Numerical Simulation Research Of The Influence Of Carbon And Silicon On The Flowability And Solidification Structure Of Ductile Iron

Ductile iron is widely used in the production of automotive castings.Nowadays,in the condition of achieving desirable mechanical properties of castings,the high surface quality requirements were new challenges for testing and improving the casting performance of ductile iron.Due to the inherent difficulty for accurately controlling the experimental parameters of traditional liquidity,the accuracy and reproducibility of the experimental results are hard to be guaranteed and traditional experiments are also time-and money-consuming.Therefore,it is necessary to conduct numerical simulation to investigate and quantitatively evaluate the flowability and solidification structure of ductile iron in industrial production.In the past,the numerical simulation research of the filling process of ductile iron mainly focused on the condition of complete filling of molten iron,while there were few research reports on the flow simulation of incomplete filling.As for the numerical simulation of solidification process,the field of cellular automata programming is usually research focus.In this way,austenite and graphite can be independently modeled which contributes to obtain highly accurate dendrite growth morphologies.However,the cellular automata programming is limited to micro-scaled simulation and hard to couple with the macroscopic flow field.Hence,it is difficult to simultaneously export simulation results including fluidity and solidification structure under the condition of a same batch of input parameters.To overcome aforementioned problems,a new parameter system of fluidity and solidification structure of ductile iron was introduced in this paper.The thermophysical parameters of hypoeutectic ductile iron with different carbon and silicon concentrations under specific composition were calculated,and the surface nucleation parameters and bulk nucleation parameters required for grain growth simulation were set and optimized.The gravity casting module and CAFE module of Procast software were used to couple the macroscopic flow and microscopic grain growth of the molten iron,and the effects of different carbon and silicon concentrations on the flow field and temperature field of the hypoeutectic ductile iron were simulated and analyzed.The simulation of the fluidity and solidification structure of hypoeutectic ductile iron under the same set of parameters was performed.Simulation results are following:1)When the carbon concentration of ductile iron is increased from 2.8 wt.% to 3.4 wt.%with the step size of 0.3 wt.%,The superheat of the molten iron increases greatly,and the remelting degree of the molten iron increases;the solid phase rate of the molten iron near the end of the straight runner decreases sharply,which strengthens the effect of the static pressure head,and the flow velocity of the molten iron near the end of the sprue and the end of the stream is greatly increased;the degree of graphitization is increased.The number of eutectic grains is greatly increased and the grain size at the end of the stream is greatly reduced,which make the end of the molten iron strand difficult to block.Therefore,the fluidity of the example is greatly improved.2)In the ductile iron calculation example,when the silicon concentration is increased from 0.35 to 3.15 wt.% with step size of 0.3 wt.%,the superheat increase of the molten iron is smaller,and the solid phase rate of the molten iron near the end of the straight runner decreases slightly.The flow velocity of the molten iron increased slightly near the end of the sprue and the end of the stream.The increase in silicon content mainly increases the degree of graphitization,causing a small increase in the number of eutectic grains.As shown in the simulation example,there were a small decrease in the grain size at the end of the stream and a slight increase in the fluidity.Under the same conditions,the effect of silicon on ductility of ductile iron is weaker than that of carbon.The reason is that the effect of silicon on the superheat of molten iron,the degree of eutectic grain refinement,and the ability to promote graphitization is weaker than that of carbon.According to the parameter system used in the numerical simulation,a fluidity verification test was conducted.The flow length data of spiral samples with various carbon and silicon concentrations were experimentally tested and analyzed.Combining with SEM results,the Voronoi eutectic distribution images were achieved by using the network formation algorithm based on single-sphere model,which can directly show the graphite phase distribution at the end of the stream.By experimental validation,the simulation results are in good agreement with experimental results.It demonstrated that the parameter system established in this paper can realize accurate quantitative simulation of ductile iron flowability and solidification structure,and a numerical simulation method for the evaluation of the ductile iron flowability and solidification structure in industrial production has been established.