Study On Laser Welding Technology And Microstructure And Properties Of Joint About Ultra-Thin Plate Hastelloy

The Hastelloy C-276 ultra-thin plate(0.381 mm)was applied to the manufacturing of the stator shielding sleeve of the nuclear main pump of the third generation nuclear power plant.This paper uses the advanced pulsed laser welding method for C-276 ultra-thin plate conducted a series of welding process test,the system to study the influences of process parameters on weld forming,analyzes the characteristics of the weld,in order to investigate the influence of process parameters on microstructure of welded joint,studied the tensile properties of the welded joint and analyzed the mechanism of joint fracture and the microhardness of the joint.The main results are as follows:1.The process test shows that pulse current has a great influence on the weld appearance.Under the premise of being able to penetrate the workpiece,the smaller the pulse current is,the better the weld appearance is;when the welding speed is too small,the weld width is too large,easy to produce defects.Excessive welding speed,heat input is insufficient,easy to form defects such as incomplete penetration;increase of pulse frequency will increase weld penetration and weld width;increase of pulse width will make the width decreases and the penetration increases.The test results shows that the pulse current is in the range of 60~65 A,the pulse frequency is in the range of 5~7 Hz,the pulse width is in the range of 8.5~9.5 ms,the welding speed is in the range of 220~240 mm/min,the weld penetration is large,the weld width is small,the weld appearance is artistic.Defects in welded joints include flat defects such as incomplete penetration,pits and slag inclusions,volumetric defects such as blowhole,shape defectand such as undercuts,corner distortions and misalignment and other defects such as spatters.2.There are three types of columnar dendrites,cell crystals and equiaxed crystals in the weld.At the position of the weld seam,the columnar dendrites alternate with the cell crystals,and the columnar dendrites grow along the weld boundary,and it points to the center of the weld and the grain size is much smaller than that of the base metal.The grain boundary of the weld is rich in Mo element,lacks Ni element and there is segregation of elements.The segregation tendency of Mo element and W element is larger than that of other elements.Compared with the base metal,the peak height of the diffraction peak in the central region of the weld changes,but the orientation of the crystallite orientation does not change.The central phase structure of the weld is still the face-centered cubic structure,the organization is austenite,and no second phase generated.3.Effect of various process parameters on the microstructure of the joint: With the increase of the pulse current,the grain size in the weld microstructure becomes larger;the increase of the pulse frequency will refine the grain;when the pulse width increases,the size of the grain increases,the equiaxed crystal transformated to columnar crystal in the meantime.With the increase of the welding speed,the number of equiaxed grains in the weld microstructure increases and the size decreases,the number of it decreases gradually,and the size of the columnar grains also decreases.4.The tensile test results show that the increase of current will reduce the plasticity of the welded joint and the tensile strength of the welded joint;the increase of the pulse frequency will reduce the elongation of the joint,increase the reduction of the area,and reduce the tensile strength of the joint.The change of the pulse width has little effect on the plasticity and the tensile strength of the joint;the increase of the welding speed will reduce the elongation and the reduction of the area of the welded joint,and the tensile strength of the joint will decrease.The difference between the yield limit of the welded street and the base metal is small,and the tensile strength of the welded joint is about 90.6% of the tensile strength of the base metal.The fracture profile of the base metal and welded joint tensile parts is mainly equiaxed dimple,but compared with the base metal,the fractures of the welded joint tensile is extremely fine and the equiaxed dimples are also relatively deep.The fracture process is plastic fracture,and dense microscopic pores are formed at the fracture.5.The hardness test results show that the microhardness of the weld is slightly higher than that of the base metal,but not much.There is no obvious "softening" area in the weld,which is very narrow in the thermal influence zone of welding joints found in the metallographic experiment,which is not easy to find.