Study On The Bonding Process Of Tungsten And 316L Austenitic Stainless Steel

In this paper,vacuum diffusion bonding and vacuum brazing are used to study the connection process of tungsten and 316L austenitic stainless steel.In vacuum diffusion bonding,the effect of diffusion temperature,interlayer and holding time on welding joint is studied.In vacuum brazing,the influence of different interlayer on the welding strength is studied.In the direct diffusion welding of tungsten and 316L steel,the FeW diffusion layer is generated in the welded joint,and the cracks in the longitudinal and transverse diffusion layer are produced.With the increase of diffusion temperature?900?and1050??,the thickness of FeW diffusion layer increased,the tendency of crack increased,the average shear strength decreased from 61MPa to 45MPa,the tendency of grain coarsening of 316L steel was increased,and the grain boundary showed a straight change.The analysis shows that the higher diffusion temperature increases the atomic diffusion capacity on the one hand,and on the other hand,the residual stress in the welded joint is larger,and the growth rate of the steel parent material is increased.In W/Ni/316L steel,with the increase of the holding time?1h,1.5h,2h?,the average shear strength by 182MPa?1h?to 286MPa?1.5h?and then down to 220MPa?2h?,the fracture of brittle+ductile mixed fracture mode and fracture position is W/Ni interface.It is considered that the main influence of the strength of W/Ni/316L steel joints is the microstructure of the W/Ni diffusion interface and the distribution of intermetallic compounds.On the one hand,the increase of holding time is beneficial to the closure of pores on the interface between tungsten and nickel,but on the other hand,Ni₄W grows from dispersed state?1.5h?to continuous layer with a certain thickness?2h?.In addition,for Ni/316L steel diffusion interface,near the side of nickel foil,the number of Kirkendall voids are proudced,the authors believe that this is because the Fe and Ni atoms with different diffusion ability.In the W/Nb/Ni/316L steel,the tungsten and niobium are not connected at 900?,15MPa,1.5h,2h,1050?,15MPa and 1.5h.At 1050?,20MPa and 1.5h.Although W/Nb/Ni/316L steel is connected,the bonding strength of the welded joint is too low and fracture occurs during the on-line cutting.It is found that Nb and W are large size atoms with higher melting points,and the diffusion channels are not easily formed at a lower temperature or pressure.The BNi-2 high temperature nickel base filler metal is applied to connect W/316L steel,W/Cu/316L steel and W/Nb/316L steel under the technological parameters of brazing temperature 1050?,25min.The average shear strength of welded joints is143MPa,197MPa and 120MPa,respectively.The author finds that the influence of the middle layer on the joint is related to the physical and chemical properties of the intermediate layer.For Cu interlayer,Cu element diffused into BNi-2 solder to improve the wettability and fluidity of solder,and Fe and Ni had good mutual solubility,so the thickness of Ni based solid solution increased,and copper has good plasticity and toughness.Benefiting from the above two points,the average shear strength of W/Cu/316L steel welded joint is increased to 197MPa.For Nb interlayer,while the thermal expansion coefficient of niobium is in steel and tungsten,but Nb-Ni is not easy to form a solid solution,after Ni and Nb interdiffusion,Ni-Nb intermetallic compounds exist in the joints,resulting in cracks,horizontal and vertical length of a joint,the average shear strength is only 120MPa.