Fundamental Study Of NiTi Shape Memory Alloy Fabricated By Selective Laser Melting

NiTi shape memory alloy(SMA)has been widely applied in areas such as biomedicine,aviation,aerospace,and energy industries for its excellent mechanical properties corrosion resistance,biocompatibility,and especially shape memory effect as well as hyperelasticity.NiTi parts are normally fabricated by casting with following mechanical process or powder metallurgy.It is likely to lead to interstitial element of C and O in the cast process.Meanwhile,it also shows poor processing performance in the following mechanical process.The demand of mold in powder metallurgy significantly increase the cost of production and result in poor mechanical properties.Relatively,selective laser melting(SLM)can effectively solve the problems in NiTi SMA fabrication combined with the characteristics of additive manufacturing and melting process.Considering the Ni-Ti atomic ratio,which can significantly affect the martensitic transformation and shape memory performance of NiTi alloy,Ni and Ti mixed powders are used as raw materials to rapidly adjust the properties of NiTi alloy.However,SLM of NiTi SMA through mixed Ni-Ti powders can be barely seen.In this paper,the forming properties,Ni-Ti reaction process,microstructural characteristics and evolution mechanism,phase transition behavior and shape memory effect of NiTi SMA are studied.The conclusions are summarized as follows:The forming quality of NiTi samples fabricated by SLM was first studied in this paper.The microstructures of single-pass cladding samples can be seperated into two parts and the distribution of elements is not homogeneous.However the homogeneity was improved due to the remelting of adjacent scanning tracks.The samples without macroscopic defect were fabricated by SLM at laser power from 160 W to 200 W,scanning speed from 5m/min to 25m/min and linear energy density from 0.38 J/mm to 1.44 J/mm.During SLM process,macro-crack is the major defect of SLMed NiTi samples.Induced by laser thermal stress,micro-cracks propagation along Ni-rich area leads to macro-cracks.Furthermore,the element content of SLMed NiTi is different from the design value of mixed powders.The loss of Ti element is greater than that of Ni element for the evaporation of metal powders and the escape of elemental powders.Phase composition and microstructures are the basis of the properties of SLMed NiTi samples.At room temperature,B2 and B19' are the principal phase of all the SLMed NiTi samples.This phase characteristic is also the premise condition of shape memory effect for NiTi alloy.However,linear energy density also strongly affects the phase composition and microstructures of NiTi samples.The results could be divided into three types,as follows.Samples fabricated at high energy density,had fine grains.In this case,Ti2Ni phase forms around NiTi phase after the solidification of NiTi phase.Moreover,a medium energy density resulted in large grain size and single NiTi phase in the samples.Samples formed at low energy density had a mass of impurity phases where the grain boundaries of NiTi phase are hard to be observed.According to the microstructures of single-pass cladding samples,the shaping mechanism of Ni-Ti mixed powders is different from that of prealloyed powders during SLM process.The chemical reaction between Ni element and Ti element and stirring effect in molten pool play important roles in the component and homogeneity of deposited layers.High laser power density penetrated several cladding layers and led to several times remelting.The microstructures of SLMed NiTi samples and the SLM processing characteristics prove that repeated melting is effective in the homogenization.As well as experimental study,the results of temperature field simulation also prove the significance of remelting to the homogenization.Moreover,the results indicate that keyhole effect is another factor for homogenization.The phase transformation behavior is influenced deeply by phase composition and microstructures.The SLMed samples formed at high linear energy density with fine grains and low residual stress show obvious martensite phase transformation characteristics.With the decrease of energy density in a certain degree,martensite phase transformation of SLMed samples is inhibited by Ni-rich NiTi phase and high residual stress.At low linear energy density processing,the formation of impure phase leads to the forming of equilibrium NiTi phase and macro cracks release partial residual stress in the samples.Thus,a small quantity of martensite phase transformation take place during heating and cooling process.In conclusion,the phase transformation behavior of SLMed samples can be divided into three types:strong phase transformation behavior,abnormal phase transformation behavior and weak phase transformation behavior.Most of the SLMed NiTi samples obtained property of shape memory effect although defects of pores and cracks are still exist.With the increasing of pre-strain,the shape recovery ratio of all the samples decreased.Samples with the pre-strain of 1%can return to original state when they were heated to above 150 ?,while the samples with pre-strain above 1%cannot.NiTi samples fabricated at laser power of 180 W has well performance on shape recovery ratio and bendability.Its shape recovery ratio is above 85%when the pre-strain is 6%.Compaired with traditional casting process and powder metallurgy method,SLMed NiTi samples show better performance on phase transformation temperature and temperature interval,especially the phase transformation energy.Its phase transformation energy varies from 19.6 J/g to 27.7 J/g,which is superior to that of casting samples and powder metallurgy samples.It proves that SLM process has technological superiority to produce NiTi shape memory alloy.