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A D V A N C E D M A T E R I A L S & P R O C E S S E S | J A N U A R Y 2 0 1 6

3 0

NEWPROCESS JOINSNITINOL

TOSTAINLESS STEEL

A new solid-state joining process for medical guidewire applications

increases joint strength, provides superior bending properties,

and does not require tertiary metals or ferrules.

Pankaj Gupta,* Arne Rimmereide, and Roger Dickenson, Lake Region Medical, Chaska, Minn.

A

guidewire isamedical deviceused

in various minimally invasive vas-

cular applications. Its foundation

is a metal core wire, typically constructed

of stainless steel or Nitinol. A metal coil,

polymer jacket, or combination of the

two covers the core wire on the distal end

in order to make the tip atraumatic, kink

resistant, and flexible.

Designing a guidewire is an in-

tricate exercise in balancing strength

and flexibility. For example, a guide-

wire with a spring-tempered stainless

steel core has good pushability and

torque transmission due to its high yield

strength and Young’s modulus. These

properties are important in order to nav-

igate to the desired treatment sites and

deliver the desired clinical therapy. How-

ever, exceeding the yield strength of the

material in a bending mode results in

permanent bends and kinks, which se-

verely reduces guidewire performance.

Nitinol is a superelastic material pro-

viding great kink resistance, but it lacks

pushability due to an inherently lower

Young’s modulus, which results in less

support in delivering therapies or devic-

es. Ideally, a guidewire core combines

the excellent mechanical properties of

stainless steel in the main body, with the

kink resistance of Nitinol at the distal tip.

A bimetal medical guidewire with

a stainless steel proximal section and

Nitinol distal section enhances perfor-

mance compared to guidewires made

of either alloy alone. However, standard

fusion welding of Nitinol (NiTi) to stain-

less steel (SS) is challenging because it

causes brittle intermetallic Fe-Ti to form,

leading to unpredictable brittle joints. To

avoid this, current joining methods use

either a transition section made of a ter-

tiary metal or a ferrule joining process.

JOINING OPTIONS

Metallurgically, joining Nitinol

to stainless steel via fusion welding is

problematic due to the formation of

brittle Fe-Ti intermetallics

[1,2]

, as pre-

viously mentioned. One method of

avoiding brittle intermetallics is to use

a tertiary metal, such as Nickel, when

joining the stainless steel to Nitinol

[3]

,

but this adds cost and complexity to the

design and can degrade performance.

Solid-state processes such as fric-

tion welding

[4]

, explosive welding

[5]

, and

ultrasonic welding

[6]

can also be used

to join dissimilar metals while avoiding

the formation of brittle intermetallics

in the joint. Another method used in

guidewire applications is to insert the

ends of the stainless steel and Nitinol

into a ferrule (a section of hypotube)

and then secure both ends using ad-

hesive or solder. This method requires

preprocessing to reduce the diameter

at the ends of each core in order to fit

the parts together, which adds cost

and complexity. Further, this decrease

in core diameter, along with the stiffer

section of hypotube, and the addition

of joint material, creates a kink point

and reduces clinical performance.

An alternative proprietary solid-

state butt joining process for Nitinol and

stainless steel wires ranging in diame-

ter from 0.013 to 0.020 in. that does not

require tertiary metals or ferrules was

developed by researchers at Lake Re-

gion Medical (LRM). The resulting joint

strength is approximately 80%of the ten-

sile strength of the raw Nitinol wire with

excellent bending properties. Complete

0.014-in. outer diameter guidewires were

built using solid-state weld technology,

tested, and compared to a competitor’s

product with a hypotube joint design.

The solid-state weld joint’s metallurgical

characteristics aswell as data fromguide-

wire functional tests are presented here.

EXPERIMENTAL PROCEDURES

Solid-state weld joints were creat-

ed using pre-straightened superelastic

binaryNitinol (54.5%-57.0%Ni) and 304v

spring-tempered stainless steel wires

with subsequent evaluation of joint

strength, durability, and microstructure.

Parts went through preconditioning

by cycling the joint 10 times through a

U-bend fixture with a 0.10-in. radius, pri-

or to obtaining tensile strength data by

pulling the joint to failure using an MTS

testing system. Joint microstructures

were examined using standard metallo-

graphic methods of polishing and etch-

ing the longitudinal joint sections. In

addition, optical microscopy and scan-

ning electron microscopy (SEM) con-

firmed overall joint quality. Energy dis-

persive spectroscopy (EDS) analysis on

the cross-section determined the weld

zone length with intermixed Nitinol and

stainless steel.

A grinding study was conduct-

ed on the solid-state welded bimetal

joints using 0.018-in. stainless steel to

0.020-in. Nitinol wires. This allowed

*Member of ASM International; now at St. Jude Medical