Study On Effect Of Plume Behavior On Weld Formation And Porosity In Laser Welding Of Al-Alloy Under Reduced Ambient Pressure

Laser welding of aluminum alloys has always been a research hotspot.Among all the difficulties,high thermal conductivity,high reflectivity to laser beam and high porosity sensitivity lead to the poor weld formation.Laser power,welding speed and other parameters have great influence on the weld formation.And welding under reduced ambient pressure can improve the laser energy efficiency,increase the penetration depth,provide a more stable environment,as well as reduce the porosity.In order to study the influence level of ambient pressure and other parameters on the aluminum alloy welding process,a new designed vacuum laser welding platform is used to perform an orthogonal experiment.The result shows that,within a certain range,ambient pressure has the greatest influence on weld formation and porosity defects,compared with laser power and welding speed.Based on the results of the orthogonal experiment,series of laser welding experiments under varied ambient pressures were further carried out.The plasma plume and its behavior were observed and the ambient pressure was proved to have great impact on the plasma plume behavior.With ambient pressure decreasing,plasma plume tended to be more stable,the splash disappeared,brightness reduced and the size decreased.When the pressure was 80 kPa,the average grayscale of the plasma plume reduced to only 26.8% of that under atmosphere.And compared with the trend of the plasma plume area,the average grayscale reduces faster when at 101 kPa~80 kPa.However,the trend changes at 80~50 kPa,which indicates that the plasma plume intensity is reduced ahead of its dimension.The spectral signals of plasma plume were collected and the electron temperature as well as electron density was calculated via Mg I and Al I characteristic spectrum.At reduced ambient pressure of 80 kPa,some characteristic spectrum disappeared and the peak of intensity reduced over 66.6%.However,the electron temperature and density of plasma plume decrease relatively small.The surface formation of the weld bead was optimized.Both the penetration depth and aspect ratio increased greatly with the ambient pressure reduce.When the pressure reduced to 15 kPa,the weld bead shape changes from inverted triangle to a slender shape and the aspect ratio was more than 1,which indicates depth > width.And when reducing to 0.2 kPa,the aspect ratio increased to 2.5,which was nearly 3.8 times of that under atmosphere,and the penetration depth reached to 7.486 mm.The ambient pressure is proven to have a strong impact on the porosity defect,including the size,distribution and the porosity rate.The pores with large size usually distributed in the upper middle part of the weld bead.With the decrease of pressure,the pores diminished with an order from the upper part to the bottom part.The porosity rate under 101 kPa was 3.741% and when the pressure decreases to 0.2 kPa,the porosity rate was only 0.039% and the porosities can’t be seen anymore.This effect can be explained by the change of the bubble formation and overflowing process under reduced ambient pressure.The results provide a reference basis for the control of laser weld forming and porosity of aluminum alloy,and provide a possibility for laser welding of aluminum alloy.