and more, the modification is insensitive to phos- phorus presence in the alloy. When adding 150 ppm strontium to a melt
already containing 100 ppm of barium and 157 ppm of calcium, a significant drop in elongation is observed (4.5%). Even though fully modified micro- structure is obtained and no second phase particles were observed, combined additions of strontium and Ca-Ba tends to lower elongations. Further investiga- tion into the Sr-Ca-Ba interactions is needed. For A356.2+Sr and A356.2 with Ca-Si-Ba addi-
tions higher than 100 ppm of barium (112 ppm of calcium), no significant difference is discern- able between any of the experiments (ANOVA (F(7,82)=1.11, p=0.36)). The main conclusion of this work can be summa-
rized as follows: • The eutectic silicon in A356.2 can be efficiently modified using Ca-Si-Ba additions.
• A minimum of 50 ppm of barium and 56 ppm of calcium added from a single addition of Ca-Si-Ba are required to obtain a fully modified microstructure.
• Phosphorus does not have any negative effect for barium concentration of 100 ppm and more (112 ppm of calcium and more).
• The presence of 30 ppm of phosphorus in the melt negatively affects the elongation of the cast parts for barium concentration below 100 ppm (calcium below 112 ppm) even though the micro- structure is still fully modified.
• Between the range of 100 ppm to 200 ppm of added barium (112 ppm to 224 ppm of added calcium), no significant differences occur between the mechanical properties or the quality index, even when taking into account phosphorus additions.
• Dissolution of Ca-Si-Ba is quite long and it can take over 6 hours to fully dissolve 200 ppm of barium and 224 ppm of calcium.
• Fading of the barium when holding the melt dur- ing an extended period of time occurs between 10 hours and 24 hours of holding time, dropping from 180 ppm to 140 ppm. There is no significant fading of the calcium even within this time frame. Fur- thermore, the microstructure is still fully modified even after 24 hours of holding time.
• Fading through remelting of the melt with initial additions of 200 ppm of bar and 224 ppm of calcium occurs.
• Even with a drop from 200 ppm of Ba to 120 ppm and 233 ppm of Ca to 187 ppm after two remelts, the microstructure is still fully modified. Furthermore, there is no significant difference in mechanical properties and Quality Index with remelting.
Tis article is based on a paper (16-076) that was presented at the 2016 CastExpo in Minneapolis.
36 | MODERN CASTING August 2016
Fig. 4: Shown here is the modified microstructure for experiment 150 ppm Ba, 30 ppm P (as polished, 500x).
Fig. 3: The modified microstructure for experiment 100 ppm Ba, 30 ppm P (as polished, 500x) is shown.
Fig. 2: Pure A356.2 (as polished, 500x) without an addition resulted in an unmodified microstructure.
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