Study On Superelasticity Of NiTi Alloys Fabricated By Laser Powder Bed Fusion

NiTi shape memory alloys(SMAs)are promising functional metallic materials due to their outstanding functional properties such as shape memory effect(SME)and superelasticity effect(SE).Based on the unique functional properties,together with excellent mechanical performance and good biocompatibility,NiTi alloys have been widely applied in various engineering fields including aerospace,biomedicine and automotive industry.However,it is quite difficult to fabricate complex NiTi alloy structures via conventional processing techniques,due to the low weldability,high work hardening and high reactivity of NiTi alloy.The majority of the conventionally produced NiTi parts have simple geometries,including wires,sheets,and rods.The simple structure and single function of these parts severely limit the application of NiTi alloy.Laser powder bed fusion(L-PBF)is an ideal method to fabricated complex NiTi structures with high precision and good surface quality.However,the L-PBF-fabricated(L-PBFed)NiTi alloys face difficulties in achieveing good superelasticity at room temperature.Low Ni content of the pre-alloyed NiTi powders and the preferential Ni evaporation during L-PBF,which leads to the phase transformation temperature above room temperature,are the main reasons.In order to solve the above problems,the effects of the process parameters,Ni element addition,size effect on the processability,microstructure,phase transformation behavior,mechanical properties and functional properties of the L-PBFed NiTi alloys are systematically investigated.The L-PBFed NiTi alloys with room temperature superelasticity are successfully fabricated by L-PBF by regulating the phase transformation temperature.The main content of this work is summarized as follows:(1)Effect of process parameters on the microstructure and performances of the L-PBFed NiTi alloys.The NiTi alloy powders with high Ni content were used as feedstock.By varying the laser power(100～300 W)and scanning speed(200～300mm/s),90 sets of process parameters were used to improve the processability of the NiTi alloys and investigated the metallurgical defect characteristics of NiTi alloys.A wide,feasible process window,which could fabricate NiTi alloys with relative density of>95%,was determined.Based on the above results,the parameters were further optimized to investigate the relationship between process parameters and phase transformation temperature.The austenite transformation finish temperature(Af)could be below room temperature by parameter optimization.The optimized NiTi samples exhibited good superelasticity at room temperature,with a recovery ratio of≥85%under tension.(2)Effect of Ni addition on the microstructure and performances of NiTi alloys.Preferential Ni element evaporation during the L-PBF process leads to an increase in phase transition temperature.To lower the phase transformation temperature,Ni powders were added in the NiTi alloy powders by mechanical mixing.The effect of Ni addition on the processability,microstructure,phase transformation behavior,mechanical properties and functional characteristics of NiTi alloys was systematically investigated.The results demonstrate that adding Ni powders could effectively lower the transformation temperature of NiTi alloys.Specifically,adding 0.30 at.%Ni powders could lower the austenite transformation finish temperature(Af)below room temperature.Therefore,the L-PBFed NiTi alloys exhibited superelasticity at room temperature,with a recovery ratio of≥90%under tension.(3)Effect of size effect on the microstructure and performances of NiTi alloys.Firstly,the effect of sample thickness on the processability,microstructure,phase transformation behavior,mechanical properties and functional properties of NiTi alloy thin-wall was investigated.The results indicate a strong size effect in the L-PBFed NiTi alloys.The decrease of the sample thickness results in the monotonous decrease of the martensitic transformation temperatures.Based on the study of size effect,the phase transformation temperature of the NiTi lattice structure is effectively reduced by reducing the strut diameter.The NiTi lattice structure exhibited good superelasticity at room temperature,with a recovery rate of up to 78%under compression.