Porosity Formation in AI-9 Wt Pct Si-3 Wt Pct Cu Alloy Systems: Metallographic Observations N. ROY, A.M. SAMUEL, and F.H. SAMUEL The formation of porosity in A1-9 wt pct Si-3 wt pct Cu-X alloys was studied as a function of (1) the hydrogen content of the melt; (2) the melt treatment additives, namely, modifier (Sr), grain refiner (TiB2), and primary silicon refiner (P); (3) alloying elements for precipitation hardening such as Mg and Zn; (4) intermetallics (a-iron,/3-iron, sludge, and AI2Cu); and (5) solidification conditions (so- lidification time and solidus velocity). The results were statistically analyzed, based on the quanti- tative image analysis data of the porosity observed in samples obtained from a set of 72 solidification experiments. Metallographic aspects of pore size and pore morphology related to the preceding parameters and the possible mechanisms of porosity formation are highlighted in this article. The results show that a melt hydrogen content of 0.1 mL/100 g AI has the same effect on percentage porosity as that obtained with an addition of 185 ppm strontium to the melt. Grain refiner particles, phosphorus, and magnesium reduce percentage porosity, although in different magnitudes. A Mg-Sr or Mg-GR combination further reduces the percentage porosity observed in the casting. The/3 needles of the AIsFeSi intermetallic phase are very active as pore nucleation sites. All intermetallics, viz. /3 needles, a-Chinese script phase, A12Cu phase, and sludge restrict pore growth and expansion. In- creasing the local solidification time or the solidus velocity increases the pore parameters. Pore growth in the two cases is attributed, respectively, to a diffusion-controlled growth process and to the formation of hot spots. I. INTRODUCTION POROSITY in aluminum alloy castings occurs because of the rejection of gas from the liquid metal during solid- ification and/or the inability of the latter to feed through the interdendritic regions to compensate for the volume shrinkage associated with the solidification, m Hydrogen is the only gas capable of dissolving to a significant extent in molten aluminum, m The dramatic decrease in its solubility at the solidification point of aluminum, resulting in outgas- sing, leads to the formation of porosity and reduced me- chanical properties and corrosion resistance, t3,4,51 Besides hydrogen concentration, the formation of poros- ity is also controlled by other factors such as grain refining and inclusion content. Grain refiners are added in small amounts to molten aluminum alloys to control the grain structure in the casting. A1-Ti and AI-Ti-B master alloys are usually employed, where TiAls particles act as nuclea- tion sites for the formation of primary a-aluminum den- drites and promote a uniform, equaxied grain structure. This leads to a finer dispersion of and, in some cases, a reduction in the amount of porosity,t6,7] Modification of aluminum alloy melts is normally car- fled out to improve the mechanical properties. In A1-Si hypoeutectic alloys, it involves the addition of strontium or sodium to the melt in order to change the shape of the eutectic Si from acicular to fibrous, ts,9] The modified eu- tectic structure enhances the mechanical properties of the casting. However, this advantage is offset by the fact that N. ROY, formerly Graduate Student, A.M. SAMUEL, Research Associate, and F.H. SAMUEL, Professor, are with the Department of Applied Sciences, University of Quebec at Chicoutimi, Chicoutimi, Canada G7H 2B1. Manuscript submitted March 17, 1995. modified castings generally exhibit an increased amount of porosity when compared to unmodified ones,rio.-.12] where the increased porosity can adversely affect these p r o p e r t i e s . [ ~3,14] The excellent foundry and mechanical properties of A1- Si-Cu alloys make them popular candidates for automotive applications. Based on the AI-Si system, these alloys con- tain copper and magnesium as the main alloying elements and varying amounts of iron, manganese, zinc, and other impurity elements that partly go into solid solution and partly form intermetallics during solidification. In the con- text of automotive applications, involving production of substantial amounts of castings, the porosity problem at- tains particular significance, and it becomes important to understand the development of porosity in relation to the various alloy, processing, and solidification parameters in- volved during the casting process. As part of an ongoing research program on the study of porosity in important automotive alloys, porosity formation in A1-9 wt pct Si-3 wt pet Cu base alloys was studied, varying the additives (to cover the wide range of alloy com- positions used in such applications), the hydrogen level, and the thermal parameters in order to obtain a statistical analysis of the resulting porosity in such alloy systems. The study was comprised of a set of 72 solidification experi- ments, where the pore size (area and length), morphology, and distribution were quantified using image analysis, as reported in detail elsewhere, t~5,16]Tables AI and AII given in the Appendix summarize the results of these porosity measurements. Metallographic aspects of porosity formation in these al- loys related to the preceding factors are highlighted in this article. The microstructural observations presented through- out are typical representations of the statistically obtained data. METALLURGICAL AND MATERIALSTRANSACTIONS A VOLUME 27A, FEBRUARY 1996--415