Lasers are an excellent method of attaching two materials together for two reasons: first that the heat added to the material can be very carefully controlled and second that the laser adds no contaminants to the melt zone. Controlling the heat input when welding nitinol is extremely important. This is partly because the restructuring caused by heat can and does dramatically alter the properties of the nitinol—usually in an undesirable way. Also, the melted nitinol is very brittle, so minimizing the amount of melt maximizes the elasticity retained in your system. Minimizing contamination in the melt zone is also extremely important because even trace amounts of contaminants cause severe embrittlement. Other welding technologies introduce contaminants from the electrode being used to strike the arc. Light, however, carries no contaminants so it does not contribute to the embrittlement of the melt zone.
Pulsed Wave vs. Continuous Wave
Lasers can generally be divided into two types based on how the laser light is delivered. The first is pulsed wave, which delivers a brief pulse of light, and the second is continuous wave, which delivers a continuous beam of light. Each of these options have strengths and weaknesses. While pulsed wave is great at spot welding, substantial time must be invested to optimize the waveform (the variation of beam intensity over the course of time). Continuous wave lasers produce excellent line welds, but lack the level of control over the melt pool that spot welders can have. For both research and production work, we have both pulsed wave and continuous wave lasers.
The earliest industrial lasers built are CO2 lasers. Despite being the oldest laser technology, these lasers still maintain the highest beam quality and beam power available. The downside is that CO2 lasers are generally limited to two axis control, greatly limiting the types of work that can be welded. We often use CO2 lasers for very high strength attachments in both research and production settings. Nd:YAG lasers, commonly referred to as YAG lasers, have good beam quality and are somewhat easier to control than CO2 lasers. For this reason, we use YAG lasers for prototyping work in the laboratory setting. Fiber lasers have a centralized light source and the beam is transmitted via optical fibers to the point of welding. Since fiber optics are quite flexible, complex robots can be used to control the beam, dramatically opening up the field of possible applications. Fiber lasers have the lowest beam quality of the three technologies, but the increased control capabilities often offsets this weakness.
Filler material is added to the melt pool to fill gaps and add additional strength when needed. While filler material often is not needed, when it is used, a wide variety of filler materials can be used. By changing the metallurgy of the filler material, the behavior of the melted zone can be controlled. This being said, filler material is another source of contaminants, so if your weld can be made without filler, it is often better to do so.