joining Technologies.