| Lightweight and high-strengthα+βtitanium alloys are commonly used in the aerospace fields,which also belong to the typical difficult-to-process materials via traditional subtractive manufacturing.Laser Powder Bed Fusion(LPBF)technology shows advantages in fabricating complex-shaped titanium alloy components.As the representativeα+βtitanium alloy,the LPBF-fabricated Ti6Al4V(TC4)alloy has been used in the aerospace field.However,their mechanical properties are still insufficient to meet the increasing application requirements.LPBF of the existingα+βhigh-strength titanium alloys is expected to further improve the mechanical properties significantly as compared with that of TC4 and achieve the engineering applications.To realize the effective control of defects,microstructure,and properties,it is necessary to understand the forming characteristics of the LPBF-fabricatedα+βhigh-strength titanium alloys.Herein,the LPBF process of high-strengthα+βphase titanium alloy(Ti-6.5Al-3.5Mo-1.5Zr-0.3Si,TC11)was investigated.The influence of process parameters and scanning strategies on the forming quality,microstructure,and mechanical properties of parts were discussed.On this basis,the composite scanning strategy was investigated.The main works are presented below.(1)The forming process of LPBF-fabricated TC11 titanium alloy was investigated.The forming process of LPBF tracks was optimized.A numerical model based on the heating,melting,and cooling processes were established.The formation mechanisms of defects,such as discontinuous track,irregular track,powder bonding,and spatters were described.A density prediction model with a model error of±1%was established,and the optimal process parameters were P=195 W,V=1250 mm/s,and H=87μm.(2)The influences of volume energy density on surface morphology and porosity were described.The high strength-plastic specimens with a tensile strength of 1207.6 MPa,yield strength of 965.0 MPa,and elongation of 14.6%were obtained.The competition between{10-12}deformation twins and dislocation on tensile properties were described.The strengthening mechanisms were analyzed,clarifying the influences of oxides and microstructure around pores on mechanical properties.When the volume energy density increased from 59.8 J/mm~3 to 85.9 J/mm~3,the strength increased and elongation decreased due to the interaction of twin boundary strengthening and porosity.(3)The scanning strategies of bulk and thin-wall structures were investigated.The correlation of laser jumps,island deformation,and peak temperature frequencies with the gully and basket-weaveα’phases in the overlapped zone was clarified.With the increase of the island size from 2 mm to 7 mm,the gully on the surface of the bulk structure decreased,and the fraction of basket-weaveα’phase structures decreased,leading to a decrease in tensile strength and increase in elongation,respectively.The influences of the zigzag scanning strategy and island scanning strategy on forming quality of thin-wall structure were also analyzed.The smaller the thin-wall thickness and island size are,the greater effect of a gully on forming quality.(4)The forming process path planning methods based on intra-layer variable scanning strategy between intra-later and inter-layers were proposed,respectively.The defect optimization and microstructure transition of the composite structure of the bulk and thin wall were realized.The sample with a feature transition distance of 4 mm and a feature boundary offset of 0.67 mm showed high tensile strength and elongation.It was found that the variable scanning strategy between interlayers could regulate the microstructure,reducing the stress-deformation and pore defects,and improving the tensile properties of the formed parts.When the interlayer ratio of zigzag scanning and island scanning was 3:1,the samples exhibited higher strength and plasticity.The results can shed light on the intelligent allocation of the scanning strategy. |
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