Damping Characteristics Of NiTi Memory Alloy Formed By Selective Laser Melting

Vibration exists widely and causes great harm to the service life,reliability and testing accuracy of instruments.Ni Ti memory alloy has good damping performance and can transform the mechanical energy of vibration into internal energy to achieve the purpose of vibration reduction and noise reduction.However,due to the poor machining performance of Ni Ti alloy,it is difficult to prepare Ni Ti alloy damping devices with complex structure.Selective laser melting（SLM）is an additive manufacturing method that uses high energy laser beam to melt metal powder and form parts by layer by layer melting and sintering.It is expected to be used to prepare high-performance Ni Ti alloy damping devices with complex structure.In this paper,Ni₅₁T₄₉（at%）memory alloy sample with high forming quality was prepared by using optimized SLM process parameters.The damping properties,microstructure and phase transformation behavior of SLM-Ni Ti alloy after solution and aging treatment were characterized in detail by means of dynamic mechanics analyzer（DMA）,differential scanning calorimeter（DSC）,transmission electron microscopy（TEM）and in-situ XRD.The internal relationship among damping characteristics,microstructure and phase transformation behavior of SLM-Ni Ti alloy was studied,and the mechanism of SLM-Ni Ti alloy with wide temperature domain and high damping characteristics was discussed.The damping results show that the initial temperature of internal friction peak decreases first and then increases with the solution time increasing at 700℃,and the initial temperature of internal friction peak is the lowest at 700℃/4.5 h.In the low temperature region,the internal friction value is higher than other solid solution samples.After that,with the decrease of aging temperature and the extension of holding time,the internal friction platform of Q^-1≥0.1 wide temperature domain gradually appeared.Among them,Q^-1=0.15 internal friction platform existed stably in the temperature range of 115℃（-10℃～-125℃）for 700℃/4.5 h+250℃/24 h samples.The grain size of the sample did not grow too large when observed by optical microscope.After mechanical test,the sample has good room temperature superelasticity.By TEM observation,SLM-Ni Ti memory alloy solution treatment at 700℃/4.5 h can eliminate the high density dislocations in the printed sample and make Ti₃Ni4redissolve.After 700℃/4.5 h+250℃/24 h treatment,homogeneous and dispersive ni-atom segregation zones were formed in the samples.DSC test showed that Mp and Ap decreased first and then increased with solution time increasing at 700℃.The700℃/4.5h sample had the lowest Mp=-95℃,and B2（?）B19’phase transition occurred in all solid solution samples.Then,with the decrease of aging temperature and the extension of holding time,the starting point of B2→R transition moved to the high temperature zone in the cooling process.All samples experienced B2（?）R（?）B19’two-step phase transition.XRD results show that R phase diffraction peak exists mainly in the temperature domain of internal friction platform.It is analyzed that the"precipitation-free"R phase transition induced by substrate heat is the main reason to improve the damping performance of SLM-Ni Ti alloy.The internal friction mechanism may be derived from the following aspects:first,the uniformly distributed ni atom segregation zone inhibits the thermally induced B19’martensitic transformation,and the temperature domain of the transformation is greatly broadened,leading to the existence of R phase interface in the wide temperature domain;Secondly,due to the limitation of the segregation zone of densely dispersed nickel atoms,the size of R phase formed by matrix heat-induced phase transformation is small,which significantly increases the amount of phase interface.Thirdly,in the temperature domain of the platform,the content of R phase with high intrinsic internal friction value shows an increasing trend and is stable.Fourthly,due to the small size of the segregation zone of the nickel atom,which is in common with the matrix and has the same crystal structure as the matrix,it can be speculated that it may not seriously damage the mobility of the phase interface or the twin interface as conventional reinforcement.