Forming metal that 'remembers'

Strips of nitinol fabricated by Memry spring back to their "remembered" shape.

Dennis Norwich revels speaking at his children's school, where he hands students bent wires thinner than paperclips and challenges the kids to twist them into another shape—and they can't. The wires bend easily enough, but no matter how hard the children try, they let go and the wires spring back to their original shape.

For eight years the engineer has immersed himself in a material that, relative to conventional steel alloys, is a metallurgical baby. Norwich is process engineering manager for Memry Corp., a high-tech manufacturer that makes its money drawing, cutting, and forming a material called nitinol. The alloy—a concoction of nickel, titanium, and trace elements—has become the most popular in a group of materials called shape-memory alloys. They seemingly "remember" their shape and spring back to it after stress is applied.

Memry has built a business around nitinol products, mainly for customers in the medical arena. It's a difficult, highly specialized field that managers at Memry are betting will grow. Today nitinol makes some of the most complex medical operations just a little easier. Tomorrow it may make a lot of everyday chores easier too.

Shape-memory Basics

Nitinol had a serendipitous discovery. In the 1960s researchers at the Naval Ordnance Laboratory in White Oak, Md., were testing a certain material making up a bowl. As the story goes, the bowl was dropped and dented. Researchers picked it up and placed it on top of a heat-treating furnace, then went on to something else that required heat treating.

"They came in the next morning, and [the bowl's] dent was gone," Norwich said, adding that the government lab's acronym served as part of the new alloy's name: Ni for nickel, ti for titanium, and nol for Naval Ordnance Laboratory.

The most common nitinol formulation has 55.8 percent nickel, with the balance being titanium and a mixture of trace elements. "It's a very precisely defined alloy, and it's a very critical mix," Norwich explained. "If you alter the weight ratio by even half a percent, you drastically alter the material properties."

Nitinol twins, meaning under stress it undergoes a phase change from austenite to martensite. As Norwich described it, "When you bend it up to about a 6 to 8 percent strain, rather than going through dislocations and permanent deformation like a piece of steel might do, you get a phase change and the restructuring of the crystals in the material. And it naturally reverses. You relieve the stress, and it will revert back to its austenitic form."

The material deforms in the martensitic form, springs back to its "remembered" shape in its austenitic form, and all nitinol varieties shift from one to the other. The temperature at which the phase shift occurs determines what type of nitinol it is and what it's used for. Superelastic nitinol springs back to its original shape if bent at room temperature. When cooled, it shifts to its martensitic form and, hence, may be bent without springing back. When heated back to room temperature, the phase shift occurs, making the material snap back to its original shape. This happens because superelastic nitinol's phase transformation occurs below room temperature.

Thermal shape-memory nitinol's phase transformation, on the other hand, happens above room temperature. This means that at room temperature, the material can be bent without springing back to its original form. When heated it undergoes phase transformation from martensite to austenite, snapping the material back to its original shape.

"You can tinker with the weight percentage of the nickel to titanium and change [the temperature at which] that transformation point occurs," Norwich said.

Click image to view largerNitinol is a twinning material, shifting phases between austenite and martensite.

Applications for nitinol go back decades. Old cell phones had antennas made of the material; no matter how much they were bent, they always returned to their original shape. Modern applications include antiscald valves on high-end faucets. If the water temperature gets too hot, the nitinol valve closes.

But it is in the medical device industry where nitinol truly has found a home, and today such companies make up about 80 percent of Memry's customer base, Norwich said. Laparoscopic, or minimally invasive, surgery using endoscopes and other devices has spurred demand for tools that fit through small spaces and then expand into their working form. Stents—tiny tubes inserted into arteries, veins, and other areas—are a perfect example. A nitinol stent is superelastic at body temperature. Compressed during insertion, it then springs to its original shape once deployed. Traditional stainless steel stents can't do this, so once they're deployed surgeons must use a small balloon mechanism to expand them.

"With a nitinol stent, you deploy it and it springs back to its original shape," Norwich explained. "There's one step."

Forming Challenges

Nitinol brings manufacturers into new territories of forming. The material can't be just placed into a forming machine and be bent into a new form. The key to forming is locking the material into a new structure, and to do that requires heat treating at high temperatures, usually around 500 degrees C. The company uses a variety of heat-treating methods, including conventional furnace heating as well as induction and salt-pot heat treating.

"When you heat-treat," Norwich said, "you relieve the stress [in the material], and it will restructure." After forming comes quenching, which quickly removes the heat and locks it into a new austenitic form that the material remembers.

Memry works with extremely small parts. Its largest tubes are just 1⁄4 inch OD, its smallest ones go down to an OD of about 0.012 in., and there's nothing easy about forming any of them.

Norwich used a small tube, formed to a 90-degree bend over a ½-in. radius, to explain. The company forms it using a custom press setup with an upper and lower clamshell. The system grabs and guides the tube into a custom die to form it, but none of this happens without prior heat treatment. These tubes can't go through a salt-pot process, a quick and consistent heat-treat of only a few minutes, because the granules can't get into such small tube IDs. So it's put through a conventional furnace, which adds variables (how long the door is open, the fixture mass inside the furnace, etc.).

Heat treating is so important because, as Norwich explained, it actually can change the material's chemical ratio. "The nickel-titanium ratio is so critical. If you heat-treat for a very long time, you start to precipitate out nickel." This can be used to an advantage, he said, because removing nickel actually raises the transition-point temperature. "This is where trial and error comes in," he said. Engineers must determine how long to keep the tube in the furnace and at what temperature. The length and intensity of heat treating directly affect the temperature at which the final product's phase transformation will occur, be it below or above room temperature.

Because temperature is so important, it's controlled throughout forming. The tubes are bent with hot forming using a custom hard die with cartridge heaters to control the temperature in-process. The tube is then quenched immediately. "If you let it cool slowly at room temperature, you start to get hydrogen embrittlement," Norwich said.

This brings up another challenge. "It's a killer on tooling," he said. "You could get these great hard alloys [to use as tooling] that would last forever, but as soon as you go from 500 degrees C to a room temperature quench bucket, the tools don't hold up so well. Some may ask why we don't use something like 303 stainless, which could handle the temperature changes. Sure it will, but it just corrodes, and the surface starts to get rough. And we can't have a rough surface, because these are implantable medical devices. The surfaces have to be smooth and pristine."

Click image to view larger Most of Memry's products are extremely small and include a variety of medical instruments, many of them designed for a single use.

The material is incredibly tough, and because of this Memry uses carbide tools on blanking presses and forming machines. "Everything has to be carbide, and the carbide does not hold up. The material is very difficult to machine, to cut, and to shear. Even when we're cutting small, 0.020-inch-diameter wires, it just tears up the tooling."

Norwich added that the company is conducting some proprietary R&D to overcome these issues, and the shop avoids hard tooling for limited applications. For certain milling work, for instance, the company uses a laser machining center when possible to avoid tearing up carbide cutters. Regardless, carbide tools and dies still dominate the shop's forming operations.

Drawing and Cutting

For the first 12 years after its founding in 1983, the company had fewer than 20 employees. Today it has about 350. Obviously, some growth happened in between.

One milestone occurred in 1996 when Memry acquired Raychem Corp.'s shape-memory business in Menlo Park, Calif., between San Francisco and San Jose. Today that West Coast operation specializes in laser cutting along with wire and tube drawing, while the East Coast office concentrates on forming and other value-added operations.

"Essentially, everything we do here is nickel-titanium," said Neal Webb, manufacturing engineering manager who oversees the company's drawing operations at Menlo Park. "You get a lot more strain hardening than you do for something like copper, so the amount of reduction you can get before you have to reanneal is considerably less."

And because drawing induces stress, phase transformations must also be taken into account. "The shape-memory properties are involved in almost everything we do [with the material]," he said.

Six years ago Memry's West Coast operation delved into laser cutting, and like any other nitinol fabricating process, it has its share of challenges. According to Nitin Sharma, new product development manager, the company cuts tube sections for stents with wall thicknesses from 0.005 in. to 0.013 in., and the smallest part geometries are in the micro realm. The operation uses a range of Nd:YAGs, as well as a green laser and fiber laser, to make these precision cuts.

The greatest challenge cutting is localized heating, Sharma said, adding that if a laser overheats the metal, the melted material resolidifies as a weld.

While he didn't reveal the details about the company's processes, he did say that operators "need to keep the material cool when running it. So coolant is used [during the process], and we need to be sure this doesn't change the thermal properties of the material.

"We use [conventional] technology," he added, "but it's a very different application of that technology. You need to have people who not only know lasers, but they need to know how to cut nitinol."

Just the Beginning

In late September Memry shareholders got some news. The company, previously listed on the American Stock Exchange, became a wholly owned subsidiary of SAES Getters, an Italian company that got its start manufacturing getters, an element used in vacuum environments such as cathode-ray television tubes. Since then SAES has diversified into various high-tech fields, including shape-memory alloys.

The acquisition came on the heels of some serious growth at Memry. Between fiscal 2004 and 2005, company revenues grew more than 30 percent after a November 2004 acquisition of Putnam Plastics Co., a polymer extrusion company.

"We were buying a lot of [Putnam's] products for use in our products," Norwich said, adding that the company also provided another revenue stream, so the move made sense from both a manufacturing and business perspective.

Norwich sees growth not only in the medical fields, but in the commercial sector as well. "The growth in the medical device industry will continue, of course. But like everything, prices have to come down eventually. I think you'll see more commercial products on the horizon, and you'll start seeing higher-volume applications."

He added that nitinol remains relatively new, and as awareness of the shape-memory alloy grows, so will applications for it.

"It's been around 40 years," he said, "but it's still a baby."