Nitinol Laser Welding and Joining for Medical Device Assemblies

Nitinol laser welding is the step where many device assemblies quietly go wrong. The alloy welds to itself reasonably well. It joins to almost everything else badly. Therefore joint strategy belongs in your design review, not in your process development phase.

This article covers what works, what fails, and what an OEM should specify. It assumes you are buying the assembly rather than developing the process yourself.

Why nitinol is difficult to weld

Three problems show up again and again.

Oxygen pickup. Titanium loves oxygen. A weld pool exposed to air becomes brittle. Consequently shielding is not optional.

Brittle intermetallics. Melting nitinol against dissimilar metals forms hard, brittle phases. Those phases crack under the strains a superelastic device sees every day.

Lost function. The heat-affected zone changes the transformation temperature. A weld can therefore leave a device that is superelastic in one place and stiff in another.

What nitinol laser welding does well

Laser welding concentrates energy in a very small volume. As a result, the heat-affected zone stays narrow and the surrounding material keeps its properties. That is the whole reason the technique dominates here.

It suits:

• Nitinol-to-nitinol butt joints in wire and tube.
• Tacking small features onto a formed component.
• Sealing and rounding wire ends.
• Joints in tight assemblies where a mechanical fitting will not fit.

It suits fine work because the beam can be positioned to a few microns and pulsed precisely.

Joining nitinol to stainless steel

This is the hard case, and it appears in most guidewire and delivery system designs. Iron and titanium form brittle compounds in the melt. A direct fusion joint therefore tends to be weak and unpredictable.

Three approaches work better:

• Use an interlayer. A nickel or niobium interlayer keeps iron and titanium apart in the melt.
• Offset the beam. Aiming slightly into the stainless side limits how much titanium enters the pool.
• Avoid fusion entirely. Crimping, swaging or an adhesive joint is often stronger and far more repeatable.

We usually recommend the third option first. A well-designed mechanical joint is easier to validate, easier to inspect, and easier to transfer between sites.

Controlling the heat-affected zone

Good nitinol laser welding is mostly heat management. The parameters that matter are pulse energy, pulse duration, spot size, focus position and shielding gas flow. We lock those per joint and then monitor them.

Fixturing matters just as much. Heat sinking through the fixture pulls energy out of the part quickly. Consequently the transformation temperature shifts less, and the device behaves the same either side of the weld.

Where a joint sits close to a shape set feature, we verify the transformation temperature after welding rather than before. ASTM F2004 gives the DSC method for that measurement.

Inspection and acceptance

A weld that looks good can still fail in service. We therefore test rather than admire:

• Pull testing to a stated minimum load, on a sampling plan.
• Metallography on setup parts, to see the fusion zone and any cracking.
• Transformation temperature checks either side of the joint.
• Fatigue testing where the device flexes in use.
• Corrosion review, because welds and their oxides are a common initiation site.

Welded assemblies also need finishing. Weld oxide must come off, which is why electropolishing normally follows welding rather than preceding it.

What to put in the specification

Write down the things you will actually reject a lot for:

• Minimum joint strength, with the test method.
• Maximum acceptable heat-affected zone length.
• Allowed discolouration, or none at all.
• Whether an interlayer is permitted, and which one.
• Post-weld finishing requirements.

Vague wording such as “weld to be sound and free from defects” gives you nothing to enforce. Clear numbers protect both sides.

Questions worth asking a supplier

Ask whether nitinol laser welding is done in house or subcontracted. Ask to see a weld cross-section from a comparable part. Ask how parameters are locked, who can change them, and how a change is recorded. Then ask what their scrap rate looks like on that joint, and whether they will share it.

Suppliers who cannot answer the last question usually have not measured it.

Our approach

We keep joining, forming and finishing under one roof and one ISO 13485 system, as part of our full nitinol processing route. As a result, nobody can blame the previous operation, because there is no handover to blame.

If your assembly involves a nitinol joint that has been giving you trouble, send us the drawing. We will tell you honestly whether we would weld it, or design the weld out.