POWER May 2010 - 12

Laser Hole-Shaping
Improves Combustion
Turbine Efficiency
Cylindrical holes were once the industry
standard for air film cooling on turbine
engine components. The cooling holes
were traditionally formed using an electro-discharge
machine (EDM) with a round
electrode that slowly eroded the component
alloy away until a complete hole was
formed. The task was time-consuming and
costly, because a typical combustion turbine
blade required hundreds of holes.
As coatings technology progressed,
turbine manufacturers added a ceramic
thermal barrier coating (TBC) to protect
blades as the pursuit of higher operating
efficiencies pushed up gas temperatures
some years ago. Unfortunately, that made
EDM shaping through a TBC coating applied
to blades and vanes impossible. One
major limitation of the EDM process is
that it requires a surface material to be
conductive, because the process requires
an electrical circuit path to function.
The process of applying shaped holes on
turbine airfoils then evolved into a twostep
machining process sandwiched around
the application of a TBC. Before a ceramic
nonconductive coating could be applied,
cylindrical airfoil holes were drilled with a
laser. Laser drilling focuses a high-energy
beam of coherent monochromatic light at
the surface of the airfoil to melt the alloy.
Next, the external TBC was applied to
the surface of the turbine components
to protect the alloy. However, the laserdrilled
holes could be blocked if overcoated,
thus reducing the efficiency of the
intended blade cooling design. Normally,
a mathematical formula was used to determine
hole coatdown based on hole size
and amount of coating thickness. The final
application of the TBC also created yet another
airflow disturbance step.
In the second machining step, a specially
shaped EDM electrode was superimposed
in each laser hole to produce a
round metering hole with a diffuser shape
at the surface. This diffuser shape (unlike
the traditional round exit hole) spreads
the exiting airflow and promotes more
uniform surface cooling effect. Because it
was a two-step process, there was a possibility
for many hole-to-shape mismatches
that impede the smooth transition of the
airflow. The entire process was time-consuming,
and rework was often required to
12
1. Full service. Today's advanced laser
drill and shape tools can produce a completed
blade by drilling through the ceramic thermal
barrier coating applied earlier. The earlier
process required the ceramic coating to be
applied after the laser drill and ship, causing
quality problems. Courtesy: Chromalloy
2. Laser hole shaping. The laser can
produce shaped and directional holes to manage
the cooling air flowing through the blade.
This blade has completed its laser drilling and
shaping and is ready for a final quality check
before shipping. The white portion of the blade
is the ceramic thermal barrier coating that protects
the blade so that it can operate at higher
temperatures. Courtesy: Chromalloy
produce a blade that met specification. At
the time, this process technology helped
to usher in the next generation of advanced
turbine engine components.
New Laser Technology
Speeds Drilling
Today, most turbine components require
some form of nonconductive TBC, thus,
final dimension shaping through the TBC
is made possible by the many new developments
in laser shaping technology. And
this laser shaping technology is not limited
by the application of coatings prior
to the hole drilling or shaping process. Laser
shaping technology has evolved from
a two-step process into a single process
that drills and shapes holes through a
TBC, bond coat, and airfoil base metal to
create a finished product (Figure 1).
This one-step process drills hole openings
shaped like a trapezoid with the
downstream portion of each hole opening
at the airfoil surface, either flaring or widening
relative to the narrower upstream
portion of each hole opening. This design
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allows the cooling air to be distributed
completely over the convex and concave
surfaces of the airfoil or platform. The increasing
cross-sectional area of the cooling
hole opening functions as a diffuser
to reduce the velocity of the cooling airstreams
exiting the holes. Lower-velocity
airstreams are more inclined to cling to
the blade surface than to separate and reduce
the effectiveness of the film cooling
of the component surface.
Laser Drilling and Shaping
The latest laser shaping processes also allow
for much more complex heat transfer
designs to be added to the surface of a
blade or other component, also in a single
step. All of the cooling holes and shape
segments are formed with a single drill
and shape process, seamlessly producing
complex diffuser shapes that previously
required multiple processes. The shape,
also perfectly superimposed over the hole,
creates the uniform airflow over the airfoil
surface, as the heat transfer engineer
originally intended. Another advantage:
POWER | May 2010
http://www.powermag.com
POWER May 2010

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