Study On Surface Strengthening Process Of Vermicular Graphite Cast Iron By Laser Cladding

In recent years,the application of vermicular graphite cast iron has been greatly developed.Vermicular graphite cast iron is a new type of cast iron material whose graphite morphology is between spherical and flaky.Because it not only has close mechanical properties to nodular graphite cast iron,but also has good casting properties and thermal conductivity similar to gray cast iron,it has attracted considerable attention of material researchers and has become the preferred material for key components such as engines.Vehicle engines with small size,light weight,high speed and high power have become a trend of development,but the heat load and mechanical load brought by them have gradually approached or exceeded the use limit of vermicular graphite cast iron materials in local areas,and the demand for local surface strengthening has become more and more intense.For vermicular graphite cast iron,single surface strengthening processes such as laser quenching,laser melting and laser cladding are easy to produce cracks,pore and other defects.In order to reduce and avoid defects,the surface modification of vermicular graphite cast iron was carried out by combining laser melting and laser cladding.In this paper,vermicular graphite cast iron was used as experimental object,CO₂ laser was used as heat source.Laser melting treatment was carried out on the surface of vermicular graphite cast iron.By changing laser power and scanning speed,the effects of different processing parameters on the macro-morphology,micro-structure and surface crack defects of the melting layer were analyzed.Laser cladding Ni201 powder with excellent toughness was used as intermediate bonding layer on defect-free solidified layer sample.The optimum parameters of laser cladding single channel Ni201 coating were determined by orthogonal experiment.The effects of different lapping rates on Ni201coating were analyzed.Finally,the macro-morphology and micro-structure of Ni201coating under the optimum lapping rate were analyzed.Finally,the Ni201 coating was stimulated on Ni201 coating.Laser cladding Ni45+5%WC wear resistant coating.The wear resistance of vermicular graphite cast iron was studied by comparing the wear resistance of three different methods:conventional quenching in reheating furnace,laser melting on the surface of vermicular graphite cast iron and laser cladding of Ni45+5%WC coating on Ni201 coating.In order to study the bonding strength between cladding layer and matrix in two different processes of direct laser cladding and laser cladding on vermicular graphite cast iron surface,a universal testing machine was used to extrude the cladding layer,observe the fracture location and fracture morphology,and analyze the fracture mechanism.The laser fusing power is 2500W,the scanning speed is 300mm/min,and the secondary heat treatment has a good surface quality of the molten layer at a power of1600W and a scanning speed of 200mm/min,and there is no crack,which has little effect on the depth of the fused layer and meets the test requirements.According to the analysis of variance,the optimum process parameters for laser cladding Ni201 coating are laser power p=1600w,scanning speed v=500 mm/min and powder feeding rate L=0.6r/min.According to the actual needs of this experiment,the laser cladding Ni201 coating at 30%overlap rate meets the requirements of this experiment.The coating surface is flat,the melting width is 8.04mm,the melting height is 0.31mm,and the dilution rate is 49.2%.According to the abrasion resistance test,the wear volume of laser cladding Ni45+5%WC coating is significantly lower than that of conventional quenching and laser melting,and the wear resistance is the best,which is 1.6 times higher than the traditional quenching wear resistance.The laser fused layer is increased by 1.9 times.According to the coating bond strength test,the average bond strength of the laser cladding coating on the fused layer is approximately 1.56 times that of the laser cladding of the vermicular graphite cast iron.