Boosting Aluminum-Silicon’s Strength, Ductility in a Permanent Mold Casting
Researchers are investigating additional lightweight alloys in permanent mold castings.
Mohammad Shamsuzzoha and Laurentiu Nastc, Univ. of Alabama, Tuscaloosa, Alabama; David Weiss, Eck Industries, Manitowoc, Wisconsin; and John T. Berry, Mississippi State Univ., Starkville, Mississippi
(Click here to see the story as it appears in the March issue of Modern Casting.)
The entire supply chain—metal casting suppliers and purchasers included—are conscious of minimizing waste and cost, while improving so-called green practices. The North American metalcasting industry faces increased government regulation for better fuel efficiency and lower emissions in the aerospace, automotive and defense industries. The challenge is to produce components that minimize weight without sacrificing mechanical or physical properties.
Hypoeutectic aluminum-silicon alloys have a silicon content less than eutectic compositions, meaning the silicon content is usually between 5-12%. These alloys are used in high strength applications that also require good ductility. This group of alloys, when treated with small amounts of barium, may further improve already desirable mechanical properties: light weight and high strength. Such alloys, containing interdendritic eutectic microstructure, may offer engineers strength and ductility not currently seen in existing cast alloys. These alloys also may be further strengthened by heat treatment.
In the research paper, “Permanent Mold Castings of High-Strength and High Ductility Ba-Treated Hypoeutectic Al-Si Alloys,” a research team investigated how barium treatments affected aluminum-silicon alloys in an effort to develop advanced alloys that could be cast.
Can the addition of barium to aluminum-silicon cast alloys increase ductility and strength in as-cast and/or T6 heat-treated conditions?
Based upon the Al-Si binary system shown in Figure 1, alloys with hypoeutectic composition have a silicon composition below 12.7 wt%. Two major components coexist in the microstructure of hypoeutectic Al-Si alloys: the primary, aluminum rich phase and the eutectic microstructure. The primary phase contains about 1.67% silicon as a solid solution and is in dendrite form. The eutectic structure, consisting of an aluminum-rich solid solution and virtually pure silicon, exists between the arms of the primary aluminum dendrites. Refinements of silicon by adding trace amount of impurities such as sodium and strontium can improve mechanical properties of resulting castings.
However, current impurity-containing hypoeutectic Al-Si cast alloys have yielded only modest improvements in ultimate tensile stress (UTS) (not in excess of 180 MPa) and ductility (roughly 10%). Two reasons explain these moderate increases:
- The silicon phase in these cast alloys is not sufficiently refined to offer a high UTS value.
- The eutectic point permits the proportion of primary aluminum and eutectic structure to promote a ductility less than 5%.
The potential exists to alter the primary aluminum to eutectic structure ratio and refine silicon morphology of Al-Si alloys with the addition of barium to improve strength and ductility. Recent work on the solidification of hypereutectic Al-Si alloys (having between 15-20% silicon) has focused on the solubility of barium in the silicon phase. This research has established primary silicon-free hypereutectic alloys with up to 17wt% silicon can be produced by directional solidifications. A shift of the normal eutectic point (shown in Figure 2) from 12.7wt% to 17.0wt% silicon caused by the addition of barium into the melt and related impurity modification mechanisms may help develop these alloys.
The same concept, which alters the ratio of the primary aluminum to eutectic phase and refines the morphology of eutectic silicon, has now been used to develop high strength, highly ductile hypoeutectic Al-Si alloys by conventional casting.
The process involved melting Al-Si alloys with 6-10% silicon in an argon-rich environment. A resistance furnace maintained a temperature of 1,418 F (770 C) for the barium treatment. After which, the resulting melt was poured into a permanent mold that was preheated to 850 F (454 C). The casting then cooled to room temperature before heat treatment. For such treatment, furnace cooled alloys were initially solution treated at 975 F (525 C) for 11 hours and then quenched in water. The quenched samples were then aged in the same furnace at 356 F (180 C) for 24 hours. Longitudinal and transverse section specimens taken from near the center of the samples were used to determine the microstructure. Visual inspection of the surface revealed negligible amounts of porosity and other casting defects.
The samples then were subjected to tensile testing. The microstructure of the alloys was studied using scanning electron microscopy (SEM). Samples were etched to remove surface aluminum and expose the topography and morphology of silicon phase.
3. Results and Conclusions
The morphology of silicon is similar to that in unmodified Al-Si alloys, but differs significantly from that of the fibrous morphology of silicon found in sodium and strontium impurity modified alloys. The proportion of eutectic silicon and primary dendrite appears consistent with what is expected of the cast alloys.
Figure 4 shows the load vs. strain plot for a typical tensile sample taken from each of the Al-6%Si-1%Ba, Al-8.5%Si-1%Ba and Al-10.5%Si-1%Ba alloys. Table 1 shows how the increase in barium from 0.5% to 2.0% improved strength and decreased ductility. Figure 4 also shows alloys with lower silicon content yielded lower UTS values but higher ductility. The UTS of 145Mpa found for the Al-10.5%Si alloy is comparable to the value of about 148 MPa reported for impurity modified Al-10%Si alloys. However, ductility of the Al-10.5%Si-Ba alloys is at least 3.5 times higher than the reported alloys of about 8% Si. In fact, the ductility of any of these alloys is at least 2.5 times higher than any current hypoeutectic casting alloys.
To see the effect of heat treatment, the microstructure (Fig. 3) and mechanical properties of T6 tempered Al-6%Si alloys were investigated. A comparison of this alloy with as-cast Al-6%Si-0.5%Ba micrograph reveals two distinct features:
- The silicon particles have increased in size by 10-15% due to heat treatment.
- The second feature relates to the background matrix, which contains less porosity in the heat treated alloys.
The microstructural features of the heat-treated samples changed the mechanical properties. The UTS and ductility of the heat treated samples, shown in Table 2, were noticeably improved compared to the as-cast samples.
The mechanical properties of Al-6%Si-Ba alloys show that heat treatment increased strength by 10-15% and ductility by 3-5%. The feature of mechanical properties for as-cast and T6 heat treated alloys is also evidenced in the load vs. strain plot of as-cast and T6 tempered Al-6%Si-1%Ba alloys shown in Figure 5.
The microstructure of the hypoeutectic Al-Si-Ba alloys cast in a permanent mold exhibit high UTS and ductility values. The silicon contents of the alloys appear uniform in size and assume sub-micron flake morphology. The primary aluminum phase in the microstructure also is very refined compared to other lightweight Al-Si alloys. The solid barium solution in silicon appears to affect the crystallization of both the primary aluminum and eutectic silicon in the hypoeutectic Al-Si-Ba alloys when cast in a permanent mold. The effect allows the hypoeutectic melt to nucleate eutectic silicon and primary aluminum crystal, resulting in the development of high-strength, highly ductile Al-Si alloys. Also, additional improvements in alloy performance may be realized through heat treatment.
This article is based on a paper (14-023) that was presented at the 2014 AFS Metalcasting Congress.