Research On Microstructure And Performance Of Ti-22Al-25Nb High-temperature Alloy Additively Manufactured By Selective Laser Melting

The aviation industry is one of the hottest topics in China.The keyways to achieve the rapid development of modern aviation technology are to develop high-temperature structural materials with high specific strength,and their advanced processing methods.Ti₂AlNb-based alloys have shown great potential to replace those well-developed but heavy-in density Ni-based superalloys,due to their excellent high-temperature specific strength,creep resistance and high fracture toughness.However,Ti₂AlNb-based alloys are difficult to be manufactured by traditional processing methods,because of the hardto-deformation nature.Selective Laser Melting（SLM）technology,emerged as a kind of advanced Additive Manufacturing technologies,has shown great advantages in fabricating the Ti₂AlNb-based alloys parts with complex structures.At present,there exist four key scientific issues to be solved in the manufacturing of the Ti₂AlNb-based alloys by SLM.The first one is about the interaction mechanism between original powder and laser.The second one is about the underlying reasons for the enhanced mechanical properties and the phase transition mechanism during SLM.The third one is about the effects of SLM processing parameters on the mechanical properties and microstructure of the as-printed samples.The last one is about performance modification of the asprinted Ti-22Al-25Nb alloy by heat treatment.This study will focus on the above four scientific issues.The main aims of this work are to solve the difficulty in workability of the Ti₂AlNb-based alloys by SLM,and obtain high-performance Ti₂AlNb-based alloys suitable for the aviation industry.The interaction mechanism between Ti-22Al-25Nb alloy powder and laser was first revealed.The surface oxide layers and the distribution along the depth direction were revealed as follows by X-ray photoelectron spectroscopy,i.e.Ti O2 + Nb2O5（as the outer surface layer）/ Ti O2+Nb2O5 + Al2O3（as the intermediate layer）/Al2O3（as the inner surface layer）/ Ti-22Al-25Nb（as the matrix）.The effect of the surface structure on the laser absorptivity was calculated based on the available model.On the basis of powder study,the effects of SLM processing parameters（hatch distance and laser power）on the densification behavior during SLM process were further studied.The processing map with regard to relative density was established.The highest relative density of 99.93% can be obtained,when the laser power was 140 W and the hatch distance was 0.12 mm.The microstructure,phase feature,and mechanical properties of the as-printed Ti-22Al-25 Nb were systematically investigated by a variety of material characterization methods.It was found that the cell or near-planar microstructures perpendicular to the printing direction are proved to be related to the microstructure growth characteristics parallel to the printing direction,which are determined by the heat flow direction,and temperature gradient of the adjacent molten pools.Two microstructural features are demonstrated as the main contributing factors to the outstanding mechanical properties（Yield strength: 960.02 MPa,Elongation: 22.73%）:（a）the high-density dislocation networks,and（b）the favorable phase constitution,including the nano-scale O phase precipitates within the disordered β phase,and disappearance of the brittle α2 phase.The phase transition mechanism of Ti-22Al-25Nb alloy during SLM process was determined by in-situ laser heating synchrotron radiation,electron back-scattered diffraction,and Thermal-Calc simulation.The B2 phase was first precipitated from the molten liquid.The O phase formation involved both displacive transition（B2 → B19）and chemical ordering（B19 → O）.The metastable B19 phase（as an intermediate stage）may be formed by shearing cubic B2 phase along（110）[（?）11]direction into an orthorhombic structure under high residual stress.The α2 phase as a primary decomposition product was suppressed by high cooling rate.Based on the above results,the effects of SLM processing parameters（hatch distance and laser power）on the microstructure and mechanical properties of the as-printed alloy were further explored.The relationships between processing parameters-microstructuremechanical properties were established.The underlying reasons for variations of the microstructure with processing parameters were clarified through the simulation of the temperature field inside the molten pool.It was found that increasing hatch distance can reduce the heat input and increase the temperature gradient of the adjacent molten pools,which increases the dislocation density and O phase content and refines the grain size.Therefore,the mechanical properties are improved.However,too large hatch distance will lead to poor metallurgy bonding between adjacent melt pools,deteriorating the performance of the as-printed alloy.The laser power mainly controls the grain size by affecting the heat input,but has little effects on the dislocation density and O phase content.In order to improve the high-temperature mechanical properties and hightemperature microstructure stability of the as-printed alloy,the heat treatments of the asprinted alloy were explored.The effects of solution treatment and aging treatment on the microstructure and mechanical properties of the as-printed alloy were systematically studied.The relationships between the microstructure and mechanical properties,and the phase transformation mechanism during heat treatment were illustrated.It was found that the mechanical properties of the heat-treated samples mainly depended on the type of precipitated phase,morphology and its volume fraction,grain size,as well as oxygen content.The acicular O phase can significantly improve the strength of as-printed alloy,but it is detrimental to the ductility.The rodlike O phase can improve the ductility and strength both at room temperature and high temperatures.The main reasons for the poor ductility of the heat-treated samples can be attributed to the grain boundary α2,fine acicular microstructure,and high oxygen content.The microstructure of solution-treated samples is rather unstable at high temperature,which can be greatly improved by aging treatment.The phase transformation mechanism during the heat treatment process is demonstrated as follows:（1）Widmanstatten precipitations of α2 and O phase are mainly determined by the lattice mismatch and concentration gradient,（2）rim O phase is formed between the B2 and α2 phase and keeps a coherent orientation relationship with the B2 and α2 phase,（3）discontinuous precipitations of plate O and disordered β along the grain boundary depend on the composition gradient,and（4）composition invariant phase transformation also occurred during heating process due to the low free energy composition curve between the B2/β and O phase.In summary,the Ti-22Al-25Nb alloy was successfully fabricated by SLM.Some key scientific issues were clarified,such as underlying reasons for the enhanced mechanical properties and phase transition mechanism during SLM and subsequent heat treatment of Ti-22Al-25Nb alloy.This work can provide theoretical guidance and new insight for the preparation of high-performance Ti₂AlNb-based alloys,and promote the industrial applications of Selective Laser Melting technology in the field of aviation industry.