Fine Welding with Lasers
Michael Müller
Table of contents
Lasers and Systems
Welding principle
Weld types and tolerances
Material selection
Influencing factors / Advanced process approaches
ISO – Standards
2
Laser Working Station
Notation according EN ISO 1145 : 1994
Beam guiding system
Beam forming
Laser source
Process gas supply
Work piece
Laser
Supply cabinet
(power,
cooling)
Handling system
(positioning, movement,
clamping, gas supply)
3
Pulsed Nd:YAG lasers
StarPulse 40 / 90 /150
Starfiber OEM
StarPulse 500
Starfiber 400 - 600
Starfiber 100 - 300
4
Class 1 Systems
Performance
Integral
MPS
Select
MPS 3D
5
Fix Optics
6
Galvo head
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
7
Welding with Galvo Head - Principle
8
Laser – As a Thermal Tool
Laser material interaction
Absorption
t0
Laser beam
Laser
beam
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
Laser beam
Laser
beam
By heat conduction from the optical penetration depth into
bulk material (temperature gradient)
Materials reaction
Solid state-, liquid state-, vapour phase processes
(e. g. recristallisation, anealing, hardening, melting,...)
depending on power density and interaction time
t1
Isotherme
9
Laser Beam Absorption
Absorption
30
Absorption in %
Cu
CO2
10,6 µm
25
20
Au
15
Ag
Al
Nd:
YAG
1,06 µm
St
Fe
10
Mo
5
0.1
0.2
0.4
0.8 1
2
4 6 8 10
Wave length in µm
20
The absorbtivity of materials at room temperature and perpendicular incidence angle
of low intensity laser radiation is strongly depending on the wave length
10
Process types – conduction welding
Heat conduction welding
1)
2)
Molten material
Weld depth
The material is heated above melting
temperature but there is no vaporization.
aspect ratio approx. 1
1)
4)
max. penetration approx. 0.5 mm
Very smooth surface
Applications:
Welding of thin workpieces, cosmetic welding
of enclosures
11
Process types – keyhole welding
Deep / Keyhole welding
1)
1)
2)
3)
4)
Plasma cloud
Molten material
Keyhole
Weld depth
Heating of the material above vaporization
temperature and formation of a keyhole
aspect ratio approx. >> 1
Keyhole diameter approx. spot
3)
2)
diameter
Cw: max. penetration depending on
laser power
Pw: max penetration approx. 3 mm
4)
12
Depth [mm]
Conduction
mode
105
Critical intensity
PROCESS TYPES
106
Plasma
shielding
Keyhole
mode
power
107
Power density [W/cm²]
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
108
Laser Parameters - pulsed
Power P
PPK
E
process threshold
PPuls
PAV
τ
1. Peak power
2. Pulse power
3. Pulse width
4. Pulse energy
5. Frequency
6. Average power
7. Pulse shape
Time t
T = 1/υ
PPK
PPuls
τ
E
υ
PAV
P(t)
E = PPuls· τ
PAV = E · υ
PPulse Spot welding
Energy too high
Energy too low
τ
14
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-10ms-Fokus 0,4 mm
1000 W-2ms-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
Pulsed Welding - Overlap
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.
ø
100%
overlap
70%
Cross section
70 %
50 %
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.
16
Laser parameters - cw
cw - Laser
Power P
E
process threshold
PAV = PPK
Time t
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
17
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.
18
Weld joint types
Butt
weld
Lap joint
Thin material should be on top
Fillet
weld
Incidence angle of the laserbeam
as much in joint direction as possible
19
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
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
Joint types – Fillet Weld
Fillet weld
Advantages:
•
easy to clamp
•
good distribution of forces
15° - 30°
Disadvantages:
•
high requirements on clamping and positioning
Please note: Angle of weld follows incidence angle of laserbeam
22
Tolerances
Butt weld
< 0,15 • d
d
< 0,1 • d
Lap joint
d
< 0,1 • d
d = 0.75 mm
23
Spot sizes
600 µm
30 µm
24
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 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.
25
Protection Gas
Goal:
Prevent oxidation
Improve seam quality
Solution: Use protection gas
Nitrogen → cheap
Argon
→ better seam quality
Helium → difficult to handle
Without gas
Nitrogen
Important:
Laminar flow
At 6 mm nozzle diameter
10 l/min are reasonable
Argon
26
Weldability of Materials
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.
27
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
Tantal
Welds well. Very good shielding with inert gas necessary
Molybdenum
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
Material Selection - Combinations
Weldability of metal combinations
poor;
good;
excellent
29
Plating Issues
Zinc coating
•
Boiling issues have to be considered
•
May cause brittle intermetallic phases with Cu
Tin
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
Steel - Basics
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.
31
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 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.
32
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.
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.
33
Suutala diagram
S+P+B [mass %]
0.1
Arc welding
Laser welding
Crack
No crack
0.05
0
1.4
1.6
1.8
Cr/Ni equivalent
J.C. Lippold, Weld. J., 73-6 (1994)129s – 139s
34
Aluminium Welding - Basics
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.
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)
35
Aluminium Welding – Hot Cracking
Al-Mg
Relative Crack Sensitivity
Al-Cu
Al-Li
Al-Mg2-Si
Al-Si
1
2
3
4
5
6
7
Percentage Alloying Element [Weight %]
36
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 % to avoid hot
cracking
soft and ductile, mainly used as
filler material
5xxx
Al-Mg
weldable using filler (Mg > 4 %)
often rough surface
higher strength due to Mg content
Heat-treatable-alloys
2xxx
Al-Cu / Al-Cu-Mg /
Al-Cu-Li
difficult to weld, exception 2219, high strength, low corrosion
2519
resistance
6xxx
Al-Mg-Si
weldable using filler (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
Weld Depth – Pulsed Laser
3,5
Stainless steel
Aluminum
3
depth in mm
2,5
2
1,5
1
0,5
0
0
5
10
15
20
25
30
35
40
45
50
Pulseenergy in J
Protection gas: Argon
38
Depth in mm
Weld depth – cw Fiber Laser
2,4
2,2
2
1,8
1,6
1,4
1,2
1
0,8
0,6
0,4
0,2
0
100 W
200 W
400 W
600 W
0
2
4
6
8
3
speed in m/min
10
Depth stainless steel
stainless steel:
aluminum:
0.5 mm/100 W
0.3 mm/200 W
Depth in mm
2,5
Rules for Laser selection:
stainless steel (3 mm thick),
welds in thinner material might be
faster.
N2 protection gas, spot Ø 20 µm
2
Depth aluminum
1,5
1
0,5
0
0
200
400
Power in W
600
800
Weld Depth - cw Diode Laser
2,5
Material: SUS304
Gas: Argon
2
Depth in mm
1kW, 400µm fiber
500µm spot size
1,5
1
0,5
0
0
2
4
Speed [m/min]
1m/min
3m/min
6
8
Focus position
-z
z=0
+z
Laser beam
inclined plane
41
Pulse Shaping
Freely programmable pulse shape
Closed loop control for accurate pulse
shaping
Green: Set point
Yellow: Actual values
42
Pulse Shape - Why
Peak Power [W]
Purpose of Pulse Shaping:
2
5
1
Pulse duration [ms]
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
Pulse Shape - Effects
Welding of Aluminum
From: Rofin - Lasag
44
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
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
Quality Aspects
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
• Smoothness, flatness of surface
47
Construction Notes
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)
Clamping related
clamping device close to joint
allow self centering
system technology related
least possible contour complexity
consider accesability
48
ISO STANDARDS
Test and inspection
ISO 15614 – 11
Seam quality evaluation
ISO 13919 part 1-2
Quality management
ISO 9000
Welding coordination
ISO 14731
Welding personnel
ISO 14732
quality standards
laser welding
Welding system
ISO 15616 part 1-3
Base material
EN 10025
Filler material
ISO 2560
Welding procedures
and -instructions
ISO 15607
ISO 15609 part 3-4
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 13919-1:1996: Welding -- Electron and laser-beam welded joints -Guidance on quality levels for imperfections -- Part 1: Steel, Part 2:
Aluminium and its weldable alloys
ISO STANDARDS
ISO 15607:2003: Specification and qualification of welding procedures for
metallic materials -- General rules
ISO 15614-11:2002: Specification and qualification of welding procedures
for metallic materials -- Welding procedure test -- Part 11: Electron and laser
beam welding
ISO 15616-1:2003: Acceptance tests for CO2-laser beam machines for high
quality welding and cutting -- Part 1 – 4
Laser standards
ANSI Z136.9 - Safe Use of Lasers in Manufacturing Environments
ISO 11145;2006: Optics and photonics -- Lasers and laser-related equipment
-- Vocabulary and symbols
Thank you for your attention.
www.rofin.com