Study Of Mechanical And Shape Memory Properties Of NiTi Alloy Fabricated By Selective Laser Melting

NiTi shape memory alloy is widely used in aerospace,biomedical,automotive engineering and other fields due to its unique shape memory effect,superelasticity,excellent biocompatibility,wear resistance,corrosion resistance and other properties.However,due to its superelastic,wear resistance,and high work-hardeing,it is difficult to prepare NiTi alloy parts with complex shapes by traditional smelting and machining,which greatly limits its further application.Selective Laser Melting(SLM)additive manufacturing technology shows great potential and prospect in the preparation of NiTi alloy parts with complex shapes due to its manufacturing characteristics of Laser scanning powder line by line and layer by layer and cumulative forming.Nevertheless,due to the holes,columnar grains,large beam diamater,etc.,the SLM-NiTi alloy still has many performance limitations and application problems.This study will focus on the following three key issues to explore and propose corresponding solutions:(1)The SLM-NiTi alloy exhibits poor tensile properties,for example,the maximum tensile strain is less than 8%,and poor shape memory effect that is not comparble to conventional wrought NiTi alloy.(2)The SLM-NiTi porous structure has poor deformation ability,of which the maximum non-damage deformation is not more than30%,as well as poor shape recoverability and deformation-recovery cycle stability.(3)The SLM-NiTi parts have a low manufacturing accuracy,of which the minimum manufacturing size is greater than 300 μm,and it is difficult to realize micro-device manufacturing with both superior mechanical and functional properties.Firstly,SLM has the characteristics of fast heating and cooling and layer by layer forming,which makes the prepared NiTi alloy inevitably have defects such as pores and coarse unidirectional columnar crystals.This causes the material is prone to experience stress concentration and early failure under load,resulting in poor tensile plasticity(<8%).In this study,by comprehensively exploring the optimal combination of intralayer laser scanning vector length and interlayer laser scanning angle,the optimal laser scanning strategy of strip partition and rotation was obtained.It is demonstrated can significantly improve the tensile plasticity of SLM-NiTi.The SLM-NiTi alloy prepared by this strategy can exhibit tensile fracture strain of up to 15.6% and fracture strength of 700 MPa,as well as excellent shape memory effect.According to the analysis,the short scan vector generated by strip partition can regulate the melting heat history of powder,prolong the residence time of high temperature section,promote the full melting of powder,decrease the solidification rate,promote the full escape of retained gas,and then improve the forming density.The rotation strategy can avoid laser overlapping scanning between layers,change the direction of heat dissipation and grain growth between layers,avoid the formation of unidirectional columnar crystals,delay crack growth and improve fracture strain.Secondly,in view of the problems of poor deformation ability and shape recovery performance of SLM-NiTi porous structure,we applied the structural design concept of"large strain at macro and small strain at local” to the structure design of NiTi porous structures.It requires that the configuration design should meet the uniform deformation,no serious deformation zone,and the local strain of the structure should not exceed the recoverable strain of the material itself(～8%).Based on this concept,the fabricated NiTi honeycomb structure can show a large deformation of 80% without fracture,as well as a high shape recovery rate of more than 98%.The fabricated NiTi three-dimensional lattice structure and negative Poisson structure can also show more than 50% large deformation and high shape recovery of more than 99%.Furthermore,it is found that the secondary hardening ability of NiTi alloy can compensate the structural weakening of porous structure under load,restrain the excessive deformation of local zone,and promote the expansion of deformation to other regions,thus achieving uniform and stable deformation behavior of the whole structure.Finally,in order to improve the manufacturing accuracy(i.e.,the minimum manufacturing size)of NiTi alloy parts prepared by SLM,we reduce spot diameter,powder particle size and powder layer thickness to a smaller size synchronously.On the basis of optimizing combined laser power and scanning rate,μ-SLM fabrication of NiTi alloy micro-devices(devices with minimum feature size <100 μm)was successfully realized.The prepared NiTi alloy micro-devices,such as thin-wall parts,micro-lattices and micro-stents,can not only achieve high manufacturing performance,such as minimum feature size of 52 μm,surface roughness of less than 2 μm,relative density of higher than 99.8%,but also show excellent mechanical and shape recovery properties.The NiTi thin-wall parts prepared by μ-SLM can exhibit tensile plasticity of >8% and superior shape memory effect.The prepared NiTi micro-lattices/stents can undergo 50%large deformation without fracture,and the shape recovery rate is more than 98% after heating.In addition,due to the unique single-track laser scanning mode,the NiTi thin-wall structure exhibits different pool morphology and grain distribution with conventional SLM-NiTi alloy,such as fan-shaped grain distribution in the center and nearly vertical grain distribution on both shoulders.Due to the weak thermal cycling process(less remelting/reheating)caused by single pass scanning,the NiTi alloy prepared by μ-SLM exhibits larger phase transformation peak width,smaller latent heat of phase transformation,lower dislocation density and smaller precipiatate size than the conventional SLM-NiTi alloy,.This study proposed innovative solutions to several key problems faced by SLM in the preparation of NiTi alloy,which significantly improved the mechanical properties and shape memory function of SLM-NiTi alloy material and structure.It will siginificantly promote the development of additive manufacturing NiTi,and promte more widely applications of NiTi alloy as functional devices,for example,repeatable cushioning energy absorbing materials,lightweight damping shock absorbing materials,self-expanding medical implants or stents,programmable intelligent drives or braking devices,etc.