| TC4 titanium alloy has large numbers of applications in the aerospace industry at present.For large-scale structural parts with complex shapes,it is mainly used for“forging+CNC machining”or“separate manufacturing+welding”and bolting methods.The material utilization rate is low,the manufacturing cost is high,and the production cycle is long.Wire arc additive manufacturing is an additive manufacture technique for fabricating high-performance complex metallic parts,which has been widely researched in the past decades.However,conventional welding power has large heat input,titanium alloy easy to form epitaxial growth of?columnar grains.Cold metal transfer?CMT?welding power source has the characteristics of low heat input,stable droplet transfer,easy connection with automation equipment,etc.CMT welding power supply is applied to additive manufacturing technology,which can realize low preparation of large structural parts.Cost,efficiency and flexibility.In this paper,the most widely used TC4titanium alloy is studied,focusing on the macroscopic microstructure evolution law of TC4 titanium alloy produced by CMT arc additive,the method of microscopic structure regulation and the measures to reduce the anisotropy of mechanical properties.The main research contents and results as follows:1.When the wire feeding speed is more than 4.0 m/min,the obtained TC4 titanium alloy exhibits epitaxially grown coarse?columnar grains,and a massive?M phase is formed on the grain boundaries of the?grains,and the massive?M phase texture strength reaches 36.8.When the wire feeding speed is less than 3.0 m/min,by adjusting the appropriate welding speed,when the heat input amount is less than 1350J/cm,the continuously growing?columnar grains disappear,and the macroscopic morphology is non-equal irregular?grains.Seven steps of different small wire feed speeds with close heat input were designed,and the original?grain profile area was the smallest when the wire feed speed was 3.0 m/min and the welding speed was 0.48m/min.The microstructure of TC4 titanium alloy under this parameter is characterized by the fact that the first layer is dominated by needle-shaped martensite?phase.After the second deposition is completed,a layer structure is formed in the lower portion of the first layer,and no obvious grains exist in the white light region of the layer structure.In the fifth layer deposition sample,the white layer of the fourth layer structure has an elongated?+?phase,and after five cycles of thermal cycling,the microstructure changes tend to be stable.2.The stepped pattern prepared at the wire feeding speed of 3.0 m/min and the welding speed of 0.48 m/min,the microstructure mainly consists of Widmanst?tten structure and martensite structure,and the?phase of the Widmanst?tten structure area is precipitated in the order of???GB??WGB??WM??S.The?phase of the martensite structure area is precipitated in the order of???'??WM.The CMT arc additive produced by continuous deposition under this parameter is a non-equally irregularly-sized?-grains under the macroscopically deposited TC4 titanium alloy.The texture strength decreased,with a maximum value of 16.2.The?phase structure has a composition of Widmanst?tten,martensite and layer band structure.3.The solid solution aging treatment has coarsening of the?phase.In the solid solution furnace cold sample,?WGB will spheroidize at 930°C,and melt at 900°C.?WGBGB maintains the original morphology at 870°C.?agglomeration occurs at 930°C and900°C,and forms a lamellar layer at 870°C on the basis of in situ;the?WM width precipitated inside the martensite becomes smaller in turn.The?WGB in the solid solution air-cooled sample was coarsened on the basis of the original morphology,and the?in the martensite region appeared agglomerated at 930°C and 900°C,and the original morphology was maintained at 870°C;the interior of the martensite The precipitated?WMM width was successively small,and precipitation at 870°C was insufficient.By comparison,the microstructure of TC4 titanium alloy produced by CMT arc additive manufacturing was uniform at 870°C,1h/FC+600°C,2h/AC.4.The tensile properties in the as-deposited state?AM?are anisotropic,and the existence of the layered structure and the inhomogeneity of the microstructure are the root causes of the anisotropy of mechanical properties.Comparison of solution aging treatment?SA1:800°C,1h/FC+600°C,1h/AC;SA2:870°C,1h/FC+600°C,2h/AC?and aging treatment?AT:600°C,2h/AC?.The tensile results show that the tensile strength decreases with increasing heat treatment temperature,but the plasticity increases with temperature.In the AT sample,the tensile strength was the highest,but the elongation was lower than the standard value.In the SA1 and SA2 samples,the tensile strength and elongation were both up to standard values.The anisotropy of the three heat treatment states was evaluated,and the anisotropy of elongation was significantly higher than the tensile strength and yield strength,showing the lowest anisotropy in the SA2 sample.5.In the heat-treated AT,SA1,and SA2 samples,the fibrils of the fiber region changed from large to small,from shallow to deep,and the fractures in the shear lip region were changed from quasi-cleavage features to dimple morphology with uniform size.The pores in the SA1 and SA2 samples were mainly found in the vicinity of the?grain boundary and the?GB phase,and inside the?-strip.Under the three heat treatment conditions,the width of the?-lath near the fracture and the tensile strength and yield strength satisfy the Hall-Petch relationship.Tensile cracks perpendicular to the load direction were observed in the SA1 and SA2 samples,and a large plastic zone was formed at the crack tip to relax the stress,and the main crack was delayed,so that the material exhibited high plasticity in the heat treatment state. |
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