A Model for Gas Microporosity in Aluminum and Magnesium Alloys

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Abstract

A quantitative prediction of the amount of gas microporosity in aluminum and magnesium-alloy castings is performed with a continuum model of dendritic solidification. The distribution of the pore volume fraction and pore size is calculated from a set of conservation equations that solves the transport phenomena during solidification at the macroscale and the hydrogen diffusion into the pores at the microscale. A technique based on a pseudo-alloy solute that is transported by the melt is used to determine the potential sites of pore growth, subject to considerations of mechanical and thermodynamic equilibrium. The modeling results for aluminum alloy A356 are found to agree well with published studies. In view of the limited availability of experimental data for Mg-alloy gravity-poured castings, the formation of porosity in AZ91 is studied qualitatively, assuming that casting conditions are similar to A356. In particular, the minimum initial hydrogen content that leads to the formation of gas porosity was compared for both alloys. It is found that the initial hydrogen content necessary for forming porosity is much higher in AZ91 than in A356. This is attributed to significant differences in the solubility of the hydrogen in both alloys.

Author(s): Sergio D. Felicelli, Liang Wang, Claudio M. Pita, Enrique Escobar de Obaldia

Introduction

MAGNESIUM cast alloys, such as AZ91, are gaining increasing attention in the struggle to decrease weight in the automobile industry. [1] However, it is difficult to consistently produce sound castings in AZ91, due to several problems that are difficult to solve: porosity, macrosegregation, oxide entrainment, irregularity of microstructure, corrosion, machining safety, etc. The formation of microporosity, in particular, is known to be one of the primary detrimental factors controlling the fatigue lifetime and total elongation in cast light-alloy components. Microporosity refers to pores that range in size from micrometers to hundreds of micrometers and are constrained to occupy the interdendritic spaces near the end of solidification. The micropores can form as a result of microshrinkage produced by the pressure drop of the interdendritic flow or because of the presence of dissolved gaseous elements in the liquid alloy.

In this study, a numerical model of hydrogen porosity formation during solidification was developed and was applied to aluminum alloy A356 and magnesium alloy AZ91. The model uses a technique based on a pseudoalloy solute that is transported by the melt in order to determine the potential sites of pore growth, subject to considerations of mechanical and thermodynamic equilibrium. The modeling results of the distribution of the pore volume fraction and pore size in A356 are compared with the published studies. In view of the limited availability of experimental data for Mg-alloy gravitypoured castings, the model is used to make a comparison study of porosity formation for aluminum alloy A356 and magnesium alloy AZ91, assuming similar casting conditions. The minimum initial hydrogen content that leads to the formation of gas porosity is compared for both alloys.

Results

Fig. 1. Porosity vs cooling rate in AZ91. Measured data approximately match simulations for a hydrogen content of 17.86 cc/100 g. Actual H content was not measured.

In this study, a numerical model was developed to calculate porosity formation during the solidification of aluminum and magnesium alloys. The model predictions of pore size and volume fraction distribution agree well with data previously published for aluminum alloy A356. The porosity model requires two parameters, the initial pore size and the concentration of inclusions, that are of a physical nature and can that be linked to measured data. The simulations with A356 show that the same set of parameters is able to reproduce with reasonable agreement the experimental data for different castings with varying levels of hydrogen content. A limitation of the method arises when pores grow at a high fraction of solid, which occurs for the lowest level of hydrogen content. In this case, the growth of pores is highly affected by the impingement on dendrites. Although the experimental data can still be reproduced through the use of a pore shape factor, a micromodel that links the pore shape parameter to physical quantities in the mushy zone would be desirable. The application of the model to the magnesium alloy AZ91 shows similar trends in porosity formation and distribution, but the threshold in hydrogen content needed to activate porosity is significantly higher than the one observed in the A356 (Figure 1). The simulations show that the minimum initial content of hydrogen needed to originate gas porosity in AZ91 is approximately 16 ppm. This indicates that porosity nucleates once the initial hydrogen content exceeds the solid solubility of hydrogen in AZ91. Although a previous work [2] has reported porosity formation in this alloy, even for a content level of hydrogen lower than the solubility, there is no independent confirmation of this study; either the porosity may have been shrinkage induced or it may be gas porosity originated by mechanisms other than the one considered in the present model. In particular, the entrainment of gas pores during pouring or during the diffusion of hydrogen into oxide bifilms pre-existing in the raw material could be considered as some alternative mechanisms of gas pore origination. The scarcity of experimental data for Mg-alloy gravity-poured castings, involving, in particular, measurement of the hydrogen content, is currently a limitation of the model validations. Controlled directional solidification experiments involving measurement of both the hydrogen content and the pre-existing inclusions in the melt are needed in order to make progress in the study of gas pore formation in magnesium alloys.

References

  1. C.H. Caceres, C.J. Davidson, J.R. Griffiths, and C.L. Newton: Mater. Sci. Eng., A, 2002, vol. 325, pp. 344–55
  2. D.M. Stefanescu: Int. J. Cast Met. Res., 2005, vol. 18, pp. 129–43

Citation: Felicelli, S., Wang, L., Pita, C., & Obaldia, E.d. (Apr 2009). A Model for Gas Microporosity in Aluminum and Magnesium Alloys. Metallurgical and Material Transactions B, 40(2), 169-181.

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