Anew tool is set to transform how temperature and strain is measured in metal casting. Researchers at Missouri University of Science & Technology (Missouri S&T) in Rolla, Missouri, have found success using fiber optic sensing technologies to accurately measure distributed temperature and strain in a wide variety of applications in the industry. This breakthrough could represent the beginning of a significant shift in industry methods to provide real-time process control and automation.

AFS Director Laura Bartlett, Ph.D., Wolf chair associate professor of Metallurgical Engineering at Missouri S&T and a Foundry Educational Foundation (FEF) key professor at the school, recently shared the findings in an AFS webinar and follow-up interview. Co-authors of the research include Jie Huang, Ph.D., Roy A. Wilkens, endowed associate professor, Department of Electrical and Computer Engineering, Missouri S&T, and Ronald O’Malley, Ph.D., F. Kenneth Iverson Steelmaking chair, professor, and director, PSMRC, as well as Jeffrey Smith, Koustav Dey, Grant Whitham, Ben Hilgers, and Rony Kumer Saha.  

Origins of the Research and Funding

According to Bartlett, research using fiber optic sensors began about five years ago, through the Peaslee Steel Manufacturing Research Center (PSMRC), a consortium of steel companies, foundries, suppliers and university researchers working together to address fundamental steel casting/manufacturing issues. 

“We had a project involving using the fiber optic sensors to sense the temperature of continuous casting steel molds,” said Bartlett.  “We found the fiber optic sensors gave very good temperature resolution, which allowed us to develop a heat map of the whole mold.” This success led to expanded research into aluminum casting, 3D printing technologies, and other applications in industry and defense, supported by approximately $11 million in total funding from the U.S. Department of Energy and the Department of Defense. 

Research takes place at the Robert V. Wolf Foundry at Missouri S&T, which is equipped with advanced solidification and thermodynamic modeling software, 3D sand printing capabilities, and pilot-scale production equipment. The foundry has up to 500 lbs. of ferrous melting capacity, as well as the ability to melt cast and process aluminum, copper, nickel, high entropy, and other technically important alloys.

Advantages of Fiber Optics in Metal Casting

Bartlett shared several advantages that fiber optics hold over traditional tools used in the metal casting industry such as thermocouples and strain gauges. While thermocouples only measure a single point, fiber optic sensors can capture measurements every half millimeter along up to 100 meters of fiber. “That means there are no dead zones,” she said. Unlike thermocouples and strain gauges that need to be attached to a wire, fiber optic cables are far less cumbersome to install since they are about the size of a human hair. They also have a much faster response time and are not affected by electromagnetic interference. Beter heat and corrosion resistance are added benefits.

How Fiber Optic Sensors Work

Fiber optic sensors consist of a core material, which is typically silica, and a stabilizing cladding material. As a laser light is transmitted through the core, temperature and strain can be measured by analyzing how light interacts with small defects in the fiber when these parameters change. The fiber is connected to a data acquisition system and a device called an interrogator, which measures the time or temperature dependent shift in the wavelength of the light that is reflected. 

Several different types of sensors will work in the demanding environments of foundries. Fiber Bragg Grating sensors, also known as quasi-distributed fiber optic sensors, use etched reflectors in the fiber to provide the sensing points and can handle temperatures up to 1800C. Rayleigh Scattering-based sensors use atomic-sized imperfections in the fiber itself to measure temperature or strain effects and can handle temperatures up to about 700C. The resolution of the Fiber Brag Grating sensors is 5 mm, meaning for each five mm across the fiber, you can get a temperature sensor or a strain sensor reading. The Rayleigh Scattering-based sensors offer considerably higher resolution (.5 mm). While most fibers are made from silica, steel casting requires the use of Sapphire fibers, specialized optical fibers made from single-crystal sapphire that can handle temperatures as high as 1800C.

Continuous Casting Applications Early testing instrumented copper molds used in continuous casting. Researchers embedded fiber sensors into grooves near the hot face of copper mold plates and emersed them into molten steels of different chemistries, simulating the industrial continuous casting process. The result: detailed heat maps that correlated strongly with shell thickness and heat transfer quality. 

“Comparing a surface scan to the actual temperature map, we see very good correlation between very good heat transfer and the thickness of the steel shell, said Bartlett. 

Bartlett stressed the importance of examining the effect of different types of steel chemistries on heat transfer though the mold. She explained that some steel grades are what are called “peritectic steels,” in which an uneven volume change during solidification causes the shell to buckle away from the mold and create an air gap, which disturbs the heat transfer. 

“There’s no solidification software or modeling tool that can accurately model that right now,” said Bartlett. “With this technology, the steel mills will be able to adjust their process parameters in real time to eliminate potential defects (like shell buckling) and improve efficiency.”

Another application in continuous casting includes embedding fiber optic sensors in holding vessels such as furnaces, ladles, and tundishes to provide real-time thermal maps of the refractory linings.  

Aluminum Casting Applications

During the AFS webinar, Bartlett explained several applications using fiber optic sensors in aluminum casting including sand casting, 3D-printed sand molds, investment casting, and die casting. For aluminum applications, researchers used Raleigh Scattering-based sensors made from silica. 

To measure temperature during aluminum solidification, researchers placed an optical fiber encased in a thin-walled 0.5-mm diameter stainless-steel tube laterally across a 60-mm-wide mold cavity and through the mold walls. By embedding the fiber inside the mold cavity, researchers were able to capture the 91 unique thermal analysis curves, “waterfall plots,” of the aluminum during solidification as a function of distance across the 60-mm mold cavity. This data can be used to improve solidification modeling software as well as predict how cooling rates affect the microstructure and ultimately the location specific mechanical properties of the casting. 

Another unique application for fiber optic sensors involved measuring filling velocity—a key parameter in gating design and metal cleanliness. By placing the fiber vertically in the mold and tracking temperature changes as metal fills the cavity, researchers can calculate fill velocity in real time. This is possible due to high sensing speed (approximately 200 Hz). According to Bartlett, velocity measurements can also be done with permanent molds. 

Improving Models for Mineral Sand Casting

Fiber optics are helping evaluate alternative sands as foundries move away from silica due to OSHA regulations. 
“The modeling software is not very good for mineral sands and 3D-printed sands,” said Bartlett. “The heat transfer is different than silica sand.” 

Working for Southern Cast Products, a steel foundry located in Jonesboro, Arkansas, Missouri S&T researchers embedded fiber optic sensors in two 3D-printed molds, one made of silica and another made of mineral sand and poured the same steel alloy. The company was then able to use the real-time temperature data to improve the accuracy of their solidification models and eliminate some of the issues they were having with casting using mineral sand. 

Investment Casting

Bartlett identified investment casting as a major opportunity for fiber optics. “Ninety percent of the defects in shell castings are the result of improper de-waxing,” she said. “It’s very hard to measure strain and temperature during the dewaxing process, and current technologies only provide localized point measurements. 

Working with AFS, researchers at Missouri S&T are embedding fiber optic sensors into the wax and the shell itself to better understand and model both the temperature and the strain that might lead to shell cracking. “This could be a cost-effective method which would allow for more accurate modeling of the process,” says Bartlett. It could also benefit lost foam casting, which faces similar measurement challenges.

Die Casting

In die casting, Bartlett believes there is an opportunity to use fiber optics to remediate thermal fatigue cracking. “The dies can cost up to a million dollars, and it’s expensive to repair and replace them,” she said. She suggests integrating fiber optics into 3D-printed dies to better monitor strain and temperature near the molten metal interface. This could reduce expensive repairs and extend tool life.

Implications for Foundry 4.0

Using fiber optics for temperature and strain measurements will play a critical role in advancing Foundry 4.0. It will provide the big data needed to help moderate, monitor, and make corrections to foundry processes in real time. The only challenge to expanding the use of fiber optics may be cost, not from the fiber optic sensors themselves, but from the cost of the interrogator—currently $80,000. “That’s the real bottleneck,” said Bartlett. To improve accessibility, Missouri S&T’s electrical engineering team is working to develop a low-cost (<$10,000) interrogator.

While Bartlett understands the industry is just getting started with foundry applications, she also thinks fiber optic sensors are here to stay, given the significant benefits they offer. “We’re getting very close to a commercial solution that we can supply to foundry producers,” said Bartlett.

Foundries should be able to make a strong business case for fiber optics because measuring distributed temperature and strain with increased accuracy will help reduce defects, improve simulation models and allow for in-situ monitoring of metal casting processes. 

Getting Started

While Missouri S&T researchers are affiliated with the Peaslee Steel Manufacturing Research Center, interested companies do not need to be a member of that group to engage the research team. Missouri S&T researchers work with private companies, associations such as AFS, the Department of Defense and other government agencies. The fiber optic research conducted for the PSMRC is available to all, but other projects are proprietary. 

To explore how fiber optics can benefit your foundry, contact Dr. Laura Bartlett at lnmkvf@mst.edu or (573) 341-4972.