| The laser welded joint of TC4 titanium alloy is studied in thesis.Firstly,the microhardness,tensile properties and microstructure of TC4 welded joints were characterized,and the influence of laser welding heat on TC4 joints was analyzed.Then TC4 welded joints were treated with ultrasonic rolling to investigate the influence of rolling times on the mechanical properties of welded joints.Secondly,the hydrogen embrittlement resistance of TC4 welded joints before and after ultrasonic rolling was evaluated by slow strain rate tensile test with electrochemical hydrogen charging on line.According to the stress-strain curves obtained,the influence of ultrasonic rolling on the tensile properties and hydrogen embrittlement behavior of TC4 joint was analyzed in combination with the fracture morphology.Finally,the finite element analysis software ABAQUS is used to simulate the ultrasonic rolling.The obtained stress distribution field was coupled with the hydrogen diffusion model to calculate the hydrogen concentration distribution inside TC4 titanium alloy in the hydrogen environment.Combined with the online hydrogen charging mechanical properties,the anti-hydrogen embrittlement mechanism of ultrasonic rolling was analyzed.The main work is as follows:(1)Different laser welding input thermal parameters were designed as 514.3J/mm533.3J/mm,600J/mm,640J/mm.The effects of laser welding heat input on microstructure and properties of TC4 welded joints were investigated.The evolution of microstructure in different regions(heat affected zone,weld zone,base metal)of TC4 joint was observed by metallographic microscope,and the change of special microstructure in weld zone under different welding heat was analyzed.The relationship between microstructure and mechanical properties was analyzed,and the influence of welding heat input on mechanical properties of TC4 joint was obtained.The results show that the weld zone of the TC4 joint with welding heat of 533.3J/mm is composed of disorganized acicular martensite distribution network basket structure,and the joint has the best comprehensive performance.(2)On-line hydrogen-charging tensile experiments of TC4 welded joints with different ultrasonic rolling cycles(0,3,6,9)were carried out.The changes of surface roughness and surface microhardness of welded joints before and after ultrasonic rolling and their effects on hydrogen embrittlement resistance were investigated.The results show that the welded joints without rolling treatment exhibit obvious brittle fracture.With the increase of rolling times,the surface roughness gradually decreases and the microhardness of the weld area increases.With the decrease of the hardness gradient in different areas,the microstructure uniformity and tensile property of the joint surface are improved,the dimples in the tensile fracture are deeper and the brittleness is weakened.The mechanical properties of TC4 joint obtained by rolling for9 times are the best.(3)Based on the finite element analysis software ABAQUS,the numerical calculation and analysis platform of ultrasonic rolling strengthening-residual stress-hydrogen diffusion model was established.The stress field obtained from ultrasonic rolling finite element analysis is used as the predefined field for hydrogen diffusion analysis.Using the residual stress induced by ultrasonic rolling to induce the hydrogen diffusion distribution in the joint,the hydrogen content in different regions of TC4 joint was calculated.The effect of ultrasonic rolling on anti-hydrogen diffusion behavior of TC4 joint was studied.The results show that the hydrogen content of the unrolled joint is concentrated in the weld zone.This strongly proves that the fracture appears in the fusion zone because of the high hydrogen content there.However,the hydrogen concentration of the joint decreases obviously after ultrasonic rolling,and the residual compressive stress generated by ultrasonic rolling can effectively reduce the hydrogen content into the joint,thus improving the hydrogen embrittlement resistance of the joint.Finally,based on the performance characterization and simulation results,the anti-hydrogen embrittlement mechanism of TC4 joint strengthened by ultrasonic rolling is proposed. |
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