| The dies are important tools to endow the products with a geometrical shape and to be used for mass production. Die forming is a manufacturing means required by about 60%-90% parts and products in modern industry. Thus the die industry is extolled as"the prime mover going into the affluent society". Since the qualities and profits of products together with the researching ability of new ones depend greatly on the dies, the technical levels of production for the dies have become an important standard for rating the country's industrialization and innovation ability. The obvious low life of hot working dies during serving in the stern conditions has been a technical difficulty for a long time. Especially the thermal fatigue caused by cyclic thermal stress and strain, responds to 60%-70% failure of hot working dies. These is consumption of about 40-50 thousands tons of hot working die steel every year in China, which equals to the RMB of about several billion yuan and produces a great loss. Because the thermal fatigue failure is mainly caused by thermal fatigue cracks occurred on the materials surface, the surface modification treatment for dies, which is expected to change the surface properties of die and keep its bore intact so that the thermal fatigue resistance is improved, is no doubt an effective way to reduce cost and conserve the resources.Through the process of evolution for millions and millions of years, the body surface of natural lives such as dung beetle, dragonfly, seashell, plant leaf and so on, with periodical array of local zones as the basic units, based on the morphologically, joining-form and structurally non-smooth characteristics, presents the maximum adaptability to the natural environment under the coupling effect of multiple functions, scales and layers. In the present paper, the common hot working die steels 4Cr5MoSiV1 (H13), 3Cr2W8V and 45# medium carbon steel were chosen as the substrate materials, on which biomimetic non-smooth units were designed and fabricated by laser technique, after imitating the non-smooth morphologies and structures on the body surface of natural lives. The purpose of doing this is to combine the unit and substrate surface following the creature coupling rules in order to create the non-smooth surface like the body surface of natural lives. By means of the modern analytical and measuring techniques, the following aspects have been addressed: the design of non-smooth morphologies and structures on the die surface; the microstructure in the unit zone, the surface morphology and depth of the unit, and the selection of laser parameters for the needed morphology and depth; the mechanical properties of die materials treated by the laser biomimetic coupling (LBC) technique and their variations after thermal fatigue; the effect of units and their size, distribution, and strength on the thermal fatigue properties of die surface treated by LBC technique; the optimization for the unit distribution and laser processing parameters of the resist-thermal-fatigue die treated by the LBC technique, and the field test in the practical application.Laser processing made the unit zone obtain the altered microstructure. The outstanding features of the microstructure include: (1) the phase transformation occurred; (2) the ultrafine grains; (3) the increased dislocation density in the unit zone. Meanwhile, due to the vaporization, liquid phase expulsion of particulates, and/or radial liquid flow toward the borders of the laser spot, the geometrically different surface morphologies of the units compared to the substrate surface were produced under the irradiation of laser. As a result, the morphologically and microstructurally non-smooth effects are created on the material surface treated by the LBC technique. The unit depth is an important scale parameter that impacts the mechanical properties of material surface treated by the LBC technique. And surface roughness is the quantitative expression of the surface morphology of the units, which is a limiting factor for the surface quality of the LBC treatment. In the present study, the influence of laser parameters on the unit depth and surface roughness of units was investigated. The results indicated that there is a steep increase in both the unit depth and surface roughness with increasing the laser energy, and a gradual reduction in those with widening the pulse duration; when the pulse frequency uplifts, they both display an increase of small amplitude; while raising the scanning speed leads to a slight decrease in unit depth and gradual climb in surface roughness. By analyzing the results, it is found that the laser power density is the most significant factor impacting the unit depth and surface roughness. So, the laser power density selection map for the LBC treatment was established in the range of this study in order to meet the needs of unit depth and surface roughness.It is well known that the mechanical properties of materials depend on their microstructures. Measurement of microhardness in the unit zone indicated that it was much higher than the substrate hardness. Tensile tests manifested that the specimen surface treated by LBC technique has the excellent comprehensive mechanical properties. For the pile-nail specimens treated by the LBC technique, the regression analysis was induced and the relationship equations of ultimate tensile strength, yield strength and elongation regarding the area ratio of units were obtained. When exerting thermal fatigue cycles on the specimens treated by the LBC technique, it is found that the changed microstructures in the unit zones occurred, which were embodied by the grains coarsening, carbides precipitation and decreased dislocation density, so that the hardness and strength of the unit even the entire specimen degraded. But due to the still higher thermal stability, dense dislocations and refined microstructure retained in the unit zone, the unit hardness and strenghth were still much higher than the substrate.It was found from the study that the units arranged on the surface treated by the LBC technique not only have the enhanced resistance to thermal crack nucleation, but also can effectively inhibit their propagation, and that the size, strength, distribution of these units have a different effect on preventing the thermal fatigue crack nucleation and propagation. In the present range of study, the gridding-form surface has the optimum thermal fatigue property; the units of large size and high strength have higher resistance to thermal fatigue crack nucleation and propagation compared to those of small size and low strength, while the surface with units of high density (small spacing) has higher thermal faigue resistance than that of low density. On the basis of individual strengthening of the units, the increase in the thermal faigue crack nucleation and propagation resistanc of surface treated by the LBC technique was concentrately attributed to the"pile-nail effect"and"dam effect"resulted from the units. On the one hand, owing to the pile-nail effect and the dam effect, the units made the thermal fatigue cracks difficult to nucleate in the unit zone, which would reduce the probability of crack initiation greatly on the surface treated by the LBC technique. On the other hand, the double-phases compound strenghtening and stress counteraction mechnism produced by the pile-nail effect and the dam effect caused the increase of thermal fatigue strenghth and the reduction of effective stress on the whole surface treated by the LBC technique, so that the thermal fatigue crack nucleation resistance enhanced. Meanwhile, due to the existance of large amount of pile-nails and dams on the surface treated by the LBC technique, the thermal faigue crack propagation was blocked effectively by these pile-nails and dams. Then all types of crack propagation inhibiting behaviors occurred in front of the units which reduce the crack growth rate greatly. As a result, the crack propagation resistance increased and thermal fatigue life retarded.Orthogonal test design is one of the advanced methods in modern experimental optimizing techniques. The optimum scheme has the minimum number of tests to be conducted and can substitute for the full factorial test. Combining with modern statistical analysis, it can complete feasibility research and reasonable prediction for the experimental results. Based on the previous study, the unit and processing parameters that have major influence on the thermal fatigue behavior were chosen as the test factors. After determining the proper levels corresponding to the factors, the parameter optimizing process were completed to investigate the effect of different factors and levels on the thermal fatigue property of die materials. The optimized parameters were applied to the actual hot working die for casting oil pump bonnet in order to perform the field test. The field test indicated that the actual die treated by LBC technique has the superior thermal fatigue resistance compared to the untreated ones. |
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