- Email: [email protected]
Some prospects for pulsed laser manufacturing processes
Robotics & Computer-Integrated Manufacturing, Vol. 4, No. 1/2, pp. 233-239, 1988
0736-5845/88 $3.00 + 0.00 Pergamon Press plc
Printed in Great Britain.
• Paper
SOME PROSPECTS FOR PULSED LASER MANUFACTURING PROCESSES JANEZ MOZINA University Edvard Kardelj, Ljubljana, Faculty of Mechanical Engineering, Ljubljana, Yugoslavia
1. INTRODUCTION Soon after the invention of the laser, a quarter of a century ago, extremely promising possibilities of using laser techniques in different areas of human activities were recognized. Among non-military applications, laser-induced fusion and the area of laser manufacturing have been studied most intensively. 1'2 Laser applications in manufacturing emanate from the ability of laser beams to deposit large quantities of energy in the thin surface layer of the workpiece. The general characteristics of laser materials pro cessing can be summarized as follows: • non-contact energy transfer, no tool wear • small heat affected zone, low thermal distortion • processing of hard, brittle or refractory materials • high processing speed • good edge quality • welding of dissimilar materials • processing of very small scale structures • creating novel surface effects and properties • amenability to automated operation. Among disadvantages of laser materials processing one must mention the absence of convenient feedback signals for adaptive control. Successful development of new monitoring methods would furthermore open the way of lasers into modern computer-controlled manufacturing systems. Today many laser processes provide appreciable economic benefits compared to their conventional counterparts. However, the benefits are expected to be much higher in the ability of laser techniques to produce entirely new properties and products, which are not achievable by conventional tools and processes. Several examples of laser processing of silicon, which all have been developed in recent years, are indicated in Fig. 1. Similar schemes can be made for a great variety of metals and other materials.
Although the improvements already made are regarded as being substantial, they represent a small fraction of the potential. The aim of this contribution is to show that the above statements apply particularly well to pulsed laser processes, thus placing them among most useful high technologies of the future. The argument is based on the differences in physical processes between continous and pulsed laser beams during their interaction with irradiated materials. Pulsed lasers operate at relatively lower mean powers but at very high peak powers. The intensities of pulsed laser beams easily exceed the absorption edge 10 MW/cm 2 above which the energy coupling becomes much more efficient than in CW operation. 4 Simultaneously, due to the short interaction times, heat losses to the surroundings are significantly diminished. Under these conditions, localized and transient thermal gradients develop, leading to the generation of high amplitude stress waves also called optoacoustic waves. 5'6 Laser-generated stress waves propagate from the interaction area into the absorbing medium and into the adjoining transparent medium. Strong optoacoustic waves modify the material's substructure and properties in a way similar to shock waves. This process has been successfully used to increase the strength, hardness and the fatigue life of several metals and alloys.7 Laser-induced stress waves are also strongly dependent upon the details of the beam-surface interaction. Applying a convenient detection and analyzing system it is, in principle, possible to use the laser induced stress waves in real-time monitoring of laser manufacturing processes. In such a way pulsed laser processes could be more effectively incorporated into modern computer-controlled manufacturing systems. 233
234
Robotics & Computer-Integrated Manufacturing • Volume 4, Number 1/2, 1988
[LASER PROCE881NG OF SILICON J [~ASER A'~ALI.a I
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Fig. 1. Block diagram showing the variety of laser processing techniques in semiconductormanufacturing.3
The above observations are elaborated in some detail in the first part of the article. In the second part, several specific applications of the optoacoustic effect in monitoring pulsed laser manufacturing processes and in non-destructive testing of materials and products are described. 2. BEAM-SURFACE INTERACTION All laser manufacturing processes depend critically upon the interaction between the incoming laser beam and the absorbing surface. To describe the interaction it is convenient to consider the following steps: 8
1. Reflection and absorption of incident photons These processes proceed in metals via free electron excitations in a thin surface layer of a width of 10 nm. The absorbed part of the incident pulse energy is proportional to the surface absorptivity which is strongly dependent upon the incoming laser beam intensity. A situation similar to that in Fig. 2, where the surface absorptivity vs incoming beam intensity of a N d - Y A G laser (a = 1.06/~m) on copper target is depicted, is valid for all combinations of incoming light and absorbing surfaces. The intensity threshold is of the order of magnitude 10 MW/cm 2. In most cases CW lasers operate below the intensity threshold while it is easily exceeded in pulsed laser operation. From what follows, the energy coupling is much better in pulsed than in CW operations. Two important facts follow from this observation: (a) operating in the pulsed mode it is possible to treat metals which cannot be treated by CW lasers due to the low absorptivity, (b) beam/surface energy coupling is a part of overall energy efficiency of the manufactur-
1.0-
t
~=1,06#m
g O,~. i
--+---o~
j
A =0.045
' 6 ' 8'10 107Wz20 Laser-Intensity .,' ----~ Fig. 2. Surface absorptivity vs incoming beam intensity for Nd-Yag laser beam on copper target.4 ing process. High values of surface absorptivity make pulsed laser processes more favorable than CW processes regarding consumption. . Electronic de-excitation In this step the absorbed energy is transferred to the lattice atoms through phonon creation by e l e c t r o n - p h o n o n collisions. The energy transfer can be characterized by the relaxation time re which is of the order of 1 ps. Except for the extremely short duration of the laser pulses, the process of energy transfer to the lattice can be considered as instantaneous. . Heat diffusion The heat flow from the thin absorption layer into the bulk can be approximated with the solutions of the linear heat diffusion equation. 9 Important parameters of this step are the surface temperature and the width of the heat affected zone. They can be calculated from the
Pulsed laser manufacturing processes • J. MOZINA
values of incident pulse intensity and duration to. Inserting typical values it is found that extreme heating and cooling rates in the range 10 9 to 1011 K/s occur in the surface layer of the width L ~ (2 Dto) 1/2, where D is heat diffusion coefficient of the absorbing medium. Melting and boiling points are easily achieved even with defocussed laser pulses. The immediate consequence is a great variety of surface treatment processes and effects. . Thermal expansion Large thermal gradients introduce severe strain in the near surface region at irradiation values well below the melting threshold. The phenomenon is much more pronounced in pulsed than in CW laser operations. Thermal expansion of heat affected zone produces a localized mechanical stress field, which propagates from the heated area into the bulk in the form of stress pulses with the velocity of sound. Laser-induced stress waves, also called optoacoustic waves, will be considered in some detail in the next section. 3. LASER-INDUCED STRESS WAVES--OPTOACOUSTIC S 3.1. Theory Laser generation of stress waves in a low intensity regime can be dealt with in a theory a dynamic thermoelasticity.1° The simple geometry depicted in Fig. 3, which corresponds to the irradiation of the metal layer by the defocussed laser pulses with intensity i(O, can be approximated with the following system of equations for the temperature
Loser - ~nduced uLtrasomc pulse m t h e gas\
///
Back s u r f a c e displacement
/ ThermoeLastic d e f o r m a t i o n of the front surface
L o s e r - induced s t r e s s wove in t h e sotid
Fig. 3. Scheme of laser induced stress wave generation on absorbing metal surface.
235
T = T(x,t) and normal displacement u = u(x,t): ,iT" = pc~7" - W(x,t) c2u" = ii + ~c~T'. Here Jl, p, c~, c and y are thermal conductivity, density, specific heat, sound velocity and Griineisen constant of the absorbing material, respectively. Of utmost importance for the generation of stress waves is the source term
W(x,t) = (1 - r) t~i(t) exp (-Ux) in which the interaction between the laser beam and the surface is described by surface reflectivity r and bulk optical absorption coefficient/a. The solution of the problem can be formally written as a convolution
u(x,t) =
[
G,(x,t - r) i (z) dr
0
of displacement of Green's function Gu(x,t) and incoming pulse intensity i(t). Taking into account the initial and boundary conditions and applying Laplace transformation techniques an analytical expression for Gu(x,t) can be obtained. 11 A theoretical solution of the problem enables one to extract the information about the beam/surface interaction from the optoacoustic signals which can be detected on the opposite surface of the irradiated layer. Theoretical and experimental results show that optoacoustic signals are proportional to the surface absorbtivity in low intensity range. Due to this unique property, the optoacoustic effect has found many interesting applications in physics, chemistry and non-destructive testing. As the optical power density is increased and the melting point is reached at the surface, optoacoustic signals show non-linear behavior connected with the ablation of the surface layers of the metal. Although the source mechanism is very complex in this case, the approximate solution was proposed and found consistent with the experiments, which assumes that the optoacoustic signal is a linear combination of pure thermoelastic and pure ablation contribution. 12 The experimental set-up for studying the laser generation of ultrasound is shown in Fig. 4. A wide range of pulsed lasers and ultrasonic transducers can be used. In addition to be PXT transducer which detects stress waves in the metal to the opposite side of the illuminated surface, the microphone takes place in front of the tanget surface, which detects the optoacoustic signal propagating in the air. Simultaneous observation of both signals gives more information about the complex source mechanism. 13
236
Robotics & Computer-Integrated Manufacturing • Volume 4, Number 1/2, 1988
~
Metallicsample
Microphone" ~
|
Phatadlode
, ransducer
'
Fig. 4. Experimental set-up for simultaneous observation of laser-induced ultrasound in the absorbing metal and in the surrounding gas.
3.2. A p p f i c a t i o n s Let us now briefly quote some applications of pulsed laser stress wave generation in the area of laser manufacturing. All pulsed laser manufacturing processes are copious sources of optoacoustic waves. Knowing that they contain information about the beam/surface interaction process, one may ask whether it is possible to use this information in monitoring the manufacturing process. As far as we know, the first attempt was made by Saifi and Vahaviolos,4 who studied laser-induced stress waves to monitor pulsed laser spot welding of insulated copper wire to terminal posts. Laser welding for such applications is very advantageous since the incident radiation can be used simulaneously to remove the insulation and to cause the weld in the precise area by means of the single laser pulse. The welding geometry and the electronic system for monitoring the quality of the welds is shown in Fig. 5. The laserinduced stress wave was detected by a differential
piezoelectric transducer attached to the post. Correlation between the parameters of the detected stress waveform and the weld quality was investigated experimentally. Comparing the magnitude of the laser-induced stress wave to the determined threshold value it was possible to discriminate in real-time between good and bad welds. This was of great value for quality control of the pulsed laser manufacturing process. Another successful application of optoacoustic effect in pulsed laser processing was reported recently by Yeack et al. 15 They developed a method for real-time monitoring of pulsed laser drilling into layered composite material. Instead of detecting the stress, wave in the absorbing medium, they measured the pressure pulse in the surrounding air, as shown in Fig. 6. By interpreting the experimental results of multiple-pulsing of the drilling site, they obtained a simple criterion for the termination of the drilling process. Each acoustic signal was used to
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Pulsed laser manufacturing processes • J. MOZINA
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determine whether the hole was drilled to completion. If not, the next pulse was applied. In this specific application, the process was not only monitored, but also controlled by means of an optoacoustic response, acting as a feedback signal in a very simple feedback loop. It is reasonable to expect that optoacoustic effect can be usefully applied to monitor many other pulsed laser manufacturing processes. Besides their use in monitoring various manufacturing processes, laser-induced stress waves are capable of producing several very interesting effects in the irradiated metals. It has been demonstrated that high energy solid state laser pulses generate stress waves with an amplitude of the order of 1012 Pa and more, which can otherwise be obtained only by strong explosions. ~6 High dislocation densities produced by laser-induced stress waves can significantly improve the properties of metallic materials. Pulsed laser shocking of several aluminum alloys showed that the 0.2% offset yield strength was increased as much as 30% over unshocked values. Large improvements in fatigue life have also been reported by using the laser shock process. 7 Repetitive illumi-
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nation of the stainless steel surface by several strong laser pulses increased surface hardness by about 40%. In addition to various surface transformation processes a special material removal process was proposed based on pulsed laser-induced fracture. 17 The striking difference between evaporation cutting which proceeds by using CW lasers and pulsed laser fracture is that the energy consumption can be diminished by several orders of magnitude in the latter case. Recent studies of laser-generated stress waves have given an important contribution also in the field of non-destructive testing. 1~'~9 The techniques has several distinct advantages over conventional ultrasonics using piezoelectric transducers. Since no physical contact with the samples is required, the problems of acoustic coupling between the transducers and the samples are eliminated. Still more advantageous is the combination of laser generation and laser detection of ultrasonic pulses, which can be achieved by constructing special displacement interferometer. 2° This is an entirely non-contact ultrasonic testing method, also called optical ultrasonics. 2~ Typical non-contact ultrasonic testing and the corresponding result of flaw detection in a 25 mm thick aluminum plate are shown in Figs. 7 and 8. Several other arrangements have been reported so far with increased signal-to-noise capabilities, thus opening the use of this method for various ultrasonic inspections under severe industrial conditions, such as during the production and processing of steel and other metals. 22-24 Another advantage of pulsed laser-induced ultrasound is that extremely short duration ultrasonic pulses can be generated and detected this way, enabling one to test very thin specimens. As an example of the extreme spatial resolution, the results of in-depth ultrasonic testing of a razor blade are shown in Fig. 9. 25 Stress pulses of the duration 2 ns were produced by using mode-lacked picosecond laser pulses. The time between two subsequent echoes corresponds to the
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238
Robotics & Computer-lntegrated Manufacturing • Volume 4, Number 1/2, 1988 0 t
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turing. C o m p a r e d to the more conventional CW, operated laser processes several advantages of pulsed laser manufacturing processes can be pointed out" • using pulsed lasers, operating at high p e a k intensities, limitations inherent to CW lasers are removed • specific energy consumption is significantly diminished • heat affected zone is diminished • broad variation of pulsed laser p a r a m e t e r s leads to a great variety of new effects and products • optically induced stress waves can be used in monitoring pulsed laser materials processes • optical ultrasonics--noncontact generation and detection of ultrasonic pulses by means of pulsed lasers is becoming a very promising nondestructive testing technique for use in hostile environments and in very small scale testing • using laser induced stress waves as feedback signals in the adaptive control loops enable one to incorporate pulsed lasers into complex and flexible manufacturing systems.
5 ns/OlV
Fig. 9. Multiple echoes of extremely short ultrasonic pulses in razor blade induced by picosecond mode-locked laser.25 acoustic transit time through the blade of the width 75/~m. These results must surely give an impetus for the further development of optical ultrasonics. 4. C O N C L U S I O N The objective of this contribution was to give some insight into the use of pulsed lasers in manufac-
5. REFERENCES 1. Ready, J. F.: Industrial Applications of Lasers. New York, Academic Press, 1978. 2. Bass, M. (Ed.): Laser Materials Processing. Amsterdam, North Holland, 1932. 3. Boyd, I. W.: Contemp. Phys. 24: 461-490, 1983. 4. Herziger, G.: Feinwerktechnik Messtechnik 91: 158, 1983. 5. Pao, Y. H. (Ed.): Optoacoustic Spectroscopy and Detection. New York, Academic Press, 1977. 6. Rosencwaig, A.: Photoacoustics and Photoacoustic Spectroscopy. New York, Wiley, 1980.
Pulsed laser manufacturing processes • J. MOZINA 7. Fairand, B. P., Clauer, A, H.: Laser generated stress waves: their characteristics and their effects to materials. In Laser-Solid Interaction and Laser Processing-1978, AIP Conf. Proc. No. 50. New York, American Institute of Physics, 1979. 8. Meyer, J. R. et al.: Phys. Rev. B 21: 1559, 1980. 9. Carslaw, H. S., Jaeger, J. C.: Conduction of Heat in Solids. Oxford, Oxford University Press, 1959. 10. Nowacki, W.: Dynamic Problems of Thermoelasticity. Leyden, Noordhoff, 1975. 11. Mozina, J.: Ph.D. Thesis, University of Ljubljana, Faculty of Mechanical Engineering, Ljubljana, 1980. 12. Scruby, C. B. et al.: J. appl. Phys. 51: 6210, 1980. 13. Mo~ina, J., Diaci, J.: J. Phys. 44: C6-73, 1983. 14. Saifi, M. A., Vahaviolos, S. J.: IEEE J. Quant. Electr. 12: 129, 1976.
239
Yeack, C. E. et al.: Appl. Phys. Lett. 41: 1043-1982. Steverding, B.: J. appl. Phys. 41: 2096, 1970. Steverding, B.: J. Phys. D: appl. Phys. 4: 787, 1971. von Guffeld, R. J., Melcher, R. L.: Appl. Phys. Left. 30: 357, 1977. 19. Hutchins, D. A. etal.: Appl. Phys. Lett. 38: 677, 1981. 20. Bondarenko, A. N., Drobot, Y. B.: Sov. J. N D T 12: 655, 1976. 21. T a m , A. C., Coufal, H: J. Phys. C6 44: C6-9, 1983. 22. Calder, C. A., Wilcox, W. W.: Materials Evaluation 38: 86, 1980. 23. Cielo, P. et al.: Ultrasonics 23: 55, 1985. 24. Monchalin, J. P.: Appl. Phys. Left. 47: 14, 1985. 25. Krautkriimer, J.: 9th World Conf. NDT, Plenary Lecture, Melbourne, 1979. 15. 16. 17. 18.
0736-5845/88 $3.00 + 0.00 Pergamon Press plc
Printed in Great Britain.
• Paper
SOME PROSPECTS FOR PULSED LASER MANUFACTURING PROCESSES JANEZ MOZINA University Edvard Kardelj, Ljubljana, Faculty of Mechanical Engineering, Ljubljana, Yugoslavia
1. INTRODUCTION Soon after the invention of the laser, a quarter of a century ago, extremely promising possibilities of using laser techniques in different areas of human activities were recognized. Among non-military applications, laser-induced fusion and the area of laser manufacturing have been studied most intensively. 1'2 Laser applications in manufacturing emanate from the ability of laser beams to deposit large quantities of energy in the thin surface layer of the workpiece. The general characteristics of laser materials pro cessing can be summarized as follows: • non-contact energy transfer, no tool wear • small heat affected zone, low thermal distortion • processing of hard, brittle or refractory materials • high processing speed • good edge quality • welding of dissimilar materials • processing of very small scale structures • creating novel surface effects and properties • amenability to automated operation. Among disadvantages of laser materials processing one must mention the absence of convenient feedback signals for adaptive control. Successful development of new monitoring methods would furthermore open the way of lasers into modern computer-controlled manufacturing systems. Today many laser processes provide appreciable economic benefits compared to their conventional counterparts. However, the benefits are expected to be much higher in the ability of laser techniques to produce entirely new properties and products, which are not achievable by conventional tools and processes. Several examples of laser processing of silicon, which all have been developed in recent years, are indicated in Fig. 1. Similar schemes can be made for a great variety of metals and other materials.
Although the improvements already made are regarded as being substantial, they represent a small fraction of the potential. The aim of this contribution is to show that the above statements apply particularly well to pulsed laser processes, thus placing them among most useful high technologies of the future. The argument is based on the differences in physical processes between continous and pulsed laser beams during their interaction with irradiated materials. Pulsed lasers operate at relatively lower mean powers but at very high peak powers. The intensities of pulsed laser beams easily exceed the absorption edge 10 MW/cm 2 above which the energy coupling becomes much more efficient than in CW operation. 4 Simultaneously, due to the short interaction times, heat losses to the surroundings are significantly diminished. Under these conditions, localized and transient thermal gradients develop, leading to the generation of high amplitude stress waves also called optoacoustic waves. 5'6 Laser-generated stress waves propagate from the interaction area into the absorbing medium and into the adjoining transparent medium. Strong optoacoustic waves modify the material's substructure and properties in a way similar to shock waves. This process has been successfully used to increase the strength, hardness and the fatigue life of several metals and alloys.7 Laser-induced stress waves are also strongly dependent upon the details of the beam-surface interaction. Applying a convenient detection and analyzing system it is, in principle, possible to use the laser induced stress waves in real-time monitoring of laser manufacturing processes. In such a way pulsed laser processes could be more effectively incorporated into modern computer-controlled manufacturing systems. 233
234
Robotics & Computer-Integrated Manufacturing • Volume 4, Number 1/2, 1988
[LASER PROCE881NG OF SILICON J [~ASER A'~ALI.a I
I ~VEL "~S='~.~ATE"ALS I
J CONTROLLED
CHIP $EPARATIONJ SCRIBING] J
J OIFFUSION OF [
I 'MPOR'T'ES I
IT,. GROWTHOf POL YSILICON LAYERS
FILM FORMATION
[
EXPLOSIVE CRYSTA~TION
[ S~RSATURArEOI | ALLOYS ]
I
I SORF,CE
I PREPARATION IJ
I REACTIONSJ I PHOTOLYTIC [(LASER PHOTODEPOSITION )J PYROLYTIC REACTIONSJ OXIDATION) I
Fig. 1. Block diagram showing the variety of laser processing techniques in semiconductormanufacturing.3
The above observations are elaborated in some detail in the first part of the article. In the second part, several specific applications of the optoacoustic effect in monitoring pulsed laser manufacturing processes and in non-destructive testing of materials and products are described. 2. BEAM-SURFACE INTERACTION All laser manufacturing processes depend critically upon the interaction between the incoming laser beam and the absorbing surface. To describe the interaction it is convenient to consider the following steps: 8
1. Reflection and absorption of incident photons These processes proceed in metals via free electron excitations in a thin surface layer of a width of 10 nm. The absorbed part of the incident pulse energy is proportional to the surface absorptivity which is strongly dependent upon the incoming laser beam intensity. A situation similar to that in Fig. 2, where the surface absorptivity vs incoming beam intensity of a N d - Y A G laser (a = 1.06/~m) on copper target is depicted, is valid for all combinations of incoming light and absorbing surfaces. The intensity threshold is of the order of magnitude 10 MW/cm 2. In most cases CW lasers operate below the intensity threshold while it is easily exceeded in pulsed laser operation. From what follows, the energy coupling is much better in pulsed than in CW operations. Two important facts follow from this observation: (a) operating in the pulsed mode it is possible to treat metals which cannot be treated by CW lasers due to the low absorptivity, (b) beam/surface energy coupling is a part of overall energy efficiency of the manufactur-
1.0-
t
~=1,06#m
g O,~. i
--+---o~
j
A =0.045
' 6 ' 8'10 107Wz20 Laser-Intensity .,' ----~ Fig. 2. Surface absorptivity vs incoming beam intensity for Nd-Yag laser beam on copper target.4 ing process. High values of surface absorptivity make pulsed laser processes more favorable than CW processes regarding consumption. . Electronic de-excitation In this step the absorbed energy is transferred to the lattice atoms through phonon creation by e l e c t r o n - p h o n o n collisions. The energy transfer can be characterized by the relaxation time re which is of the order of 1 ps. Except for the extremely short duration of the laser pulses, the process of energy transfer to the lattice can be considered as instantaneous. . Heat diffusion The heat flow from the thin absorption layer into the bulk can be approximated with the solutions of the linear heat diffusion equation. 9 Important parameters of this step are the surface temperature and the width of the heat affected zone. They can be calculated from the
Pulsed laser manufacturing processes • J. MOZINA
values of incident pulse intensity and duration to. Inserting typical values it is found that extreme heating and cooling rates in the range 10 9 to 1011 K/s occur in the surface layer of the width L ~ (2 Dto) 1/2, where D is heat diffusion coefficient of the absorbing medium. Melting and boiling points are easily achieved even with defocussed laser pulses. The immediate consequence is a great variety of surface treatment processes and effects. . Thermal expansion Large thermal gradients introduce severe strain in the near surface region at irradiation values well below the melting threshold. The phenomenon is much more pronounced in pulsed than in CW laser operations. Thermal expansion of heat affected zone produces a localized mechanical stress field, which propagates from the heated area into the bulk in the form of stress pulses with the velocity of sound. Laser-induced stress waves, also called optoacoustic waves, will be considered in some detail in the next section. 3. LASER-INDUCED STRESS WAVES--OPTOACOUSTIC S 3.1. Theory Laser generation of stress waves in a low intensity regime can be dealt with in a theory a dynamic thermoelasticity.1° The simple geometry depicted in Fig. 3, which corresponds to the irradiation of the metal layer by the defocussed laser pulses with intensity i(O, can be approximated with the following system of equations for the temperature
Loser - ~nduced uLtrasomc pulse m t h e gas\
///
Back s u r f a c e displacement
/ ThermoeLastic d e f o r m a t i o n of the front surface
L o s e r - induced s t r e s s wove in t h e sotid
Fig. 3. Scheme of laser induced stress wave generation on absorbing metal surface.
235
T = T(x,t) and normal displacement u = u(x,t): ,iT" = pc~7" - W(x,t) c2u" = ii + ~c~T'. Here Jl, p, c~, c and y are thermal conductivity, density, specific heat, sound velocity and Griineisen constant of the absorbing material, respectively. Of utmost importance for the generation of stress waves is the source term
W(x,t) = (1 - r) t~i(t) exp (-Ux) in which the interaction between the laser beam and the surface is described by surface reflectivity r and bulk optical absorption coefficient/a. The solution of the problem can be formally written as a convolution
u(x,t) =
[
G,(x,t - r) i (z) dr
0
of displacement of Green's function Gu(x,t) and incoming pulse intensity i(t). Taking into account the initial and boundary conditions and applying Laplace transformation techniques an analytical expression for Gu(x,t) can be obtained. 11 A theoretical solution of the problem enables one to extract the information about the beam/surface interaction from the optoacoustic signals which can be detected on the opposite surface of the irradiated layer. Theoretical and experimental results show that optoacoustic signals are proportional to the surface absorbtivity in low intensity range. Due to this unique property, the optoacoustic effect has found many interesting applications in physics, chemistry and non-destructive testing. As the optical power density is increased and the melting point is reached at the surface, optoacoustic signals show non-linear behavior connected with the ablation of the surface layers of the metal. Although the source mechanism is very complex in this case, the approximate solution was proposed and found consistent with the experiments, which assumes that the optoacoustic signal is a linear combination of pure thermoelastic and pure ablation contribution. 12 The experimental set-up for studying the laser generation of ultrasound is shown in Fig. 4. A wide range of pulsed lasers and ultrasonic transducers can be used. In addition to be PXT transducer which detects stress waves in the metal to the opposite side of the illuminated surface, the microphone takes place in front of the tanget surface, which detects the optoacoustic signal propagating in the air. Simultaneous observation of both signals gives more information about the complex source mechanism. 13
236
Robotics & Computer-Integrated Manufacturing • Volume 4, Number 1/2, 1988
~
Metallicsample
Microphone" ~
|
Phatadlode
, ransducer
'
Fig. 4. Experimental set-up for simultaneous observation of laser-induced ultrasound in the absorbing metal and in the surrounding gas.
3.2. A p p f i c a t i o n s Let us now briefly quote some applications of pulsed laser stress wave generation in the area of laser manufacturing. All pulsed laser manufacturing processes are copious sources of optoacoustic waves. Knowing that they contain information about the beam/surface interaction process, one may ask whether it is possible to use this information in monitoring the manufacturing process. As far as we know, the first attempt was made by Saifi and Vahaviolos,4 who studied laser-induced stress waves to monitor pulsed laser spot welding of insulated copper wire to terminal posts. Laser welding for such applications is very advantageous since the incident radiation can be used simulaneously to remove the insulation and to cause the weld in the precise area by means of the single laser pulse. The welding geometry and the electronic system for monitoring the quality of the welds is shown in Fig. 5. The laserinduced stress wave was detected by a differential
piezoelectric transducer attached to the post. Correlation between the parameters of the detected stress waveform and the weld quality was investigated experimentally. Comparing the magnitude of the laser-induced stress wave to the determined threshold value it was possible to discriminate in real-time between good and bad welds. This was of great value for quality control of the pulsed laser manufacturing process. Another successful application of optoacoustic effect in pulsed laser processing was reported recently by Yeack et al. 15 They developed a method for real-time monitoring of pulsed laser drilling into layered composite material. Instead of detecting the stress, wave in the absorbing medium, they measured the pressure pulse in the surrounding air, as shown in Fig. 6. By interpreting the experimental results of multiple-pulsing of the drilling site, they obtained a simple criterion for the termination of the drilling process. Each acoustic signal was used to
LASER
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Pulsed laser manufacturing processes • J. MOZINA
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determine whether the hole was drilled to completion. If not, the next pulse was applied. In this specific application, the process was not only monitored, but also controlled by means of an optoacoustic response, acting as a feedback signal in a very simple feedback loop. It is reasonable to expect that optoacoustic effect can be usefully applied to monitor many other pulsed laser manufacturing processes. Besides their use in monitoring various manufacturing processes, laser-induced stress waves are capable of producing several very interesting effects in the irradiated metals. It has been demonstrated that high energy solid state laser pulses generate stress waves with an amplitude of the order of 1012 Pa and more, which can otherwise be obtained only by strong explosions. ~6 High dislocation densities produced by laser-induced stress waves can significantly improve the properties of metallic materials. Pulsed laser shocking of several aluminum alloys showed that the 0.2% offset yield strength was increased as much as 30% over unshocked values. Large improvements in fatigue life have also been reported by using the laser shock process. 7 Repetitive illumi-
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nation of the stainless steel surface by several strong laser pulses increased surface hardness by about 40%. In addition to various surface transformation processes a special material removal process was proposed based on pulsed laser-induced fracture. 17 The striking difference between evaporation cutting which proceeds by using CW lasers and pulsed laser fracture is that the energy consumption can be diminished by several orders of magnitude in the latter case. Recent studies of laser-generated stress waves have given an important contribution also in the field of non-destructive testing. 1~'~9 The techniques has several distinct advantages over conventional ultrasonics using piezoelectric transducers. Since no physical contact with the samples is required, the problems of acoustic coupling between the transducers and the samples are eliminated. Still more advantageous is the combination of laser generation and laser detection of ultrasonic pulses, which can be achieved by constructing special displacement interferometer. 2° This is an entirely non-contact ultrasonic testing method, also called optical ultrasonics. 2~ Typical non-contact ultrasonic testing and the corresponding result of flaw detection in a 25 mm thick aluminum plate are shown in Figs. 7 and 8. Several other arrangements have been reported so far with increased signal-to-noise capabilities, thus opening the use of this method for various ultrasonic inspections under severe industrial conditions, such as during the production and processing of steel and other metals. 22-24 Another advantage of pulsed laser-induced ultrasound is that extremely short duration ultrasonic pulses can be generated and detected this way, enabling one to test very thin specimens. As an example of the extreme spatial resolution, the results of in-depth ultrasonic testing of a razor blade are shown in Fig. 9. 25 Stress pulses of the duration 2 ns were produced by using mode-lacked picosecond laser pulses. The time between two subsequent echoes corresponds to the
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Robotics & Computer-lntegrated Manufacturing • Volume 4, Number 1/2, 1988 0 t
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turing. C o m p a r e d to the more conventional CW, operated laser processes several advantages of pulsed laser manufacturing processes can be pointed out" • using pulsed lasers, operating at high p e a k intensities, limitations inherent to CW lasers are removed • specific energy consumption is significantly diminished • heat affected zone is diminished • broad variation of pulsed laser p a r a m e t e r s leads to a great variety of new effects and products • optically induced stress waves can be used in monitoring pulsed laser materials processes • optical ultrasonics--noncontact generation and detection of ultrasonic pulses by means of pulsed lasers is becoming a very promising nondestructive testing technique for use in hostile environments and in very small scale testing • using laser induced stress waves as feedback signals in the adaptive control loops enable one to incorporate pulsed lasers into complex and flexible manufacturing systems.
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Fig. 9. Multiple echoes of extremely short ultrasonic pulses in razor blade induced by picosecond mode-locked laser.25 acoustic transit time through the blade of the width 75/~m. These results must surely give an impetus for the further development of optical ultrasonics. 4. C O N C L U S I O N The objective of this contribution was to give some insight into the use of pulsed lasers in manufac-
5. REFERENCES 1. Ready, J. F.: Industrial Applications of Lasers. New York, Academic Press, 1978. 2. Bass, M. (Ed.): Laser Materials Processing. Amsterdam, North Holland, 1932. 3. Boyd, I. W.: Contemp. Phys. 24: 461-490, 1983. 4. Herziger, G.: Feinwerktechnik Messtechnik 91: 158, 1983. 5. Pao, Y. H. (Ed.): Optoacoustic Spectroscopy and Detection. New York, Academic Press, 1977. 6. Rosencwaig, A.: Photoacoustics and Photoacoustic Spectroscopy. New York, Wiley, 1980.
Pulsed laser manufacturing processes • J. MOZINA 7. Fairand, B. P., Clauer, A, H.: Laser generated stress waves: their characteristics and their effects to materials. In Laser-Solid Interaction and Laser Processing-1978, AIP Conf. Proc. No. 50. New York, American Institute of Physics, 1979. 8. Meyer, J. R. et al.: Phys. Rev. B 21: 1559, 1980. 9. Carslaw, H. S., Jaeger, J. C.: Conduction of Heat in Solids. Oxford, Oxford University Press, 1959. 10. Nowacki, W.: Dynamic Problems of Thermoelasticity. Leyden, Noordhoff, 1975. 11. Mozina, J.: Ph.D. Thesis, University of Ljubljana, Faculty of Mechanical Engineering, Ljubljana, 1980. 12. Scruby, C. B. et al.: J. appl. Phys. 51: 6210, 1980. 13. Mo~ina, J., Diaci, J.: J. Phys. 44: C6-73, 1983. 14. Saifi, M. A., Vahaviolos, S. J.: IEEE J. Quant. Electr. 12: 129, 1976.
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Yeack, C. E. et al.: Appl. Phys. Lett. 41: 1043-1982. Steverding, B.: J. appl. Phys. 41: 2096, 1970. Steverding, B.: J. Phys. D: appl. Phys. 4: 787, 1971. von Guffeld, R. J., Melcher, R. L.: Appl. Phys. Left. 30: 357, 1977. 19. Hutchins, D. A. etal.: Appl. Phys. Lett. 38: 677, 1981. 20. Bondarenko, A. N., Drobot, Y. B.: Sov. J. N D T 12: 655, 1976. 21. T a m , A. C., Coufal, H: J. Phys. C6 44: C6-9, 1983. 22. Calder, C. A., Wilcox, W. W.: Materials Evaluation 38: 86, 1980. 23. Cielo, P. et al.: Ultrasonics 23: 55, 1985. 24. Monchalin, J. P.: Appl. Phys. Left. 47: 14, 1985. 25. Krautkriimer, J.: 9th World Conf. NDT, Plenary Lecture, Melbourne, 1979. 15. 16. 17. 18.