Microstructural Observations of Porosity in A319.2 Alloy: Effect of Mold Type/Cooling Rate

摘要

A parametric study of porosity formation in A319.2 aluminum alloy was carried out, as a junction of hydrogen content, melt additives (Sr modifier and Al-Ti-B grain refiner) and cooling rate, using two different types of molds: an end-chill mold that provided unidirectional solidification, and in which the solidification rate decreased with increasing distance from the chill end; and a cooling fin sand mold whose different sized fins provided different cooling rates, and the means of isolating the effect of hydrogen in each fin, in the same casting. The effects of inclusions and intermetallics on porosity formation were also studied. Th is paper highlights the main micro-structural observations of porosity formation and pore characteristics noted in the two cases. It is found that, under directional solidification conditions, the hydrogen gas present in the melt is pushed upward by the moving solidification front, resulting in a gradient in the porosity observed (percentage porosity and pore size) along the height of the casting. Thus, the porosity observed at any given level above the chill end is dependent upon the hydrogen accumulated in the region just below. In the sand mold case, however, the hydrogen gas is trapped within the cast fins, and results in much greater pore sizes and porosity, compared to the end-chilled cast samples. This effect is more pronounced at the highest cooling rate (DAS approx 25 #mu#m). Inclusions and #beta#-Al_5FeSi intermetallics were frequently observed in connection with both microshrinkage and gas-type porosities. The nature of the moving solidification front in the sand mold fins causes pore coalescence due to the impingement of the gas bubbles that evolve within the liquid alloy during the solidification process. Similar effects are also observed for shrinkage pores. This linking of pores is particularly detrimental to casting soundness and strength.