Influence of the penetration depth in laser welding on surface temperatures

This study deals with penetration depth control in CO2 laser cw welding of mild steel workpieces with infrared thermography. The calculated surface temperatures are compared with the temperatures recorded by an infrared camera. The comparisons were focused on both good welds with total penetration and bad welds with incomplete penetration due to variation of laser beam power or focal position; the other welding procedure caracteristics were set at their optimal values. Then, the temperature fields were simulated using two model parameters, the absorbed energy and its deposit depth in the material, The modelling results agree with the infrared thermography results on the welded seam when the distance to the laser beam is larger than two times the thicknesses of the material being welded. For optimal and non-optimal welding, the temperature surface fields deduced from the model and from experiments have shown that the depth of penetration is in relation with the absorbed laser power., Furthermore, the results have shown that infrared thermography control method based on one point surface temperature measurements are not reliable. However, if these measurements are taken simultaneously at a minimum of two points on the weld seam interesting results are obtained :two temperature measurements make it possible to deduce whether the lack of penetration is due to insufficient laser power or defocusing.