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Intricate Titanium AM Parts Solve Cooling Challenge for CERN

Wednesday, December 2, 2020
 

Buried a football field deep in the mountains of Switzerland and France, the Large Hadron Collider (LHC) is the world’s largest and most powerful particle accelerator, used by the European Organization for Nuclear Research (CERN) to conduct high-energy physics research. The LHC’s 27-km “tube” allows particles to reach incredible speeds, enabling researchers to observe reactions of the particle beams at four beam-crossing points. At these points, large particle detectors, as part of CERN’s Large Hadron Collider beauty (LHCb) experiment, provide the means of observation. Here, a long and extremely narrow photon-detector strip―roughly 140 m in length and less than 2 mm wide―must be cooled to -40 C in order to preserve the reaction for study. The strip attaches to 3D-printed titanium cool-bars that provide 100 percent of the needed cooling.


As part of an CERN experiment, the titanium cool-bars in this assembly are responsible for keeping a long and extremely narrow photon-detector strip―roughly 140 m in length and less than 2 mm wide―cooled to -40 C in order to preserve a particle-collision reaction for study. More than 300 cool-bars were 3D printed for this application, as traditional manufacturing proved too labor-intensive and lacked needed precision and repeatability.
These cool-bars result from a collaboration between Nikhef, the Dutch National Institute for Subatomic Physics, and the 3D Systems Customer Innovation Center (CIC), and were produced using 3D Systems’ direct metal printing (DMP) technology. For its contribution to the successful upgrade of this experiment, 3D Systems earned the 2019 LHCb Industry Award.

The complexity of this experiment’s cooling system results from several unavoidable factors: the limited space in which the cool-bars are required to fit; the heat that must be dissipated within that short space; the temperature uniformity required over the length of the entire photon-detection strip; and the flatness of the cool-bars necessary to preserve detector efficiency and resolution.

“The effect of this is that you have to be very efficient in how you construct your cooling,” says Antonio Pellegrino, who works at Nikhef and is a leader on the SciFi tracker project at CERN under the LHCb experiment. 

Nikhef project engineer Rob Walet began developing the cool-bar by designing a part that ideally answered performance requirements. 

“This design was so beautiful,” says Pellegrino, “but it was not producible in the usual ways.”

One major issue complicating manufacturability through conventional means: the required wall thinness. For maximum efficacy, minimal material must be present between the coolant and the surface to be cooled. Given the length of the part, 263 mm, this thinness could not be machined.

After early experimentation with manual prototyping, CERN quickly abandoned it as too labor-intensive and too difficult to reproduce. This led to exploration of 3D printing. Though CERN had optimized its cool-bar design for final function, it was not yet optimized for additive manufacturing (AM). 

“Out of a few possible companies, we chose 3D Systems because it seemed to me that their engineers were capable of transforming our design into something that could be produced,” says Pellegrino.

CERN leveraged the application-engineering expertise housed within the 3D Systems CIC in Leuven, Belgium, to accelerate the AM path. 3D Systems CICs, in locations worldwide, advise and assist on projects at any stage, from application development and front-end engineering to equipment validation, process validation, part qualification and production. The company’s capabilities and expertise as both a manufacturer and user of AM technology enables a unique feedback loop between application engineers and machine engineering groups, which helped achieve an ideal AM solution for CERN.

Through a collaborative iterative process of design, printing and testing, the engineering teams at CERN and 3D Systems collaborated to modify the cool-bar design to meet the requirements for manufacturing as well as final function.

Performance requirements included:

 Wall thickness. A wall thickness of 0.25 mm was achieved through the dimensional accuracy of 3D Systems’ DMP machines as well as through its inhouse expertise in adjusting laser parameters with respect to the stability and width of the titanium-powder melt pool.

  • Leak-tightness. The requirement for leak-tightness guided the choice of LaserForm TiGr23 material, a high-strength titanium alloy. The custom parameter set 3D Systems developed for the project also helped attain this goal.
  • Flatness. The 263-mm-long part required a flatness tolerance of 50 microns along its entire length, achieved via various design-for-AM strategies applied by 3D Systems application engineers, as well as build-strategy recommendations, including a vertical print orientation.

Optimizing the cool-bar design for production enabled delivery of more than 300 cool-bars. The major value of using 3D printing for production included the cost-efficiency of the process relative to the extreme complexity of the components, as well as the ability to achieve the uncommon tolerances necessary for the success of the final application, according to Pellegrino.

“We needed a reliable way to get both the part and the performance we were after,” he says.

Within this project, manufacturing guidance provided by 3D Systems included:

 Design strategy. The final cool-bar was designed as a set of mirrored A and B components welded together to form a complete part. This enabled CERN to obtain required features, dimensions and quality with minimal assembly.

  • Print orientation. With AM, the orientation of a part on the build platform can impact support requirements. Based on the geometry of CERN’s design, 3D Systems engineers recommended a vertical orientation to enable the part to be as self-supporting as possible.
  • Part cleaning. The cool-bar was designed with parallel cooling channels, which can pose a challenge in controlling and ensuring complete powder removal. 3D Systems was able to assign a cleaning protocol to ensure thorough material evacuation from the parts.

Based on stress testing, the cool-bars are predicted to last at least 10 yr. Pellegrino believes that the cool-bars will prove more reliable due to the limited assembly enabled by AM, and the ability to build an optimized form in a single material.

“3D printing really brings in new possibilities,” he says, noting that AM has brought major benefits to the team at CERN, and that success in this project has sparked interest in AM from colleagues who have not used it before. In fact, Pellegrino offers, CERN application experts are considering AM on new projects.

 

See also: 3D Systems


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