Study on the effect of laser-induced plasma plume on penetration in fiber laser welding under subatmospheric pressure

Under subatmospheric pressure, the influence of a plasma plume on weld penetration depth could be greatly improved in high-power laser welding. In this paper, a series of laser welding tests under different subatmospheric pressures were performed in a vacuum chamber. The dynamic images of the plasma plume were captured by a high-speed camera and analyzed. Based on the information extracted from the images, the variation of the plasma plume in gray level and spatial size were measured. For the whole plasma plume, the internal part plays a key role in the interaction with laser. So, the variation of brightness and size of the internal part were adopted as the characteristic parameters in describing the attenuation effect of laser caused by the plasma plume. Besides, a threshold value of pressure P (cr), which is critical to the dramatic change of penetration, was found in the range of 10 to 20 kPa. As the ambient pressure was decreased to this critical range, the plasma plume was drastically shrunk, leading to a great reduction of absorption and refraction effect on laser and a resultant sharp increase of weld penetration.