When a slow-speed, high-power laser is melting metal powder during the 3D printing of a part, a keyhole-shaped cavity in the melt pool can result. Pores, i.e. defects, form at the bottom of the keyhole. New research published in Science reveals how the pores are generated and become defects trapped in solidifying metal.

“The real practical value of this research is that we can be precise about controlling the machines to avoid this problem,” says Anthony D. Rollett, a professor of materials science and engineering in Carnegie Mellon College of Engineering and a lead co-author of the paper, “Critical instability at moving keyhole tip generates porosity in laser melting.”

Building on previous research that quantified the keyhole phenomenon, the research team used extremely bright high-energy x-ray imagining to watch instabilities of the keyhole. Pores form during fluctuations of the keyhole, and it changes its shape: the keyhole tip morphs into a “J” shape and pinches off. This unstable behavior generates acoustic waves in the liquid metal that force the pores away from the keyhole so that they survive long enough to get trapped in the resolidifying metal. The team is the first to focus on this behavior and identify what is happening.

“When you have a deep keyhole, the walls oscillate strongly. Occasionally, the oscillations are strong enough at the bottom of the keyhole that they pinch off, leaving a large bubble behind. Sometimes this bubble never reconnects to the main keyhole. It collapses and generates an acoustic shock wave. This pushes the remaining pores away from the keyhole,” explains Rollett.

It’s important to note that keyholes themselves are not flaws and, e.g., they increase the efficiency of the laser. Using synchrotron x-ray equipment at Argonne National Laboratories, the only facility in the United States where the researchers could run these experiments, they noted that there is a well-defined boundary between stable versus unstable keyholes.

“As long as you stay out of the danger zone [i.e., too hot, too slow], the risk of leaving defects behind is quite small,” says Rollett.

Fluctuations in the keyhole’s depth increase strongly with decreasing scan speed and laser power on the unstable side of the boundary.

“You can think of the boundary as a speed limit, except it is the opposite of driving a car. In this case, it gets more dangerous as you go slower. If you’re below the speed limit, then you are almost certainly generating a defect,” adds Rollett.

At a broader scale, by proving the existence of well-defined keyhole porosity boundaries and demonstrating the ability to reproduce them, science can offer a more secure basis for predicting and improving printing processes. Rollett, who is the faculty co-director of Carnegie Mellon’s Next Manufacturing Center, thinks that the findings from this research will quickly find their way into how companies operate their 3D printers.

Watch Prof. Rollett discuss the process of 3D printing with metals as well as using advanced characterization techniques to solve industry challenges in the field.

The research team includes co-lead author Tao Sun, University of Virginia; Cang Zhao, Tsinghua University, China; Niranjan D. Parab and Kamel Fezzaa, Argonne National Laboratory; Xuxiao Li and Wenda Tan, University of Utah.

Funding sources include the Department of Energy, the Department of Defense, the National Aeronautics and Space Administration (NASA) University Leadership Initiative program, the National Science Foundation, and startup funds from the Department of Mechanical Engineering at Tsinghua University and the University of Virginia.

UQ PhD candidate Hubert Cheung said efforts to shift entrenched values and beliefs about Chinese medicine are not achieving conservation gains in the short term.

He said a better understanding of traditional practices was critical for conservationists to form more effective strategies.

“The use of endangered species in traditional Chinese medicine threatens species’ survival and is a challenge for conservationists,” Mr Cheung said.

“Pushing messages of inefficacy, providing various forms of scientific evidence or promoting biomedical alternatives doesn’t seem to be drastically influencing decisions and behaviours.

“And, although many practices and treatments continue to be criticised for lacking scientific support, the World Health Organization approved the inclusion of traditional Chinese medicine in its global compendium of medical practices last year.

“The challenge now is for conservationists to work proactively with practitioners and others in the industry to find sustainable solutions.

“However, most conservation scientists and organisations are unfamiliar with traditional Chinese medicine, which makes it difficult to devise effective and culturally-nuanced interventions.”

The researchers have examined the core theories and practices of traditional Chinese medicine, in a bid to make it more accessible.

They hope their study — and the nuances within — will influence policy and campaigning.

“Today, traditional Chinese medicine is formally integrated into China’s healthcare system, and has been central to China’s response to the ongoing pandemic,” Mr Cheung said.

“In fact, the Chinese government’s COVID-19 clinical guidance has included recommendations for the use of a product containing bear bile, which has raised concerns among conservation groups.”

UQ’s Professor Hugh Possingham said traditional Chinese medicine was now not only entrenched in the social and cultural fabric of Chinese society, but also gaining users elsewhere.

“A better understanding of traditional Chinese medicine will empower conservationists to engage more constructively with stakeholders in this space,” Professor Possingham said.

“We’re hoping that this work can help all parties develop more effective and lasting solutions for species threatened by medicinal use.”

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“In my daily work at the Botanical Garden, I regularly come across species names that are not clear, where existing reference lists have gaps,” said Freiberg. “This always means additional research, which keeps you from doing your actual work and above all limits the reliability of research findings. I wanted to eliminate this obstacle as well as possible.”

World’s most comprehensive and reliable catalogue of plant names

With 1,315,562 scientific names, the LCVP is the largest of its kind in the world describing vascular plants. Freiberg compiled information from accessible relevant databases, harmonized it and standardised the names listed according to the best possible criteria. On the basis of 4500 other studies, he investigated further discrepancies such as different spellings and synonyms. He also added thousands of new species to the existing lists — species identified in recent years, mainly thanks to rapid advances in molecular genetic analysis techniques.

The LCVP now comprises 351,180 vascular plant species and 6160 natural hybrids across 13,460 genera, 564 families and 84 orders. It also lists all synonyms and provides further taxonomic details. This means that it contains over 70,000 more species and subspecies than the most important reference work to date, TPL. The latter has not been updated since 2013, making it an increasingly outdated tool for use in research, according to Freiberg.

“The catalogue will help considerably in ensuring that researchers all over the world refer to the same species when they use a name,” says Freiberg. Originally, he had intended his data set for internal use in Leipzig. “But then many colleagues from other botanical gardens in Germany urged me to make the work available to everyone.”

LCVP vastly expands global knowledge of plant diversity

“Almost every field in plant research depends on reliably naming species,” says Dr Marten Winter of iDiv, adding: “Modern science often means combining data sets from different sources. We need to know exactly which species people refer to, so as not to compare apples and oranges or to erroneously lump different species.” Using the LCVP as a reference will now offer researchers a much higher degree of certainty and reduce confusion. And this will also increase the reliability of research results, adds Winter.

“Working alone, Martin Freiberg has achieved something truly incredible here,” says the director of the Botanical Garden and co-author Prof Christian Wirth (UL, iDiv). “This work has been a mammoth task, and with the LCVP he has rendered an invaluable service to plant research worldwide. I am also pleased that our colleagues from iDiv, with their expertise in biodiversity informatics, were able to make a significant contribution to this work.”

The study, which involved a collaboration with clinical teams in the Royal Victoria Eye and Ear Hospital and the Mater Hospital, also has implications for a much wider suite of neurological disorders associated with ageing.

The scientists publish their results today [Thursday 26th November 2020] in leading journal, Frontiers in Neuroscience.

Dominant optic atrophy (DOA)

Characterised by degeneration of the optic nerves, DOA typically starts to cause symptoms in patients in their early adult years. These include moderate vision loss and some colour vision defects, but severity varies, symptoms can worsen over time and some people may become blind. There is currently no way to prevent or cure DOA.

A gene (OPA1) provides instructions for making a protein that is found in cells and tissues throughout the body, and which is pivotal for maintaining proper function in mitochondria, which are the energy producers in cells.

Without the protein made by OPA1, mitochondrial function is sub-optimal and the mitochondrial network which in healthy cells is well interconnected is highly disrupted.

For those living with DOA, it is mutations in OPA1 and the dysfunctional mitochondria that are responsible for the onset and progression of the disorder.

The new gene therapy

The scientists, led by Dr Daniel Maloney and Professor Jane Farrar from Trinity’s School of Genetics and Microbiology, have developed a new gene therapy, which successfully protected the visual function of mice who were treated with a chemical targeting the mitochondria and were consequently living with dysfunctional mitochondria.

The scientists also found that their gene therapy improved mitochondrial performance in human cells that contained mutations in the OPA1 gene, offering hope that it may be effective in people.

Dr Maloney, Research Fellow, said:

“We used a clever lab technique that allows scientists to provide a specific gene to cells that need it using specially engineered non-harmful viruses. This allowed us to directly alter the functioning of the mitochondria in the cells we treated, boosting their ability to produce energy which in turn helps protects them from cell damage.

“Excitingly, our results demonstrate that this OPA1-based gene therapy can potentially provide benefit for diseases like DOA, which are due to OPA1 mutations, and also possibly for a wider array of diseases involving mitochondrial dysfunction.”

Importantly, mitochondrial dysfunction causes problems in a suite of other neurological disorders such as Alzheimer’s and Parkinson’s disease. The impacts gradually build up over time, which is why many may associate such disorders with ageing.

Professor Farrar, Research Professor, added:

“We are very excited by the prospect of this new gene therapy strategy, although it is important to highlight that there is still a long journey to complete from a research and development perspective before this therapeutic approach may one day be available as a treatment.

“OPA1 mutations are involved in DOA and so this OPA1-based therapeutic approach is relevant to DOA. However mitochondrial dysfunction is implicated in many neurological disorders that collectively affect millions of people worldwide. We think there is great potential for this type of therapeutic strategy targeting mitochondrial dysfunction to provide benefit and thereby make a major societal impact. Having worked together with patients over many years who live with visual and neurological disorders it would be a privilege to play a role in a treatment that may one day help many.”

The research was supported by Science Foundation Ireland, the Health Research Board of Ireland, Fighting Blindness Ireland, and the Health Research Charities Ireland.