Figure
7-1 : A slot machined in INVAR with nanosecond pulses.
This
sheet of INVAR was micromachined with a long-pulse laser.
It should be compared with the following picture, where
the same material was machined with an ultrafast laser.
The sample is heavily contaminated. We tried, unsuccessfully,
to clean this sample with a mild jet of dry nitrogen.
Figure
7-2: Invar sheet machined with an ultrafast laser This sheet
of INVAR was micromachined with a Clark-MXR ultrafast workstation.
It should be compared with the previous picture, where the
same material was machined with a long-pulse (10 nsec) laser.
The femtosecond-processed sample was cleaned with a mild
jet of dry nitrogen (just like the nanosecond-processed
sample). In conventional, (i.e. long-pulse), laser machining,
large amount of debris are created during the machining
process. The debris, whose form depends on the material
being machined, can be very difficult to remove.
A large heat affected zone completely surrounds the work
area. A heavy recast layer is present immediately along
the edges of the slot. Outside the recast layer, an extended
zone of debris (droplets of molten metal) is visible. This
debris was still extremely hot when it landed on the surface.
Removing this material will require substantial post processing
efforts, if it can be done at all without damaging the surface.
The situation is quite different when working with femtosecond
lasers, as shown below.
With
the ultrafast pulses very little debris was generated during
the micromachining process, and what remains is not in the
form of hot droplets that attached to the surface. Rather,
the femtosecond process creates a fine dust that does not
carry much heat, and therefore does not bind to the surface.
It
should be noted that techniques have been developed to reduce
the amount of debris created when machining with long-pulse
lasers. Using powerful gas jets one can considerably reduce
the amount of debris created, or one can push the debris
far away from the work zone before re-deposition. A significant
amount of work has gone in this debris mitigating effort.
Users have developed jets with various geometries. Various
gases have been used, including pure oxygen for rapid burn
of the debris, etc.
These
approaches can, in some cases, almost totally eliminate
contamination of the sample by debris. (This of course does
not alleviate the other problems associated with heat diffusion
as encountered in long-pulse machining.)
