LASER PROCESSING APPARATUS

Abstract: A laser processing apparatus (10; 110; 210), in particular for cutting and welding, including a focusing head (12; 112; 206) for focusing the beam on the work area (13, 13a; 113; 207) of the workpiece (15; 118; 209), and a nozzle (17) with a conduit (21; 120; 220) for directing a jet (18; 122; 213) of assist gas at a supersonic speed at least close to the work area (13, 13a; 113; 207). The work area (13, 13a; 113) is preferably covered by the walls of the conduit (21; 120) located downstream from the nozzle (33a, 33b, 33c; 132). To ensure optimum working conditions of the apparatus, the nozzle (217) is preferably supported for rotation about the focusing head (206), and is so controlled that, at least at the work area (207), the conduit is maintained tangent to the work path (T).

Description: DESCRIPTION LASER PROCESSING APPARATUS TECHNICAL FIELD The present invention relates to a workpiece laser processing apparatus, e.g. for cutting or welding. BACKGROUND ART Certain laser applications, such as cutting or welding, require an assist gas for work area subjected to the laser beam. Oxygen is an effective assist gas for cutting applications, while, for welding, any of a number of inert gases, such as helium, argon or even dry nitrogen may be used. On commonly used laser apparatus, the assist gas, which is normally supplied by the focusing nozzle together with the laser beam, is directed freely on to the work area. However, this arrangement involves a number of drawback as described in detail in the following. For example, laser cutting of metal plate is usually performed with the aid of a jet of gas directed onto the cutting " area for flushing out the liquefied material. The type of gas used depends on the workpiece material, and must be so selected as to prevent undesirable chemical reactions affecting the cutting faces. It is particularly important to prevent the formation of products affecting the metallurgical structure of the cutting faces and so resulting in hardness or fragility, which prevents post-process machining or actual use of the part if no further machining is required. Oxygen is excellent for cutting ferrous materials (alloys), due to the exothermic reaction produced at high temperatures (over 720°C ). If properly employed, the energy and fluid-thermodynamic effects so produced may result in increased cutting speed for a given power of the laser, SUBSTITUTE SHEET improved flush-out of the liquefied material, and a better surface finish of the cutting faces. To understand the chain of events occurring in this process, some knowledge is required of the fluid- thermodynamics of reactive gases, and the complex phenomena produced when such a gas is subjected to intense heat by both the laser beam and the liquefied material, and the mass of the gas stream is increased by the presence of the liquid. The latter results in a gas stream consisting of a mixture of compressible gas and (incompressible) liquid, with all the possibilities this entails. For example, the liquid may be broken down into large or small drops, or even atomized. Depending on the amount of heat, particularly that radiated by the laser beam and high-temperature solid or fluid surfaces, and the combined thermal and fluid-dynamic effects involved, the drops and atomized liquid particles may be converted into steam, which, though undesirable, is nevertheless inevitable. Moreover, due to their small size and the absence of a cooling mass (the cold faces of the workpiece) , the liquid particles within range of the laser beam may also be converted into steam, which, like the existing steam, may be energized or even ionized by the laser beam, thus resulting in several of the luminous phenomena observed during the cutting process. In dealing with the above phenomena, the reactivity of the gas must also be taken into account. In the case of oxygen, if the temperature of the liquid species exceeds a given threshold value (720°C ) , an intensely exothermic reaction is initiated, which further accentuates the intensity and instability of the above phenomena, and is what accounts for the more or less periodic oscillation of the various components of the fluid-thermodynamic field. Another important point to note is that the liquid phase subjected to the laser beam may be induced to oscillate at an entirely different amplitude and frequency as compared with SUBSTITUTE SHEET those deposited and/or formed on the cutting face. The liquid phases immersed in the gas stream may alter, in limited regions, the frequency and amplitude of both the free-flow stream at the start of the cut, and that channelled between the cutting faces. The dynamic action of the various phases so interact as to affect the end result of the process in ways and to an extent varying widely depending on the operating parameters and equipment employed. For example, on commonly used laser cutting equipment, the laser beam is normally focused in a conical nozzle, through which the oxygen is also supplied, with no separation between it and the laser beam, and directed freely on to the cutting area. Moreover, using a conical nozzle located at a given distance from the work surface, the oxygen jet forms a round impact spot about the cut, the diameter of which may be as much as 5 to 10 times the width of the cut, for which attempts are made to make as narrow as possible. The above known apparatus presents numerous drawbacks. Firstly, oxygen consumption is high, due to a substantial percentage being directed to no purpose on to the area surrounding the cut. In the above example, for instance, in which the diameter of the impact spot is 5 to 10 times the width " of the cut, consumption is respectively 8 to 30 times that actually required. Secondly, by virtue of contacting a large portion of the laser beam for a considerable length of time, the oxygen is heated to an extremely high temperature by the time it reaches the workpiece. The effects of such heating are numerous. Firstly, the coefficient of refraction varies irregularly, thus impairing focusing of the laser beam, which is further affected by the convective motion produced by heating the oxygen. SUBSTITUTE SHEET If the gas is heated long enough for it to reach, firstly, the thermal excitation and then the thermal ionization threshold, this may (even at laser cutting power levels) result in dissipation absorbing the power of the laser beam. The reduction in power and defocusing of the beam combine to reduce the power and increase the diameter of the focal spot, both of which are unfavourable for obtaining as narrow a cut as possible. Moreover, the instability caused by both convective motion and the flow phenomena required for the gas jet to penetrate inside the cut results in unsteady phenomena which also affect cutting efficiency and the quality of the cutting faces: scoring, tears, undesirable metallurgical properties. Most of these drawbacks are caused by using a nozzle which directs the gas jet freely on to the cutting groove surface. Attempts to increase flow velocity and so improve flush- out of the liquid by increasing the pressure of the jet are pointless and even counterproductive in the case of commonly used conical nozzles. When the pressure of the jet is increased over and above the critical ratio, in fact, this results in instability in the direction and velocity of the jet (wobble and pulsation) , which further affect the alteration in flow caused by internal and external aerodynamic factors (boundary layers, atmospheric air mix and drag), thus resulting in substantially uncontrollable situations. The instability of a freely-directed jet makes it even more difficult for the jet to enter and penetrate inside the cut, thus aggravating the "choking" phenomenon typical of subsonic and supersonic jets. In the conduit portion downstream from the choking section, velocity is significantly reduced. Traditional cutting faces therefore present a longitudinal line (i.e. parallel to the top and bottom SUBSTITUTE SHEET surfaces of the workpiece) caused by the choking effect, and, up and downstream from this, series of differently sloping score lines indicating a change in flow velocity. In particular, the slope of the score lines is greater downstream from the choking line, thus indicating a reduction in flow velocity. A significant reduction in flow velocity also results in a variation in scoring frequency, as well as in erosion accompanied by droplets of liquefied material, all of which are caused by the uncontrolled exothermic reaction produced by greater penetration of the face by the isotherm (about 720°C ) initiating the reaction. Now, in the case of welding, processes require an inert gas atmosphere, such as helium, argon, etc., about the area subjected to the laser beam, to prevent a chemical reaction of the material caused by the combined action of heat and atmospheric gases, of which oxygen is the most reactive. In certain cases in which the material is not negatively affected by nitriding, dry nitrogen may also be used successfully. On known devices for feeding such gas over the workpieces, the laser beam is directed and focused inside a conical nozzle together with a jet of inert assist gas, usually with no separation of the two, and the insert gas is directed freely on to the work area. Devices of the aforementioned type present numerous drawbacks. One of these is that the gas remains contacting a frequently excessive portion of the laser beam over what is usually an excessive length of time. In this respect, it is important to bear in mind that a gaseous mixture is heated by a laser beam in proportion to its capacity to absorb radiation at the laser beam wave length; in proportion to the length of time it is subjected to such radiation; and in inverse proportion to its heat capacity. The first undesired effect SUBSTITUTE SHEET of such heating is that it results in an irregular alteration of the refraction coefficient of the gas, thus distorting and impairing the focus of the laser beam, which is further impaired by the convective motion produced in the heated mixture. Should heating persist long enough for the gas to reach first the thermal excitation and then the thermal ionization threshold, this results in highly dissipative phenomena, which absorb the energy of the laser beam. In extreme cases of almost total or predominant ionization, the energy of the beam may be almost totally absorbed by the gas, thus giving rise to what is known as "blanketing" , whereby the laser beam is practically prevented from reaching the workpiece. The assist gas must therefore present a definite number of properties: 1) low radiation absorption coefficient at the laser beam wave length; 2) high heat capacity; 3) high thermal ionization threshold or ionization potential; 4) minimum interaction or transit time (the length of time it is subjected to the laser beam) ; 5) for shielding against thermochemical reactions, the gas must be immune to such reactions. For this, noble gases are preferred, especially helium (He) , the ionization potential and heat capacity of which are among the highest, and which also presents a low absorption coefficient of the most commonly used industrial laser beams. In view of the above considerations, the most logical choice is helium, providing it is economically feasible, especially in Europe. Another possibility is argon. In addition, however, to a higher radiation absorption coefficient at the wave lengths commonly used in industry, this also presents a low heat SUBSTIT T capacity and ionization potential as compared with helium. These drawbacks, however, may be overcome by proportionally reducing the interaction time, and by appropriate streamlining of the assist gas jet. Moreover, as ionization is predominantly thermal and, as such, governed by Saha's law, steps may also be taken to increase the pressure and reduce the temperature of the gas jet in the laser beam crossover region. One of the major objectives involved, therefore, is that of achieving a uniform gas jet, the properties of which remain unchanged throughout its crossing of the laser beam, i.e. uniform refraction and parallel motion, no convective motion or excitation, and no ionization. Moreover, the jet must present a poor thermophotochemical reaction with the material to be welded (Hence, no water, oxygen, hydrocarbons, acids, salts, alkalis, etc. can be used as the jet). The impact of the assist gas, in the form of plasma, on the surface of the workpiece has been found to cause a considerable thermal alteration of the surface, accompanied by vaporization of a layer of material on either side of the weld bead. In particular, in cases where the gas jet is used firstly for cooling a lens, and is then directed, together with and penetrating the length of the laser beam, into a conical nozzle, the issuing jet is substantially ionized. What is more, being directed perpendicularly to the surface of the workpiece, upon impact, the jet not only reaches the high temperatures and pressures corresponding to impact velocity 0 (zero) , but is also forced to restore the ionization energy by recombining it. By combining these effects (high Po, high To) plus the enthalpic values corresponding to restoration of the ionization energy, i.e. Po + ΔP » Po and To +ΔT >> To SUBSTITUTE SHEET it follows that the material of the workpiece vaporizes, not beneath the laser beam, but at the edge where it is struck by the plasma jet. Another point to note is that the high reflectivity of a metal surface at low temperature (practically until it becomes red hot) almost doubles the intensity of the laser beam on impact, thus resulting in ionization despite the low value of the incident laser beam. This is further assisted by gas, free radicals, and other adsorbed chemically active substances, which emit nonthermionic free electrons resulting in avalanching, and, if serious enough, even in blanketing, i.e. an unpredictable (catastrophic) increase in ionization at incident laser beam level, which prevents transmission of the beam on to the workpiece. Ionization is always accentuated by, and very often in fact due to, the surface of the workpiece, which is inevitably reflective at low temperature, and therefore reflects part of the energy of the laser beam back towards the inert gas atmosphere, thus roughly doubling the intensity of the radiant energy field in the area close to the work surface. If, on the other hand, the inert gas jet is directed on to the work surface together with the laser beam, it presents a high degree of ionization on impact with the workpiece, and, on bouncing back off the work surface, tends to mix with the atmosphere, thus reducing shielding efficiency. Moreover, ionization energy is taken from the laser beam, which therefore presents a lower power density for a given focal spot; and the variable density of the mixture so formed results in refraction and convection phenomena impairing the focus of the laser beam, thus further reducing its intensity and efficiency. According to the known state of the art, the jet is supplied freely, often even using conical nozzles. SUBSTITUTE SHEET Consequently, when supply pressure increases over and above a critical ratio value (corresponding to sonic velocity at the cone outlet, and characterized by an (outlet pressure)/(stagnation pressure) value of < 0.528 for biatomic gases such as air, nitrogen, oxygen, carbon monoxide, etc.), the jet, for lack of a stable law of motion, oscillates violently in direction (wobble) and axial velocity (pulsation) . These effects amplify those caused by air mix and drag, and by possible laminar or turbulent boundary layers established inside the nozzle and affecting uniform outflow. In view of the size and distance of commonly used nozzles from the work surface, this combined alteration in outflow, due to the internal and, even more so, external aerodynamic factors involved (boundary layer, air mix and drag), is even more serious in the case where oscillation (wobble, pulsation) is amplified by attempts to increase outflow velocity by increasing supply pressure. In the case of normal size subsonic nozzles, the instability caused by exceeding the critical pressure frequently results in such severe oscillation that the laser beam impact area is substantially uncovered, thus impairing shielding by the gas, which may be replaced by varying mixtures of air and gas. These alternate over the impact point of the laser beam, which is thus swept by a pulsating jet in which the percentage of assist gas may vary enormously and in a random manner characteristic of this type of non-stationary phenomena. Reliable control of a freely directed jet requires the use of supersonic nozzles, which, with a given configuration and pressure ratio (obviously, for a given type of gas or mixture), provide for a uniform jet. If the pressure ratio is other than nominal, however, this results in the formation, at the outlet, in the case of a supersonic nozzle (having a fixed configuration) , of what is known as Prandlt diamonds and Mach disks. Freely directed supersonic jets nevertheless provide for a better distribution of the field parameters (velocity, density, pressure, temperature), and, though subject to the above internal and external aerodynamic factors (boundary layers, air mix and drag), these are less marked and SUBSTITUTE SHEET controllable. In this case also, scale effects, expressed in Reynolds, Prandlt, Nusselt numbers, etc., must obviously be taken into account. The capacities, distances, and sizes used in a laser processing apparatus may result in field irregularities (the properties of the jet in terms of its parameters, including the incorporation of outside air) possibly affecting shielding efficiency in the laser beam-work surface impact area. Nevertheless, supersonic nozzles provide for achieving velocities and pressures otherwise unattainable using subsonic nozzles. The latter, very often, are not only badly designed, but also employed with no regard whatsoever to the basic laws of aerodynamics. Consequently, when the stagnation pressure of a freely directed jet, particularly a subsonic jet, is increased with a view to increasing crossover speed, ambient gas drag efficiency, and also surface pressure, for improving shielding efficiency, this more often than not results in uncontrollable situations the effects of which are quite the opposite to those expected. The commonest include uncontrolled ionization resulting in "nailheads" (irregular weld bead) ; vaporization and sublimation of the surface material, which often presents grooves on either side of the weld bead; and an irregular metallurgical structure, which often presents a central zone, and two surrounding zones reasonably attributable to the plasma jets on either side of the laser beam responsible for the central zone. This differs widely from the two surrounding zones, though all three are irregular. Structural and geometric irregularity clearly indicates the following, among other things: a) undesired, totally irregular stress; b) severe distortion; c) poor process efficiency; d) poor process repeatability; SUBSTITUTE SHEET e) unsatisfactory correlation with available models. The above explanation clearly indicates, therefore, the importance of correct utilization of the assist gas, for improving the process in terms of efficiency, quality and repeatability. Correct usage will also provide for a better correlation of the data involved, thus enabling a better understanding of the phenomenon and its control parameters, and more straightforward, effective models for predicting, achieving and maintaining the required results. SUMMARY OF INVENTION It is an object of the present invention to provide a straightforward, reliable laser processing method and apparatus designed to overcome the above drawbacks typically associated with known methods. According a general feature of the present invention, there is provided a laser processing apparatus including a focusing head for focusing a laser beam on to the work area of at least one workpiece; a conduit for directing a jet of assist gas in controlled manner at least close to said work area; " characterized by the fact that the conduit comprises nozzle for producing a high speed jet; and that the area of workpiece to which the jet of the assist gas is directed is substantially covered by the walls of the conduit located downstream from the nozzle. According to the first specific feature of the present invention, there is provided a method of supplying laser cutting gas, whereby the laser beam is directed on to a cutting portion of the workpiece, and characterized by the fact that it comprises a phase wherein a jet of gas is guided by solid walls on to a surface of said workpiece at least close to said portion, and at such a velocity as to control the fluid-thermodynamic effects caused by interaction with ITUTE SHEET said laser beam and with the material liquefied by the same, and to flush said liquefied material out of the cut in steady, controlled manner. According to the first specific feature of the present invention, there is also provided a laser cutting apparatus comprising a head for focusing the laser beam on to a cutting portion of the workpiece; and means for supplying a jet of gas at least close to said cutting portion; characterized by the face that said supply means comprise a solid-walled conduit extending into contact with a surface of said workpiece; said conduit being designed to supply said jet at such a velocity as to control the fluid-thermodynamic effects caused by interaction with said laser beam and with the material liquefied by the same, and to flush said liquefied material out of the cut in steady, controlled manner. According to second specific feature of the present invention τ there is provided a method whereby a laser beam is directed on to a portion of a substantially perpendicular work surface; characterized by the fact that a assist gas is fed on to said surface portion crosswise in relation to said beam and along a conduit defined by solid walls; said jet of assist gas being supplied at such a speed as to eliminate the existing ambient gas, and minimize the physical, dynamic and chemical effect of said laser beam on said gas, which thus acts substantially as an inert gas. A laser processing apparatus according to the above method is incorporated in an apparatus comprising means for directing said laser beam on to said surface portion, and comprises means for storing said assist gas; and means for supplying said gas from said storage means; characterized by the fact that said supply means comprise at least one conduit defined by solid walls and designed to supply a jet of said gas at said speed; said conduit being located so as to direct said jet on to said surface crosswise in relation to said laser beam; said conduit comprising a transverse opening for the passage of said laser beam on to said surface portion. SUBSTITUTE SHEET According to preferred embodiment of the present invention, there is provided a workpiece laser processing apparatus comprising a focusing head for focusing the beam on to the work area of at least one workpiece; a solid-walled conduit for directing a jet of assist gas in controlled manner at least close to said work area; and conveying means for effecting displacement of said at least one workpiece in relation to said focusing head and along a given work path; characterized by the fact that it comprises means for controlling and enabling controlled rotation of said conduit in relation to said focusing head, so as to maintain, at nay time and at least at said work area, a predetermined direction and position of said conduit in relation to said work path. With this arrangement, the assist gas can be supplied in a given direction and sense in relation to the cutting or welding path at the laser head or laser beam focal spot location, so that optimum performance of the laser apparatus is ensured. This is especially so in the majority of cases involving cutting or welding paths in the form of curves or broken lines. A number of preferred non-limiting embodiments of the present invention will be described by way of example with reference to the accompanying drawings, in which: BRIEF DESCRIPTION OF.DRAWING Fig. 1 shows a schematic view in perspective, with parts removed for simplicity, of a first embodiment of a laser cutting apparatus in accordance with the first specific feature of the present invention. Fig. 2 shows a partial longitudinal section along line II - II of the apparatus of Fig. 1. Fig. 3 shows a schematic, partial longitudinal section of a second embodiment of the cutting apparatus. SUBSTITUTE SHEET Fig. 4 shows a schematic view in perspective, with parts removed, of a first embodiment of a laser welding apparatus in accordance with the second specific feature of the present invention. Fig. 5 shows a schematic section of the first embodiment of the laser welding apparatus. Fig. 6 shows a schematic section of a variation of the apparatus illustrated in Fig. 5. Fig. 7 shows a schematic section of the second embodiment of the welding apparatus. Fig. 8 shows a section along line VTII-VTII in Fig. 7. Fig. 9 shows a section along line IX-IX in Fig. 8. Fig. 10 shows a schematic section of the third embodiment of the welding apparatus. Fig. 11 shows a section along line XI-XI in Fig. 10. Fig. 12 shows a schematic section of a further variation of the apparatus illustrated in Fig. 5. Fig. 13 shows a schematic view in perspective of a third embodiment of the laser cutting apparatus. Fig. 14 shows a longitudinal section of the third embodiment of the laser cutting apparatus illustrated in Fig. 13. Fig. 15 shows a longitudinal section of the forth embodiment of the welding apparatus. Fig. 16 shows a partial longitudinal section and a partial side view of the third embodiment of the cutting apparatus. SUBSTITUTE SHEET Fig. 17 shows a bottom plan view of the apparatus of Fig. 16. Fig. 18 shows a longitudinal section, as in Fig. 16, of a forth embodiment of the cutting apparatus. Fig. 19 shows a longitudinal section of a fifth embodiment of the cutting apparatus. Fig. 20 shows a longitudinal section of a sixth embodiment of the cutting apparatus. Fig. 21 shows a longitudinal section of a seventh embodiment of the cutting apparatus. Fig. 22 shows a partially-sectioned bottom plan view of the apparatus of Fig. 21. Fig. 23 shows a longitudinal section of the eighth embodiment of the cutting apparatus. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Number 10 in Fig. 1 indicates a cutting apparatus featuring a laser beam 11. Apparatus 10 substantially comprises a laser beam source (not shown) , and a focusing head 12 for focusing beam 11 on to a cutting portion 13 of the surface 14 of a workpiece 15, e.g. a metal plate of ferrous material. Apparatus 10 provides for making a very narrow, straight cut 16, with two facing surfaces 17 on the two halves of workpiece 15. According to the present invention, a jet 18 of gas, e.g. oxygen, is guided on to portion 13 of workpiece 15, on which the laser beam 11 impinges, in such a manner as to prevent it from interacting with the atmospheric air before reaching portion 13. As shown in Fig. 2, the jet 18 is supplied on to SUBSTITUTE SHEET portion 13 in an inclined direction in relation to laser beam 11. In particular, the jet 18 is supplied at high, preferably supersonic, velocity, in such a manner as to enable control of the jet and the fluid-thermodynamic action of the same, and so provide, via exothermic reaction of the jet with the ferrous material, for improved fusion of the material by the laser beam, and rapid, steady, homogeneous flush-out of the liquefied material. Cutting apparatus 10 comprises conveying means, shown schematically by 19 in Fig. 2, for moving workpiece 15 in relation to head 12 along the cutting line and in the direction of arrow F. Apparatus 10 also comprises an oxygen supply circuit (not shown) to which is connected a supply device 20. The supply device 20 presents a solid-walled inner conduit 21 curving in such a manner as to direct the jet 18 from direction A, substantially parallel to axis L of laser beam 11, to direction B lying in the plane defined by direction A and axis L, but inclined in relation to axis L by a given angle a . The angle α is such that the jet 18 presents a transverse component in relation to axis L oriented in the opposite direction to the traveling direction F of workpiece 15, so that the jet 18 flows over surfaces 17 of cut 16 through the entire thickness of workpiece 15. Flush-out is assisted by the angle of jet 18, which increases impact pressure and, consequently, penetration and removal of the liquid layer.. The supply device 20, which is substantially shaped like a shoe, presents a top base 22 in which the inlet 23 of conduit 21 is formed; and a bottom base 24 in which the outlet 26 of conduit 21 is formed close to the tapered front end portion 25 of the device 20. The device 20 is defined laterally by two closely fitted flat parallel walls 27 (only one of which is shown in Fig. 1) , SUBSTITUTE SHEET the inner surfaces of which laterally define conduit 21, which is thus narrow and substantially equal to the width of the cut 16. The length of the outlet 26 is roughly 3 to 6 times said width. The device 20 is defined at the front by a first shaped wall 29, and at the bottom and rear by a second shaped wall 30. Together with the lateral walls 27, the wall 30 defines bottom base 24, which is flat and slides over surface 14 of workpiece 15 in airtight manner via mechanical or fluid- dynamic means preventing gas in the atmosphere from mixing with the cutting gas 18. This may be achieved, for example, using a labyrinth seal or, in the case of straightforward but accurate mechanical slide,, a cutting gas pressure higher than that of the atmospheric gas. In addition to lateral walls 27, conduit 21 is thus also defined by respective inner surfaces 34 and 35 of walls 29 and 30. As described in the following, surfaces 34 and 35 are curved and so shaped as to define the aerodynamic characteristics of conduit 21. In particular, the generating lines of surfaces 34 and 35 are perpendicular to lateral walls 27, so that conduit 21 presents a rectangular cross section of constant width and varying in area. Conduit 21 is curved so as to direct jet 18 from direction A at inlet 24 to direction B at outlet 26. Conduit 21 constitutes a convergent-divergent nozzle for attaining a supersonic velocity of jet 18, and consists o.f a convergent portion 33a between inlet 23 and a contracting section (throat) 33b defined by a shaped portion 36 of curved upper surface 34; and a divergent portion 33c between throat 33b and outlet 26. At the end of tapered portion 25, front wall 29 of supply device 20 presents an opening 37 perpendicular to outlet 26 and coaxial with beam 11, which is thus allowed to pass through jet 18 in the end portion of conduit 21. Beam 11 SUBSTITUTE SHEET penetrates workpiece 15 at impact surface 13, and gradually works down to surface 13a (Fig. 2) where the cut is originated and extended. As the workpiece is cut, jet 18 only flows over a very narrow strip of original surface 14, substantially equal to the width of the cut, and penetrates between surfaces 17 over the entire thickness of workpiece 15. Closed conduit 21 guides jet 18 in controlled manner up to surfaces 13 and 13a, thus preventing it from mixing with the air; the jet 18 pass through the point at which beam 11 interacts with surfaces 13 and 13a, so as to exploit the exothermic reaction of the jet for improving liquefaction of the material, and the fluid- mechanical action of the jet for flushing the liquid material out of cut 16. Fig. 3 shows a cutting apparatus 40 according to the second embodiment of the present invention, and the following description of which is limited solely to the differences as compared with apparatus 10 in Figs. 1 and 2. Any parts similar to those of apparatus 10 are shown using the same numbering system. Apparatus 40 presents a more compact supply device 20 located next to focusing head 12 and having a conduit 21. In this case, laser beam 11 is located entirely outside conduit 21. Conduit 21 provides for directing jet 18, substantially parallel to beam 11, on to a cutting portion 16 adjacent to surface 13a in which the actual cut is originated. In this case also, conduit 21 is a convergent-divergent type for attaining a supersonic velocity of jet 18, and is defined by closely fitted flat parallel lateral walls 27 for limiting the width of jet 18 to substantially the same width as cut 16. Apparatus 40 operates in the same way as apparatus 10, and therefore requires no further description. The advantages of the cutting apparatus according to the above embodiment will be clear from the foregoing description. Firstly, the design of conduit 21 provides for significant saving by only supplying the amount of oxygen strictly required for the cutting process. Secondly, the high velocity of jet 18 provides for rapid flush-out of the liquefied material, and for controlling the fluid dynamics of the same, so as to substantially eliminate any alteration of the cutting faces, in particular, the longitudinal choking line and scoring up and downstream for the same typically associated with known methods. Thirdly, the solid wall of conduit 21 prevents the gas from mixing with atmospheric air, and all the drawbacks this entails. Finally, fluid-thermodynamic control of the jet ensures steady, controlled sweep of the oxygen over the entire thickness of the material, thus enabling steady control of the various phenomena involved in the process, and a substantially perfect finish of the cutting faces. To those skilled in the art it will be clear that changes may be made to the embodiments described and illustrated herein. For example, changes may be made to the section or design " of conduit 21, or to the way in which atmospheric air is prevented from mixing with the cutting gas where supply device 20 contacts or slides over surface 14 of workpiece 15. Now, with reference to Fig. 4, number 110 indicates the first embodiment of a laser welding apparatus in accordance with the second specific feature of the present invention. The laser welding apparatus substantially comprises a source for emitting a laser beam 111 issued from a laser head 112 and collimated on to a portion 113 of a work surface 114. In particular, apparatus 110 effects a weld 116 between two facing surfaces 117 of two metal workpieces 118, and producing a weld bead 119 on surface 114. SUBSTITUTE The apparatus 110 provides for supplying, via a conduit 120 defined by rigid walls, an inert or any appropriate shielding gas (assist gas) over portion 113 of surface 114 subjected to laser beam 111, so as to prevent atmospheric air contacting portion 113 during the welding process. The gas jet 121 is supplied over portion 113 of surface 114 crosswise in relation to the direction of beam 111, and at high, preferably supersonic, speed for substantially preventing the shielding gas from being affected by beam 111. The gas travels and is supplied in the same directions the workpiece to remove the atmospheric air; supplies fresh shielding tas over portion 113 as this moves beneath beam 111; and maintain the gas shield for some time after welding, and after both the weld and workpiece have cooled. As illustrated in Fig. 5, for maintaining constant efficiency over the entire thickness^ of jet 121, this may consist of a number of elementary currents 122 varying in speed according to the height of current 122 in relation to the conduit face adjacent to the work surface. The speed of the gas over the surface portion subjected to the laser beam must be directly proportional to the power of the beam, which, due to focusing, increases as the beam gets closer to portion 113; thus the speed of currents 122 preferably decreases upwards. In the Fig. 5, welding apparatus 110 comprises handling means, indicated schematically by 123, for moving workpieces 118 in relation to laser beam head 112; and the gas supply device comprises a gas tank 124, and a closed, solid-walled conduit 125 having a substantially rectangular-section portion 126 located over surface portion 113. To vary the heat content of elementary gas currents 122, both tank 124 and currents 122 may be separated by solid walls along the entire path of the gas, to produce a number of parallel gas circuits, in which both the stagnation parameters (pressure, temperature) and the characteristics of the gas itself (molecular weight, specific heat ratio, etc.) may be SUBSTITUTE SHEET determined as required, and even differ from one circuit to another. The portion 126 comprises a first wall 127 mating with, in this case, flat work surface 114; and a wall 128 opposite wall 127. The portion 126 slides over surface 114 of workpieces 118 as these are moved along by handling means 123. The portion 126 also presents an opening consisting of two holes 129, 131 in respective walls 127, 128, for the passage of beam 111 on to surface portion 113. The area (or, in the case of a circular beam, the smallest diameter) of hole 131, is the smallest compatible with the passage of beam 111. As beam 111 is conical, for collimating it on to portion 113, the diameter of hole 129 is smaller than that of hole 131, but conveniently larger than the impact section of the beam 111, so as to accommodate the portion of the material of workpieces 118 liquefied by beam 111 and forming weld bead 119. The same also applies to beams of other, e.g. rectangular or square, sections. In Figs. 4 and 5, the portion 126 presents a rectangular cross section, the width and height of which conveniently range respectively from 4 to 10 and 3 to 6 times the diameter of hole 129. Upstream from holes 129 and 131, the portion 126 presents gas accelerating means, e.g. nozzle means 132, for bringing jet 121 up to supersonic speed. To enable the gas, heated by the laser beam, to expand downstream from portion 113, hole 131 is located in a sloping portion 133 of wall 128, so that portion 126 presents a larger section downstream from holes 129, 131. This section must be further increased to accommodate the weld bead section downstream from hole 129, as well as any aerodynamic bodies inside the conduit. According to a first variation, the nozzle means 132 comprise a number of elementary nozzles 134 for producing respective elementary gas currents 122 at different levels in relation to portion 113, and designed to produce, in respective currents 122, speeds decreasing upwards according to the level of currents 122. The heat content and the nature of the gas in each current 122 may also differ, to enable other parameters, such as density, specific heat, absorption and ionization potential, to be so selected as to produce a gas having homogeneous optical properties and being totally unaffected, chemically and physically, by the variable- intensity laser beam, and throughout the welding process. In other words, the shielding gas (assist gas), which plays no part in the actual process, acts as an aerodynamic window, which is totally transparent to the laser beam and inert in relation to the welding process. During welding, jet 121 flows over portion 113 of work surface 114 perpendicularly to laser beam 111, thus providing for effective, constant shielding of portion 113, while beam 111 is directed through hole 131, jet 121, and hole 129 on to portion 113 for welding surfaces 117 of workpieces 118. The supersonic speed of jet 121 provides for minimum contact between the shielding gas and beam 111, so as to substantially prevent any physical or chemical change in the gas, such as density, refraction coefficient and sped, thus also eliminating convective motion, excitation or ionization of the gas. In the variation illustrated in Fig. 6, the nozzle means 132 are formed by so shaping wall 128 of portion 126 as to produce, in known manner, variable-speed gas jet 121 (multi- Mach nozzle). In particular, the wall 128 may be so shaped as to produce a number of elementary gas currents 122 decreasing upwards in speed, at the beam crossover point, in relation to the distance between current 122 and wall 127 and, therefore, surface portion 113. In a further, simplified, solution, the nozzle may provide for one supersonic sped, provided this is sufficient for achieving the objectives described above, i.e. provided a jet can withstand the maximum power of the beam without incurring drawbacks in the low-power regions. Such drawbacks may even be purely economical, such as a waste of gas, by supplying it at a higher speed than necessary over the low-power regions of the beam. The simplicity of the design and the wider margin of safety this provides for, however, may SUBSTITUTE SHEET compensate for the increase in running cost. In the embodiment illustrated in Figs. 7, 8, and 9, the wall 127 defines, upstream from hole 129, a suitably streamlined body 135 having its longer axis aligned with the flow direction of the gas, for protecting the formation of the weld bead commencing at portion 113. As the formation of the molten metal from which weld bead 119 is formed is accompanied by thermo-fluid-dynamic pulsations, the body 135 provides for: a) protecting the molten metal from impact and, consequently, expulsion or erosion by the gas jet; and b) preventing the thermodynamic pulsations of the molten metal from involving a portion of the jet larger than the section of the body, thus providing for aerodynamic shielding. The body 135 consists of a half cone 135a, the maximum end section of which continues into a half cylinder 135b of the same section, which is slightly larger than that of weld bead 119. The length of half cylinder 135b and half cone 135a is respectively 3-4 and 6-7 times the diameter of hole 129. To protect the bead 119, during welding and until it has hardened, from erosion by the gas, a hollow half cylinder 136 of the same diameter as half cylinder 135b is located downstream from hole 129 and coaxially with body 135. A sufficient, although weaker, stream of shielding gas penetrates half cylinder 136 for continued shielding of the bead as it hardens. In the embodiment illustrated in Figs. 10 and 11, formation of the weld bead is protected by the thickness of wall 127, which is comparable to the height of the bead 119, and wall 127 presents a slot 137 originating downstream from hole 129 and extending along the entire length of wall 127. In cases where appearance takes precedence over structural/mechanical performance, a flat, smooth weld is SUBSTITUTE SHEET preferred, both of which characteristics are obtainable by appropriately streamlining portion 126 of conduit 120, both up- and downstream from hole 129, and by varying the parameters of the gas jet. In Fig. 12, for example, removal of bead 119 is achieved by appropriately streamlining wall 127 upstream from hole 129. In this case, the wall 127 presents a groove 140 having its main axis in the gas flow direction, and so designed as to cause part of supersonic jet 121 to flow tangentially over the workpiece portion immediately upstream from hole 129. Downstream from hole 129, groove 140 continues in the form of slot 137 up to the end of wall 127. The groove 140 thus causes the jet to flow over the work surface, commencing upstream and continuing downstream from hole 129. Both up- and downstream from hole 129, the thermo- fluid-dynamic properties of the supersonic jet are so exploited as to distribute the action of the gas on the molten material throughout the liquefaction and subsequent hardening stages. The advantages of the welding apparatus according to the present embodiment will be clear from the foregoing description. In particular, the solid wall of the conduit directing shielding gas jet 121 on to surface portion 113 subjected to laser beam 111 prevents the gas from mixing with atmospheric air. Also, the direction and high speed of jet 121 minimizes the effect of beam 111 on the inert gas, by preventing chemical and physical changes, such as a variation in density and temperature, in turn affecting the refraction index of the gas, and resulting in convective motion seriously impairing the focus and, consequently, the power and efficiency of the laser beam. Similarly, the present apparatus provides for totally eliminating excitation and ionization phenomena absorbing and seriously impairing the power of the beam. These effects, combined with the defocusing mentioned above, may, in extreme cases, so impair the power and efficiency of the beam as to result in blanketing, whereby most of the energy of the beam is prevented from reaching the workpiece, and is dissipated in the gases adjacent to the work surface. The present apparatus also overcome the drawbacks currently posed by a larger portion of the impaired energy reaching the work surface, but in other forms, thus resulting in irregular weld beads; undesirable multiple stress states accompanied by an irregular metallurgical structure; inferior quality welds requiring greater energy; reduced output; and higher investment and running cost. In the above cutting apparatus and welding apparatus, to ensure optimum performance thereof, the assist gas must be supplied in a given direction and sense in relation to the cutting or welding path at the laser head or laser beam focal spot location. In the following, therefore, a laser processing apparatus adapted to supply the assist gas in a desired direction and sense in relation to the processing path will be described in detail. Number 201 in Fig. 13 indicates a laser cutting apparatus substantially comprising a laser beam source (not shown) , and a head 206 housing a lens (not shown) for focusing a laser beam 205 (Figs. 14 - 25) on to work area 207 of surface 208 of workpiece 209, and so effecting an extremely narrow, constant section cut 210 along any work path. A jet 213 (Figs. 14 and 16 - 25) of assist gas, e.g. oxygen, is directed in controlled manner on to work area 207 of piece 209 subjected to laser beam 205, to prevent interaction with the atmosphere prior to reaching work area 207. Jet 213 is supplied on to work area 207 in an inclined direction in relation to laser beam 205, and at high, preferably supersonic, speed, for controlling the jet and the fluid-thermodynamic action of the same, and so providing, by virtue of the exothermic reaction of the jet with the ferrous material, for improved fusion of the material by the laser SUBSTITUTE SHEET beam, and rapid, homogeneous, steady flush-out of the liquefied material. This provides for eliminating pulsation or oscillation of the jet entering the cut, as well as choking resulting in oscillation or vorticlty as the jet flows through the cutting channel, as described above. The cutting apparatus 201 comprises means, shown schematically by 215 in Figs. 14 and 16-23, for moving workpiece 209 in relation to head 206 along work path T (Fig. 13) and, therefore, with the apparatus 201 positioned as shown, in the instantaneous traveling direction indicated by arrow S. The cutting apparatus 201 also comprises an oxygen nozzle 217 extending laterally from the conical portion (tip) 206a of head 206, and designed to rotate about axis A of head 206. Rotation of nozzle 217 is controlled by means which maintain the nozzle aligned with head 206 in instantaneous traveling direction S, as shown in Fig. 13, which shows two different positions of the apparatus 201 in relation to workpiece 209. As shown in Fig. 14, nozzle 217 defines internally a stagnation chamber 219 extending substantially parallel to tip 206a. Chamber 219 is supplied at the top with the assist gas through holes 222 connected to a supply circuit (not shown) ; the cliamber 219 terminates at the bottom in a solid-walled conduit 220 for directing jet 213 on to workpiece 209. More specifically, jet 213 is supplied, at all times, substantially in direction B in the plane defined by axis A of head 206 and instantaneous traveling direction S of workpiece 209. Direction B is also inclined in relation to axis A, so that jet 213 presents a component perpendicular to axis A, the sense of which (opposite or the same as instantaneous traveling direction S) depends on the application. In the example shown, the sense of the component perpendicular to axis A is opposite that of instantaneous traveling direction S of workpiece 209. Chamber 219 and conduit 220 are defined by a wall 223 SUBSTITUTE SHEET defining a flat base 224 at the bottom, designed to slide over and in contact with surface 208 of workpiece 209, and in which is defined the outlet 225 of conduit 220. Outlet 225 is conveniently narrow, ranging in width from 0.5 to 2 times the width of cut 210, and presents a length roughly 3 - 6 times its width. Therefore the area of the workpiece 209 to which the jet 213 is directed is essentially covered by the walls of the conduit 220, as in the case illustrated in Figs. 1-3.. Conduit 220 constitutes a convergent-divergent nozzle for producing a supersonic jet 213, and consists of a convergent portion 220a between chamber 219 and a contracting section (throat) 220b; a divergent portion 220c downstream from throat 220b; and a substantially constant-section work portion 220d between divergent portion 220c and outlet 225. Nozzle 217 is supported on head 206 by supporting means 226 shown schematically in Fig. 14 and in various detailed embodiments in Figs. 16 - 23, and which enable nozzle 217 to be rotated about axis A of head 206 by control means not shown in Fig. 14 but described in detail later on. Welding unit 290 in Fig. 15 provides for welding, along a curved path, two workpieces 291 and 292 having respective facing surfaces, only one of which, 293, is shown along with a portion 294 of the weld. With the exception of conduit 220, the overall design of unit 290 is similar to that of cutting apparatus 201. Any parts common to both units are therefore shown using the .same numbering system. Welding unit 290 comprises a nozzle 217 for supplying shielding gas along solid-walled conduit 220 and so preventing atmospheric air from contacting weld area 207. For welding application, the shielding gas must be supplied so as to flow over work area 207 of surface 208 transversely in relation to laser beam 205 and at high, preferably supersonic, speed. As shown in Fig. 15, the gas jet 295 is preferably SUBSTITUTE SHEET supplied in the same direction and sense as the instantaneous travel S of workpieces 291, 292 by means 215. For this purpose, conduit 220, again in the form of a convergent- divergent nozzle, extends parallel to and in contact with surface 208 of workpieces 291 and 292, and presents, as of stagnation chamber 219, a convergent portion 220a; a throat 220b; a divergent portion 220c; and an end portion 220d, which also presents two aligned holes 296 in respective opposite walls of conduit 220 for enabling passage of laser beam 205 on to work area 207. In this case also, chamber 219 is supplied with the assist gas through holes 222 connected to a supply circuit (not shown) . The apparatus 290 is supported on head 206 by supporting means 226 (shown schematically) , which, as on apparatus 201, enable nozzle 217 to be rotated by control means about axis A of head 206. A first embodiment of the rotation control means will be described with reference to Figs. 16 and 17, in which the control means, indicated as a whole by 228 utilize pneumatic propulsion. More specifically, the control means 228 comprise a pneumatic chamber 229 (Figs. 14 and 15) formed inside nozzle 217, adjacent to the top portion of stagnation chamber 219, and separated from chamber 219 by a partition wall 230. Pneumatic chamber 229, which is radially further away from axis A as compared with chamber 219, is supplied with compressed gas along a tube 231 extending from the lateral wall of nozzle 217 and having a coiled portion enabling it to extend when nozzle 217 is rotated. The nitrogen used on head 206 for cooling the focusing lens (not shown) is conveniently employed as the compressed gas; for this purpose, tube 231 terminates at head 206 over supporting means 226. Alternatively, other compressed gases, e.g. compressed air, may be supplied from an independent source along a separate line. Pneumatic chamber 229 presents two lateral openings 232 (Figs. 14 and 15) on either side of nozzle 217 and aligned SUBSTITUTE SHEET perpendicular to axis A. Each opening 232 presents a solenoid valve 233 (Fig. 17) for selectively enabling passage of the compressed gas from pneumatic chamber 229 to one of two aligned, oppositely-oriented nozzles 234. Solenoid valves 233 are connected by electric conductors 235 to an electronic system (not shown) for controlling opening and closing of the valves. For enabling rotation of nozzle 217, supporting means 226 comprise a bush 237 connected integral with nozzle 217 and surrounding a portion of head 206. Between bush 237 and head 206, there are provided two ball bearings 238 separated by a pair of spacers 239 and locked axially by two pairs of retaining rings 240 housed inside respective grooves on head 206 and inside bush 237. A ring 241 fitted to bush 237 protects the area above bearings 238, beneath which an encoder 242 is provided for detecting the angular position of nozzle 217 in relation to head 206. Encoder 242 comprises a ring 243 fitted to the inner wall of bush 237 and projecting towards head 206; and a sensor 244 integral with head 206 and connected by conductors 245 to the electronic system (not shown ) controlling opening and closing of solenoid valves 233. Fig. 16 also shows circuit 216 for supplying nozzle 17, and which comprises two hoses 218 connecting nozzle 217 to an oxygen tank (not shown) . Hoses 218 are preferably wound about head 206, present respective coiled portions like tube 231, and, as shown in Fig. 17, terminate on either side of nozzle 217, at holes 222 (Figs. 14 and 15), substantially perpendicular to axis A of head 206, so as to compensate each other and prevent the formation of vortexes inside stagnation chamber 219. During the cutting operation, depending on the instantaneous traveling direction S of workpiece 209, one of solenoid valves 233 is opened to supply compressed gas from the respective nozzle 234 in one direction and so rotate nozzle 217 in the opposite direction. Actual rotation of SUBSTITUTE SHEET nozzle 217 is detected by encoder 242, which supplies the electronic control system with a corresponding electric signal enabling it to open and close solenoid valves 233 to the required degree of accuracy. At any given time, supply direction B of oxygen jet 213 thus presents a transverse component in relation to axis A, directed parallel to the instantaneous direction of both workpiece 209 and cut 210, so that jet 213 penetrates between and flows over the two faces of cut 210 for the entire thickness of workpiece 209. Consequently, the advantage afforded by a controlled jet, as described above with reference to Figs. 1-3, may also be exploited to the full, even in the case of a curved cutting path. In the embodiments shown in Figs. 18-20, the apparatus 201, which is otherwise identical to that of Figs. 14, 16 and 17, and therefore described wherever possible using the same numbering system, also presents means 248 for varying the height of nozzle 217 in relation to head 206. In fact, for the cut to be effected properly, laser beam 205 must penetrate inside workpiece 209, and the distance between focus F (Fig. 18) and surface 208 must be equal to a given percentage of the thickness S of workpiece 209, to which end, apparatus 201 presents means (not shown) for regulating the height of head 206. As correct operation of nozzle 217, however, depends on it being maintained contacting surface 208 of workpiece 209, said regulation also requires that the height of nozzle 217 be regulated in relation to focus F of laser beam 205, i.e. in the example shown, in relation to head 206. In the embodiment shown in Fig. 18, provision is therefore made for manual height adjusting means 248 comprising an externally-threaded sleeve 249 engaged by a thread 250 inside bush 237. In this case, ball bearings 238 are located between sleeve 249 and head 206, for enabling sleeve 249 to rotate in relation to head 206. Bush 237 is locked axially in relation to sleeve 249 and, consequently, head 206 by means of a threaded ring nut 251 screwed on to thread 250 of bush 237. SUBSTITUTE SHEET In the apparatus of Fig. 6, after adjusting the height of head 206 in relation to surface 208 of workpiece 209 and loosening ring nut 251, the height of nozzle 217 is adjusted by rotating bush 237 until base 224 of nozzle 217 again contacts surface 208. Ring nut 251 is then tightened for axially locking and rendering bush 237 integral with sleeve 249. During the cutting operation, sleeve 249 therefore rotates, together with nozzle 217 and bush 237, in relation to head 206, so as to follow cutting path T. In the embodiment shown in Fig. 19, height adjusting means 248 are in the form of an air-powered spring. More specifically, bush 237 surrounds in axially-sliding manner a sleeve 253 in turn surrounding head 206 via the interposition of bearings 238, so as to rotate substantially integral with bush 237 in relation to head 206. Over bush 237, head 206 is fitted with a boot 254 closed in airtight manner at the top and bottom by annular elements 255 and 256 inserted between boot 254 and head 206 and having respective seals 257 in the form of 0 rings. A circlip 258 is fitted about the top end of boot 254, for locking said top end and annular element 255 on to head 206. Between a portion of annular element 255 and head 206, there is conveniently inserted a ball bearing 259 locked axially by a retaining ring 260 on head 206, and a further circlip 261 is fitted about the bottom end of boot 254 for locking it on to the top end of bush 237. The chamber 262 defined between boot 254 and head 206 is filled with compressed gas, which therefore exerts a constant downward pressure on bush 237 through annular element 256, for maintaining base 224 of nozzle 217 contacting surface 208 of workpiece 209, regardless of the position of head 206 in relation to surface 208. The advantage of the above air- powered spring embodiment is that, by providing for a variable compressed gas source communicating with chamber 262, it enables the contact pressure exerted by spring 248 on surface 208 to be adapted as required. In the embodiment shown in Fig. 20, height adjusting means 248 provide for adjusting the height of nozzle 217 together with tip 206a, which, in this case, instead of being integral with the rest of head 206, is movable vertically along axis A. In the example of Fig. 20, which also shows (schematically) the focusing lens 300 fitted to head 206 by means of a support 301 (also shown schematically) , means 248 consist of a coil spring 303 inserted between a fixed stop 304, integral with head 206, and an annular projection 305 locked axially but rotating in relation to tip 206a by virtue of an axial bearing 306 between annular projection 305 and a shoulder 307 integral with tip 206a. Tip 206a thus rotates, together with nozzle 217, in relation to head 206, and the tip 206a -nozzle 217 assembly is maintained contacting workpiece 209 by spring 303, regardless of the position of lens 300 in relation to surface 208. A retaining ring 320 defines a bottom stop for an annular projection 323 on the top end of tip 206a, to prevent tip 206a and nozzle 217 from being detached from head 206. In the embodiment shown in Figs. 21 and 22, the means 264 controlling nozzle 217 are electrical. More specifically, the top end of bush 237, again integral with nozzle 217, presents helical teeth 265 meshing with a worm screw 266 on the output shaft 273 of an electric step motor 267. The free end 268 of worm 266 is supported on a bearing 269 fitted to a casing 270 in turn fitted integral with head 206 by threaded pins 271 and enclosing the top end of bush 237 and motor 267. By means of conductors 272, motor 267 is supplied and controlled by the control system (not shown) so as to rotate worm 266 and, consequently, bush 237 via teeth 265, and so set nozzle 217 to the required angular position in relation to head 206. In this case also, an encoder 242 is provided for detecting the actual position of nozzle 217 and controlling motor 267 accordingly. In the embodiment of Figs. 21 and 22, partition wall 230 and pneumatic chamber 229 are obviously dispensed with, and stagnation chamber 219 extends upwards into the portion formerly occupied by chamber 229, as shown by the dotted line in Figs. 14 and 15. SUBSTITUTE SHEET Though not shown, the embodiment in Fig. 21 and 22 may also be provided with means for adjusting the height of nozzle 217 in relation to head 206, as shown, for example, in Fig. 18, in which case, helical teeth 265 may be provided on sleeve 249. In the embodiment shown in Fig. 23, the means 274 for controlling rotation of nozzle 217 are again electrical, and comprise an electric motor 275 fitted coaxially between head 206 and a tubular element 276 rotating integral with sleeve 237 connected rigidly to nozzle 217. More specifically, the top portion of tubular element 276 supports the rotor 277 of motor 275, the stator 278 of which is supported integral with head 206. Stator 278 is connected to conductors 279, and rotor 277 to conductors 280 and a brush 281 and collector 282 system, so as to rotate tubular element 276 and set nozzle 217 to such an angular position in relation to head 206 that it is maintained tangent at all times to cutting path T. Bearings 283 are inserted between tubular element 276 and head 206, and an encoder 242 is again provided for detecting the actual angular position of nozzle 217. In the embodiment in Fig. 23, the height of nozzle 217 in relation to head 206 is adjusted by axially sliding bush 237 in relation to tubular element 276 and locking bush 237 by means of a pair of ring nuts 285. More specifically, the bottom portion of tubular element 276 presents an outer thread 286 engaged by ring nuts 285 at the top and bottom of bush 237 respectively. In this case, instead of being threaded, the inner wall of bush 237 presents an axial groove 287 engaged by a key 288 integral with the threaded portion 286 of tubular element 276. By loosening ring nuts 285, bush 237, guided axially by key 288, may thus be adjusted axially and locked in position by ring nuts 285. In place of the screw system shown, height adjustment in the Fig. 23 embodiment may, of course, be effected as shown in Figs. 18 to 20. SUBST To those skilled in the art it will be clear that changes may be made to the apparatus as described and illustrated herein without, however, departing from the scope of the present invention. In particular, the means for controlling rotation of nozzle 217 in relation to head 206 may feature pneumatic, hydraulic or electric solutions other than as described. Similarly, the means for adjusting the height of nozzle 217 may feature any number of solutions, including manual adjustment prior to start-up and alongside adjustment of the focus of the laser head; an elastic element for maintaining the base of nozzle 217 contacting the work surface throughout the process; or a pneumatic or electric solution for controlling and adapting the position of nozzle 217 and the thrust exerted on the work surface. Pre-startup or elastic height adjusting means are suitable for welding applications, or when cutting pieces with a "lead-in" edge. That is, when commencing the cut from the edge of the workpiece, or, when commencing inwards of the edge, from a through channel formed beforehand using other techniques. In either case, the molten material is flushed out by the assist gas on the opposite side of the workpiece to the laser head. Conversely, when commencing the cut inwards of the workpiece, without first providing for a through channel, the molten material must be flushed out on the laser head side until a passage Is formed through the workpiece enabling flush-out on the opposite side. Consequently, when commencing the cut, the jet must be directed on to the surface of the workpiece, at the beam impact point, whereas, under normal operating conditions, it must penetrate further inside the workpiece. As such, the height of the nozzle in relation to the surface of the workpiece must be adjustable automatically according to the cutting phase involved. Finally, the control and height adjusting means may be combined in any manner for both cutting and welding applications. SUBSTITUTE SriEET

Claims: CLAIMS 1. A laser processing apparatus (10; 110; 201, 290) comprising a focusing head (12; 112; 206) for focusing a laser beam (11;111; 205) on to the work area (13, 13a; 113; 207) of at least one workpiece (15; 118; 209, 291, 292); a conduit (21; 120; 220) for directing a jet (18; 122; 213, 295) of assist gas in controlled manner at least close to said work area (13, 13a; 113; 207); characterized by the fact that the conduit (21; 120; 220) comprises nozzle (33a, 33b, 33c; 132; 220a, 220b, 220c) for producing a high speed jet; and that the area of workpiece to which the jet (18; 122; 213, 295) of the assist gas is directed is substantially covered by the walls of the conduit (21; 120) located downstream from the nozzle (33a, 33b, 33c; 132). 2. A method of supplying laser cutting gas to a workpiece to be cut by a laser beam (11), wherein the laser beam (11) is directed on to a cutting portion (13, 13a) of the workpiece (15) , characterized by the fact that it comprises a phase wherein a jet (18) of gas is guided by solid walls on to a surface (14) of said workpiece (15) at least close to said portion (13, 13a) and at such a velocity as to control, by said jet (18) of cutting gas, the fluid-thermodynamic effects caused by interaction with said laser beam (11) and with the material liquefied by the same, and to flush said liquefied material out of the cut (16) in steady, controlled manner. 3. A method as claimed in Claim 2, characterized by the fact that the velocity of said jet (18) is supersonic. 4. A method as claimed in Claim 2 or 3 for cutting ferrous materials, characterized by the fact that said gas is oxygen. 5. A method as claimed in Claim 2, 3, or 4, wherein said laser beam (11) is focused on to said workpiece (15) by a focusing head (12), characterized by the fact that said jet (18) of gas is directed on to said cutting portion (13, 13a) in an inclined direction in relation to said laser beam (11), SUBSTITUTE SHEET so as to present a transverse component oriented the opposite way to the traveling direction of said workpiece (15) in relation to said beam (11) . 6. A method as claimed in Claims 2, 3, or 4, characterized by the fact that said jet (18) is directed on to a portion of said cut (16) adjacent to said cutting portion (13, 13a) and in a direction substantially parallel to said beam (11) . 7. A laser cutting apparatus comprising a head (12) for focusing the laser beam (11) on to a cutting portion (13, 13a) of a workpiece (15); and means (20) for supplying a jet (18) of gas at least close to said cutting portion (13) ; characterized by the fact .that said supply means (20) comprise a solid-walled conduit (21) extending into contact with a surface (14) of said workpiece (15) ; said conduit (21) supplying said jet (18) at such a velocity as to control the fluid-thermodynamic effects caused by interaction with said laser beam (11) and with the material liquefied by the same, and to flush said liquefied material out of the cut (16) in steady, controlled manner. 8. An apparatus as claimed in Claim 7, characterized by the fact that it comprises sealing means between said conduit (21) and said surface (14) of said workpiece (15) , for preventing the entry of atmospheric gas. 9. An apparatus as claimed in Claim 7 or 8, characterized by the fact that said conduit (21) defines a convergent- divergent nozzle (33a, 33b, 33c) for attaining a supersonic velocity of said jet (18). 10. An apparatus as claimed in one of the foregoing Claims from 7 to 9, characterized by the fact that said conduit (21) is defined laterally by two closely fitted parallel walls (27), for limiting the width of said jet (18) to substantially the same width as said cut (16) . 11. An apparatus as claimed in Claim 10, characterized by the SUB fact that the length of the outlet (26) of said conduit (21) is 3 to 6 times said width. 12. An apparatus as claimed in Claim 10 or 11, characterized by the fact that said conduit (21) is also defined by two shaped facing walls (29, 30) having respective curved inner surfaces (34, 35) with their generating lines perpendicular to said lateral walls (27); said conduit (21) having a rectangular inner section of constant width and varying in area. 13. An apparatus as claimed in one of the foregoing Claims form 7 to 12, characterized by the fact that it comprises conveying means (19) for moving said workpiece (15) in a given direction (F) in relation to said focusing head (129; said jet (18) being directed on to said cutting portion (13, 13a) in an inclined direction (B) in relation to said beam (11) so as to present a transverse component oriented the opposite way to said traveling direction (F) of said workpiece (15) in relation to said beam (11) . 14. An apparatus as claimed in Claim 13, characterized by the fact that said conduit (21) is so curved as to direct said jet (18) from a direction (A), substantially parallel to said beam (11), to said inclined direction (B) . 15. An apparatus as claimed in one of the foregoing Claims form 12 to 14, characterized by the fact that one (30) of said shaped walls, together with said lateral walls (27), defines the bottom base (24) of said supply means (20); said base (24) sliding over a surface (14) of said workpiece (15), and presenting said outlet (26) of said conduit (21) located facing said cutting portion (13, 13a). 16. An apparatus as claimed in Claim 15, characterized by the fact that the other (19) of said shaped walls presents an opening (37) communicating with an end portion of said conduit (21) and facing said cutting portion (13, 13a) for enabling the passage of said beam (11) on to said portion (13, 13a). SUBSTITUTE SHE 17. An apparatus as claimed in any one of the foregoing Claims from 7 to 11, characterized by the fact that said jet (18) is directed on to a portion of said cut (16) adjacent to said cutting portion (13, 13a), and in a direction substantially parallel to said beam (11) . 18. An apparatus as claimed in any one of the foregoing Claims from 7 to 17, characterized by the fact that said gas is oxygen. 19. A method of gas shielding laser processed workpieces (118), wherein a laser beam (111) is directed on to a portion (113) of a substantially perpendicular work surface (114) ; characterized by the fact that a jet (121) of said shielding gas is fed on to said surface portion (113) along a conduit (120, 125, 126) defined by solid walls, so as to flow over said surface (114) crosswise in relation to said beam (111) ; said jet (121) of shielding gas being supplied at such a speed as to eliminate the existing ambient gas, and minimize the physical, dynamic and chemical effect of said laser beam (111) on said gas, which thus acts substantially as an inert gas. 20. A method as claimed in Claim 19, characterized by the fact that said gas jet (121) is supplied at supersonic speed. 21. A method as claimed in Claim 20, characterized by the fact that said gas jet (121) is supplied in the form of a number of parallel, elementary currents (122), the speed of which decreases alongside an increase in the height of said current (122) in relation to said surface (114) . 22. A method as claimed in Claim 21, characterized by the fact that said elementary currents are supplied by complete, independent, parallel circuits; each circuit having a respective tank, and being designed to supply a gas or gas mixture having different properties such as molecular weight and structure, specific heat ratio and different enthalpic and stagnation conditions such as total temperature and pressure. SUBSTIT 23. A method as claimed in any one of the foregoing Claims from 19 to 22, wherein said process consists in the welding of two metal workpieces (118), characterized by the fact that said jet (121) of shielding gas is supplied in the same direction as that in which said workpieces (118) travel in relation to said laser beam (111). 24. A device for gas shielding laser processed workpieces according to the method claimed in the claim 19, and incorporated in an apparatus comprising means (112) for directing said laser beam (111) on to said surface portion (113); said device comprising means (124) for storing said shielding gas; and means (125) for supplying said gas from said storage means (124) ; characterized by the fact that said supply means (125) comprise at least one conduit (126) defined by solid walls and designed to supply a jet (121) of said gas at said speed; said conduit (126) being located so as to direct said jet (121) on to said surface (114) crosswise in relation to said laser beam (111); said conduit (126) comprising a transverse opening (129, 131) for the passage of said laser beam (111) on to said surface portion (113). 25. A device as claimed in Claim 24, characterized by the fact that said conduit (126) presents a bottom wall (127) having a surface resting on said surface (114) of said workpiece (118); and a top wall (128) opposite said bottom wall (127) ; said transverse opening in said conduit (126) being defined by a first hole (129) formed in said bottom wall (127), and by a second hole (131) formed in said top wall (128) and coaxial with said first hole (129). 26. A device as claimed in Claim 25, characterized by the fact that said second hole (131) presents the minimum section enabling passage of said laser beam (111); and that the section of said first hole (129) is such as to accommodate the liquefied material of said workpiece (118) subjected to said laser beam (111) . SUBSTITUTE SHEET 27. A device as claimed in Claim 25 or 26, wherein said work surface (114) is substantially flat; characterized by the fact that said conduit (126) presents a substantially rectangular section. 28. A device as claimed in Claim 27, characterized by the fact that the width of said rectangular section ranges from 4 to 10 times the diameter of said first hole (129) , and the height of said rectangular section ranges from 3 to 6 times the diameter of said first hole (129). 29. A device as claimed in one of the foregoing Claims from 24 to 28, characterized by the fact that said conduit (126) comprises nozzle means (132) for producing a supersonic gas jet (121). 30. A device as claimed in Claim 29, characterized by the fact that said nozzle means (132) produce a gas jet (121) consisting of a number of elementary gas currents (122) decreasing in speed alongside an increase in height in relation to said surface portion (113). 31. A device as claimed in Claim 30, characterized by the fact that said currents (122) produced by said nozzle means (132) are not separated physically (multi-Mach nozzle) . 32. A device as claimed in Claim 31, characterized by the fact that said nozzle means (132) comprise a number of elementary nozzles (134) for producing said elementary gas currents (122). 33. A device as claimed in Claim 32, characterized by the fact that each said elementary nozzle (134) is supplied by a respective complete circuit connected to a respective tank (124). 34. A device as claimed in one of the foregoing Claims from 25 to 33, characterized by the fact that said conduit (126) presents a larger section downstream than immediately upstream SUBSTITUTE SHEET from said holes (129, 131). 35. A device as claimed in one of the foregoing Claims from 25 to 34, characterized by the fact that, upstream from said first hole (129), said bottom wall (127) presents an inner groove (140) for directing said shielding gas on to said surface portion (113); said groove (140) extending downstream from said first hole (129) and along the entire length of said bottom wall (127) . 36. A device as claimed in one of the foregoing Claims from 27 to 35, wherein said apparatus (110) provides for welding two metal workpieces (118) along respective mating flat surfaces (117), and comprises means (123) for moving said workpieces (118) in relation to said apparatus (110); characterized by the fact that said conduit (126) is so arranged as to supply said gas jet (121) on to said surface portion (113) in the same direction as said means (123) supporting said workpieces (118) . 37. A device as claimed in Claim 36, characterized by the fact that said conduit (126) presents means (137) for directing at least part of said jet (121) from said surface portion (113) along a bead (119) of said weld (116). 38. A device as claimed in Claim 37, characterized by the fact that said directing means comprise a slot (137) parallel to the axis of said conduit (126); said slot (137) being formed in said bottom wall (127) and extending downstream from said first hole (129) along the entire length of said bottom wall (127). 39. A device as claimed in Claim 36, characterized by the fact that, upstream from said first hole (129), said bottom wall (127) presents a streamlined body (135) having its longer axis in the direction of said gas jet (121) , and consisting of a half cone portion (135a) continuing into a semicylindrical portion (135b) . SUBSTITUTE SHEET 40. A device as claimed in Claim 39, characterized by the fact that it comprises a hollow half cylinder (136) having its longer axis in the flow direction of said gas; said half cylinder (136) forming part of said bottom wall (127) and extending downstream from said first hole (129). 41. A laser processing apparatus (201; 290) comprising a focusing head (206) for focusing the beam (205) on to the work area (207) of at least one workpiece (209, 291, 292); a solid- walled conduit (220) for directing a jet (213, 295) of assist gas in controlled manner at least close to said work area (207); and conveying means (215) for effecting displacement (S) of said at least one workpiece (209; 291, 292) in relation to said focusing head (206) and along a given work path (T) ; characterized by the fact that it comprises means (228; 264; 274) for controlling and enabling controlled rotation of said conduit (220) in relation to said focusing head (206), so as to maintain, at any time and at least at said work area (207) , a predetermined direction and position of said conduit (220) in relation to said work path (T) ; and that the conduit (220) comprises nozzle (220a, 220b, 220c) for producing a high speed jet; and that the area of workpiece to which the jet (18; 122; 213, 295) of the assist gas is directed is substantially covered by the walls of the conduit (21; 120) located downstream from the nozzle (33a, 33b, 33c; 132). 42. An apparatus as claimed in Claim 41, characterized by the fact that it comprises a supporting element (226) rotating integral with said conduit (220) and about an axis (A) through said focusing head (206) and perpendicular to the surface (208) of said at least one workpiece (209; 291, 292) at said work area (207) . 43. An apparatus as claimed in Claim 42, characterized by the fact that said control means comprise pneumatic propulsion means (228). 44. An apparatus as claimed in Claim 43, characterized by the fact that said pneumatic propulsion means (228) comprises SUBSTITUTE SHEET two nozzles (234) on opposite sides of said conduit (220) and perpendicular to said axis (A); said nozzles (234) being supplied selectively with compressed gas for generating a thrust in the opposite direction and sense to the direction in which said gas issues from one of said nozzles (234). 45. An apparatus as claimed in Claim 44, characterized by the fact that said pneumatic propulsion means (228) comprise a pneumatic chamber (229) close to said conduit (220) and connected to a compressed gas supply line (231) and to said nozzles (234) via respective controlled solenoid valves (233). 46. An apparatus as claimed in Claim 45, characterized by the fact that it comprises a shielding gas nozzle (217) internally defining said conduit (220), and having a stagnation chamber (219) connected to said conduit (220) and said pneumatic chamber (229); said pneumatic chamber (229) being located radially outwards of said stagnation chamber (219) in relation to said axis (A) . 47. An apparatus as claimed in Claim 42, characterized by the fact that said control means (264; 274) comprise an electric motor (267; 275). 48. An apparatus as claimed in Claim 47, characterized by the fact that said supporting element (226) comprises a bush element (237) rotating integral with said conduit (220) and connected to said electric motor (267) by gearing (265, 266). 49. An apparatus as claimed in Claim 48, characterized by the fact that said gearing (265, 266) comprises helical teeth (265) formed externally on said bush element (237); and a worm screw (266) integral with the output shaft (273) of said electric motor (267) and meshing with said helical teeth (265). 50. An apparatus as claimed in Claim 47, characterized by the fact that said electric motor (275) is located between said focusing head (206) and a tubular element (276) rotating SUBSTITUTE SHEET integral with said conduit (220). 51. An apparatus as claimed in any one of the foregoing Claims from 42 to 50, characterized by the fact that it comprises means (242) for detecting the angular position of said conduit (220) in relation to said focusing head (206). 52. An apparatus as claimed in Claim 51, characterized by the fact that said detecting means comprise an encoder (242) between said focusing head (206) and a bush element (237) surrounding said focusing head (206) and defining said supporting means (226). 53. An apparatus as claimed in any one of the foregoing Claims from 42 to 52, characterized by the fact that it comprises means (248) for adjusting the height of said conduit (220) in relation to said focusing head (206) and parallel to said axis (A) . 54. An apparatus as claimed in Claim 53, characterized by the fact that said height adjusting means (248) comprise manual adjusting means. 55. An apparatus as claimed in Claim 54, characterized by the fact that said manual adjusting means comprise a sleeve (249) threaded externally and rotating in relation to said focusing head (206) ; and a bush element (237) integral with said conduit (220) and engaging said sleeve (249) via a screw connection (250); provision also being made for an internally- threaded ring nut (251) screwed on to said sleeve (249) for axially locking said bush element (237) , and located over said bush element (237) in relation to said workpiece (209) . 56. An apparatus as claimed in Claim 54, characterized by the fact that said manual adjusting means comprise a tubular element (276) having an outer thread (286) and rotating in relation to said focusing head (206); a bush element (237) integral with said conduit (220); means (287, 288) for axially guiding said bush element (237) in relation to said tubular SUBSTITUTESHEET element (276) ; and at least one internally-threaded ring nut (285) screwed on to said tubular element (276) for axially locking said bush element (237) at the top. 57. An apparatus as claimed in Claim 53, characterized by the fact that said height adjusting means (248) comprise elastic means (254; 303) for exerting pressure on said conduit (220) in the direction of said workpiece (209) . 58. An apparatus as claimed in Claim 57, characterized by the fact that said elastic means comprise an air-powered spring (254) surrounding said focusing head (206) and integral, at one end, with said focusing head (206), and, at the other end, with said supporting element (226). 59. An apparatus as claimed in Claim 57, characterized by the fact that said elastic means comprise a coil spring (303) surrounding said focusing head (206) and acting between said focusing head (6) and said conduit (220). 60. An apparatus as claimed in any one of the foregoing Claims from 53 to 59, characterized by the fact that said focusing head (206) presents a substantially conical end portion (206a) moving and rotating in relation to said focusing head (206) and integral with said conduit (220) . 61. An apparatus as claimed in any one of the foregoing Claims from 41 to 60, characterized by the fact that it constitutes an apparatus (201) for cutting a workpiece (209) 62. An apparatus as claimed in any one of the foregoing Claims from 41 to 60, characterized by the fact that it constitutes an apparatus (290) for welding two workpieces (291, 292). 63. An apparatus as claimed in any one of the foregoing Claims from 41 to 60, characterized by the fact that it comprises a nozzle (217) for said compressed gas; said nozzle (217) extending adjacent to said focusing head (206) and SUBSTITUTE SHEET defining internally said conduit (220); and said conduit (220) being connected to a stagnation chamber (219) supplied by a pneumatic circuit (216) including two hoses (218) terminating on opposite sides of said stagnation chamber (219) and aligned perpendicular to said axis (A) . 64. A laser processing apparatus, substantially as described and illustrated herein with reference to the accompanying drawings. SUBSTITUTE SHEET

Inventors: LA ROCCA ALDO VITTORIO, LA ROCCA, ALDO, VITTORIO
Applicants: ALTEC SRL, ALTEC S.R.L.
Application year: 1993
Main classification: B23K26
IPC codes: B23K26