Conduction Laser Welding

JOSEFINE SVENUNGSSON Licentiate Thesis Production Technology 2019 No. 27 Conduction laser welding modelling of melt pool with free surface deformation Conduction laser welding Laser welding is commonly used in the automotive-, steel- and aerospace industry. However, deeper knowledge is still needed to better control this process, improve prod- DEFORMATION POOL WITH FREE SURFACE WELDING - MODELLING OF MELT LASER CONDUCTION modelling of melt pool with free surface deformation uct quality, produce components with less material, reduce production errors and thus contribute to sustainable manufacturing production. Process knowledge can be gained through modelling and experimental observation. The present study therefore aimed at developing and testing a simulation model dedicated to the thermal flow and free surface deformation of the melt pool formed during laser welding. The physics implemented in the model includes the thermocapillary force that accounts Josefine Svenungsson for the effect of temperature gradients on surface tension, the solid-liquid phase change, and the convection of fusion enthalpy. From the computed test cases, it was found that the convection of fusion enthalpy should not be neglected. It was also found that the numerical implementation of the thermocapillary force can lead to unphysical solutions. Therefore, it is recommended to select an approach consistent for all the surface forces of the problem. Finally, free surface oscillations known from experiments to occur are also computed outputs of the model. However, it remains to investigate whether these oscillations are, or not, disturbed by numerical noise. 2019 NO.27 ISBN 978-91-88847-35-5 (Printed version) ISBN 978-91-88847-34-8 (Electronic version) 118526_HV_Josefine_Svenungsson_omslag_190516_v2.indd 1 2019-05-16 09:32:15 Tryck: BrandFactory, maj 2019. Licentiate Thesis Production Technology 2019 No. 27 Conduction laser welding modelling of melt pool with free surface deformation Josefine Svenungsson Department of Engineering Science University West SE-461 86 Trollhättan Sweden Telephone +46 (0)52 – 022 3000 www.hv.se c Josefine Svenungsson, 2019. ISBN 978-91-88847-35-5 (Printed version) ISBN 978-91-88847-34-8 (Electronic version) Typeset by the author using LATEX. Trollhättan, Sweden 2019 to my family Sammanfattning Titel: Laser konduktionssvets - modellering av smältpöl med deformation på ytan Nyckelord: Lasersvetsning, modellering, strömningslära, CFD, OpenFOAM, ytdeformation, numerisk validering ISBN 978-91-88847-35-5 (Printed version) ISBN 978-91-88847-34-8 (Electronic version) Lasersvetsning är en vanlig produktionsmetod inom t.ex. bil-, stål- och fly- gindustrin. Det är en icke-linjär och starkt kopplad process där svetsgeometrin påverkas av flödet i smältan. Experimentella observationer är utmanande då smältpölen under ytan inte är tillgänglig under svetsning. Förbättrad pro- cesstyrning skulle möjliggöra att bibehålla, eller förbättra, produktkvaliteten med mindre material och bidra vidare till hållbarhet genom minskade produk- tionsfel. Numerisk modellering med, Computational Fluid Dynamics, CFD, ger en djupare förståelse med tillgång till processegenskaper som ännu inte kan nås vid experimentell observation. De befintliga numeriska modellerna brister dock i förutsägbarhet. Arbetet som presenteras här består av utveckling av en numerisk modell för konduktionssvetsning. Genom arbetet som presenteras här har en modell för lasersvetsning utveck- lats baserat på en existerande modell. Därefter tillämpas modellen på olika testfall för att undersöka specifika delar av processfysiken som implementerats. Två fall fokuserar på termokapillär konvektion i tvåfas- och trefasflöden med ytdeformation. Slutligen betraktar det tredje testfallet smältan och flödet i denna under konduktionssvetsning. Slutsatsen av studien är att konvektion av entalpin, försummad i tidigare studier, ska inkluderas i modellen. Implementering av termokapillärverkanbör vara konsekvent med de andra ytkrafterna för att undvika icke-fysikaliska lös- ningar. Instabiliteter på den fria ytan, kända från experimentella observationer, beräknas också numeriskt. Ytterligare studier behövs för att kontrollera att dessa oscillationer inte påverkas av numeriska instabiliteter. i Abstract Title: Conduction laser welding - modelling of melt pool with free surface deformation Language: English Keywords: Conduction laser welding, numerical modelling, Computational Fluid Dynamics, OpenFOAM, free surface deformation, melt pool ISBN 978-91-88847-35-5 (Printed version) ISBN 978-91-88847-34-8 (Electronic version) Laser welding is commonly used in the automotive-, steel- and aerospace industry. It is a highly non-linear and coupled process where the weld geometry is strongly affected by the flow pattern in the melt pool. Experimental observations are challenging since the melt pool and melt flow below the surface are not yet accessible during welding. Improved process control would allow main- taining, or improving, product quality with less material and contribute further to sustainability by reducing production errors. Numerical modelling with Computational Fluid Dynamics, CFD, provides complementary understanding with access to process properties that are not yet reachable with experimental observation. However, the existing numerical models lack predictability when considering the weld shape. The work presented here is the development of a model for conduction laser welding. The solver upon which the model is based is first described in detail. Then different validation cases are applied in order to test specific parts of the physics implemented. Two cases focus on thermocapillary convection in two-phase and three-phase flows with surface deformation. Finally, a third case considers the melt pool flow during conduction mode welding. It is concluded that the convection of fusion enthalpy, which was neglected in former studies, should be included in the model. The implementation of the thermocapillary force is recommended to be consistent with the other surface forces to avoid unphysical solution. Free surface oscillations, known from experimental observations, are also computed numerically. However, further investigation is needed to check that these oscillations are not disturbed by numerical oscillations. ii Nomenclature αC omax maximum alpha Courant number α thermal diffusivity β thermal expansion coefficient ∆H 0 standard heat of adsoprtion ∆T temperature difference εn stabilization factor dσ dT thermocapillary coefficient Γ thermocapillary coefficient µ dynamic viscosity ν kinematic viscosity φf face volume flux ρ density ρ∗ dimensionless density σ capillary (or surface tension) coefficient τ stress tensor c interface curvature ~g gravitational acceleration n~f interface normal vector u~ single fluid velocity Cα compression factor iii NOMENCLATURE C omax maximum Courant number fL liquid fraction h∗ dimensionless enthalpy k1 entropy factor Ma Mach number SL∗ source term for latent heat t ∗ dimensionless time Tm melting temperature u∗ dimensionless velocity C permeability coefficient Cp specific heat Co Courant number h heat transfer coefficient k thermal conductivity Lf latent heat of fusion p static pressure T temperature V characteristic velocity of flow iv Contents Sammanfattning i Abstract ii 1 Introduction 1 1.1 Background and motivation ........................... 1 1.2 Problem and aim of work ............................ 3 1.3 Objective and research questions ....................... 5 1.4 Limitations .................................... 5 1.5 Methodology/Strategy .............................. 5 1.6 Outline ...................................... 6 2 Laser welding manufacturing process 7 2.1 Fusion welding .................................. 8 2.1.1 Arc welding ............................... 8 2.1.2 Power beam welding ......................... 9 2.1.3 Hybrid welding ............................. 10 2.2 Laser beam welding ............................... 10 2.2.1 Laser energy sources applied to welding . 11 2.2.2 Laser welding process ........................ 12 3 State of the art of laser welding models 21 3.1 Melt pool models neglecting fluid flow ..................... 21 3.2 CFD melt pool models for conduction mode laser . 22 3.3 Coupled models from other fields than laser welding . 29 3.4 Interface capturing ................................ 29 3.5 Implementation of the forces applied at the free surface . 33 4 The interFoam solver 37 4.1 MULES ...................................... 38 4.2 Pressure equation ................................ 41 4.3 Pressure-velocity coupling ........................... 43 4.4 InterFoam solution algorithm .......................... 43 v CONTENTS 5 The melt pool model for laser welding in conduction mode 46 5.1 Laser energy source model ........................... 46 5.2 Thermo-fluid model ............................... 50 5.2.1 Mass conservation equation ..................... 50 5.2.2 Momentum conservation equation . 51 5.2.3 Energy conservation equation .................... 54 5.3 New developments made in interFoam .................... 55 6 Numerical applications - results and discussion 57 6.1 Two-phase flow driven by thermocapillary force . 57 6.1.1 Test case description ......................... 58 6.1.2 Numerical setting ........................... 58 6.1.3 Results and discussion ........................ 59 6.1.4 Conclusion ............................... 64 6.2 Three-phase flow driven by thermocapillary force . 64 6.2.1 Test case description ........................

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