Laser is an efficient and widely used heat source in metal processing suchas welding and additive manufacturing. It has some great advantages compared to the other conventional heat sources like electron beam and arc namely: ability of handling complicated joint geometries and producing large components. Laser beam welding encompasses many complex physical phenomena such asheat transfer, metal melting, flow and solidification, free surface deformation, evaporation and possibly vaporization. The aim of this research work istwo-fold: gain deeper process understanding and improve the model reliability. Deeper process understanding is sought on the effect of beam shaping on themelt pool. To achieve improved model reliability, a good support consists in using qualitative experimental data representing the process. Thus, 3D validation of the melt pool geometry is performed while it was usually 2D inprevious research works. Furthermore, a new calculation procedure for laser absorption is introduced. To conduct this research work, a Computational Fluid Dynamics approach is used. A solver, capable of tracking the deformation of the melt free surface, is developed in OpenFOAM. Concerning beam shaping, it is found that not only the melt pool size as previously known but also the melt flow pattern is modified through elongating the beam shape.This last result could not be revealed by former studies as the non-transparent media hinders optical observation. New in-process quantitative measurements performed by a project partner are used to test the model. Weaknesses of the former absorptivity models are highlighted, as well as the limitations of the proposed model. Finally, the results show that the proposed absorptivity model function of local surface conditions leads to much better agreement with experimental results compared to the former constant absorptivity model. The maximum discrepancy compared to the experimental measurement, which is observed for the melt pool depth, can indeed be reduced to about 10%.

Laser är en effektiv och allmänt använd värmekälla vid svetsning och additiv tillverkning. Den har några viktiga fördelar jämfört med andra konventionella värmekällor såsom elektronstråle och elektrisk ljusbåge, nämligen: den kan ofta användas till komplicerade svetsgeometrier, och den kan producera stora komponenter. Lasersvetsning involverar olika sammansatta fysikaliska fenomen såsom värmeöverföring, metallsmältning, flöde, stelning, ytdeformation, avdunstning och i vissa fall förångning. Syftet med mitt forskningsarbete är tvåfaldigt: att få en djupare processförståelse och att förbättra modellens tillförlitlighet. Fördjupad processförståelse eftersträvades för att förstå hur formen på laserstrålen påverkar svetssmältan. För att uppnå förbättrad modellsäkerhet behövs experimentella data av hög kvalitet som representerar processen. Således utfördes 3D-validering av smältgeometrin medan det vanligtvis var 2D i tidigare forskningsarbeten. Dessutom har en ny modell för laserabsorption föreslagits. I forskningen har numerisk strömningssimulering (Computational Fluid Dynamics) använts för att simulera processen och en numerisk lösare, som kan spåra deformationen av den rörliga smälta ytan, är utveckladi programvaran OpenFOAM. Beträffande laserstrålens utbredning visar resultaten att svetssmältans storlek och även svetssmältansflöde modifieras genom att laserstråleformen förlängs. Medan den förra är känd från tidigare experimentella studier upptäcktes den senare inte före denna studie eftersomdet icke-transparenta mediet hindrar optisk observation. Nya (in-process) kvantitativa mätningar utförda av en projektpartner har använts för att testa modellerna. Svagheter i den tidigare absorptionsmodellen framhävdes, liksom begränsningarna i den föreslagna modellen. Slutligen visade resultaten att den föreslagna modellen där laserabsorptionen är en funktion av lokala ytförhållanden ledde till en bättre overensstämmelse med mätningar jämfört med den tidigare modellen med konstant laserabsorbtion. Den maximala avvikelsen jämfört med experimentell mätning, som observerades med avseende på smältbassängsdjupet, kunde reduceras till cirka 10%.

Laser metal fusion is widely used in production technology to manufacture parts, as in welding, cladding, and additive manufacturing. In this study, conduction mode laser metal fusion is applied without and with metal deposition from a wire feedstock. This manufacturing process encompasses various physical phenomena that are coupled, such as the interaction of anelectro-magnetic wave with the material, phase changes, thermal fluid dynamics, and free surface deformation, which make it complicated to comprehend.

Deeper process knowledge is thus a key to its improvement. Yet, metal is a non-transparent media, which limits experimental observation of this process.

A modelling approach that describes this multi-physics problem paying special attention to convective phenomena was used in this thesis with a two-fold aim:

1) to improve the model reliability,

2) to gain a deeper understandingof the metal fusion and deposition process.

In the first part of this research, metal fusion without wire was addressed. Different beam power density distributions (beam shapes) were investigated. Their effect on the melt pool geometry, which was known from previous experimental studies, could be predicted. Furthermore, as the simulations give access to the melt flow, it could be established that the flow pattern is modified by elongating the beam shape. In addition, a new calculation procedure was introduced to predict the fraction of laser beam energy absorbed by the metal. To validate the model, the predicted melt pool geometry was evaluated through comparison with experimental measurements. The results showed that the proposed absorptivity model that is a function of local surface conditions lead to good agreement with experimental results, with a maximum discrepancy for the melt pool depth of about 10%.

In the second part, the model was applied to study the fusion process with metal transfer from a wire feedstock without and with resistive heating of the filler wire. It was shown that the multipler eflections of beam rays could be ignored at a low laser beam angle whereas with increasing the beam angle the effect became more considerable. It was also found that the laser absorptivity varied up to 50% within the projected laser spot area. The effect of different process parameters such as depositing rate and angle, laser beam angle, position of the wire relative to the beam (offset), and ambient conditions on the metal transfer, thermal flow field, andstability of the process were studied.

The results showed that three different metal transfer modes occurred depending on the offset value. Applying resistive heating on the filler wire decreased the absorptivity. However, this decrease was compensated by the resistive heating, resulting in an increase of the volume of liquid metal. Resistive heating made the melt pool wider due to the augmented role of the thermocapillary force and also the change in flow direction because of the modified position of the melted wire front.

Applying the model at near-vacuum and no gravity conditions, it was obtained that directed energy deposition of metal with laser and wire could be used for manufacturing metal parts in space. However, the process window could need some adjustment as in-space conditions result in some narrowing of the liquid bridge between wire and workpiece compared to on-Earth.