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

Near equiatomic NiTi alloy has been widely applied in many engineering fields due to its excellent corrosion resistance,wear resistance,good mechanical properties,and biocompatibility.The most concerned is the unique shape memory effect,which is the most important functional characteristic often utilized in the aerospace,automotive,and medical instruments fields,such as an orthodontic wire.However,it isn't easy to control the composition and precise structure in the traditional manufacturing and machining process due to these advantages and chemical activity.The subsequent applications are mostly simple geometry such as wire,rod,sheet,and tube.This greatly limits the application potential of Ni-Ti shape memory alloys(SMA).This topic uses selective laser melting(SLM)to manufacture NiTi alloys.SLM is a common additive manufacturing technology,which has great advantages in manufacturing complex structural parts and can effectively solve NiTi SMA fabrication problems.However,the forming process of SLM is complicated.For example,residual stress caused by the extreme cooling rate,the pore cracks caused by excessive or insufficient energy input,and the brittle precipitation caused by improper process parameters will induce the properties of NiTi alloy parts insufficiently.Moreover,the SLM involves many process parameters and makes it difficult to find an optimal process parameter.Many scholars and engineers have given the optimal process parameter range in previous studies,but the conclusions and observations are not consistent.A large part of this is caused by the difference in SLM equipment and material supplier,and the other reason may be that there are too many process parameters to traverse completely.Therefore,it is necessary to design a complete common process flow to establish a manufacturing process window to prepare NiTi alloy parts with good performance.On the other hand,since the functional characteristics of a NiTi alloy is derived from the thermo-elastic martensite,namely the transformation between austenite and martensite,the two-phase ratio in the alloy is very important,while the chemical composition(Ni-Ti ratio)is one of the factors that can affect the behavior of phase transformation of NiTi alloy.During the SLM,the complex process parameters would cause Ni-Ti ratio changes,thereby inducing the ratio of austenite and martensite changes,which is critical for the application of NiTi alloy parts.Therefore,establishing a complete process window of phase transformation is of great significance to the design and production of NiTi alloy parts with expected phase transformation behavior.In this article,based on the principle of forming of additive manufacturing,we first studied the impact of laser power and scanning speed on the shape of the molten pool.The optimization range of key process parameters laser power and scanning speed is50 W to 90 W and 100?900 mm/s,respectively,by single factor experiment.Since the laser is the core of energy input,the impact on the molten pool is the largest.Thus,even if the linear energy density keeps constant via control of the scanning speed,the width of the molten pool also increases with the laser power increases,which may result in a situation where the calculation energy density is the same,but the fabricated NiTi alloy has different properties.In this situation,the hatch spacing can adjust the degree of molten pool overlap to optimize the alloy properties further.The optimization range of the hatch spacing was 60 to 150 ?m by experiment.On this basis,the generality of hatch spacing was studied,and it was found that "optimal energy density" was not inconstant,and the manufacturing process window was established.Subsequently,according to the distribution of "optimal energy density," the process window of phase transformation was established,and the mechanism was research.The step of establishing these two process windows provides a common method for SLM to manufacture NiTi alloys and use process parameters to control phase transformation behavior.Finally,from the perspective of scanning strategy,the remelting process' s utilization provides an effective way to solve the narrow SLM process window of NiTi alloy.