Precise Prediction of Workpiece Distortion during Laser Beam Welding. Komkamol Chongbunwatana

Transcription

1 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 2

3 Outline Laser Welding Process Overview Objective Simulation Model Work in Progress 3

4 Laser Welding: Deep Penetration Welding Characteristics Laser Beam Evaporating Material Welding Direction Workpiece 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 Solidified Weld Bead Formed Keyhole Flowing Molten Material in the Weld Pool Multiplely Reflected Laser Beam Keyhole held open by the vapour pressure 4

5 Laser Welding: Why Laser Welding? Advantages Disadvantages Low heat input P recise beam joint alignment - low thermal distortion - reduced metallurgical damage e.g. grain growth High welding speed due to high Close fitting and clamped joints beam power density - high production rates 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 5

6 Laser Welding: Weld Cross Section 5 mm 11.7 mm 1.9 mm 1.7 mm 2. Metal Arc Welding (D/H = 2.6) 6

7 Laser Welding: Cause of Distortion in Welding Stress-Strain Diagram L Stress (σ) Elas-tic ield Stress (σ ) Plastic F ΔL A F F A ΔL ε = L σ = slope = E Strain (ε) Distortion Caused by Inhomogeneous Temperature Distribution 2 3 A B C 1 Q Bar B Initial Heating Cooled Down Bars A and C 1 2 7

8 Outline Laser Welding Process Overview Objective Simulation Model Work in Progress 8

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) 9

10 Outline Laser Welding Process Overview Objective Simulation Model Coupling Principle Solid Structure Simulation Fluid Flow Simulation Alberta Implementation Work in Progress 10

11 Coupling Principle Stationary Heat Source: Fluid flow simulation with given constant boudary conditions Solid structure simulation with the temperature gradient gained from the fluid f Fluid flow simulation with current timestep boundary conditions from the solid s Dynamic Heat Source: Solid structure simulation with the temperature gradient gained from the fluid f 11

12 Outline Laser Welding Process Overview Objective Simulation Model Coupling Principle Solid Structure Simulation Fluid Flow Simulation Alberta Implementation Work in Progress 12

13 Solid Structure Simulation: Thermal Calculation Heat Equation k T T T = 0 t T c p T T = T source k T T n = h T T T 0 T x,0 = T 0 T ρ(t) k(t) h(t) : : : : cp(t) : T0 : on 0, on D 0, on N 0, on Temperature [ C] Temperature-dependent density [kg/mm3] Temperature-dependent thermal conductivity [W/mm C] Temperature-dependent thermal transfer coefficient (air) [W/mm2 C] Temperature-dependent specific heat [J/kg C] Room temperature [ C] 13

14 Solid Structure Simulation: Mechanical Calculation Momentum Equation (Displacement Calculation) = 0 on 0, u = 0 on D 0, n = 0 on N 0, p ; = u,t, σ: stress [N/mm], u: displacement [mm], εp: plastic strain [-] Plasticity Theory (Stress and Plastic Strain Calculation) 1' σ 1 0 Radial Return Mapping ε(u) ε p 0 p 1 ε ε1 Isotropic strain hardening employed Radial return mapping employed 14

15 Outline Laser Welding Process Overview Objective Simulation Model Coupling Principle Solid Structure Simulatoin Fluid Flow Simulation Alberta Implementation Work in Progress 15

16 Fluid Flow Simulation (Research State) 16

17 Outline Laser Welding Process Overview Objective Simulation Model Coupling Principle Solid Structure Simulatoin Fluid Flow Simulation Alberta Implementation Work in Progress 17

18 Alberta Implementation ALBERTA Concept Adaptive Finite Element Toolbox (simplex elements) Refinement technique: Bisectioning Error estimator type: Residual error estimator Applied Refinement and Coarsening Strategy (Solid Struture) Strategy: Implicit (time step control) and equidistribution (element size control) strategy Error estimator: Based solely on the heat equation 18

19 Outline Laser Welding Process Overview Objective Simulation Model Work in Progress 19

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 20

21 Work in Progress: Experiment and Simulation Setup Simulation Setup Analytical Axisymmetrical Keyhole Geometry Model Workpiece: 200x50x10 mm Welding Speed: 50 mm/s Laser Power: 3000 W Weld Length: 140 mm Cooling Time: 12 s Keyhole Model: Jüptner Thermal BCs: C (vapour) inside keyhole - Radiation and convection Mechanical BCs: Supports at 3 points 21

22 Work in Progress: Simulation Results Temperature Development Temperature history of three different points measured on the top surface of the workpiece at the Centre 3 mm from the C entre EXP Sim EXP Sim T emperature [ C] 400 T emperature [ C] T emperature [ C] 9 mm from the C entre T ime [s] T ime [s] EXP Sim T ime [s]

23 Work in Progress: Simulation Results Workpiece Deformation Deformation of the workpiece indicated by the displacement in Z direction measured on the top surface S imulation Distance along X-Axis [mm] = -20 = -14 = -10 = 10 = 14 = 20 Displacement in Z-Axis [mm] Displacement in Z-Axis [mm] E xperiment = -20 = -14 = -10 = 10 = 14 = Distance along X-Axis [mm]

24 Last Page Thank ou for our Attention 24