Research On Microstructure And Properties Of Laser Welded Joints Of TC4 And TA1 Titanium Alloys

With their high specific strength,low density,excellent corrosion resistance,and biocompatibility,titanium alloys have earned recognition as the emerging third metal of the 21st century.They are widely employed in aerospace,marine engineering,biomedical,and other major fields.As industrial manufacturing undergoes rapid modernization,the development and production of equipment demand higher requirements.Traditional welding methods,hindered by energy dispersion,weak penetration,and low efficiency,struggle to meet the processing needs of special components.In contrast,laser welding,known for its high energy density,narrow heat-affected zone（HAZ）,and high efficiency,has gained widespread usage.The complex welding thermal cycle during laser welding induces non-equilibrium transformations in various regions,resulting in a gradient distribution of microstructures.Optimizing the microstructure and enhancing the performance of welded joints through adjustments in laser process parameters offer a viable approach to ensure that titanium alloy components meet service conditions and engineering safety.By adjusting the welding heat input to prepare TA1 laser welded joint,the influence of welding heat input on the microstructure evolution,mechanical properties,and corrosion resistance in each zone was studied,and the corrosion mechanism of welded joint and the main factors affecting the corrosion resistance were explored.The results show that the microstructure of the fusion zone（FZ）comprises crisscrossα’martensite and lamellarα’martensite,while the HAZ exhibits a serratedαphase microstructure.With an increase in welding heat input,the width ofα’martensite and the proportion of lamellarα’martensite in the FZ increased,while the microstructure of the HAZ did not change significantly.The mechanical properties of welded joints depend on heat input.Specifically,as the welding heat input increases,the microhardness and tensile strength decreased,while the elongation increased.In a simulated artificial saliva solution,the corrosion resistance of the HAZ was better than that of the base metal（BM）but inferior to that of the FZ,and the change of welding heat input mainly affects the corrosion resistance of FZ.As the welding heat input increases,the corrosion resistance of the FZ initially increased and then decreased,which can be attributed to changes in grain size and microstructure morphology.The width ofα’martensite was identified as the main factor influencing corrosion resistance based on binary linear regression analysis.The microstructure evolution and properties of laser welded TC4 titanium alloy joints under different welding heat input were studied.The results demonstrate that the FZ consists ofα’martensite,and the size and morphology of microstructure were influenced by the heat input.At lower welding heat inputs,the HAZ underwent a complete martensitic transformation,with its microstructure comprisingα’martensite.However,as the welding heat input increases,the HAZ underwent a partial block transformation,forming a blockαphase.The proportion of the blockαphase in the HAZ was positively correlated with the welding heat input.In a 3.5 wt.%Na Cl solution,the corrosion resistance of the welded joints follows the order of FZ>HAZ>BM.Moreover,the corrosion resistance of the FZ initially increased and then decreased with an increase in welding heat input,attributed to changes in grain size and microstructure.Conversely,the corrosion resistance of the HAZ was negatively correlated with the welding heat input,as the increase in welding heat input promotes the formation of the blockαphase,thereby enhancing the galvanic corrosion effect.Furthermore,the laser beam welded joints of TC4 titanium alloy were prepared using continuous wave（CW）mode and pulsed wave（PW）mode with pulse frequencies of 5,50,and 500 Hz.The influence mechanism of pulse frequency on microstructure evolution was investigated.The corrosion resistance of each zone in a 3.5 wt.%Na Cl solution was tested using traditional electrochemical and local electrochemical impedance spectroscopy methods.The findings revealed that PW mode played a role in refining the priorβgrain,and the effect of grain refinement first increased and then decreased with an increase in pulse frequency.When the pulse frequency was 50 Hz,the priorβgrain size was only 1/4.43 of that in the CW sample.The microstructure of the FZ consists ofα’martensite,while the HAZ comprisesα’martensite and a small amount of blockαphase.The change in pulse frequency primarily affects the grain size.PW mode enhanced the meritocratic orientation of the FZ in the welded joint and reduced the average kernel average misorientation（KAM）values,with the maximum weave strength and minimum KAM values occurring at 500 Hz and 50 Hz,respectively.PW mode significantly increased the corrosion resistance of the FZ,which increased and then decreased with an increasing pulse frequency.When the pulse frequency was 50Hz,the values of charge transfer resistance(R_ct)and corrosion current density(i_corr)of the FZ were 4.17 times and 1/7.5 times bigger than those of CW mode,respectively.A mathematical model was developed to investigate the relationship between pulse frequency,microstructure,and corrosion resistance,and it is found that grain size and KAM values were the main factors affecting corrosion resistance rather than grain orientation.