Nowadays, electrical steel sheets with the thickness of 0.1 ~ 0.3 mm is being widely used to produce the core of power generators, motors and tranformers. Electrical steel is usually coated to increase electrical resistance between laminations, reducing eddy currents, to provide resistance to corrosion or rust, and to act as a lubricant during die cutting. The main problem in welding electrical sheets is the reduce of magnetic properties and losses eddy curent due to the destroy of lamination coatings. On the other hand, since the lamination coatings is evaporated during welding period, it tends to stick the tip of tungsten electrode. This leads the reduce quickly of life-time of tungsten electrodes. In other words, this causes the increase of stopping time of lines and the welding defect in welded products. In order to over this problem, we have proposed a new welding technology. In this technology, a new welding torch has been developed. In this torch, two nozzles was utiized with one of them is similar to conventional nozzle and another one is with narrowing diameter. In this presentation, the efficiency of narrowing nozzle on the life-time of tungsten electrode, the weld bead apperance, and bending test results are discussed in details.
2. Experimental method:
Main equipment for this experiment is a welding power source (Hoop welder MFW-400FT, Murata Welding Laboratory). The 1.6 mm diameter tungsten electrode mixtured 2%Thorium was utilized. The tip angle was fixed at 30 degrees. The distance between tungsten electrode tips with workpiece surface (arc length) was set up at 0.25 mm. Welding current and welding speed are 35 A and 1500 lmin-1, respectively. The shielding gas were pure Ar. The gas flow rate was 8 L min–1. Back shielding gas flow rate was about 2.8 lmin-1. Base metal was electrical sheets with the width of 400 mm and the thicknees of 0.25 mm. The narrowing nozzle with the diameter of 1.6 mm was utilized.
3. Results and discussion:
3-1. Weld bead appearance and tungsten electrode status
The width of weld bead in the case of with narrowing nozzle is nearly unchanged after thirty times of welding. The weld bead on the top surface was about 1.20 ~ 1.33 mm and on the bottom surface was about 0.75 ~ 0.93 mm. Meanwhile it is much changed in the case without narrowing nozzle, especially on the bottom surface. The weld bead was about 1.11 mm at one welding time, but it was almost zero at thirty welding times. On the bottom surface, the width was 0.49 mm at the first time, and then it decreased quickly to 0.2 mm at ten times before reach the zero at 20 times. At thirty times, no weld was done. For the tungsten electrode, the tip was corroded in both cases. However, it was much corroded in the case of without narrowing nozzle. If the first time is zero percent, then at ten times, this ratio was about 55%. It was increased to 67% and 70% at twenty times and thirty times, respectively. In the case of with narrowing nozzle, this ratio was much lower. It was about 11%, 13% and 14% corresponding to ten times, twenty times, and thirty times.
Using the narrowing nozzle is much better than without it. The weld bead is stable, the shielding effect is increased due to two shielding layers. For example, at ten times of welding, totally welding seam length is about 4000 mm. In this case, the weld bead became un-stabilization without narrowing nozzle. The weld bead on the bottom surface is much larger at ten times. The tip of tungsten electrode is much burnt out and chemically corroded due to the high temperature and the evaporation from the cathode surface (base metal). On the other hand, the black zone was seen on the top surface in the case of without narrowing nozzle due to the insufficient shielding effect. This result matches with the experimental results. At ten times after welding, the weld bead is the top surface is extremely narrow and the weld bead on the bottom surface is almost zero. At twenty and thirty times after welding, the weld bead on both the top surface and the bottom surface is zero. Meanwhile, the weld bead is nearly unchanged at the first time and thirty times after welding in the case of with narrowing nozzle. On the other hand, the surface of tungsten electrode looks cleaner in the case of using narrowing nozzle. The tip of tungsten electrode is almost corroded gradually with 0.0 mm at the first time; 0.11 mm at ten times; 0.13 mm at twenty times; and 0.14 mm at thirty times after welding. In contrast, the tip of tungsten electrode is corroded abruptly at 0 mm for the first time to 0.55 mm at ten times; 0.67 mm at twenty times; and 0.7 mm at thirty times after welding. From twenty times after welding, the corrosion is gradually reduced. In comparison to the case of without narrowing nozzle, the tip of tungsten electrode is kept constant with very low percent of corrosion ratio.
For explanation our data, the numerical simulation was developed. We have found out that using a narrowing nozzle, the plasma flow velocity is increased much, especially near the center of the arc. This can reduce the attach of evaporation on the tungsten electrode surface and reduce the temperature of tungsten electrode surface. Also due to the high arc pressure as mentioned above and high density of heat input and narrowing arc column, the weld bead on the bottom surface is increased much and it is approximately to 0.7 times of the top bead. In contrast, in the case of conventional TIG process, this ratio is very low.
It can be considered that narrowing nozzle significantly improved the production efficiency and reduce the welding defects. This suggests that develop a new TIG welding technology is really necessary to improve the efficiency of manufacturing processes related metal forming technology.
Keywords: TIG welding, narrowing nozzle, tungsten electrode, electrical steel sheets