Study On Instantaneous Interfacial Heat Transfer During Near Rapid Solidification Processes

The interfacial heat transfer during melt solidification on chiller surface is widespread in many rapid or near rapid solidification processes, such as metal mold casting, spraying casting, laser cladding, planar flow casting, single roller melt-spinning, centrifugal atomization, strip casting and so on. The heat exchange between liquid metal and substrates is a central content in these solidification processes. Its research is of great theoretical significance and engineering value. The value and change of the interfacial heat flux directly affect the cooling and solidification rate, and then affect the solidified structure,alloy element distribution, mechanical properties, surface quality, productivity and the mould life. The accurate measurement and research of the interfacial heat transfer can not only enhance understandings of the initial solidification behavior and provide a theoretical direction for the choice of process parameters, but also provide boundary conditions for the numerical simulation. The control of the interfacial heat transfer can improve the productivity, mould life and product quality.The interfacial heat transfer in traditional casting has been studied widely, but the involved heat exchange time is usually quite long, from seconds to mimutes. For many rapid or near rapid solidification processes, for example, strip casting, the total solidification time is less than 0.5s, for which the traditional interfacial heat transfer measurement is powerless. Therefore, new equipments and methods need to be designed for the more accurate measurement and research on the instantaneous interfacial heat transfer. Based on theoretical research and actual technique needs, the main purpose of this paper is to study the effects and mechanism of typical process factors (melt superheat, substrate temperature, surface roughness, alloying elements, etc.) on the interfacial heat transfer and solidified structure; design advisable methods (platings, coatings, textures, etc.) for improving and controlling the interfacial heat flux and solidification, according to the actual production situation.To achieve these goals, in this article the following work has been completed and the corresponding results obtained:1) A new interfacial heat flux measurement device has been designed, under laboratory conditions which can be more precise in the heat flux measurement of molten steel solidification on the copper substrate in the first 500ms, with millisecond resolution. Compared with other researchers'devices and methods, it has the following features: symmetrical design ensures the substrate inner surface with adiabatic boundary conditions; to avoid the drilling operation in substrates eliminates the corresponding error and thus the substrate can be considered to be strictly one-dimensional heat transfer mold; welding operation ensures that the thermocouples and the substrate is metallurgical combine and their good contact, avoiding the associated error; simple structure, high reliability. The results show that under routine conditions, the heat flux in the first time of 20 ~ 30ms can reach the peak value, 10 ~ 20MW/m2, then quickly reduce to 3 ~ 5 MW/m2 and remain stable. It is considered that the corresponding processes are liquid/solid contact and solid/solid contact respectively.2) The model of relationship between peak heat flux, substrate surface morphology, wetting angle and other factors has been established. The results show that the wetting angle between the melt and substrate is sensitive for the peak heat flux. On this basis, the effect mechanism of different types of texture on the heat flow was discussed. 3) Using AISI304 stainless steel as the experimental material, the effects of molten steel temperature, substrate temperature, roughness and other key process parameters on the interfacial heat flux and solidification have been investigated. The results show that the peak heat flux increases with the molten steel temperature increase first and then decreases with the substrate temperature, does not change significantly with the roughness. After the formation of solidified shell, the average heat flux changes little with various factors. It was observed that the large interfacial heat flux could increase the nucleation rate of solidified shell surface and produce planar structure near solidified shell surface, implying large cooling and solidification rate. The reason can be considered that the increase of superheat reduces the surface tension of the melt, so the wetting angle between the substrate and melt decreases, thus the interfacial heat transfer increases. The reasons of the increase of surface nucleation rate can be considered as the increase of the undercooling in melt surface and reduce of wetting angle between melt and substrate caused by the superheat increase. Using pure iron as mother material, the effects of alloying elements silicon, boron and phosphorus on heat flux and solidified structure were investigated. It was found that the silicon content increase reduced the peak heat flux; boron and phosphorus had no obvious effects because of the little content.4) Using low carbon steel as the experimental material, the effects of conventional metal plating and substrate surface texture on the interfacial heat flux and solidification have been investigated. Coating parameters include coating composition, coating thickness and texture includes the surface morphology of sandblasting, cold spraying and ridge-type texture. The results show that: metal platting can reduce the peak heat flux, impact insignificantly on the average heat flux, and improve the uniformity of solidification structure. Coating composition has no significant effect on the heat transfer and solidification. For texture, the peak heat flux increase with the increase of surface roughness, but in the larger roughness, the contrary will occur. Surface roughness has not significant effect on the average heat flux. High roughness easily produces air holes on the surface of the solidified shell and deteriorates the surface quality. The ridge-type texture increases the true contact area between melt and substrate, thus increases the interfacial heat transfer.5) The effects of several representative coatings on the interfacial heat transfer, solidification structure and surface quality have been investigated: 1) Inorganic non-metallic solid coatings, represented by deposited carbon, graphite and hexagonal boron nitride coatings, can reduce the peak heat flux and make it more uniform with time. As response, the solidification structure shows more uniform in the thickness direction; compared with the sample on uncoated substrate, solidified shell surface is of coarse grains, but less defects, thus the surface quality is improved significantly. 2) Liquid coatings during solidification, represented by low melting point metal Zn, can greatly improve the heat flux in the stage of solidified shell developing, solidified structure maintains high cooling and solidification rate in a wide range. 3) Gas coatings during solidification, represented by organic coating, Vaseline, can greatly reduce the heat flux in whole solidification process. The effect of carburization and remelting structure were found in solidified shell surface. The mechanism of the above phenomena was discussed.