GTA process in N2

GTA process in Nitrogen

1) High speed movies(20kfps) at arc ignition stage. Video viewrate is 30fps, so time span enlarges about 667times.
2) 2kfps movie enlarges 67times.
3) 200fps movie enlarges 6.7times.
4) Arc in air at 54kfps. Hot metal is very active for oxygen. Oxcidation reaction plays agressive.
Please feel the difference between air and nitrogen as surrounded atmosphere on oxidation by arc.
Typical high-speed movies
1)20kfps 1)2kfps 1)200fps 4)Air 54kfps
(1)20kfps (2)2kfps (3)200fps (4)Air54kfps
Right pictureas show time chart of welding process in nitrogen shield. Left hand side pictures are chart of 20ms in arc ignition stage. A left upper chart shows horisontal behavior as shown in middle upper picture. A right upper chart shows behavior in 0.2 seconds of igniton stage.
A left lower chart shows vertical behavior of arc ignition stage in 20ms. A right lower picture shows a video behavior in 200ms. Muximum intensities in each pixel are shown among 100 frames of arc ignition stage at 20kfps. Sputters can be recognized as dotted locus in the picture. Cleaning actions on the electrode surface are also recognized in the picture.
Major cleaning action on the cathode is finished in 2ms in this case. Detail analysis of the cleaning action requires more than 200kfps vido rate. Much gas generates from the base metal in arc ignition stage, so suitable arc channel generates quickly. Then the period of cleaning action is limited in this casse.

Captured video frame rate is 20kfps. Effective video rate are 2kfps in Lower videos. So, there are opotunity to proceed the image information of 10 frames.
(2-1)This movie uses maximum value among 10 frames.
(2-2)This movie uses mean value among 10 frames.
(2-3)This movie uses minimum value among 10 frames.
(2-4)This movie uses difference between maximum and minimum value among 10 frames.
2)Max 3)mean 4)min 5)dif
(2-1)2kfps_max (2-2)2kfps_mean (2-3)2kfps_min (2-4)2kfps_dif
Captured video frame rate is 20kfps. Effective video rate are 200fps in Lower videos. So, there are opotunity to proceed the image information of 100 frames.
(2-1)This movie uses maximum value among 100 frames.
(2-2)This movie uses mean value among 100 frames.
(2-3)This movie uses minimum value among 100 frames.
(2-4)This movie uses difference between maximum and minimum value among 100 frames.
2)Max 3)mean 4)min 5)dif
(3-1)200fps_max (3-2)200fps_mean (3-3)200fps_min (3-4)200fps_dif

2016.09.02open,@2016.09.02revised

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1. Top (in Japanese)
2. Outline of Labo (in Japanese)
3. Welding & cutting movies (in Japanese)
4. Visual aids training (in English) 5. movie list
6. GTAW in N2
Stainless steel

Surface of stainless steel is covered by chromium oxide. Cr2O3 is very stable, and other erosion stops anymore.
However, these oxides are easy to react nitrogen during arc welding. Nitrogen reacts with oxygen in oxide. And, this reaction produces much gas of nitrogen oxide in the molten pool. So, the welding pool moves violentaly and many sputters fly from the pool as shown in the video.
Nitrogen is sometimes used in laser welding. Penetration is deep and bead width is narrow in laser welding. That is the reason why the production of nitrogen oxide is no fatal in laser welding. However, the the molten pool size is quite large conpared with penetration in GTA. So, nitrogen is bad as shielding gas in GTA welding.