Digital Repository, CONVEGNO IGF XXII ROMA 2013

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Strength and fracture of TWIP steel dissimilar weld joints
P. Russo Spena, P. Matteis, A. Sanchez, G. Scavino

Last modified: 2013-06-27


Car-bodies are more and more frequently made with advanced high-strength steels, both to reduce
vehicles weight and to improve passenger safety. Car-body parts are usually hot or cold formed by using several
different steel sheet grades, and are then assembled by welding. In the last years, new high-manganese steels,
called TWIP, have been proposed for fabricating car body parts due to their excellent combination of strength
and toughness. These steels have an austenitic structure which is strengthened by carbon and by the possibility
to deform via mechanical twinning (TWIP: TWinning Induced Plasticity). However, a widespread usage of
TWIP steels for car body parts is conditional on the employment of appropriate welding methods, especially if
dissimilar welding must be performed with other automotive sheet steel grades. The discontinuous resistance
spot welding (RSW) is currently prevalent in assembling car body parts; however, the mechanical properties of
RSW joints are often unsatisfactory, and always much lower than those of base metals, due to the relevant notch
effects. Therefore, improvements might be obtained by welding advanced automotive steels with continuous
techniques. For these reasons, dissimilar continuous butt welding between TWIP steel sheets and two
frequently employed high strength automotive steel sheet grades is examined here. The metal active gas (MAG)
welding technique was employed to join TWIP steel with either a martensitic boron-alloyed hot-forming steel or
a Dual Phase cold-forming one. Thereafter, the weldments were characterized by metallography and
microhardness tests. A series of tensile specimens, cut perpendicular to the welding line, were also tested and
then subjected to fractographic examinations.

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