Fine WeldingwithLasers. Michael Müller

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Fine Welding with Lasers

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Lasers and Systems

Welding principle

Weld types and tolerances

Material selection

Influencing factors / Advanced process approaches

ISO – Standards

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Notation according EN ISO 1145 : 1994 Laser source Laser Supply cabinet (power, cooling) Work piece Handling system (positioning, movement, clamping, gas supply)

Beam forming Beam guiding system

Process gas supply

Laser Working Station

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StarPulse 40 / 90 /150

StarPulse 500

Pulsed Nd:YAG lasers

Starfiber 400 - 600 Starfiber OEM

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Integral Performance MPS 3D Select

Class 1 Systems

5 MPS

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Advantages

fast positioning

flexible in terms of part geometry

easy to use software

Suitable for fiber and direct beam delivery

Vision system through the lense

Galvo head

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9

Laser material interaction

Absorption

In a thin surface layer (optical penetration depth depends on material , <10 nm)

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)

t₀

t₁

Laser beam

Materials reaction

Solid state-, liquid state-, vapour phase processes (e. g. recristallisation, anealing, hardening, melting,...)

depending on power density and interaction time

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Laser – As a Thermal Tool

Laser beam

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St Fe Mo Cu Au Ag Al 30 25 20 15 10 5

Absorption

A b s o rp ti o n i n % Wave length in µm 0.1 0.2 0.4 0.8 1 2 4 6 8 10 20 Nd: YAG 1,06 µm CO2 10,6 µm

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Heat conduction welding

The material is heated above melting temperature but there is no vaporization.

aspect ratio approx. 1

max. penetration approx. 0.5 mm Very smooth surface

Process types – conduction welding

1) Molten material 2) Weld depth 1) 4) 11 Applications:

Welding of thin workpieces, cosmetic welding of enclosures

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Deep / Keyhole welding

Heating of the material above vaporization temperature and formation of a keyhole

aspect ratio approx. >> 1

Keyhole diameter approx. spot diameter

Cw: max. penetration depending on laser power

Pw: max penetration approx. 3 mm

1) Plasma cloud

2) Molten material

3) Keyhole

4) Weld depth

Process types – keyhole welding

1)

2) 3)

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Example:

Spot diameter 0.04 mm, Power 200 W -> I = 1.6 x 107 _W/cm²

Spot diameter 0.2 mm, Power 200 W -> I = 6.3 x 105 _W/cm²

Power density [W/cm²] D e p th [m m ] Conduction mode Keyhole mode C ri ti ca l in te n si ty power 105 ₁₀6 ₁₀7 ₁₀8 Plasma shielding

PROCESS TYPES

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E T = 1/υ τ P_AV P_PK P_Puls Power P Time t 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) E = P_Puls· τ

P_AV= E · υ Energy too high

Energy too low

P_Pulse

τ

process threshold

Spot welding

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1) Increase of peak power (W)

2) Increase of pulse duration (ms)

3) Increase of spot size (mm)

1000 W-2 ms-Focus 0,4 mm 2000 W-2 ms-Fokus 0,4 mm 3000 W-2 ms-Fokus 0,4 mm

1000 W-2ms-Fokus 0,4 mm 1000 W-10ms-Fokus 0,4 mm 1000 W-50 ms-Fokus 0,4 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

Effect of Parameter Changes Pulsed Laser

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ø

100%

overlap 70%

The overlap indicates which

percentage of a pulse is covered by the following pulse.

From overlap, spot diameter and velocity the necessary frequency can be calculated.

The overlap in pulsed laserwelding is usually in the range of 50 to 90 %. To achieve good strentgh a little more than 50 % are sufficient. If hermetic sealing is requiered the overlap needs to be 75 % or more.

Cross section

70 % 50 %

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Laser parameters - cw

E P_{AV =}P_PK Time t process threshold Power P cw - Laser

Peak power = average power in cw mode, peak power of a modulated puls is as maximum the max. average power Pulse width: 0.004 ms - 100 ms or cw mode

Frequency: cw (up to 170 kHz in modulated mode)

Power density (P/(π/4*D²)) has to be above process threshold

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Effects of Temperature Cycle

Laser welding has the following characteristics:

Very high gradients and heating- (>10000 K/s) and cooling rates (100 .. Some

1000 K/s). Result: high state of stress.

Material areas close to the molten zone are heated up close to the solidus temperature.

The formation of balanced microstructures is nearly impossible. Typically we find coarse grained, hard and brittle microstructures in the HAZ.

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Butt

weld

Lap joint

Fillet

weld

Thin material should be on top

Incidence angle of the laserbeam as much in joint direction as possible

Weld joint types

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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

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Joint types – Lap Joint

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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

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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

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

Tolerances

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Spot sizes

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max. weld depth = 1-2 · spot diameter

Max. gap < 0.1 · weld depth

The spot diameter should be in the range of 50 – 100 % 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.

Spot diameter – pulsed lasers

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Argon

Goal:

Prevent oxidation Improve seam quality

Solution: Use protection gas

Nitrogen → cheap

Argon → better seam quality Helium → difficult to handle

Important:

Laminar flow

At 6 mm nozzle diameter 10 l/min are reasonable

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Weldability of a material is given, if in production due to the chemical,

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.

Weldability of Materials

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Material Selection

Material Comment

Carbon steel 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 High refectivity requires high peak power

Cu-Be Welds well but particles hazardous

Bronze (Cu/Sn) Reasonable welds

Brass (Cu/Zn) Outgasing of zinc prevents good welds

Aluminium Pure Al (1xxx) welds well, only a few alloys weld crack free (2219, 3003). Filler material 4047 or combination of alloys may improve result (e. g. 6061 with 4047),

Titanium Welds well. Very good shielding with inert gas necessary

Gold, Silver, Platinum High reflectivity requires high peak power

Nickel Welds well

Ni based super alloys Welds well if Ti + Al content < 4 %

Kovar Welds well

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Material Selection - Combinations

Weldability of metal combinations poor; good; excellent

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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

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The weldabilty of steel depends strongly on the following material

characteristics:

chemical composition

metallurgical processes at melting and solidification physical properties

Material composition limits:

C- content < 0,2 %

S- and P-content as small as posible ( usually 0.035% S and 0.045 % P). (S often used to improve material suitability for milling,

e. g.: 303 = 304 with high S content, 0.15 %)

A prediction about the resulting microstructure and possible imperfections for highly alloyed steel grades can be obtained by using the Schäffler diagram.

Please note: Diagram only valid 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.

Steel - Basics

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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 weldability for 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.

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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.

• weldability for 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.

Steel Weldability – Stainless steel

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Suutala diagram

Crack _{No crack}

Arc welding Laser welding

Cr/Ni equivalent

S

+

P

+

B

[

m

a

ss

%

]

0

0.05

0.1

1.4

1.6

1.8

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Weldability of 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.

Aluminium Welding - Basics

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Solution:

Filler wire, increase flexibilty in 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)

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1 2 3 4 5 6 7 R e la ti ve C ra ck S e n si ti vi ty Al-Li Al-Cu Al-Mg₂-Si Al-Mg Al-Si

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Series

Non-heat-treatable alloys 1xxx 3xxx 4xxx 5xxx Heat-treatable-alloys 2xxx 6xxx 7xxx

Alloying elements

pure Al (> 99%) Al-Mn Al-Si Al-Mg Al-Cu / Al-Cu-Mg / Al-Cu-Li Al-Mg-Si Al-Zn, Al-Zn-Mg-Cu

Weldability

generally weldable

often weldable without filler

weldable, Si > 3 % to avoid hot cracking

weldable using filler (Mg > 4 %) often rough surface

difficult to weld, exception 2219, 2519

weldable using filler (e. g.4047)

difficult due to Zn content, filler requiered

Comment

soft material

soft , good corrosion resistance

soft and ductile, mainly used as filler material

higher strength due to Mg content

high strength, low corrosion resistance

good strength, well formable and relatively good corrosion resistance strongest Al alloy

Aluminium – Alloy series

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Weld Depth – Pulsed Laser

0 0,5 1 1,5 2 2,5 3 3,5 0 5 10 15 20 25 30 35 40 45 50 d e p th in m m Pulseenergy in J Stainless steel Aluminum

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0 0,5 1 1,5 2 2,5 3 0 200 400 600 800 D e p th in m m Power in W

Depth stainless steel

Depth aluminum

Weld depth – cw Fiber Laser

Rules for Laser selection:

stainless steel:

0.5 mm/100 W

aluminum:

0.3 mm/200 W

0 0,2 0,4 0,6 0,8 1 1,2 1,4 1,6 1,8 2 2,2 2,4 0 2 4 6 8 10 D e p th in m m speed in m/min 100 W 200 W 400 W 600 W

stainless steel (3 mm thick),

welds in thinner material might be faster.

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1m/min

3m/min

1kW, 400µm fiber 500µm spot size 0 0,5 1 1,5 2 2,5 0 2 4 6 8 D e p th i n m m Speed [m/min]

Weld Depth - cw Diode Laser

Material: SUS304 Gas: Argon

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Focus position

41

- z z = 0

+z

Laser beam

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Pulse Shaping

Freely programmable pulse shape Closed loop control for accurate pulse shaping

Green: Set point Yellow: Actual values

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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.

Pulse Shape - Why

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Purpose of Pulse Shaping:

5 2 1 Pulse duration [ms] P e a k P o w e r [W ]

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Pulse Shape - Effects

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Rofin Smart Weld Technology

Galvo Field

Workpiece Top View

Oscylation-movement perpendicular to weld seam

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

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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.

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Quality Aspects

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Weld penetration

• Easy to judge by cross section or through weld

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

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Process related

allow root fusion

support heat removal

allow degassing of the melt

new joint geometries possible (e. g. welding of several layers, weld even if lower surfece is not accesible)