10. Laser Beam Welding

Transcription

1 . Laser Beam Welding

2 . Laser Beam Welding postulate of stimulated emission by Einstein 195 work out of physical basics and realisation of a maser (Microwave Amplification by Stimulated Emission of Radiation) by Towens, Prokhorov, Basov 1954 construction of the first maser 196 construction of the first ruby laser (Light Amplification by Stimulated Emission of Radiation) 1961 manufacturing of the first HeNe lasers and Nd: glass lasers 196 development of the first semiconductor lasers 1964 nobel price for Towens, Prokhorov and Basov for their works in the field of masers construction of the first Nd:YAG solid state lasers and CO gas lasers br-er-1e.cdr 1966 established laser emission on organic dyes since increased application of CO and solid state laser 197 technologies in industry 1975 first applications of laser beam cutting in sheet fabrication industry 1983 introduction into the market of 1-kW-CO lasers 1984 first applications of laser beam welding in industrial serial production History of the Laser The term laser is the abbreviation for,,light Amplification by Stimulated Emission of Radiation. The laser is the further development of the maser (m=microwave), Figure.1. Although the principle of the stimulated emission and the quantum-mechanical fundamentals have already been postulated by Einstein in the beginning of the th century, the first laser - a ruby laser - was not implemented until 196 in the Hughes Research Laboratories. Until then numerous tests on materials had to be carried out in order to gain a more precise knowledge about the atomic structure. The following years had been characterised by a fast development of the laser technology. Already since the beginning of the Seventies and, increasingly since the Eighties when the first high-performance lasers were available, CO and solid state lasers have been used for production metal working. Figure.1 The number of the annual sales of laser beam sources has constantly increased in the course of the last few years, Figure The application areas for the laser beam sources sold in 1994 are shown in Figure.3. The main application areas of the laser in the field of production metal working are joining and cutting jobs br-er-e.cdr Figure. Japan and South East Asia North America West Europe

3 . Laser Beam Welding 135 The availability of more efficient laser beam sources opens up new application possibilities and - guided by financial considerations - makes the use of the laser also more attractive, Figure.4. Figure.5 shows the drilling 1,8% welding 18,7% characteristic properties of the laser beam. By rea- others 9,3% inscribe,5% son of the induced or stimulated emission the radiation is coherent and monochromatic. As the cutting 44,3% br-er-3e.cdr micro electronics 5,4% divergence is only 1/ mrad, long transmission paths without significant beam divergences are possible. Figure.3 Inside the resonator, Figure.6, the laser-active medium (gas molecules, ions) is excited to a higher energy level ( pumping ) by energy input (electrical gas discharge, flash lamps). laser power 4 kw CO Nd:YAG diode laser During retreat to a lower level, the energy is released in the form of a light quantum (photon). The wave length depends on the energy difference between both excited states and is thus a characteristic for the respective la- br-er-4e.cdr ser-active medium. Figure.4

4 . Laser Beam Welding 136 br-er-5e.cdr A distinction is made between spontaneous and induced transition. While the spontaneous emission is non-directional and in coherent (e.g. in fluorescent tubes) is a laser beam generated by induced emission when a particle with a higher energy level is hit by a photon. The resulting photon has the same properties (frequency, direction, phase) as the exciting photon ( coherence ). In order to maintain the ratio of the desired induced emission I spontaneous emission as high as possible, the upper energy level must be constantly overcrowded, in comparison with the lower one, the so-called laser-inversion. As result, a stationary light wave is formed between the mirrors of the resonator (one of which is semi-reflecting) causing parts of the excited laser-active medium to emit light. In the field of production metal working, and particularly in welding, especially CO and Nd:YAG lasers are applied for their high power outputs. At present, the development of diode lasers is so far advanced that their sporadic use in the field of material processing is also possible. The industrial standard powers for CO lasers are, nowadays, approximately 5 - kw, lasers with powers of up to 4 kw are available. In the field of solid state lasers average output powers of up to 4 kw are nowadays obtainable. light bulb Figure.5 polychromatic (multiple wave length) incoherent (not in phase),46",94" large divergence E E 1 Laser induced emission exited state ground state monochromatic coherent (in phase) small divergence Characteristics of Laser Beams fully reflecting mirror R = % br-er-6e.cdr Figure.6 resonator energy source active laser medium energy source Laser Principle partially reflecting mirror R < % la ser beam

5 . Laser Beam Welding 137 In the case of the CO laser, Figure.7, where the resonator is filled with a N -C -He gas mixture, pumping is carried out over the vibrational excitation of nitrogen molecules which energy again, with thrusts of the second type, transfer their vibrational energy to the carbon dioxide. During the transition to the lower energy level, CO molecules emit a radiation with a wavelength of.6 µm. The helium atoms, finally, lead the CO molecules back to their energy level. The efficiency of up to 15%, which is achievable with CO high performance lasers, is, in comparison with other laser systems, relatively high. The high dissipation component is the heat which must be discharged from the resonator. This is achieved by means of the constant gas mixture circulation and cooling by heat exchangers. In cooling water,6 ev,4,3,88 ev,,1 br-er-7e.cdr Figure.7 thrust of second type transmission of vibration energy Energy Diagram of CO Laser radio frequency high voltage exitaion transition without emission thrust of first type 1 1,9 ev E =, ev LASER λ =,6 µm N CO discharge through thrust with helium laser beam cooling water dependence of the type of gas transport, laser systems are classified into longitudinal-flow and transverse-flow laser systems, Figures.8 and.9. laser gas laser gas: CO : 5 l/h He: l/h N : 45 l/h vakuum pump gas circulation pump br-er-8e.cdr Figure.8

6 . Laser Beam Welding 138 With transverse-flow laser systems of a compact design can the multiple folding ability of the beam reach higher output powers than those achievable with longitudinal-flow systems, the beam quality, however, is worse. In d.c.-excited systems (high voltage), the electrodes are positioned inside the resonator. The interaction between the electrode material and the gas molecules causes electrode burn-off. In addition to the wear of the electrodes, the burn-off also entails a contamination of the laser gas. Parts of the gas mixture must be therefore exchanged permanently. In high-frequency a.c.-excited systems the electrodes are positioned outside the gas discharge tube where the electrical energy is capacitively coupled. High electrode lives and high achievable pulse frequencies characterise this kind of excitation principle. In diffusion-cooled CO systems beams of a high quality are generated in a minimum of space. Moreover, gas exchange is hardly ever necessary. The intensity distribution is not constant across the laser beam. The intensity distribution in the case of the ideal beam is described by TEM modes (transversal electronic-magnetic). In the Gaussian or basic mode TEM is the peak energy in the centre of the beam weakening towards its periphery, similar to the Gaussian normal distribution. In gas circulation pump end mirror br-er-9e.cdr Figure.9 d unfocussed beam br-er-e.cdr Figure. turning mirrors Cooling water cooling water f,57" focussed beam Laser Beam Qualitiy K = λ 1 π d. σ Θ σ laser gas: CO : 11 l/h He: 14 l/h N : 13 l/h <K<1 with d d. d.. Θ = Θ = Θ =c. σ σ F F laser beam mirror (partially reflecting) gas discharge laser gas Θ F d F

7 . Laser Beam Welding 139 practice, the quality of a laser beam is, in accordance with DIN EN 11146, distinguished by the non-dimensional beam quality factor (or propagation factor) K (...1), Figure.. The factor describes the ratio of the distance field divergence of a beam in the basic mode to that of a real beam and is therefore a measure of a beam focus strength. By means of the beam quality factor, different beam sources may be compared objectively and quantitaively. end mirror resonator partially reflecting mirror absorber LASER shutter beam transmission tube work piece beam divergence mirror beam creation focussing system The CO laser beam is work piece manipulator guided from the resonator br-er-11e.cdr over a beam reflection mirror system to one or sev- Figure.11 eral processing stations, Figures.11 and.1. The low divergence allows long transmission paths. At the processing station is the beam, with the help of the focussing optics, formed according to the working task. The relative motion between beam and workpiece may be realised in different ways: br-er-1e_f.cdr Figure.1 CO Laser Beam Welding Station - moving workpiece, fixed optics - moving ( flying ) optics - moving workpiece and moving optics (two handling facilities).

8 . Laser Beam Welding 14 In the case of the CO laser, beam focussing is normally carried out with mirror optics, Figure.13. Lenses reflective 9 -mirror optic α may heat up, due to absorption, especially with high powers or contaminations. As the heat may be dissipated only over the holders, there is a risk of deformation (alteration of the focal length) or destruc- br-er-13e.cdr Focussing Optics tion through thermal over- Figure.13 loading. In the case of solid state laser, the normally cylindrical rod serves only the purpose to pick up the laser-active ions (in the case of the Nd:YAG laser with yttrium-aluminium-garnet crystals dosed with Nd 3+ ions), Figure.14. The excitation is, for the most part, carried out using flash or arc lamps, which for the optimal utilisation of the excitation energy are arranged as a double ellipsoid; the rod is positioned in their common focal point. The achieved br-er-14e.cdr laser rod Figure.14 flash lamps end mirror (r = %) Principle Layout of Solid State Laser laser beam partially reflecting mirror (R < %) efficiency is below 4%. In the meantime, also diode-pumped solid state lasers have been introduced to the market. The possibility to guide the solid-state laser beam over flexible fibre optics makes these systems destined for the robot application, whereas the CO laser application is restricted, as its necessary complex mirror systems may cause radiation losses, Figure.15.

9 . Laser Beam Welding 141 Some types of optical fibres allow, with fibre diameters of 1 mm bending radii of up to mm. With optical switches a multiple utilisation of the solid state laser source is possible; with beam splitters (mostly with a fixed splitting proportion) simultaneous welding at several processing stations is possible. The disadvantage of this type of beam projection is the impaired beam quality on account of multiple reflection. The semiconductor or diode lasers are characterised by their mechanical robustness, high efficiency and compact design, Figure.16. High performance diode lasers allow the welding of metals, although no deep penetration effect is achieved. In material processing they are therefore particularly suitable for welding thin sheets. br-er-15e_f.cdr Figure.15 Nd:YAG Laser Beam Welding Station Energy input into the workpiece is carried out over the absorption of the laser beam. The absorption coefficient is, apart from the surface quality, also dependent on the wave length and the material. The problem is that a large part of the radiation is reflected and that, for example, steel which is exposed to wave lengths of.6 µm reflects only % of the impinging radiation, Figure.17. As copper is a highly reflective metal Figure.16 br-er-16e_f.cdr Diode Laser

10 . Laser Beam Welding 14 with also a good heat conductivity, it is frequently used as mirror material. absorption A,3,5,,15,,5 br-er-17e.cdr Figure.17 and workpiece surface must be maintained within close tolerances. At the same time, highest accuracy and quality demands are set on all machine components (handling, optics, resonator, beam manipulation, etc.). Al Ag Nd:YAGlaser Cu,1,,3,5, µm wave length λ Wave Length Dependent Absorption of Various Materials W/cm power density Fe Mo Stahl CO- laser shock hardening glaze ISF 6 4 drilling remelting coating 6 Intensity adjustment at the working surface by the focal position with a simultaneous variation of the working speed make the laser a flexible and contactless tool, Figure.18. The methods of welding and cutting demand high intensities in the focal point, which means the distance between focussing optics energy density [J/cm²] welding cutting martensitic hardening Steel materials with treated surfaces reflect the laser beam to a degree of up to 95%, Figures br-er-18e.cdr Figure s acting time When metals are welded with a low-intensity laser beam (I 5 W/cm ), just the workpiece surfaces and/or edges are melted and thus thermal-conduction welding with a low deeppenetration effect is possible. Above the threshold intensity value (I 6 W/cm ) a phase transition occurs and laser-induced plasma develops. The plasma, whose absorption characteristics depend on the beam intensity and the vapour density, absorbs an increased quantity of radiation.

11 . Laser Beam Welding 143 laser beam molten pool br-er-19e.cdr Figure.19 heat conducting welding soldified weld metal laser beam keyhole (vapour-/ plasma cavity) deep penetration welding Principle of Laser Beam Welding A vapour cavity forms and allows the laser beam to penetrate deep into the material (energy input deep beyond the workpiece surface); this effect is called the deep penetration effect. The cavity which is moved though the joining zone and is prevented to close due to the vapour pressure is surrounded by the largest part of the molten metal. The residual material vaporises and condenses either on the cavity side walls or flows off in an ionised form. With suitable parameter selection, an almost complete energy input into the workpiece can be obtained. metal vapour blowing away laser-induced plasma molten pool soldified weld metal % steel penetration depth t reflection R mm 1 material: steel wave length λ:,6 µm laser power: kw welding speed: mm/s working gas: helium laser intensity I W/cm r F = µm =.6 µm plasma shielding working zone plasma threshold A =,1 A = 1 br-er-e.cdr 5 6 W/cm 7 laser intesity I Reflection and Penetration Depth in Dependence on Intensity ISF 6 br-er-1e.cdr s radiation time t Calculated Intensity Threshold for Producing a Laser-Induced Plasma Figure. Figure.1

12 . Laser Beam Welding 144 However, in dependence of the electron density in the plasma and of the radiated beam intensity, plasma may detach from the workpiece surface and screen off the working zone. The plasma is heated to such a high degree that only a fraction of the beam radiation reaches the Transformation of electromagnetic energy into thermal energy within nm range at the surface of the work piece by stimulation of atoms to resonant oscillations "normal" absorption: depandent on laser beam intensity: I < W/cm² dependent on wave length dependent on temperature dependent on material absorption at solid or liquid surface: A < 3% formation of a molten bath with low penetration depth br-er-e.cdr Figure. 6 heat conducting welding "abormal" absorption: dependent on laser intesity: 6 I W/cm² heating up to temperature of evaporation and formation of a metal vapour plasma almost complete energy entry through absorption by plasma: A > 9% formation of a vapour cavity deep penetration welding Interaction Between Laser Beam and Material workpiece. This is the reason why, in laser beam welding, gases are applied for plasma control. The gases ionisation potential should be as high as possible, since also the formation of shielding gas plasmas is possible which again decreases the energy input. Only a part of the beam energy from the resonator is used up for the actual welding process, Figure.3. Another part is absorbed by the optics in the beam manipulation system, another part is lost by reflection or transmission (beam penetration through the vapour cavity). Other parts flow over thermal conductance into the workpiece % beam energy -,5%,5-1,5% work piece ca. 5% ca. % diagnostics beam transmission focussing system reflection transmission Figure.4 shows the most important advantages and disadvantages of the laser beam welding method. ca. 3% -15% ca. 4% recombination heat convection heat conduction metal vapour plasma fusion energy br-er-3e.cdr Scheme of Energy Flow Figure.3

13 . Laser Beam Welding 145 Penetration depths in dependence of the beam power and welding speed which are achievable in laser beam welding are depicted in Figure.5. Further relevant influential process work piece advantages - high power density - small beam diameter - high welding speed - non-contact tool - atmosphere welding possible - minimum thermal stress - little distortion - completely processed components - welding at positions difficult to access - different materials weldable disadvantages - high reflection at metallic surfaces - restricted penetration depth ( 5 mm) - expensive edge preparation - exact positioning required - danger of increased hardness - danger of cracks - Al, Cu difficult to weld factors are, among others, the material (thermal conductivity), the design of the resonator (beam quality), the focal position and the applied optics (focal length; focus diameter). installation br-er-4e.cdr Figure.4 - short cycle times - operation at several stations possible - installation availability > 9% - well suitable to automatic function Advantages and Disadvantages of Laser Beam Welding - expensive beam transmission and forming - power losses at optical devices - laser radiation protection required - high investment cost - low efficency (CO-Laser: < %, Nd:YAG: < 5%) Figure.6 shows several joint shapes which are typical for car body production and which can be welded by laser beam application. penetration depth penetration depth br-er-5e.cdr 8 mm ,5 kw,6 1, 1,8 m/min 3, welding speed 15 mm 5,% C-steel CO-laser (cross flow) laser power: 15 kw kw 8 kw 6 kw 4 kw 6 kw kw 4 kw 1 kw m/min 9 welding speed Penetration Depths X 5 CrNi 18 CO-laser (axial flow) laser power: The high cooling rate during laser beam welding leads, when transforming steel materials are used, to significantly increased hardness values in comparison with other welding methods, Figure.7. These are a sign for the increased strength at a lower toughness and they are particularly critical in circumstances of dynamic loads. The small beam diameter demands the very precise manipulation and positioning of the workpiece or of the beam and an exact weld preparation, Figure.8. Otherwise, as result, lack of fusion, sagged welds or concave root surfaces are possible weld defects. Figure.5

14 . Laser Beam Welding 146 butt weld fillet weld at overlap joint lap weld at overlap joint flanged weld at overlap joint br-er-6e.cdr Figure.6 5 HV,4 WMA MAZ MAZ laser beam weld hardness MAZ MAZ 1 distance from the weld centre weld submerged arc weld submerged arc weld br-er-7e_f.cdr Figure.7 edge preparation misalignment (e,1 x plate thickness) gap beam mispositioning (a,1 x plate thickness) br-er-8e.cdr Welding Defects Figure.8

15 . Laser Beam Welding 147 Caused by the high cooling rate and, in connection with this, the insufficient degassing of the molten metal, pore formation may occur during laser beam welding of, in particular, thick plates (very deep welds) or while carrying out welding-in works (insufficient degassing over the root), Figure.9. However, too low a weld speed may also cause pore formation when the molten metal picks up gases from the root side. gas filler wire weld metal br-er-9e.cdr Figure.9 molten pool forward wire feeding br-er-3e.cdr Figure.3 v w =,7 m/min laser beam keyhole plasma work piece v w =,9 m/min material: P46N (StE46), s = mm, P = 15 kw Porosity welding direction laser beam plasma weld metal molten pool v w = 1,5 m/min backward wire feeding keyhole filler wire gas work piece The materials that may be welded with the laser reach from unalloyed and lowalloy steels up to high quality titanium and nickel based alloys. The high carbon content of the transforming steel materials is, due to the high cooling rate, to be considered a critical influential factor where contents of C >.% may be stipulated as the limiting reference value. Aluminium and copper properties cause problems during energy input and process stability. Highly reactive materials demand, also during laser beam welding, sufficient gas shielding beyond the solidification of the weld seam. The sole application of working gases is, as a rule, not adequate.

16 . Laser Beam Welding 148 The application of laser beam welding may be extended by process variants. One is laser beam welding with filler wire, Figures.3 and.31 which offers the following advantages: - influence on the mechanic-technological properties of the weld and fusion zone (e.g. strength, toughness, corrosion, wear resistance) over the metallurgical composition of the filler wire - reduction of the demands on the accuracy of the weld preparation in regard to edge misalignment, edge preparation and beam misalignment, due to larger molten pools increase of gap bridging ability material: S38N (StE 38) gap:,5 mm P L = 8,3 kw V W = 3 m/min E S = 166 J/min s = 4 mm Possibility of metallurgical influence material combination: CrMo9-/ X6CrNiTi18- P L = 5, kw br-er-31e.cdr Figure.31 without filler wire gap: mm v w = 1,6 m/min weld zone - filling of non-ideal, for example, V-shaped groove geometries with filler wire gap:,5 mm wire: SG-Ni Cr1 Fe18 Mo - a realisation of a defined weld reinforcement on the beam entry and beam exit side. filler wire: Sg d w =,8 mm weld zone v w = 1, m/min d = 1, mm w The exact positioning of the filler wire is a prerequisite for a high weld quality and a sufficient dilution of the molten pool through which filler wire of different composition as the base can reach right to the root. Therefore, the use of sensor systems is indispensable for industrial application, Figure.3. The sensor systems are to take over the tasks of - process control, with sensing device; fill factor 1 % br-er-3e.cdr KB 46/9 :1 /9.1 mm. mm.3 mm.4 mm.5 mm.6 mm KB 46/1 :1 /9 Figure.3 KB 46/6 :1 /9 Probe MS1-6C Probe MS1-5A Probe MS1-4CProbe MS1-3A Probe MS1-B Probe MS1-1C KB 46/17 :1 /9 KB 46/4 :1 /9 KB 461/15 :1 /9 Probe OS1-6A Probe OS1-5C Probe OS1-4C Probe OS1-3B Probe OS1-B Probe OS1-1B without sensing device; wire speed v D = 4 m/min constant KB 46/ :1 /9 KB 461/1 :1 /9 KB 46/41 :1 /9 KB 461/9 :1 /9 KB 46/38 :1 /9 KB 461/7 :1 /9 1 mm

17 . Laser Beam Welding weld quality as surance - beam positioning and joint tracking, respectively. The present state-of-the-art is the further development of systems for industrial applications which until now have been tested in the laboratory. Welding by means of solid state lasers has, in the past, mainly been applied by manufacturers from the fields of precision mechanics and microelectronics. Ever since solid state lasers with higher powers are available on the market, they are applied in the car industry to an ever increasing degree. This is due to their more variable beam manipulation possibilities when automotive industry - gear parts (cog-wheels, planet gears) - body-making (bottom plates, skins) - engine components (tappet housings, diesel engine precombustion chambers) medical industry - heart pacemaker cases - artificial hip joints br-er-33e.cdr Figure.33 aerospace industry - engine components - instrument cases plant and apparatus engineering - seal welds at housings - measurement probes Practical Application Fields steel industry - pipe production - vehicle superstructures - continuous metal strips - tins electronic industry - PCBs - accumulator cases - transformer plates - CRTs comparing with CO lasers. The CO laser is mostly used by the car industry and by their ancillary industry for welding rotation-symmetrical massproduced parts or sheets. Figure.33 shows some typical application examples for laser beam welding.