Fine WeldingwithLasers. Michael Müller

Transcription

1 Fine WeldingwithLasers Michael Müller

2 Table of contents Lasers and Systems Welding principle Weld types and tolerances Material selection Influencing factors / Advanced process approaches ISO Standards 2

3 Laser Working Station Notation according EN ISO 1145 : 1994 Laser Laser source Beam guiding system Work piece Beam forming Process gas supply Supply cabinet (power, cooling) Handling system (positioning, movement, clamping, gas supply) 3

4 Pulsed Nd:YAG lasers StarPulse 40 / 90 /150 Starfiber OEM Starfiber StarPulse 500 Starfiber

5 Class 1 Systems Performance Integral Select MPS MPS 3D 5

6 Fix Optics 6

7 Galvo head Advantages fast positioning flexible in termsofpart geometry easy tousesoftware Suitable for fiber and direct beam delivery Vision system through the lense 7

8 Welding with Galvo Head - Principle 8

9 Laser As a Thermal Tool Laser material interaction Absorption In a thinsurfacelayer(opticalpenetrationdepthdepends on material, <10 nm) t 0 Laser beam Generation of heat By transition of the energy of the light (photons) to the electrons of the material within the optical penetration depth Heat transport By heat conduction from the optical penetration depth into bulk material (temperature gradient) Laser beam Materials reaction Solid state-, liquid state-, vapour phase processes (e. g. recristallisation, anealing, hardening, melting,...) depending on power density and interaction time t 1 Isotherme 9

10 Laser Beam Absorption Absorption in % Al Ag Cu Au Absorption Nd: YAG 1,06 µm Mo Fe CO2 10,6 µm St Wave length in µm The absorbtivity of materials at room temperature and perpendicular incidence angle of low intensity laser radiation is strongly depending on the wave length 10

11 Process types conduction welding 1) Molten material 2) Weld depth Heat conduction welding The material is heated above melting temperature but there is no vaporization. 1) 4) aspectratioapprox. 1 max. penetration approx. 0.5 mm Very smooth surface Applications: Welding of thin workpieces, cosmetic welding of enclosures 11

12 Process types keyhole welding 1) 1) Plasma cloud 2) Molten material 3) Keyhole 4) Weld depth 3) 2) 4) Deep / Keyhole welding Heating of the material above vaporization temperature and formation of a keyhole aspectratioapprox. >> 1 Keyhole diameter approx. spot diameter Cw: max. penetrationdependingon laserpower Pw: maxpenetrationapprox. 3 mm 12

13 PROCESS TYPES Depth[mm] Conduction mode Critical intensity Keyhole mode Plasma shielding power Power density [W/cm²] Example: Spot diameter0.04 mm, Power 200 W -> I = 1.6 x 10 7 W/cm² Spot diameter0.2 mm, Power 200 W -> I = 6.3 x 10 5 W/cm²

14 Laser Parameters - pulsed Power P P PK P Puls E process threshold P AV 1. Peak power P PK 2. Pulse power P Puls 3. Pulse width τ 4. Pulse energy E 5. Frequency υ 6. Average power P AV 7. Pulse shape P(t) τ T = 1/υ E = P Puls τ P AV = E υ P Pulse Time t Spot welding Energy too high Energy too low τ 14

15 Effect of Parameter Changes Pulsed Laser 1) Increase of peak power (W) 1000 W-2 ms-focus 0,4 mm 2000 W-2 ms-fokus 0,4 mm 3000 W-2 ms-fokus 0,4 mm 2) Increase of pulse duration (ms) 1000 W-2ms-Fokus 0,4 mm 1000 W-10ms-Fokus 0,4 mm 1000 W-50 ms-fokus 0,4 mm 3) Increase of spot size (mm) 1000 W-2 ms-fokus 0,4 mm 1000 W-2 ms-fokus 0,8 mm 1000 W-2 ms-fokus 1,2 mm 15

16 Pulsed Welding- Overlap ø 100% overlap 70% The overlap indicates which percentageofa pulse iscoveredby thefollowingpulse. From overlap, spot diameter and velocity the necessary frequency can be calculated. Cross section 70 % 50 % The overlapin pulsedlaserweldingisusuallyin therangeof50 to90 %. Toachievegoodstrentgha littlemorethan50 % aresufficient. Ifhermetic sealingisrequieredtheoverlapneedstobe75 % ormore. 16

17 Laser parameters- cw cw - Laser Power P E process threshold P AV = P PK Time t Peak power = averagepower in cwmode, peakpower ofa modulated puls is as maximum the max. average power Pulse width: ms-100 msorcwmode Frequency: cw(upto170 khz in modulatedmode) Power density(p/(π/4*d²)) has to be above process threshold 17

18 Effects of Temperature Cycle Laser welding has the following characteristics: Very high gradientsandheating-(>10000 K/s) andcoolingrates(100.. Some 1000 K/s). Result: high state of stress. Material areasclosetothemoltenzoneareheatedupclosetothesolidus temperature. The formation of balanced microstructures is nearly impossible. Typically we find coarsegrained, hardandbrittlemicrostructuresin thehaz. 18

19 Weld joint types Butt weld Lap joint Thinmaterial shouldbeon top Fillet weld Incidenceangle ofthelaserbeam as much in joint direction as possible 19

20 Joint types Butt Weld Butt weld Advantages: optimum distribution of forces optimum solution for light weight structures no problems at welding coated material Disadvantages: high requirements on tolerances high requirements on clamping and positioning 20

21 Joint types Lap Joint Lap joint Advantages: low requirements in tolerances and positioning accuracy low distortion indistribution of forces more than 2 layers possible Disadvantages: risc of crevice corrosion difficult degassification Please note: Thin material should be on top. 21

22 Joint types Fillet Weld Fillet weld Advantages: easy to clamp good distribution of forces Disadvantages: high requirements on clamping and positioning Please note: Angle of weld follows incidence angle of laserbeam 22

23 Tolerances < 0,15 d Butt weld d < 0,1 d Lap joint d < 0,1 d d = 0.75 mm 23

24 Spot sizes 600 µm 30 µm 24

25 Spot diameter pulsed lasers max. weld depth = 1-2 spot diameter Max. gap < 0.1 weld depth The spot diameter should be in the range of % of the requiered weld depth and 10 times the maximum gap. The spot diameter results from the beam expansion rate and the focal length of the focussing lens when using direct beam delivery. Using fiber delivery the spot diameter depends on fiber diameter, upcollimation and focal length of the focussing lens. 25

26 Protection Gas Goal: Prevent oxidation Improve seam quality Solution: Use protection gas Nitrogen cheap Argon better seam quality Helium difficult to handle Important: Laminar flow At6 mm nozzlediameter 10 l/min are reasonable Without gas Nitrogen Argon 26

27 Weldability of Materials Weldabilityofa material is given, ifin productiondue tothechemical, metallurgical and physical properties a weld according to the requirements can be done. from: DIN 8528 Teil 1 Possible requirements: static strength dynamic strength heremtical sealing electrical conductivity reproducability process stability Weldability is no material specific value. Due to this most often tests need to be done. 27

28 Material Selection Material Carbon steel Comment Welds well. If carbon content > 0.2 % brittle welds. Stainless steel 300 series welds well except alloys with S > 0.05 % 400 series welds brittle Copper Cu-Be Bronze (Cu/Sn) Brass (Cu/Zn) Aluminium Titanium Gold, Silver, Platinum Nickel High refectivity requires high peak power Welds well but particles hazardous Reasonable welds Outgasing of zinc prevents good welds Pure Al (1xxx) weldswell, onlya fewalloysweldcrack free(2219, 3003). Fillermaterial 4047 orcombinationofalloysmayimproveresult(e. g with4047), Welds well. Very good shielding with inert gas necessary High reflectivity requires high peak power Welds well Ni based super alloys Welds well if Ti + Al content < 4 % Kovar Tantal Molybdenum Welds well Welds well. Very good shielding with inert gas necessary Usually welds brittle may be acceptable where high strength is not required Plating may cause cracks e.g. electroless nickel plating due to its phosphorous content 28

29 Material Selection - Combinations Weldability of metal combinations poor; good; excellent 29

30 Plating Issues Zinc coating Boiling issues have to be considered Tin May cause brittle intermetallic phases with Cu Nickel Electroless-> leads to cracking due to P in plating process Electrolytic-> to be preferred Gold Often with Ni underplating, avoid electroless Ni plating Shiny Au more difficult than dull Au Silver Tends to spatter 30

31 Steel - Basics The weldabiltyofsteeldependsstronglyon thefollowingmaterial characteristics: chemical composition metallurgical processes at melting and solidification physical properties Material composition limits: C-content< 0,2 % S-andP-content assmallasposible( usually0.035% S and0.045 % P). (S often used to improve material suitability for milling, e. g.: 303 = 304 withhigh S content, 0.15 %) A prediction about the resulting microstructure and possible imperfections for highly alloyed steelgrades canbeobtainedbyusingtheschäffler diagram. Pleasenote: Diagramonlyvalid for< 0,2 % C, < 1,0 % Si, < 4 % Mn, < 3 % Mo, < 1,5 % Nb. Schäffler diagram was created for non laser welding processes with lower cooling rates, use with great care. 31

32 Steel Weldability Low alloyed Steel non- and low alloyed steel General structural steel: hardening in HAZ possible. Tough at subzero steel and heat resisting structural steel: Weldabilty good besides martensitic heat resisting structural steel. Case hardening-, nitriding heat treatable steel: good weldabilityfor CE = C+Mn/20+Mo/15+Ni/40+Cr/10+V/10+Cu/20+Si/25 < 0,35, limited weldability for 0,35< CE < 0,5. 32

33 Steel Weldability Stainless steel Stainless steel Ferritic chrome steel (12 % < Cr < 17%, C < 0,1 %): weldability has to be proved. Martensitic chrome steel (10 % < Cr < 14%, 0,1 % < C < 1,2 %): Danger of cold cracking, increase of hardness and brittleness. weldability has to be proved. weldabilityfor martensitic chrome nickel steel with 1 % < Ni < 6% and C < 0,05 % is better. Austenitic chrome nickel (-molybdenum)-steel: mainly good weldabilty. Austenitic ferritic steel (duplex steel): Cool down time not sufficient for complete change of microstructure. 33

34 Suutala diagram S+P+B [mass%] Arc welding Crack Laser welding No crack Cr/Ni equivalent J.C. Lippold, Weld. J., 73-6 (1994)129s 139s 34

35 Aluminium Welding- Basics Weldabilityof aluminium depends strongly on the composition of the alloy. Pure aluminium is for example well weldable. When using Al alloys containing Si, Mg and Cu care should be taken to avoid the peak of hot crack sensitivity. Solution: Filler wire, increase flexibiltyin material selection but difficult handling Choose the material of one of the parts to weld in a way that the resulting microstructure in the weld seam is not critical. (e. g.: 5052 and 4047) 35

36 Aluminium Welding Hot Cracking Al-Mg Al-Cu Relative Crack Sensitivity Al-Si Al-Mg 2 -Si Al-Li Percentage Alloying Element [Weight%] 36

37 Aluminium Alloy series Series Alloying elements Weldability Comment Non-heat-treatable alloys 1xxx pure Al (> 99%) generally weldable soft material 3xxx Al-Mn often weldable without filler soft, good corrosion resistance 4xxx Al-Si weldable, Si > 3 % toavoidhot cracking soft and ductile, mainly used as filler material 5xxx Al-Mg Heat-treatable-alloys 2xxx Al-Cu/ Al-Cu-Mg / Al-Cu-Li weldableusingfiller(mg > 4 %) often rough surface difficulttoweld, exception2219, 2519 higher strength due to Mg content high strength, low corrosion resistance 6xxx Al-Mg-Si weldableusingfiller(e. g.4047) good strength, well formable and relatively good corrosion resistance 7xxx Al-Zn, Al-Zn-Mg-Cu difficult due to Zn content, filler requiered strongest Al alloy 37

38 Weld Depth Pulsed Laser 3,5 3 2,5 Stainless steel Aluminum depth in mm 2 1,5 1 0, Pulseenergy in J Protection gas: Argon 38

39 Weld depth cwfiber Laser Depth in mm 2,4 2,2 1,8 2 1,6 1,4 1,2 0,8 1 0,6 0,4 0, speed in m/min 3 2,5 100 W 200 W 400 W 600 W stainless steel(3 mm thick), welds in thinner material might be faster. N 2 protectiongas, spotø 20 µm Depth stainless steel Rules for Laser selection: stainless steel: 0.5 mm/100 W aluminum: 0.3 mm/200 W Depth in mm 2 1,5 1 0,5 Depth aluminum Power in W

40 Weld Depth-cwDiode Laser 2,5 1kW, 400µm fiber 500µm spot size Material: SUS304 Gas: Argon Depth in mm 2 1,5 1 0, Speed [m/min] 1m/min 3m/min

41 Focus position - z z = 0 +z Laser beam inclined plane 41

42 Pulse Shaping Freelyprogrammablepulse shape Closedloopcontrolforaccuratepulse shaping Green: Set point Yellow: Actual values 42

43 Pulse Shape -Why Purpose of Pulse Shaping: Peak Power [W] 2 1 Pulse duration[ms] 5 1. Fast keyhole opening using steepest rising slope and high peak intensity 2. Option: Prevention of melt expulsion (depending on viscosity of melt) 3. Adjust penetration and volume of keyhole (deep penetration/ keyhole welding ) 4. Step down or ramp down intensity to avoid overheating of melt (spatter!) 5. Continuous absorption of radiation into still open or just closing keyhole (medium intensity, transition to heat conduction welding ) Smoothening effect. 43

44 Pulse Shape - Effects Welding of Aluminum From: Rofin- Lasag 44

45 Rofin Smart Weld Technology Workpiece Top View Galvo Field 1. Application: Fine seam welding 2. Laser: Fiberlaser with Scanner Optics 3. Technology: Galvo used to move small spot perpendicular (programmable) to the welding direction Oscylation-movement perpendicular to weld seam

46 Influence oscillation width Oscillation width 700 µm Oscillation width 350 µm With increasing oscillation width the weld gets wider but less deep. Max process speed depends on oscillation width and frequency. 46

47 Quality Aspects Weld penetration Easy tojudgebycrosssectionorthroughweld Weld strength Determined by destructive testing(pull test.) Cracking Visual inspection, ultrasonic, dye pentration Porosity Cross section, various causes Hermetical sealing Determined by leak test Weld cosmetics Smoothness, flatness of surface 47

48 Construction Notes Process related allow root fusion support heat removal allowdegassingofthemelt new joint geometries possible (e. g. welding of several layers, weld even if lower surfece is not accesible) Clamping related clamping device close to joint allow self centering system technology related least possible contour complexity consider accesability 48

49 ISO STANDARDS Test and inspection ISO Seam quality evaluation ISO part1-2 Welding system ISO part1-3 Quality management ISO 9000 quality standards laser welding Base material EN Filler material ISO 2560 Welding coordination ISO Welding personnel ISO Welding procedures and-instructions ISO ISO part3-4

50 ISO STANDARDS ISO 4063:2009: Welding and allied processes --Nomenclature of processes and reference numbers EN 10025: Steel Specifications ISO 2560:2009: Welding consumables --Covered electrodes for manual metal arc welding of non-alloy and fine grain steels -- Classification ISO 14731:2006: Welding coordination -- Tasks and responsibilities ISO 14732:2013: Welding personnel --Qualification testing of welding operators and weld setters for mechanized and automatic welding of metallic materials ISO :1996: Welding -- Electron and laser-beam welded joints -- Guidance on quality levels for imperfections --Part 1: Steel, Part 2: Aluminium and its weldable alloys

51 ISO STANDARDS ISO 15607:2003: Specification and qualification of welding procedures for metallic materials -- General rules ISO :2002: Specification and qualification of welding procedures for metallic materials --Welding procedure test --Part 11: Electron and laser beam welding ISO :2003: Acceptance tests for CO2-laser beam machines for high quality welding and cutting --Part 1 4 Laser standards ANSI Z Safe Use of Lasers in Manufacturing Environments ISO 11145;2006: Optics and photonics --Lasers and laser-related equipment -- Vocabulary and symbols

52 Thank you for your attention.