Alternative Modification of A356 for Ductility Studied


Hypoeutectic aluminum alloys such as A356.0 (Al-7Si-0.3Mg) are usually subjected to strontium additions in order to obtain a chemically modified eutectic morphology. The modification of the eutectic silicon from sharp acicular plates to a fibrous structure is normally applied to improve the alloy ductility. However, strontium additions are relatively costly and less expensive alternatives are being investigated.

One such addition, Ca-Si-Ba, which is traditionally used as a deoxidizer in the steel industry, is the focus of this study. The effects of the combination of the modifying elements calcium and barium through a Ca-Si-Ba addition have been investigated, and it has been found that a fully modified microstructure can be obtained. The elongation values obtained for Ca-Si-Ba modified A356.0 are equivalent to values from traditional strontium additions.

Furthermore, barium and calcium additions have been observed to be resistant to extensive fading even after holding the melt for 24 hours and also during multiple remelting.

Strontium modification of hypoeutectic aluminum alloys is suppressed by a small concentration of phosphorus. In contrast, Ca-Si-Ba modification appears to be insensitive to the presence of phosphorus. If Ca-Si-Ba and strontium are simultaneously found in the alloy, the alloy is still modified.

Background
A356 alloy contains 7% weight of silicon and 0.3% weight of magnesium. During solidification, the aluminum phase forms first and the silicon is rejected in the liquid ahead of the dendritic solidification front. The composition of the interdendritic zone is then close to the eutectic composition and the resulting microstructure is eutectic. When solidification rates are relatively slow, the silicon in the eutectic possesses a coarse acicular morphology. When faster cooling rates are achieved, the eutectic silicon has a fibrous morphology, in which the structure is smaller and less acicular.

The acicular morphology is considered to have a negative effect on mechanical properties. The elongated and angular shape of the silicon acts as crack initiator, which in turn decreases the elongation and ultimate strength of cast parts. The fibrous morphology allows improved ductility and strength in cast parts. In order to obtain the modified structure without affecting the solidification rates, small additions of alloying elements are made. This chemical modification is used for Al-Si hypoeutectic alloys. Phosphorus presence in the melt negates the modification as it reacts with strontium and sodium to form phosphates. The modifying elements are thus rendered nonavailable for the change of the eutectic silicon.

Strontium is the most commonly used modifying element. Additions of 80 to 200 ppm usually are used to obtain a modified eutectic silicon structure. Even though the fading phenomenon is not as important with strontium as it is with sodium, it will still decrease in concentration due to oxidation and the modifying effect will disappear with time and re-melting.

Procedure
The optimal barium and calcium concentration, the effect of phosphorus on such addition, and the fading of the modification through holding the melt at the high temperature and through remelting have all been evaluated in this study. The composition of the alloy used is presented in Table 1. For strontium additions, Al-10%Sr master alloy was used.

The barium and calcium additions were done using a Ca-Si-Ba for which the composition is presented in Table 2. The additions of calcium and barium for modification purposes were calculated using a specific barium concentration.

The first step of the study was to evaluate the optimal concentration of barium and calcium in order to obtain complete modification and optimal mechanical properties. The effect of the phosphorus content on the modification using barium and calcium also was evaluated. Moreover, A356.2 with strontium was cast to compare the properties of the parts. Finally, A356.2 with barium and calcium was remelted and strontium was added to evaluate their interaction.

For this set of experiments, loads of 187.4 lbs. (85 kg) were melted in a resistance furnace. The metal was held at 1,391F (755C).

The melt was degassed using dry argon and a rotary impeller. A reduced pressure test was performed to validate the effectiveness of degassing. Once satisfactory, the modification elements of Ca-Si-Ba, phosphorus or strontium were added.

The casting was in a modified ASTM B108 mold. This modified mold was designed to reduce the variability of the casting operator (Fig.1).

For each composition, 20 ASTM B108 test bars were cast. To measure the calcium and barium concentration, optical emission spectrometry disks were cast right before the first, the fifth and the last casting. A metallography specimen was cut from tensile specimens from the first, fifth and 10th castings.

The tensile specimens were heat treated using a T61 treatment and solution heat treated at a temperature of 1,004F (540C) for four hours, followed by a quench in water at 122-140F (50-60C). The specimens were held for 24 hours at room temperature and then were aged at 320F (160C) for six hours. Out of the 20 specimens cast, 10 were randomly selected and tested using the ASTM E8 standard.
The second experiment was the fading set of experiment. A356.2 with Ca-Si-Ba additions of 200 ppm of barium (224 ppm of calcium) was held at 1,391F (755C) for 24 hurs. Optical emission spectroscopy and metallography samples, using the modified B108 mold, were cast at 0 minutes, 30 minutes, 1 hour, 1.5 hours, 2 hours, 3 hours, 4 hours, 6 hours, 8 hours, 10 hours and 24 hours. Sets of five modified B108 castings were cast after two hours, six hours and 24 hours to evaluate the evolution of mechanical properties.

The last experiment was the remelting experiment. An A356.2 melt charge of 264.6 lbs. (120 kg) with 200 ppm of barium added (224ppm of calcium) was prepared (Melt 1). Ten ASTM B108 molds were cast out of melt 1. The rest of melt 1 was cast into ingots to be remelted (remelting 1). Ten ASTM B108 molds were cast from remelting 1. The rest of remelting 1 was cast into ingots (remelting 2) to be remelted again. Ten ASTM molds were cast from remelting 2. The same degassing and casting procedure that was mentioned before was used again with the exception that no waiting time for dissolution was used for remelting 1 and remelting 2.

Results and Conclusions
The experiment of using pure A356.2 without any addition resulted in a completely unmodified microstructure (level of modification 1 from AFS rating), as can be observed by the coarse and angular eutectic silicon in Fig. 2.

Following additions of phosphorus with the barium and calcium, there was a small to negligible effect on the contamination of the modification. For additions of 50 ppm of barium (56 ppm of calcium) with 30 ppm of phosphorus, a fully modified microstructure is obtained (level 4). The same can be noted for larger additions of barium and calcium with additions of phosphorus (Figs. 3–4). This demonstrates that barium and calcium through Ca-Si-Ba additions are not susceptible to phosphorus contamination and as thus, they can act as a viable replacement for strontium additions.

While the yield strength and ultimate tensile strength values do not vary much, elongation measurements varied significantly. As expected, pure A356.2 presented a lower elongation with an average of 3.85%. This shows the effect of the angular and coarse silicon eutectic. With the 50 ppm of barium addition experiment, the 6.34% elongation is similar or greater than what can be obtained in a higher concentration of Ca-Si-Ba additions. However, when 30 ppm of phosphorus is added, elongation drops to 4.93%. This indicates that even though a fully modified microstructure is obtained, the phosphorus still negatively affects the mechanical properties. However, this effect disappears for additions of barium of 100 ppm and more. For higher additions of barium than 100 ppm and 112 ppm of calcium, there is no difference in elongation between the different experiments.

An ANOVA test was used to compare the different results for experiments with additions of 150 ppm of strontium without barium/calcium and experiments with additions of barium higher than 100 ppm with and without phosphorus additions. No significant differences between the different means could be observed (ANOVA (F (6,64)=1.36, p=0.24)). This signifies that when using additions of 100 ppm of barium and 112 ppm of calcium and more, the modification is insensitive to phosphorus 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 microstructure is obtained and no second phase particles were observed, combined additions of strontium and Ca-Ba tends to lower elongations. Further investigation into the Sr-Ca-Ba interactions is needed.
For A356.2+Sr and A356.2 with Ca-Si-Ba additions higher than 100 ppm of barium (112 ppm of calcium), no significant difference is discernable between any of the experiments (ANOVA (F(7,82)=1.11, p=0.36)).

The main conclusion of this work can be summarized 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 microstructure 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 six hours to fully dissolve 200 ppm of barium and 224 ppm of calcium.

Fading of the barium when holding the melt during 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. Furthermore, 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.

This article is based on a paper (16-076) that was presented at the 2016 CastExpo in Minneapolis.