The influence of strontium on porosity formation in Al-Si alloys
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I. INTRODUCTION
STRONTIUM is added to hypoeutectic aluminum-silicon alloys in order to transform the morphology of the eutectic silicon from faceted acicular flakes to a fibrous rodlike form, thereby improving mechanical properties, especially fracture toughness and elongation.[1] However, modification is often associated with an increase in porosity above that of the unmodified alloy[2–9] and also a redistribution of porosity from macroscopic shrinkage porosity to well-dispersed microporosity,[5–11] which may negate the beneficial effects of the morphological change of silicon on the mechanical properties.[12] It is well established that porosity forms through the effects of solidification shrinkage and hydrogen rejection, either individually or in combination. The growth of a pore is determined by a balance between the pressures that promote and oppose pore formation. The condition required for pore formation is described by Eq. [1]. Pg PS Patm PH Pst
[1]
where Pg is the equilibrium pressure of dissolved gases, PS is the pressure drop due to solidification shrinkage, Patm is the atmospheric pressure, PH is the pressure due to metallostatic head, and Pst is the pressure due to pore-liquid surface tension. Theoretically, a pore can form if the combined effect of the gas pressure and the solidification shrinkage is larger than the sum of atmospheric pressure, metallostatic head, and surface tension. With reference to Eq. [1], it can be reasoned that impurity or alloying elements may affect porosity formation in a given
C.M. DINNIS, Postgraduate Researcher, A.K. DAHLE, Associate Professor, and J.A. TAYLOR, Senior Research Fellow, are with the Cooperative Research Centre for Cast Metals Manufacturing, Division of Materials, School of Engineering, University of Queensland, Brisbane, Queensland, Australia 4072. Contact e-mail: [email protected] M.O. OTTE, formerly with the Hydro Aluminium, Bonn, Germany, is with Karl Schmidt Unisia, Ft. Wayne, IN 46803. Manuscript submitted August 11, 2003. METALLURGICAL AND MATERIALS TRANSACTIONS A
hypoeutectic aluminum-silicon alloy casting in a number of ways. Impurity or alloying elements may alter surface tension and, therefore, alter the term Pst. All other parameters being equal, reduced surface tension should make pore formation easier. The hydrogen solubility may be affected by alloying and impurity elements. This would have the effect of changing Pg. Reduced solubility of hydrogen in the solid implies that the amount of hydrogen segregated during solidification would increase, while reduced solubility of hydrogen in the liquid implies that supersaturation of the liquid would occur at lower hydrogen levels. The stability of the protective oxide layer may be affected by impurity and alloying elements. A less stable oxide layer may allow higher rates of absorption of hydrogen from the atmosphere, possibly affecting the term Pg. A less stable oxide layer may also allow more oxide films to become entrained in the melt. Folded oxide films with nonwetting surf
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