Microstructural Development And Wear Properties Analyses Of Modified Layer On Bainitic Steel By Laser Remleting/cladding

In this study,a wear-resistant coating on bainitic steel is prepared by laser remelting/cladding technology.The microstructure evolution was characterized by optical microscope and scanning electron microscope.The flow field state of molten pool and the microstructure after solidification at different scanning speeds were simulated and analyzed by finite element method.The distribution of elements and phase composition in the modified layer were analyzed by energy dispersive spectrometer and X-ray diffraction.The evolution of crystal structure of the modified surface was analyzed by transmission electron microscope.The hardness gradient and wear resistance of the modified layer were studied by means of nano indentation,microhardness and dry sliding friction and wear test.In the laser remelting experiment the surface roughness and the depth of the modified layer are gradually roughened and deepened with the decrease of scanning speed,and the maximum depth is 152?m.The phenomenon of separation of the modified layer in the single laser melting experiment was observed,which is closely related to the temperature gradient and cooling rate of various parts.The abrupt change in hardness in the surface region is due to the formation of fine martensite.The internal characteristics of the martensite in different depths have changed significantly from the surface to the bottom,and high density dislocations and twins are observed.In the multi-channel melting experiment with the same lap ratio,the wear loss of the laser modified layer decreases by 1/2 as much as that of the matrix,and decreases gradually with the decrease of the scanning speed.In the experiments with the same scanning speed and different lap ratio,the effect of the melting of the back channel on the heat of the front channel is shown by the hardness distribution of the modified layer.At the same time,by means of properly increasing the scanning distance the modified layer can be obtained with improved wear resistance and production efficiency,which is essential to the efficiency of surface modification.In the laser cladding experiment,with the increase of laser scanning speed,the microstructure in the middle of the cladding layer is gradually refined and densified.The area of dendrites in the cladding layer gradually decreases from 77%to 69.4%.Through the research of the temperature field simulation and the flow field distribution in the cross section of the molten pool,it is found that the larger the scanning speed is,the smaller the molten pool is generated due to the low energy density input,so that the curvature of the convection ring generated by Marangoni stress in the molten pool is smaller,and the shear force along the scanning direction is larger.Since the primary dendrite growth is blocked,more nucleation points are generated,and the crystal growth is gradually disordered.Furthermore,high laser scanning speed leads to the acceleration of the cooling rate of the molten pool.The distribution of elements between the dendrites and interdendrites is inhomogeneous,and this effect is more obvious with the increase of scanning speed.However,the phase component in the cladding layer does not change over the laser scanning speed.The eutectic phase composed of M₂₃C₆carbide and?-Fe is the key factor to the improvement of the hardness of the cladding layer.By calculating the friction coefficient,measuring the volume loss and characterizing the wear surface morphology,it is found that the wear resistance of the coating increases with the increase of scanning speed,which is due to the formation of fine and uniform grains of the laser cladding coating,and the increase of the eutectic area of M₂₃C₆ carbide and?-Fe in the cladding layer.