  This thesis presents a novel multiphysics solver, named gtawFoam, for Gas
Tungsten Arc Welding (GTAW) that is applied to simulate orbital GTAW on
ultra-thin-walled titanium tubing. In this thesis, ultra-thin-walled tubing
refers to tubing where the wall thicknesses are less than 500 $\mu m$. Orbital
welding of tubing with this wall thickness requires both a sufficient heat
input to weld the tubing and an internal buttressing gas flow to ensure the
tube retains its geometrical integrity. The specific use case is for the
commercially pure grade 2 titanium tubing used in the ATLAS ITk cooling system
which is 2.275 $mm$ outer diameter and 300 $\mu m$ wall thickness at the weld.
The solver is created using the open source computational fluid dynamics
library OpenFOAM and each component of the solver is benchmarked against an
appropriate case. With the solver established, it is used to simulate a series
of welding procedures that were performed experimentally on the aforementioned
titanium tubing. Both the experimental and simulation results show a
`goldilocks' region where the weld heat input and inner buttressing gas flow
are moderated to a level where a fully penetrating weld is created but the
geometric integrity of the tube is not compromised. gtawFoam is then used to
simulate hypothetical tubing with larger and smaller wall thicknesses between
250 $\mu m$ and 350 $\mu m$. The results suggest that the required buttressing
gas pressure once achieved is relatively transferable between wall thickness
changes but applying enough heat so as to achieve full penetration is critical.
These results are then used to predict effective welding procedures for this
hypothetical tubing. gtawFoam is subsequently applied to the welding of turbine
blades. This includes the addition of multiple layers of filler metal to mimic
additive manufacturing.
