Monitoraggio e Controllo nei Processi di Saldatura Laser ... · PDF fileMonitoraggio e...

Monitoraggio e Controllo nei Processi di Saldatura Laser:

nuove soluzioni e prospettive

Daniele Colombo, Barbara Previtali - SITEC Laboratorio per le Applicazioni Laser

Luca Cevasco, Claudio Cenati, Diego Parazzoli, Nicola Pedrocchi – CNR ITIA

Seminario ESI-AITeM “Saldatura e Trattamenti Termici” - Brescia – 16 Ottobre 2014

Our major equipment: •  Fiber Laser IPG YLR1000 •  Fiber Laser IPG YLS3000 •  Fiber Laser IPG YLP-1/100/50/50 •  Trumpf Nd:YAG HL 124P – PowerWeld •  Robots ABB IR 2400 and IR 4400 •  Robot COMAU Smartlaser •  BLM AdigeSys LTCombo fiber laser cutting system •  Aerotech ALS and ACS moving stages

Our competencies in laser material processing: •  Laser Welding •  Laser Cutting •  Laser Hardening •  Laser Cladding •  Laser Micromachining (drilling, cutting, texturing) •  Monitoring and Control of Laser Processes

www.sitec.mecc.polimi.it


IRAS group competencies •  Artificial Vision:

•  development of 3D laser scanning systems •  development of stereoscopic vision systems for optical tracking

•  Industrial Controls: • development of embedded Linux images with Xenomai and RTAI realtime extension • development of modules for interfacing with advanced sensors (vision, laser scanner, manual guidance, etc. ..) • design and configuration of Embedded Systems • study and implementation of devices for data collection, remote configuration and management (GSM Ehternet or encrypted), advanced web interfaces etc. ..

!  Mechanical design with 3D solid modeling CAD-CAM. !  Electronic boards design and programming (signal

conditioning, IO management, interfaces to devices, battery management, FPGA, etc. ..)

www.itia.cnr.it


Indice della presentazione

1.  INTRODUZIONE

2.  LA SALDATURA LASER

3.  LE TECNOLOGIE ABILITANTI LA SALDATURA LASER 3D

3 A DI PROCESSO

3 B DI SISTEMA 4. CONCLUSIONI


Settori Applicativi della Saldatura Laser


Tecnologie di giunzione tradizionale

MIG/MAG

Cold Metal Transfer

TIG/Plasma

Hybrid (MIG/MAG + Laser)

Laser con filo d’apporto

Laser autogeno

Remote Laser Welding

aumento della QUALITA’ del giunto saldato, in termini di minor deformazione termica del componente,

maggior ripetibilità e controllo del processo;

aumento della PRODUTTIVITA’ in termini di velocità di saldatura



1.  INTRODUZIONE



3 A DI PROCESSO



La saldatura laser

Sorgenti Laser ad Elevata Brillanza: Sorgenti Laser in Fibra: λ = 1070 nm Sorgenti Laser a Disco: λ = 1030 nm Sorgenti Laser a Diodi: λ ≈ 800 ÷ 1000 nm

[W.Steen J.Mazumder, 2010]


La saldatura laser

cortesia IPG Photonics IT [W.Steen J.Mazumder, 2010]


The higher power value that reaches the monitoring side in free space emission at the laser wavelength is 0.68 mW. This radiation is comparable with the mean radiation values that are generally used in spectrographic tests so its presence should limit the S/N ratio in the Visible spectrum. Another unwanted presence of laser radiation during process monitoring is related to the backscattering of the laser beam radiation coming directly from the workpiece surface during the welding process. To analyse this effect, bead on plate welds were performed on AISI 316L plates (4 mm in thickness) in different gas environments, such as Nitrogen, Helium and Argon. Except for the laser power and the shielding gas, all the other process parameters were kept at fixed values.

Power measerements

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

0 100 200 300 400 500 600 700 800 900 1000 1100 1200Laser power (W)

mW

Nitrogen

Helium

Argon

Figure 5 Laser power on the monitoring side of the combiner during welding in different gas environments.

As can be seen in the graph, backscattering radiation is superimposed to the laser emission that comes from the laser modules. Backscattered laser wavelength increases with the emitted power till its maximum value that is reached at 800W where, in Nitrogen environment, a mean laser power of 4.5mW is measured at the SMA gate. These radiation levels overfill the saturation levels of common spectrographs, requiring the use of an optical filter for laser wavelength cut off. For this sake a broadband filter will be used in the further tests, allowing the monitoring of the wavelength range between 300 nm and 900 nm.

Comparison between Off-axis and T.O.C. Monitoring Performances

Performances of the T.O.C.M. technique were also tested on welding trials in which spectroscopic monitoring was performed both in combiner and off axes. In this last configuration, the probe was fixed at the focusing head with an angular position referred to the laser beam axis of 70° and with a distance to the welding spot of 90 mm. The material used in the experimentation was Titanium alloy Ti6Al4V (ASTM B265 Gr5) in form of plates of 2 mm in thickness. Butt joint welds with a total length of 150 mm were

performed at different sets of increasing laser speed and power. A shielding gas trailer was used to protect the upper side of the welds with Helium (flow rate 40 nl/min) while the root side was protected with an Argon chamber [gas pressure =1.2 atm]. As for the weld bead geometry, all the tested sets of process parameters satisfied the weld quality criteria for Aerospace application [20], while no X-ray analysis were done for holes and pores detection. An Avantes 2048 USB2 300-1100 nm spectrograph (wavelength resolution of 0.6 nm) acquired the spectra at a speed of 6 samples per second both in the T.O.C. and in the off axis configuration. A 2 identical optical filters were used to cut off the laser wavelength, allowing a monitored wavelength range between 300 nm and 900 nm. For briefness, only the welding condition at higher speed is presented. Table 1 shows the laser parameters used in this condition while figure 6 depicts the related weld cross section.

Table 1 Process parameters used in the discussed welding configuration

Process parameters Value Laser power [P] 1000 W

Travelling speed [v] 4000 mm/min Focus position [z] 0 mm

Shielding gas He / Ar Collimating focal

length 60 mm

Focusing focal length 125 mm Beam spot diameter 114 µm

elapsed mean irradiance

7.7 x 1010 W/m2

Depth of field 1,5 mm

Figure 6 Weld bead cross section of the welding condition used in the off-axis/T.O.C. monitoring

comparison. Ti6Al4V, s=2mm λ=1070nm, P=1kW

5

First Screening Experiments for Magnesium Overlap JointAZ31 – 2.5 mm

FP = -2.5 mm

Power [kW]

v [mm/s]

40

60

2.6 3

He/He

He/HeAr/He

Ar/He

FP = 0 mm

Power [kW]

v [mm/s]

40

60

2.6 3

He/He

He/He

Ar/He

Ar/He AZ31B, s=5mm λ=1070nm, P=2.6kW

Cu60Zn40, B.o.P. λ=1070nm, P=1kW

La saldatura laser (λ = 1070 nm)

Cu60Zn40 SMA, B.o.P. λ=1070nm, P=1kW

AISI, s=2mm λ=1070nm, P=0.6kW

!

AISI, butt- λ=1070nm, P=1.0kW

Acciaio, s=6mm λ=1070nm, P=4.0kW

SITEC / CNR-IENI SITEC SITEC

SITEC SITEC SITEC


SALDATURA LASER (componente in Ti6Al4V, s=2su6mm, λ=1070nm @ SITEC)



Saldatura di acciaio zincato: 1.  Laser Dimpling [Ferrarini, 2013] Premio Laurea Mag. AITeM 2013

2.  Remote Laser Welding [Gattere, 2014] Premio L.M. UCIMU 2014

3.  Monitoraggio del gap [Colombo, Colosimo, Previtali, 2013]


Author's personal copy

can lead to results similar to those achievable using one or morephotodiodes opportunely filtered in the selected emission ranges.The third approach is based on the application of PCA to thenormalised spectra and is useful for data reduction and theidentification of physical phenomena that govern the opticalemission.

To evaluate the ability of each method to identify the effect ofthe parameters (gap and location), the analysis of variance(ANOVA) is performed using the set of indicators provided bythe three approaches as response variables. The ANOVA resultsare used as performance indicators to compare the effectivenessof the three approaches in analysing emission spectra. In parti-cular, the adjusted coefficient of determination R2-adj and theF-value of each factor (gap and location) in the analysis ofvariance are used as indicators. The coefficient of determinationdescribes the proportion of variability in a data set that iscollected by the statistical model and provides a measure ofhow well future outcomes are likely to be predicted by the model.The use of the adjusted form of the coefficient of determinationenables one to take into account the effects of a number of

explanatory terms in the model. Unlike R2, the adjusted R2

increases only if the new term improves the model. The F-valuerelates the amount of the explained variance of a factor to thefactor’s unexplained variance. The higher the F-value, the higherthe amount of process variance due to the considered variablefactor. The comparison of the F-value is used in heuristicapproaches for model selection (as stepwise regression) and iseffective for models that have approximately the same number ofsignificant factors.

The full results from the statistical analysis are presented in anappendix. The main hypotheses of the ANOVA test (residuals thatare independently and identically distributed as a normal variablewith constant variance) have always been checked but are notreported here for the sake of brevity.

4. Results and discussion

4.1. Joints analysis

The quality of the remote laser welding process was evaluatedby visual inspection of the weld seams and metallurgical analysesperformed on cross and longitudinal sections of the weldedsamples.

Fig. 3 shows representative macro-photographs and sectionsof the welding condition at each of the three gaps.

With an optimal gap value of 200 mm, the weld bead is regularand does not present local defects. Welding defects such as poresor cracks are not present and the weld bead is sound. Furthermorea constant penetration is observed along the whole welding seam.On the contrary, when the gap is moved from its reference value,

Table 5Data-analysis methods used in the present paper.

Data-analysis method Indicators

Analysis of the overall emission Overall emission

Analysis of the selected emission ranges Plasma emissionLaser emissionThermal emission

Spectra normalisationþprincipal component analysis PCs

Fig. 3. Different views of the analysed welding conditions taken at different gap values: gap¼100 mm (top), gap¼200 mm—reference welding condition (centre),gap¼300 mm (below). ((a): cross section of the weld bead, (b): longitudinal section of the weld bead, (c): macro-view of the weld bead).

D. Colombo et al. / Optics and Lasers in Engineering 51 (2013) 34–4640

SITEC

IPG YLR1000 + ElEn Thetafiber + ABB IRB2400 @ SITEC

IPG YLS3000 + COMAU SmartLaser @ SITEC



1.  INTRODUZIONE



3 A DI PROCESSO



Criticità nella saldatura di assemblati


Criticità nella saldatura di assemblati

Precisione dell’Attrezzaggio

Tolleranze Dimensionali

Parte B

Precisione del Robot

Robustezza del Processo di Saldatura


Parte A


La saldatura laser 3D: le tecnologie abilitanti



Parte B




Parte A


Le Tecnologie Abilitanti: Technology Layers

Livello 0: Processo Livello 1: Sistema

Livello 1: Sistema

Livello 2: Ciclo Produttivo Livello 2: Ciclo Produttivo



Parte B




Parte A


Le Tecnologie Abilitanti: Technology Layers

Livello 0: Processo di Saldatura •  Process Knowledge •  Process Monitoring •  Laser Beam Shaping •  Diagnosi sistemi accessori al processo • …

Livello 1: Sistema di Saldatura •  Programmazione mediante Manual Guidance •  Visione: Inseguigiunto • …(programmazione off-line, machine Vision,

interazione uomo/macchina, etc)



1.  INTRODUZIONE



3 A DI PROCESSO



Le Tecnologie Abilitanti

• Livello 0: Processo di Saldatura

• Process Knowledge • Process Monitoring • Diagnosi sistemi accessori al processo • Laser Beam Shaping

cortesia SITEC/HZG/IPG

Robustezza del processo di saldatura a variazioni di - gap tra i lembi - posizione del piano di fuoco Saldatura di lega AZ31B s = 2 mm butt-joint P = 3 kW diametro fibra = 50 µm spot size = 100 µm





Robustezza del processo di saldatura a variazioni di - gap tra i lembi - posizione del piano di fuoco Saldatura di lega AZ31B s = 2 mm butt-joint P = 3 kW diametro fibra = 50 µm spot size = 100 µm






Variazione del diametro della fibra ottica " effetto del diametro del fascio (in prima approx) Saldatura di lega AZ31B s = 2 mm butt-joint M = 1 E [J/mm] = P/v = 40 J/mm (cost) Gap = 0 mm






Variazione del diametro della fibra ottica " effetto del diametro del fascio (in prima approx) Saldatura di lega AZ31B s = 2 mm butt-joint E [J/mm] = P/v = 40 J/mm (cost) Gap = 0 mm

2 4 6 8

180

200

220

240

260

280

UTS

(MP

a)

Power (kW)

200 400 600 1000

Heat Input 40 j/mm

Base material UTS 276 MPa





• Process Knowledge • Process Monitoring • Diagnosi sistemi accessori al processo • Laser Beam Shaping (wobbling)

Sovraimporre alla velocità di avanzamento del fascio laser una oscillazione (circolare) ad elevata frequenza per generare uno spot laser apparente

cortesia IPG Photonics IT




• Process Knowledge • Process Monitoring • Diagnosi sistemi accessori al processo • Laser Beam Shaping (wobbling)

Sovraimporre alla velocità di avanzamento del fascio laser una oscillazione (circolare) ad elevata frequenza per generare uno spot laser apparente D = diametro di wobbling


D = 177µm D = 355µm D = 557µm

D = 952µm D = 1445µm D = 1950µm





Optical Emission Spectroscopy (O.E.S.)

Optical Emission Monitoring (O.E.M.)

Visual Monitoring (V.M.)

spectral domain [nm] time domain [t] spatial domain [m]

Fig. 3: Comparison of images of the welding zone and its surrounding as taken with a Si- (left) or an InGaAs- (right) camera.

The weld pool is more clearly observable in the InGaAs (=NIR) image, the thermal trace is virtually only detectable in the NIR. (Welding direction is from right to left here and in all following images. The laser spot in the Si-image is not saturated, but its appearance is just an effect of the print. The left quarter of the InGaAs-image is attenuated by a filter.)

4. EXPERIMENTS

We have conducted many sets of experimental welds of ferrous materials in various configurations that reflect industrial relevance. In TRUMPF´s application laboratory we used commercial equipment, like a TruDisk 16002 thin disk laser with up to 16 kW cw output power and an industrial grade beam delivery system. The process sensor head SeamLine Pro was connected to the laser by a standard industry D-type fiber connector. The core diameter of the used optical fiber determines the spot size on the work piece: The BEO welding optics that we used has a magnification of 1.4, which results in a 280 µm or 560 µm spot size with 200 µm or 400 µm core diameter fiber, respectively. The smaller spot of 280 µm corresponds to typical industrial welding applications of several millimeter thicknesses, whereas a 560 µm spot represents the typical spot for lap welding of sheet metals of approx. 1 mm thickness.

Fig. 4: The NIR-image of a laser beam welding process shows three significant regions. The area of the laser spot and the keyhole is attenuated by a neutral density filter (left quarter of the image) to increase the overall dynamic of the image. (Material: Mild steel S235)

The NIR-image of the welding zone reveals different features that can be evaluated (see fig. 4):

4.1. Laser spot and keyhole area When the applied laser power is high enough, the laser beam fully penetrates the materials, the keyhole opens to the bottom surface and a dark spot becomes visible within the laser spot5,6. This well-known “full-penetration hole” occurs in the VIS as well as in the NIR spectral range and we did not observe a significant difference in this feature in the two spectral ranges.

Si InGaAs

weld pool (liquid)

thermal trace (solidified)

laser spot / keyhole

attenuated

attenuated

welding direction

Proc. of SPIE Vol. 8239 82390T-4





Identificazione di variazioni di potenza laser e portata di gas di protezione mediante O.E.S. Saldatura di lega Ti6Al4V s = 2 mm lap-joint

0,0250,0200,0150,0100,0050,000-0,005-0,010-0,015

0,0050

0,0025

0,0000

-0,0025

-0,0050

-0,0075

-0,0100

pc1

pc2

gas100gas50p15p30rz

z z

z

z

z

zzz

zz

z

zzzz

gas100gas100gas100gas100gas100gas100gas100gas100

gas100gas100

gas50

gas50gas50gas50

gas50

gas50gas50

gas50gas50

gas50

gas50

gas50gas50gas50

gas50

p30p30 p30p30

p30

p30p30 p30p30

p30

p30p30 p30 p30

p30p15p15

p15

p15

p15

p15p15p15p15

p15

p15p15

p15

p15

p15

rr

r

r

r

r rr r

r

rr r

r

r

Scatterplot of pc2 vs pc1

[Colombo, Capello, Previtali, 2009]





Identificazione del piano di fuoco mediante automatizzazione della procedura secondo UNI 10300

[Colombo, Previtali, Riva, Semeraro, 2013]







[Colombo et al, US Patent 036664, 2012]



1.  INTRODUZIONE



3 A DI PROCESSO




• Livello 1: Sistema di Saldatura

• Programmazione mediante Manual Guidance • Visione: Inseguigiunto

MGD: E’ un dispositivo che facilita il movimento intuitivo del robot, remotando a tutti gli effetti gli scomodi tasti di movimentazione del Jpad del Teach Pendant

L.M.Tosatti et al, 2009





L'operatore può programmare il robot, trascinando intuitivamente l'end-effector attraverso la traiettoria di lavoro, salvando i punti caratteristici per creare il path program

link al video sulla PMG

L.M.Tosatti et al, 2009





Saldatura di testa o di spigolo

lega AA7020, s = 2mm

P = 3 kW





Saldatura di testa di acciaio Fe P04 zincato s = 0.8/0.8 mm

Gap = 0 mm ! #

P = 1 kW

v = 2 m/min




• Programmazione mediante Manual Guidance • Visione: inseguigiunto

Lo scanner laser 3D a triangolazione per l'inseguimento di un giunto, riconosce un profilo (es. uno scalino o un gap, un cordone, etc) come feature chiave a cui agganciarsi, con triplice funzionalità:

•  Programmazione: ausilio alla generazione del programma

•  Correzione programma robot on line: segue deformazione della lamiera durante la saldatura

•  Controllo Qualità : analisi 3D del giunto a saldatura eseguita

L.M.Tosatti et al, 2010 Cevasco et al, 2011

Cevasco et al, 2012





Inseguigiunto per programmazione

la scrittura del programma di saldatura vine ulteriormente facilitata con l'utilizzo combinato di uno scanner inseguigiunto durante la generazione della traiettoria di saldatura 3D tramite la manual guidance

In particolare il robot viene “costretto” ad agganciarsi al giunto (centraggio sulla feature), alla distanza Z necessaria al processo di saldatura, correggendo anche l'angolo di inclinazione del polso in modo da mantenere costante l'angolo di attacco al piano di saldatura impostato.

L'unico grado di libertà che rimane è l'ascissa curvilinea del giunto da seguire


Cevasco et al, 2012





Inseguigiunto per correzione del programma robot on-line

… in processi la dove è attesa la deformazione termica del componente che si sta saldando


Cevasco et al, 2012


Le Tecnologie Abilitanti (CNR)



Controllo Qualità : analisi 3D del giunto a saldatura eseguita

Il controllo del giunto saldato può essere fatto anch'esso on line utilizzando un secondo scanner che inquadri il giunto appena saldato, oppure offline ripercorrendo la traiettoria a valle del processo.

Con la modalità online si possono retroazionare i parametri di processo in un loop che mantenga la forma del cordone entro certe tolleranze, ma spesso l'ingombro di 2 teste di scansione ne preclude l'utilizzo

Il controllo può essere fatto anche indirettamente monitorando la forma della pozza durante il processo


Cevasco et al, 2012



1.  INTRODUZIONE



3 A DI PROCESSO



Conclusioni

La saldatura laser è un processo maturo e robusto. Tecnologie abilitanti sono disponibili per la saldatura laser 3D. I robot di saldatura non saranno più solo “tools” ma veri “assistenti di processo”.


Gli autori ringraziano per la collaborazione:


Ing. Daniele Colombo, PhD Dipartimento di Meccanica Politecnico di Milano via La Masa, 1 - 20156, Milano (IT) website: http://sitec.mecc.polimi.it/ e-mail: [email protected]

Ing. Luca Cevasco Consiglio Nazionale delle Ricerche

Istituto di Tecnologie Industriali e Automazione via Bassini, 15 – 20133, Milano (IT)

website: http://www.itia.cnr.it e-mail: [email protected]

Grazie per l’attenzione !


Riferimenti Bibliografici [Steen, Mazumder, 2010] W.Steen J.Mazumder, “Laser Material Processing”, Springer, 2010 [Ferrarini, 2013] Tesi di Laurea Magistrale: Characterization of the laser dimpling process on galvanized steels for

automotive applications;Supervisor. Prof.Barbara Previtali (PoliMi), Co-Superviosor: Prof.Darek Ceglarek (WMG-UK) Premio di Laurea Magistrale UCIMU 2014

[Gattere, 2014] Tesi di Laurea Magistrale: Development of an Optical Based Monitoring Device for Remote Laser Welding of Zinc Coated Steels. Supervisor: Prof.Barbara Previtali (PoliMi),Co-Superviosor: Prof.Darek Ceglarek (WMG-UK) Premio di LaureaMagistrale AITeM 2013 “F.Soavi”

[Colombo, Colosimo, Previtali, 2013] D.Colombo, B.M.Colosimo, B.Previtali Comparison of methods for data analysis in the remote monitoring of remote laser welding Optics and Lasers in Engineering 51 (2013) 34-46 D.Colombo, B.Previtali Laser dimpling and remote welding of zinc coated steels for automotive applications International Journal of Advanced Manufacturing Technology (2014) (DOI: 10.1007/s00170-014-5690-1) - Febbraio 2014


Riferimenti Bibliografici [Colombo, Capello, Previtali, 2009] E.Capello, D.Colombo, B.Previtali Process monitoring through the optical combiner in fiber laser welding applications Proceedings of the Fifth international WLT-Conference on Laser in Manufacturing 2009, Munich, June 2009 [Colombo, Previtali, Riva, Semeraro, 2013] D.Colombo, B.Previtali, G.Riva, Q.Semeraro Procedure for the identification of the focal plane position in high power fiber laser systems Proceedings del convegno AITeM 2013 - San Benedetto del Tronto (IT), Settembre 2013 [Colombo et al, US Patent 036664, 2012] No. WO/2012/036664, 22 March 2012. Industrial high power fiber laser system with optical monitoring assembly Applicant: IPG Photonics Corp. (US). Inventors: G. Moroni, D. Colombo, B. Previtali, S. Cattaneo, L. Rossotti


Riferimenti Bibliografici [Cevasco et al, 2011] C. Cenati & G. Borroni & L. Cevasco & D. Parazzoli & M. Danesi National Research Council (CNR)

Institute of Industrial Technologies And Automation (ITIA), Milano, Italy "An innovative methodologie for laser scanner integration in a robot cell for small batch production of sculpture artworks"

[Cevasco et al, 2012] C. Cenati & L. Cevasco & G. Borroni & N. Pedrocchi & L.M. Tosatti National Research Council

(CNR) Institute of Industrial Technologies And Automation (ITIA), Milano, Italy "Low Cost Scanning Device Application for Footwear Industry"

[Tosatti et al, 2010] Deposito di Brevetto 23/07/2010 CIA S.r.l. & L.M. Tosatti & C. Cenati & G. Borroni & L. Cevasco & D.

Parazzoli & M. Danesi "Sistema e metodo per la ricostruzione tridimensionale di oggetti”

[Tosatti et al, 2009] Deposito brevetto 16/11/2009 - COMAU S.p.a. & L.M. Tosatti & D. Parazzoli & M. Danesi & E. Scari

"Robot System"

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