Investigation of Hot cracking behaviour based on thermo-mechanical aspects of Laser spot welded Advanced High strength steels
A. Dixit (TU Delft - Mechanical Engineering)
M.J.M. Hermans – Mentor
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Abstract
The mechanical properties of advanced high strength steels (AHSS) for automotive applications are sensitive to their multi-phase microstructures, which are carefully designed and produced by selection of the composition and production route. A combination of alloying elements and complex heat treatment generates microstructures with designed fractions, dimensions, compositions, morphologies and spatial distribution of phases. However, the high alloying content, required to stabilize the multi-phase microstructure of these steels, makes the material sensitive to hot cracking/solidification cracking. Hot cracking of laser welded components is a serious problem that is often reported by automotive manufacturers using AHSS. The increasing use of thinner gauge steels in order to reduce the weight of the automobile demands the application of laser welding at the edge of the steel flanges, which often leads to severe cracking. During the commercial steel making process, laser welding is applied to join steel coils. Therefore, in several industrial processes for advanced high strength steels, hot cracking can lead to serious disturbances, down time and production loss.
An experimental study was performed for the graduation project based exclusively on thermo-mechanical aspects. External tensile loading was applied during lase spot welding to relate the stress and strain based models designed specifically for hot cracking. In situ strain measurements were extracted, using digital imaging correlation (DIC). All the laser welds were made using Nd:YAG laser. The experimental study focused majorly on single pulsed spot welds with few bead-on-plate welds for comparison. Only two kinds of AHSS were investigated namely, Dual phase (DP) steels and Transformation induced plasticity (TRIP) steels, due to their high demands for laser weld automotive applications. The project includes welding with external tensile loading by using a modified version of controlled tensile weldability test (CTW) and characterization of welds to study hot cracking behaviour based on thermo-mechanical aspects. A 2-D finite element model was made by the modeling software, COMSOL multiphysics to depict the temperature profile.
Keywords: Nd:YAG laser welding, Dual phase and Transformation induced plasticity steels, Single pulsed laser spot welds, Hot cracking/weld solidification cracking, Thermal modeling, DIC measurements