Study On Microstructure And Mechanical Properties Of Pulse Laser Welded Joints Of C-276/C-276 And C-276/304 Stainless Steel

C-276 nickel-based superalloy has stable internal structure,higher corrosion resistance and toughness.It is often used in chemical and power equipment components,and is the main material of nuclear stator shielding sleeve of AP1000 nuclear power unit.304 stainless steel has good corrosion resistance and economic practicability,which is a common material for the end flange of nuclear stator shield sleeve of AP1000 nuclear power unit.Due to the different physical and chemical properties of two materials,the forming quality of the joints obtained by welding C-276 alloy and 304 stainless steel by conventional fusion welding method is not easy to control,and there are some problems such as wide heat-affected zone and coarse weld grain.Pulse laser welding has the advantages of energy concentration and high efficiency,which has a wide application prospect in the jointing of C-276 alloy and 304 stainless steel.Pulse laser welding was used to weld C-276 alloy and 304 stainless steel in this paper.The forming mechanism,microstructural characteristics,element distribution,phase composition and mechanical properties of lap joints of C-276/C-276 and C-276/304 stainless steel were studied.The relationship between forming morphology,microstructure and mechanical properties,as well as the fracture characteristics of two kinds of joints were discussed.The effects of welding process parameters?pulse current,welding speed,pulse width,pulse frequency,etc.?on the forming morphology,microstructure and mechanical properties of the joints were systematically analyzed,and the process parameters were rationally optimized.The experimental results show that:?1?The lap joints of C-276/C-276 and C-276/304 stainless steel obtained by pulsed laser welding have good weldability.However,due to improper welding process,the C-276/C-276 joints have defects such as undercut,spatter and incompletely filled weld,and the obvious eddy phenomenon exists in C-276/304 stainless steel joints.As the pulse current,pulse width,and pulse frequency increase or the welding speed decreases,the morphology of joints changes in the form of"U"-''V"-''Y''.?2?Under the condition that the defocusing amount is+0.5mm and the shielding gas flow rate is 15L/min:The lap joints of C-276/C-276 with flat weld surface and no obvious welding defects can be obtained when pulse current is 70A⁷5A,welding speed is 180mm/min²00mm/min,pulse width is 6ms⁶.5ms,and pulse frequency is6Hz⁷Hz;When pulse current is 90A⁹5A,welding speed is 160mm/min¹80mm/min,pulse width is 7ms,and pulse frequency is 8Hz,the lap joints of C-276/304 stainless steel with flat weld surface,deep penetration and no obvious welding defects can be obtained.?3?There is not any obvious HAZ in C-276/C-276 joints.The weld structure is composed of cellular,columnar and equiaxed crystals.Increasing pulse current,pulse width and pulse frequency can increase grain size,while increasing welding speed can refine grain.Compared with the C-276/C-276 joints,the C-276/304 stainless steel joints also have no obvious HAZ,but there are plane crystal structure and strip structure caused by the eddy in the weld.These strip structure mainly concentrate on the upper and lower parts of the weld.?4?The macrosegregation of C-276/C-276 joints is not obvious,but there is microsegregation.The grain boundary of the weld is rich in Mo and poor in Ni.The macrosegregation of C-276/304 stainless steel joints is serious,and the variation of elements in the strip structure is large.The results of EDS and XRD test in the weld show that the microstructure of two kinds of joints is austenite,C-276/C-276 joints have no second phase,and C-276/304 stainless steel joints have no ferrite structure.?5?The microhardness of the weld zone of C-276/C-276 joints is in the range of240²80 HV_0.1,and that of C-276 base metal is in the range of 220²50 HV_0.1.The microhardness of the weld zone of C-276/304 stainless steel joints is in the range of220²95 HV_0.1,and that of 304 stainless steel base metal is in the range of 180²00HV_0.1.The grain refinement is the main reason why the microhardness of the weld zone is higher than that of the base metal.?6?There are two fracture modes in C-276/C-276 and C-276/304 stainless steel joints:interface fracture and HAZ fracture.Under the interface fracture mode,the average maximum tensile-shear load of C-276/C-276 joints can reach 3.52KN,and that of C-276/304 stainless steel joints can reach 3.20KN,which is 90.9%of that of C-276/C-276 joints.Under this mode,both joints show ductile fracture characteristic.Under HAZ fracture mode,the average maximum tensile-shear load of C-276/C-276joints can reach 3.95KN,and that of C-276/304 stainless steel joints can reach 3.50KN,which is 88.6%of C-276/C-276 joints.Under this mode,the former joint features both ductile and brittle fracture,while the latter joint only presents ductile fracture characteristic.