金属3D打印关键科学问题 || 阿贡高级光子源超强X射线揭示气孔的产生机制 || 双语

Finding keyholes in metals 3D printing

找出3D打印金属中的关键孔洞

 

New research identifies causes for defectsin 3D printing and paves way for better results

最新研究发现了3D打印金属材料缺陷产生的原因，并为获得更好的增材制造效果铺平了道路。 

 

Source: College of Engineering, CarnegieMellon University

来源：卡内基梅隆大学工程学院

Summary:New research has identified how andwhen gas pockets form, as well as a methodology to predict their formation -- apivotal discovery that could dramatically improve the 3D printing process.

摘要：新研究已经确定了气孔形成的方式和时间并提出了预测气孔形成的方法——这是一个极其重要的发现，可以极大地改进3D打印工艺。

 

Additive manufacturing's promise torevolutionize industry is constrained by a widespread problem: tiny gas pocketsin the final product, which can lead to cracks and other failures.

增材制造能够变革工业的承诺受到一个普遍存在的问题的制约：最终产品中存在微小的孔洞，这可能导致裂纹和其他失效。

 

New research published today in Science (willhave hyperlink to paper), led by researchers from Carnegie Mellon Universityand Argonne National Laboratory, has identified how and when these gas pocketsform, as well as a methodology to predict their formation -- a pivotaldiscovery that could dramatically improve the 3D printing process.

今天在《科学》杂志上发表，由卡内基梅隆大学和阿贡国家实验室的研究人员主导的一项新的研究，已经确定了这些气孔是如何形成和何时形成的，以及预测它们形成的方法——这一关键性的发现可以极大地改进3D打印工艺。

Fig.1 Evolutions of melt pool andvapor depression under stationary laser illumination.  (A)Initial formation of a melt pool. (B) Formation of a small, stable vapordepression. (C) Steady growth of the vapor depression. (D) Instabilities formin the vapor depression. (E and F) Rapid change in the vapordepression shape. (G and H) Periodic fluctuation of the vapordepression. (I and J) Change of the melt pool shape fromquasi-semicircular to bimodal with a bowl on top and a spike in the middle atthe bottom. The sample is a Ti-6Al-4V bare plate. The laser spot size is 140μm, and the laser power is 156 W. The images have been background-corrected bythe image collected before the laser illumination. The shape of the melt poolis marked with a red shade in (E) and (J).

图1稳态激光照射下熔池和蒸汽压陷的演变。（A）熔池的最初形成。（B）形成一个小的、稳定的蒸汽压陷。（C）蒸汽压差的稳定增长。（D）不稳定性形成于蒸汽压陷。（E和F）蒸汽压陷形状的快速变化。（G和H）蒸汽压降的周期性波动。（I和J）熔池形状从准半圆到双峰的变化，顶部有碗，底部中间有尖峰。样品为Ti-6Al-4V裸板。激光光斑尺寸为140μm，激光功率为156w，图像经激光照射前采集的图像进行背景校正。熔池的形状在（E）和（J）中用红色阴影标出。

"The research in this paper willtranslate into better quality control and better control of working with themachines," said Anthony Rollett, a Professor of Materials Science andEngineering at Carnegie Mellon University and an author on the paper. "Foradditive manufacturing to really take off for the majority of companies, weneed to improve the consistency of the finished products. This research is amajor step in that direction."

卡内基梅隆大学材料科学与工程教授、论文作者安东尼·罗利特说：“本文的研究将转化为更好的质量控制和对机器工作的更好控制。”为了让增材制造真正助力大多数公司腾飞，我们需要提高成品的一致性。这项研究是朝着该方向迈出了的重要一步。”

The scientists used the extremely brighthigh-energy X-rays at Argonne's Advanced Photon Source (APS), a DOE Office ofScience User Facility, to take super-fast video and images of a process calledLaser Power Bed Fusion (LPBF), in which lasers are used to melt and fusematerial powder together.

科学家们在，美国能源部科学用户设施，阿贡高级光子源（APS），使用了超亮高能X射线，拍摄了一个叫做激光能量床熔融工艺（LPBF）的超高速视频和图像，在这个过程中，激光被用来熔化和融合材料粉末。

Fig.2 Keyhole drilling understationary laser illumination. (A and B) Penetration depth of vapordepression over time at different powers for a spot size of 95 and 140 μm,respectively. The transition occurs at approximately the same vapor depressiondepth for a given spot size, with the smaller spot size having a shallowercritical depth. (C) Drill rate of the laser as a function of power densityafter the transition. The black dashed line is the linear fitting to theprefluctuation drill rates.

图2固定激光照射下锁孔钻孔。（A）和（B）在不同功率下，在光斑尺寸分别为95μm和140μm的情况下，随着时间的推移，蒸汽压陷的穿透深度。对于给定的光斑尺寸，这种转变发生在几乎相同的蒸汽压陷深度处，较小的光斑尺寸具有较浅的临界深度。（C）激光钻速与功率密度的函数关系。黑色虚线是预测钻速的线性拟合。

The lasers, which scan over each layer ofpowder to fuse metal where it is needed, literally create the finished productfrom the ground up. Defects can form when pockets of gas become trapped intothese layers, causing imperfections that could lead to cracks or otherbreakdowns in the final product.

激光扫描每一层粉末，在需要的地方熔化金属，从基底上直接制造成品。当气泡被困在这些层中时，会形成缺陷，导致最终产品出现裂纹或其他失效。

Until now, manufacturers and researchersdid not know much about how the laser drills into the metal, producing cavitiescalled "vapor depressions," but they assumed that the type of metalpowder or strength of laser were to blame. As a result, manufacturers have beenusing a trial and error approach with different types of metals and lasers toseek to reduce the defects.

直到现在，制造商和研究人员还不太清楚激光是如何钻入金属的，产生了称为“蒸汽压陷”的空洞，他们认为是金属粉末的类型或激光的强度造成的。因此，制造商一直在使用不同类型的金属和激光的试错方法来减少缺陷。

In fact, the research shows that thesevapor depressions exist under nearly all conditions in the process, no matterthe laser or metal. Even more important, the research shows how to predict whena small depression will grow into a big and unstable one that can potentiallycreate a defect.

事实上，研究表明，在这个过程中，无论是激光还是金属，几乎所有的条件下都存在这些蒸汽压陷。更重要的是，这项研究显示了如何预测小压陷何时会发展成一个大而不稳定的大压陷，从而可能产生缺陷。

Fig.3 Keyhole morphologies across P-Vspace. (A) Tableau of representative radiographs in P-V space of Ti-6Al-4V bareplate for a laser spot size of 95 μm, showing the variation in vapor depressionsize and morphology. The vapor depression and melt pool transitions, measuredin the stationary beam experiment (Fig. 2A and fig. S2), are marked withblue and red dashed lines, respectively. (B and C) Vapor depressiondepth as a function of laser power at different scanning velocities for laserspot sizes of 95 μm (B) and 140 μm (C). Error bars indicate SD.

图3穿过P-V空间的锁孔形态。（A）激光光斑尺寸为95μm的Ti-6Al-4V裸板P-V空间的代表性射线照片，显示了蒸汽抑制尺寸和形态的变化。在固定束实验（图2A和图S2）中测量到的蒸汽压陷和熔池转变分别用蓝色和红色虚线标记。（B和C）95μm（B）和140μm（C）激光光斑在不同扫描速度下的蒸汽压陷深度随激光功率的变化。误差线SD。

"We're drawing back the veil andrevealing what's really going on," said Rollett who is also a co-directorof the NextManufacturing Center at Carnegie Mellon. "Most people think youshine a laser light on the surface of a metal powder, the light is absorbed bythe material, and it melts the metal into a melt pool. In actuality, you'rereally drilling a hole into the metal."

“我们正在揭开面纱，揭示出真正发生的事情，”同时也是卡内基梅隆下一代制造中心联合主任的罗莱特说，大多数人认为你用激光照射金属粉末的表面，光被材料吸收，然后熔化成熔池。实际上，你其实是在金属上钻了一个洞。”

By using highly specialized equipment atArgonne's APS, one of the most powerful synchrotron facilities in the world,researchers watched what happens as the laser moves across the metal powder bedto create each layer of the product.

通过世界上最强大的同步加速器设备之一阿贡高级光子源使用高度专业化的设备，研究人员观察了激光穿过金属粉末床制造每一层产品时所发生的事情。

Under perfect conditions, the melt poolshape is shallow and semicircular, called the "conduction mode." Butduring the actual printing process, the high-power laser, often moving at a lowspeed, can change the melt pool shape to something like a keyhole in a wardedlock: round and large on top, with a narrow spike at bottom. Such "keyholemode" melting can potentially lead to defects in the final product.

在完美的条件下，熔池的形状是浅而半圆形的，称为“传导模式”。但是在实际的打印过程中，高功率激光通常以低速移动，可以将熔池的形状改变为类似于锁中的钥匙孔形状：顶部是圆的，顶部是大的，底部是窄的尖峰。这种“小孔模式”熔化可能导致最终产品出现缺陷。

"Based on this research, we now knowthat the keyhole phenomenon is more important, in many ways, than the powderbeing used in additive manufacturing," said Ross Cunningham, a recentgraduate from Carnegie Mellon University and one of the co-first authors ofthis paper. "Our research shows that you can predict the factors that leadto a keyhole -- which means you can also isolate those factors for betterresults."

卡内基梅隆大学应届毕业生、本文第一作者之一罗斯•坎宁安说：“根据这项研究，我们现在知道，在许多方面，锁孔现象比增材制造中使用的粉末更为重要。”我们的研究表明，你可以预测导致锁孔的因素——这意味着你也可以分离这些因素，以获得更好的结果。”

Fig.4 Relationships between keyholedepth, front wall angle, and laser power density.(A) Representative x-ray imageof the vapor depression in a Ti-6Al-4V bare plate, labeling the depression zonedepth, d, and the front keyhole wall angle, θ. (B) Schematic of keyholedepth and front keyhole wall angle, adapted from Fabbro et al.. (C)Comparison of the front keyhole wall angles between theoretical predictions(dashed and dash-dotted lines) and experimental measurements (open and solidsymbols) for selected beam velocities with spot sizes of 95 and 140 μm. (D)Keyhole depth as a function of tangent of the front keyhole wall angle for the95-μm laser spot size. Two equivalent plots are shown in figs. S5 and S7, whichreveal that adding powder on top of the plate has only a small effect. Errorbars indicate SD.

图4 小孔深度、前壁角和激光功率密度之间的关系。（A）Ti-6Al-4V裸板中蒸汽压陷的代表性x射线图像，标记压陷区深度d和前锁孔壁角θ。（B）锁孔深度和前锁孔壁角示意图，改编自Fabbro等人。（C）理论预测（虚线和虚线）和实验测量（开放和固体符号）中选择的光斑尺寸为95和140μm的光束速度之间的前锁孔壁角比较。（D）锁孔深度是95μm激光光斑尺寸的前锁孔壁角正切的函数。两个等效图如图S5和S7所示，这表明在板的顶部添加粉末只有很小的效果。误差线SD。

The research shows that keyholes form whena certain laser power density is reached that is sufficient to boil the metal.This, in turn, reveals the critical importance of the laser focus in theadditive manufacturing process, an element that has received scant attention sofar, according to the research team.

研究表明，当达到一定的激光功率密度足以使金属沸腾时，就会形成小孔。研究团队称，这反过来揭示了激光聚焦在增材制造过程中的重要性，它是在增材制造过程迄今为止很少受到关注的重要因素。

"The keyhole phenomenon was able to beviewed for the first time with such details because of the scale andspecialized capability developed at Argonne," said Tao Sun, an Argonnephysicist and an author on the paper. "The intense high-energy X-ray beamat the APS is key to discoveries like this."

阿贡实验室物理学家、论文作者陶荪说：“由于阿贡的规模和专业能力，人们首次能够用这样的细节来观察锁孔现象。”高级光子源超强高能X射线束是取得该发现的关键。

The experiment platform that supports studyof additive manufacturing includes a laser apparatus, specialized detectors,and dedicated beamline instruments. In 2016, the Argonne team, together withtheir research partners, captured the first-ever X-ray video of laser additivemanufacturing at micrometer and microsecond scales. That study increasedinterest in the impact Argonne's APS could have on manufacturing techniques andchallenges.

支持增材制造研究的实验平台包括激光装置、专用探测器和专用光束线仪器。2016年，阿贡团队与他们的研究伙伴一起，首次拍摄了激光增材制造在微米和微秒级的X光视频。这项研究增加了人们对阿贡高级光子源可能对制造技术和挑战产生影响的兴趣。

"We are really studying a very basicscience problem, which is what happens to metal when you heat it up with ahigh-power laser," said Cang Zhao, an Argonne postdoc and the otherco-first author of the paper. "Because of our unique experimentalcapability, we are able to work with our collaborators on experiments that arereally valuable to manufacturers."

“我们真的在研究一个非常基础的科学问题，那就是当你用高功率激光加热金属时，金属会发生什么，”阿贡博士后、论文的另一位第一作者仓昭说，由于我们独特的实验能力，我们能够与合作者合作进行对制造商真正有价值的实验。”

The research team believes this researchcould motivate makers of additive manufacturing machines to offer moreflexibility when controlling the machines and that the improved use of themachines could lead to a significant improvement in the final product. In addition,if these insights are acted upon, the process for 3D printing could get faster.

研究团队认为，这项研究可以激励付材制造设备制造商在控制设备时提供更大的灵活性，设备的改进使用可以使最终产品显著改进。此外，如果针对这些观点采取行动，3D打印的过程可能会更快。

"It's important because 3D printing ingeneral is rather slow," Rollett said. "It takes hours to print apart that is a few inches high. That's OK if you can afford to pay for thetechnique, but we need to do better."

罗利特说：“这一点很重要，因为3D打印通常速度很慢。”打印几英寸高的零件需要几个小时。如果你能付得起技术费，那没关系，但我们需要做得更好。”

(平台原创翻译，英文来源：Sciencedaily, 2019年2月22日)

英文来源:  https://www.sciencedaily.com/ releases/ 2019/ 02/ 190222101232.htm.

图片来源： 封面图片来源： X-ray CT: a powerful partner in the 3Dprinting revolution（https://blog.nikonmetrology.com/x-ray-ct-for-3d-printing/），文中图片来源于参考文献，版权归杂志和文献作者。

参考文献: Ross Cunningham, Cang Zhao, Niranjan Parab, Christopher Kantzos,Joseph Pauza, Kamel Fezzaa, Tao Sun, Anthony D. Rollett. Keyhole thresholdand morphology in laser melting revealed by ultrahigh-speed x-ray imaging. Science,2019; 363 (6429): 849 DOI: 10.1126/science.aav4687.