| With the continuous improvement of social requirements for energy conservation and emission reduction,further improving the performance and lightweight of diesel engines has become the main research direction of diesel engines.Vermicular graphite cast iron(VGI)has excellent comprehensive performance due to the particularity of graphite morphology.At the same time,compared with the gray cast iron currently used in diesel engines cylinder block and head,VGI is more able to meet the requirements of lightweight.Therefore,VGI has become an ideal material for a new generation of diesel engines cylinder block and head.The working principle of diesel engines requires that the material of cylinder block and head should not only have enough strength and stiffness,but also have enough good thermal conductivity.The combustion temperature of the internal fuel of the diesel engines is very high.Good thermal conductivity can ensure the cooling efficiency of the cooling system,make the temperature of the diesel engines tend to be uniform,and reduce the thermal stress,which has a significant impact on the working performance and service life of the diesel engines.Therefore,it is of great significance to study the thermal conductivity of VGI.At present,online control technology is usually used to control the quality of liquid iron creeping to ensure the performance of VGI cylinder block and head castings in industrial production.For the cylinder block and head with complex structure,different solidification conditions of each position lead to different performance,which requires more accurate prediction of the performance of each position in design and production,and thus provides basic data for lightweight design.Therefore,it is necessary to obtain a low cost,high efficiency and accurate method to predict the performance of VGI.In this paper,the cylinder block and head of VGI six-cylinder diesel engine were taken as the research object.The thermal conductivity of VGI cylinder block and head with vermicularity greater than 80%was quantitatively studied by combining simulation and experiment.And the relationship models of microstructure and performance based on cooling rate were obtained.The unit cell model based on the thermal resistance network method was used to quantitatively study the influence of microstructure factors on the thermal conductivity of VGI by numerical calculation when the vermicularity was 0-100%.Finally,the mathematical models were verified by experimental data,and the results were in good agreement.Firstly,the cooling curves of different positions in VGI cylinder block and head were obtained by numerical simulation of MAGMA solidification process.The microstructure and properties of the corresponding positions of the cooling curves were quantitatively analyzed and characterized by optical microscope(OM),scanning electron microscope(SEM),universal tensile test machine,laser thermal conductivity and tomography.Through the quantitative analysis of the experimental data,it was found that for qualified castings with vermicularity greater than 80%,when the eutectic cooling rate was within the range of 2-9℃/s,the vermicularity increased first and then decreased with the decrease of the eutectic cooling rate.For the hot spot with eutectic cooling rate of 0.56℃/s,although the vermicularity was at a high level,graphite was as slender as flake graphite.At the same time,when the vermicularity was above 80%,the proportion of vermicular graphite was positively correlated with the creep degree of a single graphite.In addition,the average area of graphite particles obtained by two-dimensional metallographic statistics decreased with the increase of eutectic cooling rate.The change of pearlite fraction can be well correlated with the total area of all graphite particles and eutectoid cooling rate.Finally,the quantitative relationship models of vermicularity and pearlite fraction were established based on cooling rate and verified by the data point used for non-modeling.The errors of the results were within 5%,which proved the reliability of the models.Secondly,through the correlation analysis of experimental data,it was determined that when the vermicularity was greater than 80%,the two-dimensional microstructure characteristics affecting the thermal conductivity of VGI were the average area of graphite particles and the pearlite fraction.The thermal conductivity increased with the increase of the average area of graphite particles and decreased with the increase of pearlite fraction.The connection conditions between graphite were observed by SEM.It was found that the connection between graphite was more in the specimen with higher thermal conductivity.With the increase of the average area of graphite particles,the possibility of contact and connection between graphite increased.In addition,three specimens with the same pearlite fraction were tested by Micro-CT.The results shown that thermal conductivity of specimens increased with increasing graphite volume fraction and decreasing total surface area.Finally,the quantitative relationship models of thermal conductivity and tensile strength were established based on eutectic and eutectoid cooling rate.The standard sampling position used to detect the microstructure and performance in actual production was used for model verification.The data were from the production process records,and the errors were within 5%,which proved the reliability of the models.Finality,using the unit cell model based on the thermal resistance network method,the quantitative effects of graphite volume fraction,shape and matrix thermal conductivity on the thermal conductivity of VGI were studied by numerical calculation when the vermicularity was in the range of 0-100%.In the calculation process,the cast iron cell model with different shape and volume fractions of graphite was constructed,and the thermal conductivity anisotropy of graphite and the interface between graphite and matrix were treated.Finally,the thermal conductivity prediction model of VGI containing three independent variables of graphite volume fraction,shape and matrix thermal conductivity was established.The three experimental data in this paper and the four experimental data in the literature were used for verification,and the maximum error was about 3%,which proved the reliability of the prediction model. |
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