Study Of Microstructure,Phase Transformation And Properties Of NiTi Alloy Formed By Selective Laser Melting

NiTi shape memory alloy has good prospects for application because of its excellent functional properties,but its poor machinability makes it difficult to prepare complex structural parts.The selective laser melting（SLM）technology has demonstrated significant advantages in preparing small batch,customized,and complex structural parts.In the previous study of SLM-manufactured NiTi（SLM-NiTi）alloys,our team prepared NiTi alloys with excellent tensile properties by a stripe rotation scanning strategy.Based on this scanning strategy,the following work was carried out.Firstly,the junction zone between adjacent stripes,which necessarily exists under the stripe scanning strategy,was investigated.Because the thermal history in the junction zone differs from other regions,it can be inferred that this causes differences in local microstructure and mechanical properties.However,it has not been revealed yet.Moreover,the study of this region is instructive and useful for multi-laser processing and gradient material processing.Secondly,the effects of laser scanning speed,a key processing parameter,on the microstructure,pore defects,internal stresses,phase transformation behavior,mechanical properties,and shape memory effect of SLM-NiTi samples were investigated.This is because the scanning speed profoundly affects the energy input to the SLM process,the temperature and volume of the molten pool,the fluidity,viscosity and solidification of the melt.In addition,laser scanning speed also determines the manufacturing efficiency and throughput of SLM-fabrication.Finally,hot isostatic pressing of SLM-NiTi alloy was investigated,expecting to improve the properties of NiTi alloy by eliminating the unavoidable pore defects in SLM products.The following conclusions are drawn from the above study.1.In the junction zone of adjacent stripes,there are a large number of metal vapor pores.During remelting,the junction zone is mainly surrounded by the solidified bulk and dissipates heat quickly,so its grains are finer.Whilst,the bidirectional scanning path and the complex heat dissipation in the junction zone lead to a more random grain orientation.The fracture strength of SLM-NiTi alloy increases as the size of the junction zone increases.2.The contributions of metallurgical factors under different scanning speeds,such as Ni evaporation,Ni₄Ti₃ precipitation,dislocations and internal stress,to the transformation temperature and transformation latent heat were clarified through specially designed experiments.Ni evaporation is found to have the most profound effect,followed by precipitation.Increasing scanning speed is found to reduce the Ni loss,thus cause less increase in the transformation temperature and transformation heat of the SLM-NiTi alloys.Increasing scanning speed also increases the microstructure non-uniformity and thus widens the transformation temperature interval.3.There is an optimum scanning speed for minimum porosity and smallest average pore size.Increasing or decreasing scanning speed led to increase of porosity and pore size.In addition,pore defects is found to have weak influence on the tensile properties of the SLM-NiTi alloys,which exhibited a high fracture strains（>13.4%）for all SLM-NiTi alloys with different scanning speeds.This apparent insensitivity to pore defects is attributed to the fact that the lattice shear deformation mechanism by the martensitic transformation helps to relax the stress concentration around the pores,and the significant strain strengthening presented after reorientation inhibits crack（pore defects）growth.4.Hot isostatic pressing（HIP）causes a large number of ellipsoidal Ti₄Ni₂O_x brittle phases with lengths of 200–500 nm and widths of 150–250 nm along the grain boundaries of the SLM-NiTi alloy,which severely weakens the grain boundaries.Whilst,the growth and disappearance of the“dual microstructure”on the scanning plane and the subgrain on the vertical plane,which again weakens the NiTi alloy matrix.In addition,the HIP causes the generation of surface oxide layers and Ti-depleted zone.All of above seriously affect the mechanical properties of SLM-NiTi alloy.Therefore,although the HIP effectively eliminates the pore defects of the SLM-NiTi alloy,the tensile mechanical properties of the alloy become worse instead.