Local formability and strength of TWIP-TRIP weldments for stamping tailor welded blanks (TWBs)

The assessment of the local formability and mechanical behavior of weldments in tailor welded blanks (TWBs) made of advanced high strength steels is of crucial importance in the automotive industry, both for the mechanical design of tailored components, and to set up accurate sheet forming numerical simulations. This work investigates the formability of TWBs made of a twinning induced plasticity (TWIP) and transformation induced plasticity (TRIP) steel, determining the local elastoplastic behavior of the different regions of the weldment. To this purpose, several micro-samples were extracted along the weld seam and heat affected zones and then subjected to tensile tests. Tensile tests were also performed on macro-samples to assess the overall mechanical behavior of the TWBs during plastic deformation. A digital image correlation (DIC) technique was used to measure the surface strain field of both micro- and macro-samples, to calculate the plastic strain ratio, through-thickness strain, and to point out possible inhomogeneities in the plastic behavior of the TWB weld beads. The TWB joint exhibited a significant formability of the fusion zone and of the TWIP heat affected zone, whereas it was rather limited at the TRIP side. Locally, an uneven plastic behavior within the fusion zone was observed, with a variable plastic strain ratio (i.e., thinning) in both axial and transverse direction. The other regions of the weld, instead, deformed more homogeneously. Preliminary considerations about the numerical modeling of TWIP/TRIP TWBs and their prospective use in sheet forming operations have been outlined.