Welding Technology And Microstructure And Properties Of Welded Joints For TC4/TA7 Dissimilar Titanium Alloys

Titanium alloys have excellent comprehensive properties such as low density,high specific strength,good corrosion resistance and anti-fatigue properties.They are widely used in aerospace,petrochemical,shipbuilding and biomedical fields,etc.In industry field,the combined component of dissimilar titanium alloys not only possesses the advantage of different alloys,but also its product cost can be greatly reduced.Vacuum electron beam welding(EBW)has the characteristics of high energy density,narrow heat-affected zone(HAZ),large depth-to-width ratio of weld,good protection atmosphere,etc,thus it is suitable for the welding of active metals such as titanium alloys.Based on these,in this paper,the MSC.Marc software is used to simulate the welding temperature field for the TC4/TA7 dissimilar titanium alloys during EBW process.According to the simulation results,the welding parameters are optimized.The microstructure and mechanical properties of dissimilar joints are systematically analyzed and measured,and the formation mechanism of dissimilar joint is discussed.As a contrast,the TIG welding is also carried out for the TC4/TA7 dissimilar titanium alloys,and the effect of different filler metals on microstructure and mechanical properties of welded joint is investigated.The simulation results show that the welding temperature field presents asymmetrical distribution during the EBW of TC4/TA7 dissimilar titanium alloys.The high temperature region in TC4 base metal(BM)side is wider than that of in TA7 BM side.The peak temperature of nodes which have the same distance away from the weld center is different,and the peak temperature on the weld thermal cycle curves of TC4 side is higher than that of TA7 side.With the distance away from weld center,the peak temperatures in both TC4 and TA7 side are gradually decreased.The shorter the distance away from heat source is,the larger the temperature gradient is.The morphology of molten pool under different welding parameters is analysed.Compared with that of welding speed,the effect of electron beam current on the morphology of molten pool is more obvious.The first bead welding parameters are optimized as follows: accelation voltage 50 k V,welding speed 13mm/s,and electron beam current15 m A.The modification welding parameters are optimized as follows: accelation voltage 50 k V,welding speed 12mm/s,and electron beam current 12 m A.The microstructure analysis shows that the microstructure in different zones of TC4/TA7 dissimilar joint is obviously different.The fusion zone(FZ)is mainly composed of fine α′ martensite.The HAZ of TC4 side is composed of martensite,blocky α phase,original α phase and β phase.The HAZ of TA7 side consists of a small amount of martensite,blocky α phase and original α phase.SEM analysis indicates that there are mainly the parallel martensite colony and basket-weave structure with interlaced arrangement in FZ.TEM analysis shows that,due to the effect of modification welding,the martensite becomes coarser in the upper part of joint,and dislocations are generated in FZ.Element line scanning and elemental maps analyses indicate that the distribution of alloying elements is relatively uniform at joint interface,and no obvious segregation is occurred.The distribution of hardness in weld zone is ranked as follows: FZ > HAZ of TC4 side > TC4 BM > HAZ of TA7 side >TA7 BM.The results of tensile tests show that the tensile strength of welded joint is close to that of TA7 BM,while its elongation is slightly lower than those of the two BMs.The joint fails in the side of TA7 BM during stretching,and the morphology dominantly presents the characteristic of ductile fracture.The different filler metals involving welding wires TC4 and TA28 are used respectively during the tungsten inert gas arc welding(TIG)of TC4/TA7 dissimilar titanium alloys.The joint metallographic observation shows that,compared with that of EBW joint,the microstructure of TIG joint is relatively coarse,and the parallel martensite colony with the same orientation is generated in FZ.Tests of mechanical properties indicate that the hardness distribution of TIG joint with the filler metal of TC4 is ranked as follows: FZ > HAZ of TC4 side > TC4 BM > HAZ of TA7 side > TA7 BM.However,the hardness distribution of TIG joint with the filler metal of TA28 is ranked as follows: HAZ of TC4 side >TC4 BM > HAZ of TA7 side > FZ > TA7 BM.Both the hardness and tensile strength of the TIG joint are slightly lower than those of the EBW joint.During welding,when the heat input is large,the welding defect such as gas pores is easily generated in TIG joint,which is disadvantageous to the mechanical properties of welded joint.