For many years, solid state lasers have been used successfully in machining processes such as drilling cooling holes in jet engine components. Nevertheless, industry demands improvements in the quality of machined features. More specifically, the drilled holes have to be more consistent in geometry, and should exhibit minimal recast layers and heat-affected zones. The application of hard-to-machine materials makes laser drilling even more challenging. Nanosecond laser pulses with a short wavelength are highly suitable for processing of various types of material [1, 2], mainly as a result of a higher absorption by the material and a lower absorption by the plume [3, 4]. To achieve the highest processing speed and to control the quality of the drilled holes, it is essential to achieve a fundamental understanding of the underlying drilling process. In the laser drilling process, material is removed by vaporisation and by melt ejection [5, 6]. The latter is the expulsion of molten material, driven by the recoil pressure generated by vaporisation of the material. Experimental studies about material removal during drilling and about the transparency of the plume can be found in literature for two applications. One application is evaporation of material for vapour deposition of thin metal and dielectric films [7]. In this case melt droplets have to be avoided. The other application is drilling, in which melt ejection is helpful because the energy required for melting is significantly less than for vaporisation [8]. However, melt ejection during drilling of metals with an excimer laser is not well understood for lack of detailed experimental investigations. Excimer lasers with a pulse length of 20-30 ns have been used only occasionally for the applications as studied in this paper.

1.
D.
Bäuerle
.
Laser Processing and Chemistry
.
Springer
, 3rd edition,
2000
.
2.
J.F.
Ready
and
D.F.
Farson
.
LIA Handbook of Laser Materials Processing
.
Laser Institute of America
,
2001
.
3.
H.
Schittenhelm
,
G.
Callies
,
A.
Straub
,
P.
Berger
, and
H.
Hügel
.
Measurements of wavelength-dependent transmission in excimer laser-induced plasma plumes and their interpretation
.
J. Phys. D: Appl. Phys.
,
31
:
418
427
,
1998
.
4.
D.
Breitling
,
H.
Schittenhelm
,
P.
Berger
,
F.
Dausinger
, and
H.
Hügel
.
Shadowgraphic and interferometric investigations on Nd:YAG laser-induced vapor/plasma plumes for different processing wavelengths
.
Appl. Phys. A
,
69
:
S505
S508
,
1999
.
5.
A.
Schoonderbeek
,
C.A.
Biesheuvel
,
R.M.
Hofstra
,
K.-J.
Boller
, and
J.
Meijer
.
The influence of the pulse length on the drilling of metals with an excimer laser
.
Journal of Laser Applications
,
16
(
2
):
85
91
,
2004
.
6.
A.
Schoonderbeek
,
C.A.
Biesheuvel
,
R.M.
Hofstra
,
K.-J.
Boller
, and
J.
Meijer
.
Shadowgraphic imaging of material removal during laser drilling with a long pulse excimer laser
.
Appl. Phys. A
, online first,
2004
.
7.
G.
Koren
,
R.J.
Baseman
,
A.
Gupta
,
M.I.
Lutwyche
, and
R.B.
Laibowitz
.
Particulates reduction in laser-ablated YBa2Cu3O7-delta thin films by laser induced plume heating
.
Appl. Phys. Lett.
,
56
(
21
):
2144
2146
,
1990
.
8.
K.T.
Voisey
,
W.
Rodden
, and
T.W. D.
Hand
.
Melt ejection characteristics during laser drilling of metals.
In Proceedings of ICALEO 2001
,
2001
.
9.
J.H.
Yoo
,
S.H.
Jeong
,
R.
Greif
, and
R.E.
Russo
.
Explosive change in crater properties during high power nanosecond laser ablation of silicon
.
J. Appl. Phys.
,
88
(
3
):
1638
1649
,
2000
.
10.
H.
Schittenhelm
,
G.
Callies
,
P.
Berger
, and
H.
Hügel
.
Investigations of extinction coefficients during excimer laser ablation and their interpretation in terms of rayleigh scattering
.
J. Phys. D: Appl. Phys.
,
29
:
1564
1575
,
1996
.
11.
A.
Bejan
.
Heat transfer
.
John Wiley and Sons, Inc
.,
1993
.
12.
D.F.
de Lange
,
S.
Postma
, and
J.
Meijer
.
Modelling and observation of laser welding: The effect of latent heat.
In Proceedings of ICALEO 2003
, volume Section C, pages
154
162
,
Jacksonville FL, October
2003
.
13.
A.
Kar
,
T.
Rockstroh
, and
J.
Mazumder
.
Two-dimensional model for laser-induced materials damage: Effects of assist gas and multiple reflections inside the cavity
.
J. Appl. Phys.
,
6
:
2560
2569
,
1992
.
14.
M.
Von Allmen
and
A.
Blatter
.
Laser-Beam Interactions with Materials
.
Springer
, 2nd edition,
1995
.
15.
Comsol AB
.
Manuals FemLab 2.2 & 2.3
,
2001
.
16.
D.
Breitling
,
A.
Ruf
,
P.W.
Berger
,
F.H.
Dausinger
,
S.M.
Klimentov
,
P.A.
Pivovarov
,
T.V.
Kononenko
, and
V.I.
Konvov
.
Plasma effects during ablation and drilling using pulsed solid-state lasers.
In Proceedings of Laser Processing of Advanced Materials and Laser Microtechnologies
, volume
5221
, pages
24
33
,
2003
.
17.
F.
Dausinger
,
T.
Abeln
,
D.
Breitling
,
J.
Radtke
,
V.I.
Konov
,
S.V.
Garnov
,
S.M.
Klimentov
,
T.V.
Kononenko
, and
O.
Tsarkova
.
Drilling of ceramics with short-pulsed solid-state lasers
.
LaserOpto
,
31
:
78
85
,
1999
.
18.
K.H.
Song
and
X.
Xu
.
Mechanisms of absorption in pulsed excimer laser-induced plasma
.
Appl. Phys. A
,
65
:
477
485
,
1997
.
This content is only available via PDF.
You do not currently have access to this content.