Laser metal fusion and deposition using wire feedstock : Process modelling and CFD simulation

Abstract: 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.

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