IL40703A - Method and apparatus for welding with a high power laser beam - Google Patents

Description

mtya *iin»n ipnm no's? nasi? a*i -IT»'V let od arid apparatus for welding with a high power laser beam A?CO ERBfT HlSli¾ECH ¾aBORAW>RT» IMC.

C:38717 DKT. AERL 117 This invention relates to a method and apparatus for welding with a laser beam.

Prior art welding techniques include oxyacetylene welding, arc welding and certain newer methods involving un-usual heat sources, such as electron beams and lasers.

In oxyacetylene welding, acetylene is burned with oxygen and the flame melts the work causing it to flow together. Additional metal to form a sound joint may be added from a wire or rod. The process is too slow and is insuffi-ciently precise for large-scale industrial use.

In arc welding, metals are fused together by utilizing the heat generated by an electric arc. Arc welding includes the MIG (metal inert gas) process and the "short-arc" and "spray-arc" variations of the MIG process, the hand held stick electrode technique, the submerged arc process and the TIG (tungsten inert gas) process.

The MIG process utilizes a consumable electrode 'of material similar to that of the work. The electrode material is transferred through a hot arc to the work while an gas forms a protective atmosphere. Disadvantages of the MIG process are that it is difficult to control manually, that the deposition of filler material from the electrode is not independent of heat input and that the protective atmosphere is expensive to provide. The "short-arc" variation differs from the ordinary MIG process in that the voltage-current controls are set so that the arc shuts off intermittently f & a series of molten drops become the heat source. The "short-arc" variation has the disadvantage that it cannot be used with highly reactive metals, it permits only a moderate pro-duction rate, its penetration is shallow and it tends to produce fusion defects as a result of the rapid solidification of the molten drops after they strike the work. The "spray-arc" variation differs from the ordinary MIG process in that the voltage-current controls are set so that the arc causes the metal to be deposited as a spray of fine particles. The "spray-arc" variation has the same disadvantages as the "short-arc" variation (except that fusion defects are not a problem) and it is moreover difficult to use in manual welding.

In the hand held stick electrode technique, a con-sumable welding electrode of a material similar to that of the work is utilized which is about 30 centimeters long, coated with flux and fixed to a hand held insulated grip. Disadvantages of this technique are that it has a low production rate and a high labor cost, that the flux coating must be cleaned and that the technique is not amenable to automatic welding.

The submerged arc process also utilizes a consumable electrode of a material similar to that of the work, but the actual weld itself occurs under a molten pool of flux and slag. This process is completely unsuitable for manual operation, it is not suitable for the welding of thin (less than 7 mm) work, it is restricted to the welding of horizontally disposed work and it gives off unpleasant fumes.

The TIG process is unique in arc welding in that the electrode is not consumed. The filler material is fed in separately. Its disadvantages are low production rates, low depths of penetration, a need for expensive shielding gases having very low dew points and a tendency to excessively heat the work surrounding the weld site.

In electron beam welding, metals are fused together by the energy provided by an electron beam. As generally practiced, it has the disadvantage of having to be performed in vacuum chambers. If performed in air, the voltage has to be increased and the beam becomes diffused. In either case, it has the further disadvantages of being susceptible to magnetic fields and to contamination.

In contrast with the welding techniques thus far described, high power laser welding has the following unique advantages: (1) the laser beam can be focused by means of an optical system to an area as small as 1 mil in diameter, so that the deep penetration mode of welding, similar to vacuum electron beam welds, can be achieved. (2) The optical system and workpiece can be located in atmospheric pressure air since this does not attenuate or scatter the laser beam. (3) The diameter of the beam focus may be in-creased to any desired size above 1 mil by appropriate adjustment of the optical system. (4) Because of the high radiant intensity of the laser beam, it is possible to drill small holes as small as 1 mil in diameter in sheets of metals, glass, and in ceramics. (5) Micromachining of thin films and microminiature electronic or mechanical components is possible by vaporizing the material in accordance with a desired pattern. (6) The welded joint by a laser is strong and there is less grain growth in the metal surrounding the weld area, and, therefore, in many cases, no heat treatment of the workpiece is necessary. (7) The welding energy of laser can be varied by control settings of the machine. (8) In the case of aircraft, rocket, or any other bulk structure, the welding can be accomplished from the out-^ side of the .structure by focusing the laser beam on the structure. (9) Because the laser beam can be "piped" through periscope optical systems without substantial attenuation of its intensity, the welding energy can be carried to parts and areas inaccessible by means of conventional welding equipment.

Unfortunately, difficulties have arisen due to using a high power laser beam in that in some cases the material as it is being welded creates an absorption cloud immediately above the interaction zone. This cloud absorbs and scatters the beam and thereby reduces the efficiency of the laser welding. It is therefore an aim of the invention to remove or reduce this cloud during the laser welding operation.

According to the invention there is provided a method in which a laser beam is used to form a weld along a line of contact between two pieces of metal, comprising reflectively focusing said laser beam to a focal point disposed at said line of contact, controlling the power of said laser beam to be in excess of 8 kilowatts and sufficient successively to form from said metal a cloud of vapor in the path of said beam and then to ionize said vapor, moving said focal point and line of contact relatively to one another in a direction aligned with said line of contact, directing a stream of inert gas to flow through said beam in a direction orthogonal to the beam path and in a stream path above and . spaced from said focal point, and controlling. the gas flow so that said gas and vapor do not remain in said beam path for a time sufficient for ionization thereof to occur, thereby preventing radiation of said laser beam away from said focal There is also provided apparatus in which a laser beam is used to form a weld along a line of contact between two pieces of metal, comprising a laser beam source for pro- viding an output laser beam, reflecting means for receiving and focusing said laser beam to a focal point at said line of contact, said laser beam source being controllable to provide said laser beam with a power in excess of 8 kilowatts and sufficient successively to form from said metal a cloud of vapor in the path of said beam and then to ionize said vapor, said reflecting means and pieces of metal having respective supports which permit movement of said focal point and line of contact relatively to one another in a direction aligned with said line of contact, and a source of inert gas connected to directing means for directing a stream of said gas to flow through said beam in a direction orthogonal to the beam path and in a stream path above and spaced from said focal point, the connection between the gas source and stream directing means including metering means for controlling the gas flow so that .said gas and vapor do not remain in said beam path for a time sufficient for ionization thereof to occur, thereby preventing radiation of said laser beam away from said focal point.

Other aims and advantages of the invention will become apparent from the following description taken in conjunction with the accompanying drawings, wherein: Figures la and lb are perspective views of laser welding apparatus for carrying out the method according to the invention; Figure 2 is an expanded perspective view of the -4b- interaction zone; Figure 3 is another expanded perspective view of the welding interaction zone in another embodiment of the laser apparatus? Figures 4a, 4b and 4c illustrate three plane cross-sectional views of materials welded according to the method of the invention; and Figure 5 is a graphic representation of the effect of laser power on welding speed.

With reference now to the drawings and more particularly to Figures la and lb, there is shown a preferred embodiment of the apparatus. A source of high laser energy capable of generating powers in excess of 8 kilo- watts is positioned so that the beam of energy II4. enters an optical system generally designated as 12e Characteristics of two typical lasers which may comprise the- source 11 are listed in the Table below* One such laser is of the gas dynamic type. A known gas dynamic laser which delivered 20 kilowatts of continuous wave (cw) power, had an optical cavity which was an unstable oscillator and operated with an output coupling of 60%, The resulting beam was an annular beam with the missing area in the center representing k.0% of the total area of the beam. Independent measurements of the beam quality of this device obtained by focal point scans indicated beam divergence which was close to the defraction limit. Another known laser which may be used in carrying out the method of this invention is of the electric discharge laser type. While this laser has been operated at a continuous power of llj. kilowatts, the test results reported herein were performed at an average output power less than II. kilowatts but greater than 8 kilowatts, continuous wave. The known electric discharge laser is a closed cycle, continuous wave device with a total electric efficiency of approximately l!$. The optical cavity was also an unstable oscillator. The output coupling was only 1+0 , since the annular output beam had an obstruction of 60$.

TABLE: lASER CHARACTERISTICS Gas Dynamic Laser Power: 15-20 kw Optical Cavity: unstable oscillator, 60% coupling Beam quality: near diffraction limited Electric Discharge Laser Power: 8-lIj. kw Optical Cavity: unstable oscillator, \0% coupling Beam quality: near diffraction limited The optical system 12 illustrated in Figure lb may comprise optics to provide control, focus, and/or W direction control. In the preferred embodiment as illustrated in Figure la, the beam II . emerging from the laser source 11 reaches a primary focal point just outside the laser device itself. The optical system 12 comprises a Cassegrain optical system which receives the beam II . on the receiver reflective elements 13. The beam II . is then reflected from elements 13 to transmitter elements 16 so that the focal point is reimaged on to the two parts 1$ and 17 which comprise the workpiece to be welded. The parts 1 and 17 are butted together and transported in a direction indicated by the velocity vector l8 through the secondary focal point at weld site 19 by a conventional translational table (not shown). Rather than moving the workpiece as previously described, the beam II4. may alternatively be optically moved by a conventional mirror system contained within the optical system 12.

In order to prevent absorption of the impinging rei aged beam II . by the absorption cloud created by the interaction of laser energy and material in weld site 19, a flow of gas above the weld is provided. This flow may be created by a suction pump drawing air across the weld site 19 and into the pump. Alternatively, the flow of gas may be provided from a source of gas 20, such as bottled argon, nitrogen or air. The amount of gas flow at the we^d site 19 may be controlled by a standard metering system 21 placed in the gas line. A gas nozzle 22 is positioned near weld site 19 to control the direction and quantity of gas flow across the weld site 19.

Attention is now directed to Figures 2 and 3 which illustrate two possible arrangements of apparatus ~ for carrying out the method in accordance with the invention. In Figure 2, one arrangement of apparatus is shown using cylindrical copper tubing as a gas nozzle 22 emitting less than three grams per second of Argon close to and preferably parallel to workpiece, but at an angle to the velocity vector l80 As shown in Figure 2, nozzle 22 is directed at the laser beam lij. which impinges on the parts 1$ and 17 at the weld site 19. The interaction of the laser beam on the workpiece generates a weld 2$ as the workpiece moves in the direction of the velocity vector 18. A resulting weld in 30I4. stainless steel may be made at £0 inches per minute, approximately 3/K inches deep and 5/32 inches wide as illustrated in Figure l+a. Another arrange-ment of apparatus is illustrated in Figure 3, wherein a rectangular tube 1/1+ inch wide was used as a gas nozzle 22 emitting less than 30 grams per second of Argon gas. In this configuration, nozzle 22 was positioned parallel to the workpiece, but orthogonal to both the laser beam direc-tion and velocity vector direction. A resulting weld in 30i stainless steel, made at 100 inches per minute was approximately 1/2 inch deep with an average width of 3/32 inches as shown in Figure 1+b. The results of the welding operation in 30i|. stainless steel without a gas jet is illustrated .in Figure I.C which shows a shallow and wide weld which was I/I . inch in width and only 1/8 inch in deptho This clearly illustrates that a weld created by very intense radiation without the use of gas to continuously remove the absorption cloud will create unsatisfactory welds. Similar effects were observed when welding 1020 partially killed carbon steel. In this case, the differejlfee between using a gas assist and not using a gas assist were significant.

Theoretical calculations of the penetration to be expected in laser welding are in reasonably good agreement with the experimental data. Penetration up to 3/J. inch has been achieved as illustrated in Figure i+a at a rate of 0 inches per minute in a 6 to 1 depth to average width fusion zone in 30lj. stainless steel at a 20 kw power level. Penetration of nearly 3/8 inch in a ij.:l aspect ratio fusion zone was achieved at 8 kw. Although deeper, narrower welds might be possible with the smaller beam spot sizes consistent with less obscuration in the laser beam, the moderately narrow welds produced in these experiments minimize the fit-up requirements of the pieces to be Joined. In one experiment at the 8 kw level, two pieces of I/I. inch stainless steel with commercially sheared edges, and spaced O.OI inch apart were Joined with a nearly full thickness fusion zone of O.O80 inch average width.

The results of various laser welding tests performed while carrying out the method of the present invention are correlated in Figure 5. In this Figure, the non-dimensionalized welding speed is plotted as a function of the nondimensionalized laser power. The power is normalized with respect to the power conducted into the parent material, and the welding speed is normalized with respect to the speed of the characteristic isotherm as heat is conducted into the adjacent material. The solid line shown in this Figure is a correlation obtained for E-beam welding data. This correlation has been shown to be accurate over many orders of magnitude in these two nondimensional parameter^.

At values of P/Qtk (where P is laser power, 9 is melting temperature, t is plate thickness illustrated in Figure ib and k is the thermal conductivity) greater than 100, heat conduction (k) is unimportant and the welding speed is a linear function of the power (P). However, at values of P/9tk less than 100, heat conduction becomes important and the welding speed drops with the square of the power. Hence, for any particular weld, the ratio of the actual power needed to the power needed without any heat conduction is a measure of the efficiency. It is seen that for the fastest welds obtained in the test at 20 kw an efficiency of approximately was obtained.

At the lower power levels of 8 kw, the experimentally obtained efficiency was about 28 . For welding at 3.8 kw, the efficiency was about 22%.

It should be pointed out that deep penetration welds can be obtained at much lower powers, but with correspondingly low efficiencies. For example, welds in 0. cm copper have been obtained at i| kw, where the efficiency is only 1%.

As previously mentioned, during the testing of the laser welding apparatus at power levels greater than 8 kw, an interesting phenomenon occurred when no gas was used. The interaction zone of the laser beam with the workpiece was observed to be only slightly incandescent. However, a region of very high luminosity was seen to be located in the laser beam but displaced well away from the workpiece. Apparently, a significant amount of the material to be welded was vaporized at the interaction zone, ejected backward into the incoming laser beam, and ionized by the^ laser beame This standing absorption cloud radiated away the incident laser energy and only allowed enough of the laser beam to penetrate to the workpiece to maintain the vaporization.

This phenomenon of a standing absorption cloud with no gas assist and the difference in types of welds depending on the type of gas assist used clearly demonstrate the significance of a proper gas assist for laser welding. At powers below 8 kw, no ionization was observed. However, at powers above 8 kw precautions must be taken such as that taught herein to prevent the formation of this absorption cloud for laser welding. The type of gas assists used to prevent the formation of this absorption cloud relies on the dynamic pressure of the gas assist to blow awa the material ejected from the interaction zone. When the gas assist of the type illustrated in Figure 2 was used in accordance with the method of this invention, the penetration was the deepest. However, the weld was not uniform and had less penetration at some locations along the length of the weld. In addition, the head of the weld was blown off because of the dynamic pressure exerted on the molten metal by the gas assist. With a gas assist of the type previously discussed and illustrated in Figure 3, there is a considerably greater degree of luminosity at the interaction point. However, from the resulting weld it is obvious that most of the laser beam penetrated through to the material to be welded. This type of weld in which the gas assist was displaced slightly off the surface resulted in a very continuous, even, smooth weld with uniform properties along the length of the weld. Thus, it was found that the optimum method for applying the gas assist was slightly above the interaction zone and parallel to the workpiece. The direction of the gas flow may be applied at almost any angle radial from the interaction point .

The prior art welding devices have introduced gas flow into the interaction zone primarily to shield the material being welded. This shielding gas prevented impurities such as oxides from forming within the welded material, whereas the present invention may utilize the shielding effect of the gas, but more importantly provides an optimum environment for the laser welding to be accomplished. As previously pointed out, without such a gas assist high power laser welding would not be feasible.

It should be further noted that all power levels previously noted have been for a continuously operating laser. However, it is envisioned that a similar type of gas assist is required if a laser operated in the pulsed mode is utilized wherein the average power level exceeds 8 kw or in a pulsatile mode wherein the peak power is greater than 8 kw and the pulse width is greater than 3 seconds .

Claims (10)

40703/3 CLAIMS :

1. A 'method in which a laser beam ist used to form a weld along a line of contact between two pieces of metal, comprising reflectively focusing said laser beam to a focal point disposed at said line of contact, controlling the power of said laser beam to be in excess of 8 kilowatts and sufficient successively to form from said metal a cloud of vapor in the path of said beam and then to ionize said vapor, moving said focal point and line of contact relatively to one another in a direction aligned with said line of contract, directing a stream of inert gas to flow through said beam in a direction orthogonal to the beam path and in a stream path above and spaced from said focal point, and controlling the gas flow so that said gas and vapor do not remain in said beam path for a time sufficient for ionization thereof to occur, thereby preventing radiation of said laser beam away from said focal point.

2. A method according to claim 1, wherein the stream of inert gas is caused to flow in a direction substantially orthogonal to the direction of relative movement of said focal point and line of contact.

3. A method according to claim 1 or 2, wherein said laser beam is continuous.

4. A method according to claim 1 or 2, wherein said laser beam is pulsed.

5. · A method according to any of claims 1 to , wherein said inert gas is argon or nitrogen.

6. Apparatus in which a laser beam is used to form a weld along a line of contact between two pieces of metal, comprising a laser, beam source for providing an output laser 40703/3 beam, reflecting means for receiving and focusing said laser beam to a fqcal point at said line of contact^ said laser beam source being controllable to provide said laser beam with a power in excess of 8 kilowatts and sufficient successively to form from said metal a cloud of vapor in the path of said beam and then to ionize said vapor, said reflecting means and pieces of metal. having respective supports which permit movement of said focal point and line of contact relatively to one another in a direction aligned with said line of contact, and a source of inert gas connected^ to directing means for directing a stream of said gas to flow through said beam in a direction orthogonal to the beam path and in a stream path above and spaced from said focal point, the connection . between the gas source and stream directing means including metering means for controlling the gas flow so that said gas and vapor do not remain in said beam path for a time sufficient for ionization thereof to occur, thereby preventing radiation of said laser beam away from said focal point.

7. Apparatus according to claim 6, wherein the source of inert gas is a bottle or other device for supplying compressed gas, the stream directing means being a nozzle.

8. Apparatus according to claim 6 or 7» wherein said reflecting means includes a Cassegrain optical system.

9. · Apparatus according to any of claims 6 to 8, wherein the laser beam source is a gas dynamic laser or electric discharge laser.

10. A method in which a laser beam is used to form a weld along a line of contact between two pieces of metal, according to claim 1, substantially as herein described. 40703-3 weld along a line of contact between two pieces of metal, constructed and arranged substantially as herein described with reference to the accompanying drawings . For the Applicants DR. R.iNHOLD COHN AND PARTi S