Investigation into the influence of pulse shaping on
drilling efficiency
P W French1, M Naeem2, M Sharp1and K G Watkins3
1Lairdside
Laser Engineering Centre, Campbeltown Rd, Birkenhead, CH41 9HP,
ENGLAND
2 GSI GROUP, Cosford Lane, Swift Valley, Rugby, CV21 1QN, ENGLAND
3 Laser Group, Department of Engineering, University of Liverpool, Brownlow
Street Liverpool, L69 3GH, ENGLAND
Abstract
Laser percussion drilling is now a well established material processing technique for
producing holes in certain aerospace components for boundary layer cooling. Percussion
drilling allows a components cycle time to be reduced compared with the other laser
drilling technique, laser trepanning. This is a very important factor as some components
can have up to 40,000 holes. One of the major factors that concern the aerospace
industry is the quality of the holes. Hole geometry such as taper, roundness and variation
between holes must be within certain limits if the component is going to be used in an
aero engine. Other important factors with respect to hole quality are metallurgical issues
concerning recast layer and micro cracking with in the hole. This paper investigates how
temporal pulse shaping and other factors such as focal position; gas pressure and gas
type can improve laser-drilled holes. The study uses laser percussion drilling technique
and four different pulse shapes, the square shaped pulse, the ramping up pulse, the
ramping down pulse and the pulse train. A sensor designed especially to measure the
drilling velocity was used to determine the drilling efficiency of each laser drilling pulse.
Interactions between the different pulse shapes and other important drilling parameters
were also examined and best-drilling practices is discussed.
1. Introduction
There are two basic techniques for producing holes within an aerospace component with a
laser, trepanning and percussion drilling. Trepanning is were the laser beam pierces the centre
of the hole and then moving to the holes circumference the laser beam or the component
rotates producing a hole. The second basic method called laser percussion drilling, here neither
laser beam nor component is moved but by firing a continual series of laser pulses a hole is
produced. The hole diameter is controlled by the amount of energy used in the drilling pulse.
Percussion drilling is a very important enabling technology within the aerospace industry as it
allows for the cycle times on a component to be reduced.
The issue of hole quality is very important but is a subjective one. The qualities of a hole
produced by laser drilling are judged on a number of different characteristics. The geometric
factors are hole roundness, hole taper and variation in hole entrance diameter. The
metallurgical factors are oxidation and recast layer. The recast layer, melted material that was
not ejected from the hole by vapour pressure generated by the laser pulse, coats the wall of the
hole leaving a thin layer of solidified metal. This layer can generate micro-cracks, which can
propagate into the parent material
2. Experimental Set-up
Using full factorial experimental design [1] the experiments were performed on a pulsed NdYAG JK704TR laser. The factors that were kept constant were the focal length of the lens at
120mm. The material, which were coupons of uncoated aerospace nickel based alloy, C263.
The thickness of the coupons were 3mm thick. The pulse shapes investigated in this study are
given in figure 1 with a list of other factors used in this investigation with their low and high
settings given in table 1. All of the coupons were drilled normal to the surface.
An experimental run consisted of 5 holes drilled using the factor settings taken from the
experimental design. After a design had been completed the entrance and exit diameters were
measured using an optical based measurement software and microscope. These values were
then used to calculate the response variables. These experiments were based on 5 holes for
the following reason. If more than 5 holes were drilled at low gas pressure setting then there
was a danger that the contamination of the cover slide by ejected melted material from the hole
would mask the affect of gas pressure on the response variable. The factor gas pressure would
be confounded with cover slide transmission quality.
1
2
3
4
Figure 1, schematic of the different pulse shapes. (1) Square shape pulse, (2) Ramping-up
Pulse, (3) Ramping-down pulse, (4) Pulse burst.
The JK704TR laser was mounted on a sturdy drilling stand with the samples held in a sample
holder placed in the laser beam at the focal position under study. The focus was found using the
Kapton film technique. All of the gas pressures were measured at the drilling nozzle assembly
Table 1. A list of other factors used in this study.
Factor
Pulse
Energy
Gas
Pressure
Gas Type
Peak
Power
Focal
Position
Lens
Type
Nozzle
Exit
Diameter
Low
Setting
10J
1 bar
Oxygen
14kW
Surface
Achromat
1.5mm
High
Setting
20J
5bar
Compressed 23kW
Air
+2.5 mm
Gradium
3.0mm
2.1 Response Variable Definitions
2.1.1 Hole Taper. Hole taper is an important factor that can influence the performance of an
aerospace component. The taper of the hole will affect the airflow over the surface of the
component, which in turn will affect the lifespan of a component. The definition of hole taper is
given in equation 1 and in figure 2.
Entrance
Diameter

Exit Diameter
Figure 2. Diagram showing hole taper definitions.
d1
 d2
 100 %
2 h
[1]
 100 tan 
Where:
d1 = Average entrance diameter.
d2 = Average exit diameter.
h = Thickness.
(mm)
(mm)
(mm)
2.1.2 Hole Roundness. The roundness of a hole is a parameter that will again affects the flow
characteristics of a component. Hole roundness is a measure of the entrance hole. The
definition of hole roundness is given below in equation 2.

1 



  100
d1 y 
d1
x
[2]
%
Where:
d 1 x = Average entrance diameter in the x axis. (mm)
d 1 y= Average entrance diameter in the y axis. (mm)
2.1.3 Hole Variance. The control of the variation in entrance hole diameter is an important
factor, as this will affect the flow characteristics of the component. Another good reason for the
control of entrance diameter variance is that two adjacent holes can encroach on each other. If
this was to happen with a number of holes in a particular row of a combustion ring, the
component may crack along the holes and the component would fail. The term for this failure is
the component has become unzipped. The hole variance is given by equation 3.
. s2  
d
1
j
 d1

2
j
n  1
[3]
Where:
d1
j
= the average hole diameter of the jth hole.
d = The average hole diameter for the set of holes drilled with the same conditions.
n = sample size.
In general the response variables of interest in laser percussion drilling are hole geometry,
for example variation in entrance hole diameter, taper and roundness, and the metallurgical
properties of the hole such as recast layer, micro cracking, oxidation and delamination for
coated materials. In this paper we limit ourselves to geometric aspects of the percussion-drilled
hole.
A full factorial designed experiment describes a series of experiments that when run will
identify which are the most important factors operating singly or interacting with others that
affect a response variable. The method is not explained here but the following reference gives
very good guidance on the subject [1].
3. Results and Discussion
The square shaped pulse has become the aerospace industries standard for laser drilling
components. In this study four different pulse shapes were investigated the square pulse, the
ramping–up pulse, the ramping–down pulse and the pulse burst, figure 1.
The hole geometry with respect to hole taper, entrance diameter variation and hole
roundness were the response variables of interest. As was stated in the section above other
factors were also ran during the factorial designs to see if there was any interactions between
pulse shape and these other factors.
3.1 Hole Taper
Taper is an important feature of hole quality, so it was selected as a response variable. It was
found that compared with the industry standard square pulse the pulse burst whether consisting
of 2 pulses or 4 pulses gave very little advantage. Examination of the hole entrance diameters
showed that the pulse burst drilling did not produce a smaller initial hole. The gaps between
pulses were 0.3 and 0.6ms. An experimental design looked at the effects of the ramping-up and
ramping-down pulse. During this study an interesting interaction between the factor pulse shape
and focal position was observed, figure 3.
Interaction Graph
Experimental design 21-7-98
Pulse Shape / Energy
Interaction Graph
Experiment Design 6-1-98
Focal Position/Pulse Shape
5.5
6
5
5.8
4.5
5.6
4
Taper %
Taper %
Pulse Shape (High)
Ramping-down
5.4
5.2
3.5
3
2.5
5
2
4.8
1.5
Pulse Shape (Low)
Ramping-up
4.6
Energy (High)
20 J
Energy (Low)
10 J
1
0.5
- 1
4.4
- 1
Focal position(Low)
Surface
- 0. 5
0
0. 5
Focal position(High)
+2.5 mm
1
Pulse Shape (Low)
Square
- 0. 5
0
0. 5
1
Pulse Shape (High)
Ramping-up
Figure 3. Pulse Shape-Focal Position interaction graph. Figure 4. Pulse Shape-Energy interaction graph.
The graph clearly shows that at the low setting for focal position, on the surface of the
sample the better taper was achieved with the ramping-up pulse, a value of 4.8% as opposed to
5.8% for the ramping-down pulse. At the high focal position there was very little difference
between the two pulse shapes, but assuming a linear response the ramping-up pulse gives a
more consistent low taper value compared to the ramping-down pulse. I believe that this result
can be understood in terms of the power density at the focused spot. The ramping-up pulse has
been tailored to take advantage of the drilling mechanics of melt expulsion. The initial part of the
pulse creates a melt but keeps below the plasma threshold. The second part of the ramping-up
pulse has a higher peak power, which creates a plasma and strong recoil pressure to help flush
the liquid melt from the hole. Across the focal position range from the surface to 2.5mm above
the sample this mechanism is active, the only difference being a scaling in power intensity at the
two different positions. The ramping-down pulse has its high peak power section of the pulse at
the beginning so the drilling mechanism starts in a vaporisation regime creating a recoil
pressure, which scours the holes entrance producing more taper. At the higher focal position the
spot size increases and the power density drops giving a similar performance to the ramping-up
pulse.
The ramping-up pulse was also compared with the square shaped pulse in another
experimental design. Here an interaction between pulse shape and pulse energy was observed,
figure 5. The better hole taper was observed at lower pulse energy, peak power value for the
two different pulse shapes. At the lower energy the ramping-up pulse gave a better performance
the taper being less than 1%. At the higher pulse energy the taper increases for both pulse
shapes, but again the ramping-up pulse produces less taper. The interaction graph clearly
shows that the range over which taper varies from low to high energy is smaller for the ramping
up pulse.
3.2. Hole Variance
As with the previous response variable the factor pulse shape showed an interaction with
focal position and pulse energy. Figure 6 shows the interaction graph for pulse shape and focal
position. The pulse shape in this particular study was a pulse burst consisting of two pulses with
a 0.3 ms separation. At the lower focal position the square shaped pulse gave the better
performance but over the range of focal position it showed the most variation. The drilling
mechanism between the two different focal positions is different [2]. A focal position below the
surface of the sample the energy partition is in favour thermal processes and hence produces
more melt. At a focal position above the sample the pulse energy partition gives the melt more
kinetic energy from the hole. This increase in kinetic energy for the square shaped pulse causes
increased scouring at the holes entrance and hence causes an increase in the variability of the
entrance diameter from hole to hole.
Interaction Graph
Experimental Design 5-8-97
Pulse Shape / Focal Position
Interaction Graph
Experimental Design 2-7-98
Pulse Shape / Energy
0.06
0.021
0.02
0.019
Pulse Shape (High)
Double Pulse
0.05
Hole Variance mm
Hole Variance mm
0.055
0.045
0.04
- 0. 5
0
0.016
0.015
Pulse Shape (Low)
Square
0.013
0.03
- 1
0.017
0.014
Pulse Shape (Low)
Square
0.035
Pulse Shape (High)
Ramping-up
0.018
0. 5
1
0.012
- 1
Focal Position (low)
-0.5 mm
Focal Position (High)
+2.5 mm
Energy (Low)
10 J
- 0. 5
0
0. 5
1
Energy (High)
20 J
Figure 6. Pulse Shape-Focal Position interaction graph. Figure 7. Pulse Shape-Energy interaction graph.
Figure 7 shows the interaction graph for pulse shape and pulse energy. For a low energy
pulse, the square pulse gives a good drilling performance therefore when drilling small holes a
square shaped pulse will give good hole variation. As the pulse energy is increased however to
produce larger diameter holes the control over the entrance diameter variation is lost. The graph
shows that the ramping-up pulse gives better overall control in the variation of the entrance
diameter.
3.3. Hole Roundness
With respect to hole roundness one interactions was observed. Not surprisingly this interaction
involved gas dynamics. Figure 8 shows the interaction between pulse shape and gas pressure.
Interaction Graph
Experimental Design 6-8-97
Pulse Shape / Gas Pressure
4.5
Hole Roundness %
4
Gas Pressure (Low)
1 Bar
3.5
3
2.5
2
Gas Pressure (High)
2 Bar
1.5
- 1
- 0. 5
Pulse Shape (Low)
Square
0
0. 5
1
Pulse Shape (High)
Double Pulse
Figure 8. Pulse Shape-Focal Position interaction graph.
The graph shows that at the lower gas pressure there is a greater variation in the roundness
of the holes. This may be down to melt ejection dynamics. At the lower pressure individual pulse
shapes ejection mechanisms are more pronounced. At higher gas pressures the melt dynamics
of the pulse shapes are less sensitive and in a sense are overwritten by the gas flow dynamics.
This may also include shockwave formation and shockwave instability, which have been
observed by the author. This shockwave instability that develop in a drilling set-up are
dependent on nozzle stand-off and gas pressure.
4. Conclusion
Temporal shaping of a percussion drilling pulse has been shown to improve hole quality. Pulse
shaping allows for the control of geometric aspects of hole quality. In terms of hole taper the
ramping-up pulse gave the best performance showing that it is immune to focal position change
above the surface of the sample. The ramping-up pulse also showed that in the pulse energy
range of 10 – 20 J, taper only changed by less than 2%. The ramping-up pulse also gave a
consistent performance with respect to entrance diameter variation for the same range in pulse
energy.
With respect to hole roundness the evidence for better hole quality was not as strong, but
during the course of these experiments there was an indication that the entrance hole diameter
in one of the axis was greater than the other. This discrepancy in the distribution of hole
diameter readings may be due to a generic design feature of solid state laser cavities but further
experimentation is need to investigate this proposition.
5. References
[1] G.E.P. Box, W.G. Hunter, J.S. Hunter. Statistics for Experimenters. ISBN 0-471-09315-7
Wiley & Sons Inc. ISBN 0-471-38897-3
[2] M.Bass, M.A.Nassar, R.T.Swimm, Impulse coupling to aluminium resulting from Nd:glass
laser irradiation induced material removal. Journal of Applied Physics 61 (3) 1987 American
Institute of Physics. Pages 1137 – 1144.