Microstructure and Mechanical Behavior of Ti-6Al-2Mo-2Sn-2Zr-2Cr-2V Titanium Alloy Fabricated by Laser Deposition with Change of Solution Temperature
Yang Guang Shao Shuai Wang Chao Wang Xiangming Liu Yanmei
Key Laboratory of Fundamental Science for National Defence of Aeronautical Digital Manufacturing Process,Shenyang Aerospace University
Shenyang Institute of Aircraft Design,China Aviation Industry Group Corporation
Shenyang Aircraft Corporation
Abstract：
Ti-6Al-2Mo-2Sn-2Zr-2Cr-2V titanium alloy spherical powder was used as raw material to prepare titanium alloy samples by laser deposition. The size of titanium alloy spherical powder ranged from 80 to100 μm. The effects of different solid solution temperatures on the microstructure and mechanical behavior of laser-deposited titanium alloy were analyzed by optical microscopy(OM),scanning electron microscopy(SEM),hardness test and tensile test at room temperature. The results showed that in the process of laser deposition manufacturing,the metal powder was melted in the dynamic molten pool formed by laser beam irradiation. Most of the heat dissipated vertically in the molten pool and downward through the substrate in the form of heat conduction when the laser molten pool solidified,and resulting in the small melt in the molten pool epitaxial grown against the heat flow direction,formed coarse columnar β grains penetrating multiple deposition layers. A uniform and orderly basket structure braided in the columnar β grains,which was composed of a large number of slender α lamellae and a small amount of β phase. The macrostructure of the alloy was still coarse columnar β grains after solution and aging treatment in the α+β phase field. The intragranular structure was mainly composed of primary α phase and β transformation structure,and a small amount of lamellar secondary α phase. With the increase of solution temperature,the content of primary α phase decreased,and ratio of secondary α phase increased gradually,which increased the inter-tissue interface and decreased the compatibility of deformation,resulting in the increase of strength and a slight decrease of plasticity and hardness of the alloy. After solution and aging treatment in the β phase region,the macrostructure of the alloy was transformed from columnar crystals to equiaxed crystals,and the primary α phase disappeared completely,all of which were thin lamellar secondary αphases. With solution temperature increased,the width of the secondary α phase layer became larger,the grain boundary was reduced,and the plastic deformation ability was further weakened,resulting in the decrease of strength,plasticity and microhardness.At 900 ℃/2 h,AC(air cooling)+540 ℃/4 h,AC solution aging,the tensile strength and yield strength increased from 1024.2 and 933.1 MPa to 1177.1 and 1064.8 MPa respectively,and the plasticity also increased. The elongation increased from 6.0% to 7.5%,and the reduction of area increased from 14.1% to 20.4%;When the temperature exceeded the transformation point,at 920 ℃/2 h,AC+540 ℃/4 h,AC solution aging,the tensile strength and yield strength further increased to 1185.6 and 1078.4 MPa respectively,and the elongation and reduction of area decreased to 6.2% and 11.5%,respectively. At 960 ℃,the tensile strength and yield strength decreased slightly,reaching 1172.0 and 1057.1 MPa,and the plasticity decreased obviously. The elongation and reduction of area were only 2.6% and 6.2%. Compared with conventional TC4 titanium alloy,the strength of TC4 titanium alloy increased obviously,but the plasticity decreased slightly. And at 920 ℃/2 h,AC(air cooling)+540 ℃/4 h,AC solution aging,the alloy tensile strength at room temperature was 1183.5 MPa,yield strength was 1066 MPa,elongation was 7.1%,the area shrinkage was 18.2%,and the alloy had the best strength-plasticity matching. With the increased of solution temperature,the microhardness value of titanium alloy increased first and then decreased,because the microstructure of primary α phase was very important to the hardness of titanium alloy.When the titanium alloy was solution aged in the α+β two-phase region,the lamellar size of the primary α phase was significantly reduced compared with that of the deposited phase,and a small amount of acicular secondary α phase precipitated,which led to the increase of interphase interface ratio. Under the same pressure,the grain slip distance decreased,which was reflected in the increase of micro hardness. When the solution temperature rose to above the phase transformation point,the secondary α phase grew into a thin lamellar layer,the proportion of interphase interface increased,and the deformation coordination ability of secondary α phase was lower than that of primary α phase,so that the microhardness of titanium alloy at 940 ℃ was significantly higher than that at 920 ℃. When the solution temperature increased to 960 ℃,the secondary α phase layer coarsens,the interface proportion and the hardness of the alloy decreased slightly. The fracture mechanism of the sedimentary sample was the same as that of the two-phase solution aging sample.A large number of dimples were distributed in the micro morphology,which was a typical ductile fracture feature. Compared with the as deposited dimples,the dimples in the solution and aging state were larger and deeper,and the plasticity was better. The results showed that no obvious necking phenomenon in the β single-phase region,a small amount of dimples existed in the micro morphology,and obvious cleavage plane appeared in some regions,resulting in poor plasticity,and the fracture mode was quasi cleavage fracture.
