Abstract
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.
Highlights
Gas metal arc welding (GMAW) uses a flow of argon (Ar), carbon dioxide (CO2), or a mixture thereof, to limit chemical reactions of the molten metal with the surrounding air
The refractive index gradients are primarily due to variations in temperature, pressure and gas concentration averaged through the measurement region. The interdependence of these three parameters makes it difficult to draw quantitative data directly from the schlieren images, they reveal a great deal of qualitative information regarding the flow
The MHD model was validated against the schlieren images and shown to calculate the underlying temperature, pressure and concentration gradients with acceptable accuracy to describe the main features of the flow
Summary
Gas metal arc welding (GMAW) uses a flow of argon (Ar), carbon dioxide (CO2), or a mixture thereof, to limit chemical reactions of the molten metal with the surrounding air. Excess CO is released after solidification of the metal, forming pores Such discontinuities in welded joints reduce the effective cross section and accumulate stresses, constituting potential crack initiation sites. A shield gas flowrate of 15–20 L per minute (l/min) is often used in GMAW, but in practice welders sometimes use as much as 36 l/min Such overuse of shield gas is wasteful, impacts negatively on the environment and can lead to turbulence induced porosity in the weld. Reducing shield gas usage is important in the additive manufacture of metals via directed energy deposition processes based on welding, where localised trailing shield units use flow-rates as high as 195 l/min (Ding et al, 2015). Many models of welding are reported in the literature, and shield gas coverage has been visualised by experimental techniques, there has been no systematic study reported that aims to optimize the shield gas usage
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