The objectives of reactive chemical and nonreactive thermal processing with laser radiation are outlined giving indication that processing with laser radiation is governed by a hierarchy of time constants originating from photon-matter interaction, phase transition dynamics, laser source excitation fluctuations, and optical feedback in combination with the influence of beam delivery systems, processing/shielding gas flow configurations, robotics, production lines and environment. The minimization of losses by heat flow, reflection and transmission and the stringent need for quality assurance require as first approach the control of processing, which is mainly due to the capability of laser radiation source. The current status of laser radiation sources is reported giving information on the state of the art of processing with laser radiation in combination with subsequent demonstration of future trends and developments with respect to radiation sources, beam delivery, beam shaping, materials, processing and quality assurance.

At the second EUREKA ministerial conference held in Hannover on November 5 and 6 1985, the participating governments agreed to launch 10 projects one of which was the EUROLASER project (EU 6). The EUROLASER project comprised firstly a definitionsphase, which was based on a joint proposal put forward by industry and scientific institutes in Austria, Belgium, Denmark, Federal Republic of Germany,France, Greece, Italy, Netherlands, Spain and United Kingdom. The project covers the evaluation and development of industrial lasers and applications for material processing in the following areas: - CO2 laser systems in the power range of 10-100 kW, - high-power solid state laser systems in the power range of 1-5 kW, - excimer lasers with an output power of up to 10 kW, - other laser systems to be defined. The definitionphase ended in December 1987 with the result of feasibility studies. Out of the project EU 6 several realisationsphase projects emerged: CO2 laser: EU 83, EU 180, EU 214 / solid state laser: EU 226 / Excimer laser: EU 205, EU 213 / laser applications: EU 194.

The results of an EUROLASER definition phase study on high-power lasers are reported. Volume and structure of future demand, and required performance characteristics are identified

It has been recognised for some time that whilst high-power lasers are suitable for cutting, welding and heat treatment, different beam mode may be required for optimum use in a particular process. However, up to now industrial lasers have generally been designed to provide only one mode. Typically this has been TEM00 at powers up to 1 kW, TEM01* at 1 - 3 kW and Multimode at 3 - 5 kW. This paper, however, describes the development of a modular industrial laser with rated powers from 1.5 to 6 kW, with a novel optical design which allows user selection of beam mode. TEM0 ' in cutting, and Multimode are all available from this laser design for optimum efficiency in cutting, welding and heat treatment applications. Further, the modular construction technique allows a user to upgrade the maximum power of his system from 1.5 kW to 6 kW to any time after installation.

The development of CO2 waveguide lasers with ceramic bodies has led to the wide availability of rugged, low to medium power CW sources at 10.6 μm. Utilising a new glaze bonding technique we have manufactured an alumina based, CO2 waveguide laser having a single, linear, 2.5 mm square bore and an overall length of 1 metre. With this straight forward approach to scaling up the active gain length, CW output power up to 58 watts has been demonstrated.

Important features of an industrial CO2 laser to be considered during design are pointed out. An excellent overall efficiency of this laser is reported. Additional requirements are listed which allow for successful integration of laser technology in industry. Differences between DC and RF excitation are discussed.

High power lasers are more and more equipped with unstable resonators. Since an exact analytical treatment is not feasible in all cases numerical calculations are the best way to get beam quality, output power and workload of the resonator mirrors. A typical design process for unstable resonators using a wave propagation program is demonstrated. This programm uses a Fast Fourier Transform together with a Phase Sheet Technique. It allows to calculate the outcoupling factor and the near- and farfield distribution due to local medium inhomogeneities. The influence of potential intracavity adaptive optics is discussed as well.

The rf-excitation technique bases on the application of a sinusoidal voltage in the regime above 10 MHz to obtain a glow discharge. Specific requirements are necessary to ensure an efficient and reflection free power transmission from the generator to the load. In case of power matching the maximum power transmission is obtained, whereas impedance matching avoids reflection of the incident rf-power which would cause a standing wave at the line. Two or three element lumped circuits are sufficient to match the impedance of an rf-plasma to the characteristic impedance of the transmission line. In our case the eight laser discharge sections were driven by their own generators, each connected by a 50 Ω transmission line and a Collins filter. This Collins filter ensures an ideal matching for the total range of input power so that the total incident rf-power is utilized in the plasma.

Energy coupling , heat conduction, melting, evaporation and plasma generation including melt-, vapor- and plasma dynamics as basic physical processes for surface processing with laser radiation are reviewed. For low power densities energy transfer by heat conduction and mass transfer by diffusion are dominant with pronouncing convection and evaporation at increasing power densities. All the processes with different time constants govern the properties of the processed surface layers. High surface finish and improvement in wear-resistance, fatigue life or corrosion-resistance require adaptive control for optimum surface processing. Different approaches of mathematical analysis of the multiparameter problem surface processing with laser radiation provide a testing comparison with the physical picture and an easy estimation of process parameters in applications. Typical examples are reported to highlight the state of the art and to demonstrate the future trends in surface procesing with laser radiation.

The laser-induced plasma which is essential for the laser welding process absorbs a fractional part of the beam power above the workpiece as well as inside the "keyhole". The absorption above the surface of the workpiece always reduces the welding efficiency. It is demonstrated at which processing parameters this absorption takes place and how it can be reduced by shielding gases. Inside the keyhole, different absorption mechanisms could take place. 1st: Fresnel-absorption at the keyhole walls is the substantial absorption mechanism. In this case the direction of the beam polarization shows a strong effect on the welding result. The absorption length of the laser induced plasma Labs is well above the welding depth d (Labs >> d) 2nd: Plasma absorption inside the keyhole takes place. In this case, the welding depth is a function of the plasma absorption length. These absorption mechanisms are described and examples are shown.

The spectral emission of the laser induced plasma is investigated during the welding process of steel. In this study emission spectroscopy in the visible spectral range was used. A method is presented for determining the electron temperature, the electron density as well as the density of the neutral atoms in the welding plasma. In these metallic studies the relative line intensity technique was used, in which the radiation from a pair of spectral lines is compared as a function of electron temperature. The technique eliminates the need for a calibration source and can be used easily in the plasma monitoring applications. Some examples of the estimations of the plasma temperature, the electron and the neutral atom density are demonstrated. Time and space resolved measurements of the keyhole emission are presented. The application of the spectral diagnostic during the keyhole formation at the beginning of the metal sheet as well as at the end of the welding process at the end of the sheet is shown. The results help to explain the plasma absorption inside the keyhole. Some aspects of the spectral diagnostic as a basic method for a welding control are discussed.

In the frame of work of these studies of material hardening using intense shock wave produced during laser material interaction, the knowledge of thermal coupling requires determination of plasma characteristics. Optical emission spectra (3800-7100 Å) of 10,6 μm TEA pulsed CO2 laser produced plasma on aluminum and carbon targets have been studied. CO2 laser laser radiation intensities of 108-109 W/cm las ing for about 1.6 to 8 μs induced plasma formation on aluminum and carbon targets situated in vacuum chamber (10-5 Torr). Spatial and temporal evolution of emission spectra are obtained with a map of different plasma states. Absorption lines, continuum and emission lines were found to predominate successively the spectrum as we get closer to the target.

In laser processing a major physical phenomenon is the coupling of the laser energy into the workpiece. As high power infrared laser beams are directed to metallic targets, the reflectivity of the target shows a decrease. This paper describe some theoretically investigations in the laser light coupling to metallic targets. The validity of some of the reflectivity theories in relation to high power laser irradiation will be discussed based upon numerical calculations of the theoretical coefficients of reflectivity. Some possible mechanisms responsible for the changes in the optical properties of metal surfaces irradiated by high intensity laser beams, such as multi-photon absorption and effects of inhomogenous target heating will be discussed. One simple model describing the reflectivity as a function of material properties, commonly used in the litterature, is based upon the Hagens-Rubens equations. However, in this paper will be shown, why this model is not valid in the wavelength spectrum of high power lasers.

The absorption behaviour of metallic materials during laser beam cutting is examined under the presumption of a three dimensional cutting front model by applying the Fresnel formalism. The resulting theoretical influence of the laser beam parameters (power, mode, polarization, focal position and beam divergence) on the integral and homogeneity of the absorption distribution within the cutting front co-ordination system is described and compared with experimental results concerning the practicable cutting speed and the obtainable quality of the cut. Calculations of the power balance, considering also an estimation of heat losses, and appropriate cutting experiments are in accordance with the predicted average absorptivity of the idealized cutting front.

Laser materials processing shows a special pecularity compared to other customary techniques: the (generally reflecting) target introduces optical feedback into the system. This feedback changes the mode properties of the laser radiation according to the targets dynamics. We report on one of these aspects of laser-target interaction resulting in the change of the polarization of the incident light. Based on rate equations, a theoretical model is presented that allows the calculation of this change with respect of the target properties, yielding a simple relation for the two orthogonal planes of polarization of a laser mode. This relation turns out to be linearly dependent of a function ψ(t) which describes the optical feedback. The relation holds for target reflexions of up to 10% and for times larger than T1 • T2/T1 - T2 (where T1, T2 are the time constants of the passive resonator for the two orthogonal Planes of polarization). Experiments supporting the model are presented. The model offers a method for the modulation of laser radiation without change of frequency or intensity. It might also be of interest for high-power CO2 laser cutting and welding of metals.

During laser beam welding, a laser induced metal vapour plasma is formed. The plasma shows strong fluctuations combined with sound emission. The origin of these fluctuations is discussed. On-line control of the welding process is possible by measuring and analyzing the plasma emission with a photodiode and the sound emission with a microphone. Optical diagnostic methods are compared with acoustical methods. The results show that both diagnostic methods yield complementary information on the process. On-line control of the welding process is demonstrated for the influence of the process gas, hole formation in the seam and welding through.

Controlled laser processing is required to improve and ensure the quality of welding and heat treatment. A sensor collects informations from the processing zone and passes them to a controller. The controller generates a signal adjusting the beam power according to the present situation. This feed-back structure using a pyrometer as sensor for the surface temperature was successfully employed for transformation hardening of workpieces with complex geometries. The complex phenomena in laser induced plasma require a modified sensor. The plasma signal detected by a photodiode contains implicit informations about the quality of the welding process. These informations have to be worked up before they can be passed to a controller. An experimental approach of process modelling, which requires a wide banded (0..300kHz) on-line mesurement of beam power for a recursive system identification algorithm, will be described.

This paper describes the mechanisms by which non-ferrous metals can be cut by CO2 laser. Materials dealt with include titanium, alumimium, copper and nickel alloys. The differences between inert gas and oxygen assisted laser cutting are explained and examples of cut qualities and speeds are given. Cutting efficiencies are related to the physical properties of the various alloy systems with particular emphasis placed upon reflectivity and conductivity. Specialised techniques developed for non-ferrous metal cutting are described and commercial applications of the process are discussed.

In industrial production, CO2-lasers have been used in welding steel components for several years. The use of CO2-lasers in deep penetration welding of aluminium alloys was hindered up to now due to the properties of these materials. Due to low absorption, high thermal conductivity and low ionisation energy respectively, the width of variating the process parameters is reduced considerably compared to the welding of steel. Experimental investigations in welding AL 99,5 and aluminium alloys show that it is possible to reach welding results with acceptable and reproducible quality. The presented investigations are an attempt to analyse the difficulties in CO2-laser welding of aluminium systematically.

Analysis of the different factors which affect the improvement of high power laser systems are presented. It concerns ; the laser sources and their power and mode structure stability, their pointing stability and their efficiency, the beam transport and handling with different morphologies of machines according to the dimensions of the parts and the production rate ; the control of the industrial process through a central computer which takes in charge all the functions of the machine. Next, factors to be considered in the economical analysis for installing high power laser systems are discussed ; this covers the investment cost for the source and the associated system, the cost for consummables, for maintenance for training and labor. Comparison of the investment cost as function of power for both laser and electrons beam welding, shows that, generally speaking, there are two regions of beam power : the first located at less than 3 to 4kW where the laser is more advantageous economically, the second located at higher than 10kW where the E.B. is more advantageous. An example of a multi-stations mock-up (12 welding heads) supplied by a single laser beam is finally presented. Located at about 20m from the laser source the welding heads receive the laser beam through a distributor of high alignement precision. A spot weld similar to what is usually obtained with resistance spot welding can be achieved in about 0,5 second. Some technical and scientific problems relative to this application and concerning the transfer of laser beam are presented.

A critical factor in the performance of high power industrial CO2 lasers is the substrate material used for optical components in the laser and beam delivery system. The choices for transmissive optics such as lenses and output couplers can be narrowed down to GaAs or ZnSe with low absorbing coatings, however, determining which one of these two materials will work best in a particular application can be difficult and in some cases ambiguous. This paper compares ZnSe and GaAs as used in high power laser applications by use of thermal finite element analysis, laser calorimetry data, and a relative figure of merit function.

In applications of high power CO2-lasers which require very high power concentration and thus wide aperture focussing optics, the use of retrofocus type optical systems is advantageous in that it allows relatively long distances between the focus and the last optical surface. Relative apertures around f/1.2 have been achieved with diffraction limited systems consisting of a single negative and (symmetrical) dual positive lens elements made from ZnSe. An alternative approach combines the dual positive lenses in to a single aspheric element. The practical consequenses and tradeoffs are discussed.

The near surface microstructure of 3 austempered ductile irons has been modified using laser surface melting (LSM). A white cast iron layer was produced to depths of up to 500 microns. The microstructure of this region varied depending upon the parameters used. A heat affected zone (HAZ) appeared at an abrupt transition between the LSM zone and the substrate. The size of the HAZ, which varied between 20 and 200 microns was predominately affected by the pass velocity in the range of energies used. The hardness profiles showed large variations in the Haz probably due to varing tempering effects from the subsequent passes.