Femtosecond Lasers for Use in Additive Manufacturing

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“All I ask for is one simple request, and that is to have sharks with frickin’ laser beams attached to their heads!” (Credit: Dr. Evil, Austin Powers Goldmember). Perhaps, if Dr. Evil was an Additive Engineer, he may have rephrased as such: “All I ask for is one simple request, and that is to have Additive Machines with frickin’ femtosecond lasers attached to their optic systems!”  Pretty cheesy, but you get the gist.

Femtosecond lasers have been used for decades in micro-machining to achieve machining with nearly zero thermal stresses and precise dimensional tolerance. Due to the short pulse width and high energy of a femtosecond laser, thermal stresses are kept local and do not deform surrounding areas of the metal.

Representative thermal picture of femtosecond laser (fs) vs current nanolaser (ns) in an additive machine

PolarOnyx, an additive manufacturing company based out of San Jose, California, has created a first-of-its-kind femtosecond laser-based additive system. Traditional DMLM (Direct Metal Laser Melting) machines use what’s known as CW or Continuous Wave lasers. These lasers, although ideal for low-temperature parts such as aluminum and titanium, have shown to have challenges with higher-temperature materials such as tungsten and iron. Higher-temperature materials require quite a bit more energy to bond metal particles together as compared to their lower temperature counterparts. Due to this increase in energy, CW lasers must output much more laser power. However, their pulse duration (i.e. how long the laser stays on) does not change. Thus, surrounding metal particles are affected and what are known as “thermal stresses” are built into the part itself.

Samples of tungsten parts on tungsten substrates with various shapes and density. The gear has a 1/2-in. diameter (left), while the thin wall (right) has a thickness of 100 µm. [Image: PolarOnyx]

In comparison, femtosecond lasers with their much shorter pulse duration are able to instantaneously ionize and bond the metal particles together with nearly zero thermal stresses. By being able to instantaneously melt particles together, femtosecond lasers also have an innate advantage in building denser parts.

Additionally, PolarOnyx was also able to successfully print iron powder directly on glass. Iron and glass have different but very close melting temperatures. With traditional CW lasers, the thermal buildup would have caused the glass to crack. However, with the femtosecond laser process and its ability to quickly fuse the iron powder, the iron was able to melt without causing any damage to the glass substrate.

Iron and Glass Property Comparison

Most interesting is PolarOnyx’s vision for a process whereby both the additive and subtractive properties of femtosecond lasers are integrated into one machine. Although metal additive has come a long way, there are still cases where complex features must be machined post-print. With femtosecond lasers, this task could be done all in one process with one machine. The femtosecond lasers could first additively build a layer, followed by a subtractive ablation of the same layer where tight machining dimensional tolerances are required.

If only Dr. Evil would have known about femtolasers, he may have been more specific in his request!

 

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