A Laser Beam Welding Efficiency ( L.B.W.E. ) Measurement Unit : 1970 2005 A Laser Beam Welding Efficiency ( L.B.W.E. ) Measurement Unit : The Dau Genova 22 – 24 Giugno 2005 G. Daurelio I. N. F. M. – National Institute for the Physics of Matter - Regional Lab. L I T 3 - Laser Innovation Technology Transfer and Training - c/o Physics Department “ M. Merlin “ of the University and Polytechnic of Bari , Via Amendola , 173 , I – 70126 Bari - Italy - Phone +39 080 544 23 81 , Fax +39 080 544 22 19 , E-Mail daurelio@fisica.uniba.it - g.daurelio@virgilio.it ( home ) Summary After 40 years of the LMP ( Laser Material Processing ) and LBW ( Laser Beam Welding ) studies, researches and industrial applications, nowadays it is possible to undertake at “ the Laser People “ attention the possible adoption of some common stated basic rules and a measurement unit ( the Dau ) for the LBW technology. This unit , corresponding to 1 Dau = 1 mm3 / kJ, defines the LBWE ( Laser Beam Welding Efficiency) unit as the the Melted Volume( MV ) per time unit (MVs) per incident Laser Power unit .The Dau unit has been already undertaken to International “ Laser People “ and orally referred for the first time during the last XV Intern. Symposium on Gas Flow and Chemical Lasers & High Power Laser Conference ( GCL – HPL ) , held on the last 30 August to 03 September 2004 in Praha ( Czech Republic ). This unit has been carried out by an innovative LBW efficiency method ( the DA.LU. one ) that was orally accepted and presented for the first time at LASER ‘ 97 – European Symposium on Lasers and Optics in Manufacturing , held in Munich ( Germany ) on June 1997. A DA.LU. method Base Plot has been reported. During the last seven years of the application of this method the same one has still implemented and applied on the LBWE of some different materials ( Stainless Steels, Aluminium and Titanium and their Alloys ). These last results have been recently referred during the same Int. Symposium XV GCL – HPL , previously reported . With this method the evaluation occurs on the “product” (weld bead) of the welding process and not on the “process” as usually in the theoretical models (mathematical or thermal balance ones ). To do this a schematic layout has been reported; so that the Melted Area (MA) – Melted Volume (MV) and Melted Volume per second (MVs) are automatically relieved and calculated on the base of an IMAGE SOFTWARE ANALYSIS, by means a Microscope + Digital Camera + PC + Software (NIKON LUCIA- LIM ) + Monitor . This paper presents a schematic overview from 1970 to 2004 on the Laser Welding Process as Parameters as Efficiencies as well as the problems, data and parameters that have always strongly influenced and let to failure many theoretical models . At the end if the all boundary conditions are fully respected and satisfied, then it is possible to define a Laser Beam Welding Efficiency measurement unit : 1 Dau = mm 3 / kJ that it is suggested and undertaken to the Research Colleagues, Technologists , Design Engineers, Laser End-Users, Industries and SME’s for a daily use. So they all can “ speech” the same language . Introduction On the following a schematic overview from 1970 to 2005 year on the Laser Welding Process as Working Parameters as Weld Characterization have been reported. The laser sources considered have been: WORKING PARAMETERS Laser Type Lens Type Laser Beam Dimension Focused or Defocused Spot Focal Length Relative Focus Position Covering Gas Type (N2, Ar, He) Laser Power Level Laser Power Density Welding Speed Tilted Laser Beam Autogeneous Welding Filler Material WELD CHARACTERIZATION Sample THickness (mm) - TH Melted Height (mm) - MH Melted Width (mm) - MW Melted Area (mm2) - MA Melted Volume (mm3) - MV Full Penetration depth (mm) - FP Incomplete Penetration depth (mm) - IP Aspect Ratio (MH/MW) - AR Laser Sources Co2 ( a gas laser ) Nd:YAG ( a solid-state laser ) H.P.D.L. ( a solid state diode array laser ) These last parameters have always measured on the cross section of the laser welded beads after the treatment by usual metallographic techniques and related chemical etching to reveal the bead macro and micro metallurgical structure The weld characterization has always been carried out by considering the above cited parameters versus Laser Power level ( LPL ) and Welding Speed ( WS ) parameters. PROCESS EFFICIENCY: Theoretical Models On the last 30 years for the Laser Welding Process Efficiency some diverse Theoretical Models ( mathematical or thermal-balance ones ) have been considered as following : SWIFT-HOOK & GICK (1973) CLINE & ANTONY (1977) KLEMENS (1977) MAZUNDER & STEEN (1980) SCHUOKER & ABEL (1984) KI, MOHANTY, MAZUNDER (2000) ESPOSITO & DAURELIO (1980) DAURELIO et All. (1988) DAURELIO et All. (1990) DAURELIO et All. (1992) DAURELIO & LUDOVICO (1997) These last five ones have to be considered as some following implementations of the SWIFT-HOOK & GICK Model . All these mathematical and thermal-balance models are strongly influenced by : thermo-physical properties of the metals or alloys or steels to be welded vs temperature change of the phase state (solid to liquid to vapour) real laser energy absorbed into the sample surface comparing to the laser energy illuminating the surface versus surface Temperature THESE MODELS FAILURE for the following problems : It is difficult to know the thermo-physical parameters for many alloys, steels and metals vs temperature The energy absorbed is much more different to that impinged on the sample The weld joint has obtained between two dissimilar metals and/or alloys and/or steels One o two changes of the phase state occur The measurement on the weld cross-sections of the Melted Height / Melted Width / Melted Area are NOT “FULLY CORRECT” but are “Personalized” ( different one person by person) Two laser sources are employed at once A filled material is added to the laser process The use of these models in Industries and SME’s is very difficult The impossibility to compare different data on the LBWE, carried out by diverse Laser centers and Researche even if they were obtained by using the same Laser Sources ( Type, model and manifacturer) D A . L U . m e t h o d On the light of the above reported problems, a new easy method ( DA. LU. method ) for a quantitative evaluation of the welding process was carried out. It was presented for the first time during the Int. Conference on Laser Material Processing in Munich 1997 . With this method the evaluation occurs on the “product” ( weld bead cross-section ) and not on the “ welding process” as usually in the previous models. So, all the above cited “problems” are fully solved. A welding bead cross-section has displayed on a monitor where the MA ( mm2 ) is automatically measured by using a special software. The result is normalized on the calibrated scale used to set the screen area. The MA value is multiplied by the WS ( mm/s ) and divided by LPL values KW = KJ / s . So, it defines the Welding Efficiency WE (MVs vs Power) in mm3 / kJ as the melted volume per time unit once fixed the incident power. In this way it can run a shortcut to evaluate the process efficiency regardless to absorbed on incident energy ratio and thermo-physical parameters. If the all boundary conditions above reported are fully respected and satisfied, then it is possible to define a Laser Welding Efficiency measurement unit : 1 Dau = mm 3 / k J that it is suggested to the Research Colleagues, Laser End-Users, Industries and SME’s for a daily use. The Complete system set-up MA – Melted Area MV – Melted Volume MVs - Melted Volume per second are automatically relieved and calculated on the base of an IMAGE ANALYSIS SYSTEM LUCIA Measurement, Version 4.82 (by LIM Laboratory Imaging S.P.O. ) by means: Microscope + Digital Camera + PC + Software (NIKON LUCIA- LIM ). + Monitor Penetration welding Mixed regime welding Conduction welding Laser Power [kW] Laser Power [kW] Laser Power Plots for AISI 304 weld beads , produced by a CO2 laser source and a laser beam tilted 80° ( on the left ) and 70° ( on the right ) and diverse relative laser focus positions vs two laser power levels ( 2 and 2.5 kW ) – Ref. Laser Power [kW] Laser Power Plots for a Titanium Alloy - Ti 6Al4V weld beads , produced by a CO2 laser source and a laser beam angle 90° vs four laser power levels (1 - 1.5 - 2 and 2.5 kW ) - Ref. Some plots regarding different stainless steels (austenitic, ferritic and martensitic), a Ti alloy Ti6Al4V and some Al-alloys (AA2024,5056, 5083, 5383, 6061, 6063, 6082, 8090 and Al99)- even if these last ones are nowadays in progress, are obtained. CONCLUSIONI In questo lavoro si sono volute riportare e dimostrare le ragioni che hanno condotto alla ricerca di un nuovo metodo (il DA.LU. method) facile, di immediata applicazione sia in ambito di ricerca che industriale, per una valutazione quantitativa dell’efficienza del processo di saldatura realizzati con i seguenti laser (a CO2, a Nd-YAG, ad array di diodi-H.P.D.L.). Partendo da diversi modelli teorici (matematici e di bilancio termico), susseguitisi in un trentennio (1970-2000), si sono sottolineate le limitazioni che hanno impedito a detti modelli di essere realmente utilizzati per lo scopo previsto e come il nuovo DA.LU. method ha risolto tutte quelle limitazioni, prendendo in esame non il “processo” bensì il “prodotto” (il cordone) della saldatura a laser. Stabilita l’esistenza di tre range diversi in cui si realizza solo un processo di “deep penetration welding”, un mixec regime ed un ultio di sola “conduction welding” si è sintetizzato il tutto in un DA.LU. method Base Plot, che riporta le tre regioni, prima accennate, con in ordinata il parametro MVs (Melted Volume per second, mm3/s) e in ascissa la potenza laser in kW( KJ/s). Le tre semirette, con origine nello 0, delimitano le tre regioni di operatività ad efficienza molto alta, media e bassa. La pendenza invece di dette semirette determina il WE (Welding Efficiency), in mm3/KJ, che, se sono rispettate tutte le condizioni al contorno del DA.LU. method imposte, può essere considera come unità di misura per il WE e L.B.W.E. , cioè 1 Dau= mm3 / KJ. Sia il DA.LU. method che il Dau sono stati applicati e verificati su una gamma molto estesa di materiali ( acciai Inox, acciai al carbonio, metalli puri, leghe, superleghe e giunzioni di materiali dissimili), utilizzando sorgenti laser diverse, un range di potenza laser molto ampio (da 500W a 14000W) differenti tipologie di giunzione (butt, lap, corner, T joints), diversi sistemi di focalizzazione fascio laser (a specchi o a lenti- KCl o ZnSe), diversi ugelli ( coassiali o laterali o due cornette), diversi gas di copertura (He, Ar, N2), diversi gradi di defocalizzazione, diversi gradi di incidenza-materia (90°, 80°, 70°), ampio range di spessori (da 0.4 a 15 mm ), mostrando sempre una piena applicabilità e facilità di impiego. L’abbinamento poi del DA.LU. method e del Dau ad un IMAGE ANALYSIS SYSTEM-LUCIA Measurament , Version 4.82 (o precedente), by LIM Laboratori Imaging S.P.O., connesso ad un Pc, monitor, telecamera digitale ad alta risoluzione, microscopio ottico stereo o metallografico, fanno, di detto metodo di valutazione della efficienza della saldatura a laser, uno strumento di lavoro molto facile da usare, da personale facilmente addestrabile, di immediata valutazione dei risultati e confronti degli stessi per differenti materiali, visto che il sistema, oltre che a fornire in automatico la misura esatta del parametro MA (Melted Area), permette anche la gestione, acquisizione e storage di immagini. Lo stesso sistema poi può anche essere usato, sempre in accoppiamento con il DA.LU. method e unità Dau, per valutare l’efficienza WE di cordoni di saldatura a laser, non solo direttamente ad inglobamenti metallografici con cordone in sezione trasversale, ma anche da foto, fotocopie, lavori scientifici purché riportino le sezioni metallografiche (dei cordoni oggetti di studio), la potenza laser, la velocità di saldatura, lo spessore del materiale e l’ingrandimento delle foto. Da quanto sopra si risale a valori di MA e di WE in Dau. AKNOLEDGEMENTS The Author wishes to thank Prof. Ing. A. LUDOVICO, Prof. Ing. F. MEMOLA CAPECE , MINUTOLO , Ing. F. CURCIO for their scientific – technological discussions on the experimental data . A particular thank has due to the friend Dr. E. ANDRIANI for his precious help, encouragement during the whole preparation and data discussion of this paper . REFERENCES Swift-Hook, A.E.F. Gick : Penetration Welding with laser, Welding Research Suppl.,9 ,1973, pp. 492 to 499. G. Daurelio, A.D. Ludovico : An easy method for a quantitative evaluation of the laser welding efficiency on austenitic, ferritic and martensitic stainless steels, Lasers in Material Processing, Munich Proceedings Europto, 1997, pp. 80 to 95 . F. Caiazzo, F. Curcio, G. Daurelio, F. Memola Capece Minutolo, F. Ottonelli : Lap and butt joining by a CO2 laser of titanium alloys for civil and military high speed aircrafts - 10th Int World . Conf. Ti 2003 Science and Technology , Hamburg,13 – 18 July 2003 , Proceedings WILEY –VCH Verlag , Vol.4 , pp.2651 to 2658 . F. Caiazzo, F. Curcio , G. Daurelio, F. Memola Capece Minutolo : Ti6Al4V sheets lap and butt joints carried out by CO2 laser: mechanical and morphological characterization, Journal of Material Processing Technology, Vol.149, pp. 542 to 548. G. Daurelio, A.D. Ludovico, M.Lugarà, L.A.C. De Filippis, A.M. Spera, S. 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