Many project teams in the medical device industry undertake laser welding tasks within the broader scope of developing, or iterating, a medical device without formal training in welding or metallurgy. One of the common alloys welded in the medical device field is Nitinol because of its unique physical properties, including the shape memory and superelasticitic effects. Many medical device manufacturing teams laser weld Nitinol without understanding the importance of shielding it from the environment during the welding process. A Nitinol bead-on-tube fiber laser welding study was conducted to determine the deleterious effects of welding Nitinol without Argon shielding gas. Two populations of Nitinol samples were welded with a fiber laser. One with Argon shielding gas, and the other without. Samples from each population were subjected to tension testing, fractography, optical microscopy, and 2D nano-hardness testing.
A single-pass circumferential bead-on-tube, pulsed fiber laser weld, on two sample populations of Nitinol tubing was conducted. One population was welded without any protection from the environment, welded in air. The other population used Argon shielding gas on the OD and ID. Samples from each population were tension tested, followed by fractography, and samples from each population were cross-sectioned and examined with optical microscopy and tested for nano-hardness.
Results and discussion:
Optical microscopy showed two significant differences between the Ar shielded and unshielded welds. Although the laser welding parameters were kept the same between the two populations, greater penetration and larger grains in the fusion zone were noted on the welds made without Ar.
The mean peak load of the samples welded with Ar was more than 2 times greater than the mean for the group welded without Ar. The mean cross-head strain at break for the batch welded with Ar was 7 times greater than the mean for the batch welded without Ar. The mean energy-to-break for the population welded with Ar was 13.8 times greater than the mean for the population welded without Ar.
The mechanical sacrifice made when welding Nitinol without proper protection from the environment is significant. The loss of ultimate tensile strength is outstanding, but the loss of toughness and ductility is devastating, especially for a material that is sought out for its ability to absorb enormous amounts of strain without permanent damage.
Fractography of the tension tested samples showed distinct differences between the two welded populations. The fractures of the Nitinol welded with Argon showed indications of ductile failure, namely, cup and cone, and shear lips. Conversely, cracks and flat fracture features were observed in the samples welded without Argon.
For both populations nano-indentation showed softening in the fusion zone and an increase in hardness of the fusion zone near the OD surface, but this was more pronounced in the samples welded without Ar.
As a result of this work, the data provides the following conclusions when comparing Nitinol welded without Argon with Nitinol welded with Argon shielding gas.
1. Nitinol welded without Ar shielding gas has a 7X reduction in ductility as shown by comparing the cross-head strains at failure in tension.
2. Nitinol welded without Ar shielding gas has a 2.1X reduction in ultimate tensile strength.
3. Nitinol welded without Ar shielding gas has a 13.8X reduction in toughness as shown by comparing the energy, measured in in*lbf, at tension failure.
4. Both cases show a fusion zone that is softer when compared with the base material, and an increase in hardness near the surface of the weldment adjacent to the atmosphere. Nitinol welded without Ar shielding gas has a softer fusion zone but shows a higher hardness near the surface exposed to the atmosphere.
5. Nitinol welded without Ar has greater penetration and larger grains in the fusion zone, given the same laser parameters as Nitinol welded with Ar.