The solidification expansion of cast iron can also cause penetration. .... Bounds et al.8 developed a 3-D code that predicts pipe shrinkage and macroporosity.
Proc. 3rd Int. Conf. Computational Model. Sim. of Materials, Acireale, Sicily (2004)
COMPUTER SIMULATION OF MICRO- AND MACRO-SHRINKAGE AND OF METAL PENETRATION DEFECTS IN METAL CASTING
Doru Michael STEFANESCU Professor, The University of Alabama, PO Box 870202, Tuscaloosa, Alabama 35487, USA
Penetration of liquid metals into sand molds is a common casting defect that results in millions of dollars in economic losses for the metal casting industry. The paper will review the physics of chemical and mechanical penetration of molten iron-base alloys in sand molds, as well as the mathematical models developed to describe the physics. Simulation of shrinkage defects in shape castings has been extensively studied because of its potential contribution to quality improvement of cast products. Although the location of macroshrinkage can be estimated rather easily by present solidification models, it is still difficult to quantify its size. In spite of concentrated efforts and numerous claims of success, microporosity prediction is still a subject looking for a solution. After a succinct analysis of the physics of the problem, the paper reviews the various approaches to predicting shrinkage evolution, from simple thermal models and criteria functions, to channel and porous medium models based on hydrogen diffusion, and finally to the most recent model based on oxide entrapment.
1. INTRODUCTION The interaction between the liquid metal and the molding aggregate before and during casting solidification is responsible for a number of casting defects such as metal penetration into the molding aggregate, and a series of shrinkage-induced defects. Such defects have a negative economic impact on casting production. Metal penetration is a surface condition in castings in which metal or metal oxides fill the voids between sand grains without displacing them (FIGURE 1). It develops before the beginning of solidification. Mechanical penetration occurs when the pressure exerted by the metal exceeds the pressures that oppose penetration. Chemical penetration occurs because of a chemical reaction between the metal and the atmosphere or the molding aggregate. Casting soundness depends on continuous liquid metal flow into the region that solidifies to feed the mass deficit resulting from solidification contraction. Failure to feed the mass deficit produces shrinkage defects. The terminology is rather ambiguous. In this paper we will use the classification and definitions in FIGURE 2. Shrinkage cavities are open to the at-
Proc. 3rd Int. Conf. Computational Model. Sim. of Materials, Acireale, Sicily (2004) mosphere and are a consequence of metal contraction while cooling in liquid state and during solidification. The mass deficit is compensated by atmospheric gasses, a process that is independent of the gas content of the metal and which does not require gas pores nucleation and growth. On the contrary, shrinkage porosity depends heavily on pores nucleation and growth in the mushy zone, and thus on the impurity level and the amount of gas dissolved in the metal.
2. METAL PENETRATION Mechanical penetration can be understood in terms of the pressure balance at the metal-mold interface (FIGURE 3).2 If the pressure on the metal side exceeds that on the mold side, the liquid metal will penetrate between the sand grains. The issue is to formulate this threshold (critical pressure). Depending on the molten metal/sand contact angle, the capillary pressure can be negative or positive. Thus it can be beneficial or detrimental to penetration. The capillary pressure, the main force opposing penetration, is a function of the liquid-vapor surface energy and of the size of the pores between the sand grains. shrinkage cavity
pipe oxidized steel
caved surface
shrinkage porosity
macroporosity (closed shrinkage) microporosity (microshrinkage)
sand grain
x25
non-oxidized steel
FIGURE 1 Chemical penetration region in low alloys steel.1
FIGURE 2 Definition and classification of shrinkage defects.
The solidification expansion of cast iron can also cause penetration. Since Pexp depends on casting and gating system design, and Pg is negligible for cast iron in sand, they will not be considered. From the pressure balance equation an equation for the length of penetration, Lp, is derived. The critical angle for penetration, θcr, was defined as the angle above which Lp = 0. Depending on the metal composition and pouring temperature, the
Proc. 3rd Int. Conf. Computational Model. Sim. of Materials, Acireale, Sicily (2004) actual metal/mold contact angle, A, will usually be different from θcr. If A>θcr no penetration is expected. This can be expressed as a Mechanical Penetration Index, Pmech=A/θcr. When Pmech
