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?

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 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.

2. Procedure

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. 

ncountering a scenario in which you are forced to suddenly and immediately suspend melting operations for an extended period can be a death sentence for many metalcasting facilities. Small to mid-size businesses are the backbone of the industry, but many do not survive when forced into extended downtime. One disaster-stricken metalcaster, however, found resilience through its own perseverance and a circle of support from peers, friends, suppliers, teams from installation and repair providers, an original equipment manufacturer and even competitors.
Tonkawa Foundry, a third-generation, family-owned operation in Tonkawa, Okla., was entering its 65th year of operation this year when a significant technical failure ravaged the power supply and melting furnaces on January 17. Thanks to the textbook evacuation directed by Operations Manager Carrie Haley, no one was physically harmed during the incident, but the extent of emotional and financial damage, and just how long the event would take Tonkawa offline, was unclear.
Tonkawa’s power supply and two steel-shell furnaces would have to be rebuilt. No part of the reconstruction process could begin until the insurance company approved removal of the equipment from the site. The potential loss of Tonkawa’s employees and customers to competing metalcasters seemed inevitable.
Within two days of the incident, repair, installation and equipment representatives were on site at Tonkawa to survey the damage. Once the insurance company issued approval to begin work, the installation team mobilized within 24 hours to remove the equipment and disassemble the melt deck.
Since the damaged equipment was installed in the 1980s and 1990s, Tonkawa and an equipment services and repair company quickly strategized a plan and identified ways to enhance the safety, efficiency and overall productivity of Tonkawa’s melt deck.
“The most critical issue was for our team to organize a response plan,” said Steve Otto, executive vice president for EMSCO’s New Jersey Installation Division. “We needed to arrive at Tonkawa ready to work as soon as possible and deliver quickly and thoroughly so they could get back to the business of melting and producing castings, and minimize their risk of closing.”
Several years after Tonkawa’s melt deck was originally installed, an elevation change was required to accommodate the use of a larger capacity ladle under the spout of the furnaces. Rather than raising the entire melt deck, only the area supporting the furnaces was elevated. As a result, the power supply and workstation were two steps down from the furnaces, creating a number of inconveniences and challenges that impacted overall work flow in the melt area. Additionally, the proximity of the power supply to the furnaces not only contributed to the limited workspace, but also increased the odds of the power supply facing damage.
The damage to the melt deck required it to be reconstructed. It was determined to be the ideal opportunity to raise the entire deck to the same elevation and arrange the power supply, workstation and furnaces onto one level. The furnace installation company provided the layout concepts, and with the aid of Rajesh Krishnamurthy, applications engineer, Oklahoma State Univ., Tonkawa used the concepts to generate blueprints for the new deck construction. The results yielded a modernized melt system with an even elevation, strategically placed power supply, enhanced worker safety and increased operator productivity.
“Eliminating the steps and relocating the power supply farther from the furnaces was a significant improvement to our melt deck,” Tonkawa Co-Owner Jim Salisbury said.
Within four days of insurance company approval, all damaged equipment had been removed and shipped for repair.
The insurance company required an autopsy on the damaged furnace before any repair work could begin. The forensic analysis was hosted by EMSCO in Anniston, Ala., in the presence of insurance company personnel, as well as an assembly of industry representatives from the companies who had received notices of potential subrogation from the insurance company.
Tonkawa’s furnace was completely disassembled while the insurance company’s forensic inspector directed, photographed, cataloged and analyzed every turn of every bolt on the furnace over a nine-hour workday. The coil was dissected, and lining samples were retained for future reference.
While the furnace sustained extensive damage, it did not have to be replaced entirely.
Structural reconstruction was performed to address run-out damage in the bottom of the furnace, a new coil was fabricated and the hydraulic cylinders were repacked and resealed. Fortunately, the major components were salvageable, and ultimately, the furnace was rebuilt for half the cost of a new furnace.
“The furnace experienced a significant technical failure,” said Jimmy Horton, vice president and general manager of southern operations, EMSCO. “However, not only was the unit rebuilt, it was rebuilt using minimal replacement parts.”
Though work was underway on the furnaces, Tonkawa was challenged with a projected lead time of 14 weeks on the power supply.
When accounting for the three weeks lost to insurance company holds and the time required for installation, Tonkawa was looking at a total production loss of 18-20 weeks. From the perspective of sibling co-owners Sandy Salisbury Linton and Jim Salisbury, Tonkawa could not survive such a long period of lost productivity. After putting their heads together with their furnace supplier, it was determined the reason for the long turnaround on the power supply could be traced to the manufacturer of the steel cabinet that housed the power supply.
The solution? The existing cabinet would be completely refurbished and Tonkawa would do the work rather than the initial manufacturer. This reduced the 14-week lead time to just five weeks.
Tonkawa is the single source for a number of its customers. Although lead-time had been significantly reduced, the Tonkawa team still needed a strategy to keep the single source customers in business as well as a plan to retain their larger customers.
Tonkawa pours many wear-resistant, high-chrome alloys for the agriculture and shot blast industries. Kansas Castings, Belle Plaine, Kan., which is a friendly competitor, is located 50 miles north of Tonkawa. Kansas Castings offered Tonkawa two to three heats every Friday for as long as it needed.
“We made molds, put them on a flatbed trailer, prayed it wasn’t going to rain in Oklahoma, and drove the molds to Kansas Castings. We were molding, shot blasting, cleaning, grinding and shipping every Friday,” Salisbury Linton said.
Others joined the circle of support that was quickly surrounding the Tonkawa Foundry family.
Modern Investment Casting Corporation (MICC) is located 12 miles east of Tonkawa in Ponca City, Okla. Though MICC is an investment shop and Tonkawa is a sand casting facility, MICC’s relationship with Tonkawa dates back years to when Sandy and Jim’s father, Gene Salisbury, was at the helm.
“Gene was always willing to help you out,” said MICC owner, Dave Cashon. “His advice was invaluable for us over the years, so when the opportunity arose to support Sandy and Jim, we volunteered our help.”
 MICC offered to pour anything Tonkawa needed every Friday in its furnace. Tonkawa brought its alloy, furnace hand and molds, while MICC provided its furnace and a furnace hand for three heats. Many of the specialty parts Tonkawa produces were completed with MICC’s support.
When Salisbury Linton approached Cashon and asked him to issue her an invoice to cover the overhead Tonkawa was consuming, Cashon told her if she brought in six-dozen donuts every Friday morning they’d call it even.
“We’re all kind of like family,” Cashon said. “We’re all part of the same industry and though we may be friendly competitors at times, you don’t want to see anybody go through what they’ve gone through and it could have just as easily been our furnace that failed. While we all take the appropriate measures and perform maintenance to prevent these scenarios from occurring, they unfortunately still occur from time to time in our industry.”
Tonkawa had recently added steel work to its menu of services and Central Machine & Tool, Enid, Okla., was able to take Tonkawa’s patterns and fulfill its steel orders so it would not fall behind with those customers, while CFM Corporation, Blackwell, Okla., took three of Tonkawa’s employees on a temporary basis and kept them working during the downtime. Additionally, a couple of Tonkawa’s major suppliers extended their payables terms.
Thanks to Tonkawa’s suppliers, friends and its personnel’s own passion, persistence and dedication, the business is up, running and recovering—placing it among the few shops of its size to overcome the odds and remain in business after facing calamity.
 Nearly eight months after that devastating Saturday evening in January, Salisbury Linton reflected on the people and events that helped Tonkawa rise from the ashes. “We certainly would not have the opportunity to see what the future holds for Tonkawa if it weren’t for all the kind-hearted people who cared about what happened to us. Everyone still checks in on us.”