The existence of flake graphite in the middle layer of gray cast iron seriously affects the improvement of its properties. In this paper, the composite modification methods of laser melting, laser shock and graphitization annealing are used to change the structure of graphite on the surface of gray cast iron, refine the structure of modified layer, improve the performance of gray cast iron surface, reduce the probability of surface cracking of gray cast iron and increase the wear resistance of gray cast iron. HT200 gray cast iron is selected as the research object. A method to effectively improve its surface properties is established through simulation and experiment. By studying the change and distribution law of graphite phase in the process of thermal action, it provides a certain reference for improving the surface properties of gray cast iron.
Through the method of laser melting, under the action of laser rapid heating and rapid cooling, without adding other elements, the phase transformation and microstructure transformation of the surface material of gray cast iron are made, and the melting layer with high strength is obtained on the surface of gray cast iron. The microstructure, graphite morphology, hardness and wear resistance of the fused layer were studied by adjusting different laser parameters. Then, through the method of laser shock, the melting layer is induced to produce a certain depth of plastic deformation, so that the residual tensile stress in the melting layer is transformed into compressive stress, which reduces the generation of surface cracks in the melting layer, refines the microstructure of the melting layer and improves the strength of the melting layer. Through graphitization annealing, the hard brittle cementite in the melting layer is transformed into fine flocculent graphite, so as to improve the comprehensive properties. The changes of microstructure and graphite of laser melting impact layer after annealing at different temperatures are analyzed, and the reasonable graphitization annealing process parameters are obtained.
(1) According to the principle of laser melting and laser shock, the numerical model of laser melting and laser shock process of gray cast iron is established with the help of ABAQUS finite element analysis software. The temperature gradient and stress changes under different laser parameters are solved to obtain the temperature and stress distribution under different parameters, and provide more appropriate experimental parameters for subsequent experiments. The changes of residual stress in the fused layer before and after laser shock are compared, and the influence of laser shock on the cracking behavior of the fused layer is analyzed.
(2) Through the laser melting experiment of gray cast iron, the macro surface quality and molten pool morphology under different laser melting parameters are analyzed, and the laser melting parameters are optimized. X-ray diffraction (XRD) and scanning electron microscope (SEM) were used to observe and analyze the microstructure of the melting zone and heat affected zone, and the changes of graphite phase in the melting layer were analyzed. The hardness changes under different laser melting parameters were tested.
(3) The plastic deformation depth of the fused layer under different laser shock energy was studied. The changes of metallographic structure and graphite phase of fused layer after laser shock were analyzed. The residual stress on the surface of the material before and after laser shock was measured, and the inhibition effect of the change of residual compressive stress on the surface crack after laser shock was analyzed. The microhardness of the fused layer after laser shock was tested, and the friction and wear experiments were carried out on the samples after laser fusion and laser shock.
(4) The graphitization annealing experiment of gray cast iron samples after laser shock was carried out. The changes of microstructure and graphite phase of modified layer of gray cast iron under different annealing temperatures were analyzed. The effects of annealing temperature on microstructure transformation and the size, morphology and quantity of graphite were explored. At the same time, the effects of laser shock on graphitization annealing process were discussed. The hardness of the modified layer after graphitization annealing was tested, and the friction and wear properties of the modified surface were tested.
The research can not only further enrich the theory of thermal composite surface modification, but also has important engineering practical significance for exploring and developing high strength and toughness surface modification methods and their popularization and application.