Whenever possible, it is best to make nitinol components without any welds. This is because, no matter how hard we try, nitinol welds are more brittle than the surrounding materials and, therefore, prone to failure. In applications where the cost of a broken weld is high—such as a surgical implant—it is highly advisable to never use a weld. However, sometimes, there is no other choice than to weld the nitinol to itself or another material. In that case, you must eat crow and accept the weld. Let’s take a look at some of the different welding technologies and the pros and cons associated with them.
Preparation: Surface preparation of nitinol is critical to obtaining a high quality weld. Ideally, the material should be electropolished prior to welding and then stored in an oxygen free environment until welding occurs. If this is not feasible, then a strong wire brushing until the surface is satin followed by cleaning with acetone and wiping with a lint free cloth will yield acceptable results.
Shielding Gas: For optimum results, nitinol should be welded in a vacuum. If a vacuum isn’t available, then pure argon should be used as a shielding gas. Typically, a flow rate 2-3 times that used to weld steel is recommended. Also, the shielding gas must continue to flow until the nitinol has cooled until it stops glowing. As with welding titanium, shielding gas is required on the back side of the weld.
Filler Material: For nitinol:nitinol bonds, we have found that the best results come from using no filler material. When bonding nitinol to other materials, filler material selection can get very tricky—especially for welds that have high intermetallic content.
- MIG welding: We MIG welded nitinol to prove that it can be done. If that statement doesn’t immediately tell you anything: don’t try it, it won’t give you good results.
- TIG welding: Nitinol TIG welds similar to titanium and the weld quality can be evaluated in a manner similar to titanium. The ideal weld color is straw colored and a blue weld is likely to fail. TIG welding is generally reserved for larger (>1mm) materials. It is important to use the least amount of heat possible when TIG welding.
- Pulsed Wave (PW) laser: PW laser is best suited for spot welding butt joints. The major advantage of the PW laser of continuous wave (CW) laser is that the waveform can be carefully controlled for nearly perfect joints. Microfractures are almost always inherent in a nitinol weld. However, with PW laser, you can adjust the waveform so that the nitinol cools slowly, resulting in a crack free weld. It should be noted that, while overlapping welds is required to get the strongest possible weld, the overlap should be limited to 30% by area of each successive spot.
- Continuous Wave (CW) laser: The major downside to PW laser is that there is always an air gap between the welds. CW produces a perfect, air tight seal with the smallest possible heat affected zone. With only two variables (speed and intensity), the amount of experimentation necessary for a good outcome is minimal.
- Diffusion Bonding: Diffusion bonding produces an atomically perfect seal between the two pieces that were attached—even between dissimilar metals. Since the material is never melted, the attachment doesn’t have the brittleness inherent to other technologies. However, the high temperatures associated with diffusion bonding tend to minimize the shape memory and superelastic properties of nitinol.