激光增材成形纯锌的微观组织及力学性能各向异性研究(特邀)特邀研究论文亮点文章
Selective laser melting (SLM) is a widely popular metal additive manufacturing technique that offers distinct advantages in fabricating bone implants with customized shapes and internal bionic porous structures. In particular, using a very high cooling rate (103?108 K·s-1) during the SLM process can inhibit the grain growth of pure Zn and confer good mechanical properties. This study reveals the internal relationship between the microstructure and mechanical anisotropy of SLM-fabricated pure Zn. We also report the influences of the grain characteristics and texture on the anisotropy.
The purity (mass fraction) of Zn powder used in this experiment is 99.9% and the sizes of particles are 7.2?29.7 μm. Pure Zn samples are fabricated using a commercial SLM printing device equipped with a 200 W fiber laser. The density of a pure Zn sample is greater than 99.5% when using optimized forming parameters (laser power P=80 W, and scanning speed VS=900 mm·s-1). To investigate the mechanical anisotropy, the fabricated Zn samples with dimensions of 8 mm×8 mm×8 mm are microscopically characterized in the horizontal and vertical directions. After etching with the 4% (volume fraction) nitric acid solution for 5 s, the microstructures on both the horizontal and vertical planes of the Zn samples are characterized using a metallographic optical microscope (OM) and scanning electron microscope (SEM). The grain orientation, grain size, and texture information are analyzed using electronic backscattered diffractometer (EBSD). Moreover, tensile samples with a gauge length of 22.0 mm, width of 3.0 mm, and thickness of 2.8 mm are fabricated for tensile tests.
Significant differences are observed in the microstructures of Zn samples formed by SLM on horizontal and vertical planes. A large number of equiaxed grains are observed on the horizontal plane in the OM and SEM images. In contrast, fish-scale molten pools with depth of 30?50 μm and width of 100?150 μm are found on the vertical plane. Furthermore, most of the grains exhibit preferred orientations along
This study reports the investigation results on the microstructure and mechanical properties of SLM-fabricated Zn in both the horizontal and vertical directions, with a particular focus on the grain morphology and orientation. Furthermore, the relationships between these microstructural aspects and mechanical properties are discussed. The SLM-fabricated Zn exhibits pronounced anisotropy in its tensile strength and ductility. Specimens fabricated on the horizontal plane exhibit a higher yield strength and ultimate tensile strength but a lower elongation rate compared to those fabricated in the vertical direction. The greater strength of horizontally fabricated specimens is primarily attributed to their finer grain size and higher initial dislocation density, which hinder dislocation movement. Conversely, specimens fabricated on the vertical plane demonstrate enhanced ductility because they contain a higher proportion of high-angle grain boundaries, which effectively impede crack propagation and thereby prevent premature fracturing.
1 引言
生物可降解金属具有优异的力学性能,在骨植入体和血管支架等医疗植入物中具有广阔的应用前景[1]。与不锈钢、钛合金、钴铬合金等永久性医用金属相比,可降解金属可在人体内降解,无需担心长期损伤和二次手术[2-3]。理想的生物可降解金属应当具有良好的生物相容性、合适的降解速率以及良好的力学性能,典型材料包括镁(Mg)、锌(Zn)和铁(Fe)。Mg降解时会释放氢气且降解速率快,使得骨骼的再生时间不足,进而影响愈合过程中骨的机械完整性[4]。Fe降解太慢,且降解产物可能导致代谢并发症[5]。Zn是人体必需的微量元素之一,电极电位(-0.762 V)处于Mg(-2.037 V)和Fe(-0.44 V)之间,表现出更适宜的降解速率[6]。此外,已有研究通过体内和体外试验证明,锌具有良好的生物相容性,能通过增强成骨细胞基因与抑制破骨细胞分化来促进骨修复[7]。
由于锌晶格属于密排六方(HCP)结构,常温加工性能较差,常用传统方法(如铸造)制备的锌的力学性能通常较弱(极限抗拉强度为20 MPa,延伸率为0.3%)[8]。另一方面,骨植入物需要个性化的定制形状与内部互通的多孔结构,为骨组织再生提供必要的空间[9]。激光选区熔化(SLM)增材制造技术采用高能激光选择性地熔化粉末颗粒形成熔池,已被广泛用于制造尺寸精度高、力学性能优异的金属骨植入物[10]。SLM过程中极高的冷却速率(103~108 K·s-1)可以抑制晶粒生长[11-12],使成形的纯锌获得良好的力学性能(极限拉伸强度超过100 MPa,延伸率超过10%)[13],SLM技术在制造Zn植入体方面表现出独特优势。
近年来,SLM成形纯Zn的研究主要集中在工艺参数对成形件致密化和力学性能的影响方面。例如,米兰理工大学Montani等[14]首次利用SLM成形纯Zn,采用的激光功率为300 W、扫描间距为100 μm、扫描速度为600~1900 mm/s,获得的成形试样的致密度最高达88%,表现出比铸态Zn更高的机械强度。由于Zn的熔沸点低,在成形过程中易蒸发,蒸发烟雾将阻碍激光能量的有效传递,最终降低试样的致密度。暨南大学Wang等[15]优化了SLM成形Zn的工艺参数,探讨了工艺参数对其力学性能的影响。他们发现,随着激光能量密度的增加,Zn试样的强度和延展性逐渐提高,最优参数下得到的SLM成形Zn试样的致密度为93.04%,极限抗拉强度和延伸率分别为95.93 MPa和11.73%。江西理工大学Yang等[16]研究了SLM成形纯Zn工艺参数、成形质量和力学性能之间的关系,发现成形质量随着激光功率或扫描速度的增加而提高,高激光功率导致织构强度增大和晶粒粗化。当激光功率为800 W、扫描速度为800 mm/s时,其相对密度达到99.5%以上,最优工艺参数下得到的纯Zn试样的平均硬度、机械强度和伸长率分别达到50.2 HV、127.8 MPa和7.6%。
然而,关于SLM成形纯Zn的微观组织和力学性能的各向异性的系统研究较少。比利时鲁汶大学Lietaert等[17]研究了不同扫描策略对SLM成形纯Zn微观织构和力学性能各向异性的影响,沿垂直和水平方向的抗拉强度分别为~100 MPa和~79 MPa,延伸率分别为10%和12%。清华大学Qin等[18]也研究了不同加工参数与构建方向对成形纯Zn的力学性能和腐蚀行为的影响,发现在相同参数下,沿垂直平面成形的试样比沿水平平面成形的试样表现出更高的强度和延展性,表明纯锌的力学性能存在各向异性。然而,在SLM快速凝固条件下,成形纯锌各向异性的产生机理仍需进一步分析。
本文重点研究了SLM成形纯Zn的微观组织和力学性能的各向异性及形成机理。首先,通过SLM分别沿“水平平面”(垂直于激光方向)和“垂直平面”(平行于激光方向)成形纯Zn试样。采用扫描电子显微镜(SEM)和电子背散射衍射仪(EBSD)对两种试样进行微观表征,以揭示微观组织的各向异性特征。通过拉伸测试研究了晶体学特征对SLM成形纯Zn力学性能的各向异性的影响。最后,分别对拉伸件的断口形貌进行了SEM显微观察和分析,并结合微观组织讨论了产生各向异性的原因。研究旨在阐明SLM成形纯Zn的微观结构与力学性能各向异性间的内在联系,探索晶粒特征和织构对各向异性的影响规律。
2 材料与方法
2.1 原料与工艺
本试验所用气雾化纯锌粉末的纯度(质量分数)为99.9%。扫描电子显微镜显示粉末具有良好的球形度,如
图 1. 纯锌粉末特征。(a)SEM形貌;(b)粒径分布
Fig. 1. Characteristics of pure Zn powder. (a) SEM morphology; (b) particle size distribution
2.2 制备方法
采用商用SLM成形设备成形纯锌试样。该设备配备功率为200 W的光纤激光器,光斑直径为70 μm。采用45号钢作为成形基板,经打磨和清洁处理后开展试验。关键工艺参数包括激光功率(P)、扫描速度(VS)、扫描间距(HS)和层厚(DS)。激光能量密度(EV)的计算公式为
优化成形工艺参数,使试样致密度大于99.5%。优化参数为P=80 W,VS=900 mm·s-1,HS=55 μm,DS=30 μm。在垂直和水平平面上成形了尺寸为8 mm×8 mm×8 mm的方块试样用于微观表征,同时成形了标距长度为22.0 mm、宽度为3.0 mm、厚度为2.8 mm的拉伸试样用于机械拉伸测试,尺寸如
图 2. SLM成形纯锌试样。(a)拉伸试样的尺寸示意图;(b)沿水平平面成形的Zn试样;(c)沿垂直平面成形的Zn试样
Fig. 2. Pure Zn samples formed by SLM. (a) Dimensional diagram of tensile specimen; (b) Zn sample formed along horizontal plane; (c) Zn sample formed along vertical plane
2.3 表征和测试方法
首先使用碳化硅砂纸研磨试样,然后用二氧化硅悬浊液进行抛光,最后用体积分数为4%的硝酸乙醇溶液腐蚀2 s左右。通过光学显微镜(OM)和扫描电子显微镜观察成形方块试样的形貌。在得到高质量电子背散射图案前,使用宽束氩离子抛光系统进行氩离子抛光。氩离子研磨的参数包括:喷枪与试样表面成2°角度放置,并在6 keV下粗抛光2 h,然后在1 keV下抛光1 h。试样准备完成后采用电子背散射衍射仪表征晶粒的微观结构特征,扫描在电压20 kV下进行,扫描步长为0.5 μm。晶界是根据其取向差角定义的:取向差角在2°~15°区间的晶界被定义为低角度晶界(LAGBs),而取向差角大于15°的晶界被定义为高角度晶界(HAGBs)。纹理强度由极图(PF)和反极图中最大均匀密度(d)的倍数表示。通过EBSD生成核平均取向错位角(KAM)分布图,量化每个像素与其最邻近像素的平均取向误差(即局部取向误差分布),并排除超过5°的误差。使用万能材料试验机在室温下以0.5 mm/min的恒定速度进行拉伸试验。测试前对试样表面进行清洁和干燥处理。每组参数下测试5个试样,得到平均值和标准差。拉伸测试后,使用SEM观察其断面形貌。
3 分析与讨论
3.1 微观组织
图 3. SLM成形Zn试样在不同平面上的微观结构。水平平面上的(a)(b)OM图和(c)SEM图;垂直平面上的(d)(e)OM图和(f)SEM图
Fig. 3. Microstructures of Zn samples formed by SLM on different planes. (a)(b) OM images and (c) SEM image on horizontal plane; (d)(e) OM images and (f) SEM image on vertical plane
图 4. SLM成形Zn试样在不同平面上的EBSD结果。垂直平面上的EBSD晶粒(a)取向图与(c)尺寸分布图;水平平面的EBSD晶粒(b)取向图与(d)尺寸分布图
Fig. 4. EBSD results of Zn samples formed by SLM on different planes. (a) Orientation map and (c) grain size distribution map of EBSD grains on vertical plane; (b) orientation map and (d) grain size distribution map of EBSD grains on horizontal plane
为进一步研究微观组织的各向异性,分析了Zn试样垂直平面和水平平面的极图与反极图,结果如
式中:f为织构取向分布;g为欧拉坐标系;
图 5. SLM成形Zn试样在不同平面上的极图与反极图。垂直平面上的(a)反极图和(b)极图;水平平面上的(c)反极图和(d)极图
Fig. 5. Pole and inverse pole figures of Zn samples formed by SLM on different planes. (a) Inverse polar figures and (b) polar figure on vertical plane; (c) inverse polar figures and (d) polar figure on horizontal plane
图 6. SLM成形Zn试样在不同平面上的晶界错位角。垂直平面上晶界错位角的(a)分布及(b)统计图;水平平面上晶界错位角的(c)分布及(d)统计图
Fig. 6. Grain boundary misorientation angles of Zn samples formed by SLM on different planes. (a) Distribution and (b) statistical diagram of grain boundary misorientation angle on vertical plane; (c) distribution and (d) statistical diagram of grain boundary misorientation angle on horizontal plane
图 7. SLM成形Zn试样在不同平面上的KAM分析结果。垂直平面上KAM的(a)分布图及(b)相应的直方图;水平平面上KAM的(c)分布图及(d)相应的直方图
Fig. 7. KAM analysis results of Zn samples formed by SLM on different planes. (a) Distribution and (b) corresponding histogram of KAM on vertical direction; (c) distribution and (d) corresponding histogram of KAM on horizontal plane
3.2 力学性能
进一步测试了SLM 沿水平与垂直平面成形的Zn试样的拉伸性能,如
式中:
图 8. SLM成形Zn试样的拉伸性能。(a)应力-应变曲线;(b)应变加工硬化率曲线;(c)极限强度、屈服强度和延伸率;(d)不同Zn试样的力学性能对比
Fig. 8. Tensile properties of Zn samples formed by SLM. (a) Stress-strain curves; (b) strain hardening rate curves; (c) ultimate strength, yield strength, and elongation; (d) comparison of mechanical properties of different Zn samples
通过量化拉伸性能,可得到沿水平平面成形的试样的屈服强度、极限抗拉强度和延伸率分别为(108.0±0.9) MPa、(123.5±2.1) MPa和(11.7±0.9)%,而沿垂直平面成形的试样的屈服强度、极限抗拉强度和延伸率则分别为(90.2±1.2) MPa、(108.0±2.4) MPa和(14.1±0.7)%[
SLM沿水平与垂直平面成形的Zn试样的拉伸断口形貌如
图 9. SLM 沿水平与垂直平面成形的Zn试样的拉伸断口形貌。(a)(b)垂直方向;(c)(d)水平方向
Fig. 9. Tensile fracture morphologies of Zn samples formed by SLM on horizontal and vertical planes. (a)(b) Vertical plane; (c)(d) horizontal plane
4 结论
从晶粒形貌和取向等方面研究了SLM沿水平与垂直平面成形的Zn试样的微观结构和力学性能,并讨论了微观结构和各向异性力学性能之间的关系,主要结论如下:
1) 在SLM沿垂直平面成形的Zn试样中,可观察到
2) 量化了高低晶界角和初始位错密度分布,其中SLM沿水平平面成形的Zn试样的低角度晶界的体积分数(69.4%)高于SLM沿垂直平面成形的Zn试样(61.6%),前者的平均KAM值(0.84°)大于后者(0.79°),说明SLM沿水平平面成形的Zn试样具有更高的初始位错密度。
3) SLM沿水平与垂直平面成形的试样的力学性能表现出明显的各向异性。SLM沿水平平面成形的试样的屈服强度和极限抗拉强度分别比SLM沿垂直平面成形的试样高7.2%和12.5%,延伸率低17%,SLM沿水平平面成形的试样具有高强度,主要归因于其具有更细的晶粒尺寸和更高的初始位错密度,能够阻碍后期位错运动,而SLM沿垂直平面成形的试样具有强延伸性,主要归因于其具有更多的高角度晶界,能够有效地阻碍裂纹偏转,试样不易过早断裂。
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