Study On The Mechanism Of Porosity Formation And Laser Welding Technology Of Commercial Pure Titanium TA2

TA2 is one of the most widely used commercial pure titanium,because of its excellent specific strength and corrosion resistance,especially strong corrosion resistance for Cl-1.Therefore,the use of industrial pure titanium TA2 as a raw material for equipment manufacturing exhibits significant economic value which extends the equipment life and reduces equipment maintenance costs.TA2 welding includes TIG welding,electron beam welding,laser weldingand so on,in which the laser welding has advantages on concentrated energy,well weld joint,small deformation,high production efficiency making it as the leader of titanium and titanium alloywelding.In this paper,2 mm and 4.2 mm thick commercial pure titanium TA2 as the research material,using fiber laser welding and fiber laser-MIG hybrid welding method.With the design of the front and back protector and fixture forTA2 welding which shows good protective effect.We investigated the characteristics and mechanism of the porosity formation in 2 mm TA2 laser welding.We also inspected the laser welding process of laser autogenously welding and laser-MIG hybrid welding of 4.2 mmTA2,so as to obtain well formation of welding joints.OM,XRD,EBSD and other analytical methods were employed to systematic analyze two welding joints microstructures,texture and mechanical properties,meanwhile,comparative studies on relationship between regional microstructural evolution and mechanical properties are analyzed and discussed.The mechanism of the porosities formation in 2 mm TA2 laser autogenously welding process shows that two types of porosities: one is hydrogen porosity,which is the small size with the smooth inner wall,resulting from H gatheringin molten pool forming bubbles.The other is process porosity with large size and irregular shape,which is caused by the instability of the keyhole in air or gas.laser autogenously welding experimental results for 4.2 mm TA2 show that with the increase of defocusing amount,the next-part of weld bead width decreased greatly while the upper weld width and penetration depth decreases slightly.With the increase of laser power,the upper weld bead width,the next-part weld width and penetration depth were increased of different trend.With the increase of welding speed,weld bead width and weld width are reduced greatly.Laser-MIG hybrid welding experimental results for 4.2 mm TA2 show that when gas shield residence time is 90 s and laser heat source is in front,weld joint formation is great,which is not oxidized,the color is silver white.With the increase of defocusing amount,the weld width slightlybroaden,next-part weld width and penetration depth show different degrees of decline,light wire spacing has little effect on weld formation.With the increase of welding speed,upper weld bead width,next-part bead width,penetration depth decreased to different extent.With the increase of laser power,upper weld bead width,next-part bead width and penetration depth increase ina basic linear relationship.With the increase of arc current,upper weld width,weld penetration,and weld reinforcement increase first and then decrease,the next-part weld bead widthincrease.The experiments results show that compared to uniformequiaxed α grainof the base material,the αgrainin the weld zone and heat-affected zoneof welded jointsof laser autogenously welding and hybrid weldingbecomemore coarsened and with columnar grains,irregular serrationα grains and a small amount of acicular shapesα grains and show αtwins grain.The texture characteristics in weld zone of laser autogenously welding joint is not obvious,however the hybrid weldingexhibit an opposite tendency to strong texture.When far away from the center of two kinds of welded joints,the diameter of grain is smaller,and its average length in the heat-affected zone size is relatively small.The diameterof grain in weld zoneand heat-affected zone of hybrid welding joint is significantly higher than that of laser autogenously welding jointand the number of twins is also greater.In the prismatic slipsystem,Schmidfactors of other zones of laser autogenously weldingand hybrid welding are less than the base material,according to the Schmidt’s law,the material with bigger Schmid factor is more prone to sliding.There are two twins in the weld zone,the {11-22} compression twin and the {10-12} tensile twin which lead to the texture change of the base material,result in the work hardening.The micro-hardness of weld zone and heat-affected zoneof hybrid weldingjoints of are significantly higher than that of base metal,the micro-hardness of weld area and heat-affected areawas essentially the same.However the micro-hardness from weld zone of laser autogenously welding joint to the base metal is tend to decreased,also the weld zone decrease from the upper to the bottom.The tensile fracture positions of the two welded joints are all in the base metal,but the elongation is lower than that of the base metal.