The Study On The Characteristics Of Molten Pool And Temperature Field During Laser Full Penetration Welding Of 304 Stainless Steel

As a new technology of material processing, laser welding has advantages, such as high energy density, high welding speed, high depth-to-width ratio, narrow heat affected zone and small welding distortion. In recent years, due to the rapid development of high-power laser, the laser welding is becoming an important processing method in the field of modern industrial manufacturing, and has vast potential for development. Features of the molten pool and temperature field have close relationship with welding parameters and weld quality during laser welding. Therefore, in this paper, both experimental and calculated methods were used in order to investigate the changes of features of the molten pool and temperature field in different laser welding conditions. The research not only can benefit to increase the understanding of the relationship among molten pools, welding parameters and welding results, but also achieve prediction models used to predict molten pool and temperature field by relatively simple and applicable methods.In this paper, based on the analysis of the main mathematical model of laser deep penetration welding, combined with research objectives, Rosenthal moving line heat source model was selected. At the same time, limitation of Rosenthal moving line heat source model was analyzed in laser full penetration welding. Equivalent parameters were introduced to the analytical solution of the moving line heat source model to make the calculation results of the model were close to the experimental results, and it was used as prediction model of the molten pool and temperature field.First of all, CO2 laser welding process and welding results for 304 stainless steel under different welding conditions were analyzed by the experiments. The research results show that the heat input is closely related to the weld shape and the weld microstructure. At the same time, the equivalent parameters in the model which was based on moving line heat source model were determined by the features of the molten pool which were measured by a high-speed video camera, and then the corresponding prediction models for the molten pool and temperature field were established. Research results show that the calculated results agree well with experimental results when the heat inputs are in a certain range, and the equivalent parameters need to be modified when heat inputs are too high or too low, which was mainly due to the changes of the energy absorption of weldment caused by the changes of keyhole characters during different penetration modes. The weld thermal cycle of weld center were calculated by prediction models under the corresponding heat inputs. The calculation results show that the higher the heat input, the smaller the cooling rate at the melting point, and the longer the cooling time at 800℃500℃(t8/5),which well explain why weld crystalline grain get thicken under high heat input.Further, the effects of laser power and welding speed, sheet thickness and shielding gas on molten pool and temperature field were studied, the effects of application of the prediction models for the molten pool and the temperature field under different welding parameters were discussed, the equivalent parameters in the corresponding models were analyzed, and then the weld thermal cycles of weld center were calculated by the corresponding prediction models under the corresponding welding conditions.At the same time, the prediction models for the molten pool and the temperature field under different heat inputs for YAG laser full penetration welding of 304 stainless steel were established. The weld thermal cycles of weld center were calculated by the prediction models under the corresponding heat inputs, and the relationships between heat input and molten pool, the temperature field, the weld thermal cycle of weld center, the weld microstructure were analyzed. It is found that the predictions of the models are consistent with experimental results.Comparisons between CO2 laser welding and YAG laser welding in term of the weld shapes, the features of the molten pool, and the predictions models were made. It is found that all of these are different because the effects of plasma and vapor are different during the two kinds of laser welding, therefore, predictions model should be established respectively according to the process of the two kinds of laser welding.