Study On Process And Performance Of Laser Cladding Nickel Based Composite Coatings

Laser cladding is a promising process using high energy laser beam to modify the metal surface to meet user requirements.And the state of molten pool influences the quality of the coating.In this paper,the strengthening mechanism of cladding NiCrBSi alloy coating was studied by adding zirconia（ZrO₂）.The formed carbides in the coating as enhancement elements are to improve the hardness and wear resistance.Further,the W-Zr material system was introduced into cladding to improve the wear resistance under high temperature.The microstructure,composition and particle distribution of cladding coatings were observed and analyzed by optical microscope（OM）,electron scanning microscope（SEM）,energy dispersive spectrometer（EDS）and X-Ray diffraction system.Besides,the laser cladding monitoring system is constructed to obtain the key information of the cladding process to stable the cladding process.Finally,the composite alloy coating was fabricated onto the Cr-Zr-Cu substrate with desired properties.The abrasion resistance of the coating was tested at room temperature and at high temperature by the disc-pin friction experiment,and the wear mechanism was analyzed as well.Three types of composite alloy coating,such as ZrC/Ni25,ZrC/Ni45and ZrC/Ni60,were successfully fabricated by adding mixed powder of ZrO₂and C into Ni25,Ni45 and Ni60 in accordance with the molar ratio.In laser cladding process,the reactions between ZrO₂ and carbon happens and finally generates the ZrC particles.The synthesize ZrC will prevent the growth of coarse carbide of M₇C₃ andγ-Ni+M₂₃C₆ eutectic,refine the microstructure of the coating.Most of ZrC particles are grown into independent particles and presented as triangular,quadrilateral or long axis shapes.While some particles will collide with each other and sintering together.Finally,the larger size particle forms.Based on the theory of ZrC enhanced nickel-based coating and experiments,composite coatings of Ni60-WC-ZrO₂ were produced by adding WC and ZrO₂ into Ni60 alloy.In the Ni60-WC-ZrO₂ alloy coating,it consists of a variety of carbides like WC,ZrC,（W,Zr）C,M₂₃C₆ and so on,which are as the hardening phase.These carbides form by gradual decomposition of WC,and afterward,generate the new ZrC particles.Finally,ZrC and WC will diffuse each other,and then the carbide of（W,Zr）C will be generated.The matrix was enhanced by Zr due to solid solution strengthening,the ability to resist plastic deformation were then improved as well.In addition,Zr can be as beneficial element to refine the structure by inhibiting the growth of primary phase and eutectic phase.Furthermore,the synthesized ZrC particles evenly distribute in the coating and have dispersion enhancement effect.So,the coating offers good anti-cutting ability,high hardness and improving the wear resistance.The friction mechanism of Ni25/ZrC alloy coating changes from adhesive wear to abrasive wear,which implies the good anti-adhesion ability and anti-cutting ability of ZrC.Morphologies show that there were some plowed grooves on the worn surface of Ni45/ZrC composite coating.The primary wear mechanism of ZrC/Ni60 composite coating was abrasive wear.Because the internal stress in Ni60 alloy coating is large,and it will release at high temperature,thus wear resistance at high temperature is better than that at room temperature,while wear resistance of ZrC/Ni60 composite coating under high temperature is better than that of Ni60 alloy coating.The hardness of Ni60-WC-ZrO₂ was improved obviously;it can reach up to HV1000 above.Additionally,the wear resistance at room temperature and high temperature were enhanced effectively.Compared to Ni60 alloy coating,the volume of carbide in Ni60-WC-ZrO₂ composite coating was increased,and the structure of that was refined distinctively.Therefore,the wear resistance was improved significantly compared to Ni60 alloy coating.To stable the laser cladding process under different parameters,materials,and conditions,the monitoring system was established to detect the molten pool and extract the key information.Afterward,the numerical finite element method（FEM）is utilized to simulate the laser cladding process with the same parameters in accordance with the experiments.The results obtained from the simulation and extracted from monitoring system were compared under different conditions such as different scanning speed,power input and pre-placed powder thickness.The results verified the validation of the monitoring system and correctness of numerical model.At last,the Zr-reinforced composite coating was fabricated onto the Cr-Zr-Cu substrate based on the exploratory researches and the monitoring system.The nickel coating was electroplated firstly,and then,Zr-reinforced composite coating was cladded.Finally,the wear testes were carried on under room and high temperature.The results shown that the wear resistance of composite coating for copper was improved about 20 times compared to the electroplating coating.