微观孔洞和逆偏析缺陷的形成机理与耦合预测研究进展

Microporosity and inverse segregation are two common casting defects mainly caused by solidification shrinkage, which are detrimental to the mechanical properties of components, especially to the fatigue performance and ductility. Numerous efforts have been put into the investigation on microporosity and inverse segregation independently. However, few work has been done to establish a theoretical model for predicting the two defects simultaneously, whereas they often interact with each other and the formation of microporosity may exert a beneficial effect on inverse segregation. In this review, the coupling models for prediction of microporosity and inverse segregation were introduced. Firstly, the mechanisms and the predicting models for the two defects were summarized separately. Microporosity is a resultant of solidification shrinkage and gas segregation. Therefore, the porosity was previously categorized into two types: shrinkage porosity and gas porosity. More recent porosity models have combined the effect of pressure drop induced by feeding, the evolution of pores radius, the decrease of gases solu-bility in the liquid and the gas rejection at the solid/liquid interface, which provide rather good approximation to experimental results. As for inverse segregation, it is mainly caused by the suction of interdendritic liquid which is generally rich in solute. Therefore, determination of the feeding velocity is crucial for most inverse segregation models. Then, through the analysis of the underlying interaction between microporosity and interdendritic feeding flow, the coupling methods for prediction of the two defects were reviewed. Most of the models have added porosity into the continuity equation to amend the feeding velocity and utilized the "local solute redistribution equation" to get the solute concentration profiles. A new coupling model recently proposed by the present authors, based on analyses of the redistribution of gases element as well as the alloying element, is also in this route. The result shows that for Al-4.5%Cu (mass fraction) alloy solidified in a columnar dendrites structure, the predicted fraction of microporosity is a little smaller than that of Poirier's model, and the increase of initially dissolved hydrogen in the melt will decrease the solute enrichment in the interdendritic liquid. Microporosity seems to reduce the flow needed to compensate the solidification shrinkage, thus the solute segregation gets reduced. Finally, several suggestions were proposed, including the treatment of pore radius, eutectic shrinkage and gas porosity precipitated during eutectic reaction, etc.