Skip to main content
SpringerLink
Log in

Arc Welding and Hybrid Laser-Arc Welding

  • Chapter
  • First Online:
The Theory of Laser Materials Processing

Part of the book series: Springer Series in Materials Science ((SSMATERIALS,volume 119))

  • 2003 Accesses

Abstract

Laser-arc hybrid welding has developed into a viable industrial technology in recent years with a number of technological applications. The physics of the underlying interactions between the laser beam and arc plasma is quite complex and, in order to explore the relationships involved, it is useful first to consider important aspects of arc and laser welding separately. The physics of laser welding has already been examined in Chaps. 4 and 5. A generic description of welding arcs is therefore provided here, which forms a basis for interpretation of laser-arc interactions and the hybrid welding conditions discussed in the final section of this chapter.

This is a preview of subscription content, log in via an institution to check access.

Access this chapter

Log in via an institution
Chapter
USD 29.95
Price excludes VAT (USA)
  • Available as PDF
  • Read on any device
  • Instant download
  • Own it forever
eBook
USD 229.00
Price excludes VAT (USA)
  • Available as EPUB and PDF
  • Read on any device
  • Instant download
  • Own it forever
Softcover Book
USD 299.99
Price excludes VAT (USA)
  • Compact, lightweight edition
  • Dispatched in 3 to 5 business days
  • Free shipping worldwide - see info
Hardcover Book
USD 299.99
Price excludes VAT (USA)
  • Durable hardcover edition
  • Dispatched in 3 to 5 business days
  • Free shipping worldwide - see info

Tax calculation will be finalised at checkout

Purchases are for personal use only

Institutional subscriptions

References

  1. Guile AE (1969) Arc cathode and anode phenomena. International Institute of Welding (IIW) Document 212-170-69

    Google Scholar 

  2. Lancaster J (1984) The physics of welding. Pergamon Press

    Google Scholar 

  3. Ducharme R, Kapadia P, Dowden J, Thornton M, Richardson IM (1995) A mathematical model of the arc in electric arc welding, including shielding gas flow and cathode spot location. J Phys D Appl Phys 28(9):1840–1850

    Article  ADS  Google Scholar 

  4. Kovitya P, Lowke JJ (1985) Two-dimensional analysis of free burning arcs in argon. J Phys D Appl Phys 18:53–70

    Article  ADS  Google Scholar 

  5. Haddad GN, Farmer AJD (1984) Temperature determinations in a free-burning arc: I. Experimental techniques and results in argon. J Phys D Appl Phys 17:1189–1196

    Article  ADS  Google Scholar 

  6. Olsen HN (1959) Thermal and electrical properties of an argon plasma. Phys Fluids 2(6):614–623

    Article  ADS  Google Scholar 

  7. Greim HR (1964) Plasma spectroscopy. McGraw-Hill

    Google Scholar 

  8. Degout D, Catherinot A (1986) Spectroscopic analysis of the plasma created by a double-flux tungsten inert-gas (TIG) arc plasma torch. J Phys D Appl Phys 19(5):811–823

    Article  ADS  Google Scholar 

  9. Thornton MF (1993) Spectroscopic determination of temperature distributions for a TIG Arc. PhD thesis, Cranfield Institute of Technology, UK

    Google Scholar 

  10. Cram LE, Poladian L, Roumeliotis G (1988) Departures from equilibrium in a free-burning argon arc. J Phys D: Appl Phys 21:418–425

    Google Scholar 

  11. Gomés AM (1983) Criteria for partial LTE in an argon thermal discharge at atmospheric pressure; validity of the spectroscopically measured electronic temperature. J Phys D Appl Phys 16:357–378

    Article  ADS  Google Scholar 

  12. Drellischak KS, Knopp CF, Cambel AB (1962) Partition functions and thermodynamic properties of argon plasma. Gas Dynamics Laboratory, Nothwestern University, Illinois, USA. Report No A-3-62

    Google Scholar 

  13. Farmer AJD, Haddad GN (1988) Rayleigh-scattering measurements in a free-burning argon arc. J Phys D Appl Phys 21(3):426–431

    Article  ADS  Google Scholar 

  14. Farmer AJD, Haddad GN (1984) Local thermodynamic-equilibrium in free-burning arcs in argon. Appl Phys Lett 45(1):24–25

    Article  ADS  Google Scholar 

  15. Bakshi V, Kearney RJ (1989) An investigation of local thermodynamic equilibrium in an argon plasma jet at atmospheric pressure. J Quant Spect Rad Trans 41(5):369–376

    Article  ADS  Google Scholar 

  16. Snyder SC, Lassahn GD, Reynolds LD (1993) Direct evidence of departures from local thermal equilibrium in a free-burning arc-discharge plasma. Phys Rev E 48(5):4124–4127

    Article  ADS  Google Scholar 

  17. Kitamura T, Takeda K, Shibata K (1998) Deviation from local thermal equilibrium state in thermal plasma. ISIJ Int 38(11):1165–1169

    Article  Google Scholar 

  18. Thornton MF (1993) Spectroscopic determination of temperature distributions for a TIG Arc. J Phys D Appl Phys 26:1432–1438

    Google Scholar 

  19. Rat V, Aubreton J, Elchinger MF, Fauchais P, Murphy AB (2002) Diffusion in two-temperature thermal plasmas. Phys Rev E 66:056407

    Google Scholar 

  20. Murphy AB (1996) Modelling and diagnostics of plasma chemical processes in mixed-gas arcs. Pure Appl Chem 68(5):1137–1142

    Article  Google Scholar 

  21. Murphy AB (1993) Diffusion in equilibrium mixtures of ionized gases. Phys Rev E 44(5):3594–3603

    Article  ADS  Google Scholar 

  22. Murphy AB (1996) A comparison of treatments of diffusion in thermal plasmas. J Phys D Appl Phys 29(7):1922–1932

    Article  ADS  Google Scholar 

  23. Murphy AB, Arundell CJ (1994) Transport coefficients of argon, nitrogen, oxygen, argon-nitrogen, and argon-oxygen plasmas. Plasma Chem Plasma Process 14(4):451–490

    Article  Google Scholar 

  24. Devoto RS (1966) Transport properties of ionized monatomic gases. Phys Fluids 9(6):1230–1240

    Article  ADS  Google Scholar 

  25. Murphy AB (1996) The influence of demixing on the properties of a free burning arc. Appl Phys Lett 69(3):323–330

    Article  ADS  Google Scholar 

  26. Murphy AB (1997) Demixing in free-burning Arcs. Phys Rev E 55(6):7473–7494

    Article  ADS  Google Scholar 

  27. Greses-Juan J (1999) Examination of adaptive control strategies for hyperbaric keyhole plasma arc welding. MSc thesis, Cranfield University, UK

    Google Scholar 

  28. Aubreton A, Elchinger MF (2003) Transport properties in non-equilibrium argon, copper and argon–copper thermal plasmas. J Phys D Appl Phys 36(15):1798–1805

    Article  ADS  Google Scholar 

  29. Rethfeld B, Wendelstorf J, Klein T, Simon G (1996) A Self-consistent model for the cathode fall region of an electric arc. J Phys D Appl Phys 29:121–128

    Article  ADS  Google Scholar 

  30. Chen MM, Thorne RE, Wyner EF (1976) Resolution of electron emission mechanisms in an argon arc with hot tungsten cathode. J Appl Phys 47(12):5214–5217

    Article  ADS  Google Scholar 

  31. Klein T, Paulini J, Simon G (1994) Time-resolved description of cathode spot development in vacuum arcs. J Phys D Appl Phys 27:1914–1921

    Article  ADS  Google Scholar 

  32. Morrow R, Lowke JJ (1993) A one-dimensional theory for the electrode sheaths of electric-arcs. J Phys D Appl Phys 26(4):634–642

    Article  ADS  Google Scholar 

  33. Spataru C, Teillet-Billy D, Gauyacq JP, Teste P, Chabrerie JP (1997) Ion-assisted electron emission from a cathode in an electric arc. J Phys D Appl Phys 30:1135–1145

    Article  ADS  Google Scholar 

  34. Hsu KC, Pfender E (1983) Analysis of the cathode region of a free burning high intensity argon arc. J Appl Phys 54(7):3818–3824

    Article  ADS  Google Scholar 

  35. Delalondre C, Simonin O (1990) Modeling of high-intensity arcs including a non-equilibrium description of the cathode sheath. J De Phys 51(18):C5199–C5206

    Google Scholar 

  36. Wendelstorf J (2000) Ab initio modelling of thermal plasma gas discharges (electric arcs). PhD thesis, University of Braunschweig, Germany

    Google Scholar 

  37. Murphy EL, Good RH (1956) Thermionic emission, field emission and the transition region. Phys Rev 102(6):1464–1473

    Article  ADS  Google Scholar 

  38. Coulombe S, Meunier JL (1997) A comparison of electron-emission equations used in arc-cathode interaction calculations. J Phys D Appl Phys 20(30):2905–2910

    Article  ADS  Google Scholar 

  39. Quigley MBC, Richards PH, Swift-Hook DT, Gick AEF (1973) Heat-flow to the workpiece from a TIG welding arc. J Phys D Appl Phys 6(18):2250–2258

    Article  ADS  Google Scholar 

  40. Ushio M, Tanaka M, Lowke JJ (2004) Anode melting from free-burning argon arcs. IEEE Trans Plasma Sci 32(1):108–117

    Article  ADS  Google Scholar 

  41. Tanaka M, Ushio M, Wu CS (1999) One-dimensional analysis of the anode boundary layer in free-burning argon arcs. J Phys D Appl Phys 32:605–611

    Article  ADS  Google Scholar 

  42. Lowke JJ, Morrow R, Haidar J (1997) A simplified unified theory of arcs and their electrodes. J Phys D Appl Phys 30:2033–2042

    Article  ADS  Google Scholar 

  43. Hinnov E, Hirschberg JG (1962) Electron-ion recombination in dense plasmas. Phys Rev 125(3):795–801

    Article  ADS  Google Scholar 

  44. Jenista JJ, Heberlein VR (1997) Numerical model of the anode region of high current electric arcs. IEEE Trans Plasma Sci 25(5):883–890

    Article  ADS  Google Scholar 

  45. Zhu P, Lowke JJ, Morrow R, Haider J (1995) Prediction of anode temperatures of free burning arcs. J Phys D Appl Phys 28:1369–1376

    Article  ADS  Google Scholar 

  46. Sanders NA, Pfender E (1984) Measurement of anode falls and anode heat transfer in atmospheric pressure high intensity arcs. J Appl Phys 55(3):714–722

    Article  ADS  Google Scholar 

  47. Yang G, Heberlein J (2007) Anode attachment modes and their formation in a high intensity argon arc. Plasma Sources Sci Technol 16:529–542

    Article  ADS  Google Scholar 

  48. Kim W-H, Na S-J (1998) Heat and fluid flow in pulsed current GTA weld pool. Int J Heat and Mass Trans 41:3213–3227

    Article  Google Scholar 

  49. Zhang W, Kim C-H, DebRoy T (2004) Heat and fluid flow in complex joints during gas metal arc welding part I: numerical model of fillet welding. J Appl Phys 95(9):5210–5219

    Article  ADS  Google Scholar 

  50. Wu CS, Yan F (2004) Numerical simulation of transient development and diminution of weld pool in gas tungsten arc welding. Modell Simul Mater Sci Eng 12:13–20

    Article  ADS  Google Scholar 

  51. Mishra S, DebRoy T (2005) A heat-transfer and fluid-flow based model to obtain a specific weld geometry using various combinations of welding variables. J Appl Phys 98:044902

    Article  ADS  Google Scholar 

  52. Chakraborty N, Chakraborty S, Dutta P (2004) Three-dimensional modeling of turbulent weld pool convection in GTAW processes. Numer Heat Transf. A 45:391–413

    Article  ADS  Google Scholar 

  53. Jaidi J, Dutta P (2004) Three-dimensional turbulent weld pool convection in gas metal arc welding process. Sci Technol Weld Join 9(5):407–414

    Article  Google Scholar 

  54. Goodarzi M, Choo R, Toguri JM (1997) The effect of the cathode tip angle on the gas tungsten arc welding arc and weld pool: I. Mathematical model of the arc. J Phys D Appl Phys 30:2744–2756

    Article  ADS  Google Scholar 

  55. Goodarzi M, Choo R, Takasu T, Toguri JM (1998) The effect of the cathode tip angle on the gas tungsten arc welding arc and weld pool: II. The mathematical model for the weld pool. J Phys D Appl Phys 31:569–583

    Article  ADS  Google Scholar 

  56. Tanaka M, Ushio M, Lowke JJ (2004) Numerical study of gas tungsten arc plasma with anode melting. Vacuum 73:381–389

    Article  Google Scholar 

  57. Tanaka M, Ushio M, Lowke JJ (2004) Numerical analysis for weld formation using a free-burning helium arc at atmospheric pressure. JSME Int J Series B 48(3):397–404

    Article  ADS  Google Scholar 

  58. Lowke JJ, Tanaka M, Ushio M (2004) Mechanisms giving increased weld depth due to a flux. J Phys D: Appl Phys 38:3438–3445

    Article  ADS  Google Scholar 

  59. Lowke JJ, Tanaka M (2007) Predictions of weld pool profiles using plasma physics. J Phys D: Appl Phys 40:R1–R23

    Article  Google Scholar 

  60. Tanaka M, Tashiro S, Lowke JJ (2007) Predictions of weld formation using gas tungsten arcs for various arc lengths from unified arc-electrode model. Sci Technol Weld Join 12(1):2–9

    Article  Google Scholar 

  61. Lago F, Gonzalez JJ, Freton P, Gleizes A (2004) A numerical modelling of an electric arc and its interaction with the anode: part I. The two-dimensional model. J Phys D Appl Phys 37:883–897

    Article  ADS  Google Scholar 

  62. Gonzalez JJ, Lago F, Freton P, Masquère M, Franceries X (2005) Numerical modelling of an electric arc and its interaction with the anode: part II. The three-dimensional model—influence of external forces on the arc column. J Phys D: Appl Phys 38:306–318

    Article  ADS  Google Scholar 

  63. Tanaka M, Yamamoto K, Tashiro S, Nakata K, Ushio M, Yamazaki K, Yamamoto E, Suzuki K, Murphy AB, Lowke JJ (2007) Metal vapour behaviour in thermal plasma of gas tungsten arcs during welding. International Institute of Welding (IIW) Document 212-1107-07

    Google Scholar 

  64. Hu J, Tsai HL (2007) Heat and mass transfer in gas metal arc welding. part I: the arc. Int J Heat Mass Trans 50:808–820

    Article  MATH  Google Scholar 

  65. Hu J, Tsai HL (2007) Heat and mass transfer in gas metal arc welding. part II: the metal. Int J Heat Mass Trans 50:833–846

    Article  MATH  Google Scholar 

  66. Hu J, Tsai HL (2006) Effects of current on droplet generation and arc plasma in gas metal arc welding. J Appl Phys 100:053304

    Google Scholar 

  67. Quinn TP, Szanto M, Gilad I, Shai I (2205) Coupled arc and droplet model of GMAW. Sci Technol Weld Join 10(1):113–119

    Google Scholar 

  68. Steen WM, Eboo M (1979) Arc Augmented Laser-Welding. Metal Construction 11(7):332–335

    Google Scholar 

  69. Steen WM, Eboo M, Clarke J (1978) Arc augmented laser welding. Proceedings 4th international conference advances in welding processes, Harrogate, UK, 9–11 May. Paper 17: 257–265

    Google Scholar 

  70. Deron C, Rivière P, Perrin M-Y, Soufiani A (2006) Coupled radiation, conduction, and joule heating in argon thermal plasmas. J Thermophys Heat Trans 20(2):211–219

    Article  Google Scholar 

  71. Hughes TP (1975) Plasmas and laser light, Adam Hilger

    Google Scholar 

  72. Paulini J, Simon G (1993) A theoretical lower limit for laser power in laser-enhanced arc welding. J Phys D Appl Phys 26:1523–1527

    Article  ADS  Google Scholar 

  73. Seyffarth P, Krivtsun IV (2002) Laser-arc processes and their applications in welding and material treatment. Weld Allied Process vol 1. Taylor & Francis

    Google Scholar 

  74. Wang T-S, Rhodes R (2003) Thermophysics characterization of multiply ionized air plasma absorption of laser radiation. J Thermophys Heat Trans 17(2):217–224

    Article  Google Scholar 

  75. Stallcop JR, Billman KW (1974) Analytical formulae for the inverse bremsstrahlung absorption coefficient. Plasma Physics 16:1187–1189

    Article  ADS  Google Scholar 

  76. Devoto RS (1973) Transport coefficients of ionised argon. Phys Fluids 16(5):616–623

    Article  ADS  Google Scholar 

  77. Mathuthu M, Raseleka RM, Forbes A, West N (2006) Radial variation of refractive index, plasma frequency and phase velocity in laser induced air plasma. IEEE Trans Plasma Sci 34(6):2554–2560

    Article  ADS  Google Scholar 

  78. Howlader MK, Yang Y, Roth JR (2005) Time-resolved measurements of electron number density and collision frequency for a fluorescent lamp plasma using microwave diagnostics. IEEE Trans Plasma Sci 33(3):1093–1099

    Article  ADS  Google Scholar 

  79. Lacroix D, Jeandel G, Boudot C (1998) Solution of the radiative transfer equation in an absorbing and scattering Nd:YAG laser-induced plume. J Appl Phys 84(5):2443–2449

    Article  ADS  Google Scholar 

  80. Kokhanovsky AA (1999) Optics of light scattering media. Wiley and Praxis Publishing

    Google Scholar 

  81. Greses J (2003) Plasma/plume effects in CO2 and Nd:YAG laser welding. PhD thesis, Cambridge University, UK

    Google Scholar 

  82. Greses J, Hilton PA, Barlow CY, Steen WM (2002) Plume attenuation under high power Nd:YAG laser welding. J Laser Appl 16(1):9–15

    Article  Google Scholar 

  83. Zimmer AT, Biswas P (2001) Characterization of the aerosols resulting from arc welding processes. J Aerosol Science 32:993–1008

    Article  Google Scholar 

  84. Hewett P (1995) The particle size distribution, density, and specific surface area of welding fumes from SMAW and GMAW mild and stainless steel consumables. American Industrial Hygiene Association, Journal 56:128–135

    Article  Google Scholar 

  85. Thomas ME (2006) Optical propagation in linear media. Oxford University Press

    Google Scholar 

  86. Hansen F, Duley WW (1994) Attenuation of laser radiation by particles during laser materials processing. J Laser Appl 6(3):137–143

    Article  Google Scholar 

  87. Tu J, Miyamoto I, Inoue T (2002) Characterizing keyhole plasma light emission and plasma plume scattering for monitoring 20 kW class CO2 laser welding processes. J Laser Appl 14(3):146–153

    Article  Google Scholar 

  88. Kozakov R, Gott G, Uhrlandt D, Emde B, Hermsdorf J, Wesling V (2015) Study of laser radiation absorption in a TIG welding arc. Weld World 59(4):475–481

    Article  Google Scholar 

  89. Kozakov R, Emde B, Pipa AV, Huse D, Uhrlandt D, Hermsdorf J, Wesling V (2015) Change of electrical conductivity of Ar welding arc under resonant absorption of laser radiation. J Phys D Appl Phys 48:095502

    Google Scholar 

  90. Miller R, DebRoy T (1990) Energy absorption by metal-vapour-dominated plasma during carbon dioxide laser welding of steels. J Appl Phys 68(5):2045–2050

    Article  ADS  Google Scholar 

  91. Bagger C, Olsen F (2005) Review of laser hybrid welding. J Laser Appl 17(1):2–14

    Article  Google Scholar 

  92. Chen Y, Li L, Fang J, Feng X, Wu L (2003) Temperature field simulation of laser-TIG hybrid welding. China Weld 12(1):62–66

    Google Scholar 

  93. Chen Y, Li L, Fang J, Feng X (2003) Numerical analysis of energy effect in laser-TIG hybrid welding. J Mater Sci Technol 19(Suppl 1):23–26

    Google Scholar 

  94. Hu B (2002) Nd:YAG laser-assisted arc welding. PhD thesis Delft University of Technology, The Netherlands

    Google Scholar 

  95. Reutzel EW, Kelly SM, Martukanitz RP, Bugarewicz MM and Michaleris P (2005) Laser-GMA [MIG/MAG] hybrid welding: process monitoring and thermal modelling. In: David SA, DebRoy T, Lippold JC, Smartt HB, Vitek JM (eds) Proceedings of the 7th international conference trends in welding research, pine mountain, GA, USA, 16–20 May 2005. Publ, Materials Park, OH 44073-0002, USA, ASM International, 2006, 143–148

    Google Scholar 

  96. Goldak J, Chakravarti A, Bibby M (1984) A new finite element model for welding heat sources. Met Trans B 15B:299–305

    Article  Google Scholar 

  97. Zaĭkin AE, Katulin VA, Levin AV, Petrov AL (1991) Hydrodynamic processes in a melt bath under laser-arc interaction conditions. Sov J Quantum Electron. 21(6):635–639

    Article  ADS  Google Scholar 

  98. Gratzke U, Kapadia PD, Dowden J (1991) Heat-conduction in high-speed laser-welding. J Phys D: Appl Phys 24(12):2125–2134

    Article  ADS  Google Scholar 

  99. Cline HE, Anthony TR (1977) Heat treating and melting material with a scanning laser or electron-beam. J Appl Phys 8(9):3895–3900

    Article  ADS  Google Scholar 

  100. Hu B, den Ouden G (2005) Synergic effects of hybrid laser/arc welding. Sci Technol Weld Join 10(4):427–431

    Article  Google Scholar 

  101. Dowden J, Kapadia PD (1998) The use of a high power laser to provide an electrical path of low resistance. J Laser Appl 10(5):219–223

    Article  Google Scholar 

  102. Startsev VN, Martynenko DP, Leonov AF (2000) Investigation of characteristics of an arc column in laser arc welding using numerical simulation. High Temp 38(1):24–29

    Article  Google Scholar 

  103. Mahrle A, Rose S, Schnick M, Pinder T, Beyer E and Fussel U (2102) Improvements of the welding performance of plasma arcs by a superimposed fibre laser beam, Proc SPIE 8239 83290D-1:13

    Google Scholar 

  104. Reisgen U, Zabirov A, Krivtsun I, Demchenko V, Krikent I (2015) Interaction of CO2-laser beam with argon plasma of Gs tungsten arc. Weld World 59(5):611–622

    Article  Google Scholar 

  105. Piekarska W, Kubiak M, Bokota A (2011) Numerical simulation of thermal phenomena and phase transformations in laser-arc hybrid welded joints. Arch Metall Mater 56(2):409–421

    Article  Google Scholar 

  106. Zhou J and Tsai HL (2012) In: Kovacevic R Hybrid laser-arc welding, in book “Welding Processes”, ISBN 978-953-51-0854-2, pub In-Tech

    Google Scholar 

  107. Kong F, Ma J, Kovacevic R (2011) Numerical and experimental study of thermally induced residual stress in the hybrid laser–GMA welding process. J Mater Process Technol 211:1102–1111

    Google Scholar 

  108. Zhou J, Tsai HL (2012) Modeling of transport phenomena in hybrid laser-MIG keyhole welding. Int J Heat Mass Transfer 51:4353–4366

    Article  MATH  Google Scholar 

  109. Le Guen E, Carin M, Fabbro R, Coste F, Le Masson P (2011) 3D heat transfer model of hybrid laser Nd:Yag-MAG welding of S355 steel and experimental validation. Int J Heat Mass Transfer 54:1313–1322

    Article  MATH  Google Scholar 

  110. Zijp JP (1990) Heat transport during gas tungsten arc welding. PhD thesis, Delft University of Technology, The Netherlands

    Google Scholar 

Download references

Author information

Authors and Affiliations

  1. Department of Materials Science and Engineering, Delft University of Technology, Mekelweg 2, 2628 CD, Delft, The Netherlands

    Ian Richardson

Authors
  1. Ian Richardson
    View author publications

    You can also search for this author in PubMed Google Scholar

Corresponding author

Correspondence to Ian Richardson .

Editor information

Editors and Affiliations

  1. Mathematical Sciences, University of Essex, Colchester, Essex, United Kingdom

    John Dowden

  2. Fraunhofer-Institut für Lasertechnik, RWTH Aachen, Aachen, Germany

    Wolfgang Schulz

Rights and permissions

Reprints and permissions

Copyright information

© 2017 Springer International Publishing AG

About this chapter

Cite this chapter

Richardson, I. (2017). Arc Welding and Hybrid Laser-Arc Welding. In: Dowden, J., Schulz, W. (eds) The Theory of Laser Materials Processing. Springer Series in Materials Science, vol 119. Springer, Cham. https://doi.org/10.1007/978-3-319-56711-2_7

Download citation

Publish with us

Policies and ethics

Search

Navigation