| 其他摘要 | China has been the largest annual producer of castings in the world since 2000. However, China is not a foundry powerful country because of high consumption of resources and energy together with serious pollution, correspondingly poor casting technology and quality, and low value added castings in the foundry industry, which dictates that most of the key castings of large and heavy equipment are imported from developed countries. In order to change this situation, it is crucial to improve the casting technology, especially the critical technologies of key castings used in large and heavy equipment. The principal method is to integrate the numerical simulation with the traditional casting technology, and further create a virtual casting technique that can be used to solve the practical problem. Therefore, this thesis studies the casting processes of several key castings from the aspects of solidification-heat transfer, fluid flow, thermal stress and macrosegregation by numerical simulation. Based on the simulation results, these key castings were successfully trial manufactured in the foundry. At the same time some critical technologies was integrated to present a general approach, combining the simulations with the practical pilot production of castings.
Based on the measurement of the temperature field in a bolster casting of a railroad boxcar bogie, the interface heat transfer coefficients (IHTC) between the casting and several mould materials were obtained by inverse calculation. The locations of porosity in the steel bolster casting were accurately predicted using the calculated IHTCs. These interface heat transfer coefficients provided a precondition for other casting simulations.
For a backup roll casting the electro-slag hot topping (ESHT) was modeled using a moving heat source, which accurately predicted the porosity. The biggest backup roll produced in China, weighing 65t, was successfully trial manufactured with the optimum parameters calculated by this model. A new design procedure for the ESHT was formulated, which provides a technical reference for the production of higher tonnage castings with high length-diameter ratio.
A mold filling and solidification model for a super-alloy master ingot was established, predicting the origins of the primary and second porosity in this casting. Based on the simulation results, the optimum gating system, iron mold and insulating feeder were designed, which completely eliminated the primary and secondary porosity. Yield increases were increased from 72% (not completely removing the primary and secondary porosity) to 77% (without any porosity), forming a casting process for the superalloy ingot.
A solidification model of a 100t hollow steel ingot was established, which simulated and analyzed the effects of process parameters on the location of the end-point of solidification. It was found that the cooling capacity of a mixture of liquid nitrogen and air was higher than the compressed air alone by a physical modeling experiment. The heat transfer between the core and the casting was studied and found to depend on the engineering design of the core, the proportion of gases in the mixture, and the rate of flow. A 45t hollow ingot was successfully pilot produced using an optimum solution which was found from the simulation results, demonstrating a new technology for hollow ingot production and provides a technical reference for the production of higher tonnage hollow ingots.
A thermal stress model of a 310t heavy gray iron ingot mould with variable cross-section in the ingot-casting process was established. The simulation found the initial positions, time and form of cracks, the mechanics of crack expansion. All of the simulated results agreed well with the practical situation. An optimized structural design was proposed that the simulation indicated would possess a lower crack-sensitivity. This technology has been used in several hundred tons of heavy ingot moulds.
An integrated solidification/cooling/de-gating/heat treatment/thermal stress model was established, which simulated the distortion of an impeller during casting, de-gating and heat treatment. The geometry of the final part was obtained. Comparison of the experimental measurements with the model predictions showed good agreement. From the calculated displacements of key points of the blade, the proper inverse displacements were determined to provide an optimum casting pattern and to achieve a uniform and reasonable machining allowance for both faces of the blade. On the basis of this exercise with the thin-walled impeller casting, a deformation controlled method for thin wall complex geometry castings was demonstrated.
A novel method to suppress the macrosegregation of large scale steel ingots by reducing the solidification time was suggested. A simple simulation model estimated the effect of small solid additions on the solidification time of ingots, avoiding the traditional complex macrosegregation models. The parameters of adding steel particles were obtained by simulations. The simulative results of 500 kg steel ingot in a sand mould proved that a small ingot had sufficient time to generate significant macrosegregation. Based on the simulation results, the small ingot with the complete range of macrosegregation structures was successfully verified. The experiments with the addition of solid steel particles showed that the macrosegregation was suppressed deeply. Moreover the microstructure was refined and the mechanical properties were improved. A technical reference for large scale ingot with low macrosegregation was thereby defined.
These examples demonstrate the value of an integrated modelling technique, in which solidification, heat transfer, fluid flow, thermal stress and macrosegregation are simultaneously introduced. |
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