A cookbook for additive manufacturing

With the ADDvance™ O₂ precision in place to control the atmosphere in a 3D printing chamber, Linde experts are now working to define the ideal gas mix for each printing job.

Hundreds, if not thousands, of metallic cubes no bigger than the tip of your finger fill the racks, the desks and occasionally the floors in Linde’s 3D printing laboratory. The flight of offices, housed on the premises of the company’s gas filling plant just north of Bavaria’s capital Munich, oozes a spirit of relentless research and contagious curiosity.

Kai Dietrich and Camille Pauzon, both in PhD programmes at their alma mater, churn out some 40 of these cubes every day. While the pieces may look rather unassuming to a layperson, Dietrich and Pauzon draw academic excitement from their rigorous search for minuscule differences between the cubes’ microstructure, texture, hardness and porosity.

The samples come from one of the laboratory’s two 3D metal printers. While the term “print” echoes the common office task, additive manufacturing (AM), as it is known to experts, is in fact a highly complex process. “Joining materials to make parts from a three-dimensional digital model, usually by adding layer after layer in an inert printing chamber, depends on more than 200 parameters for its success,” says Pierre Forêt, responsible for Linde’s research in the field and managing the Global Development Centre Additive Manufacturing (GDC AM), as the laboratory is formally known. With Linde’s ADDvance™ O₂ precision, manufacturers are now able to control at least those factors that define the atmosphere in the printing chamber.

Image of the space shuttle, used for the additive manufacturing campaign
Offering greater liberty to designers at lower costs, additive manufacturing is set to grow
Taken from the "Linde ADDvance O2 precision." Folder (id: 105698, the image describes the creation of the AM-campaigns starship. Image one shows a perfect atmosphere with ADDvance
Traces of nitrogen, carbon dioxide, hydrogen and water vapour can influence the material properties of a 3D printed product

Just the right challenge

Opened in 2017, the lab was set up to provide innovative technology solutions to support the additive manufacturing industry in powder production and storage, laser powder bed fusion, laser metal deposition, wire arc 3D printing and post treatments.

As 3D printers take no more than a few hours to turn digital data into physical objects while reducing material waste and the need for tools, the technology is likely to see significant growth over the coming years. It is already used in the construction of jet engines and medical applications, for example, increasing the durability of parts and bringing down their costs.

One of the technique’s greatest challenges, however, is to reproduce identical samples. The material properties of 3D printed objects can be influenced by its geometry and the way the printer has been operated, to name but two factors.

The raw materials as well as the actual printing process in the chamber are particularly sensitive to the surrounding atmosphere. Take titanium, for example. Often used in laser powder bed fusion, a technique for printing metals, it slowly reacts with the hydrogen in the air.

“Humid titanium powder behaves like wet flour, it coagulates and loses the ability to close its pores,” Forêt says. “If wet titanium powder is fed into a 3D printer, the printed product will inevitably be of lower quality.” Linde therefore developed a powder cabinet with a controlled atmosphere to keep titanium and other metal powders at optimal conditions.

This is just the right challenge for Forêt and his team. “It is Linde’s core expertise to measure and control the atmosphere. Whether in a printing chamber or elsewhere, we can make sure that the mix of gases is according to specifications.”

Devil in the detail

Such expertise is needed rather urgently. Additive manufacturers long assumed that the atmosphere in the printing chamber consists mainly of the gas used for inertisation, for example Argon, at a share of 99.9%, oxygen (0.1%) and little else. Yet air also contains nitrogen and traces of carbon dioxide, hydrogen and water vapour. Contributing less than one percent by volume, this “little else” together with oxygen can make a significant difference to the material qualities of a 3D printed object. “The interplay between the atmosphere in the printing chamber and the final product is significantly more complex than many in the industry thought,” says Forêt. 

Forêt, Dietrich and Pauzon aim to shed some light on these dependencies. They currently work to provide customers with “a cookbook” that describes the preferred material-specific gas mixture in a printing chamber. This is also the limiting factor in their daily production of the cubes. The 3D printer could easily provide them with more than 40, but the time-consuming task is to analyse the cubes’ qualities in an effort to understand what works and what doesn’t.

In the meantime, Linde’s ADDvance™ O₂ precision, can be plugged into any 3D printer. It allows operators to define and control the atmosphere in their printing chamber by adjusting the level of oxygen. Impurities in the inert chamber due to incomplete purging, machine leakage and unclean raw material can all influence the oxygen level. Such variation can result in inconsistent mechanical properties or chemical compositions of the product, for example a decrease in fatigue resistance.

ADDvance O2 precision box with EOS 3D printer and employee holding/looking at a 3D printed object. Picture taken by Bernhard Rohnke. Downsized version of ID: 110848 (930px x 768px, 72dpi) for presentations.
The ADDvance™ O₂ precision (right) with the EOS 3D printer at work
ADDvance O2 precision box (semi-profile from the left). Picture taken by Bernhard Rohnke. Downsized version of ID: 110827 (930px x 768px, 72dpi) for presentations.
Linde’s ADDvance™ O₂ precision can be plugged into any 3D printer to define and control the atmosphere in its printing chamber

Replicable conditions

The O₂ precision recognises gas concentrations as small as 10 parts per million without cross-sensitivity. It continuously monitors and adjusts the atmosphere. In other words: The gas atmosphere, the share of oxygen in particular, is kept constant throughout multiple additive manufacturing processes, creating identical preconditions for each printing procedure.