10. Laser Beam Welding

The term laser is the abbreviation for 1917 postulate of stimulated emission by Einstein ,,Light Amplification by Stimulated 1950 work out of physical basics and realisation of a maser (Microwave Amplification by Stimulated Emission of Radiation”. The laser is Emission of Radiation) by Towens, Prokhorov, Basov

1954 construction of the first maser the further development of the maser

1960 construction of the first ruby laser (Light Amplification (m=microwave), Figure 10.1. Al- by Stimulated Emission of Radiation)

1961 manufacturing of the first HeNe lasers and Nd: glass lasers though the principle of the stimulated 1962 development of the first semiconductor lasers emission and the quantum- 1964 nobel price for Towens, Prokhorov and Basov for their works in the field of masers mechanical fundamentals have al- construction of the first Nd:YAG solid state lasers and CO gas lasers 2 ready been postulated by Einstein in 1966 established laser emission on organic dyes the beginning of the 20th century, the

since increased application of CO2 and solid state laser 1970 technologies in industry first laser - a ruby laser - was not 1975 first applications of laser beam cutting in sheet fabrication industry implemented until 1960 in the Hughes

1983 introduction into the market of 1-kW-CO lasers 2 Research Laboratories. Until then 1984 first applications of laser beam welding in industrial serial production numerous tests on materials had to br-er10-01e.cdr be carried out in order to gain a more History of the Laser precise knowledge about the atomic structure. The following years had Figure 10.1 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,

CO2 and solid state lasers have been used for production metal working.

The number of the annual sales of la- 3 ser beam sources 109 € has constantly in- 2 creased in the 1.5 course of the last 1 few years, Figure 0.5 10.2. 0 1986 1988 1990 1992 1994 1996 1998 2000

Japan and South East Asia The application ar- North America West Europe br-er10-02e.cdr eas for the laser beam sources sold Figure 10.2 10. Laser Beam Welding 130 in 1994 are shown in Figure 10.3. The main application areas of the laser in the field of production metal working are joining and cutting jobs.

The availability of more efficient laser drilling welding 1,8% 18,7% beam sources opens up new ap- others inscribe 9,3% 20,5% plication possibilities and - guided by financial con- siderations - makes cutting micro 44,3% electronics the use of the laser 5,4% also more attrac- br-er10-03e.cdr tive, Figure 10.4.

Figure 10.3

Figure 10.5 shows the characteristic properties of the laser beam. By reason of the induced or stimu- 40 lated emission the kW CO radiation is coher- 20 2 10 ent and mono-

r 5 chromatic. As the e w

o 4 p

divergence is only r e

s 3 a

l 1/10 mrad, long 2 Nd:YAG 1 transmission paths diode laser 0 without significant 0 5 0 5 0 5 0 7 7 8 8 9 9 0 9 9 9 9 9 9 0

1 1 1 1 1 1 2 beam divergences

br-er10-04e.cdr are possible.

Figure 10.4

10. Laser Beam Welding 131

Inside the resonator, Figure 10.6, the laser-active medium (gas molecules, ions) is excited to a higher energy level (“pumping”) by energy input (electrical gas discharge, flash lamps).

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 laser-active medium. 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 gener- light bulb Laser ated by induced induced emission emission when a

E particle with a 2 higher energy level 0, E1 46" exited ground state state is hit by a photon. 0 ,9 4" The resulting pho- polychromatic monochromatic ton has the same (multiple wave length) coherent incoherent (in phase) properties (fre- (not in phase) small divergence large divergence quency, direction,

br-er10-05e.cdr © ISF 2002 phase) as the excit- Characteristics of Laser Beams ing photon (“coher- ence”). In order to Figure 10.5 maintain the ratio of the desired induced resonator energy source emission I spontaneous emission as high as possible, m a active laser medium e the upper energy b

r e s a level must be con- l stantly over- crowded, in com- fully reflecting partially reflecting mirror mirror parison with the R = 100% energy source R < 100%

br-er10-06e.cdr © ISF 2002 lower one, the so- Laser Principle called “laser- Figure 10.6 10. Laser Beam Welding 132 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 CO2 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 CO2 lasers are, nowadays, approximately 5 - 20 kW, lasers with powers of up to 40 kW are available. In the field of solid state lasers average output powers of up to 4 kW are nowadays obtainable.

In the case of the 0,6 thrust of second type 2 002 CO2 laser, Figure eV transition without transmission of emission 10.7, where the vibration energy 0,4 y g r resonator is filled e

n thrust of first type

e 0,3 0,288 eV 1 001 0,290 eV DE = 0,002 eV LASER l = 10,6 µm with a N2-C02-He 0,2 100 gas mixture, pump- discharge through 0,1 thrust with helium ing is carried out 0 0 000 over the vibrational N2 CO2 br-er10-07e.cdr © ISF 2002 excitation of nitro- Energy Diagram of CO Laser 2 gen molecules Figure 10.7 which again, with thrusts of the sec-

radio frequency high voltage exitaion ond type, transfer their vibrational laser beam energy to the carbon dioxide. During cooling water cooling water the transition to the lower energy level, laser gas CO2 molecules laser gas:

CO2: 5 l/h vakuum pump He: 100 l/h emit a radiation

N2 : 45 l/h

gas circulation pump with a wavelength

br-er10-08e.cdr of 10.6 µm. The helium atoms, fi- Figure 10.8 10. Laser Beam Welding 133

nally, lead the CO2 Cooling water molecules back to laser gas:

CO2: 11 l/h He: 142 l/h turning their energy level. N: 130 l/h mirrors 2 gas circulation pump laser beam The efficiency of up mirror (partially reflecting) to 15%, which is

gas discharge achievable with

CO2 high perform- laser gas end mirror ance lasers, is, in

cooling water comparison with br-er10-09e.cdr other laser systems, relatively Figure 10.9 high. The high dis- sipation 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 dependence of the type of gas transport, laser systems are classified into longitudi- nal-flow and transverse-flow laser systems, Figures 10.8 and 10.9.

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 longitudi- nal-flow systems, the beam quality, however, is worse. In d.c.-excited systems (high voltage), the f2,57" electrodes are po-

d0 sitioned inside the

QF unfocussed beam focussed beam resonator. The in-

dF teraction between the electrode material and the gas molecules causes K = 2l 1 p d.Q 0

with dd.Q=.Q=Qd. =c. ss00FF In addition to the

br-er10-10e.cdr © ISF 2002 wear of the elec- Laser Beam Qualitiy trodes, the burn-off Figure 10.10 10. Laser Beam Welding 134 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 shutter beam divergence mirror kind of excitation resonator partially reflecting principle. In diffu- end beam transmission mirror mirror absorber tube sion-cooled CO2 LASER systems beams of a high quality are generated in a beam creation focussing system minimum of space. work piece Moreover, gas ex- work piece manipulator change is hardly br-er10-11e.cdr ever necessary.

Figure 10.11 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 TEM00 is the peak energy in the centre of the beam weakening towards its periphery, similar to the Gaussian normal distribution. In 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 (0...1), Figure 10.10. The factor describes the ratio of the distance field divergence of a

br-er10-12e_f.cdr © ISF 2002 beam in the basic CO Laser Beam Welding Station 2 mode to that of a Figure 10.12 10. Laser Beam Welding 135

real beam and is reflective 90°-mirror optic therefore a meas- ure of a beam fo- a cus strength. By means of the beam quality factor, different beam sources may be compared objec- tively and quanti- br-er10-13e.cdr © ISF 2002 taively. Focussing Optics

Figure 10.13

The CO2 laser beam is guided from the resonator over a beam reflection mirror system to one or several processing stations, Figures 10.11 and 10.12. 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 mo- tion between beam and workpiece may be realised in different ways: - moving workpiece, fixed optics - moving (“flying”) optics - moving workpiece and moving optics (two handling facilities).

In the case of the

CO2 laser, beam

flash lamps focussing is normally

laser rod carried out with mirror optics, Figure laser beam 10.13. Lenses may

partially reflecting mirror (R < 100%) heat up, due to absorption, especially end mirror (r = 100%) with high powers or

br-er10-14e.cdr © ISF 2002 contaminations. As Principle Layout of Solid State Laser the heat may be dis-

Figure 10.14 10. Laser Beam Welding 136 sipated only over the holders, there is a risk of deformation (alteration of the focal length) or destruction through thermal overloading.

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 Nd3+ ions), Figure 10.14. 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 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 des- tined for the robot

br-er10-15e_f.cdr © ISF 2002 application, Nd:YAG Laser Beam Welding Station whereas the CO2 laser application is Figure 10.15 restricted, as its necessary complex mirror systems may cause radiation losses, Figure 10.15. Some types of optical fibres allow, with fibre diameters of £ 1 mm bending radii br-er10-16e_f.cdr © ISF 2002 of up to 100 mm. Diode Laser With optical Figure 10.16 10. Laser Beam Welding 137 switches a multiple utilisation of the solid state laser source is possible; with beam splitters (mostly

Nd:YAG- CO-2 laser laser with a fixed splitting 0,30 proportion) simulta- 0,25 neous welding at 0,20 Cu A

several processing Al n Stahl o

i Ag t

p 0,15 stations is possible. r o s

b Fe a The disadvantage 0,10 of this type of beam 0,05 Mo projection is the 0 impaired beam 0,1 0,2 0,3 0,5 0,8 1 2 4 5 8 10 µm 20 wave lenght l br-er10-17e.cdr quality on account of multiple reflection. Figure 10.17

The semiconductor or diode lasers are characterised by their mechanical robust- ness, high efficiency and compact design, Figure 10.16. 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.

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 1010 en 6 e and the material. 10 rg y W/cm2 d shock hardening en s it The problem is that y 4 [ 10 J/ 108 cm ²] a large part of the y

t 2

i drilling 7 10 s 10

n gla radiation is re- e ze d

e 6 0

w 10 10 remelting welding flected and that, for o

p coating cutting

105 example, steel

4 which is exposed 10 m art ens itic ha rde nin to wave lengths of 103 g 10-8 10-6 1-40 -20 10 s 10 10.6 µm reflects

br-er10-18e.cdr acting time only 10% of the impinging radia- Figure 10.18 10. Laser Beam Welding 138 tion, Figure 10.17. As copper is a highly reflective metal with also a good heat conductivity, it is frequently used as mirror material.

Intensity adjust-

heat conducting deep penetration ment at the work- welding welding ing surface by the metal vapour laser beam blowing away focal position with laser beam laser-induced plasma a simultaneous

soldified molten pool weld metal keyhole variation of the molten pool (vapour-/ plasma cavity) soldified weld metal working speed make the laser a flexible and con- tactless tool, Fig-

br-er10-19e.cdr © ISF 2002 ure 10.18. The Principle of Laser Beam Welding methods of welding Figure 10.19 and cutting demand high intensi- ties in the focal point, which means the 100 distance between focussing optics and % workpiece surface must be maintained R

n 60 o i within close tolerances. At the same t c e l f

e 40 time, highest accuracy and quality r demands are set on all machine com- 20 ponents (handling, optics, resonator, 0 beam manipulation, etc.).

4 t

h Steel materials with treated surfaces t p

e mm d

n o

reflect the laser beam to a degree of i t a

r 2 t e up to 95%, Figures 10.19 - 10.22. n e p 1

0 When metals are welded with a low- 105 160 W/c27m 10 laser intesity I intensity laser beam (I £ 105 W/cm2), br-er10-20e.cdr © ISF 2002 Reflection and Penetration Depth just the workpiece surfaces and/or in Dependence on Intensity edges are melted and thus thermal- Figure 10.20 10. Laser Beam Welding 139

conduction welding with a low deep-

1010 penetration effect is possible. Above

steel the threshold intensity value (I £ 106 rF = 100 µm l = 10.6 µm 2 W/cm2 W/cm ) a phase transition occurs and laser-induced plasma develops. The

8 plasma, whose absorption character- 10 I

y t

i istics depend on the beam intensity s n e t

n plasma shielding i

and the vapour density, absorbs an r 7 e 10 s a l working zone increased quantity of radiation.

plasma threshold A vapour cavity forms and allows the 106 A = 0,1 laser beam to penetrate deep into the material (energy input deep beyond

5 10 A = 1 the workpiece surface); this effect is 10-10 10-8 10-6 s 10-2

radiation time t called the “deep penetration effect”. br-er10-21e.cdr © ISF 2002 The cavity which is moved though the Calculated Intensity Threshold for Producing a Laser-Induced Plasma joining zone and is prevented to close due to the vapour pressure is sur- Figure 10.21 rounded 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.

However, in dependence of the Transformation of electromagnetic energy into thermal energy within nm range at the surface of the work piece by electron density in stimulation of atoms to resonant oscillations the plasma and of "normal" absorption: "abormal" absorption: - depandent on laser beam intensity: - dependent on laser intesity: 6 the radiated beam I < 10W/cm² I ³ 106 W/cm² - dependent on wave length - heating up to temperature of evaporation intensity, plasma - dependent on temperature and formation of a metal - dependent on material vapour plasma almost complete energy entry through - absorption at solid or liquid surface: - absorption by plasma: may detach from A < 30% A > 90% - formation of a molten bath with low - formation of a vapour cavity the workpiece sur- - penetration depth face and screen off heat conducting welding deep penetration welding the working zone.

br-er10-22e.cdr © ISF 2002 The plasma is Interaction Between Laser Beam and Material heated to such a Figure 10.22 10. Laser Beam Welding 140 high degree that only a fraction of the beam radiation reaches the 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 in- beam energy put. 0-2,5% diagnostics

beam transmission 2,5-12,5% focussing system Only a part of the beam energy from 85-95% work piece the resonator is used up for the actual

ca. 5% reflection welding process, Figure 10.23. Another

ca. 10% transmission part is absorbed by the optics in the beam manipulation system, another

heat convection heat conduction part is lost by reflection or transmission ca. 40% metal vapour plasma (beam penetration through the vapour cavity). Other parts flow over thermal

ca. 30% 10-15% recombination conductance into the workpiece.

fusion energy Figure 10.24 shows the most important br-er10-23e.cdr

Scheme of Energy Flow advantages and disadvantages of the laser beam welding method.

Figure 10.23

Penetration depths in depend- advantages disadvantages

- high power density ence of the beam - small beam diameter process - high welding speed - high reflection at metallic surfaces - non-contact tool - restricted penetration depth ( 25 mm) power and welding - atmosphere welding possible speed which are - minimum thermal stress - expensive edge preparation - little distortion - exact positioning required - completely processed components achievable in laser work piece - danger of increased hardness - welding at positions difficult to access - danger of cracks - different materials weldable beam welding are - Al, Cu difficult to weld depicted in Figure - expensive beam transmission and forming - short cycle times - power losses at optical devices - laser radiation protection required installation - operation at several stations possible 10.25. Further rele- - installation availability > 90% - high investment cost - well suitable to automatic function - low efficency (CO-Laser: < 20%, Nd:YAG: < 5%) vant influential 2

br-er10-24e.cdr © ISF 2002 factors are, among Advantages and Disadvantages others, the material of Laser Beam Welding Figure 10.24 10. Laser Beam Welding 141

(thermal conductivity), the design of 28 0,2% C-steel the resonator (beam quality), the CO-laser mm 2 (cross flow) h t

p focal position and the applied optics

e 20 d

n o i laser power: (focal length; focus diameter). t 16 a r

t 15 kW e

n 12 e

p 10 kW 8 8 kW 6 kW 4 kW 4 1,5 kW Figure 10.26 shows several joint 0 0 0,6 1,2 1,8 m/min 3,0 shapes which are typical for car body welding speed production and which can be welded 15

X 5 CrNi 18 10 by laser beam application. h

t CO2-laser

p (axial flow) e

d mm

n o i t a r

t e

n laser power:

e 5 p 6 kW 2 kW 4 kW 1 kW 0 0 1 2 3 4 5 6 7 m/min 9 welding speed br-er10-25e.cdr

Penetration Depths

Figure 10.25

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 butt weld fillet weld at overlap joint 10.27. These are a sign for the increased strength at a lower toughness and they are par- lap weld at flanged weld at ticularly critical in overlap joint overlap joint circumstances of dynamic loads.

br-er10-26e.cdr

Figure 10.26 10. Laser Beam Welding 142

The small beam diameter demands the very precise manipulation and positioning of the workpiece or of 500 WMA the beam and an HV 0,4 MAZ MAZ exact weld prepa- s s e

n ration, Figure d r a laser beam weld h 10.28. Otherwise, as result, lack of MAZ MAZ 0 12 fusion, sagged distance from the weld centre welds or concave

weld submerged arc weld root surfaces are

submerged arc weld possible weld de-

br-er10-27e_f.cdr fects.

Figure 10.27

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 10.29. However, too low a weld speed may also cause pore formation when the molten metal picks up gases from the root side. The materials that may be welded edge preparation misalignment with the laser reach from unalloyed and

(e £ 0,1 x plate thickness) low-alloy steels up to high quality tita-

gap beam mispositioning nium and nickel based alloys. The high carbon con-

(a £ 0,1 x plate thickness) tent of the trans-

br-er10-28e.cdr © ISF 2002 forming steel mate- Welding Defects rials is, due to the Figure 10.28 10. Laser Beam Welding 143 high cooling rate, to be considered a critical influential factor where contents of C > 0.22% 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.

Porosity

Figure 10.29

The application of laser beam welding may be extended by process variants. One is laser beam welding with filler wire, Figures 10.30 and 10.31 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 - “filling” of non-ideal, for example, V-shaped groove geometries - a realisation of a defined weld reinforcement on the beam entry and beam exit side.

10. Laser Beam Welding 144

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 indis- pensable for industrial application, Figure 10.32. The sensor systems are to take over the tasks of - process control, - weld quality as surance - beam positioning and joint tracking, respectively.

welding direction

filler wire laser beam laser beam filler wire

gas gas plasma plasma

weld metal work piece weld metal work piece

molten pool keyhole molten pool keyhole

forward wire feeding backward wire feeding

br-er10-30e.cdr

Figure 10.30

without filler wire with filler wire

increase of gap bridging ability

material: S380N (StE 380) filler wire: Sg2 gap: 0,5 mm dw = 0,8 mm PL = 8,3 kW

VW = 3 m/min

ES = 166 J/min s = 4 mm

weld zone Possibility of weld zone metallurgical influence

material combination: 10CrMo9-10/ X6CrNiTi18-10

PL = 5,0 kW gap: 0 mm gap: 0,5 mm vw = 1,0 m/min

vw = 1,6 m/min wire: SG-Ni Cr21 Fe18 Mo dw = 1,2 mm

br-er10-31e.cdr

Figure 10.31 10. Laser Beam Welding 145

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 vari- able beam manipulation possibilities when comparing with CO2 lasers. The CO2 laser is mostly used by the car industry and with sensing device; fill factor 120 % by their ancillary KB 4620/9 KB 4620/6 KB 4620/4 KB 4620/0 KB 4620/41 KB 4620/38 20:1 20:1 20:1 20:1 20:1 20:1 industry for welding 10/92 10/92 10/92 10/92 10/92 10/92 Probe MS1-6CProbe MS1-5A Probe MS1-4CProbe MS1-3A Probe MS1-2B Probe MS1-1C rotation- 0.1 mm 0.2 mm 0.3 mm 0.4 mm 0.5 mm 0.6 mm symmetrical mass- produced parts or

KB 4620/12 KB 4620/17 KB 4621/15 KB 4621/12 KB 4621/9 KB 4621/7 sheets. Figure 20:1 20:1 20:1 20:1 20:1 20:1 10/92 10/92 10/92 10/92 10/92 10/92

10.33 shows some Probe OS1-6A Probe OS1-5C Probe OS1-4CProbe OS1-3B Probe OS1-2B Probe OS1-1B 1 mm without sensing device; wire speed v = 4 m/min constant typical application D

br-er10-32e.cdr examples for laser beam welding.

Figure 10.32

aerospace industry - engine components - instrument cases automotive industry steel industry - gear parts - pipe production (cog-wheels, planet gears) - body-making - vehicle superstructures (bottom plates, skins) - continuous metal strips - engine components - tins (tappet housings, diesel engine precombustion chambers)

electronic industry - PCBs medical industry - accumulator cases - heart pacemaker cases - transformer plates - artificial hip joints - CRTs plant and apparatus engineering - seal welds at housings - measurement probes

Practical Application Fields

Figure 10.33