THE MECHANISM OF WELDING WITH A SEALED-OFF CONTINUOUS CO2-GAS LASER*)

Transcription

1 R673 Philips Res. Repts 23, , 1968 THE MECHANISM OF WELDING WITH A SEALED-OFF CONTINUOUS CO2-GAS LASER*) by J. G. SIEKMAN and R. E. MORIJN Abstract A very simple high-power sealed-off COrgas laser with an output of about 100 W has been developed. Some experiments were performed on drilling and welding with the outcoming laser beam collimated by a Ge lens. The drilling rate and hole dimensions were measured as a function of the distance between work piece and the lens. From these results the optimum working distance was obtained. The welding phenomenon was investigated using a low-reflectivity material with poor heat conduction. The welding mechanism is described. On the basis of these results requirements can be drafted for the laser welding of solids of higher reflectivity and higher heat diffusivity. 1. Introduetion Fusion welding by means of the absorption of intense light beams as a technical application of lasers is for various reasons of great importance. Many of the advantages of this technique are the same as those of electron-beam welding. In this connection the remarkably rapid development of the electronbeam-welding technique, in which the maximum energy rose in the space of ten years from 1 kw (l50 kv) to 25 kw (lso kv) 1) with simultaneous extension of the possibilities, is well known. We are now witnessing the beginning of the development of a welding technique in which the continuous laser beam is used as the source of energy. In this respect the CO 2 -gas laser offers attractive possibilities. In the course of the last two years Witteman and co-workers of our laboratories 2) have developed sealed-off C02lasers of increasing power and longer life. Their most recent sealed-off laser is 3 m long and supplies a maximum continuous power of 180 W in a practically cylindrical beam (15 mm diameter) of monochromatic light (it = 10 6 flm),with beam spread ~ 4 m rad. Focussing of this beam on a small point of a surface provides at that point a high luminous-flux density, i.e. locally a great energy density. If this energy is strongly absorbed, either naturally or by means of an artificial device, in the surface of a solid, this radiant energy can be converted into thermal energy. In this case a very small and quite sharply limited volume at the surface of the solid is heated at least to the melting temperature. In a number of cases even the boiling point is reached in the centre of this small-surface volume element. The absorbed fraction of the total amount of light emitted by the laser is (1 - R), where R is the reflection at the surface of the material. This reflection, *) Paper read at the Conference of Laser Applications, Paris, July 1967.

2 368 J. G. SIEKMAN and R. E. MORIJN depends not only on the wavelength of the light of the laser but also on the angle of incidence, on the kind of material and its surface state; R is practically a constant for temperatures lower than the melting point, but it may change when this point is reached and exceeded. Thus R and hence (1 - R) may change during irradiation of the material. Unfortunately there is not a great deal of information to be gleaned from the literature about R for Ä = 10 6 (Lm at low temperatures and none at all at melting-point temperatures. At room température the absorption (1 - R) of a 10 6-(Lmlight beam incident normal to the surface of the solid lies for most metals between 0 01 and For graphite (1 - R) is about 0 4, for silicon carbide about 0 7 and for silicon oxide (fused silica) about As soon as a hole is formed by evaporation of some irradiated material, the total absorption of the light increases due to successive reflections by the walls of the hole. In this case the magnitude of the absorption coefficient of silicon oxide for the laser beam is comparable with that of an electron beam. For electron beams (1 - R) lies between 0 95 and 1 0 in practical cases. Owing to the large degree of extinction the intensity of the laser radiation decreases very rapidly beneath the surface of most materials (metals, oxides). The penetration is 10-3 _10-1 (Lm.Compared with practical cases of electronbeam welding this penetration is several orders of magnitude smaller. The series of experiments to be described in the next sections were designed mainly for investigating the welding process in which a continuous gas laser was used and the laser-energy-absorption rate was relatively high. First of all the "working distance" of a given lens had to be determined. For this purpose we made a series of test drillings in fused silica *). Thereafter a series of test weldings were made with the same material, in order to determine the effect of energy and of welding velocity on the depth of the weld. The gas laser used has been described earlier by Witteman 2). The laser beam is focussed with a positive lens made from a wafer of single-crystal germanium (n-type) having a specific resistance of more than 40 Q cm. On either side this lens is provided with an anti-reflection coating. Infrared absorption in the lens is low at or slightly above room temperature. The requisite cooling was ensured by a fan. *) A block of polished fused silica was chosen as workpiece for the following reasons: (1) the absorption of the i.r. laser light is relatively great, as stated above; (2) the melting temperature is fairly high and comparable with that of a number of metals; (3) the thermal conductivity is very poor, which makes the thermal input very effective; (4) the coefficient of expansion is very low, so that the resistance to thermal shocks is great, and therefore: (5) the drilling and welding process can be followed easily with a magnifying glass or a simple microscope; (6) the results can be measured immediately without the tedious work of making microscope specimens.

3 } WELDING WITH A SEALED-OFF CONTINUOUS C02-GAS LASER The optimum working distance; some remarks about drilling The optimum working distance for a lens with focallengthj R::J 50 mm and a laser beam of 40 W was determined by drilling a series of holes in fused silica at various distances d between lens and work piece. Then we measured among other things: (a) the drilling rate h, defined here as the depth of the holes achieved after 5 seconds; (b) the ellipticity e of the holes, e being defined as the quotient of the maximum and minimum dimensions at the aperture of the drilled hole *). Plotting both hand e as a function of the distance d (see fig. 1, lower and upper J I Shape and orientation of the hole Fig. 1. The hole depth achieved after 5 seconds of laser-beam drilling (lower curve) and the eiiipticity of the entrance of the holes (upper curve) measured as a function of the distance from the lens. curve, respectively) we observe that in d w = 46 7 mm, e is a minimum, while h is maximum. This distance d; we now define as our working distance. Compared with holes obtained by applying an electron beam the holes are less uniform in shape and the ratio of depth to width is less. Just as with electron-beam drilling, the dimensions of the holes depend on the amount of energy used and on the drilling time. Using a constant energy the drilling rate decreases with incrèasing depth for two reasons: (1) the beam diverges, so that the energy density further down the hole decreases **); *) The eiiipticity is caused mainly by the intensity distribution in the laser beam. **) The incident laser light does of course undergo reflections and absorptions. This causes the energy density to decrease less quickly than with the square of the distance.

4 370 J. G, SIEKMAN and R. E. MORlJN (2) the chance of escape of the vapour molecules at the bottom of the drilling hole decreases with an increase in the depth of the hole *). Therefore the depth of the holes does not continue to increase with time in either technique, but after a fairly short time a limiting value is reached (see fig. 2). If we plot the ratio of ultimate depths of laser-beam-drilled hole to electronbeam-drilled hole as a function of the energy used, we obtain a curve as shown in fig. 3. This graph shows that for materials with high (1 - R) values the limiting values ofthe depth become the same, either with the laser-beam technique or with the electron-beam technique, at higher beam energies. ] -c:: :Ei g. "0.!!l ~ 1 6~ W _-_ ,/' / I Drilling time {s} Fig. 2. Dependence of hole depth on drilling time. The drawn curves refer to electron-beam drilling, the dashed curves to laser-beam drilling. 3. The welding mechanism Our investigation shows that the mechanism of the butt-welding process can be described very simply, using in principle the same picture as Schwarz 3) who investigated the electron-beam-welding mechanism a few years ago. Let us assume that the work piece is moving at a uniform velocity v. Simultaneously with the switching on of the laser, welding commences with the drilling of a fairly narrow channel to a depth h (h = depth of welding seam). This depth *) From earlier measurements and calculations of drilling with a focussed electron beam as the energy source it was found that the rate of flow of the vapour molecules from the hole as a function of time could be approximated reasonably well if the flow of vapour was considered as "molecular flow". In other words, the rate of outflow of the vapour molecules was determined largely by collisions with the wall ofthe hole and less by mutual collisions of the vapour molecules.. Moving the work towards the laser during drilling, so that the focal spot comes to lie deeper in the hole, is found to yield no better result. The aperture of the hole then works as a diaphragm, so that photons are intercepted and the intensity of the penetrating radiation decreases.

5 WELDING WITH A SEALED-OFF CONTINUOUS C02-GAS LASER 371 ' j L., ~~ 0 8 ::~ 0,., 10'" :S'..,... ~~ 0 6 r' 15~ 0" ~"tj 1, , Fig. 3. Ratio of ultimate depths of laser-beam-drilled hole to electron-beam-drilled a function of the energy used. hole as is determined principally by the beam energy and the rate of welding. During this brief starting process a plume of vapour of the material is seen to be ejected from this hole. The plume vanishes as soon as the depth h has been reached. During the further process the drilling hole is drawn through the solid material, the hole being lined by a thin film of molten material. At the leading edge, where fresh material is being heated all the while by the laser beam, a great deal of evaporation takes place. Sometimes signs of boiling can be observed there (in the practice of welding, however, endeavours will generally be made to limit such boiling as far as possible, in order to limit the losses of material). Since the channel is narrow and rapidly cools at the trailing edge of the drilling hole, the major part of the vapour molecules will condense on this cooler part of the wall of the hole, so that very little material will be lost during the further welding process. Only a shallow groove remains to indicate the welding site. At welding rates of a few centimetres per minute, or more, the drilling hole exhibits an obvious bend on account of the time required for the heating and displacing of the material, and also because of the reflection of the laser light at the hot, liquid leading edge of the drilling channel. Figure 4 shows the shape of the hole during the drawing of the weld seam at various values of v. It can readily be seen that h diminishes as v increases. Figure 5 indicates how the laser beam penetrates the hole and how part of the light is reflected at the wall. In the hole the shaded area represents at the same time the brightest region in the hole *). In consequence ofthe process of evaporation and condensationjust described there will be marked mixing of the material in the weld seam. This mixing will *) The same phenomena: curvation of the hole and brightness distribution were observed when welding with the electron beam in a block of fused silica.

6 372 J. G. SIEKMAN and R. E. MORIJN Fig. 4. Hole shape during the drawing ofthe weld seam, from left to right at increasing values of welding velocity v. For the hole to the extreme left v = O. Fig. 5. The way the laser beam penetrates the hole. The shaded area in the hole represents the brightest (hottest) region in the hole; h = depth of welding seam, v = velocity of work piece relative to laser beam. be increased even further by the convection currents in the liquid material along the wall of the hole *). Strong evidence of this picture has been given by a number of welding experiments carried out with thin molybdenum foils pressed between plates of fused silica. In one case the laser beam was parallel to the foil and crossed it perpendicularly when welding. In the weld seam the foil material was V-shaped. In another case the laser beam pierced the foil perpendicularly and molybdenum was observed through the whole weld seam in decreasing concentration from the foil. *) In favourable cases this mixing of material in the weld seam may be an advantage if in this way an intermediate layer is produced with a coefficient of expansion intermediate between those ofthe two materials. With certain groups of glasses and ceramics this does not seem at all improbable.

7 WELDING WITH A SEALED OFF CONTINUOUS C02 GAS LASER The depth of welding,.' The effect of the welding rate and the applied energy on the depth of the weld seam infused silica has been measured. For this purpose, a number of test blocks (4 x 10 x 40 mm 3) were welded to each other by their 10 x 40 mm".sur- ' faces. The results of these measurements are summarized in fig. 6. ~ , E.. "ti i Fig. 6. Weld depth as a function of welding speed and beam energy. Here it should be noted that variations in the depth of the weld seam are found, even in a particular weld seam. This variation depends on the constancy of the beam energy, mechanical vibrations, width of the slit, and also on the extent to which boiling takes place in the drilling channel. The latter also depends on the causes of the above-mentioned variation and in addition on the welding rate. All these effects were not specially investigated, but the variations actually found have been indicated in the graph. 5. Welding of solids of higher reflectivity and higher heat diffusivity The investigations just described were carried out on fused silica which has a very low reflection coefficient and a relatively low heat-diffusion constant. For solids with higher reflection coefficient than that of fused silica more laserbeam input energy is required to obtain the same result. In some cases the welding process can be started from a mechanically predrilled narrow hole that acts as a "black box", i.e. the greater part of the laser-beam energy is absorbed by successive reflections. This hole lasts during the further process. Another method of diminishing the reflection losses is to cover the solid with a thin surface layer Which has a high absorption coefficient (e.g. a thin layer of Si02

8 374 J. G. SIEKMAN and R. E. MORIJN powder). This has proved to be very helpful in butt welding, e.g. steel plates of 0 3 mm thick (see fig. 7). Fig. 7. Butt welding of 0 3 mm thick steel plates. Solids which have a higher heat diffusivity than fused silica lose a greater amount of heat by conduction and therefore need a higher laser-beam input. It will be more favourable then to inject the laser-beam energy not as a continuous energy flow but as a pulsed flow with high repetition rate. This latter effect could be obtained by Q-switching. The fascinating history of electron-beam techniques shows that in industry welding is the most widespread of all applications of the electron beam. We expect that welding will be one of tbe most useful applications for tbe CO 2 laser as well. In spite of generally higher reflection losses, the laser may well develop into a serious rival of the electron beam as it needs no vacuum chamber and as the apparatus is far less complex and more convenient to handle. REFERENCES 1) J. W. Meier, Welding J. 43, , ) W. J. Witteman, J. Chimie phys. 64, , ) H. Sc h w a r z, J. appl, Phys. 35, ,1964. Eindhoven, June 1967