Above: The Vaporized Foil Actuator pushes two pieces of dissimilar metal together at speeds exceeding Mach 2. The bond is often stronger than the parent metals.
New welding technique joins temperamental high-strength alloys without changing their base properties
April 2017 - “This looks like a job for Superman.” Automakers are calling on advanced high-strength steels (AHSS) to help them tackle multi-material vehicle designs and lightweighting challenges. And for good reason. The sustainable metals pair exceptional strength with reduced mass, are affordable, crash resistant and can add muscle to thin-gauge or fatigue-sensitive parts.
Unlike the Man of Steel, who gets his abilities from the Earth’s sun, AHSS grades derive their special powers from chemical compositions and multiphase microstructures. But the delicate architecture also makes the super materials vulnerable to their own form of kryptonite: welding.
Dr. Glenn Daehn, an Ohio State University professor of materials science and engineering, explains why. “These microstructures are highly engineered for strength,” he says. “Yet the very mechanical properties designed into these materials—such as yield strength, tensile strength, fatigue strength and crack resistance—can be reduced by as much as 50 percent when exposed to the heat-affected zones of spot welding.”
To counteract this problem, Daehn—teamed with research scientist Anupam Vivek and a group of engineers—developed a new technique called Vaporized Foil Actuator Welding (VFAW). The method uses a high-voltage capacitor bank (which can store electrical energy at thousands of volts) to generate a short electrical pulse that is used to vaporize a small section of wafer-thin aluminum foil with explosive force. In microseconds, a burst of gas expanding at speeds exceeding Mach 2 (1,534 miles an hour) pushes the two pieces of metal together to create a bond stronger than each base metal.
A stronger bond
“The basic phenomenon of electrically vaporized conductors isn’t new, but we were the first ones to use it for metalworking purposes in the way that we do, especially for impact welding,” says Vivek, the primary inventor of the welding technique. The platform technology for VFAW was developed by Daehn’s research group, Impulse Manufacturing Laboratory (IML) part of Ohio State’s Department of Materials Science and Engineering. Vivek joined the department in 2007 as a graduate student and was then hired as a researcher. Daehn’s team holds several patents for its work in impulse manufacturing, including techniques based on VFAW, laser shock and electromagnetic induced forces, much of it for joining and forming applications.
“We’ve worked in the impulse manufacturing space for more than 25 years,” says Daehn. “Explosive welding dates back some 50 years. The process uses a controlled explosion to blast one piece of metal into another metal part at high velocity to create a bond. We figured out how to miniaturize the process, control the event, exchange a massive detonation for a minuscule amount of metallic vapor and perform the procedure safely in a factory environment.”
The research team celebrates successful low-energy, high-strength aluminum-steel bonding with an automated system. From left, under-graduate researcher Brian Thurston, Daehn, Vivek and graduate student Yu Mao.
Although the foil strips must be replaced with each weld, the item is an inexpensive consumable. Because VFAW bonds metal without melting it, the technique will help the industry overcome a major challenge associated with developing lightweight, cost-effective parts.
“Different metals have different melting points, which is one of the reasons joining these materials is so problematic,” explains Vivek. “VFAW can be used to weld a wide range of disparate metals with nearly 100 percent joint efficiency, which means we can design based on reliable base metal properties instead of joint strength. This will allow designers to optimize use of materials with a high strength-to-weight ratio like aluminum, magnesium and AHSS in the body of a vehicle where it makes the most sense.”
The introduction of VFAW is timely because the appetite for new grades of very high strength AHSS and the need to weld dissimilar materials has continued to grow. The Steel Market Development Institute reported last year on a FutureSteelVehicle project that unveiled 19 new grades of AHSS and achieved a 29 percent reduction in mass. The next generation of cars is expected to be multi-material—making the ability to join dissimilar materials like AHSS, magnesium, carbon fiber composites and aluminum more critical than ever.
Vivek explains how VFAW stacks up against other welding methods. “Minimizing the creation of brittle intermetallic compounds, while welding aluminum and steel, has been a major hurdle for any fusion-based welding technology, be it resistance spot or metal inert gas welding,” he says. “This has prompted the industry to look at solid state welding technologies such as friction stir, ultrasonic and impact welding, all of which have pros and cons.”
VFAW uses a high-voltage capacitor bank to generate a short pulse that vaporizes a small section of aluminum foil with explosive force, fusing dissimilar pieces of metal.
Friction stir welding requires high clamping pressures and creates a softened thermomechanically affected zone (TMAZ), he says, while ultrasonic welding is best suited for joining thin sheets.
“Impact welding has been traditionally practiced with explosives, making the method difficult to scale down and automate. Impact welding technologies, like Magnetic Pulse welding, can be used for smaller scale applications but have serious limitations in terms of the driving pressures that can be repeatedly produced, tool longevity and material combinations that can be joined,” Vivek says. VFAW, on the other hand, “is capable of welding anything that explosives can weld, but in a much more civilized manner.”
Solving a puzzle
Daehn breaks it down further. “We chose to start with the automotive market because a single vehicle can require thousands of welds. The cost has to be low. High-end custom military and aerospace fabricators are challenged to join wildly dissimilar material such as niobium and titanium. It’s a puzzle that has to be solved.”
[ABOVE] Impact weld between titanium and copper develops joint strength from fluid-shear instability similar to the Kelvin-Helmholtz instability principle that produces waves in clouds [BELOW].
Fasteners pose another issue. “A typical sedan has 3,000 spot welds, an SUV could have up to 5,000 spot welds or rivets,” Daehn continues.
“Fasteners aren’t the best option because they add weight and they are an extra consumable that can act as a source for corrosion. If you don’t weaken the joint region up front by softening, loss of thickness, or corrosion, a design engineer’s problems become easier because he or she doesn’t have to worry about joint strength.”
A Customer Discovery Program called I-Corps@Ohio also helped the team fine-tune its’ focus. More than 100 customers in the automotive industry were asked about the types of challenges they face with the push toward lightweighting. The need to have a flexible and robust technology capable of joining different types and gages of material without reducing their base properties emerged as the top contender.
Grants have helped
Daehn, Vivek and their colleagues have some muscle behind them to answer that call. In 2013, the team received a Department of Energy (DOE) grant to study “Breakthrough Technologies for Dissimilar Material Joining.”
In 2015, they received a Technology Validation grant from the state of Ohio to build a VFAW prototype, including a welding head and power source, to demonstrate the technology’s versatility across industries.
In October 2016, the team received a $2.7 million grant from DOE’s Vehicle Technologies Office with a $300,000 cost share for a total of $3 million. Grant monies are being used to further develop VFAW as a production-ready technology. The group also partnered with Honda Motors Manufacturing, Coldwater Machine Co., OSU’s Center for Design and Manufacturing Excellence (CDME), Alcoa (now Arconic), Ashland Pacific Northwest Laboratory, Cosma International and Fontana Corrosion Center to help scale up the technology and investigate costs associated with making car parts out of aluminum and steel versus an all-steel component.
Through Ohio’s Edison Advanced Manufacturing Program, a $500,000 grant was issued in 2016 to cover design/build of three near-production grade VFAW systems. CDME will produce the capacitor banks. Coldwater Machine Co., will manufacture the output welding cells. Based in Coldwater, Ohio, the company manufactures and integrates assembly automation and specialty machines for such industries as automotive, aerospace, energy and appliance.
Coldwater Machine President Tim McCaughey and his team began the design process in December 2016 to build three near-production ready VFAW systems to attract early adopters.
An Edison Welding Institute member and part of LIFT (Lightweighting Innovations for Tomorrow), the company also researches high-strength, lightweight steels and the joining of dissimilar aluminum alloys.
“We have extensive experience in providing friction welding solutions,” says Coldwater Machine President Tim McCaughey. “In the last decade, we have focused our attention on the automotive industry’s drive to lightweight vehicles and increase the use of aluminum, magnesium and composites.”
The company developed a SpotMeld system to join aluminum to aluminum and aluminum to magnesium. Coldwater contacted Ohio State to perform some testing and met the VFAW team.
“Their option had the potential to allow us to join different aluminum alloys with boron steel, something our system couldn’t do,” McCaughey says. “We saw we could help each other and joined forces.”
With the VFAW team at Ohio State acting as the technology provider, Coldwater has become the manufacturing partner. “The system they have at the lab is able to conceptually demonstrate the VFAW process,” says McCaughey. “It was a basic machine that allowed them to prove their concept and their process. They were able to make welds for customers for testing purposes.”
Calling all manufacturers
Coldwater will build a near-production-ready unit that can be demonstrated for early adopters.
“They will be able to see firsthand how a production-ready machine would look and operate,” says McCaughey. “Our goal is to build a system that is replicable and competitive. We will also assist them in taking the technology to the open marketplace.”
Design work on the three machines began in December 2016. Safety was a primary objective. Coldwater engineers also considered screen displays, programming options and how the machine will provide real-time feedback to the operator. Coldwater expects to complete the first of three prototypes in April.
Of the three near-production-ready systems, one machine will be installed at OSU, one will remain with Coldwater and the third machine will be delivered to the Tri-Rivers Careers Center in Marion, Ohio, to train welders on how to operate it.
VFAW joins 1⁄4 in. aluminum to 1 in. steel. Destructive testing revealed failure in the parent material rather than the weld.
Daehn and Vivek are also seeking out manufacturers in the automotive, aerospace, appliance and power generation industries to adopt the technology for prototype work, “even if it is in the form of a semi-automated pedestal system,” according to Daehn.
In collaboration with CDME and Coldwater Machine, the Impulse Manufacturing Lab is building fully automated welding heads that can be mounted on the end of a robot. This feature expands the flexibility of this joining technology.
Coldwater will build production-ready machines for customers when they order them. “Initially with the VFAW unit, we may start with standard sizes,” and custom units would come later, says McCaughey.
In a quest to turn ideas into valuable industrial applications, the VFAW team traveled far in the last few years.
“Working with DOE, designing and building equipment is different from our primary roles,” Daehn acknowledges. “VFAW has the potential to reduce the use of fasteners by as much as 70 percent. It consumes 80 percent less energy than resistance spot welding and creates a bond that is 50 percent stronger.”
For the new kid in town, those are pretty good numbers. FFJ