Lasers in metallurgy and technology of inorganic materials

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Lasers in metallurgy and technology of inorganic materials (Review)

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1974 Sov. J. Quantum Electron. 4 564

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Lasers in metallurgy and technology of inorganic materials(Review)A. A. UglovA.A. Baikov Institute of Metallurgy, USSR Academy of Sciences(Submitted November 12, 1973)Kvant. Elektron. 1, 1037-1055 (May 1974)A review is given of the applications of high-power cw and pulsed lasers in thetechnology of inorganic materials, including local hardening, welding, cutting of metalsand dielectrics, film processing, drilling of holes, and so on. An analysis is made ofthe potential applications of high-power cw lasers in metallurgical processes, includingpreparation of refractory metals, remelting of high-temperature materials, growth ofsingle crystals, processing of powder materials, etc.

Lasers can be used in various technological processes(cutting, welding, surface-hardening, hole drilling, etc.)because of the special features of the interaction betweenhigh-power radiation and matter. These features includethe precision of processing, high rates of heating and hightemperature gradients in the interaction zone, possibilityof local destruction of practically any material, simplicityof control of the energy input, and so on. Lasers emittingat different wavelengths can be used in a flexible mannerin processes with a variety of end products, including ini-tiation of chemical reactions. Only electron-beam pro-cessing and, partly, low-temperature plasma units cancompete with lasers in the universality of their processingapplications.

Some of the promising applications of laser radiationoccur in metallurgy. Various metallurgical processes in-volve the synthesis of new and known materials, surfaceoxidation, formation of protective coatings, decompositionof complex compounds, chemical reduction in controlledmedia, conversion and heating of gases, etc. The metal-lurgical applications of lasers in industry have been lessdeveloped than those in the technology of inorganic materials.

The review is divided into four sections, in accordancewith the various applications of lasers in technologicalprocesses. In Sec. I (Introduction) we shall formulate themain problems encountered in laser technology and metal-lurgy. In Sec. II we shall consider the application ofpulsed lasers in technology, including heat treatment,welding, and drilling of small holes. We shall also dis-cuss film processing, radiation-initiated thermochemicaltreatments, and several other applications. Section IIIdeals with the use of cw lasers. In this section we shallconsider particularly the welding and cutting of inorganicand other materials by carbon dioxide laser beams. Weshall discuss the initiation of chemical and thermochem-ical reactions in gases and on the surfaces of condensedmedia. We shall consider briefly the laser separation ofisotopes. In Sec. IV we shall discuss potential applicationsof lasers in metallurgy. The purpose of the review is toidentify the problems whose solution will help to extend therange of practical applications of laser radiation.I. INTRODUCTION

In the technology of inorganic materials, which in-cludes the traditional material processing treatments (forexample, welding), an important role is played by heat

sources located on the surface or in the bulk of a materiaacted upon by a flux of particles (electron, ion, and photonbeams or low-temperature plasma jets or blobs, etc.).

The theory of the thermal phenomena which occur inthe zone of interaction between highly concentrated (fluxdensity q > 102 W/cm 2) energy sources has been developedto a considerable degree by N. N. Rykalin and his school,1particularly for the processes of welding, and of mechan-ical and heat treatments.

Laser radiation focused into a small spot of size r^on the surface of a material opaque to a given wavelengthcan raise the upper limit of the attainable flux density toq > 1015 W/Cm2. This considerable increase in the energyflux density (by four or five orders of magnitude comparedwith the densities attainable for the older energy sources)is frequently unnecessary. Moreover, it is sometimes undesirable. This is because the radia tion incident on a material interacts with the gaseous and liquid products of thetreatment. In this way, a considerable proportion of theenergy is wasted in increasing the temperature and kinet-ic energy of the products of the treatment, which reducesthe effective efficiency of the process as a whole and re-sults in a loss of precision of localization. Therefore, inmost technological processes, the upper limit to the fluxdensity is q ^ 109W/cm2. Only in some cases, when thetime of interaction is short (< 10"8 sec), is it desirable touse radiation of q^ 109 W/cm2 density.

The developmentof high-power cw lasers has raisedthe possibility of their use in technology and metallurgy.The advantages of lasers and other highly concentratedenergy sources in large-scale metallurgy include the con-servation of the environment because in spite of theeconomic advantages of modern large-scale metallurgyover the laser or even plasma metallurgy the problemsof pollution of the environment by modern metallurgy re-quire the developmentof new processes. These new pro-cesses may result in a reduction of the pollution of theatmosphere and oceans. These comments are applicableto the future development of metallurgy in the next de-cade or so.

It is useful to divide the existing high-power lasersinto two groups: These are the pulsed and cw lasers. Theconcept of "high power" in the case of applications ofpulsed lasers in technology may, for example, mean thatduring a pulse of r\ ~ 1 msec duration a zone of radius

564 Sov. J. Quant. Electron., Vol. 4, No. 5, November 1974 Copyright 1974 American Institute of Physics 564


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r^ ~ 10~2 cm on the surface of a material reaches a tem-perature close to its melting point. In the case of iron,this definition applies to lasers which ca n provide powerdensities q $ 104 W/cm2. The concept of "high-power" cwlasers is more conveniently based on the assumption thatthe radiation acts for an indefinite time. Then, in the caseof iron, a cw laser can be regarded as a high-power ener-gy source if P ^ 10 W. We shall consider the principalapplications of the pulsed and cw lasers separately, but wemust remember that such a division is quite arbitrary.II. APPLICATIONS OF PULSEDL A S E R S I N T E C H N O L O G YOF I N O R G A N I C M A T E R I A L S

1. H e a t t r e a t m e n t . T h e focusing of a laserbeam on the surface of a material opaque to laser radia-tion results in the absorption of some part of the radiationenergy. The reflection coefficient R of a materia l dependson the state of its surface (polished, ground, etc.), angleof incidence of the radiation, temperature of the surface,and density of the radiation flux. When heat treatmentsare performed using radiation of q < 104 W/cm2 density,we can assume approximately that R is independent of thetemperature and the intensity of the heat source on thesurface of the material is governed by the spacetimecharacteristics of the radiation. In a large number ofpractical cases1'2 we can assume that the spatial charac-teristics of pulsed laser radiation can be described by thenormal distribution law

q(f) =


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11. The mechanism of this increase in microhardness isrelated to the formation of nonequilibrium structures, gen-eration of large numbers of dislocations because of con-siderable thermal stresses, fragmentation of structureblocks in the thermal influence zone, etc. Topochemicalreactions and the diffusion of nitrogen from the atmo-sphere, resulting in the formation of nitrides, can play animportant role in such treatments.

In machine building it is very desirable to achieve lo-cal hardening in those cases when a localized hard regionis produced in a relatively cheap material because thisreduces the cost of the final product. Important practicalapplications include hardening of the cutting edges ofdrills, cutters, and other machining tools. Surface hard-ening can be produced by doping the liquid phase formedin this way 12 or without such doping. Work on these ap-plications has started relatively recently and should even-tually produce results suitable for adoption in industry.

2 . L a s e r w e l d i n g . Welding is one of the mosteffective applications of lasers in technology. In laserwelding, one does not have to place the workpiece in vac-uum (as has to be done in electron-beam welding) but theprocess can be carried out in a controlled medium pass-ing laser radiation through walls (windows) transparentto a given wavelength. In view of the short duration of thepulses, one can avoid damage to such heat-sensitive ele-ments as p n junctions and so on, and one can also avoiddistortion in welding of thin plates.

Laser welding methods cover a wide range, includingwelding of thin-walled parts, wires to plates and largerparts, wires to one another, etc. Investigations of themechanical properties of laser-welded parts have shownthat the strength of the welded joints is usually not infe-rior to the strength of the bulk metal.13

Although laser welding is basically a melting process,one can also visualize processes in which radiation ac-tivates the surface, raising its temperature to values atwhich the material does not melt but becomes plastic (sol-id-state welding). A quartz or sapphire capillary can beused to apply pressure to the parts being welded together.

Th e knowledge of the laws governing heating, whichare used in the selection of the parameters of the radia-tion pulses, plays an important role in laser welding.There have been many investigations of the characteristicsof heat sources and temperature fields in laser weldingbut most have provided only estimates.14"24

Laser melting can be considered (like laser heattreatment) as a problem of heat conduction in three di-mensions with a specified surface source of heat. In de-scribing the heating processes, considerable difficultiesare encountered in allowing for the thermal contact be-tween materials of different kinds.18 Allowance for theheat of melting complicates considerably the solution ofthe problem because three-dimensional Stefan problemscan be analyzed only by numerical methods25 (the Stefanproblems are those involving calculations of the rates ofmotion of the boundaries of phase transitions and of thedistribution of the potentials, including temperature, inboth phases; the Stefan problems are strongly nonlinear).Experiments show that in the case of typical metals (steel,

copper, etc.) it is important to allow not so much for theheat of melting as for the occurrence of a situation inwhich surface evaporation deforms the liquid phase andthe source of heat penetrates deeper into the material.26

The critical flux density qc, which does not result in asignificant deviation of the depth and shape of the moltenzone from that predicted by the linear theory of heat con-duction, can be estimated employing a relationship similarto Eq. (4):qe = (6)where TD is the boiling temperature under normal pres-sure. For example, in the case of copper with T^ =2300C, we!have qc = 2 105 W/cm2. The experimentalvalues of qc are close to those found by calculation em-ploying Eq. (6).

The kinetics of melting depends on the temporal structure of a laser radiation pulse. The splashing of liquid ouof the molten zone, which usually impairs the quality of awelded joint, is due to several factors. They include ir-regularity of the pulse (spike structure), insufficient cleanness of the materials being welded and their saturationwith gases, conditions of focusing of the incident radiationand contacts between the materials. The causes of splashing of the liquid phase have not yet been investigated insufficient detail. The use of regular pulses or quasicon-tinuous radiation27 increases the stability of the meltingprocess and reduces the probability of the formation ofdefects in the welding zone. The spike structure of a pulsemay tend to cause deeper melting because of deformationof the liquid phase.

Th e size of the thermal influence zone can be calcu-lated allowing for the boundary conditions on free surfacesand on surfaces in contact. In calculations relating to la-ser welding, the assumption of a point source of heat mayresult in considerable errors even if the size of the weld-ing zone is only several tens of microns.28

Spot laser welding is used industrially in electronictechnology, microelectronics, instrument construction,the watch and clock industry, precision machine construc-tion, etc. It has to compete with other methods for joiningmaterials, which include electron-beam welding, heat-compression welding and its various varieties, ultrasonicwelding, etc. However, in some cases, laser welding isthe only acceptable method because ofa combination of sev-eral factors (the need to limit the thermal influence zone, im-possibility of applying mechanical stresses, presence ofa closed volume, etc.). Spot laser welding can also beused to produce welded seams if the repetition frequency/ of separate pulses, producing a seam by a partial overlaof the separate melting zones, is sufficiently high (f > 1Hz). Laser welding can be performed no t only when thematerials are in direct contact but also when they are separated by an easily fusible material (weldingsolderingoperation) or through a spacer made of a material whichensures the weldability of different materials, and so on.The number of various laser melting methods is very largbu t still does not cover all the various cases specific toparticular situations.

3 . Dr i l l ing o f s m a l l h o l e s . If q exceeds thecritical value for the onset of damage, a through or blind

566 Sov. J. Quant. Electron., Vol.4, No. 5, November 1974 A. A. Uglov 566


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hole is produ ced. The physical processes which occur inthe interaction zone above the critical intensity are knownonly quali tat ively. In considering the interaction process(i.e., in estimating temperatures, pressures, depth and di-ameter of the damage zone or in evaluating the proportionsof the liquid and gaseous fractions in the ejected products,etc.) in the q ^ 109 W/cm range, it is usual to employ thethermal damage theory29 or the gas-dynamic theory of theinteraction between radiation and condensed media.30 Inspite of the different approach to the damage caused byhigh-power fluxes, these two theories predict approximate-ly the same results in the technological range of flux den-sities (106-109 W/cm2). Neither of these theories givesquantitative estimates because the thermal theory is lim-ited to the one-dimensional approximation and ignores thepresence of the liquid phase on the surface of the material(this is allowed for in ref. 31) and the gas-dynamic theoryis also limited to the one-dimensional approximation, ig-nores the presence of the condensed medium, and is con-cerned only with the motion of a plasma and its interactionwith the incident radiation.In some estimates concerned with hole drilling, both

approximations are satisfactory only during th e initialstage of the interaction, when the process can be regardedas one-dimensional.

Thus, there is practically no theory which can give aconsistent description of all the phenomena resulting fromthe interaction of high-power radiation with metals andother materials.The format ion of holes is accompanied by the ejectionof the liquid phase and, according to the thermal theory,this is due to the meltingerosion mechanism. 29 Th e pres-ence of various inclusions, dissolved gases, particles ofcontamination, etc. in a material , which are all conse-quences of the technology of preparation, may favor addi-tional ejection of liquid and solid matter from the inter-action zone.32"35 A preliminary hardening of a material(for example, by quenching) also tends to increase theproportion of the condensed material ejected from theinteraction zone.11 '36A physical analysis of the processes of interactionbetween laser radia tion and matter in the q~ 109 W/cm2range predicts interesting co nsequences in the case whenthe temperature exceeds the critical value. In drillingholes a considerable increase in the flux density in milli-second pulses is usually undesirable because of a consid-erable enhancement of the interaction between the incidentradiation and the ejection products. The screening of theradiation by these products becomes considerable fo rsteels even for q ~ 105 W/cm 2, i.e., in the "welding" range.If q ~ 109 W/cm 2, the hole drilling becomes ineffective be-cause of the stron g screening. On the other hand, thepresence of a liquid phase on the walls of a hole, set inmotion by the interaction with radiation of q ~ 106 W/cm2intensity, is a frequent hindrance in precision drilling,partic ularly in the case of materials with a high thermalconductivity. Under these co nditio ns, a drilled hole fillsup with a liquid during a pulse and the process is quasi-periodic.37 The motion of the liquid phase frequently pre-vents the format ion of complex and noncircular holes be-cause, after a time, the melting destroys the intended

shape of the hole.

The solution to this problem in the precision drillingof small-diameter holes is provided by many-pulsetreatment.38 In such treatment, the main parameterwhichis controlled is the pulse repetition frequency, but thereare some industrial processes in which the energy of theindividual pulses is varied in accordance with a specifiedprogram. Obviously, in order to obtain holes of specifiedshape, depth, diameter, and surface finish, one has toemploy amplitudefrequency modulation of a train ofpulses. This method is already employed in other formsof processing of very hard and refractory materials byhighly concentrated energy sources such as the electronbeam.The drilling of a hole in a plate is usually accompan-ied by changes in the composit ion and structure of thematerial in the interaction zone.11 '39 This is due to themass transfer processes, diffusion in liquid an d solidphases, differences between the vapor pressures of thecomponents of the material, etc. When holes are drilledin steels of complex composit ion, it is usual to hardensuch steels before drilling.40We shall now mention some applications of laser holedrilling in metal and dielectric plates. One of the prom-ising applications of lasers is in the dril l ing of a largenumber of holes of small diameter ( ~ 10~2-10~3cm), suchas are needed for the processing of ferrite plates used incomputer memory cells. The difficulties in this processarise from the brittleness of ferrites and the appearance ofcracks due to thermal stresses.Th e drilling of holes in various hard iron alloys whichare used in dies in the manufacture of chemical (synthe-tic) fibers is one importan t application. It has been mas-tered only partly: Only rough holes can be drilled by alaser.38 Th e laser dril l ing mu st be followed by additionalgrooving.Another important application is the dril l ing of smallholes in ruby, diamond, an d other hard materials em -ployed in watches, clocks, an d other industrial products.4. F i l m p r o c e s s i n g . Laser radiation has alsoimportant applications in the processing of thin films.This type of processing is encountered in electronic tech-,nology and microelectro nics. The impo rtant tasks are thepreparation of photographic masks, removal of excess ma-terial from the surface of an insulating substrate (makingwindows), recrystallization of films, 41 trimming (adjust-ment of nominal values) of resistors, etc. Although thereare other ways of perform ing these tasks, the use of la-sers promises to increase the efficiency.We shall consider briefly only the thermal action oflaser radiation onfilm materials because the processesof initiation of thermochemical reactions on the surfacesof thin f i lms are discussed elsewhere.Radiation produced by free-oscillation and Q-switchedlasers is used in processing thin films.4 2 The simplertasks, such as the t r imming of resistors, can be performedby removing a part of a film until the required value of theresistance is reached. This process can be automated.Another method fo r adjusting the values of resistors is toalter the structure of the film by heating. The thermal

physics problems encountered in processing films by la-567 Sov. J. Quant. Electron., Vol. 4, No. 5. November 1974 A. A. Uglov 567


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ser radiation are considered in refs. 43-46. Estimatesmake it possible to specify the characteristics of a radia-tion source needed for the performance of a given opera-tion. Since the threshold den sity of the radiation flux re-quired for the destruction of thin film s is relatively low,one can use gas,47 semiconductor, and other lasers.48'49A projection method , suggested by V. P. Veiko andM. N. Libenson,50 and some of its variants51 are amongthe most prom ising. In the projection method, a workpiece

is placed in the plane of the image of a mask with a pre-scribed profile which must be transferred to the w ork-piece. The mask is placed between the exit ob jective ofthe laser and the objective which transfers the image (tothe required scale) to the plane of the sub strate car ryin gthe f i lm. Th e main advantage of the method is that it canbe used to transfer a complex figure. Moreov er, the re-solving power is sufficiently high. The projection methodpermits treating up to several square centimeters of thesurface simultaneou sly and the minimum size of an ele-ment of a figure is ~ 1 /*. The main distortions in the filmare due to the melting of the boundaries of the f igure, whichoccurs when heat travels during the action of a pulse. Aradical metho d for avoid ing such melting is to reduc e theduration of radiation pulses. Q-switched lasers are usual-ly employed in the projection method.5 . T h e r m o c h e m i c a l p r o c e s s e s i n i t i a t e db y p u l s e d l a s e r r a d i a t i o n . Thechemical inter-action between laser radiation and matter may be due to avariety of causes. They include: local heating, absorptionof photons by molecules which become dissociated or ion-ized, or are excited to a level at which a reaction with an -other molecule requires only a small activation energy,all of which may happen without a significant rise of thegeneral temperature of the me dium ; excitation of radia-tionchemical transformations analogous to the processesresulting from the passage of fast charged particlesthrough a medium which generate ions, radicals, and ex-cited atoms and molecules; macrosco pic heating of a me-dium as a whole.Surface thermochemical reactions initiated by pulsedlaser radiation are of considerable interest in variousbranches of technology: These reac tions include therm aldecomposition, oxidation and reduction of metals, andsynthesis of substances from simpler components. Inpractice, such reactions alter the properties of the sur-faces of bulk or film materials by changing the electricalconductivity, 52 '53 mechanical strength,11 resistance tochemical action, etc.Surface thermochemical reactions proceed generallyas follows.54 A workpiece whose surface is subjected tolaser radiation is placed in a chem ically active gaseousmedium. The laser radiation raises the temperature an dthis activates a surface chemical reaction whose rate isproportional to exp(u/kT), where u is the activation en-ergy of the process and k the Boltzmann constant. In thesubsequent stage, the rate of reaction is controlled to agreat extent by the rate of supply of the reagents via alayer of the reacted substance (this is limited by diffusion)rather than by the rate of the reaction itself.Pulsed and continuous laser radiation provides apromising means fo r surface chemical (topochemical) re -

actions on substrates of materials of the type used inelectronic technology and micro electron ics. In this case,it is usual to combine laser illumination and subsequentchemical treatments.We shall consider some examples of the use of suchthermochemical reactions. One of the simplest reactionsis thermal decomposition.54 The original substance inthe form of, for example, a suspension is deposited on asubstrate heated by laser radiation within the desired area

by the projection method. The flux density is selected soas to ensure a temperature sufficient for the decomposi-tion of the substance throughout the thickness of the sam-ple. Th e flux density should be within the range q j < q

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Lasers in metallurgy and technology of inorganic materials