Microstructure And Properties Of Laser Cladding Ni/Ni-WC Gradient Coating On Cr12MoV Steel Surface

As a traditional cold work die steel,Cr12MoV steel exhibits high hardness and wear resistance.It can be often employed to make various cold punching dies and tools with large cross-sections,complex shapes and withstand large impact loads,such as punching recesses,trimming die,edge die and standard tools.These molds often fail due to long-term withstand reciprocating impact and friction as well as wear in production practice.In order to extend the service life of the mold or repair the worn area of the mold,this article uses a pulsed Nd:YAG laser to laser cladding Ni60A and Ni60A+?%WC alloy powder on the surface of Cr12MoV mold steel to prepare a gradient wear-resistant coating.The phase,microstructure and composition distribution of the coating were analyzed using X-ray diffractometer,scanning electron microscope and energy spectrometer.At the same time,the micro-Vickers hardness tester and high-speed reciprocating friction and wear testing machine were used to compare the hardness,wear resistance of the coating and the substrate.The wear scar morphology was analyzed by a white light diffractometer combined with a scanning electron microscope.The results are as follows:?1?By changing the scanning speed and laser current of the main laser process parameters,the following optimized process parameters were obtained:scan speed120mm/min,laser current 160A,pulse width 6ms,frequency 8Hz,defocus amount 0mm,protective gas flow 12L/min,powder thickness 0.5mm,overlap rate 40%.?2?Laser cladding Ni20Cr alloy powder on the surface of Cr12MoV steel as a base layer makes the metallurgical combination of the coating and the substrate strong,and can effectively reduce cracks,pores and other defects in the coating.The surface of the multilayer Ni-based coating and the Ni60A+?%WC gradient coating is smooth and continuous,and the forming quality is good.After the cladding layer is polished,it can become a good strengthening layer or repair layer.?3?The phases on the surface of Ni60A coating are mainly?-?Fe,Ni?solid solution,FeNi₃,BNi₃,Cr₃C₂ compounds and Ni-Cr-Fe phase.The structure from the bottom to the surface of the coating is composed of cell dendrite,columnar dendrite,preferential growth dendrite and equiaxed dendrite successively.The branch structure is composed of Cr₃C₂ compound and?-?Fe,Ni?solid solution and the interdendritic structure is composed of FeNi₃ and BNi₃ compounds as well as a small amount of?-?Fe,Ni?solid solution and Ni-Cr-Fe phase.The surface phase of Ni60A+35%WC coating is mainly composed of?-?Fe,Ni?solid solution,WC,W₂C,Cr₇C₃,Cr₂₃C₆ carbide and WSi₂ phase.The coating stem structure is composed of WC,W₂C,Cr₇C₃,Cr₂₃C₆ and WSi₂ and the interdendritic structure is composed of?-?Fe,Ni?solid solution.There is significant element diffusion between coatings and the coating of the substrate,and the metallurgical bonding is good.?4?Ni60A coating and Ni60A+?%WC gradient coating have greatly improved the hardness and wear resistance of Cr12MoV die steel.Among them,the average microhardness of the Ni60A coating is about 1.3 times that of the substrate,and the average microhardness of the Ni60A+35%WC coating is about 1.7 times that of the substrate.As the content of WC in the coating increases,the wear resistance gradually increases.However,with the increase of the wear time,the improvement of the wear resistance represented by the wear volume decreased.When the wear time is 75min,the wear resistance of Ni60A coating is improved by 1.0 times compared with the substrate,and the wear resistance of Ni60A+35%WC coating is improved by 3.8 times compared with the substrate.?5?When the wear time is 75min,the grinding surface of the substrate is flat and smooth without obvious grooves,mainly composed of adhesive wear with a small amount of abrasive wear.The grinding surface of the Ni60A coating is relatively smooth,and there are a few shallow and narrow grooves.There is a small amount of adhesive wear and abrasive wear,the grinding surface of the Ni60A+35%WC coating is smooth,with wide and deep grooves,and the micro-cracks on the grinding surface of the coating gradually disappear.In summary,the multilayer wear-resistant coating and gradient wear-resistant coating prepared by laser cladding Ni60A and Ni60A+?%WC alloy powder on the surface of Cr12MoV steel can improve the mechanical properties of Cr12MoV steel surface and carry out on its surface for repairing requirements.Ni60A+35%WC wear-resistant coating is proved possess the best effect.The various hard phases formed in each coating layer can effectively improve the hardness and wear resistance of the substrate surface,and reduce the probability of scrapping due to friction and wear during the use of the mold,thereby further extending the service life of the mold.