激光焊接NiTi合金的性能研究
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摘要
NiTi形状记忆合金集形状记忆效应、超弹性、良好的生物相容性于一体，使它成为理想的医用材料，成功地应用于介入医疗领域中各种支架的制作。目前对于直径较小、厚度较薄的血管支架的需求量日益增加。但是，现有的加工技术，激光雕刻或手工编织，成本高且难以加工此类支架。另外，手工编织的大尺寸外周管腔支架，接头部分目前往往用套管来连接，寻求更好的连接方法也是所面临的难题之一。激光微焊接被认为是解决这些难题适合的、有希望的方法之一。
本论文系统地研究了超弹性Ti-50.6at．％Ni合金丝脉冲激光焊接接头的组织和性能，目的在于揭示用激光焊接方法加工微小医疗器械的可行性。主要结论如下：
NiTi合金细丝激光点焊时需选择小电流、大脉宽、负离焦量，同时采用Ar气保护；激光点焊接头的熔化区由树枝晶组成，热影响区靠近熔池的部分为粗大等轴晶，靠近母材的部分为细小等轴晶；激光焊接会影响到NiTi合金丝的相变行为，使NiTi合金丝的相变过程由B2-R-B19′转变为B2-B19′且相变温度发生变化；接头中碳化物的数量明显少于母材；接头的抗拉强度和延伸率低于母材。在37℃时，接头的抗拉强度可达母材的60％，加载到应变为7％后卸载，接头和母材的残余应变为分别为1.06％和0.45％，接头性能降低的原因是由于激光焊接所形成的树枝晶。
NiTi合金激光焊接组织的耐蚀性和血液相容性比母材好。由于激光焊接组织中夹杂物的数量较少，表面易形成均匀、连续的钝化膜，从而提高了NiTi合金的耐蚀性和血液相容性。在相同条件下，NiTi合金激光焊接组织的钝化电位区间比母材的大，击穿电位比母材的高，凝血时间较长，血小板粘附量较少，变形小。
热循环和退火会对NiTi合金激光点焊接头的相变行为产生影响。热循环后接头中出现R相变，随着热循环次数的增加，R相变向高温方向移动，M相变向低温方向移动，R相存在的温度区间变宽，其主要原因是热循环在基体中引入位错。激光焊接NiTi合金经退火后，也出现R相变，随着退火温度的升高，R相变峰向低温方向移动，M相变峰也向低温方向移动，但移动的非常小，R相变和M相变之间的距离缩小，其主要原因是退火过程中产生了Ti_3Ni_4析出相。
NiTi合金激光点焊接头具有较好的超弹性疲劳性能。在加载卸载循环时，尽管开始时接头的残余应变比母材的大，但随着循环次数的增加，残余应变趋向稳定。在旋转弯曲疲劳中，当应变量大于0.4％时，在相同的应变下，接头的旋转弯曲疲劳寿命低于母材；当应变量小于0.4％，接头和母材的疲劳寿命都超过100万次。溶液中Cl~-浓度和溶液的pH值对NiTi合金丝的旋转弯曲疲劳性能没有显著的影响。
退火温度对激光点焊接头的性能有影响。焊接状态的NiTi合金在退火的过程中会析出Ti_3Ni_4相。400℃退火时，析出的Ti_3Ni_4相呈细小的圆形颗粒状，平均直径为10nm，弥散分布，对基体强化作用很大，能提高NiTi合金激光点焊接头的性能；而退火温度较高时，析出的Ti_3Ni_4相粗化，弥散度大幅度下降，对基体的强化作用很弱，接头的性能降低。
A combination of shape memory effect, superelasticity and good biocompatibility makes NiTi alloy a desirable candidate material for certain biomedical device applications, successfully used in the field of interventional therapy to fabricate different stents. There is a growing demand for smaller and thinner endocascular stents. However, present technique, laser cutting or hand made technique, usually takes high costs and it is difficult to fabricate these stents. In addition, it is one of the difficulties to find better joining method for hand made peripheral endoluminal stent, which is confined to mechanical fastening by tube at the present time. Micro laser welding is considered to be one of the available and promising methods for resolving these difficulties.
Process, microstructures and properties of pulse laser spot-welded NiTi shape memory alloy have been studied in this paper in order to reveal the applicability of laser welding to the fabrication of medical devices. Superelastic Ti-50.6at %Ni wires were welded using pulsed YAG laser. The primary conclusions are listed as follows:
Theoretic analysis and experimental results show that welding parameters possessing lower current, higher pulse duration, defocusing distance and Ar protecting atmosphere are advantageous to getting better joint. The fusion zone features dendrite structure, the microstructures of heat-afected zone can be divided into two parts: coarse equiaxial crystals near the fusion zone and fine equiaxial crystals near the base metal. Laser welding affects the transformation behavior of NiTi alloy wire, which varies from B2→R→B19' to B2→B19'. Carbide in laser spot-welded joint is lower than that in base metal. The ultimate tensile strength and elongation of the joint are lower than that of the base metal. When unloading at the 7% strain, the residual strain is 1.06% for laser spot-welded joint and 0.45% for base metal, respectively. This is mainly caused by coarse-grained and dendritic microstructure in the welded joints.
The corrosion resistance and hemocompatibility of laser-welded NiTi are better than that of base metal. The improvement of the corrosion resistance and hemocompatibility of laser-welded NiTi is due to the sharp decrease of surface defects. At the same condition, laser-welded NiTi exhibits a higher breakdown potentials、wider passive range、longer cruor time.
The transformation behavior of joint changes with thermal cycling and annealing, varying from B2→B19' to B2→R→B19'. With increasing thermal cycles, the R phase transformation start temperature increases and the temperature region of the R phase is broadened. But the martensitic transformation start temperature decreases with increasing thermal cycles. It is thought that dislocations play important role in the transfer from one-step to the two-step transformation and in the shifts of transformation temperature. R phase can also appear in the laser-welded NiTi after annealing. R phase and martensite transformation temperature move to the low temperature side with increasing annealing temperature. The variations in transformation behavior are attributable to the presence of Ti_3Ni_4 precipitates after annealing.
The laser spot-welded NiTi has good superelastic fatigue property. Although it has a larger residual strain than base metal at the first cycling, the residual strain decreases with the increase of the cycle number. The rotation-bending fatigue test results show significantly reduced numbers of cycles to failure when strain amplitude remains above 0.4%. This is mainly caused by coarse-grained and dendritic microstructure in the welded joints. Both the welded microstructure and the unwelded one is able to be loaded up to 10~6 cycles without failure if the strain amplitude remains below 0.4%. The fatigue life of laser spot-welded joint and base metal has no significant changes with increasing of Cl concentration and pH value.
The properties of laser-welded NiTi are affected by annealing temperature because of the presence of Ti_3Ni_4 precipitates during annealing. Smaller cohe, rent Ti_3Ni_4 precipitates (10nm) produced at 400℃annealing have a dispersive distribution and can strongly strengthen the joint. Therefore, it can improve the properties of the joint; When annealing at 500℃, the Ti_3Ni_4 precipitates become coarse, so the properties of joint decreases.
本论文系统地研究了超弹性Ti-50.6at．％Ni合金丝脉冲激光焊接接头的组织和性能，目的在于揭示用激光焊接方法加工微小医疗器械的可行性。主要结论如下：
NiTi合金细丝激光点焊时需选择小电流、大脉宽、负离焦量，同时采用Ar气保护；激光点焊接头的熔化区由树枝晶组成，热影响区靠近熔池的部分为粗大等轴晶，靠近母材的部分为细小等轴晶；激光焊接会影响到NiTi合金丝的相变行为，使NiTi合金丝的相变过程由B2-R-B19′转变为B2-B19′且相变温度发生变化；接头中碳化物的数量明显少于母材；接头的抗拉强度和延伸率低于母材。在37℃时，接头的抗拉强度可达母材的60％，加载到应变为7％后卸载，接头和母材的残余应变为分别为1.06％和0.45％，接头性能降低的原因是由于激光焊接所形成的树枝晶。
NiTi合金激光焊接组织的耐蚀性和血液相容性比母材好。由于激光焊接组织中夹杂物的数量较少，表面易形成均匀、连续的钝化膜，从而提高了NiTi合金的耐蚀性和血液相容性。在相同条件下，NiTi合金激光焊接组织的钝化电位区间比母材的大，击穿电位比母材的高，凝血时间较长，血小板粘附量较少，变形小。
热循环和退火会对NiTi合金激光点焊接头的相变行为产生影响。热循环后接头中出现R相变，随着热循环次数的增加，R相变向高温方向移动，M相变向低温方向移动，R相存在的温度区间变宽，其主要原因是热循环在基体中引入位错。激光焊接NiTi合金经退火后，也出现R相变，随着退火温度的升高，R相变峰向低温方向移动，M相变峰也向低温方向移动，但移动的非常小，R相变和M相变之间的距离缩小，其主要原因是退火过程中产生了Ti_3Ni_4析出相。
NiTi合金激光点焊接头具有较好的超弹性疲劳性能。在加载卸载循环时，尽管开始时接头的残余应变比母材的大，但随着循环次数的增加，残余应变趋向稳定。在旋转弯曲疲劳中，当应变量大于0.4％时，在相同的应变下，接头的旋转弯曲疲劳寿命低于母材；当应变量小于0.4％，接头和母材的疲劳寿命都超过100万次。溶液中Cl~-浓度和溶液的pH值对NiTi合金丝的旋转弯曲疲劳性能没有显著的影响。
退火温度对激光点焊接头的性能有影响。焊接状态的NiTi合金在退火的过程中会析出Ti_3Ni_4相。400℃退火时，析出的Ti_3Ni_4相呈细小的圆形颗粒状，平均直径为10nm，弥散分布，对基体强化作用很大，能提高NiTi合金激光点焊接头的性能；而退火温度较高时，析出的Ti_3Ni_4相粗化，弥散度大幅度下降，对基体的强化作用很弱，接头的性能降低。
A combination of shape memory effect, superelasticity and good biocompatibility makes NiTi alloy a desirable candidate material for certain biomedical device applications, successfully used in the field of interventional therapy to fabricate different stents. There is a growing demand for smaller and thinner endocascular stents. However, present technique, laser cutting or hand made technique, usually takes high costs and it is difficult to fabricate these stents. In addition, it is one of the difficulties to find better joining method for hand made peripheral endoluminal stent, which is confined to mechanical fastening by tube at the present time. Micro laser welding is considered to be one of the available and promising methods for resolving these difficulties.
Process, microstructures and properties of pulse laser spot-welded NiTi shape memory alloy have been studied in this paper in order to reveal the applicability of laser welding to the fabrication of medical devices. Superelastic Ti-50.6at %Ni wires were welded using pulsed YAG laser. The primary conclusions are listed as follows:
Theoretic analysis and experimental results show that welding parameters possessing lower current, higher pulse duration, defocusing distance and Ar protecting atmosphere are advantageous to getting better joint. The fusion zone features dendrite structure, the microstructures of heat-afected zone can be divided into two parts: coarse equiaxial crystals near the fusion zone and fine equiaxial crystals near the base metal. Laser welding affects the transformation behavior of NiTi alloy wire, which varies from B2→R→B19' to B2→B19'. Carbide in laser spot-welded joint is lower than that in base metal. The ultimate tensile strength and elongation of the joint are lower than that of the base metal. When unloading at the 7% strain, the residual strain is 1.06% for laser spot-welded joint and 0.45% for base metal, respectively. This is mainly caused by coarse-grained and dendritic microstructure in the welded joints.
The corrosion resistance and hemocompatibility of laser-welded NiTi are better than that of base metal. The improvement of the corrosion resistance and hemocompatibility of laser-welded NiTi is due to the sharp decrease of surface defects. At the same condition, laser-welded NiTi exhibits a higher breakdown potentials、wider passive range、longer cruor time.
The transformation behavior of joint changes with thermal cycling and annealing, varying from B2→B19' to B2→R→B19'. With increasing thermal cycles, the R phase transformation start temperature increases and the temperature region of the R phase is broadened. But the martensitic transformation start temperature decreases with increasing thermal cycles. It is thought that dislocations play important role in the transfer from one-step to the two-step transformation and in the shifts of transformation temperature. R phase can also appear in the laser-welded NiTi after annealing. R phase and martensite transformation temperature move to the low temperature side with increasing annealing temperature. The variations in transformation behavior are attributable to the presence of Ti_3Ni_4 precipitates after annealing.
The laser spot-welded NiTi has good superelastic fatigue property. Although it has a larger residual strain than base metal at the first cycling, the residual strain decreases with the increase of the cycle number. The rotation-bending fatigue test results show significantly reduced numbers of cycles to failure when strain amplitude remains above 0.4%. This is mainly caused by coarse-grained and dendritic microstructure in the welded joints. Both the welded microstructure and the unwelded one is able to be loaded up to 10~6 cycles without failure if the strain amplitude remains below 0.4%. The fatigue life of laser spot-welded joint and base metal has no significant changes with increasing of Cl concentration and pH value.
The properties of laser-welded NiTi are affected by annealing temperature because of the presence of Ti_3Ni_4 precipitates during annealing. Smaller cohe, rent Ti_3Ni_4 precipitates (10nm) produced at 400℃annealing have a dispersive distribution and can strongly strengthen the joint. Therefore, it can improve the properties of the joint; When annealing at 500℃, the Ti_3Ni_4 precipitates become coarse, so the properties of joint decreases.
引文
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