In order to make gray cast iron more widely used, the graphite morphology in gray cast iron must be strictly controlled in order to effectively improve its mechanical properties. In recent years, many scholars have studied the improvement of mechanical properties of gray cast iron. Inoculation treatment and heat treatment are usually used to refine the graphite phase in gray cast iron. In the casting process of grey cast iron, the composition of molten iron is improved by adding alloy elements (Cu, Ni, Mo, etc.) or rare earth elements to the molten iron, or the inoculation treatment is optimized by improving the inoculant, so as to promote the solidification process of molten iron, increase the number of eutectic clusters and improve the graphite morphology, so as to make grey cast iron have good properties. In recent years, many scholars have studied the improvement of gray cast iron casting process. By adding element zinc to QT400-18L molten iron, Yang Huilong et al. [8] reduced the size of graphite ball, improved the graphitization grade, and reduced the pearlite content in the matrix by 30%. When the increased zinc content reached 0.33%, the tensile strength of gray cast iron increased to 425 MPa, optimizing the toughness. After adding trace elements Mn and Cu to gray cast iron, Hu Yong and others found that adding an appropriate amount of Cu to molten iron when the Mn content is certain can refine the graphite in nodular cast iron and increase the tensile strength of nodular cast iron to 473 MPa.
Although the optimization of casting process can significantly improve the strength of gray cast iron. However, the casting process has many limitations, because the action mechanism of alloy elements is very complex, there are many factors affecting the improvement of casting process, and the research on the control of element proportion is not perfect. In addition, the price of alloy elements is high, which makes the cost of improving the casting process high. For small gray cast iron parts, it is usually only necessary to improve the performance of their surface or local position to meet the actual use needs. The method of improving casting process is not only inefficient, but also has limited effect on improving the strength of specific position. In modern material treatment technology, surface modification is usually used.
According to the surface properties of gray cast iron, a composite modification process combining laser melting, laser shock and graphitization annealing is designed. Through this process, the surface strength of gray cast iron can be effectively improved to meet the specific service properties. Using laser melting process, the material surface can melt and solidify rapidly, and the non-equilibrium high hardness structure with fine grains can be obtained in a certain depth of gray cast iron parts, so that the surface hardness can be improved rapidly. However, after laser melting, the generation of surface cracks in the melting layer is a major bottleneck restricting the application of laser melting technology. The generation of cracks is due to the large residual tensile stress in the melting layer in the process of laser melting, so it is difficult to solve the problem of melting
The influence of residual tensile stress on the surface after laser melting is of great significance to improve the strength of gray cast iron after laser melting. Laser shock processing can effectively refine the surface grains of metal materials, improve the strength of the surface layer, and transform the residual tensile stress into residual compressive stress, which plays a good role in inhibiting the initiation and propagation of cracks in the fused layer. Graphitization annealing process can reduce the hard and brittle phase in white cast iron, change the matrix structure, and optimize the morphology of graphite phase, which plays an important role in improving the properties of gray cast iron. Therefore, this paper mainly takes HT200 gray cast iron as the research object. Through the combination of theoretical simulation and laser melting, laser shock and graphitization annealing process, the thermal stress response process of gray cast iron surface is studied, and the change of residual stress in the melting layer is analyzed. Through the parameter optimization of the simulation and experimental process, the change laws of microstructure and graphite phase in the modified layer of gray cast iron under different process parameters are analyzed. The best composite process parameters are explored in the simulation and experiment, and the changes of the shape, size and quantity of graphite phase in the composite modification process are clarified. The research results have important significance and practical application value for improving the surface properties of gray cast iron.