Rasearch On Joining Of W/CuCrZr And W/steel Using Cu-TiH₂-Ni Based Fillers

Joining of W facing plasma material to heat sinks?CuCrZr alloy and steel?is essential for the preparation of W-bsed facing plasma components.However,large residual stresses are generated in the W/CuCrZr and W/steel joints due to the difference in physical properties between W and heat sinks.In this study,the active filler of Cu-TiH₂-Ni has been used to join W to CuCrZr and to 316L stainless steel,and the characterizations of microstructure and properties of the joint have been performed.Then,metal foils have been introduced in the filler for the joining,and the effects of the metal foils on the strength and microstructure of joints as well as the joint residual stress have been studied.The W/CuCrZr joint brazed with Cu-TiH₂-Ni filler at 1203 K exhibits a sound interfacial bonding between the filler layer and the base materials.The joining layer is mainly composed of Cu?ss?,TiCu₄,Ti₂?Cu,Ni?,?₁-Ti(Cu_xNi_1-x)₂ and TiCu phases.The microhardness results of the joint indicate that the hardness values of the W and CuCrZr substrates after joining are lower than those of as-received substrates.The main reason for the decrease in the hardness of the W substrate is the unilateral diffusion of W atoms into the joining layer due to the existence of Ni in the filler.The W diffusion gives rise to the formation of vacancies in the W base material,leading to the decrease of the hardness in W.The decrease in the hardness of the CuCrZr base material is attributed to the recrystallization and grain growth during the joining process.Joining of W to CuCrZr has been realized with the Cu-TiH₂-Ni filler and a soft Cu foil.The filler layer comprises two filler layers and a Cu interlayer.The filler layers are mainly composed of Cu?ss?,TiCu₄,Ti₂?Cu,Ni?and?₁-Ti(Cu_xNi_1-x)₂ phases.Furthermore,the thickness of Cu interlayer is reduced by 50%due to its dissolution in the liquid filler during the joining process.The addition of Cu foil to the filler contributes to the improvement of shear strength in the joint.On the one hand,the Cu foil with excellent plastic deformation ability can absorb partial residual thermal stresses of joint;on the other hand,large dissolution of Cu foil can effectively inhibit the growth of intermetallic compounds in the filler layer.In the case of other metal foils?such as Ni,Nb,Ta,et al.?,the joint strength is reduced,mainly caused by the complexity of the joint interfaces with the addition of the foils.The W/316L joint brazed with Cu-TiH₂-Ni filler reveals a poor interfacial joining.The joining is successful with the introduction of a Cu foil or Nb foil in the Cu-TiH₂-Ni filler.A diffusion layer is formed close to the 316L substrate due to the diffusion of Fe and Cr in the 316L base material into the filler layer.The diffusion layer is mainly composed of a diffusion layer I consisting of TiCu₄,?₁-Ti(Cu_xNi_1-x)₂,and?Fe,Cr?₂Ti,a layer II of Ti₂?Cu,Ni?and?Fe,Cr?₂Ti,and a layer III of?phase.The filler layer for the W/316L joint brazed with Cu-TiH₂-Ni filler and a Nb foil is mainly composed of Ti₂?Cu,Ni?,?₁-Ti(Cu_xNi_1-x)₂ and Nb?ss?.