金屬3D列印關鍵科學問題 || 阿貢高級光子源超強X射線揭示氣孔的產生機制 || 雙語

Finding keyholes in metals 3D printing

找出3D列印金屬中的關鍵孔洞

 

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

最新研究發現了3D列印金屬材料缺陷產生的原因，並為獲得更好的增材製造效果鋪平了道路。 

 

Source: College of Engineering, CarnegieMellon University

來源：卡內基梅隆大學工程學院

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：新研究已經確定了氣孔形成的方式和時間並提出了預測氣孔形成的方法——這是一個極其重要的發現，可以極大地改進3D列印工藝。

 

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.

增材製造能夠變革工業的承諾受到一個普遍存在的問題的制約：最終產品中存在微小的孔洞，這可能導致裂紋和其他失效。

 

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《科學》雜誌上發表，由卡內基梅隆大學和阿貢國家實驗室的研究人員主導的一項新的研究，已經確定了這些氣孔是如何形成和何時形成的，以及預測它們形成的方法——這一關鍵性的發現可以極大地改進3D列印工藝。

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).

圖1穩態雷射照射下熔池和蒸汽壓陷的演變。（A）熔池的最初形成。（B）形成一個小的、穩定的蒸汽壓陷。（C）蒸汽壓差的穩定增長。（D）不穩定性形成於蒸汽壓陷。（E和F）蒸汽壓陷形狀的快速變化。（G和H）蒸汽壓降的周期性波動。（I和J）熔池形狀從準半圓到雙峰的變化，頂部有碗，底部中間有尖峰。樣品為Ti-6Al-4V裸板。雷射光斑尺寸為140μm，雷射功率為156w，圖像經雷射照射前採集的圖像進行背景校正。熔池的形狀在（E）和（J）中用紅色陰影標出。

"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."

卡內基梅隆大學材料科學與工程教授、論文作者安東尼·羅利特說：「本文的研究將轉化為更好的質量控制和對機器工作的更好控制。」為了讓增材製造真正助力大多數公司騰飛，我們需要提高成品的一致性。這項研究是朝著該方向邁出了的重要一步。」

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.

科學家們在，美國能源部科學用戶設施，阿貢高級光子源（APS），使用了超亮高能X射線，拍攝了一個叫做雷射能量床熔融工藝（LPBF）的超高速視頻和圖像，在這個過程中，雷射被用來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.

圖2固定雷射照射下鎖孔鑽孔。（A）和（B）在不同功率下，在光斑尺寸分別為95μm和140μm的情況下，隨著時間的推移，蒸汽壓陷的穿透深度。對於給定的光斑尺寸，這種轉變發生在幾乎相同的蒸汽壓陷深度處，較小的光斑尺寸具有較淺的臨界深度。（C）雷射鑽速與功率密度的函數關係。黑色虛線是預測鑽速的線性擬合。

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.

雷射掃描每一層粉末，在需要的地方熔化金屬，從基底上直接製造成品。當氣泡被困在這些層中時，會形成缺陷，導致最終產品出現裂紋或其他失效。

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.

直到現在，製造商和研究人員還不太清楚雷射是如何鑽入金屬的，產生了稱為「蒸汽壓陷」的空洞，他們認為是金屬粉末的類型或雷射的強度造成的。因此，製造商一直在使用不同類型的金屬和雷射的試錯方法來減少缺陷。

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.

事實上，研究表明，在這個過程中，無論是雷射還是金屬，幾乎所有的條件下都存在這些蒸汽壓陷。更重要的是，這項研究顯示了如何預測小壓陷何時會發展成一個大而不穩定的大壓陷，從而可能產生缺陷。

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.

圖3穿過P-V空間的鎖孔形態。（A）雷射光斑尺寸為95μm的Ti-6Al-4V裸板P-V空間的代表性射線照片，顯示了蒸汽抑制尺寸和形態的變化。在固定束實驗（圖2A和圖S2）中測量到的蒸汽壓陷和熔池轉變分別用藍色和紅色虛線標記。（B和C）95μm（B）和140μm（C）雷射光斑在不同掃描速度下的蒸汽壓陷深度隨雷射功率的變化。誤差線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."

「我們正在揭開面紗，揭示出真正發生的事情，」同時也是卡內基梅隆下一代製造中心聯合主任的羅萊特說，大多數人認為你用雷射照射金屬粉末的表面，光被材料吸收，然後熔化成熔池。實際上，你其實是在金屬上鑽了一個洞。」

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.

通過世界上最強大的同步加速器設備之一阿貢高級光子源使用高度專業化的設備，研究人員觀察了雷射穿過金屬粉末床製造每一層產品時所發生的事情。

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.

在完美的條件下，熔池的形狀是淺而半圓形的，稱為「傳導模式」。但是在實際的列印過程中，高功率雷射通常以低速移動，可以將熔池的形狀改變為類似於鎖中的鑰匙孔形狀：頂部是圓的，頂部是大的，底部是窄的尖峰。這種「小孔模式」熔化可能導致最終產品出現缺陷。

"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."

卡內基梅隆大學應屆畢業生、本文第一作者之一羅斯•坎寧安說：「根據這項研究，我們現在知道，在許多方面，鎖孔現象比增材製造中使用的粉末更為重要。」我們的研究表明，你可以預測導致鎖孔的因素——這意味著你也可以分離這些因素，以獲得更好的結果。」

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.

圖4 小孔深度、前壁角和雷射功率密度之間的關係。（A）Ti-6Al-4V裸板中蒸汽壓陷的代表性x射線圖像，標記壓陷區深度d和前鎖孔壁角θ。（B）鎖孔深度和前鎖孔壁角示意圖，改編自Fabbro等人。（C）理論預測（虛線和虛線）和實驗測量（開放和固體符號）中選擇的光斑尺寸為95和140μm的光束速度之間的前鎖孔壁角比較。（D）鎖孔深度是95μm雷射光斑尺寸的前鎖孔壁角正切的函數。兩個等效圖如圖S5和S7所示，這表明在板的頂部添加粉末只有很小的效果。誤差線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，當達到一定的雷射功率密度足以使金屬沸騰時，就會形成小孔。研究團隊稱，這反過來揭示了雷射聚焦在增材製造過程中的重要性，它是在增材製造過程迄今為止很少受到關注的重要因素。

"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."

阿貢實驗室物理學家、論文作者陶蓀說：「由於阿貢的規模和專業能力，人們首次能夠用這樣的細節來觀察鎖孔現象。」高級光子源超強高能X射線束是取得該發現的關鍵。

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.

支持增材製造研究的實驗平臺包括雷射裝置、專用探測器和專用光束線儀器。2016年，阿貢團隊與他們的研究夥伴一起，首次拍攝了雷射增材製造在微米和微秒級的X光視頻。這項研究增加了人們對阿貢高級光子源可能對製造技術和挑戰產生影響的興趣。

"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."

「我們真的在研究一個非常基礎的科學問題，那就是當你用高功率雷射加熱金屬時，金屬會發生什麼，」阿貢博士後、論文的另一位第一作者倉昭說，由於我們獨特的實驗能力，我們能夠與合作者合作進行對製造商真正有價值的實驗。」

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.

研究團隊認為，這項研究可以激勵付材製造設備製造商在控制設備時提供更大的靈活性，設備的改進使用可以使最終產品顯著改進。此外，如果針對這些觀點採取行動，3D列印的過程可能會更快。

"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."

羅利特說：「這一點很重要，因為3D列印通常速度很慢。」列印幾英寸高的零件需要幾個小時。如果你能付得起技術費，那沒關係，但我們需要做得更好。」

(平臺原創翻譯，英文來源：Sciencedaily, 2019年2月22日)

英文來源: https://www.sciencedaily.com/ releases/ 2019/ 02/ 190222101232.htm.圖片來源： 封面圖片來源：X-ray CT: a powerful partner in the 3Dprinting revolution（https://blog.nikonmetrology.com/x-ray-ct-for-3d-printing/），文中圖片來源於參考文獻，版權歸雜誌和文獻作者。參考文獻: 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.

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