The Optimization Of Forming Process And Performance Of Selective Laser Melted NiTi Shape Memory Alloys

Due to its unique shape memory effect,superelasticity,good biocompatibility,corrosion resistance and damping properties,NiTi shape memory alloys exhibit huge application potential in the fields of aerospace,biomedicine,automobiles and machinery manufacturing.However,the shape and structure of NiTi alloy parts manufactured by traditional methods are relatively simple,which greatly limits the application and promotion of NiTi alloys.Selective Laser Melting(SLM),as a typical additive manufacturing technology,can achieve high-efficiency and high-precision integrated near-net forming of complex structures.It provides an effective way to fabricate complex structures of NiTi alloys.This has become one of the research hotspots in the material field in recent years.At present,domestic and foreign research on NiTi shape memory alloy formed by SLM is still focused on the exploration stage of the preparation process.There is still a lack of systematic research on the influence of hatch distance and post-processing technology.Therefore,the influence of laser power and scanning speed on the width,continuity and stability of NiTi alloy single track was systematically studied.The effect of hatch distance on forming quality,density,phase composition,phase transformation behavior,mechanical properties and the evolution law of superelasticity of NiTi alloy bulk was deeply explored.The influence mechanism of annealing treatment,solution treatment and aging treatment on the microstructure,phase composition,phase transformation behavior,mechanical properties and superelasticity was also systematically explored.The research results show that the laser power and scanning speed directly affect the forming quality of the NiTi alloy single track fabricated by SLM.Among them,the increase of the laser power or the decrease of the scanning speed can increase the width and the continuity of the single track.The forming quality of the single track is also related to the line energy density of the laser.When the line energy density is low(0 J/m～100 J/m),the formed single track is discontinuous or droplet-like,and even cannot be formed;When the line energy density is high(250 J/m～500 J/m),the single track is continuous and stable.But the widening phenomenon is serious,and even defects such as keyholes appear.Moderate line energy density(100 J/m～250 J/m)can form a single track with moderate width,good continuity and stability.The better single track forming process is laser power with 150 W,scanning speed with 1100 mm/s.With the increase of the hatch distance,the roughness of the SLMed NiTi alloy bulk sample formed by SLM gradually increases and the density gradually decreases.Moreover,the defect type gradually changes from a small keyhole defect to a large porosity defect.The SLMed NiTi alloy bulk samples with different hatch distance,are mainly composed of NiTi(B2)phase and a small amount of NiTi(B19’)phase.As the hatch distance decreases,the content of NiTi(B2)phase decreases.The hatch distance can affect the shape and size of the grains.When the hatch distance is large,the XY plane of the SLMed NiTi alloy bulk sample is composed of small rectangular grains and the XZ plane is composed of coarser columnar grains.When the hatch distance is small,the crystal grain size on the XY plane is larger with irregularly shaped,and the columnar crystals on the XZ plane are densely arranged vertically and parallelly.As the hatch distance decreases,the phase transformation temperature gradually increases,and the phase transformation enthalpy first increases and then decreases.When the hatch distance h=90μm,the phase transformation enthalpy reaches the maximum.The austenite phase transformation enthalpy and martensite phase transformation enthalpy is 23.4J/g and 26.4 J/g,respectively.With the decrease of the hatch distance,the compressive performance of the SLMed NiTi bulk sample gradually decreases,and the tensile performance and superelasticity gradually increase.Among them,When the hatch distance h=77μm,the compressive strength and tensile strength are 3351 MPa and 839 MPa,respectively.And the strain recovery rate is as high as 97%after the first compression cycle.After the 20th compression cycle,the recoverable strain remains 5.75%.The SLMed NiTi alloy bulk samples after heat treatment are still composed of NiTi(B2)phase and NiTi(B19’)phase.But after the content of NiTi(B19’)phase with the sample after annealing treatment increases significantly,The solid solution and aging treatment exhibit limited effect on the phase content.The solution treatment dissolves the grain boundaries and increases the grain size,while after aging treatment and annealing treatment,there is no significant change in the grain size and shape.Annealing treatment presents slight effect on the phase transformation enthalpy of SLMed NiTi alloy,but it can increase the phase transformation temperature.The M_s,M_f,A_s and A_f points increase by 5.3℃,20.1℃,20.9℃and 25.4℃,respectively.The solution treatment and aging treatment both reduced the phase transformation enthalpy of the SLMed NiTi alloy,but the phase transformation temperature change slightly.Solution treatment can reduce the compressive and tensile strength,but increase its elongation.The solution-aging treatment can simultaneously increase the compression,tensile strength and elongation of the SLMed NiTi alloy.The tensile strength and elongation of the samples treated with S+A＿550℃-8h increased by 5.5%and 36%,respectively.Heat treatment can improve the superelasticity of SLMed NiTi alloy samples.After 20 compression cycles,the recoverable strains of annealing treatment,solution treatment and S+A＿550℃-3h treatment samples increased by 81%,31%and 39%,respectively.