Study On The Basic Properties Of Cu-Al-Ni-Ti SMAs Fabricated By Selective Laser Melting

Cu-Al-Ni-X SMAs is a kind of copper-based SMAs with high phase transition temperature and thermal stability.Its phase transition temperature can be adjusted from-180to 400°C,so it has good potential for high temperature application.When forming by the casting method,impurities are easily introduced,and the crystal grains of the alloy are coarse and the material is brittle.Selective laser melting（SLM）technology is one of the metal additive manufacturing processes.It uses high-energy beam laser to melt fine metal powder point by point and layer by layer,and can manufacture complex fine metal parts.During the SLM process,the material cooling rate can reach 10⁶¹0⁸°C/s,which has obvious fast cooling characteristics,and it is easy to form fine crystal grains,which is beneficial to improve the performance of the alloy.In view of forming complex structure and excellent metallurgical properties,this paper explores the use of SLM to prepare Cu-Al-Ni-X shape memory alloys.The microstructure,performance characteristics and process rules of copper-based memory alloys under the new process are studied,which laid a theoretical and technical foundation for SLM forming complex copper-based memory alloy parts.Cu-13.5Al-4Ni-0.5Ti prealloyed powder is used as raw material,and Ti element is added to refine grains.The main findings and conclusions of the paper are summarized as follows:（1）By analyzing the continuity,flatness and spheroidization of the melting tracks,the bulk density and the microstructure of the alloy under different laser power and scanning speed.The optimal process parameters of SLM-fabricated Cu-13.5Al-4Ni-0.5Ti memory alloy are determined.The optimized process parameters are:laser energy density of about107 J/mm³（power of 310 W,scanning speed of 800 mm/s,layer thickness of 0.04 mm,and scanning space of 0.09 mm）.The alloy with good forming quality and relative density over99%can be obtained under this process parameter.Excessive or low energy density will lead to voids or spheroidization in the alloy.When the scanning speed is 600⁷00 mm/s,the density substantially decreases with the increase in laser power but increases with the increase in scanning speed.The combination of the processing parameter,a high laser power and a low scanning speed easily makes the metal solution boil in molten pools due to an extremely high temperature,which causes the splashing of small metal droplets and finally the formation of small pores in the samples and a lower density.When the scanning speed is improved to 800⁹00 mm/s,the relative density exceeds 99%.In this case,the density only slightly varies with the laser power and scanning speed.Thus,the laser energy is sufficient and appropriate to completely melt powder to form a continuous molten pool.（2）Based on the optimized process parameters,the phase composition and microstructure of the alloy at room temperature are studied.Compared with castings,due to the rapid cooling rate during SLM,the precipitation ofαphase andγ₂ phase are inhibited in SLM-prepared Cu-13.5Al-4Ni-0.5Ti alloy,and onlyβ₁’martensite phase is produced.The average grain size is 43μm,which is only 1/10 of the cast.Martensite is plate-like and needle-like,interlaced within the grain.At the same time,the granular Cu₂TiAl phase with a size of about 20⁵0 nm is found,which is dispersed on the surface of the substrate,hinderes grain growth and has the effect of refining grains.（3）Based on the optimized process parameters,the macroscopic properties of the alloy are studied.The hardness of the alloy at room temperature（267.1²89.1 HV）is higher than that of casting,exceeding 30 HV,the elongation（7.63±0.39%）is increased by 5.41%,and the tensile strength（541±26 MPa）is reduced by 12%.The high cooling rate of SLM and the fine-grained effect of Ti lead to the refinement of alloy grain.Meanwhile,the high cooling rate leads to the suppression of brittleγ₂ phase precipitation,so the hardness and fracture elongation of the alloy are increased.The existence of pores in the sample results in the decrease of the tensile strength of the sample.The DSC experimental result showes that the martensite transformation starting temperature Ms of the alloy is about 83°C,which indicates that the alloy is martensite phase at room temperature,and it has the feasibility of constructing shape recovery at high temperature.The strength of the alloy exceeds 600 MPa at 300℃,and the fracture elongation can reach 12%.The alloy has good high temperature application potential.In addition,the shape recovery experiment showes that the shape recovery rate of the alloy in the deformation range is close to 90%,which verifies the memory characteristics of the material.（4）The complex porous structure of the Cu-13.5Al-4Ni-0.5Ti memory alloy is formed by SLM.The volume fraction of the porous structure is 10%,the space size is 25 mm×25mm×12.5 mm,and the unit sizes are 3.5 mm,4.5 mm and 6.5 mm,respectively.The prepared porous structure had no obvious defects and the forming precision is high.The average deviation is+0.168/-0.057 mm.With the same porosity,the larger the unit size,the lower the stress-strain concentration level and the better the compression performance of the porous structure.The compressive strength of porous structure with 10%volume fraction and 6.5 mm unit size is 9.45±0.23 MPa,the elastic modulus is about 6%,and the elastic modulus is 249±0.23 N/mm².The feasibility of SLM forming complex parts of Cu-13.5Al-4Ni-0.5Ti memory alloy is preliminarily verified.