摘要
采用激光粉末床熔融(LPBF)技术制备了超弹性骨植入NiTi合金,研究了激光功率和扫描速度对LPBF成形NiTi合金致密度和相变温度的影响规律,建立了NiTi合金的LPBF成形工艺窗口,提出了致密度与相变温度协同调控的方法,揭示了不同扫描速度下微观组织、力学性能与超弹性的变化规律。研究结果表明:当激光功率为250 W,扫描速度为900~1300 mm/s时,制备的NiTi合金致密度大于99.5%,奥氏体转变终止温度在39℃左右,接近人体体温;随着扫描速度的增加,合金的相变温度与致密度先升高后降低,晶粒逐渐细化,平均晶粒尺寸由34.4μm减小至20.5μm;当扫描速度为1100 mm/s时,拉伸性能与超弹性能最优,抗拉强度和伸长率分别为625.6 MPa和14.67%,变形回复率达到99.51%。研究结果为LPBF成形NiTi合金骨组织工程支架的工艺性能调控提供了理论参考。
Objective Bone tissue engineering is a crucial approach for treating bone defects.NiTi alloys are regarded as highly promising materials for bone tissue engineering due to their exceptional superelasticity,shape memory effect,high specific strength,excellent corrosion resistance,and biocompatibility.However,their further development is constrained by traditional fabrication methods.Laser powder bed fusion(LPBF)technology can precisely control the porosity,phase transition temperature,and comprehensive properties of NiTi alloys,providing a new approach for preparing NiTi alloy bone scaffolds.Based on the application requirements of bone implants,this study systematically investigates the phase transformation temperature,mechanical properties,and superelastic behavior of LPBFfabricated NiTi alloys,aiming to provide a theoretical foundation for the development of LPBFfabricated NiTi alloy bone implants.Methods In this study,nearequiatomic NiTi alloy powder(Ni atomic fraction of 50.8%)is fabricated using an LPBF system.During the printing process,a process window is established by varying the laser power(P of 190‒310 W)and scan speed(v of 900‒1300 mm/s)to produce NiTi alloy samples with dimensions of 8 mm×8 mm×8 mm.The density of the samples is measured using the Archimedes drainage method,and the phase transformation temperatures are determined using differential scanning calorimetry(DSC),with the results visualized in a contour map.Additionally,the phase composition and microstructural evolution are analyzed using Xray diffraction(XRD)and electron backscatter diffraction(EBSD).The mechanical properties and superelastic behavior are evaluated through uniaxial tensile tests and cyclic loadingunloading experiments.Results and Discussions An analysis of the contour maps of phase transformation temperature and relative density shows that under a constant laser power of 250 W,the phase transformation temperature of the samples is slightly higher than body temperature(37℃).Additionally,under these conditions,the relative density exceeds 99.5%,meeting the requirements for both phase transformation temperature and relative density(Fig.2).When the laser power is kept constant at 250 W,with increased scanning speed,the phase transition temperature first increases and then decreases(Fig.3).Three groups of representative samples(samples 1#,2#,and 3#,with scanning speeds of 900,1100,and 1300 mm/s,respectively)are selected for subsequent research.As the scanning speed increases,the grain size of the columnar crystals gradually decreases,accompanied by an increase in grain boundaries.Consequently,the average value of the Kernel average misorientation(KAM)progressively increases,indicating higher dislocation density and more pronounced changes in crystal orientation(Fig.5).As the scanning speed increases,the cooling rate accelerates,temperature gradient decreases,and interface velocity increases,promoting the formation of finer grains.Sample 2#exhibits the highest tensile strength(625.6 MPa)and yield strength(336.8 MPa),demonstrating the best overall mechanical performance.However,excessively high scanning speeds lead to the formation of a small amount of unmelted powder and pores during the melting process.These defects act as stress concentration points during tensile testing,initiating and propagating cracks at lower stress levels and ultimately causing premature fracture,thereby reducing both the tensile strength and elongation at break of the material(Figs.6 and 7).During the cyclic tensile loadingunloading tests,after the first cycle,sample 1#exhibits the highest recovery strain(9.38%),while sample 2#shows the lowest total strain and recovery strain compared to the other two samples.However,sample 2#demonstrates the highest deformation recovery rate of 94.02%.By the 10th cycle,samples 2#and 3#exhibited the same deformation recovery rate(99.51%).Considering the overall cyclic recovery performance,sample 2#exhibits the best deformation recovery behavior and optimal superelasticity(Figs.8 and 9).Under the same stress level,sample 2#experiences less dislocation slip and martensitic plastic deformation,but a higher proportion of stressinduced martensitic transformation occurs.This results in a higher superelastic recovery rate for sample 2#compared to the other two samples.Conclusions This study investigates the influence of laser power and scanning speed on the relative density and phase transition temperature of an LPBFfabricated NiTi alloy.A process window for LPBF of the NiTi alloy is established,and a method for optimizing the relationship between density and phase transition temperature is proposed.In addition,the impacts of different scanning speeds on the microstructure,mechanical properties,and superelasticity are analyzed.The findings indicate that at a laser power of 250 W and scanning speeds ranging from 900 mm/s to 1300 mm/s,the printed NiTi alloy exhibits a relative density higher than 99.5%,and its austenitic transformation finish temperature is approximately 39℃,close to human body temperature.As the scanning speed increases,the transition temperature and relative density of the alloy first increase and then decrease,while the grain size is gradually refined.The average grain size decreases from 34.4μm to 20.5μm.Both B2 austenite phase and B19'martensite phase are observed in the NiTi alloy formed by LPBF,but the peak of B19'martensite phase is weak.When the scanning speed is 1100 mm/s,the alloy exhibits optimal mechanical properties,with a tensile strength of 625.6 MPa and fracture strain of 14.67%.At a scanning speed of 900 mm/s,the sample exhibits the highest recovery strain(9.38%),while a scanning speed of 1100 mm/s results in a higher deformation recovery rate(99.51%),with the deformation recovery rate reaching 94.02%during the first cycle of tensile testing,demonstrating optimal superelastic performance.
作者
郑旭
宋军
宋波
张磊
钟诗宇
刘燕
章媛洁
史玉升
Zheng Xu;Song Jun;Song Bo;Zhang Lei;Zhong Shiyu;Liu Yan;Zhang Yuanjie;Shi Yusheng(State Key Laboratory of Materials Processing and Die&Mould Technology,Huazhong University of Science and Technology,Wuhan 430074,Hubei,China;Department of Mechanical Engineering,City University of Hong Kong,Hong Kong 999077,China)
出处
《中国激光》
北大核心
2025年第12期154-164,共11页
Chinese Journal of Lasers
基金
国家自然科学基金(52275331,52201041)
国家重点研发计划(2023YFB4604800)。
关键词
激光技术
激光粉末床熔融
NITI合金
超弹性
相变温度
显微组织
laser technique
laser powder bed fusion
NiTi alloy
superelasticity
phasetransition temperature
microstructure
