Originally Published MPMN April 2007

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Welding Technology Fuses Nitinol to Stainless Steel

Shana Leonard

For three years, engineers at Creganna Medical Devices (Galway, Ireland; www.creganna.com) scratched their heads over how to join nitinol to stainless steel to produce a catheter that exhibited the performance of nitinol without the associated high costs. For the past year, the company has actually been developing the nitinol–to–stainless-steel-tube welding technology that provides the elusive answer to a question posed for years by manufacturers.

“They’re such dissimilar materials in terms of metallurgy,” says Creganna product manager Maura Leahy. “Whenever you read about nitinol and challenges in manufacturing, one of the issues is how difficult it is to weld to any sort of dissimilar type of material.”

Welding nitinol to stainless steels tends to be extremely difficult due to the formation of intermetallics in the weld zone, which results in brittle joints. As an alternative, nitinol can be joined to dissimilar metals by way of such mechanical processes as crimping or swaging, bonding, or heat shrinking. However, these operations do not have the strength of the join zones achieved by welding, according to the company. Creganna’s Fusion Technology incorporates a filler material in the join zone, thus avoiding the interference of intermetallics and ensuring strong joints.

“We’ve developed the capability, which has been a holy grail for design engineers for quite some time, to be able to put these two materials together,” adds Alan Crean, business developer for Creganna.

When designing a catheter, engineers often pass over nitinol in favor of a more economical material, despite its desirable properties for the application. However, Creganna’s joining technology may enable catheters to reap the benefits of the nickel-titanium alloy without the high cost.

In addition to its shape-memory characteristics, nitinol offers crush resistance, superelasticity, and flexibility. Because of these properties, the alloy is suited specifically for use in the distal section of the catheter, which is the end that first enters the patient’s body. Fusion Technology enables engineers to employ nitinol solely at the distal end of the catheter. Reducing materials costs, this design approach requires only 100 millimeters or so of nitinol out of the one or two meters of material that comprise the catheter, according to the company. The remaining proximal section of the device can consist of such cost-effective alloys as stainless steel, joined to the nitinol through the firm’s technology.

As the only company currently offering this capability, Creganna has remained tight-lipped as to how the dissimilar metals can be fused. However, the company does reveal that the welded material in the middle is a common medical-grade material, so it will not present any new challenges to OEMs. Fully biocompatible in a single-use implantable device, the technology offers pull forces ranging from 300 to 370 Mpa, joint strain ranging from 0.5 to 1.5%, and a joint hardness of 410 Hv maximum.

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