The Fundamental Process Study Of NiTi Shape Memory Alloy Manufactured By Selective Laser Melting

Near equiatomic NiTi shape memory alloy has been commonly employed in mechanical and electrical,biomedical sciences,and aerospace areas due to its unique form memory effect,super elasticity,high wear and corrosion resistance,and mechanical qualities.Selective Laser Melting（SLM）is used to manufacture NiTi alloy parts.Although it has obvious advantages in the manufacture of parts with complex shapes and structures,inappropriate process parameters can cause porosity cracks in the formed parts,or even failure to form,resulting in the limitation of the excellent properties of NiTi alloy.At the same time,the functional properties of NiTi alloy are essentially the mutual transformation between martensite and austenite,and the phase transformation temperature,which can significantly affect the behavior of both phase transformations,is highly dependent on the Ni/Ti composition ratio.In traditional manufacturing methods,precise control of the composition ratio of NiTi alloy often involves a wide range of parameters and is difficult.Therefore,in order to effectively control the phase transformation and properties of NiTi alloy,SLM technology is utilized to form NiTi alloy.Combining simulation and experiments,the forming process is studied to establish a complete phase transformation and manufacturing process window.The main research contents and conclusions are as follows:（1）The temperature field model of SLM additive manufacturing of NiTi alloy was established by ANSYS finite element software platform to study the effects of laser power and scanning speed on the temperature field distribution,thermal behavior,and molten pool structure.The results show that by increasing the laser power or decreasing the scanning speed,the melt pool size and temperature can be increased,and the melt pool existence time can be prolonged.At the same time,the increase of laser power or scanning speed can increase the maximum cooling rate of the molten pool.The scanning speed alteration can obviously influence the molten pool cooling rate,but the impact of scanning speed on the molten pool size was no significant compared with laser power.Through numerical simulation,it can be obtained that when the laser power and scanning speed are 150 W and 1000 mm/s,the molten pool size（width 103.34μm,depth:54.8μm）is conducive to good bonding between adjacent molten layers and adjacent scanning channels.（2）Based on the range of process parameters obtained from numerical simulation,single factor experiments were conducted to optimize the process parameters with the goal of increasing density.At the same time,the influence of laser power and scanning speed on the microstructure and phase transition behavior of the formed samples was explored.The results show that most of the samples consist of B2 phase and a small amount of B19’phase,and the B19’phase content decrease with a decrease in laser power or an increase in scanning speed.The grain morphology on the section parallel to the deposition direction gradually changes from irregular shape to nearly quadrangular shape with the increase of laser power,and the grain size gradually increases.However,the grain morphology tends to be regularized first and then to be fine and disordered because scanning speed affects the temperature of the molten pool.The phase transition temperature of NiTi alloy shows a gradual increasing trend with the increase of laser power or the decrease of scanning speed,but there is no obvious rule between the phase transition enthalpy and process parameters.When the scanning speed is 1400 mm/s,the enthalpy of martensite transformation is approximately 0,and its transformation is suppressed.（3）The influence mechanism of process parameters on the microhardness,tensile properties,wear resistance,and shape memory effects of NiTi alloy was explored based on the influence law of process parameters on the microstructure and phase transition temperature of NiTi alloy.The results show that the microhardness and wear resistance of the sample increase first and then decrease with the increase of laser power or the decrease of scanning speed.When the laser power is 100 W and scanning speed is 1200 mm/s,the above two properties are the best.In this case,the microhardness is 251.2 HV_0.2,and the wear volume is 0.055 mm³.Under the condition of small density difference,higher laser power or scanning speed can improve the strength and the elongation of sample,and the laser power can improve the tensile strength and elongation to a greater extent than the scanning speed.The maximum values of 675.14 MPa and 8.38%can be obtained when the laser power,scanning speed and hatch spacing were 150W-1000 mm/s-0.05 mm.The shape memory effect of NiTi alloy is judged by the shape recovery rate,indicating that higher energy density can improve the shape memory effect of NiTi alloy.