| Permanent cathode copper electrolysis technology has gradually replaced the traditional starting plate technology because of its advantages of short process,high efficiency,long life,high output and low energy consumption.The key of this technology is to use stainless steel cathode plate instead of the electrolytic starting plate which can only be used once.At present,permanent cathode plate copper electrolysis technology can be divided into three processes: ISA,KIDD and OT.The structure of these cathode plate are all copper coated with steel and then connected with stainless steel.This structure makes it difficult to manufacture cathode plate.At the same time,the conductivity of the joint is greatly reduced and the power consumption is increased.If the cathode plate and copper/steel conductive rod can be connected directly,not only the manufacturing efficiency can be improved,but also the the resistance of the one-connection joint is lower than that of the two-connection joint in the original process,which improves the conversion efficiency of electricitychemical energy.Therefore,in view of the special requirements of the welding joint of the new copper/steel direct-connected electrolytic copper permanent cathode plate,the connection process,properties and welding structure of T2 copper and 316 L stainless steel were studied in this paper.Firstly,the effects of laser welding and tungsten argon arc welding on the weld formation,microstructure and mechanical properties of copper/steel joints were compared and analyzed.In order to improve the service life of cathode plate under corrosive environment,three kinds of copper-nickel welding wires with different Ni contents were used to conduct TIG fillet welding on copper/stainless steel to study the influence of Ni on the electrical conductivity and corrosion performance of copper/steel joint.At the same time,the corrosion resistance of copper/steel joint under TIG welding and laser welding without filler wire was studied comparatively.According to the specific requirements of cathode plate manufacturing structure,the welding parameters of copper/steel joint were obtained,and the micromorphology,mechanical properties and conductivity of copper/steel joint were studied.Firstly,T2 copper and 316 L stainless steel were joined by TIG welding and laser welding.By optimizing the welding parameters,the ideal copper/steel welded joints were obtained,in which the width of copper/steel welding seam obtained by laser welding was significantly narrower than that of TIG welding.The study on the microstructure of copper/steel joints shows that there is a clear boundary between the weld seam and the steel interface of the copper/steel joints obtained by TIG welding and laser welding,and a certain amount of copper penetrates into the steel matrix,especially the copper/steel joint corresponding to TIG welding.The structure of the central zone of the copper/steel weld is mainly composed of two-phase solid solution of copper in iron and iron in copper.The solid solution is mostly in the form of spheres.There is no obvious boundary between the weld and the copper interface of the copper/steel joint corresponding to TIG welding and laser welding,but there are obvious iron-rich bands,in which the iron structure of the TIG welding corresponds to the copper/steel joint is obviously involved in the copper matrix.The results show that the average tensile strength of copper/steel joints corresponding to the two welding processes can reach 205 MPa,and the tensile fracture points are in the heat-affected zone on the copper side.It is found that the conductivity of laser welded copper/steel joints is slightly higher than that of TIG welded copper/steel joints by measuring the conductivity of copper/steel joints.Because the working environment of electrolytic copper cathode plate makes the copper/steel joint exposed to acidic electrolytic corrosion environment directly,the corrosion resistance of copper/steel joint is studied.Firstly,compared with the corrosion resistance of copper/steel joints obtained by TIG self-melting welding and laser self-melting welding,considering the higher efficiency of TIG wire-filling welding in the manufacture of permanent cathode plate,the TIG wire-filling welding of copper/steel was studied.Ni is an element that can improve the corrosion resistance of copper-steel alloy materials,and nickel has good weldability with iron and copper,Therefore,TIG filler wire was used to weld T2 copper and 316 L stainless steel with different copper-nickel wires of S201,S231 and NiCu-7.The effects of Ni on the microstructure,mechanical properties,electrical conductivity and corrosion resistance of copper/steel joints were studied.The research shows that good copper/steel joints can be obtained by using three kinds of welding wires with different Ni content.The microstructure of joints is observed,it is found that within a certain range,the presence of a small amount of Ni in the weld will cause a small number of micro-cracks in the weld near the steel side.The presence of Ni in the weld wire will lead to the appearance of flaky and massive Ni-Cr-Fe phase in the weld.With the increase of Ni in the wire,the Ni-Cr-Fe phase in the weld increases and the microcracks disappear.The mechanical properties,electrical conductivity and corrosion properties corresponding to copper/steel joints were tested.The results show that the mechanical properties of copper/steel joints are excellent.The fracture positions of copper/steel joints all occur in the heat-affected zone on the copper,and there is obvious necking phenomenon at the fracture site.The conductivity of copper/steel joints decreases with the increase of Ni in the joints.The increase of Ni in copper/steel joints with TIG wire filling welding can obviously improve the corrosion resistance of joints under acidic conditions.The corrosion resistance of copper/steel joints obtained by laser self-melting welding is better than that of copper/steel joints obtained by TIG self-melting welding in acidic conditions,and the corrosion resistance of copper/steel joints obtained by TIG selfmelting welding is better than that of copper/steel joints obtained by TIG wire filling welding in acidic conditions.Considering the experimental results of good electrical conductivity and corrosion resistance of copper/steel joint obtained by laser self-fusion welding and the advantages of fillet welded structure itself,the laser welding of copper/steel fillet welded structure is studied.The structure of copper/steel fillet welding is grooved in the copper rod and 316 L stainless steel plate is inserted into the copper groove.The results show that the copper/steel double-sided fillet welded joint can be formed well by laser power 3000 W,defocusing 0 mm,welding speed 4 mm/s and continuous welding on both sides,i.e.the welding process of one side without cooling down the sample and then welding on the other side.The tensile tests of copper/steel fillet welded joints show that the optimal nominal tensile strength of copper/steel fillet welded joints is about 400 MPa,and the mechanical properties of the joints are excellent.The mechanical properties of the joints meet the requirements of the actual use of electrolytic copper cathode plate.The tensile fracture location is at the junction of the weld and stainless steel.The copper/steel fillet welded joints obtained under the optimum parameters have excellent electrical conductivity.The analysis shows that the existence of welding cracks in copper/steel joints has a significant effect on the electrical conductivity of joints.The welding cracks are caused by the tensile stress in the welded joints during the cooling and solidification process of molten pool,and the increase of tensile stress during solidification process leads to the occurrence of welding cracks finally.In order to ensure the electrical conductivity of the copper/steel joint,copper/steel can take preheating before welding method to reduce the appearance of weld crack. |
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