Numerical Simulation Of Residual Stress In Tungsten/304 Stainless Steel Diffusion Bonded Joints

Due to the large difference in physical properties between tungsten and steel,the joints will fail due to extreme residual thermal stress during connection and service,which restricts the development of tungsten/steel materials and has become a bottleneck for the commercial use of nuclear fusion reactors.In this paper,the research on diffusion bonding of tungsten and steel is carried out from two aspects of numerical simulation and experiment.The finite element method was used to numerically analyze the residual stress distribution of the tungsten/steel joint.The high-quality dif fusion bonded tungsten/steel joint was obtained by Cur or Ni interlayer.Current research results indicate:The maximum tensile stress ?X,+max in the tungsten/steel joint is located in the steel matrix near the interface and free boundary,and the maximum compressive stress ?X,-max is located at the tungsten/middle layer interface near the free boundary.The axial residual stress ?? is much less than the radial residual stress ?X in the tungsten/steel joint,and the radial residual stress is the main factor affecting the residual stress of the joint.The equivalent plastic strain area of tungsten/steel is mainly concentrated in the steel matrix near the interface.The change in bonding temperature has a limited effect on the residual stress distribution of the joint.For tungsten/copper/steel joints,the maximum residual stress rises only 70 MPa when the bonding temperature changes between 980-1080 ?.For tungsten/nickel/steel joints,the maximum residual stress increases by less than 40 MPa when the bonding temperature changes from 850-1050 ?.With the increase of the thickness of the interlayer(0.02-0.6 mm),the maximum tensile stress ?X,+max and the maximum compressive stress ?X,-max in the tungsten/copper/steel joints both decrease significantly(over 200 MPa).The maximum tensile stress ?X,max transfers from the steel matrix to the interlayer copper as the thickness of the interlayer increases;As the thickness of nickel increases,there is only a 50 MPa change in tungsten/nickel/steel joints.Tungsten/steel can be effectively connected by adding 20 ?m Cu interlayer,and no obvious compound is formed on the joint.A complex solid solution phase composed of multiple elements is formed on the Cu/W interface,which will strengthen the interface.The defects of the joint mainly exist as cracks on the SS/Cu interface.The crack penetrates the entire SS/Cu interface at the bonding temperature of 1060?,causing the joint performance to decline sharply.The tungsten/copper/steel joint with the best performance can be obtained at 1020?.The increase of the thickness of the middle layer will limit the diffusion of elements.Fe,Ni,Cr and other elements in the steel cannot diffuse to the Cu/W interface,which weakens the performance of the Cu/W interface and leads to the decrease of the strength of the whole joint.High quality tungsten/steel joints can be obtained from the nickel interlayer.The different bonding temperatures result in different phase compositions of the Ni/W interface.Only solid solution phase Ni(W)is present at the Ni/W interface at low bonding temperature,but Ni(W)?Ni4W?W(Ni)at high bonding temperature.The j oints with the connection temperature of 950?have the maximum shear strength,and the fracture positions of the joints all occur at the Ni/W interface.The extended holding time makes the element diffusion behavior full.The Fe in the steel is able to diffuse into the Ni/W interface,forming a solid solution W(Ni,Fe)and Ni(W,Fe)at the interface.The thickness of the diffusion layer at the Ni/W interface increase,but obvious cracks appear at the Ni/W interface,and the compound at the Ni/W interface is found to crack in some areas.