Study On The Preparation And Properties Of The Micro-nano Composite Coating On The Surface Of Copper Crystallizer

As the "heart" of continuous casting machine,crystallizer is an important link in the process of continuous casting,but its working environment is very rough.As a carrier of the heat transfer,the outer surface of crystallizer is affected by the cooling water.On the other hand,the inner surface is also washed by high temperature molten steel and worn by high frequency friction of solidified shell.It is easy to cause crystallizer copper plate deformation,crack and wear,then result in failure.In order to prolong the service life of the crystallizer and improve the quality of the casting billet,the surface treatment of copper crystallizer is usually carried out.In industry,the crystallizer is usually strengthened by plating a layer of nickel and nickel alloy on the surface of the copper plate.But the coating has relatively low hardness,insufficient abrasion resistance and poor high temperature mechanical properties.In order to further solve the problem,this study trys to use composite plating technology to add nanoparticles into the routine nickel plating bath.Co-deposition of particles and anodic metal will achieve the effect of strengthening the coating properties,which will have stronger hardness,wear resistance and high temperature performance.In this paper,Ni-Al₂O₃ composite coating was prepared on the surface of copper crystallizer by micro nano composite plating technology.The preparation technology and related properties of Ni based composite coatings were systematically analyzed.The following results have been achieved.?1?Determination of the optimum preparation process of high hardness nano composite coatingBased on the micro-hardness of nano composite coating,orthogonal experiments and single factor experiment were used to obtain the optimum process parameters of preparing the Ni/Al₂O₃ nano composite coatings with high hardness: 4A/dm2 of cathode current density,50? of electroplating bath temperature,4 of electroplating bath pH,30g/L of the addition of Al₂O₃ nano particles.The micro-hardness of pure Ni coating and Ni/Al₂O₃ nano composite coating prepared by the optimum process parameters is 214 HV and 376 HV,respectively.?2?Study on properties of Ni/Al₂O₃ nano composite coatingsCompared with the pure Ni coating,the nano composite coating has finer grain,smoother surface and no obvious depression or bulge.The microstructure is more compact and the nano Al₂O₃ dispersed uniformly in the matrix metal.There is no obvious agglomeration of nanoparticles in the nanocomposite coating obtained by mechanical and ultrasonic fragmentation.The friction coefficient of nano composite coating is less than 0.4.And the wear loss is less than that of pure Ni coating.In the 3.5Wt.% NaCl solution,the corrosion weightlessness of Ni/Al₂O₃ nano composite coating is a quarter of that of only pure Ni coating.?3?Effect of Al₂O₃ particle size on properties of Ni/Al₂O₃ composite coatingNi/Al₂O₃ micron composite coating and nano composite coating were prepared under the optimum process parameters.And their performance is also compared and analyzed.Compared with the micron composite coating,the performance of the Ni/Al₂O₃ nano composite coating is better.And the smaller the size of the composite nano particles is,the better the performance of the coating is.?4?Effect of heat treatment on properties of Ni based coatingsThe reasonable heat treatment of Ni based composite coatings can improve the microstructure and mechanical properties.The hardness of Ni/Al₂O₃ micron composite coating reaches the peak value at about 200?,and its hardness decreases after above 200?.The hardness of Ni/Al₂O₃ nano composite coating with composite 50 nm Al₂O₃ particles reaches the peak at about 400?,and its hardness decreases after above 400?.The hardness of Ni/Al₂O₃ nano composite coating with composite 30 nm Al₂O₃ particles reaches the peak at about 500?,and its hardness decreases after above 500?.At the same heat treatment temperature,the high temperature oxidation resistance of Ni/Al₂O₃ micron nano composite coating is better than that of pure Ni coating.The smaller the Al₂O₃ particles is,the better the high temperature oxidation resistance of the composite coatings is.