Papers by Author: Ossama Dreibati

Abstract: The paper presents a numerical and experimental approach for the quantification of the thermo-mechanical properties in multi-pass welds heat affected zone (HAZ) of low alloy steel S355J2+N. First, the characteristic temperature cycles for multi-pass welds were identified by FE temperature field simulations of welding. Based on the identified temperature cycles, the microstructure in the HAZ has been physically simulated with the simulation and testing system Gleeble 3500 to investigate the influence of multi thermal exposure on the thermo-mechanical properties. Thus, the thermo-mechanical material properties including thermal strain and temperature dependent stress strain behaviour as function of peak temperatures and cooling rates have been determined. These material properties were used to calibrate a developed model for numerical prediction of the material properties of multi-pass weld HAZ.

Abstract: The thermomechanical properties of an AlMg0,8Si1 (corresponds to EN AW-6181) alloy in T4 state (Ecodal^® 608) were investigated under coldArc^® welding conditions using Gleeble 3500 concerning the numerical simulations of residual welding stresses and distortions. Thereby, tensile tests were carried out during the heating phase, cooling phase and after natural aging up to 10 days. Thus, the effect of weld cycle and corresponding dissolution of β''(Mg₂Si)- precipitates on the mechanical properties was physically simulated. Furthermore, two phase transformation models were used to simulate the dissolution of hardening precipitates as a result of weld temperature cycles. The used models were compared, considering their capabilities and accuracy.

Abstract: Cold cracks occur during the cooling down of welded joint at low temperatures or later at room temperature after the end of welding. It is associated with the formation of brittle microstructures as martensite in the presence of diffusible hydrogen as well as of tension stresses. By using an enhanced Simulation-und Testing Center Gleeble 3500, a procedure for physical simulation of cold cracking under laser beam welding conditions is suggested. The approach reproduces combinations of the cold crack main parameters, a brittle microstructure, tension stress and high local hydrogen concentration under welding conditions which induce a cold crack. A specimen geometry and technique were developed to enable the gaseous hydrogen charging from pure hydrogen atmosphere. The amount of charged hydrogen can be adjusted through varying the charging parameters like temperature, gas pressure and charging time. The hydrogen charging technique and the cold crack testing procedure were proven with high strength steel specimens.