WO1997025178A1

WO1997025178A1 - Laser cutting method and device

Info

Publication number: WO1997025178A1
Application number: PCT/FR1997/000004
Authority: WO
Inventors: Gérard PACARY
Original assignee: Litech S.A.R.L.
Priority date: 1996-01-04
Filing date: 1997-01-02
Publication date: 1997-07-17
Also published as: AU1382297A; EP0874712A1; FR2743318B1; FR2743318A1

Abstract

A laser cutting method using a cutting head (10) comprising a laser beam source (F) such as a carbon dioxide laser, and an oxygen feed nozzle (13). To cut thick products (1), e.g. made of a ceramic or iron-carbon alloy, a cooling fluid (19) such as water is delivered onto the surface of the product to be cut on the cutting head side thereof and a negative pressure of at least 0.25 bars relative to the pressure on the cutting head side is generated on the other side of said product. The negative pressure is generated, e.g., by means of a pressure-regulated suction chamber (20) in sealing engagement with the product surface. The method is useful for laser cutting thick products such as steel sheets having a thickness of 10-30 mm.

Description

LASER CUTTING METHOD AND DEVICE

The present invention relates to the technique of laser beam cutting of parts in sheets or the like, of relatively large thickness, of iron-carbon alloy (steel or cast iron), or of other materials such as ceramics.

Numerous thermal energy cutting processes are already known using an oxygen supply to carry out an exothermic reaction for iron oxidation, in particular the long-known processes of oxy-acetylene cutting or similar, of cutting with electric arc and, more recently, cutting by laser beam, plasma, summer, making it possible to obtain significant energy densities at the cutting point. The performance of these processes depends on the nature and density of the energy source, and on the oxygen supply conditions (type and diameter of the gas supply nozzle, pressure and flow rate of the gas).

It will be noted that these are processes which all use an oxygen supply as an agent generating an energy complementary to that supplied, for example in the case of laser cutting, by the laser radiation itself. This additional energy, resulting from the exothermic reaction caused by the oxygen supplied, is, in the field targeted by the invention, necessary to ensure the desired cutting in very thick products. Are therefore excluded from the field covered by the invention processes which do not use said exothermic reaction, such as for example laser cutting processes with nitrogen supply, used for example for cutting in stainless steels.

The use of suction means placed on the side of the sheet opposite to the cutting head is already known, in particular in laser sheet cutting methods. These means are intended to evacuate the gases resulting from the cutting and the slag formed during it.

The known methods, with oxygen supply, mentioned above, however, cannot be used in practice since the cutting requires prolonged maintenance of the energy source in the same area of the part to be cut.

This is particularly the case for cuts made in products of large thickness, such as for example steel sheets of thickness approaching 10 mm or more than 10 mm, and for cutting in such sheets of parts to sharp angles, or the drilling of holes of small diameters or small section, for example typically having a transverse dimension less than or equal to the thickness of said sheet. Indeed, in such cases, there occurs at the cutting line, and in the thickness of the sheet, a concentration of energy which excessively heats the material, produces an excess of fusion of the latter unacceptable 'obtaining a fine and regular cut, or at least destroys the mechanical characteristics of the materials in the vicinity of the cut line.

Similar problems arise when the energy accumulated by the workpiece cannot be dissipated sufficiently by thermal conductivity. This is particularly the case when cutting near an edge of the sheet, of cuts in such a sheet of parts whose edges are very close from one part to another {typical case when looking for 'optimization of the distribution of the parts to be cut in a sheet to reduce metal falls), and in general, when two cutting lines are very close together.

Another problem arises specifically from the thickness of the workpiece. The more the thickness of the part increases, the more the width of the cutting groove must increase to facilitate the formation of the front of cutting and removal of slag generated by cutting. It therefore becomes almost impossible to make small diameter holes in sheets of thickness greater than the diameter of the hole. In such a case, in addition to the specific problem of the necessary width of the groove, the known methods of laser cutting lead, due to the excess energy brought in the vicinity of the cut, to flaring of the cut line towards the opposite side of the cutting head. This flaring is due, not to the effect of the laser radiation itself, but to the propagation, towards the said opposite face, of the exothermic reaction initiated in the thickness of the sheet and which, due to 1 ' self-combustion of the resulting iron and the kinetics of the oxygen supplied, affects an area which widens in the direction of the gas jet.

It is therefore impossible, with the known laser cutting technique, to make fine and regular cuts, by controlling the surface condition and the geometry of the sides of the cutting line, since the thickness of the sheet is large and that the cut pieces are small or that the cut lines are very close together.

The object of the invention is to solve the problems indicated above. With these objectives in view, the invention relates to a laser cutting method using a cutting head comprising a laser beam source and an oxygen supply nozzle, characterized in that, in order to perform cuts in thick iron-carbon alloy or ceramic products, a coolant is added to the surface of the product to be cut located on the side of the cutting head and a vacuum is generated on the other side of the product existing pressure on the side of the cutting head, greater than or equal to 0.25 bars. Preferably, for a product with a thickness greater than 10 mm, the depression is greater than or equal to 0.01 xe + 0.15 where e is the thickness of the product in mm.

The addition of a cooling fluid on the side where the laser beam acts directly on the product to be cut, combined with the suction caused by the depression exerted on the other side of the product, creates in the groove produced, and in particular at behind the point of application of the laser beam, a circulation of fluid at a speed and a flow rate which results in very efficient cooling of the cut piece, directly at the cutting line and over the entire height of this one.

The coolant can generally be any fluid, at low temperature. Preferably, a liquid will be used, having better heat exchange capacities than a gas, such as water, or an aqueous solution or emulsion, such as for example an oil-water emulsion of the type conventionally used in metal machining by cutting tool.

It is important to note that the suction carried out on the side opposite to the cutting head in certain known laser cutting processes has the purpose of removing the slag formed during cutting as well as the gases brought in or generated by the cutting, but , even if this suction implicitly causes a certain circulation of gas in the cutting line, the depressions used are not sufficient to cause sufficient cooling of the material, on the two sides of the cutting line, typical of the present invention.

The laser used is a gas laser, continuous or pulsed, of the conventional type such as a CO ₂ or CO laser. Such a laser makes it possible to obtain a power density of the order of 1000 KW / cm ² at the focal point of the laser beam and, in cutting applications, this is assisted, in known manner, by an oxygen supply, under a relatively low pressure (of the order of 1 bar), brought by a nozzle into the orifice from which the laser beam passes. This supply of oxygen is necessary to cause oxygen cutting. Given the large thicknesses to be cut, the laser focusing device will be adjusted to obtain a long focal distance, of the order of 250 to 350 mm. It will be noted that the flaring effect of the cutting line mentioned above is not so much due to the proper geometry of the laser beam as to the excessive heating of the material caused by the supply of oxygen. To illustrate this phenomenon, there is shown by way of example in FIG. 1, in section, the region of the cutting line in a sheet 1, obtained by a conventional laser cutting process from a thick steel sheet. 10 mm, the groove having a width of about 0.3 mm. The shaded areas 11, which would delimit the parallel flanks of the ideal cutting line, have in fact disappeared by fusion under the effect of the energy supply of the oxygen flow (arrow 12) provided by the nozzle 13 of the cutting head. . It will be noted that these zones are all the more important when the material is less thermally conductive, or when it is already strongly heated by a cut made previously in the vicinity, since the dissipation by conductivity of the thermal energy supplied is then hampered. It will also be noted that the flaring of the cutting line is all the more important the thicker the sheet, which has in practice limited until now the use of laser cutting to steel sheets of lower thickness. about 10 mm. From 10 to 20 mm, the geometry of the cutting line is of poor quality, limiting the production of the cut pieces to simple shapes. Beyond 20 mm, cutting is impossible due to an enlargement of the heat affected area, widening such that the molten metal can no longer be removed to make an effective cut.

It will also be noted in FIG. 1 the presence of cutting burrs 14, of a dimension of the order of 0.3 mm, resulting from the resolidification on the cooler underside of part of the molten metal coming from the bleeding.

To improve the geometry of the cutting line, and in particular to avoid these burrs, it has already been mentioned to direct a jet of cooling fluid towards the groove, on each side of the latter, substantially tangentially to the lower surface of the sheet. The implementation of such a process certainly makes it possible to reduce the width of the flaring, but turns out to be notably insufficient when the sheet is thick to avoid the harmful effects of excess energy at the cutting line, in the thickness of the material. The drawing of Figure 2 illustrates what would happen on the sides of the cutting line, assuming that the cooling of the underside of the sheet may be sufficient to avoid flaring the cutting line. We see there on each side of the cutting line and in the thickness of the sheet, the zones 16 affected thermally whose importance, at the level of the middle zone in the thickness of the sheet, results from the concentration in this area cutting energy. On the one hand, this concentration of heat can cause significant heating of the sheet in the vicinity of the cutting line, which is troublesome for making other cuts nearby. On the other hand, the metal of these zones can be melted and entrained by the flow of the oxygen supplied, causing a digging of the sides of the groove, or even risking closing the latter on the side of the underside in s' resolidifying there. On the other hand, and certainly, the mechanical characteristics of the heat affected areas are deteriorated (weakening of the edge of the cut parts, reduction in their resistance to wear, summer).

The method according to the invention makes it possible, thanks to the supply of cooling fluid to the upper face of the sheet, on the side of the cutting head, and to the forced circulation of this fluid in the cutting line, under the effect of the depression created on the other side of the sheet, effectively cooling the sides of the groove over the entire thickness of the sheet, and therefore limiting the extent of the heat affected zone.

In addition, the strong depression created between the two faces of the sheet allows better evacuation of the slag from the groove than in the methods according to the prior art. This slag entrainment effect is favorably influenced by the kinetic energy of the fluid molecules passing through the bleeding, energy which is all the higher as the depression, and therefore the speed of the fluid, is high, the kinetic energy also being increased when the coolant is a liquid.

The process according to the invention makes it possible, by the cooling effect of the fluid circulating in the bleeding, to keep the cut product at low temperature (for example around 70 to 80 °) and, in combination with an efficient removal of slag, stabilizes the cutting front, even in the case of a narrow width, less than 1 mm and typically of the order of 0.3 mm, when cutting from a sheet of very thick material, for example around 20 mm. It makes it possible to keep the width of the groove practically constant over the entire thickness of the sheet, and consequently allows the production of holes of small section, of dimension significantly less than the thickness of the sheet, for example the production in a sheet metal 20 mm thick, oblong holes in length

10 mm and 2 mm wide, 5 mm circular holes diameter, of holes having a section in the shape of a teardrop or ovoid of length 18 mm and of width varying over the length from 3 to 5 mm, or of square holes of

3 mm side. The heating of the sheet near the cutouts being low, the method according to the invention allows the successive production without delay of cut lines very close to each other, the production of holes or recesses close to the edge of the sheet metal, and the production of parts with sharp angles, all these cuts being impracticable in thick products by the cutting methods according to the prior art.

By reducing the extent of the heat affected zone, the method according to the invention reduces the risk of embrittlement of steel by hydrogen by preventing the formation of an austenization zone capable of absorbing hydrogen. .

By limiting the overall heating of the cut sheet, it greatly reduces the residual stresses after cutting and promotes the maintenance of the flatness of the sheets or cut pieces in these sheets.

By allowing a precise geometry of the cutting line to be obtained, it allows the cutting head to tilt relative to the surface of the cut piece and thus enables chamfer cuts to be made, on a closed or non-closed contour. , guaranteeing compliance with the desired geometry.

The reduction in the heat affected zone allows cutting into alloys which have already undergone a heat treatment (for example steels of high hardness, hardened and tempered) without altering the mechanical characteristics, in particular the hardness, of the parts thus cut.

As a consequence of the various advantages mentioned above, the method according to the invention is for example particularly suitable for the manufacture of sheets perforated and very thick screening grids

(of the order of 10 to 30 mm) in high mechanical strength quenched and tempered steel, in which are made many orifices of small section, with or without draft, and close to each other.

The invention also relates to a laser cutting device which is particularly suitable for implementing the method mentioned previously. This device is characterized in that it comprises: - a cutting head comprising a laser beam source and an oxygen supply nozzle, - a conduit for supplying a cooling fluid to the surface of the product to cut, opening next to the cutting head, - a suction box connected to a suction group and having an opening located in front of the cutting head and provided at its periphery with a seal intended for be placed against the surface of the product to be cut opposite the cutting head, - means for regulating the vacuum generated in the said box by the said suction group to maintain the said vacuum at a value greater than or equal to 0.25 bars .

Preferably, the means for regulating the vacuum comprise a relief valve connected to the box, to put the inside of the box in communication with the ambient atmosphere and means for regulating the opening of said relief valve in function of the open section of the orifices or cutting lines produced by the cutting in the product.

This regulation system has the advantage of being able to let the suction unit operate at its optimum efficiency permanently, by compensating for the increase in fluid flow rate passing through the cut lines or orifices already made in the sheet metal by reducing the flow through the relief valve. A another advantage is that the discharge valve allows, even at the start of cutting, to let enter into the box a quantity of ambient air suitable for ensuring in the suction circuit an oxygen concentration lower than the rate likely to cause the ignition of the gas mixture sucked in by the pumping unit.

Other characteristics and advantages will appear in the description which will be given by way of example of a laser cutting installation in accordance with the invention, and of its use for making cuts in a thick steel sheet.

Reference will be made to the appended drawings in which: FIG. 1 and 2 illustrate the problems encountered during laser cutting according to the prior art, already explained previously,

- Figure 3 is a schematic representation of the laser cutting installation according to the invention, Figure 4 is a simplified representation of a mask system intended to close the grooves or orifices already made, to limit the section of suction of the coolant, FIG. 5 is a graph illustrating the field of application of the method, limited by a curve indicating, as a function of the thickness of the sheet to be cut, the value of the absolute pressure to be maintained under the sheet , when the pressure above the sheet on the side of the cutting head is atmospheric pressure.

The laser cutting installation shown in FIG. 3, for cutting from a steel sheet 1, comprises a laser cutting head 10 of a type known per se. This cutting head shown diagrammatically in the drawing, comprises an oxygen injection nozzle 13 with a diameter of 2 mm, supplied under a relative pressure of 1 bar, at the center of which passes the laser beam F. This beam is generated by a laser continuous gas, for example a C0 ₂ laser, having a power of 2 to 3 KW, and a focal length set to 250 mm.

Under the cutting head 10, is disposed a suction box 20 open upwards and carrying at the periphery of its opening a seal 21. During cutting, the box 20 is applied against the underside of the sheet, the seal 21 ensuring the seal between the latter and the box. For localized cuts of small dimensions smaller than those of the opening of the box, the sheet 1 and the box can be fixed, and the cutting head is then moved along the outline of the cuts to be made. For cuts on larger areas, the cutting head and the box can be fixed, and it is then the sheet which is moved while remaining held in contact with the seal. For cuts in thick and large sheets, for example 2 mx 6 m, it is preferable to keep the fixed sheet resting on suitable supports, and then the cutting head and the box will be moved simultaneously, parallel to the plane of the sheet . A pipe 19 for supplying a cooling fluid such as water opens above the sheet to be cut, next to the cutting head, the water flow being sufficient for the water to cover the surface sheet metal in a sufficiently large area around the laser beam.

The box 20 is connected by a suction line 22, provided with an isolation valve 23, to an assembly of accumulation tank 24, itself connected to a suction group 25, via a separator filter 26 impurities and liquids. This group is equipped with a suction pump having a large flow rate, for example from 500 to 700 m ³ / h, and making it possible to obtain an absolute pressure at suction of 0.5 bars or less. This suction group makes it possible to obtain in the accumulation tanks 24 and in the box 20 a absolute pressure adjustable for example from 0.7 to 0.5 bars depending in particular on the thickness of the sheet 1. The large volume of the accumulation tanks makes it possible to maintain them there and in the box 20 of a depression substantially constant despite the variations in flow rate that can occur during cutting, and in particular makes it possible to very quickly generate the required vacuum in the box 20 as soon as the isolation valve 23 opens. The accumulation tanks are connected via d valves insulation 27 to a condensate recovery tank 28, to which the filter 26 is also connected.

The box 20 is also connected by a valve 29 to a device, not shown, making it possible to inject into the box a cooling fluid, for example a water mist, which is used for cooling the box 20 and the gases resulting from the cutting operation, which also has a moderate cooling effect on the underside of the sheet. It will be noted that this injection of cooling fluid is in no way similar to the cooling, mentioned in the introduction to this memo, of the cutting zone by jets directed towards this zone from below. The use of such jets of liquid under a vacuum as strong as that according to the invention would lead to having to suck excessively large quantities of liquid. The invention precisely overcomes these problems.

Another valve 30 puts the box 20 in communication with the ambient atmosphere, and allows the entry of ambient air into the box under the effect of the vacuum which is generated there by the suction group. The opening of this valve, and therefore the air flow entering the tank can be regulated to maintain the vacuum in the box substantially constant. This regulation can be carried out directly from a pressure measurement in the box. It will be noted that this regulation is necessary, taking into account the constant flow rate of the suction group, to maintain constant during cutting the conditions for circulation of the cooling water in the cut groove, despite the variation in open cross section of the line. cutting holes or holes made in the sheet, resulting from cutting progress.

According to a particular regulation mode, the opening of the valve 30 will be adjusted as a function of the position of the laser relative to the sheet by virtue of a prior programming linking this position to the linear length of the cutting line made, or more generally, in the open section in the sheet by cutting.

A typical process for making a cut is divided into two phases.

A priming phase is carried out prior to the actual cutting. During this phase, a priming hole of small diameter, for example from 0.5 to 0.6 mm, is made in the sheet. For this ignition, the CO2 laser cutting head is temporarily replaced by a pulse laser, for example a YAG laser capable of providing a very high specific energy (10 ³ to 10 ⁴ J / cm ² ) with a power density of 10 ⁷ to on

10 W / cm, favoring the drilling of the priming hole in full sheet metal, by limiting the heating thereof. This priming could also be carried out by means of a CO2 laser in impulse mode, but one could not then make a hole of such small diameter.

The cutting phase is then carried out, starting from the priming hole, in accordance with the method according to the invention. For starting the cut, the suction unit 25 being in service and creating a vacuum in the suction box 20, the valve 30 is opened to obtain the required vacuum in the box, by example 0.5 bars, then, during cutting, the regulating device then gradually closes this valve, depending on the pressure measured in the box or the length of the cutting line produced, as indicated above. As an indication, the cutting speed is of the order of 0.7 m / min in steel sheets 10 mm thick, 0.5 m / min for a thickness of 20 mm, and 0, 3 m / min for 25 mm sheets. According to one embodiment, particularly suitable when it is desired to avoid moving the box relative to the sheet, which implies the use of a box having a large opening, the device comprises around the cutting head masks for covering the product to seal the orifices or cutting lines already made. This embodiment makes it possible to move the cutting head, with respect to the box and to the sheet metal, over great distances, by limiting the open section of the cutting lines or orifices already made in the sheet metal, since all the cutouts distant from the cutting heads are covered by said masks. FIG. 5 schematically illustrates such an embodiment, in which the masks consist of mats 40, for example made of rubber, which are wound or unwound on drums 41 linked to the cutting head 10 as a function of the displacements of the latter .

The drawing of FIG. 6 illustrates the field of application of the method according to the invention as a function of the thickness of the cut product. Curve 60 established experimentally, defines the minimum depression required (the values indicated on the ordinate are the absolute pressure values P under the sheet, considering that the pressure above the sheet is the atmospheric pressure of 1 bar) as a function of the thickness e of prison. The hatched area under this curve is the area of validity of the process.

A vacuum of 0.5 bar gives the best results in the case of a steel cut thicker than 15 mm. A vacuum of 0.25 bars (i.e. an absolute pressure of 0.75 bars) nevertheless makes it possible to make small diameter holes, for example 3 mm, in a steel thickness of 10 mm.

Although the above description has been made with respect to the cutting of flat steel sheets, the method according to the invention also applies to cutting from other non-planar products, such as tubular or complex shaped parts. . It also applies to cutting in other materials, such as low-alloy steels, manganese steels, coated steels (for example 8 mm thick E36 steel sheet with a layer of 5 mm chromium cast iron ), or ceramics.

By way of examples, the method according to the invention has made it possible to produce, in 10, 15 and 20 mm thick sheets of highly alloyed abrasion-resistant steel and of manganese steel, and in composite sheets ( steel sheets reloaded with a layer of chromium cast iron) of thickness 5 + 3 mm, 8 + 4 mm and 10 + 5 mm, the following cuts:

- oblong holes of 30 X 15 mm, with 7 ° clearance, separated by 8 mm of metal on the upper surface (side of the cutting head); oblong holes of 30 X 8 mm, without clearance, separated by 4 mm of metal;

- oblong holes of 15 X 5 mm, with relief of 7 °, separated by 5 mm of metal on the upper surface;

- oblong holes of 8 X 3 mm and 13 X 3 mm, without draft, separated by 2 mm of metal; - teardrop-shaped holes, 26 mm long and 5 mm in diameter at one end and 3 mm at the other end, with 7 ° clearance, separated by 6 mm of metal on the upper surface;

- teardrop-shaped holes, 26 mm long and 5 mm in diameter at one end and 3 mm at the other end, without undercut, separated by 3 mm of metal;

- square holes of 15 X 15 mm with rounded corners of 1.5 mm radius, with 7 ° clearance, separated by 8 mm of metal on the upper surface;

- square holes of 15 X 15 mm with rounded corners of 1.5 mm radius, without undercut, separated by

6 mm of metal;

- 6 mm diameter holes in 8 mm steps, without clearance;

- 10 mm diameter holes with 7 ° clearance, 17 mm pitch; as well as oblong holes of 30 X 15 mm with relief of 7 ° in high alloy steel sheets resistant to abrasion of 30 mm thick.

Claims

1) Method of laser cutting using a cutting head comprising a laser beam source and an oxygen supply nozzle, characterized in that, in order to make cuts in thick products of iron-carbon alloy or ceramic , a coolant is added to the surface of the product to be cut located on the side of the cutting head and a vacuum is generated on the other side of the product, with respect to the pressure existing on the side of the cutting head or equal to 0.25 bars.

2) Method according to claim 1, characterized in that, for a product with a thickness greater than 10 mm, the depression is greater than or equal to 0.01 x e + 0.15 where e is the thickness of the product in mm.

3) Method according to claim 1, characterized in that the laser is a Œ> 2 or CO, or YAG laser.

4) Method according to claim 1, characterized in that the cooling fluid is an aqueous solution or emulsion.

5) Method according to any one of claims 1 to 4, characterized in that, prior to cutting, a priming hole is made by means of a pulse laser.

6) Laser cutting device for implementing the method according to any one of claims 1 to 5, characterized in that it comprises: - a cutting head (10) comprising a laser beam source and a nozzle d '' ¹ oxygen supply (13), - a duct (19) for supplying a cooling fluid to the surface of the product to be cut, opening out next to the cutting head,

- a suction box (20) connected to a suction group (25) and comprising an opening located opposite the cutting head and provided at its periphery with a seal (21) intended to be placed against the surface of the product (1) to be cut opposite the cutting head, - means for regulating the vacuum generated in the said box by the said suction group to maintain the said vacuum at a value greater than or equal to 0.25 bars.

7) A laser cutting device according to claim 6, characterized in that the means for regulating the vacuum comprise a discharge valve (30) connected to the box, to put the inside of the box in communication with the ambient atmosphere and means for regulating the opening of said discharge valve as a function of the open section of the orifices or cutting lines produced by the cutting in the product.

8) Device according to claim 6, characterized in that the box (20) comprises a valve (29) inlet of a cooling fluid.

9) Device according to claim 6, characterized in that it comprises around the cutting head masks (40) for covering the product to close the orifices or cutting lines already made.

10) Application of the method according to claim 1 for cutting into thick orifice products, the section to a dimension less than or equal to the thickness of said product.