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3D printing: Inner workings come into view

Researchers from Argonne National Laboratory, Carnegie Mellon University, and the Missouri University of Science and Technology team up to examine phenomena at the microstructural level. High-speed X-ray microscopy lets them watch -- in real time -- as 3D printers add layers.

The development of additive manufacturing (AM) -- often referred to as 3D printing -- means that engineers' imaginations are soaring -- only to be brought back to earth by limitations in the quality and reliability of the parts they can produce. In 3D printing, complex structures are built up on a layer-by-layer basis with different types of materials. Scientists have already used AM to build cars, pedestrian bridges, and even artificial jawbones. Engineers can use titanium and other metal alloys in metal AM as a way to tap raw materials more efficiently. Other advantages include reducing product costs and weight, shortening supply chains, and accruing environmental benefits.

Argonne researchers are able to monitor the laser powder bed fusion process in real time using X-ray microscopy.

 

 

However, engineers are also grappling with issues associated with 3D printing, including deformations in the printed product that can lead to greater and greater variations from design, as well as difficulties in repairing 3D products. Researchers at the U.S. Department of Energy's (DOE's) Argonne National Laboratory near Chicago, along with colleagues at Carnegie Mellon University and the Missouri University of Science and Technology, are using the facilities and resources at Argonne to peer at the complex dynamics occurring at the microstructural level in 3D printing, with a view to predicting and improving 3D-printing outcomes.

Issues and challenges and the 3D-printing research effort
Yet, there are obstacles to overcome in metal AM. When 3D-printed materials contain structural defects and vary from their designs, engineers are forced to repair their finished pieces or start over from scratch. In addition, aspects of the physics behind the process are not well understood, so much of the research in this area involves trial and error -- a costly and time-consuming way to innovate. According to Argonne physicist and project team member Tao Sun, scientists and engineers are wrestling with how to:

  • Print a part with precise dimensions reliably, given that the residual stress may lead to distortion.
  • Print a part without major defects, that is, without porosity or cracks.
  • Print a part with the desired microstructures, given the extreme thermal conditions in the metal 3D-printing process.
  • Use raw feedstock and energy more efficiently.
  • Realize closed-loop, in-process control to achieve high reliability and stability.
  • Predict optimized design and printing conditions via simulation and modeling.

To address these problems, Sun and colleagues from Argonne, Carnegie Mellon, and the Missouri University of S&T are investigating the entire 3D-printing process to discover both how defects form and what methods are needed to avoid them. From their front-row seats at beamline 32-ID-B at the Advanced Photon Source (APS), a DOE Office of Science User Facility, for the first time these scientists are peering inside materials formed by 3D printing to witness the inner workings in real time via the APS's intense synchrotron X rays.

VIDEO: Argonne National Laboratory researchers are gaining a deeper understanding of the 3D-printing process, and as a result, they are helping industries quickly and economically manufacture 3D-printed products that are truly reliable.

What happens in 3D printing
The team's efforts resulted in the 2017 Scientific Reports article, "Real-time monitoring of laser powder bed fusion process using high-speed X-ray imaging and diffraction," which describes the complex interplays occurring in materials under the extremely high heating and cooling rates involved in 3D printing.

The Advanced Photon Source, a U.S. Department of Energy Office of Science National User Facility at Argonne, provides ultra-bright, high-energy storage ring-generated X-ray beams for research in almost all scientific disciplines.

 

 

 

 

Most metal AM systems are a type called laser powder bed fusion (LPBF), which features a "superior capability to make geometrically complex parts."1 In LPBF:

  • A laser beam scans across a thin layer of metallic powders and locally melts the powders through to the layer below. (For their research, the team used a titanium, Ti-6Al-4V, powder bed system.)
  • Many highly dynamic and transient physical phenomena come into play because of the extremely hot and cold temperatures: melting and partial vaporization of metallic powders, flow of the molten metal, powder ejection and redistribution, rapid solidification, non-equilibrium phase transition, and the like.
  • Characterizing these LPBF dynamics is extremely challenging experimentally because the laser beam has highly localized and very fast interaction with the metal powders. The (previously) most advanced method uses high-speed visible light imaging and can monitor only what happens above the powder bed's surface.
  • These complex interactions often result in a product that has a rough surface, significant porosity in terms of both size and number density, residual stress, and unfavorable phase and grain structures, with an attendant impact on its properties.

The team showed they can observe and quantify many metal 3D-printing characteristics -- including melt pool size/shape, powder ejection, solidification, porosity formation, and phase transformations. "The laser-metal interaction happens very quickly," says Sun. "Fortunately, we captured the process at 50,000 frames a second using the high-speed X-ray instrument at the APS. We can study the resulting movie frame by frame to examine how the material's microstructure, especially defects and pores, form." Recently, Sun's team demonstrated the ultra-high-speed X-ray imaging of the LPBF process with 10 million frames per second at the APS.

Findings, implications, and future aims
Sun shares his conclusions with partners in academia and other national laboratories who are building models to predict the characteristics of the printed materials more reliably. These models also predict the dynamics of the process -- such as how the laser melts the powder, when the powder changes into gases and liquids, and so on.

Sequence 1: Argonne researchers have provided a first-of-its-kind in-situ observation and measurement of the metal additive manufacturing process. Shown here are dynamic X-ray images of laser powder bed fusion process of Ti-6Al-4V. (Image by Argonne National Laboratory.)

 

 

 

 

Sequence 2: In these high-speed X-ray images, the 3D printer is using a laser to melt metal powder, which causes a "keyhole" defect within the cooled material. Researchers at Argonne are studying this process and developing guidelines to avoid such errors. (Image by Argonne National Laboratory.)

 

 

 

 

Meanwhile, Aaron Greco, a principal materials scientist at Argonne and project co-leader for Argonne's additive manufacturing effort, enhances the models from a different angle. "After printing, we examine the product's resulting microstructure and defects," says Greco. "We use a variety of techniques, including optical and electron microscopy and even tomography at the APS, to validate the models."

The result is a virtuous loop in which the experimental data feeds into AM models, and then the improved models are tested by more elaborate and insightful experiments. This interplay has been essential to gaining a clear and accurate understanding of the underlying materials physics required to make 3D printing truly reliable.

Although this loop is vital to their fundamental understanding of metal AM, the researchers' ambitions extend further. "Our goal is to explore new possibilities," says Greco. "Industries are currently limited to a certain set of metal alloys. But what about identifying new materials that we can print? If you understand the physical properties related to how to print new alloys, you can adopt these into the process and speed up the reliability of 3D printing."

Industries are also limited by the extremely detailed models currently required to define the printing process for complex parts. By reducing these models to just the handful of elements that affect quality and reliability, the team hopes to make the 3D-printing process faster and more suitable for industry. Another challenge is determining how to print a functional part with composition and structure gradients.

Ultimately, Argonne's efforts will achieve the best of both worlds: scientists will uncover the dynamic mysteries of metal AM, while industries will thrive with blueprints that rapidly print cost-effective and reliable products. "Our work will not only help industries improve efficiency and performance, but increase the likelihood that metal additive manufacturing will be more widely adopted in other applications," Greco adds.

To learn about tapping into Argonne's facilities and expertise in this area, contact partners@anl.gov.

This research has been supported by the U.S. Department of Energy's (DOE's) Office of Science, DOE's Office of Energy Efficiency and Renewable Energy, and the National Nuclear Security Agency's Kansas City National Security Campus. The research used resources of the Advanced Photon Source, a DOE Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory.

1. https://www.nature.com/articles/s41598-017-03761-2.

Source: Argonne National Laboratory

Published November 2017

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