Influence of solidfication variables on the microporosity formation of aluminum-copper (4.5 wt%) alloy with axial heat processing

The purpose of this study is to investigate the effect of solidification variables on porosity formation in Al-Cu (4.5 wt%) alloy. These variables include thermal gradient (from 19 to 25 K/cm), solidification rate (from 0.00007 to 0.15 cm/sec), initial hydrogen concentration (from 0.09 cc/100 g to 0.27 cc/100 g), and level of convection. The effect of the convection on porosity formation, which has not yet been studied, was investigated by comparing two different solidification techniques: the normal Vertical Bridgeman (DS) technique and the Axial Heat Processing (AHP) technique.;The AHP technique makes use of a graphite disk (baffle) immersed in the melt. The baffle is made of high density graphite, which can conduct the heat axially and distribute it over the entire growing solid/liquid (= s/l) interface. The melt is separated into two zones when the baffle is brought close to the s/l interface. The small zone below the baffle reduces the melt height, leading to a reduction of convection. During the solidification, the solutes are rejected from the liquid due to their inherent lower solubility within the solid. These rejected solutes can be piled up locally in this small region, resulting in the change of solute concentration in the liquid and in the solid.;The microstructures produced by the AHP and DS techniques do not show a noticeable difference according to the dendrite arm spacing measurement. However, samples produced by the AHP technique show a porosity 20 to 40% lower than those prepared by the DS technique, and its effect is more pronounced with decreasing cooling rates and increasing initial hydrogen concentration. Along the height of the AHP sample, the volume percent of microporosity exhibits a maximum in the early stage of solidification, and then drops considerably. In the DS samples, it is almost constant regardless of the sample height. Additionally, the pore size in AHP samples was 5 to15% smaller than in DS samples.;The level of convection has been estimated by Rayleigh number, where h is melt height: h (DS) = 8 cm and h (AHP) = 0.5 to 0.7 cm. The convection led to the composition change at the dendrite tip, which could be confirmed by Electron Micro Probe Analysis (EPMA). The liquid copper composition at the head of the tip in the AHP sample increased to 8.9 wt%, compared to 5.32 wt% for the DS samples when the solidification rate is 0.00007 cm/sec. Thus, the suppressed convection increased reduced the dendrite height in the AHP samples compared to the DS samples. Additionally, local buildup of hydrogen below the baffle in the AHP technique could be calculated by Tiller's approach and confirmed by the Inductively Coupled Plasma (ICP) analysis. A hydrogen bubble can nucleate near the dendrite tip when the amount of hydrogen accumulates below the baffle exceeds its solubility.;Accordingly, decreasing the dendrite height and increasing the hydrogen concentration near the dendrite tip are both helpful in removing gas bubbles from the dendrites. Thus, it is believed that the final gas porosity is lower in the AHP samples compared to the DS samples.