Microstructure And Properties Of Ti-6Al-4V Alloy Based On Laser Powder Bed Fusion Technology

The development of the transportation industry often depends on the progress of the material,especially titanium alloy is the ideal medium and low-temperature material,which has the advantages of high strength,low density,and strong corrosion resistance.Among titanium alloys,Ti-6Al-4V（Ti64）alloy accounts for more than 50%of the usage,and it has been widely used in the transportation industrial fields.With the fast development of additive manufacturing technology in recent years,Laser powder bed fusion technology（LPBF）plays a pivotal role.And the LPBF is expected to become one of the main forming technology in aerospace because of its characteristics of high melting temperature,high precision,and high density.Therefore,the development of Ti64 alloy in LPBF technology has become one of the first research topics.LPBF-Ti64 alloy has different microstructure characteristics in traditional forming methods.The structure of the as-built LPBF-Ti64 alloy can be divided into three layers of columnar parent grain,martensitic lath and twin from the outside to the inside,which can be called hierarchical structure.Giving full play to its microstructure advantages while avoiding its defects is the critical problem in process control,microstructure optimization,and performance improvement.In this study,according to the hierarchical structure characteristics of LPBF-Ti64 alloy,the process,microstructure,and strength-ductility mechanism were studied.Firstly,the machine learning method was used to optimize LPBF process parameters to solve the problem of low ductility in the as-built LPBF-Ti64 alloy.The machine learning model gives the following hints for controlling process parameters.Among the selected process features,the influence of hatching space on ductility is second only to heat treatment.When the linear energy density is0.1 J/mm,optimized ductility can be obtained by adjusting the hatching space.When the hatching space was 90μm,the excellent performance of strength-ductility synergy was obtained(yield strength,Ys_0.2=1044±10MPa,uniform elongation,UEL=10.5±1.5%).The martensite colony microstructure（MCM）of the as-built LPBF-Ti64 alloy was further described.MCM can cause local stress concentration and thus lead to low ductility.It is proved that the unique Less martensite colony microstructure（LMCM）can be obtained by adjusting the hatching space,which can de-localize the stress,thus improving ductility with little loss of strength.This proves that theα’-LPBF-Ti64 alloy is not intrinsically brittle and explains the large plastic fluctuation in different processes.In addition,the alloy has a high yield strength（>1GPa）by maintaining theα’phase,and finally,a strength-ductility synergistic optimization is obtained.Secondly,according to the hierarchical structure characteristics of the LPBF-Ti64 alloy,the microstructure and properties of the LPBF-Ti64 alloy under multiple optimization strategies were studied.Ti64 alloy has a very narrow solidification front,making it have coarse columnar parent phase grains during the rapid cooling forming process of LPBF.This can lead to problems with the mechanical anisotropy of printed parts.The martensitic morphology inside the parent phase grains also plays an important role in mechanical properties.For this hierarchical structure,combined with the previous empirical basis that reducing MCM can improve ductility,we propose a“Refining&Splitting”（R&S）strategy.The specific process includes the refinement of martensitic lath through high energy density,namely the R process.The optimized process can increase the yield strength by nearly 30%,thus reaching～1.3GPa.At the same time,the control of the laser scanning strategy can achieve similar splitting of the coarse parent phase grains and MCM structure,namely the S process,thereby increasing the elongation at the break by nearly86%.This R&S method has obvious process advantages because it does not require the introduction of additional equipment conditions and the addition of alloying elements.Finally,the relationship between the variant interface and the deformation mechanism of LPBF-Ti64 alloy was investigated.Previous studies focused on the relationship between parent grains and martensite morphology and mechanical behavior,and finally,the deeper variant interfaces were studied.We expounded that the 63.26°/[10 5 5 3]interface with the most content among the LPBF-Ti64 alloy variants is the essence of the asymmetric tilting grain boundary and proved the existence of（3 1 4 1）//（4 1 3 1）relationship,which lays the foundation for the interface deformation behavior.In addition,the behavior of the interface under different strains was studied by the quasi-in-situ electron back scattering diffraction（EBSD）technique,which showed that the interface would gradually annihilate with the straining process even its almost disappearance is consistent with the strain at which the mechanical instability occurs.In addition,the phase and microstructure of the LPBF-Ti64 alloy did not change significantly after heat treatment at 400℃,but the formed parts will have an obvious brittle effect.It is observed that the interface content of63.26°/[10 5 5 3]decreases at this temperature,which suggests that the interface has a great influence on the mechanical behavior.In summary,this study explores the process,microstructure optimization,and performance mechanism of LPBF forming Ti64 alloy technology.It focuses on solving the strength-ductility synergy,variant interface,and mechanical performance of the as-built LPBF-Ti64 alloy.This study provides theoretical guidance and new ideas for preparing high-performance LPBF-Ti64 alloys and promotes the application of LPBF technology in the aerospace field.