Research On Formation And Mechanical Properties Characteristic Of Laser Welded Frame-covering Structures Of Titanium Alloy

The main application backgrounds of this paper are frame-covering structures of titanium alloy, and its research subjects are titanium alloy laser welded lap joints. By using high power YAG laser beam and CO₂ laser beam, the influence of laser welding parameters were studied, and the microstructure and mechanical properties of the joints were also analyzed.By using YAG laser beam, the surfacing welding of titanium alloy sheets with 6 mm thickness is carried out. It is found that when the laser head lean backward, both weld penetration and width increased compared to the laser head lean forward. Within 6¹0 degree range, the angle of inclination of laser head had little influence on variation of weld penetration and width. The flowrate of axial protection gas have significant effect on weld protection. With the defocusing value changing, the fusion width and depth in a small scope changes. Both weld penetration and width increased with the laser power increasing and welding speed decreasing. The laser welding experiments of TC4 titanium lap joints indicates that the width and thickness of the skeleton, the gap between covering and skeleton and laser transverse offset had little influence on variation of weld penetration and width. Gap and transverse offset increase made bite side and hollow obvious.The CO₂ laser welding experiments of TC4 titanium lap joints show that the weld depth, width and two board connection length decreased with reducing welding heat input. When welding heat input was fixed for a value, with increasing laser power and welding speed by the same proportion, the weld depth, weld width and two board connection length all increased obviously. It manifested that laser power variation played a more important role in increase of weld size.The weld shape both of penetration weld and partial penetration weld are like nails form.βcolumnar crystals in different zones have different direction of growth, and the upperβcolumnar crystals dimensions is greater than the lower. The root of partial penetration weld has smooth transition by YAG laser welding, however, the root has cusp by CO₂ laser welding. The microstructure of fusion zone is like network made up of acicularαˊ martensite, the base metal is composed of darkβphase in the brightαmatrix, the HAZ is a mixture of primaryαandαˊ. With the weld heat input increased, the grains in weld joints became coarse. When weld heat input was fixed for a value, larger laser power has resulted in grain growth.The microhardness gradually reduced from fusion zone to the HAZ and base metal due to the interface enhancement effect of martensites. Covering have higher microhardness in the FZ and HAZ than the skeleton has.Both shear strength and microhardness increased with the weld heat input increasing. However, when the weld heat input is excessive, microstructure of weld joint became coarse and microhardness and shear strength decreased.When the weld heat input was fixed for a value, larger laser power has resulted in grain growth and decrease of shear strength and microhardness of lap joints.