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Abstract
An overview is given of the state of the art of laser beam machining in general with special emphasis on applications of short and
ultrashort lasers. In laser welding the trend is to apply optical sensors for process control. Laser surface treatment is mostly used to apply
corrosion and wear resistant layers, but also for repair of engine and machine parts. In micro-machining, shorter pulses reduce heat-affected
damage of the material and opens new ways for nanometer accuracy. Even 40 years after the development of the laser there is a lot of
effort in developing new and better performing lasers. The driving force is higher accuracy at reasonable cost, which is realised by compact
systems delivering short laser pulses of high beam quality. Another trend is the shift towards shorter wavelengths, which are better absorbed
by the material and which allows smaller feature sizes to be produced. Examples of new products, which became possible by this technique,
are given. The trends in miniaturisation as predicted by Moore and Taniguchi are expected to continue over the next decade too thanks to
short and ultrashort laser machining techniques.
Introduction
Photons are in this century. They are replacing electrons
as the favourite tool in modern industry. Light is used for
everything from eye surgery to telephone technology and
materials processing. Photons are applied in an increasing
number of topics addressed by this ISEM 14. An important
property of light is that it has no volume, photons have no
charge, so when concentrated into a very small space, they
do not repulse each other like negative charged electrons do.
This is an important property especial for ultrashort machining.
Light moves through space as a wave, but when it encounters
matter it behaves like a particle of energy, a photon.
Not all photons have the same amount of energy. The visible
part of the spectrum contains wavelengths from 400 to
750 nm. Radiation below 400 nm includes the harmful frequencies
of UV and X-rays, while above 750 nm the invisible
infrared, microwave and radio frequencies are included.
The energy of photons is E = hν. For the visible 500 nm
wavelength this is 4 × 10−19 J or 2.5 eV per photon, which
is not enough to break the chemical bonds in the material,
which requires 3–10 eV. In the laser materials processing
this can be overcome in different ways. The first solution is simply heating the material by absorption of laser energy,
which is a thermal or pyrolytic process. Secondly higher
energy photons (UV) can be used with photon energies of
3–7 eV, which is used to break the chemical bonds directly
(especially plastics). This is a photolytic process. For metals
even more energy is required (up to five times the sublimation
energy of about 4 eV for most metals). The third
option is using lasers that deliver so many photons on a time
that electrons are hit by several photons simultaneously. Absorption
of multi-photons has the same result as single high
energetic photons. In this case the photon energy, thus the
wavelength, is less important because energy is transferred
by multi-photons simultaneously. This is a reason that such
lasers are preferable operated in the visible part of the spectrum
with relatively simple optics. In this paper, some comments
are made about developments in welding and surface
treatment and deals in more detail with the challenges of
applications using short and ultrashort pulsed lasers.
2. Laser welding
After the maturation of laser cutting also welding is becoming
everyday technology. For 10 years people said ‘we
can weld any material, as long as it is stainless steel’. Now
indeed almost all materials can be laser welded. Challenges are found in the area of beam and product handling. In the
high volume market, e.g. in the electronics industry special
machines are built around one product for just a few welds
(Fig. 1). In such applications, the required accuracy for laser
welding is built-in in the machine.
The same is seen in welding of tailor made blanks, which
become more and more popular in car manufacturing. It is
the production of blanks of steel used to press sheet metal
parts like doors, hoods, suspensions, etc. The sheet elements
are chosen of different thickness or different strength to meet
the requirements of the composed part. Here the welds are
mostly linear while the accuracy is obtained by the design
of the machine and the clamping tools. In this area there is
trend to inspect each weld by CCD cameras directly after
welding and even to observe the seam before in order to
correct for instance for uneven cutting edges. In automotive
applications, in special in bodywork, it is hard to meet the
tolerances. This problem is often solved by the construction,
using overlap seams, which are more faults tolerant. The
effects of some errors in are shown in Fig. 2. Gaps should be
restricted to 10–20% of the sheet thickness. The process is
more tolerant for displacement tolerances. Aluminium is less
sensitive for adjustment errors, probably due to the better
heat conductivity.
A new area is welding of dissimilar materials like steel and
aluminium, which is known to be impossible in conventional
welding. Fig. 3 shows an overlap weld of a 1 mm thick
steel plate to aluminium. For a good wettability the material should be free from oxides before welding and the speed and
temperature should be balanced to reduce the intermetallic
layer (less than 5 m). It is shown that the aluminium flows
nicely over the steel. This type of welds are of growing
interest in shipbuilding and automotive applications.
The high welding speeds in laser welding, e.g. 100 mm/s
and the small tolerances require a high degree of automation.
In particular robots are applied more and more. Developments
are ongoing on real time process sensing and
control as well as on-line quality inspection. Postma et al.
[1] have developed a system for measuring and control the
welding process by sensing the plume radiation optimising
the welding speed. Such a systems can be fully integrated
in laser-robot welding systems with the sensor measuring
through the same optical fibre as used for beam delivery