The objective of the welding simulation is to study the temperature generated during the welding process and to investigate residual stresses in the component after welding. Such results give the possibility to determine properties of materials in welding zones, stress and strain state of welded parts. From other side it gives the possibility to perform optimization process looking for welding parameters (welding speed, welding power source, etc.) or initial shape of welded sheets according to displacement state (welding of thin metal sheets with stiffeners - T joints). Those results are the base for fatigue analysis too. A grid based evolutionary optimization of welding parameters influences on strength parameters in the heat affected zone (HAZ) is shown in the paper. Numerical simulation of a welding process using the finite element method is applied. Results for coupled thermo-mechanical problem are prescribed.