Microstructure and Mechanical Properties of TC4-DT/TC21 Linear Friction Welded Joint with Duplex Annealing
As a solid-state connection technology,linear friction welding is widely used in the connection of various alloys due to its advantages of fewer defects and low cost.However,the mechanical properties of the joints will be significantly reduced by residual stress and metastable phases,so the heat treatment of the joints is necessary.Recently,the post-weld heat treatment of TC4-DT/TC21 linear friction welded joints was mainly focused on simple annealing at low temperatures,which couldn't significantly change the mi-crostructure characteristics of the joints.In order to fully understand the effect of post-weld heat treatment on TC4-DT/TC21 linear fric-tion welding joints,two different duplex annealing(PWHT 1:840 ℃/1 h,AC+590 ℃/4 h,AC;PWHT 2:870 ℃/1 h,AC+590 ℃/4 h,AC)was applied to the linear friction welded joint of TC4-DT/TC21 dissimilar titanium alloy,and the effect of duplex annealing on the microstructure and mechanical properties of the joint was analyzed.Optical microscope(OM),scanning electron microscope(SEM)and Vickers hardness tester were used to compare and analyze the microstructure,microhardness and fracture of the joints un-der different duplex annealing.According to the structural characteristics of TC4-DT/TC21 linear friction welding joint,the joint could be divided into weld center zone(WCZ),thermo-mechanical affect zone(TMAZ)and base metal(BM).After duplex annealing,the structure of each zone had undergone significant changes,and the duplex annealing had great influence on the microstructure of the joint.The average width of the acicular α phases in the weld center zone increased from 0.2 to 0.5～1.0 μm after duplex annealing,in the thermal-mechanical affected zone the deformation of the primary wavy α phases in the oscillating direction decreased under the ef-fect of distortion energy release and element diffusion,and the acicular α phases around the primary wavy α phases grew into bamboo leaf shape,in the base metal,the spheroidization of primary α phases occurred and the acicular α phase precipitated on β matrix.Compared with the mechanical properties of as-welded joint(tensile strength(Rm)=911 MPa,elongation(A)=8.5％),the strength and ductile of the joint decreased under the combined effect of microstructure coarsening and acicular α phase precipitation(Rm=886 MPa,A=6.0％)after the duplex annealing at lower temperature(PWHT 1).However,after duplex annealing at higher temperature(PWHT 2),the strength of the joint was significantly improved due to the dispersed acicular secondary α phase,and the plasticity of the joint remained at a high level(Rm=925 MPa,A=6.2％).For the tensile test of the joints after duplex annealing,the crack propaga-tion zones were covered by the dimples and cleavage facets,and the cracks propagated at a certain angle with α phases during propaga-tion,indicating that the joints failed in the form of hybrid fracture of toughness and brittleness during the tensile test.Compared with the impact energy of the joint(14.8 J)without heat treatment,the impact toughness of the joints was significantly improved by the du-plex annealing,and with the increase of the first annealing temperature,the impact energy of the joints increased from 38.6 to 41.2 J,indicating that the impact toughness of the joint was further improved under the duplex annealing at the higher temperature.
linear friction weldingpost-weld heat treatmentmicrostructuretensile strengthhardness distributionimpact toughness
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