Numerical Analysis of the Heating Characteristics of Magnetic Oscillation Arc and the Fluid Flow in Molten Pool in Narrow Gap Gas Tungsten Arc Welding

Magnetic oscillation arc (MOA) technology was developed to avoid insufficient fusion defects appearing at the sidewalls in narrow gap gas tungsten arc welding (NG-GTAW). In this work, a unified model was developed to simulate the process of MOA assisted NG-GTAW. The model included the MOA, welding pool, workpiece and the coupling interaction between them. The heating characteristic of the MOA and the flow of liquid metal were simulated, and the mechanism of forming a uniform welding bead under MOA was investigated. It was found that if the magnetic flux density increased to 9 mT, the MOA could point to the sidewall directly; the maximum heat flux at the bottom declined by almost half and at the side, it increased by more than ten times. Additionally, the heat flux was no longer concentrated but dispersed along the narrow groove face. Under the effect of MOA, there were mainly two flow vortexes in the molten pool, which could further increase the heat distribution between the bottom, sidewall and corner, and was beneficial for the formation of a good-shape weld. The model was validated by experimental data.

CO line and radio continuum study of elephant trunks: the Pillars of Creation in M16

ABSTRACT Molecular line and radio continuum properties of the elephant trunks (ET, Pillars of Creation) in M16 are investigated by analysing 12CO(J = 1−0) , 13CO(J = 1−0) and C18O(J = 1−0) line survey data from the Nobeyama 45-m telescope and the Galactic plane radio survey at 20 and 90 cm with the Very Large Array. The head clump of Pillar West I is found to be the brightest radio source in M16, showing a thermal spectrum and the properties of a compact H ii region, with the nearest O5 star in NGC 6611 being the heating source. The radio pillars have a cometary structure concave to the molecular trunk head, and the surface brightness distribution obeys a simple illumination law from a remote excitation source. The molecular density in the pillar head is estimated to be several 104 H2 cm−3 and the molecular mass is $\sim 13\!-\!40 \, \mathrm{M}_\odot$. CO-line kinematics reveals random rotation of the clumps in the pillar tail at ∼1–2 km s−1, comparable with the velocity dispersion and estimated Alfvén velocity. It is suggested that the random directions of the velocity gradients would manifest as torsional magnetic oscillation of the clumps around the pillar axis.