Gas metal arc is a high-efficiency and widely used heat source for metal processing applied predominantly in welding and additive manufacturing. In this study, it was applied to welding. It offers high productivity, low production and investment cost, as well as suffers from some drawbacks such as humping or undercut when welding large parts that are curved and impose changing the orientation of the welding torch along the joint path. Deeper process understanding was therefore sought to mitigate these drawbacks. The difficulty is then the non-lineardependence of the process to the welding parameters and material properties. Besides, visual observation of this process is also difficult. For instance, the elevated temperature and the intense radiative emission from the electric arc, smoke, spatter, as well as the non-transparency of the processed alloy can hinder in-process observation or limit it. Process simulation provides a complementary means to reach process knowledge. It was thus the approach used in this study. For this, a thermo-fluid melt pool model that can predict melting and solidification, track free surface deformation, metal transfer, and coalescence with the melt pool was developed. Two main research questions were identified and addressed.The first one led to studying the effect of the substrate orientation during multilayer welding of a V-groove joint with INVAR and gas metal arc. It was foundthat the force balance in the melt pool changes significantly when the workpieceorientation is changed, resulting in distinct melt flow patterns, melt pool and bead geometries, and in some conditions defect initiation such as humping, undercut, and lack of fusion. As a result, multi-layer welding with flat substrate and downhill welding of a 20◦ inclined substrate are recommended with these process conditions. On the contrary, welding of a side inclined substrate and uphill welding of a 20◦ inclined substrate are not recommended. The second question gave rise to the comparative investigation of the three electromagnetic force models commonly used when modelling a melt pool produced by an electric arc. The underlying modelling assumptions were retrieved and investigated. It was found that each of these three models predicts a different melt flow pattern, different heat convection, melt pool shape, free surface oscillation, and interaction with the transferred metal drops, and thus result in different bead geometry. All these models can be adjusted to predict the penetration depth, however, only the most complete of them is recommended for developing a predictive melt pool model. For this, it is proposed as a future work to improve this model through predicting an electromagnetic force that takes also into account the local deformation of the free surface.

Gasmetallbåge är en effektiv och allmänt använd värmekälla vid svetsning och additiv tillverkning. I denna studie tillämpas den på svetsning. Den erbjuder hög produktivitet, låg kostnad vid inköp och användning, såväl som vissa nackdelarsom ojämn "bucklig" svetssträng och smältdiken vid svetsning av stora komponenter som är krökta och medför att svetsbrännarens orientering ändras utmed fogen. Bättre processförståelse eftersträvas därför för att mildra dessa nackdelar. En utmaning är processens icke-linjära beroende av svetsparametrarna och materialegenskaperna. Dessutom är experimentell optisk övervakning svår. Till exempel kan den höga temperaturen och den intensiva elektromagnetiska strålningen från ljusbågen, rök, sprut, såväl som legeringens ogenomskinlighet, förhindra observation under processen eller begränsa den. Processimulering erbjuder en komplementär metod för att nå processkunskap. Det är alltså detta tillvägagångssätt som används i denna studie. För detta har en modell av värme och materialflödena i smältan utvecklats som kan prediktera smältning och stelning, spåra smältytans deformation, metallflöde och koalescens med smältan.Två huvudsakliga forskningsfrågor har identifierats och adresserats.

Den första studerade gravitationens påverkan vid flersträngs-, gasmetallbågsvetsning av V-fogar i INVAR. Olika svetslägen har visat sig ha en betydande påverkan på kraftbalanserna i svetssmältan vilket resulterar i distinkta smältflöden, smält- och svetsförbandgeometrier, och under vissa förhållanden svetsdefekter såsom ojämn "bucklig" svetssträng, smältdiken och bindfel. Som ett resultat rekommenderas horisontellt och 20◦ fallande läge vid flersträngssvetsning, medan 20◦ stigande och sidolutande inte rekommenderas.

Den andra frågan undersökte inverkan av de tre huvudsakliga modellerna för den elektromagnetiska kraften som idag används vid svetssimuleringar. För modelleringen har antaganden lagts fram och undersökts. Det visade sig att de tre modellerna predikterar olika flödesmönster i smältan, olika värmekonvektion, smältgeometri, ytvågor och interaktion med de överförda metalldropparna, och därmed också predikterar olika svetsstränggeometrier. Alla tre modeller kan justeras för att prediktera svetspenetrationen, men endast den mest kompletta av dessa rekommenderas för sant prediktiv modellering. Det föreslås också att ytterligare förbättra den mest kompletta modellen så att det elektromagnetiska kraftfältet följer deformationen av den fria smältytan.

Pulsed gas metal arc is a highly efficient technique used in manufacturing processes like welding and additive manufacturing. It offers high productivity and cost benefits but it is also prone to defect formation when process parameters are not properly controlled and optimized. A deeper process understanding can support achieving improved process control and mitigate these potential drawbacks. Nevertheless, there are still several challenges. For instance, the correlation between the input and output process parameters is non-linear and complex due to the multi-physics nature of the process. In addition, the elevated temperature and the intense radiative emission from the arc, along with the smoke, and the non-transparency of metal, make in-process observation challenging. Modelling and simulation offer a complementary approach to gain a deeper process understanding. In this study, a thermo- and fluid dynamics model was developed, focusing on the melt pool and metal deposition, while simplifying the arc to boundary conditions (decoupled approach). This model incorporates various forces and phenomena such as thermocapillary and electromagnetic forces, melting and solidification, and tracking of surface deformation and droplet coalescence.

In the first part of the thesis, the developed model was applied to investigate the effect of workpiece orientations on the melt pool dynamics and reinforced bead geometry in multi-layer gas metal arc welding of a V-groove joint. The comparison of the predicted fusion zone with macrographs obtained from the experiments showed good qualitative agreement. It was found that the force balance in the melt pool changes significantly when changing the workpiece orientation by as little as 20◦ relative to the flat position. This results in distinct melt flow patterns, melt pool shapes, bead geometries, and in some cases, defect formation such as humping, undercut, and insufficient fusion. It was concluded that to avoid these defects a lower angle range is necessary for multilayer welding with the uphill orientation and side inclination.

The second part of the thesis focused on analyzing different variants of the model for the electromagnetic force with a decoupled approach. Three commonly used models were compared: (1) the analytical models proposed by Kou and Sun inintegral form, (2) by Tsao and Wu in algebraic form, and (3) the partial differential equations governing the electric and magnetic fields. The comparative investigation was supported by experimental tests that also provided estimates of unknown model parameters and validation data. It was found that the distinct assumptions on which these models rely are not all justified. They resulted inpredicting different melt flow patterns and amplitude of the free surface oscillations, as well as different melt pool shapes and bead geometries. Model (3) is recommended to advance to a predictive melt pool model and was subsequentlyused in the remaining work of the thesis.

Furthermore, the literature shows that modeling the effect of pulsed arc on the melt pool using a decoupled approach involves various simplifications. Arc pulsation affects energy and force balance in the melt pool through arc heat flux, arc pressure, and electromagnetic force. A systematic investigation of model variants considering pulsing was conducted using previously documented experimental test cases. The results showed that the influence of arc pressure was insignificant in those cases. However, model variants simplifying arc pulsing to a time-averaged effect underestimated the amplitude of the Marangoni flow and downward flow compared to a more comprehensive approach that considered the time dependence of arc pulsation. Thus, it is recommended to use a meltpool model that accounts for the time-dependent arc pulsation, which was also subsequently utilized in the remaining work of the thesis.

The electromagnetic force models discussed earlier assume a stationary free surface when computing the electromagnetic force. However, this force is often at leading order in the vicinity of the arc. In the same region, the metal drop transfer leads to a periodic deformation of the melt pool free surface. In the final part of the thesis, the model was extended to account for free surface deformation when computing the electromagnetic force. This extension was applied to experimental test cases, and a comparison was made with simulation results obtained using the stationary electromagnetic force model. Significant differences in the results were observed, particularly in predicting the experimentally observed fingertip-shaped fusion zone geometry. The proposed improvement in the electromagnetic force model provided better predictions in this regard.

Gasmetallbågsprocessen har revolutionerat metallbearbetning och produktionsteknik under ett århundrade med sin påfallande effektivitet och mångsidighet, speciellt vid svetsning. På senare år har denna teknik också tillämpats alltmer inom additiv tillverkning (AM), även känt som 3D-printing. Gasmetallbågsbaserad AM har väckt ett stort industriellt intresse på grund av dess förmåga att tillverka stora och komplexa komponenter. Det finns dock problem, dels kring defekter i processen, dels kring numeriska modellers förmåga att simulera processen. Följaktligen finns det ett behov av en djupare förståelse och förbättrade modeller för att övervinna dessa utmaningar och frigöra den fulla potentialen i denna teknologi. För att angripa detta problem utvecklades och tillämpades modellering med hjälp strömningsmekaniska beräkningar (CFD) i detta avhandlingsarbete, tillsammans med fysiska experiment för att komplettera modelleringsarbetet. Det utförda modelleringsarbetet har gjort det möjligt att förklara hur flödet i smältan och dess geometri orsakar defekter vid ändringar av ett arbetsstyckesorientering så lite som 20◦ jämfört med horisontell positionering vid svets iV-fog. Dessutom använder moderna numeriska modeller för smältans fysikolika delmodeller för att beräkna den elektromagnetiska kraften och tidsberoendet hos en pulserande ljusbåge. Dessa delmodeller jämförandes i en analys föratt förklara deras signifikanta skillnader vid simulering av smältflödet, termisk konvektion, ytvågor, smältans form och stelnade geometri. De föreslagna förbättringarna i modelleringen baserat på denna analys har gett mer noggranna förutsägelser av processens smältzon, vilket bidrar till utvecklingen av en sant prediktiv simuleringsmodell som kommer att vara användbar i den efterfrågade utvecklingen av gasmetallbågsprocessen.