Precise Prediction of Workpiece Distortion during Laser Beam Welding
Komkamol Chongbunwatana
MZH Gebäude, Universität Bremen, 14 July 2009
2
Outline
Laser Welding Process Overview
Objective
Simulation Model
Work in Progress
3
Outline
Laser Welding Process Overview
Objective
Simulation Model
Work in Progress
4
Laser Welding: Deep Penetration Welding Characteristics
Definition: A keyhole fusion welding technique achieved with the very high power density obtained by focusing a beam of laser light to a very fine spot
Energy absorption by multiple beam reflection inside the keyhole
Keyhole held open by the vapour pressure
MultiplelyReflectedLaserBeam
FormedKeyhole
Laser Beam
Workpiece
Solidified WeldBead
EvaporatingMaterial
Flowing MoltenMaterial in theWeld Pool
Welding Direction
5
Laser Welding: Why Laser Welding?
Advantages Disadvantages
Low heat input - low thermal distortion - reduced metallurgical damage e.g. grain growth
Precise beam joint alignment
High welding speed due to high beam power density - high production rates
Close fitting and clamped joints
High process flexibility - applicable to all welding position
High equipment and operating costs
Welding thick workpiece in one pass - weld penetration depth limited by available laser power (not by conductivity of the workpiece material)
Not portable (workshop-based) due to the large size of the equipments (e.g. power supplies) and compulsarily controlled environment due to safety reasons
6
Laser Welding: Weld Cross Section
2. Metal Arc Welding (D/H = 2.6)
5 mm1.7 mm
1.9
mm
11.7
mm
7
Laser Welding: Cause of Distortion in Welding
Stress-Strain DiagramStress (σ)
PlasticElas-tic
Yield
Stress
(σ )Y
slope = E
A FF
L ΔL
σ =FA
ε =ΔLLStrain (ε)
1
QA B C
1
2
3
1
2
3
Bars A and C
Bar B
Distortion Caused by Inhomogeneous Temperature Distribution
2 3
Initial Heating Cooled Down
8
Outline
Laser Welding Process Overview
Objective
Simulation Model
Work in Progress
9
Objective
Developing a precise simulation model for workpiece distortion prediction
Determination of relationship between weld geometry and degree of workpiece deformation
Solution:
„Coupling of fluid dynamics simulation (accurate heat source)and solid structure simulation (calculating workpiece distortion)”
10
Outline
Laser Welding Process Overview
Objective
Simulation ModelCoupling PrincipleSolid Structure SimulationFluid Flow SimulationAlberta Implementation
Work in Progress
11
Coupling Principle
Stationary Heat Source:
Dynamic Heat Source:
Fluid flow simulation with given constant boudary conditions
Solid structure simulation with the temperature gradient gained from the fluid flow simulation
Fluid flow simulation with current timestep boundary conditions from the solid structure simulation
Solid structure simulation with the temperature gradient gained from the fluid flow simulation
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Outline
Laser Welding Process Overview
Objective
Simulation ModelCoupling Principle Solid Structure SimulationFluid Flow SimulationAlberta Implementation
Work in Progress
13
Solid Structure Simulation: Thermal Calculation
Heat Equation
T : Temperature [°C] ρ(T) : Temperature-dependent density [kg/mm3] k(T) : Temperature-dependent thermal conductivity [W/mm°C] h(T) : Temperature-dependent thermal transfer
coefficient (air) [W/mm2°C]
∂T∂ t
− ∇⋅kT
T cpT ⋅∇ T = 0 on × 0,
T = T source on D × 0,
−kT ⋅∇T ⋅n = hT ⋅T−T0 on N × 0,
T x ,0 = T 0 on
cp(T) : Temperature-dependent specific heat [J/kg°C]T
0: Room temperature [°C]
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Solid Structure Simulation: Mechanical Calculation
Momentum Equation (Displacement Calculation)
σ: stress [N/mm], u: displacement [mm], εp: plastic strain [-]
Plasticity Theory (Stress and Plastic Strain Calculation)
−∇⋅ = 0 on × 0, ; = u,T , p
u = 0 on D × 0,
⋅n = 0 on N × 0,
σ
εp1
ε1
εp0
ε(u)
0
1'
1 Radial Return
Mapping
Isotropic strain hardeningemployed
Radial return mappingemployed
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Outline
Laser Welding Process Overview
Objective
Simulation ModelCoupling Principle Solid Structure SimulatoinFluid Flow SimulationAlberta Implementation
Work in Progress
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Fluid Flow Simulation (Research State)
17
Outline
Laser Welding Process Overview
Objective
Simulation ModelCoupling Principle Solid Structure SimulatoinFluid Flow SimulationAlberta Implementation
Work in Progress
18
Alberta Implementation
ALBERTA ConceptAdaptive Finite Element Toolbox (simplex elements)Refinement technique: BisectioningError estimator type: Residual error estimator
Applied Refinement and Coarsening Strategy (Solid Struture)Strategy: Implicit (time step control) and equidistribution (element size control) strategyError estimator: Based solely on the heat equation
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Outline
Laser Welding Process Overview
Objective
Simulation Model
Work in Progress
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Work in Progress: Project Status
Modified solid structure simulation model from the forerunner project obtained with a simplified analytical keyhole model (calculated keyhole geometry)
Fluid flow simulation model not yet created (research stage)
Coupling not yet researched
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Work in Progress: Experiment and Simulation Setup
Simulation Setup
Workpiece: 200x50x10 mm Keyhole Model: Jüptner Welding Speed: 50 mm/s Thermal BCs: Laser Power: 3000 W - 3000 °C (vapour) inside keyhole Weld Length: 140 mm - Radiation and convection Cooling Time: 12 s Mechanical BCs: Supports at 3 points
Analytical Axisymmetrical Keyhole Geometry Model
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Work in Progress: Simulation Results
Temperature Development
0 5 10 15 20 250
50
100
150
200
250
300
350
400
9 mm from the Centre
EXPSim
Time [s]
Tem
per
atu
re [°
C]
0 5 10 15 20 250
50
100
150
200
250
300
350
400
3 mm from the Centre
EXPSim
Time [s]
Tem
per
atu
re [°
C]
0 5 10 15 20 250
50
100
150
200
250
300
350
400
at the Centre
EXPSim
Time [s]
Tem
per
atu
re [°
C]
Temperature history of three different points measured on the top surface of the workpiece
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Work in Progress: Simulation Results
Workpiece Deformation
-100.00 -50.00 0.00 50.00 100.00-0.15
-0.10
-0.05
0.00
0.05
0.10
Experiment
Y = -20Y = -14Y = -10Y = 10Y = 14Y = 20
Distance along X-Axis [mm]
Dis
pla
cem
en
t in
Z-A
xis
[mm
]
-100.00 -50.00 0.00 50.00 100.00-0.15
-0.10
-0.05
0.00
0.05
0.10
Simulation
Y = -20Y = -14Y = -10Y = 10Y = 14Y = 20
Distance along X-Axis [mm]
Dis
pla
cem
ent
in Z
-Axi
s [m
m]
Deformation of the workpiece indicated by the displacement in Z direction measured on the top surface
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