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

12

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]

14

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

15

Outline

Laser Welding Process Overview

Objective

Simulation ModelCoupling Principle Solid Structure SimulatoinFluid Flow SimulationAlberta Implementation

Work in Progress

16

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

19

Outline

Laser Welding Process Overview

Objective

Simulation Model

Work in Progress

20

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

21

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

22

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

23

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

24

Last Page

Thank You for Your Attention