View source for A Model for Gas Microporosity in Aluminum and Magnesium Alloys

==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): [http://www.me.msstate.edu/people/meFaculty/felicelli/felicelli.html Sergio D. Felicelli], [http://www.cavs.msstate.edu/information.php?eid=235 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.
<ref> C.H. Caceres, C.J. Davidson, J.R. Griﬃths, and C.L. Newton:
Mater. Sci. Eng., A, 2002, vol. 325, pp. 344–55 </ref>
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==
[[image:Fig.1.Porosity.vs.cooling.rate.in.AZ91.jpg|thumb|350px|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 <ref> D.M. Stefanescu: Int. J. Cast Met. Res., 2005, vol. 18, pp. 129–43 </ref> 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.


[[Category:Research Paper]]
[[Category:Casting]]
[[Category:Aluminum]]
[[Category:Magnesium]]

== References ==
<references/>

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.