Computational Mechanics and Materials Lab, Department of Mechanical Engineering, Ostbayerische Technische Hochschule (OTH) Regensburg, Regensburg, Germany.
Technology Campus Neustadt a. d. Donau, Department of Mechanical Engineering, OTH Regensburg, Regensburg, Germany.
PLoS One. 2020 Dec 29;15(12):e0244463. doi: 10.1371/journal.pone.0244463. eCollection 2020.
Advances in additive manufacturing enable the production of tailored lattice structures and thus, in principle, coronary stents. This study investigates the effects of process-related irregularities, heat and surface treatment on the morphology, mechanical response, and expansion behavior of 316L stainless steel stents produced by laser powder bed fusion and provides a methodological approach for their numerical evaluation. A combined experimental and computational framework is used, based on both actual and computationally reconstructed laser powder bed fused stents. Process-related morphological deviations between the as-designed and actual laser powder bed fused stents were observed, resulting in a diameter increase by a factor of 2-2.6 for the stents without surface treatment and 1.3-2 for the electropolished stent compared to the as-designed stent. Thus, due to the increased geometrically induced stiffness, the laser powder bed fused stents in the as-built (7.11 ± 0.63 N) or the heat treated condition (5.87 ± 0.49 N) showed increased radial forces when compressed between two plates. After electropolishing, the heat treated stents exhibited radial forces (2.38 ± 0.23 N) comparable to conventional metallic stents. The laser powder bed fused stents were further affected by the size effect, resulting in a reduced yield strength by 41% in the as-built and by 59% in the heat treated condition compared to the bulk material obtained from tensile tests. The presented numerical approach was successful in predicting the macroscopic mechanical response of the stents under compression. During deformation, increased stiffness and local stress concentration were observed within the laser powder bed fused stents. Subsequent numerical expansion analysis of the derived stent models within a previously verified numerical model of stent expansion showed that electropolished and heat treated laser powder bed fused stents can exhibit comparable expansion behavior to conventional stents. The findings from this work motivate future experimental/numerical studies to quantify threshold values of critical geometric irregularities, which could be used to establish design guidelines for laser powder bed fused stents/lattice structures.
增材制造技术的进步使得定制化的晶格结构的生产成为可能，从而在理论上也使得冠状动脉支架成为可能。本研究调查了与工艺相关的不规则性、热和表面处理对激光粉末床熔合（laser powder bed fusion，LPBF）制造的 316L 不锈钢支架的形态、力学响应和扩张行为的影响，并为其数值评估提供了一种方法。本研究采用了一种基于实际和计算重建的 LPBF 支架的组合实验和计算框架。研究观察到了与设计相比，实际的 LPBF 支架存在与工艺相关的形态偏差，导致未经表面处理的支架直径增加了 2-2.6 倍，而经过电化学抛光的支架直径增加了 1.3-2 倍。因此，由于几何刚度的增加，在两块板之间压缩时，未经过处理的 LPBF 支架（7.11±0.63N）或经过热处理的支架（5.87±0.49N）表现出更大的径向力。经过电化学抛光后，热处理后的支架表现出的径向力（2.38±0.23N）与传统金属支架相当。LPBF 支架还受到尺寸效应的影响，与从拉伸试验获得的块状材料相比，未经过处理的支架和经过热处理的支架的屈服强度分别降低了 41%和 59%。本研究提出的数值方法成功地预测了支架在压缩下的宏观力学响应。在变形过程中，观察到 LPBF 支架内部的刚度增加和局部应力集中。在之前验证的支架扩张数值模型中对衍生的支架模型进行的后续数值扩张分析表明，经过电化学抛光和热处理的 LPBF 支架可以表现出与传统支架相当的扩张行为。本工作的结果激励未来进行更多的实验/数值研究，以量化关键几何不规则性的临界值，这可用于为 LPBF 支架/晶格结构建立设计指南。
