| During deep penetration laser welding, the plasma plume has an effect of absorption, refraction and scattering on laser beam, which changes laser energy distribution on workpieces and affects the weld formation and weld quality. Besides, the attenuation effect of the plasma plume increases more significantly as the laser energy gets higher. All of these problems will restrict, to some extent, the application of deep penetration laser welding of thick plate. So it is of necessity to suppress the plasma plume, the laser welding under subatmospheric pressure is one of the effective methods.In this research, a practicable experiment method was designed for study on laser welding under subatmospheric pressure. On this basis an experiment platform was set up, and a series of tests with different pressures, laser powers and welding speeds were performed. Meanwhile, a high-speed camera was employed to capture the plasma plume image. Their characteristic parameters were extracted by image processing, which could be used to measure the variations of plasma plume in density and spatial sizes. The results revealed that the influence of ambient pressure on the formation of laser welding bead was nonlinear, there was a critical value of pressure located near 20.0 kPa. When the ambient pressure dropped below the value, both the penetration depth and depth to width ratio of weld seam would increase drastically, and the cross section would turn from a nail-shape to a needle-shape. As the ambient pressure decreased, the plasma plume would shrink. Under the pressure of 3.0 kPa, the plasma plume was fully suppressed and disappeared.At last, the interaction between laser light and plasma plume was studied by theoretical analysis and calculating characteristic parameters. It turned out that with reduction of ambient pressure, their interaction became lower, which helped to diminish porosity defects, improve the welding stability and weld formation. |
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