Technical Field

The invention relates to a laser apparatus comprising a laser means for directing a light beam through a first nozzle towards a workpiece to be machined, said nozzle having an orifice, through which the light beam and a first gas jet may pass and a further orifice for a further gas jet, both orifices possibly being supplied with gas from the same chamber.

Background Art

From the article "Dual Gas- Jet Laser Cutting of Thick Stainless Steels" in "Proceedings of LAMP 92, Nagaoka (June 1992) a dual gas-jet cutting technique is known employing coaxial and off-axial oxygen jet for cutting various types of stainless steel of a plate thickness of up to 6.4 mm. Good edge qualities have been obtained at cutting speeds up to 100 cm per min.

Disclosure of the invention

The object of the invention is to provide an even better machining quality without reducing the machining speed.

According to the invention, a laser apparatus of the type stated in the introduction is characterised in that the orifices are so closely interspaced that the flows merges, whereby a even better machining quality than hitherto is obtained , utilizing the fact that flows from the closely interspaced orifices merge so that mterferring gasses cannot reach the area of machining.

European publication No. 430.234 discloses a laser nozzle provided with several

orifices for outflowing gas. However, said orifices are not arranged sufficiently close for the flows to merge.

Moreover, according to the invention the jet orifices may form an acute angle with each other. As a result, it is ensured that the flows merges.

Furthermore, according to the invention the directions of the flows may form an angle of approximately 45° with each other.

Moreover, according to the invention, the interspace between the orifices may be less than the diameter of each of the orifices.

Brief Description of the Drawings

The invention is explained in details below with reference to the drawings, in which

Fig. 1 illustrates a known laser apparatus having a nozzle with two orifices,

Fig. 2 is an illustration of the interaction between the gas jet and d e erosion front,

Fig. 3 shows a laser apparatus according to the invention,

Fig. ^' 4 is a bottom view of the nozzle for the laser apparatus of Fig. 3,

Fig. 5 is another embodiment thereof, and

Fig. 6 is a top view of the nozzle of Fig. 5.

Best Mode for Carrying out the Invention

The laser apparatus shown in Fig. 1 comprises a laser means for directing a light

beam through a nozzle. The light beam is focused by means of a lens 1, before it reaches the upper face of a workpiece 3 to be machined. The nozzle is provided with an orifice 4, through which the ligth beam and a gas jet may pass, and a further orifice 5 for a further gas jet, said cutting gasses may for instance be O ₂ nitrogen or argon, depending on the material to be machined. The workpiece 3 may for instance be a plate of stainless steel.

A laser cutting technique employing a co-axial and an off-axial oxygen jet has been developed to cut various types of stainless steel with a thickness of 6.35 mm. Good egde qualities have been achieved at cutting speeds up to 100 cm per min. by the use of a CO ₂ laser of 1.2 kW. The conventional laser cutting method with single coaxial gas jet under the same process conditions does not produce a "dross-free" cut edge, even if the speed is reduced to a few cm per minute.

A model based on the momentum transfer of gas jets and thermodynamics of chemical reactions at the erosion front has been used to explain the effectiveness of the new technique in cutting thick plates of stainless steel.

Lasers have been developed for cutting, welding, drilling and heat treatment. Laser cutting has proved particularly advantageous. Due to its non-contact and thermal process characteristics, laser cutting provides an exceptionally good convertibility to CAD/CAM environments and allows precision machining without the use of hard tooling and complex fixturings. Moreover, the laser cutting speed may be increased, if a coaxial oxygen jet is arranged to assist in the cutting operation by means of a release of combustion heat. For industrial use, oxygen-assisted laser cutting is limited to machining of carbon steel with a thickness of approximately 9.5 mm and stainless steel plates with a thickness of 2 to 3 mm, as the edge quality otherwise deteriorates significantly.

The problems in connection with laser cutting of stainless steel may be attributed to the formation of chromium oxide dross clinging to the lowermost edge during

cutting and making deburring difficult. Chromium oxide has a high melting point (2300°C) and a high viscosity, which neutralizes oxygen diffusion in the molten layer of the erosion front. To overcome said problems, a high gas pressure of approximately 150 psi is used to obtain a clean cutting edge when machining 2 to 3 mm thick plates of stainless steel and superalloys.

A high pressure laser has the drawback that any shock effects may damage the optics.

Two techniques have been suggested; viz. pile cutting and tandem nozzle cutting for obtaining dross-free machining of 2 mm stainless steel of the type AISI 304. It was concluded that dilution of chromium oxide by means of excess iron oxide results in an increase in the fludity of the molten layers at pile cutting, while an off-axial gas jet at tandem nozzle cutting renders a dynamic force effectively removing the molten metallic oxides. The applicable thickness of mild steel at pile cutting is limited at the use of this technique, which can only be used for machining of thin stainless steel. An off-axial gas jet appears to be promissing, but has not been studied in detail for cutting of thicker sections. The focus has been on dual gas jet laser cutting of various types of stainless steel to obtain a faster cutting of thick sections with dross-free edge quality. The dynamics of gas flow and the combustion mechanisms in connection with dual gas jet laser cutting have been studied.

The materials studied are commercially available hot-rolled stainless steel of the type AISI 304, 410, 430, and 440 C with a thickness of 6.35 mm. The composition of the materials is stated in table 1.

Table 1 Chemical compositions of selected types of stainless steel

Composition Material Cr% Ni% C% (max) Fe% Type

AISI 304 19 9 0.08 balance austenitic

AISI 410 12.5 - 0.15 martensitic

AISI 430 17 - 0.12 ferritic

AISI 440C 17 - 1.07 martensitic

Table 2 Optimized conditions for dual gas jet laser cutting

Impinging angle Coaxial jet Off-axial jet Jet targeting pressure pressure positon

38° ~ 40° 0.15 MPa 0.31 MPa — 1/3 thickness (21 psi) (45 psi) from the the top

A CO ₂ laser of 1.2 kW was used. The laser beam showed a nearly Gaussian energy distribution, before it was focused by means of a ZnSe lens to a spot diameter of 0.1 mm. The setup is shown in Fig. 1. The optimum process parametres in dual gas jet laser cutting of AISI 304 stainless steel with respect to the pressure of the coaxial and the off-axial nozzle, the diameter of the off-axial nozzle, the impinging angle and the tageting position have been studied previously. The same process parametres were used to study the cutting of AISI 410, 430, and 440 C stainless steel plates. Table 2 illustrates the process parametres of dual gas jet laser cutting. The cutting faces were examined by means of an electron microscope to determine the quality and any combustion droplets.

The theory of homgeneous jets and characteristics of impinging gas jet have been discussed in the technical litterature. In general, the initial flow area is limited to a length of about four to six orifice diametres along the axis. Since the configuration and set-up of the gas nozzles according to the invention with regards to the erosion front is inside the initial flow area, it may be assumed that the gas jet velocity at the erosion front is identical to the gas jet velocity at the orifice and has a uniform velocity profile in radial direction.

Fig. 2 shows, how the gas flows interacts with the erosion front at dual gas jet laser cutting. Based on an analysis of a control volume, the momentum and the energy balance in a direction parallel to the erosion front may be expressed as

M _co ^• N _co + M _off ^• N _off ^• COS = M _e ^• N _e + M _ex ^• V, (1)

M _∞ ' ^ + M _off ^• _0{{ = M _e ^• N ² _e + M _ex ^■ N ² _ex (2)

where M is the mass flow rate and V is the gas velocity. The subsequent mass flow rates on the erosion front from the coaxial and the off-axial nozzles are:

M C_O- - wδ λ (3)

^} co ' V πd C. O

\ J

(4)

where P the gas density

N the volume gas flow rate at the nozzle

W the kerf width δ the molten film thickness d the inside diameter of the nozzle

When considering the mass balance in the molten erosion front, the material fed into the control volume in solid state should be equal to the ejected droplets in liquid state. Then

p _s ^• u ^• w ^• £ = p _e ^• V _e w δ F = M _e ' F (5)

where p _s and p _e are the densities of the workpiece and the droplets.

u is the laser cutting speed and £ is the thickness of the workpiece. A parametre F is introduced at this stage related to the composition of the droplets due to the addition of oxygen during the combustion. The momentum transfer and the shearing force (friction) of the gas flow at laser cutting have been studied and it was concluded that the velocity of the ejected droplets can be expressed as:

V. = p _β - ό ' W

(6)

where μ = the viscosity of the gas.

By substituting equations 3, 4, and 5 by equations 1 and 2, the gas velocity N _e

(7) is obtained.

As it appears from Fig. 7, the contribution of kinetic energy from the off-axial nozzle is dominant. The velocity of the exhaust gas N _ex is essentially equal to the off-axial gas velocity N _off . At the use of the off-axial velocity N _off as the velocity N in equation 6, the machining speed at dual gas jet laser cutting may be expressed as:

The thickness δ of molten film in equation 8 is to be determined. In conventional laser cutting of carbon steel with a thickness of 6.35 mm, δ is about 0.1 mm. This value is used to evaluate the machining speed during dual gas jet laser cutting. When cutting stainless steel, the XRD-analysis indicates a dominant composition of FeCr ₂ O ₄ in the combustion droplets corresponding to the parameter F, which is 0.714. The machining speed in equation 8 was calculated to be approximately 50 cm per min. The underestimation of the machining speed can be explained as follows:

1. The thickness δ of the molten film may be thicker at dual gas jet cutting of stainless steel compared to the thickness at conventional laser cutting of carbon steel of the same thickness. However, further examination thereof is required.

2. In the model according to the present invention, vaporization of material has been disregarded at the analysis in connection with equation 5. Besides the ejection of droplets, a term related to the vaporization of the workpiece should be added to the right-hand side of the equation. The machining speed may be increased accordingly.

In conventional CO ₂ assisted laser cutting of 6.35 mm stainless steel at a power of 1 kW, the machining speed is very low (2 to 12 cm per min.) and the edge quality is poor.

Laser cutting tests with dual gas jet (coaxial and off-axial) have showed a considerable improvement for various types of stainless steel. It was possible to

increase the cutting speed to 12.7 mm per second and still obtain clean edge qualities. Similar results were obtained at the use of the laser cutting technique on superalloys. An exeption was 440 C stainless steel, where excess dross formation and consequently poor cutting was found. It is believed that a high carbon content in 440 C stainless steel plays an important role in excess dross formation, as a clean cut was found on 430 stainless steel which has the same content of Cr. The mechanisms related to carbon in dross formation are discussed below. In oxygen- assisted laser cutting of stainless steel, the following chemical reactions occurred.

2Fe + O ₂ → 2FeO

4Cr + 3O ₂ 2 Cr ₂ O ₃

2Ni + O ₂ — 2NiO

2C + o ₂ → 2CO

2CO + O ₂ → 2CO ₂

2Fe + O ₂ + 2CR ₂ O ₃ — > 2FeCr ₂ O ₄

FeO + Cr ₂ O ₃ — -» FeCr ₂ O ₄

It is evident that the formation of carbon monoxide is favoured by a high temperature. It has been reported that decarburization in a basic-oxygen furnace steelmaking process is due to slag metal emulsions and pure oxygen reacting with liquid iron metal to form iron oxide. The iron oxide then reacts with the carbon in the steel to form carbon monoxide: FeO + C → Fe + CO.

Furthermore, it was concluded that decarburization is proportional to the carbon content in the metal and is independent of the rate of the oxygen supply.

A laser cutting technique combining coaxial and off-axial oxygen jets was used for machining thick steel plates with a low carbon content. The addition of an off-axial gas jet not only provides an enormous momentum force during dross removal, but also changes the chemical reactions during combustion. The formation of Cr ₂ O ₃ at conventional laser cutting was completely eliminated at me dual gas jet laser cutting.

Considerable improvements in the cutting speed and the surface quality are attributed to absence of Cr ₂ O ₃ and the momentum transfer from an off-axial gas jet to the erosion front.

The conclusion of the above is thus that gas jets must have essentially the same speed. Thus, both orifices may advantageously receive gas from the same chamber, confer Fig. 3. Moreover, the orifices 4,5 may be so closely interspaced that the flows 7 merge, whereby interf erring gasses are not able to reach the area of machining. As a result, the machining quality is improved at the same time as the machining speed may be increased. Furthermore, the laser apparatus has been simplified. The flow directions may form an acute angle with each other, preferably an angle of 45°, confer Fig. 5, whereby it is ensured that the flows merge. The interval between the orifices 4,5 is preferably less than the diameter of each of the orifices 4,5. The interval between the orifices 4,5 may be 0.2 to 0.8 mm and each orifice may have a diameter of 0.2 to 1.0 mm. The orifices are preferably circular or oval. Other embodiments may, however, be possible.