Study On The Thermal Fatigue Property Of The Chilling Roll Materials In Rapid Solidification Techniques

The single roll rapid solidification technique is an important method in the preparation of rare earth permanent magnetic materials, while the chilling roll is the core component in the preparation equipment. The strip yield and production efficiency of rare earth permanent magnetic materials are directly determined by duty cycle (fatigue life) of the chilling roll. In the process of rapid quenching, the liquid molten alloy was pouring onto the rotary chilling roll. The heat that the melted alloy gives out in the course of cooling and solidification is mostly transferred to the chilling roll, and then is absorbed and taken out of by the cooling water. So, the chilling roll is impacted by the periodic thermal load which causes thermal fatigue of chilling roll material. As the chilling roll is very expensive, it will increase the production costs if the roll fails ahead of service time. Besides, frequent repair of the roll is not conducive to continuous large-scale industrial production. So, the research of improving the chilling roll’s thermal fatigue life time has important engineering significance and economic value.The present paper uses the FEA software to simulate the temperature field of roll surfaces during the service time. Besides, thin film was prepared on molybdenum substrate by magnetron sputtering, and then thermal shock and fatigue resistance tests were carried out by high power laser irradiation. The main findings are as follows:(1) Base on the heat transfer analysis of the chilling roll, the3-dimensional finite element analysis model of the chilling roll in Strip Casting process was established. Using the ANSYS parametric design language and the method of "killing and activating elements" to program, and the automation of solution was realized. The distribution of temperature field of copper roll and molybdenum roll were obtained. The results show that the temperature fluctuation of roll’s surface is very intense which leads to thermal fatigue failure in chilling roll material.(2) The different rapid quenching technical parameters, such as pouring temperature of alloy, strength of cooling water and velocity of roll, are studied respectively during the analysis of roll’s temperature field. The maximum temperature of roll surface increases with the increasing of casting temperature of alloys, decreasing of roll speed and heat transfer coefficient of cooling water. With the same set of parameters, the temperature fluctuation range of molybdenum roll is larger than copper. Compared with the copper roll, the surface temperature of molybdenum roll increases faster with increasing of alloy pouring temperature, decreasing of roll speed and heat transfer coefficient of cooling water.(3) Prepared by radio frequency magnetron sputtering, the thickness of Mo film increases in a similar straight line with the increasing of deposition time. The deposition rate increases slightly with the increasing of deposition time when the deposition time is less than30min.(4) FESEM pictures show that the Mo film is smooth, dense, and defectless. The surface topography of the film is in triangular pyramid-like pattern whose particle size is about100nm. These triangular pyramid-like particles were arranged closely and disorderly with each other. Therefore, the triangular pyramid is considered to be the morphology in the front of grain growth interface. Cross-sectional topography of the thin film sample shows that the grains are in growth of columnar crystal. In addition, the parallelism of grain growth in Mo film on glass substrate is more obvious than the one on the Mo substrate. Beside, Mo grains grew in a random orientation during the initial sputtering, and later the random orientation growth was inhibited while the grains grew in a strong orientation with the increasing of the deposition time.(5) When the sputtering deposition time is in the range of5to30min, the lattice constant of Mo(110) gradually decreases and the grain size increases in the direction perpendicular to the surface with the increasing deposition time. And in the direction paralleled to the surface, the lattice constant of Mo(110) first decreases and then increases, while the grain size maintains a relatively stable range. Laser thermal conductivity tests show that there is no significant difference in thermal conductivity between the samples of Mo films on Mo substrate and the Mo substrate. The thermal conductivity of two samples both decrease with the increasing testing temperature.(6) On the basis of existing experimental platform of laser thermal load, The present paper initially proposes a new method of testing the thermal shock and fatigue properties of chilling roll materials by laser heating under vacuum or inert gas protection conditions. The laser thermal fatigue device is carried out to assess the molybdenum thermal fatigue resistance fast and accurately.(7) During the laser power increased from500W to1500W, laser thermal shock and fatigue experimental results of copper and molybdenum specimens show that, the molybdenum sample performs better fatigue crack propagation resistance while the laser power is lower. And the copper sample shows better thermal fatigue resistance when the laser power is higher. From the tests of molybdenum and film coating molybdenum specimens under the same laser parameters, it was found that the crack growth at the intersection region of the beam spot of molybdenum sample is more dramatic than the coated one, and molybdenum coated film specimen performs better thermal fatigue properties. Thermal fatigue crack initiation and propagation mechanisms of copper and molybdenum were discussed.(8) A layer of molybdenum film was prepared by magnetron sputtering on the molybdenum roll material. The laser thermal fatigue tests showed that the homogeneous film coating improved the material’s resistance to cracking. A useful exploration is conducted to improve the thermal fatigue properties of chilling roll for rapid solidification technology, and it also provides a new way to improve the duty cycle of the chilling roll.