Determining the Effect of Bi-Sr Interactions on Si Morphology in 319 Al.

Through experimental data, this article details the Sr/Bi ratio required to effectively modify Si morphology in 319 aluminum and improve melt quality and casting grain structure.

Several elements have been found to refine the size and morphology of eutectic silicon (Si) in hypoeutectic aluminum alloys. This refinement (or modification of the eutectic Si) is defined as a change in the morphology of the eutectic Si from a coarse plate (acicular) to lamellar or fibrous morphologies of varying degrees. For many years, strontium (Sr) has been the most popular choice by North American foundries to modify the eutectic Si phase. The effect of bismuth (Bi) on modification and its interactions with Sr are not as well-documented. Bi is added to aluminum casting alloys to impart anti-seizing characteristics and to improve machinability. Several investigators have found that Bi is capable of producing a slight modification effect while others reported no effect.

Through an experimental study, this article quantifies the effect of the Bi-Sr interaction on the Al-Si eutectic reaction using thermal analysis to characterize solidification and metallography to characterize the resulting morphology of the Si phase.

Experimental Procedure

Aluminum alloy 319 ingot was melted and held in an electric resistance furnace to which Sr and Bi were added to achieve the desired compositions. For this set of experiments, Bi was varied from 0.014 to 0.055 wt %, and Sr was varied from less than 0.001 to 0.020 wt %. Ten minutes after each addition, the melt was stirred. The melt then was left to settle for another 10 min before sample collection began.

At each Sr-Bi level, two spectrometer disks were poured for chemical composition and two thermal analyses were performed. Thermal analysis was conducted by recording temperature vs. time during solidification of a sample. The Al-Si eutectic temperature for each thermal analysis sample was defined as the maximum temperature observed during the Al-Si eutectic arrest on the cooling curve. The chemical composition of each heat was determined using optical emission spectroscopy. The samples were tested for the following elements: Si, copper, magnesium, iron, manganese, chromium, nickel, zinc, Sr, titanium, sodium, calcium, phosphorus, and Bi. The remainder of each sample was assumed to be Al. Selected samples were prepared for metallographic inspection to determine the level of Si modification in each.

Results

Thermal Analysis--Data was plotted in several ways in order to clarify the various effects of interactions between Bi and Sr. Figure 1 is a contour plot of the eutectic temperature as a function of Sr and Bi concentrations. Contour lines were drawn to illustrate that as Bi decreases and Sr increases, the Al-Si eutectic temperature decreases. Figure 2 is a plot of wt % Bi vs. eutectic temperature at various Sr concentrations. At Sr levels less than 0.002 wt %, Bi additions have no effect on eutectic temperature. At the highest Sr levels examined, Bi has no effect on eutectic temperature. At Sr levels of 0.002-0.004 wt %, increasing the Bi from 0.014 to 0.02 wt % results in a dramatic increase in eutectic temperature. Increasing the Bi concentration beyond 0.02 wt % at the 0.002-0.004 wt % Sr level does not cause further increases in the eutectic temperature. At the highest Sr levels examined, Bi has no effect on eutectic temperature.

Figure 2 indicates that at low Sr levels, the eutectic temperature is unaffected by Bi additions. Conversely, at high Sr levels, the eutectic temperature is low regardless of the Bi concentration within the range in this study. The lower eutectic temperature indicates modification of the Si structure has occurred. The eutectic temperature of mid-range levels of Sr appears to be affected by the amount of Bi present in the melt. This behavior suggests that a Bi-Sr interaction is affecting the Si modification in the alloy.

To further understand the Bi-Sr interaction on the Al-Si eutectic temperature, the Sr/Bi ratio was plotted vs. the Al-Si eutectic temperature and showed three distinct regions. When the Sr/Bi ratio is less than 0.2, the eutectic temperature remains high. When the Sr/Bi ratio range increases from 0.2 to more than 0.4, the eutectic temperature falls rapidly. At Sr/Bi ratios greater than 0.45, the eutectic temperature remains constant. A one-to-one atomic ratio of Sr/Bi would give a mass ratio of 0.41. This suggests that Sr and Bi may form a compound, which reduces the amount of Sr available to induce modification of the Si. It appears that the observed ratio of 0.45 provides enough Sr to react with the Bi with sufficient excess Sr to induce modification of the eutectic Si.

Metallography--Based upon the results of the thermal analysis data, 11 samples were chosen for metal-lographic inspection. The samples' Simorphologies were classified by visual inspection based upon the presence of plate, acicular, lamellar or fibrous particles. Descriptions of the Si morphology for each sample are listed in Table 1. The samples were sorted in order of apparent modification. These visual results then were compared with the recorded eutectic temperature, the wt % Bi, the wt % Sr and the Sr/Bi ratio for each sample.

AFS modification ratings then were assigned to the samples based upon comparison to AFS publications. Determining Si modification levels for our samples based on the AFS chart raises two issues. The first issue is that modification ratings are made by visual comparison between the samples and the AFS chart and are therefore subjective in nature. Second, the alloy depicted in the AFS chart is a binary Al-Si alloy that produces a more uniform structure than the 319 alloy used in this study. The range in morphologies observed in a single sample of 319 aluminum alloy introduces another level of subjectivity to the analysis. The general trend is that with decreasing Al-Si eutectic temperature, the Si modification level increases. A comparison between Sr/Bi ratio and Si modification shows that as the Sr/Bi ratio increased, the level of Si modification also increased.

An examination of the microstructure indicates that the Sr/Bi ratio directly affects the level of Si modification in 319 aluminum alloy. Figures 3a and c show two different compositions, both having Sr/Bi ratios of 0.7. Both of these samples exhibit similar levels of Si modification, even though they have different amounts of Sr and Bi. In contrast, samples that have either similar amounts of Sr (Figs. 3f and j) or similar amounts of Bi (Figs. 3g and j) do not necessarily exhibit similar levels of modification. Therefore, in order to predict the Si morphology, both the Sr concentration and the Sr/Bi ratio must be determined.

Conclusions

Examination of the relationship between the Sr/Bi ratio, eutectic temperature and Si morphology shows an interesting relationship. At Sr/Bi ratios below 0.2, the Si structure was unmodified and the eutectic temperature remained high [above 1048F (564C)]. At Sr/Bi ratios between 0.2 and 0.45, the eutectic temperature drops dramatically from 1051F (566C) to 1037F (558C), and the modification level increases dramatically in this region with increasing Sr/Bi ratios. Above ratios of 0.45, the Si structure was modified and eutectic temperatures remained consistent between 1051-1037F.

In alloys containing Bi, in order for Sr to modify the Si eutectic morphology, a Sr/Bi ratio of at least 0.45 must be maintained.

This article was adapted from a paper (00076) presented at the 2000 AFS Casting Congress and is available from the AFS Library at 800/537-4237.