| Laser welding is a welding fabrication technology with extensive application value,featured with high power density,high welding efficiency and small heat-affected zones and thermal deformation,which is suitable for occasions of high depth-to-width t ratio.This paper focuses on the machining process of laser welding of stainless steel sheet of heat exchanger,analyzes the performance of welded joint on the basis of experiment,and studies the dynamic process of thermal cycle of welding process combined with numerical analysis model.The microstructures and mechanical properties of laser welded joints were investigated by metallographic experiment and tensile strength experiment.In the metallographic experiment,the sizes of a batch of solder joints processed by a 3000 W fiber laser were measured from the cross-sectional view of a set of solder joint specimens,and the appearance defects such as surface pits were observed in the welded joints,also,a small number of specimens have internal defects that affect the mechanical properties of weldments,finally,the microstructure characteristics of welded joints were analyzed.The results showed that,the cooling speed is fast after being welded,the microstructure of the seams is a cellular austenite with dendrite tendency,and the corrosion resistance is slightly lower than that of the base metal.By tensile strength test the tensile strength of welding joint is 236.8 MPa and the calculated pressure capacity of single heat exchanger plate is 3.35 MPa.The welding quality is optimized by changing the welding parameters,and the results showed that the microstructure of the laser welding seam after reducing the pulse width is more homogeneous and the surface quality of welding joint is better after reducing the welding power density.Through the establishment of transient behavior of the laser spot welding molten pool,the welding fusion nuclear growth,the cooling process after welding and the keyhole formation were studyed.,also,the flow field of the welding process was analyzed emphatically and it was considered that the disturbance of metal vapor is prone to engendering welding defects such as porosity.And the thermal cycle curve of various points in the molten pool was figured and the solidification rate of each point was between 5 mm / s and 25 mm / s.According to the melt solidification speed,solidification pattern of melt was speculated asfollow: the solidification of molten pool tends to F model horizontally and tends to FA model vertically.Finally,the numerical simulation of the actual laser welding process of the stainless steel sheet of heat exchanger was carried out and the numerical models of spot welding and linear welding were established respectively to analyze the temperature field and stress field of the welding process,and the influence of welding speed and welding power on the linear welding was analyzed emphatically.The results showed that the preheating and slow cooling effect between the two adjacent solder joints are limited,and the thermal impact range of welding is small.The stress concentration area in the spot welding process is mainly in the distribution areas with large temperature gradient around the solder joint.While when linear welding is at different welding speed,each point has a similar thermal cycle curve,and the stress concentration area is in the welding pool around the distribution areas with large gradient temperature and contact position between the plate and the fixture.Finally,according to the size of molten pool obtained by different welding speeds and welding powers,the relationship between welding power density,welding line energy and the size of molten pool is predicted through linear fitting.When the welding power is 2500 w and welding speed is 100 mm/s,the weldment is right penetrated. |
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