Study On The Process,Microstructure And Properties Of NiTi Shape Memory Alloys Fabricated By Laminar Plasma Additive Manufacturing

The laminar plasma additive manufacturing has the advantages of high deposition efficiency,low cost,and no need for molds.It has broad application prospects in the fields of aerospace and rail transportation,etc.In this thesis,the process,microstructure and properties of NiTi shape memory alloy fabricated by laminar plasma additive manufacturing with bypass wire feeding were discussed.The samples of NiTi shape memory alloys were successfully prepared by laminar plasma additive manufacturing technique.The microstructure distribution feature and phase composition of NiTi alloy were analyzed.Finally,the two-way memory effect of NiTi alloy was achieved by constraint aging.The influence of the constraint aging process on the two-way memory effect,the two-way deformation feature and electric heating feature of the shape memory alloy were studied.Firstly,preliminary experiments were carried out using 308L stainless steel as raw material to explore the correlation between the fabricating process and microstructures by means of laminar plasma additive manufacturing technique.The experimental results show that in the bottom and middle regions of the specimens,there exhibit a microstructure of columnar grain growing across the layers,but in terms of the transition from the middle region to top region,the transition from the columnar to equiaxed grains can be observed,and in the top region,each layer is dominated by the equiaxed grain structure,which can also be observed at the layer boundary.The density of the bottom,middle and top region of the 308L stainless steel sample is basically the same.The average density is about 98.3%.The parts have no obvious defects.The tensile test results show that the bottom and middle regions of the sample are anisotropic in mechanical property.On the basis of preliminary experiments,the nickel-titanium alloy samples were successfully fabricated by laminar plasma additive manufacturing technique,and the process,microstructures and properties were investigated.The experimental results showed that the microstructure distribution feature is similar with 308L that the grain microstructure is columnar at the bottom and middle region,which was growing consistently across the deposited layers,and gradually transformed into equiaxed grain structure from the middle to top region.The average density of the nickel-titanium alloy sample is more than 99.6%,without obvious cracks and pores.The phase composition mainly consists of austenite,a small portion of martensite and precipitated phase existing in Ti Ni₂,Ni₃Ti and Ni₄Ti₃.The two-way memory effect was achieved by solution treatment and constraint aging for nickel-titanium shape memory alloy samples fabricated by laminar plasma additive manufacturing.The phase composition of the sample after constraint aging is mainly austenite.There are evenly distributed Ti₂Ni phase and Ni₄Ti₃ phase.The nickel-titanium shape memory alloy is treated with constraint aging at450?.As the constraint aging time increases from 1h to 10h and 20h,it can be observed that the recovery ratio of the two-way memory effect gradually increases.And at 20h,It can be seen that the phase transition temperature range becomes narrower and the response speed of phase transition becomes faster.When energizing a nickel-titanium shape memory alloy with the current of 5A,6.5A and 8A respectively,its temperature continues to rise with the extension of the energizing time and eventually tends to be stable.The higher the current,the higher the temperature of nickel-titanium shape memory alloy in the steady state.The mechanical properties of the nickel-titanium shape memory alloy manufactured by laminar plasma additive manufacturing and the heat-treated nickel-titanium shape memory alloy are different.The maximum tensile strength of the printed state sample is 990 MPa and the elongation is 8.3%.The tensile strength of the sample after heat treatment is 840 MPa and the elongation is 9.1%.