Laser machining

Details Laser machining Laser machining

1999-01-22

2004-02-17

Hail, III, Joseph J. (Department: 3723)

Abrading

Abrading process

Utilizing fluent abradant

C219S121840

Reexamination Certificate

active

06692337

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to laser cutting and machining and, in particular, the use of lasers to cut or machine thick materials or reflective materials such as aluminium and copper. The invention is especially beneficial in cutting or machining hard materials, such as titanium alloys, for example.
BACKGROUND OF THE INVENTION
Several laser cutting and machining processes are known as described below.
The thermal cracking process generates controlled fracture with brittle material such as ceramics and glass. Thermal stress is caused by the absorption of a laser beam and resultant rapid heat generation so that the material can be broken apart in a controlled manner.
The vaporisation process makes use of a focused laser beam to vaporise a material such as wood, copper, rubber or concrete so that the material can break apart in a controlled manner.
Photochemical ablation breaks the bonds between molecules of a material by high energy photons such as those produced by an Excimer laser which gives an ultraviolet laser beam. This process does not involve melting and vaporisation and accordingly little heat is generated. The process is mainly used for micro-machining.
Gas assisted laser cutting involves melting material with a laser beam and blowing the melted material away from the material with gas which is usually applied in a direction coaxial with the laser beam. This is the most efficient and widely used method for metal cutting. Oxygen is often used since additional energy can be produced due to the combustion of the material in the oxygen. Other gases such as nitrogen, argon, helium and carbon dioxide are also used for cutting materials where oxidation is not desired.
Water assisted laser cutting makes use of water instead of gas to blow away the laser generated melt in the cutting of materials.
GB-A-2230222 discloses a method and device for making filters by laser machining. The method makes use of a laser beam and a jet of a working gas. In addition, abrasive particles are introduced, in a periodically pulsed manner, into the jet of the working gas at the moment when the maximum amplitude of the melt metal volume oscillations in the cutting area is obtained. The abrasive action of the solid particles enables the formation of a “Y” shaped cut (drill) slot in the filter manufacturing. A rotating mechanism was used to direct the jet nozzle in the desired direction.
EP-A-129603 describes a laser machining process which uses in combination a laser beam and auxiliary energy (eg plasma).
FIG. 5
of EP-A-129603 illustrates an apparatus for performing a version of the process in which a halogen gas or gaseous halogen compound is used to discharge abrasive grains at the workpiece. The abrasive grains contact the workpiece in front of the laser beam. It is taught that the workpiece is effectively worked in a short time under the action of (i) the auxiliary energy (specifically, glow discharge), (ii) the thermal and chemical actions caused as a result of the halogen or halogen compound being radiated by the laser beam, and (iii) the abrasive grains, apparently as well as by the laser beam itself. This complex process involving chemically reactive species is performed within a closed chamber under a pressure of no more than 10 torr.
In prior art processes, the limitation of the thickness of cut and the cut quality, when using a laser, are largely dependent on the gas or water pressure which produce the required momentum to blow the molten material away from the cut zone. Higher laser power does not necessarily guarantee a deeper cut, if the cut quality to be achieved is similar to that with thinner material cutting. For the cutting of material of thick section, the cut quality cannot be maintained using existing methods due to reduced pressure and drag capability of the gas or water vapour at the deeper end of the cutting slot.
The process of EP-A-129603 requires a combination of many techniques, including indispensably a working gas to create chemically reactive species.
It is an object of the present invention to improve laser cutting quality and extend the thickness of the material which may be cut.
According to the present invention there is provided a method of treating an area of a material (eg a workpiece), comprising directing a laser beam onto said area and at the same time directing substantially continuously a stream of high speed solid particles onto said area or the vicinity thereof. The invention includes a method of cutting or machining a material by directing a laser beam into the cut zone and at the same time injecting substantially continuously a stream of high speed solid particles around and into said cut zone. In this way molten material is dragged or blasted away by the solid particles as a result of kinetic energy and momentum transfer. As an alternative to cutting the material, the method may machine it, eg mill it. The term “machining” includes in particular generating a groove, which in practice will be of a desired (predetermined) width and depth.
The present invention also provides apparatus for treating materials including means for performing the invention. The invention therefore includes an apparatus comprising means directing a laser beam into the cut zone and at the same time substantially continuously injecting high speed solid articles around or into said cut zone.
The solid particles should preferably be abrasive particles. Examples of particles which may be used are aluminium oxide (Al
2
O
3
), silicon carbide (SiC) or zirconium oxide (ZrO) as well as other solid particles such as glass beads or metallic powders. Such particles have a much higher density than the gases and water vapour which are currently used. They therefore provide higher momentum if they travel at a similar speed as the gases and water vapour.
Due to the high speed of the injective particles, their absorption of energy from the laser beam is small. To prevent the melting of the injective particles, it is preferred that the particles are formed of material or materials with a higher melting point than the material which is being cut.
Preferably, the size of the particles is in the range 1-250 &mgr;m and more preferably 1-150 &mgr;m (eg 1-50 &mgr;m), the particular size depending on the application. The bottom end point of these ranges is suitably 5 &mgr;m.
Preferably, the speed of travel of the particles prior to reaching the cut zone is in the range from 50-300 m/s, although higher speeds, including supersonic speeds, can be utilised. The speed of the particles is selected partly with a view to minimising the residue of the injected particles remaining on the workpiece at the end of the cutting process.
The particles can be accelerated and delivered to the cut zone by means of a gas stream, for instance, an air stream or water vapour or a suitable solution. As an alternative, particles charged with electricity may be injected making use of an electrostatic accelerator. If this method is used, discharge of the electric charge on the particles may be carried out before they are delivered to the cut zone. In one preferred class of methods, the particles are accelerated to form the particle stream by means which itself substantially does not work the material, such non-working means include compressed air or another effectively inert gas (eg nitrogen) or electric charge. An auxiliary energy source is not necessary.
Where a gas is used for particle delivery and acceleration, it may be a single gas such as oxygen, nitrogen, argon, helium or carbon dioxide or it may be a mixture of gases. Preferably the gas pressure is in the range from 0.5 to 30 bars.
When water vapour is used dust emission can be controlled so that a cleaner process results. It is not an essential feature of the invention that the method be performed in a closed space. The method may therefore be performed in an open atmosphere and the apparatus need not include a closure for separating the workpiece from the environment.
A preferred method of the invention comprises concurrently directing at a ma

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