Study On The Structure And Properties Of The Brazed Joint Of Cu/HF Cutting Electrode

Plasma cutting has the advantages of simple operation,good working environment,and excellent cutting quality,and it is widely used in the current social industry.The quality of plasma cutting is closely related to the quality of the cutting electrode.This subject uses the vacuum brazing connection method to connect the rare metals Hf and Cu,studies the changes in the microstructure and hardness of the brazed joints at three different brazing temperatures.Consider that during service,the electrode is in the process of alternating cold and hot cycles.The electrode is subjected to thermal fatigue testing to study its thermal fatigue performance and thermal fatigue behavior mechanism.In order to optimize the performance of the electrode,it was cryogenically treated to study the effect of cryogenic treatment on the microstructure and hardness of the material,and its mechanism was analyzed.The research results are as follows:Using 810?,828?,835?as the welding temperature to achieve the connection of copper and hafnium,molding quality is good,no defects.The brazing seam consists of a black band-shaped diffusion zone on the Hf side,a non-isothermal solidification zone,an isothermal solidification zone,and a peak-shaped diffusion-affected zone on the Cu side.The black ribbon on the Hf side is composed of the ternary solid solution phase of Ag,Cu,and Hf and the compound containing Hf.The non-isothermal solidification zone is composed of the Hf compound.The isothermal solidification zone is composed of the Cu-rich solid solution and the Ag-rich solid solution.The Cu side The diffusion zone is composed of Cu-rich solid solution.There are oceans of compounds in the non-isothermal solidification area,this area is the hardest in the brazing seam.As the brazing temperature increases,the brazing filler metal diffuses more thoroughly,the elements in the weld are fully reacted,the non-isothermal solidification zone decreases,the isothermal solidification zone becomes wider,and the Hf side diffusion zone becomes wider.The hardness of each part of the brazing seam also increases with the increase of this temperature.The thermal fatigue test was carried out on the brazed joint at 600?,with cold water as the cooling medium,the upper limit heating time t₁=120s,the lower limit cooling time t₂=20s,and 200,500,800 and 1100 thermal fatigue cycle tests were performed respectively.At a brazing temperature of 810°C,a crack first sprouts in the non-isothermal solidification zone at a thermal cycle of 200 times.The length is about 5?m.With the accumulation of thermal fatigue stress,the Hf-side diffusion zone cracks,because the Hf-side diffusion zone has a small range,so crack propagation is not obvious.In this joint,the cracks in the non-isothermal solidification zone are the main cracks.When the number of fatigue times is1100,the length reaches 40?m.The brazing temperature is 828?and 835?.The thermal fatigue performance is similar.Due to the increase of the welding temperature,the range of the diffusion zone on the Hf side becomes wider.Here,the compound is mainly used,so the fatigue stress is easy to accumulate here.Cracks are generated,and fatigue tests are conducted to cause the cracks to continue to expand,and then cracks are generated in the non-isothermal solidification zone.The brazing temperature is 835?,and the crack initiation is later,the crack propagation rate is slower,and the thermal fatigue performance is better.The cryogenic treatment test uses liquid nitrogen as the cooling medium,and the holding time is 2h,4h,and 6h.With the extension of the cryogenic treatment holding time,the grain volume shrinks and the arrangement is closer,which plays a remedial role for the defects such as some vacancies in the microstructure and promotes the precipitation of the strengthening phase.Cryogenic treatment makes the grains closer and improves the hardness of the joint.The thermal fatigue test was carried out on the joints with brazing temperature of 835?and cryogenic treatment for 6h.When the number of thermal fatigue cycles was 500,the cracks in the non-isothermal solidification zone of the brazing seam appeared about 6?m.When the cycle was 1000 times,the non-isothermal solidification zone A crack parallel to the direction of the brazing line appeared at the junction of the diffusion zone on the Hf side,and the length was about 6?m.