Formation Mechanism and Coupling Prediction of Microporosity and Inverse Segregation: A Review

doi:10.11900/0412.1961.2017.00501

Acta Metall Sin

2018, Vol. 54

Issue (5): 717-726 DOI: 10.11900/0412.1961.2017.00501

Special Issue for the Solidification of Metallic Materials

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Formation Mechanism and Coupling Prediction of Microporosity and Inverse Segregation: A Review

Zhiming GAO¹, Wanqi JIE¹(

), Yongqin LIU², Haijun LUO¹

1 State Key Laboratory of Solidification Processing, School of Materials Science and Engineering, Northwestern Polytechnical University, Xi'an 710072, China
2 School of Materials Science and Chemical Engineering, Xi'an Technological University, Xi'an 710021, China

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Zhiming GAO, Wanqi JIE, Yongqin LIU, Haijun LUO. Formation Mechanism and Coupling Prediction of Microporosity and Inverse Segregation: A Review. Acta Metall Sin, 2018, 54(5): 717-726.

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Abstract

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 solubility 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.

Key words: microporosity inverse segregation pressure drop gas precipitation feeding flow solute redistribution

Received: 27 November 2017

ZTFLH:

TG21

Fund: Supported by Major International (Regional) Joint Research Program of National Natural Science Foundation of China (No.51420105005)

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https://www.ams.org.cn/EN/10.11900/0412.1961.2017.00501 OR https://www.ams.org.cn/EN/Y2018/V54/I5/717

Fig.1 Defects in as cast A357 alloy
(a) shrinkage porosity (b) gas bubbles

Fig.2 The concentrations of hydrogen in the liquid during solidification (broken curves) and the hydrogen solubility (solid curves) ([H]₀ is the initial concentration of hydrogen in the melt, T_L and T_E are the melting point and eutectic isotherm of Al-4.5%Cu alloy, respectively)

Fig.3 Schematic of the condition for pore formation (P_atm is the ambient pressure, P_st is the static pressure by molten metal, P_σ is the liquid-gas surface tension, P_shr is the pressure drop induced by solidification shrinkage and P_ε is the pressure drop contributed by mechanical deformation, P_g is the pressure in bubble)^[11]

Fig.4 Various developmental stages of a pore
(a) nucleation on an folded oxide film/inclusions/dendrite arms
(b) sustained expansion in the interdendritic space
(c) when it is constrained by the dendrite arms

Fig.5 Calculated pressures within the mushy zone of Al-4.5%Cu alloy. Separate pressure contributions are shown along with total pressure in gas bubbles. A Scheil-type solidification path is also demonstrated (f_e is the liquid fraction when it reaches eutectic concentration)^[11]

Fig.6 Calculated porosity with the initial hydrogen contents^[11]. The present calculation is compared with Poirier's model^[9] for an initial hydrogen content of 0.30 cc/100g

Fig.7 Schematic for the calculation of inverse segregation during directional solidification (L is the width of the mushy zone; x_tip and x_root are the positions of the unidirectional dendrites tip and root, respectively)

Fig.8 Numerical results of concentration profile for Al-4.5%Cu in directional solidification with a constant thermal gradient (G) and a constant solidification rate (R)

Fig.9 Calculated velocity of interdendritic liquid flow of Al-4.5%Cu alloy with the initial hydrogen contents. The results are compared with the case of no pore^[11]

Fig.10 Profiles for mass fraction of Cu in Al-4.5%Cu alloy at late stage of solidification with the initial hydrogen contents. Profile for case with no pore formation is also shown^[11]

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