Porosity formation in Al-9 wt pct Si-3 wt pct Cu alloy systems: Metallographic observations

The formation of porosity in Al-9 wt pet Si-3 wt pet 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 (alpha-iron, beta-iron, sludge, and Al2Cu); and (5) solidification conditions (solidification time and solidus velocity). The results were statistically analyzed, based on the quantitative 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 Al 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 beta needles of the Al5FeSi intermetallic phase are very active as pore nucleation sites. All intermetallics, viz. beta needles, alpha-Chinese script phase, Al2Cu phase, and sludge restrict pore growth and expansion. Increasing 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.