Research On Vacuum Brazing Technology Of CZ-Zr Alloy With 316L Stainless Steel

Zirconium alloy is widely used in many fields because of its corrosion resistance,high temperature resistance,mechanical properties and machining properties.In particular,the composite component formed by the connection of expensive zirconium alloy and cheap stainless steel can obtain greater economic and social benefits by combining the advantages of both.However,brittle intermetallic compounds are easily formed when zirconium alloy is welded to stainless steel,which makes the direct connection between them very difficult.In this paper,vacuum brazing of CZ-Zr alloy and 316L stainless steel was carried out using BAg72Cu binary eutectic filler metal and BNi7 ternary eutectic filler metal.The effects of different brazing processes on the microstructure and mechanical properties of the joints were studied.The main results are as follows:The brazing joints of CZ-Zr alloy and 316L stainless steel were brazed with BAg72Cu and BNi7 solders under vacuum condition.The brazing joint is composed of a continuous dense layer parallel to the interface.The reaction layer is formed between the solder and the base metal on both sides,and the metallurgical effect is sufficient.The microstructure of CZ-Zr/BAg72Cu/316L brazed joint is mainly Fe(Cr,Zr)₂,Zr(Fe,Cr)₂,(Zr,Cu),Zr₂Ag+Zr Ag,?-Zr+Zr₂Cu+Zr₂Ag and Zr₃Fe phases.At different brazing temperatures and holding time,the brazing joints harden remarkably.The hardness of the Zr(Fe,Cr)₂ diffusion layer near the stainless steel side is the largest,which is 790.8 HV,and the hardness of the base metal on both sides is the smallest,and the center area of the brazing joint is between the two.The shear strength increases first,then decreases with the increase of brazing temperature,and the maximum shear strength is 125 MPa at 820?.When the brazing temperature is constant,the shear strength of the joint increases slowly at first,but it is redused greatly with the extension of holding time.The fracture morphology of the joint is typical river pattern and step,which belongs to cleavage brittle fracture.The XRD results show that the weakest area of the joint is at the interface between zirconium alloy and solder metal,where cracks are prone to initiate and expand rapidly in the weld,and brittle fracture occurs in the stainless steel.The typical microstructure of CZ-Zr/BNi7/316L brazing joint is(Cr,Fe)solid solution,Zr(Fe,Cr)₂ intermetallic compound,Ni(Cr,Zr)-P compound,?-Zr solid solution.The joints were hardened significantly under different conditions.The hardness of the Ni(Cr,Zr)-P black block in the weld center is the highest,followed by the Zr(Fe,Cr)₂ intermetallic compound layer,and the hardness of the interface zone of the brazing joint is in the middle.The smaller the brazing gap,the greater the joint shear strength.When holding time is constant,the shear strength increases with the increase of brazing temperature.The joint shear strength reaches the maximum of 150 MPa when the brazing gap is 100?m,1050?and 20 min.The joint fracture is layered brittle fracture,and the weld center area is the weakest area in the joint.This area is easy to initiate cracks,and then rapidly expand to both sides along the weld interface,resulting in fracture.The residual stress analysis of CZ-Zr/BAg72Cu/316L brazing joint was carried out based on ABAQUS software.It was found that the stress was mainly distributed in the brazing joint and the base metal near the joint,showing the law of horizontal micro-arch vertical gradient shape,and decreasing step by step from the weld to the base metal on both sides.The residual stress in the joint is mainly tensile stress,and there is a small compressive stress at the top,around and center of the base metal far away from the weld.The peak stress of the joint is located at the right angle around the solder metal and is 246 MPa.