Laser Cladding Process And Performance Characterization Of NbC Particle Composite Nickel-based Amorphous Coating

The nickel base amorphous composite coating was deposited on the low carbon steel by using the high power fiber laser. It’s trying to adopt one-step method, which is to obtain the amorphous composite coating on the low carbon steel by coaxial powder feeding, to improve the hardness and wear resistance of the low carbon steel surface. The main research contents are included as following: the influence of macro forming and dilution rate in the cladding layer by the changes of laser power; the influencing rules of macro forming, microstructure, phase structure and amorphous content in the cladding layer by the changes of heat input; the mechanism of microstructure formation from the outside position to the inside position of the coating section; the influence of microstructure and mechanical properties in the previous cladding laser which is overlapped; the test and analysis of microhardness and wear resistance in amorphous composite coating with different parameters, etc.The results of the study are as follows:Laser power will affect the dilution rate between coating and substrate. As the research results show that with the increasing of laser power, dilution rate of the coating will augment and the volume content of amorphous phase which is got in the cladding layer will gradually decrease, on the other hand, under the lower dilution rate, coating can obtain more amorphous phase. Therefore,what can be got by optimizing laser cladding parameters is that the cladding laser should be obtained with low dilution rate. On this basis, this article has used the Optical Microscope, SEM, XRD and TEM to observe the cladding layer, and find the influence of the microstructures and phase composition in the cladding layer under different heat input.The results show that with the decreasing of the heat input, the height of cladding layer will reduce, but the content of amorphous will increase. When the heat input is 131.3 J/mm, there would have no amorphous phase in the coating. With the decreasing of the heat input, amorphous phase will be seen in the coating, and the content of amorphous will gradually increase. When the heat input is less than 50.0 J/mm, there is only a small amount of fine grain phase and some white particle phase in the coating, and the proportion of amorphous can reach 80.9%.The hardness and wear resistance of the cladding layer can be tested by using the Vicker micro-hardness and the friction and wear tester. It can be found that with the decreasing of heat input, the micro-hardness of the coating will gradually increase, and the downward trend would be seen in the hardness value after testing the section from the outside to the inside in every coating, and the hardness peak is about 20 um away from the surface of the cladding layer. As the results of friction and wear test show that due to the effect of high hardness and reinforcing phase NbC in coating, the wear resistance of the coating has a lot of improvement with the matrix. The matrix has the largest friction coefficient value which has a larger fluctuation and the wear loss is also large. With the decreasing of the heat input, the wear extent of the coating will gradually decrease, the friction coefficient will show a decreasing trend and the amplitude of fluctuation will also decrease.By using the ANSYS finite element software and the method of “birth and death of element” to simulate the process of the laser cladding nickel based amorphous composite What can be got the temperature field distribution and the different conditions of laser cladding heat input and the thermal cycle curve of different positions in the coating. Results indicate that when the heat input is 50.0J/mm, the temperature peak will decrease with the increasing of the distance to the surface of coating which has a temperature of 1578.53 ℃, otherwise, the cooling rate of melt isalso gradually decline from the surface to the center. And the rate at the surface can reach to 16086.5 K/s, which is far greater than the critical cooling rate oftraditionalfast quenching technology in copper mold. In addition, what can be found through the research is that the temperature peak of the coating will drop with the decreasing of the laser cladding heat input, and the cooling rate has a increase trend. Therefore, it is benefit for the formation of amorphous phase in the coating when the heat input is decreasing.For the heat input of 50.0 J/mm, different microstructures were formed in the coating gradually. At the coating/substrate interface, the coating is mainly composed of the planar phase and column phase, and then the transformation from column phase to equiaxed phase. At the middle of the coating, there were amorphous phase, NbC particles and equiaxed phase formed.From the interface between cladding layer and matrix to the top of cladding layer, the microstructuredisplays a graded distribution, and evolvement mechanism of the microstructure can be explained by rapid solidification theory. The variation of microstructure is based on the change of the temperature gradient(G) and solidification rate(R). In addition, G x R determines the microstructure size.With the increase of G/R, the structure morphology changes from planar phase, cell phase, column dendrites to equiaxed phase. When the cooling rate was higher than the critical cooling rate needed for the amorphous phase formation, the melt was “frozen” in to amorphous phase. An amorphous phase composite coating was obtained using laser cladding technology.