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Authors:
Steve Norman,
René Siebert,
Andy Appleyard,
Harry Thonig,
Andreas Wetzig,
Eckhard Beyer
Application and performance of kW-class single-mode fibre lasers in the cutting of non-oriented electrical steel
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Content
1. Introduction: Rationale and scope of study
2. Application Overview / Technical challenges
3. Singlemode kW OEM Laser Beam Source & Characterisation
4. Cutting Trials and Results Analysis
5. Conclusions
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Rationale & Scope
1. Rationale
E-mobility projects focused on energy efficient drive trains- Battery technology / electric motors / drive electronics
Typical process for stator / rotor involves mechanical punching
Previous studies with CO2 & Disc lasers have demonstrated impact of HAZ on laser-cut laminations
Future trend towards thinner laminations / higher Si-content (more brittle) for lower losses / higher speeds
2. Scope
Investigation & Optimisation of cutting process forprototyping / batch manufacturing non-oriented electrical steels using SINGLEMODE kW-class fibre lasers
Comparison of magnetic performance against conventional guillotined parts
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Application introduction
electrical machine application – core manufacturing
cut rotor & stator laminations
stack production: cut electrical steel laminations are assembled into magnetic cores by automated stacking, riveting, welding or sticking
copper/aluminium wire inserting
Reference Sample geometry, parallel / perpendicular to rolling direction (RD)
Chosen to represent stator equivalent geometry
Reference sample geometry and cutting direction Figure: example core design
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SPI’s kW OEM Laser Platform
Pumped laser cavity with single mode/ multimode delivery fibre.
Integrated driver boards and local control unit provide status and integrity monitoring and self protection capability.
Customer supplied PSU and laser control system.
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kW Single-Mode Power scaling: 2-Stage GTWave® Architecture
All-fibre, integrated monolithic design:-
High-brightness beam-combined pump modules (~400W / module)
Power Scaled to 500W Oscillator and 1kW 2-stage MOPA
HR OC
Seed Laser Power Amplifier
QBH / LLK-DBDO
HB PumpModule
HBPump
Module
HB PumpModule
HB PumpModule
HB PumpModule
RAL
PD PD
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kW OEM Laser: Power Linearity
Instantaneous output power vs set current
Output Power vs Pump Current (%)
0
200
400
600
800
1000
1200
0% 20% 40% 60% 80% 100%
Percentage Drive Current
Out
put P
ower
, Wat
ts
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0
200
400
600
800
1000
0 200 400 600 800 1000
Pow
er(W
)
Time (hrs)
kW OEM Laser: Open-Loop Power Stability(1000hr soak test @ rated power)
Open-loop operation for 1000hrs+ at rated current / power
Output Power vs Pump Current (%)
0
200
400
600
800
1000
1200
0% 20% 40% 60% 80% 100%
Percentage Drive Current
Out
put P
ower
, Wat
ts
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Digital Modulation: 1000W modulation response @ 10kHz
Analogue Modulation: 1000W SineWave response @ 5kHz
kW OEM Laser: Modulation response(for PSO-controlled cutting)
0
0.05
0.1
0.15
0.2
0.25
0.3
0 50 100 150 200Time (usec)
Pow
er (a
rb u
nits
)
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
Volta
ge (V
)
SignalTrigger
0
0.02
0.04
0.06
0.08
0.1
0.12
0.14
0.16
0.18
0.2
0 100 200 300 400 500
Time (usec)
Pow
er (a
rb u
nits
)
0
2
4
6
8
10
12
Volta
ge (V
)
SignalSet Point
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Precitec
Laser Mechanisms Inc (LMI)
Laser Cutting Test Cell & Cutting Heads
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Process Investigations
Configuration / Parameter Defined value / range Process Rationale
Spot Size, 1/e2, µm Nominal 25µm Optimised kerf width, low HAZ
Process Assist gas Nitrogen, pressure controllable over range 0 bar to 14 bar
Inert gas to avoid additional thermal input / oxidisation
Beam Source Power Range, Watts
200W 1000W Key variable for process optimisation
Cutting Speed, m/min 20m/min 35m/min Key variable, targeting maximum
Cut sample dimensions, mm x mm x mm
250 x 30 x 0.35 and 60 x 60 x 0.35
Standard sample size for subsequent magnetic characterisation
Cut quality assessment Visual (Cut edge / HAZ / dross) Subjective based on criteria for cutting non-electrical steels
> 40 different process conditions were tested, always on M330 steel, 0.35mm thick
Preferred conditions chosen on the basis of cut quality / process stability / speed
For the selected processes, full range of samples at varying angles to the material’s rolling direction were produced for subsequent characterisation by Fraunhofer IWS
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Beam Characterisation: Focused Spot Measurement using PRIMES MicroSpot Monitor
Cutting Head Optical ConfigurationCollimator: 100mm FLFocus lens: 125mm FL
Nominal Spot Size: 25μm
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Fine Kerf Precision Cutting of Magnetic steels:Advantages of Single-Mode
-0.10-0.050.000.050.100.150.200.250.30
Bea
m D
iam
eter
Distance above Focus, mm
Beam Quality Advantage of Singlemode for Fine Kerf Cutting
25um Spot, BPP = 5, M2 ~1525um spot, BPP = 1, M2 ~ 325um spot, BPP = 0.35, M2 ~ 1.0525um Spot, Rayleigh Diameter (35.0um)
Rayleigh Range
0.31mm
0.11mm 0.03mm
25.0µm beamwaist
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Focused Beam Source Characterisation:Thermal Lensing & Rayleigh Range
0.4
0.42
0.44
0.46
0.48
0.5
25
26
27
28
29
30
0 200 400 600 800 1000
Rayleigh
Range, m
m
1/e2spot size, µm
Power, Watts
Thermal Lensing Characterisation(100/125 Lens configuration)
1/e2 Spot Size, um
Rayleigh Range, mm
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Cut-Sample Characterisation: Experimental strategy
Hysteresis loops of various samples
1. fibre laser cut
2. conventionally cut by guillotine
Measuring strategy
1. specific core loss – index for efficiency
2. required magnetic field strength to reach certain polarisation – index for maximum torque & power increase
Cross section investigation
Edge quality / HAZ / metallurgical tests
Hysteresis loop measured with Brockhaus SST
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Cutting Results
Lateral Cross-Section M330 Steel sheet
Fibre laser cut Conventionally cut by guillotine
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Magnetic Parameter Determination
acquired from Hysteresis Loops
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Discussion of results – Magnetic Performance
1. The variation of specific core losses with magnetic polarisation was not
influenced by the choice of the cutting technique employed
2. SM FL processed samples might require a lower magnetic field to achieve
certain polarisation (“torque”)
consequently less electrical current in the copper windings;
this potential benefit would be achieved without any further
significant core losses in the corresponding range of
polarisation
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Conclusions
1. Singlemode fibre-laser-cut parts can match or exceed the magnetic
performance of conventionally processed guillotine-cut parts.
2. Cut-edge quality of the FL-cut parts was dross-free and burr-free, and the
corner edge of the laser-cut parts showed less deformation than
mechanically stamped material.
Further work is required to assess downstream wire-winding
process / cut-through risks etc
3. The assembly of multi-layer laminations should be significantly improved
using laser-cut parts (“stacking factor”).
