Page 31 - MetalForming April 2020

ond range (10-9 sec. or a billionth of a second). But, these lasers can be designed to produce pulse lengths in the picosecond (10-12 sec.) or even fem- tosecond (10-15 sec.) range.
Generally, shorter pulse lengths deliver a smaller HAZ. Furthermore, the impact of decreased pulse width generally is much more dramatic than that of decreased wavelength for a cou- ple of reasons.
First, nanosecond pulse lengths tend to remove material by thermal means— heating the material until it boils off. But, at shorter pulse lengths (especially below 1 picosec.), another mechanism called photoablation starts to occur.
In photoablation, the use of very short pulse lengths produces very high peak fluences that can directly break the molecular or atomic bonds holding the material together, rather than sim- ply heating it. Plus, with the material exposed to the laser energy for such a short time, the energy can’t be carried beyond the area of impact, thus the
surrounding area stays cold. Energy remaining after the bond-breaking process is carried away with the expelled particles. Together, these effects result in an inherently colder process with a significantly reduced HAZ (Figs. 2 and 3). Plus, as a very clean process, photoablation leaves no recast material, thereby eliminating the need for elaborate post-processing.
But, making DPSS lasers operate at shorter pulse lengths (called ultrashort pulse, or USP) requires more system complexity, and, therefore, greater cost. Thus, cost/part versus feature necessity should be weighed. With this in mind, USP lasers generally are reserved for the most demanding tasks where pre- cision is absolutely critical. USP lasers also enable machining of traditionally difficult materials such as glass, ceram- ics, sapphire and diamond.
Help With Particular Applications
Most manufacturers seek to achieve
RAMPING
the most cost-effective process. Besides laser purchase and operating costs, many other factors play into this, including throughput, scrap and rework rates, and consumables costs (e.g., assist gas). It’s a relatively large parameter space to explore, making assistance from the laser supplier or systems integrator a practical matter. Specifically, it can be helpful to partner with a supplier for some applications development, particularly on the work- piece material. Ideally, the supplier should have a variety of different laser sources available in order to determine a best choice for a given application. In the best-case scenario, the supplier may even develop an optimum process recipe for the specific application. MF
Author’s Note: An excellent treatment of this topic can be found in Funda- mentals of Laser MicroMachining, a book by Ronald Schaeffer, Ph.D. The author wishes to thank Dr. Schaeffer for his assistance in the preparation of this article.
Fabrication: Laser Cutting
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