Microstructure And Properties Of Ti6Al4V Alloy Prepared By Wire And Laser Additive Manufacturing

Ti6Al4V alloy has the characteristics of low density,high specific strength,good corrosion resistance and good biocompatibility.It is currently the most widely used titanium alloy and has broad applications in aerospace and marine fields.For example,Ti6Al4V alloy can be used in the manufacture of pressure-resistant shells of deep submersibles,one of the important tools for human exploration of the ocean.The manufacturing process of the traditional pressure shell is first rolling and then welding.The existence of the welding interface affects the overall integrity of the pressure shell and has a negative impact on its mechanical properties.The additive manufacturing technology can realize the overall rapid prototyping of complex parts,and is an ideal manufacturing technology for key components such as pressure-resistant shells.In this paper,single-layer and multilayer Ti6Al4V alloy samples were prepared by WLAM,and the effects of laser power,scanning speed and wire feeding speed on the microstructure,tensile properties,and impact properties of Ti6Al4V alloy were systematically studied.Subsequently,heat treatments such as recrystallization annealing,annealing,solid solution,aging,and solid solution + aging were carried out on the Ti6Al4V alloy prepared by WLAM,and the regulation mechanism of the heat treatment process on the microstructure and properties of the titanium alloy was studied.Research indicates:For a single-layer deposition sample,the increase of laser power increases the melting width of the sample and decreases the melting height,and the content of martensite α’ inside the sample decreases with the increase of laser power.The increase of wire feeding speed increases the melting height of the sample,refines β grains,reduces defects and increases the content of martensite α’.The increase of scanning speed reduces the melting height and width of the deposition layer at the same time,and makes the structure of the single layer deposition layer more uniform.The microstructure of the multilayer deposition sample is composed of martensite α’,αbundle and basket-weave microstructure.The increase of laser power reduces lack-of-fusion,increases the size of β grains,and increases the degree of decomposition of martensite α’.When the laser power increases from 3000 W to 3500 W,the tensile strength of the sample decreases by 4% due to the decomposition of martensite α’.But the elongation increased by50%,and the impact toughness increased by about 6%.The increase of wire feeding speed increases the average size of β grains of the sample,and multilayer deposition can improve the defects of samples with low wire feeding speed.As the wire feeding speed increased from10 mm/s to 30 mm/s,the martensite α’ inside the sample decomposed,the tensile strength decreased by 2%,the elongation increased by 67%,and the impact toughness increased by11%.When the scanning speed increases,the lack-of-fusion and residual martensite α’ in the sample will increase.The sample with a scanning speed of 8 mm/s contains more defects,but the primary β grains are relatively fine.The Widmanstatten structure of the sample with a scanning speed of 6mm/s is closer to the basket-weave microstructure,so compared with the sample with a scanning speed of 4mm/s,the elongation rate increased by about 45%,the tensile strength decreased by 2%,and the impact toughness increased by 11%.Under the same deposition parameters,the upper part of the sample has more residual martensite α’,and the lower part of the primary β grains are relatively fine,so the upper part has about 1.4%higher tensile strength and 8% lower elongation than the lower part,and the lower part has about 3% higher impact toughness than the upper part.After recrystallization annealing,the top structure of the sample is a mixed structure of long needle-shaped and short rod-shaped α grains formed by recrystallization,the middle part is short rod-shaped α grains and sheet-like structure,and the bottom is mainly sheet-like structure,which aggravated the unevenness of the organization.The decomposition of martensite α’ inside the recrystallized annealed sample led to a 10% decrease in tensile strength,and the coarse recrystallized α grains may be the cause of the decrease in elongation and impact toughness,which decreased by 20% and 13% respectively.The microstructure heterogeneity of the sample after annealing has been improved,the martensite α’decomposition inside the sample reduces the tensile strength by about 8%,and the coarse basket-weave microstructure generated reduces the elongation by 42%,and the impact fracture mode is brittle fracture,the impact toughness decreases by about 37%.After solid solution treatment,the structure of the sample is completely transformed into acicular martensite α’,and the uneven structure is completely eliminated.The widely distributed martensite α’ increased the tensile strength of the specimen by 7.4%,while the elongation decreased by about 20%.The impact toughness decreased by 11% due to the disappearance of α bundle Widmanstatten structures,and the fracture mode became closer to brittle fracture.It is difficult to eliminate the heterogeneous structure of the sample after aging treatment,and the aging treatment cannot completely decompose the martensite α’ of the sample,and the original coarse Widmanstatten structure continues to grow during the heat preservation.The decomposition of martensite α’ slightly decreased the strength of the aged specimen,but increased the elongation by about 20%.Although α clustering has a positive effect on hindering impact crack propagation,the coarse grain boundary α phase formed by heat preservation is also likely to become the source of failure.The interaction between the two causes the crack propagation work of the aging treated sample to decrease by about 13%,but the overall impact toughness increased by 6%.After solid solution + aging treatment,the sample structure is uniform,and the structure is composed of residual martensite α’ and coarse basket-weave Widmanstatten structure,and the hardness increases by about 11%.The tensile strength and elongation of the samples increased by 5% and 33%,respectively.However,the martensite α’ and the coarse basket-weave microstructure are not conducive to hindering crack growth,resulting in a decrease in the crack propagation work,which reduces its impact toughness by about 28%.