| AISI 304 stainless steel(SS) is a kind of the most widely used austenitic stainless steel. In many situations, the applications of 304 SS need to use the important processing method—welding. Comparing to the conventional welding methods, laser welding has the advantages of high-intensity focused heat source, narrow HAZ, small distortion, high welding speed and so on, resulting in the high quality of 304 SS joints. So, it's necessary to investigate on the macro-morphology, microstructure and mechanical properties of the laser welded 304 SS joints, which will have a great influence on the quality of joints.The objective of this paper is to study the effect of welding parameters on the macro-morphology, microstructure and mechanical properties of laser welded 304 SS joints, and find out the appropriate processing parameters for the welding of 304 SS. In addition, the study also aims at establishing a three-dimensional(3D) heat source model and a finite element(FE) analytical model which are suitable for laser welding to realize the 3D and dynamic simulation of welding temperature distribution. The simulation results are related to the experimental results from the aspect of macro-morphology and microstructure, which will provide guideline for selecting and optimizing welding parameters.Firstly, the experiment of laser welding was conducted on the 0.7-mm-thick 304 SS sheets, and the effect of processing parameters on the macro-morphology of joint was analyzed. It is found that the joints welded at appropriate processing parameters have the appropriate macro-dimensions and free of macro-defects. Subsequently, the welded joints were made into the metallographic analysis sample and then electrolytic etched. The microstructure of the joints were captured by an optical microscope. It is found that the microstructure of fusion zone are different from that of the base metal, and the processing parameters have an influence on the microstructure of fusion zone, which mainly reflects on the change of band width of columnar dendrite zone near the fusion boundary and the size of equiaxed grains in the weld center. The microhardness of different zones of the joints was tested, and found that the microhardness of fusion zone is higher than that of the base metal. In addition, the tensile test of joints welded under different processing parameters were conducted, and the tensile properties were related to the macro-morphology and microstructure of joints. When the joint obtains the appropriate macro-dimensions and fine grains in the fusion zone, it gets the best tensile properties and fractures in the base metal after tensile test. The morphology of fracture surface was examined via a scanning electron microscope. It is visible that large quantity of dimples is formed on the fracture surfaces, revealing that the welded joint fails in a ductile mode.Finally, according to the feature of weld shape of laser welding, a combined heat source model of double ellipsoid body heat source and gauss surface heat source was established. The dynamic simulation of temperature distribution of laser welding was realized by using FE software ABAQUS and the user subroutine which is in the FORTRAN language. From the aspect of macro-morphology, the welding pool boundary in simulation is in accordance with the fusion boundary in experiment, which verify the reliability of model. From the aspect of microstructure, the temperature distribution can predict the temperature gradient of liquid and the cooling rate of weld center, and further predict the tendency of solidification mode and the relative size of grains qualitatively. So the relation between the temperature distribution and microstructure of weld is studied. |
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