Study On Superelastic Properties Of NiTi Memory Alloys Fabricated By Selective Laser Melting

NiTi shape memory alloys have unique shape memory and superelastic properties,and have been widely used in biomedical,aviation,aerospace and other fields.In recent years,Selective Laser Melting(SLM)has been used as an additive manufacturing technology to fabricate NiTi devices with complex structures.However,due to the coarse grains of SLM-fabricated NiTi alloys,its yield strength is low,and it is easy to produce large plastic deformation during the deformation process,which seriously deteriorates its superelastic properties and cyclic stability.How to realize the excellent superelastic properties of SLM-NiTi alloy has become a difficult problem in the field of SLM-NiTi alloys.In this paper,high-quality SLM-NiTi alloy samples were first obtained by optimizing the parameters of the selective laser melting process,and then a nano-scale Ni atomic segregation region structure was fabricated in the SLM-NiTi alloy by a unique heat treatment process,which significantly improved the superelastic properties of SLMNiTi alloys.The main research contents and conclusions of this paper are as follows:In this paper,the effects of SLM process parameters on the microstructure,phase transformation behavior and superelastic properties of NiTi alloys were investigated.It was found that there are many kinds of pore defects in SLM-NiTi alloys.Keyholes will be formed when the input energy density is high,irregularly unfused pores will be formed when the input energy density is low,and tiny pores will be formed when the input energy is moderate.The martensitic transformation temperature of SLM-NiTi alloy decreases with the increase of laser scanning speed and scanning distance,and increases with the increase of laser power.This phenomenon is mainly related to the burning loss of Ni element during the SLM forming process.The tensile superelastic properties of the SLMNiTi alloy is much lower than the compression superelastic properties.The main reason is that the tensile is more sensitive to pore defects,and crack initiation and propagation are easy to occur,resulting in premature fracture of the sample and unable to exhibit excellent superelasticity properties.In this paper,the effects of heat treatment process on the phase transformation behavior,microstructure and mechanical properties of SLM-NiTi alloys were investigated.It was found that after the SLM-NiTi alloy was solution treated at 700°C for4.5 h and aged at 250℃ for 24 h,a uniformly dispersed nanoscale Ni atomic segregation zone structure was fabricated in the sample,and this nanostructure would inhibit the the B2→B19’ martensitic transformation during the cooling process,resulting in the R-phase transformation,which can effectively improve the mechanical property.In this paper,the effect of heat treatment process on the superelastic properties of SLM-NiTi alloy was studied.It was found that the combination of low temperature solid solution and low temperature aging could form a uniformly dispersed nickel atom segregation region in SLM-NiTi alloy,which can significantly improve the tensile superelastic strain and the cyclic stability of tensile superelasticity.The SLM-NiTi alloys treated by solution treatment at 700°C for 4.5 h and aged at 250°C for 24 h exhibited tensile superelastic strain as high as 7%,and the superelastic strain was still as high as5.2% after 10 cycles of 6% tension.The tensile superelastic strain and cyclic stability are close to the levels of NiTi alloys used by traditional manufacturers.