Visualisation of shielding gas flows during high-value manufacture

This thesis is a collection of experimental and theoretical analyses of the behaviour of inert gases during material processing. The approach taken is the use of a combination of schlieren imaging and numerical simulations to understand the physical mechanisms associated with the gas flows in each process. The visualisations carried out experimentally were used to validate the models, while the models aided in the interpretation of the imaged refractive index gradients. For gas metal arc welding (GMAW), the variation of Ar input flowrate with varying torch angle, standoff and joint type was investigated. Magnetohydrodynamic (MHD) models of the arc and electrodes showed that air entrainment was determined by the interplay between the momentum in the shielding gas stream and the inwards pull of Lorentz forces which develop within the plasma jet. Good agreement was found between the images and model, showing that gas coverage decreased at values below 9 l/min. Torch angle and standoff were shown to not significantly influence coverage. Similar coverage was found to occur in bead on plate and fillet welds under the same conditions. Further experiments using flux-cored, gas shielded arc welding (FCAW-G) and 80% Ar / 20% CO2 gas allowed good quality welds to be deposited with flowrates as low as 3 l/min. These results supported the use of flow controllers in production welding units at BAE systems Govan, leading to cost savings and reduced environmental impact by locking the gas flowrate to 12 l/min. A study of gas tungsten arc welding (GTAW) using alternating Ar and He shielding gases as a method of arc pulsing was also investigated. The effects of pulsing frequency and input flowrate were investigated. When pulsing, it was found that alternating the gases resulted in He constriction close to the arc region due to the preceding Ar pulse. Comparison of weld macrographs showed that He can be used more efficiently through alternating technique compared to a premixed gas with the same He content. The schlieren system was used to analyse the flow of Ar from a trailing shield device and plasma arc welding (PAW) torch in the context of wire-arc additive manufacture (WAAM). Flow characterisation with changes in standoff and welding configuration showed that air entrainment can be minimised when using a trailing shield. However, increasingly tall parts were insufficiently covered due to interactions between fast jets from the torch and shielding gas streams. MHD modelling of the torch allowed the characterisation of heat transfer and O2 levels with varying input current or over different geometrical features. The final study in this thesis concerns the fluid-particle interactions in laser powder-bed fusion (LPBF). High-speed direct and schlieren imaging showed that differences in laser plume orientation arise from different process settings, even under the same energy input. It was shown that the denudation of the powder bed was caused by drag forces acting on particles, due to atmospheric gas flow induced by the plume. Numerical modelling was in good agreement with the experiments, indicating that evaporative phenomena are an integral part of the heat and mass transfer in LPBF

Visualisation and optimisation of shielding gas coverage during gas metal arc welding

The density gradients and flow characteristics of the gas shield during gas metal arc welding (GMAW) of DH36, higher strength ‘construction steel’ were visualised using schlieren imaging. A systematic study was undertaken to determine the effect of shielding gas flow rate, as well as changes in the nozzle stand-off and angle, on the weld quality. The schlieren images were used to validate 2D and 3D magnetohydrodynamic (MHD) finite element models of the interaction between the Ar shielding gas, the arc and the ambient atmosphere. Weld porosity levels were determined through x-ray radiography. Sufficient shielding gas coverage was provided at a minimum of 9 l/min pure Ar, irrespective of relatively large increases in the nozzle stand-off and angle. Using 80% Ar/20% CO2 shielding gas, and 86% Ar/12% CO2/2% O2 shielding gas with flux cored arc welding (FCAW-G), achieved good quality welds down to 5 l/min. The introduction of 12 l/min in production welding has been implemented with no compromise in the weld quality and further reductions are feasible