| TC4 titanium alloy is an α+β duplex titanium alloy,which is widely used in aerospace and aviation,biomedical and other fields because of its high specific strength,low density and good heat resistance.Laser welding has the characteristics of small welding deformation,small heat-affected zone and good welding stability,which has become the main method of welding TC4 titanium alloy.However,due to the physical properties of the titanium alloy itself,it leads to problems such as coarse columnar crystal and porous cracking during laser welding.Therefore,the ultrasonic assisted introduction of laser welding,the use of ultrasonic energy in the melt pool cavitation effect,acoustic flow effect and mechanical and thermal effects to improve the titanium alloy laser welding problems,and thus improve the comprehensive mechanical properties of welded joints.This thesis takes 2.5 mm thick TC4 titanium alloy as the research object,and conducts ultrasound-assisted laser self-fusion welding and butt welding tests on it on an ultrasound-assisted working platform to investigate the effects of ultrasound-assisted laser welding on the macroscopic morphology,weld seam formation,microstructure and mechanical properties of welded joints under different variable amplitude rod pressure,ultrasound power and welding speed.The results show that changing the ultrasonic amplitude rod pressure will affect the ultrasonic energy loss during the propagation of ultrasonic energy to the molten pool,when the pressure is less than 81 N or more than 191 N,the ultrasonic energy loss in the propagation process,the effect of ultrasonic assisted is not obvious,in the appropriate pressure can enhance the transmission efficiency of ultrasonic energy,strengthen the effect of ultrasound,when the amplitude rod pressure is 191 N when the weld forming coefficient is the smallest,the grain in the weld Refinement is obvious.In the low welding speed when the application of ultrasound,can reduce the weld in the porosity defects,and weld forming coefficient and grain size significantly reduced,but with the increase in welding speed,the molten pool solidification cooling speed,the more the effect of ultrasound is not obvious,melt depth,melt width and grain size no significant changes.Compared with laser welding of titanium alloys without ultrasound,the weld forming factor and weld columnar crystal size were reduced and the microhardness of the welded joints increased;with the increase in ultrasound power,the weld columnar crystal size tended to decrease and the pin-like martensite content was higher,and the orientation and alignment were more disordered.When the ultrasonic power of 1.8 k W weld area grain size is the smallest,the average width of the columnar crystal for 50.05 μm,compared with no ultrasound reduced by 28.7%,indicating that the application of ultrasound can effectively refine the weld grain.The tensile strength of the welded joint increased with the ultrasonic power and then decreased,reaching a peak at 1.6 k W,with a tensile strength of1003 MPa,reaching 95% of the tensile strength of the base material,an increase of 32.5%compared to the laser welded joint without ultrasound,with a post-break elongation of8.6% and a hardness of 420 HV in the weld area,improving the mechanical properties of the welded joint.The optimum process parameters for ultrasound-assisted laser welding of TC4 titanium alloy are ultrasonic power of 1.6 k W,ultrasonic frequency of 20 k Hz;laser power of 1.8 k W,out-of-focus volume of 0,welding speed of 0.03 m/s;ultrasonic variable rod pressure of 191 N. |
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