Study On Two-way Memory Effect Of TiNi Shape Memory Alloy Fabricated Selective Laser Melting

The two-way shape memory effect of TiNi alloy enables it to realize spontaneous shape change,output force and displacement at high and low temperature,so it is considered to have broad application prospects in the field of actuation.However,it is difficult to prepare high-precision TiNi memory alloy drive devices with complex configurations with traditional machining methods.Selective Laser Melting(SLM),as a kind of additive manufacturing technology,has the ability to prepare complex structures.At the same time,due to its high surface finish and high dimensional accuracy,it has been widely used in TiNi alloy in recent years.In this paper,the two-way shape memory effect of SLM formed TiNi alloy is systematically studied,and the development law of its twoway memory effect is explored through the methods of matensite deformation and thermomechanical training.Through the design of elementary cubic truss lattice structures,a large two-way memory strain(11.93%)was obtained for the first time in the TiNi alloy porous structure fabricated by SLM,which is expected to provide new ideas for the design and preparation of new complex-structure TiNi memory alloy drive devices.The main research contents and results of this paper are as follows:Ti-50.2 at.%Ni alloys samples with different building orientations were fabricated by selective laser melting,and the two-way memory effect was studied by the method of matensite deformation.The research results show that the building orientations has no obvious effect on the two-way strain obtained by the pre-deformation.The maximum two-way strain obtained by the pre-deformation method is 2.24%(with a pre-strain of12%),which is lower than that of the conventional commercial TiNi alloys’ 4% to 5%.The large plastic deformation during the deformation process is the main reason for the smaller two-way strain.After ten times of shape memory training with a strain of 12%,the irreversible strain of the TiNi alloy formed by selective laser melting increased from5.53% to 7.67%,but at the same time the two-way strain was improved to a certain extent(2.24%→2.72%).The analysis shows that the newly generated dislocation structure and the stress field during the cycling process is generally beneficial to the nucleation and growth of the preferentially oriented martensitic variant during the cooling process,which promotes the increase of the two-way memory strain.In addition,after 10 cycles of heating and cooling with different constant stress,the maximum two-way memory strain obtained by the sample was 1.90%,indicating that the two-way memory effect of TiNi alloy prepared by SLM is closely related to the constant stress value.Four elementary cubic truss lattice structures with different configurations were designed and fabricated,and their mechanical properties and two-way memory effects were tested.The experimental results show that,in the elementary cubic truss lattice structure system,the pre-strain value of the shape memory loop training is the most important factor affecting the obtained two-way strain.After 10 times of shape memory cycle training with a pre-strain of 65%,the two-way strain of the BCC lattice structure can reach 11.93%,which provides a new idea for the design and fabrication of complexstructure TiNi alloy driven devices.