Boosting Aluminum-Silicon's Strenth, Ductility in a Permanent Mold Casting
Researchers are investigating additional lightweight alloys in permanent mold castings.
Mohammad Shamsuzzoha & 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 November/December issue of Metal Casting Design & Purchasing.)
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.
Question
Can the addition of barium to aluminum-silicon cast alloys increase ductility and strength in as-cast and/or T6 heat-treated conditions?
1. Background
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 in 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 Ba 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.
2. Procedure
The process involved melting Al-Si alloys with 6-10% silicon in an argon-rich environment. A resistance furnace was used to maintain a temperature of 1,418F (770C) for the Ba treatment. After which, the resulting melt was poured into a permanent mold that was preheated to 850F (454C). The casting then cooled to room temperature before heat treatment. For such treatment, furnace cooled alloys were initially solution treated at 975F (525C) for 11 hours and then quenched in water. The quenched samples were then aged in the same furnace at 356F (180C) 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 Si 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.
In August, Lightweight Innovations for Tomorrow (LIFT) announced a new project intended to advance technologies for diecasting and heat treating aluminum parts, primarily for aerospace, defense and automotive applications.
“If we can reduce just a few ounces of metal from automobile engine mounting cradles or the housings that hold transmissions, we can deliver an impact that is multiplied by the millions,” said Larry Brown, executive director, LIFT. “In aerospace, an added benefit might lower manufacturing costs as well as increase fuel savings from the lighter weight designs.”
LIFT is operated by the American Lightweight Materials Manufacturing Innovation Institute and is one of the founding institutes in the National Network for Manufacturing Innovation, which is a federal initiative to create regional hubs to accelerate the development and adoption of cutting edge manufacturing technologies. It was formed in 2014, and the vacuum diecasting project is one of the first two started. The other project focuses on thin-walled gray iron parts.
Lead partners for the project are Boeing and The Ohio State University. The focus is to develop key process technologies (super vacuum diecasting and a shortened heat treatment) and computer engineering tools for 300 series aluminum diecasting alloys to improve mechanical properties and reduce the minimum wall thickness (up to 40%) and weight (up to 20%). According to Alan Luo, professor of materials science and engineering and integrated systems engineering, The Ohio State University, the project will reduce the variability in quality and improve the mechanical properties of high pressure die castings. The project also will explore new design methods of lightweight castings using local mechanical properties predicted by the new computer engineering tools, as opposed to the current casting design using minimum properties of cast alloys.
“If you can take a common part, such as an access panel you see on the wing of an airplane and use high integrity die castings, it could reduce weight and manufacturing costs,” said Russ Cochran, associate technical fellow, Boeing. “We hope to demonstrate that advances in high vacuum diecasting will produce parts that meet all the rigorous performance specifications we require, while realizing weight and efficiency goals.”
In current high-speed aluminum diecasting, microscopic air bubbles can form inside the part as the molten metal races through the mold. These tiny bubbles are not an issue for most diecast parts in typical applications, and engineers allow for them by using more metal and making parts thicker to meet strength and other performance requirements. These aluminum parts can achieve tensile strengths up to 47 ksi and minimum wall thicknesses of 0.04 in.
For this project, however, researchers are looking at methods to cast thinner walls with increased strength for structural applications, and for that, the bubbles can be detrimental. By applying a vacuum to the mold, diecasters remove air from the environment. Air is the culprit for porosity.
“We know in the laboratory that if we pull all the air out of the mold just before the molten metal flows in, we can eliminate the bubbles,” Luo said. “Without bubbles, we can design thinner parts that are just as strong and durable, but with less metal and lighter weight. There are other benefits, as well, because the new process allows us to heat treat parts after they are cast, which will improve their performance in service.”
The group also will be working on a shortened solution heat treatment to improve mechanical properties cost efficiently. A simple T5 heat treatment (where castings are cooled from an elevated temperature and artificially aged) has shown in preliminary work to increase yield strength by 40% for E380-type alloys. Now researchers want to see if a shortened T6 heat treatment (where castings are solution heat treated and artificially aged) can be developed to achieve even better properties in 300 series aluminum.
An important part of the two-year project will be enhancing the ability of computer models to predict the performance of aluminum diecast parts by combining information about the microstructure of the metal with a host of design and production parameters. The process, called integrated computational materials engineering (ICME), has great potential for reducing the time it takes to design and qualify new components for vehicles and will address some of the key challenges in implementing thin-wall diecasting technologies: die design, process control, casting design and process simulation.
Currently, castings are designed using the minimum properties of alloys as a baseline for the entire part. The ICME approach will allow designers to pinpoint higher or lower minimum properties to localized regions.
The aim is to connect the thermodynamic prediction of alloy composition and heat treatments to process modeling, which will enable designers to locate specific properties in specific areas of a part to meet service loading conditions. When load paths are clearly defined, the research group also plans on establishing topology optimization techniques to enhance design optimization.
In the first three months of the project, the group has selected the baseline alloy (A380) and identified an experimental high strength aluminum alloy for structural casting development for aerospace and automotive applications. A concept design on a thin-wall casting die also has been started. The die is based on a thin-walled zinc die casting, which is being redesigned for aluminum. Researchers have begun mechanical property evaluation and ICME model validation in thin-wall casting development. Key real-world applications, such as an aerospace wing fuel door, to demonstrate the benefits of vacuum diecasting and ICME have been identified.
“What we are doing here is bridging the gap between great research in laboratories and great manufacturing skills in private industry,” Brown said. “Once you bring these innovations into production, the results just multiply.”
Lightweight aluminum diecast components have a significant market to fill, particularly in the transportation industries, including aerospace, automotive, military and marine. Industry partners from these markets include Eaton Corp., Comau and Nemak. On the research side, Worcester Polytechnic Institute, Southwest Research Institute, the University of Michigan and Massachusetts Institute for Technology have joined the effort, while the American Foundry Society and North American Die Casting Association are assisting with modeling, technology oversight and dissemination of knowledge on how to manage the new thin-wall aluminum diecasting process in a production environment.
LIFT is operated by the American Lightweight Materials Manufacturing Institute and was selected through a competitive process led by the U.S. Department of Defense. It is one of the founding institutes in the National Network for Manufacturing Innovation, a federal initiative to create regional hubs to accelerate the development and adoption of cutting-edge manufacturing technologies.
After the two-year project is concluded, the group plans to deliver design guidelines and property specifications for thin-walled aluminum diecasting and ICME models for thin-wall casting design with location-specific properties. As these technologies and guidelines are incorporated into production diecasting operations, the options for aluminum components in structural applications will expand. ■