The time-honoured question, 'What exactly is happening in the molten weld pool' ...has faced welding engineers for decades.
Now, through collaboration between TWI and the University of Liverpool, a unique viewing system has been designed to provide reliable real time measurements of the molten pool's behaviour.
The vision system provides an affordable solution to removing the arc light almost totally and produce reliable and high quality real-time welding images. It uses a CMOS camera with a lens and narrow band pass filter along with a frame grabber and an illumination source.
Several laser illumination light sources were used during the system's development, including a pulsed laser, a continuous wave laser and laser diodes. It was found that low-cost laser diodes provide an extremely efficient and economic alternative illumination system to the bulky and expensive laser systems usually adopted.
During the development programme both arc and weld pool emissions were studied to isolate the image of the weld pool. It was found that arc emissions are at the lower part of the spectrum, and the weld pool emissions are at upper end of the spectrum, towards the infra red region. The noise level is dependent on the shielding gas composition.
If the shielding gas is argon its emissions are within the range 700nm to 900nm wavelength, and increase the noise level at this region. Operating at wavelengths greater than 1000nm would be preferable because arc and argon emissions are low and weld pool emissions are high. However, since the camera's spectral response is low, more laser energy is required.
Although there are now high power, super-radiant and high flux LEDs, the power output is insufficient for this application which requires high illumination power. Other considerations such as emission pattern, spectral width and pulse width make it unlikely that LEDs will be the illumination source for such applications in the near future.
On the other hand, the laser diode system constructed in this study is a promising alternative. As well as the relatively high pulsed output power of these laser diodes, other crucial factors such as pulse width control, spectral width, emission pattern and efficiency of operation have made laser diodes ideal for many applications including weld pool monitoring.
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