Additive Manufacturing and Metallic Microstructures (AM3)

“Industry knows the additive manufacturing technology has now reached a stage where it can no longer be ignored.” Roland Logé, EPFL

“Additive manufacturing” (AM) is used to build 3D parts layer by layer out of metallic, ceramic or polymer powders. The techniques involve depositing a coat of powder directly onto a substrate, melting it selectively and allowing it to cool and consolidate—the process is then repeated until the part is complete.

Advantages over traditional methods include the ability to create complex geometries; to mix materials during the layering process to produce structures with varying functions; to make new composite microstructures and maybe even new material classes. It is not straightforward though.

High heating and cooling rates lead to harder, less ductile microstructures with high residual stresses and residual porosity, affecting both function and aesthetics. The processing conditions are so different from those that have shaped standard approaches that researchers will need to develop new design principles. This will allow them to produce parts with specific mechanical and aesthetic properties and which meet high-quality speci¬fications. The team behind CCMX Materials Challenge “Additive Manufacturing and Metallic Microstructures” wants to address these needs.

“From the materials science point of view, additive manufacturing presents a number of challenges in terms of both experiments and modelling due to the fact that this is effectively a very fast quenching process, very strongly out of equilibrium,” said Michele Ceriotti, head of the Laboratory of Computational Science and Modelling at EPFL. “The idea of this Challenge is to start investigating the materials science problems related to the theory and practice of additive manufacturing.”

The team—which includes Roland Logé, head of EPFL’s Laboratory of Thermomechanical Metallurgy, Christian Leinenbach, head of Alloy Design and Processing Technologies at Empa and Helena van Swygenhoven at the Paul Scherrer Institute, as well as a number of industrial partners, will look at the fundamental materials science problems underlying the transition from traditional manufacturing techniques to additive manufacturing, focussing on metallic alloys and the correlation between process, microstructure and properties.

The researchers will tackle the problem from multiple angles, including optimising alloys and part production by additive manufacturing, multi-scale modelling of the out-of-equilibrium processes involved in the technique, in-situ and ex-situ characterisation of the microstructure, and thermo-mechanical treatments that may be used to enhance the properties of the material. They will look at the microscopic processes that control the formation of defects, the development of alloys specifically designed for additive manufacturing processing, the characterisation of the microstructure that arises because of the peculiar aspects of AM as well as the microstructural changes due to the thermal and mechanical fields, and the resulting properties.

The aim, ultimately, is to design alloys optimised for the process.

“Right now there are very few alloys that are processed by additive manufacturing, and in most cases these are alloys that have been tuned and optimised and designed for traditional manufacturing processes,” said Ceriotti.

“The idea would be to develop the insight that would allow us to design alloys speci¬fically for additive manufacturing applications rather than using materials that have been around for quite some time and have been optimised having in mind the traditional way of making objects.”
The work will initially focus on three classes of materials: solution-strengthened, precipitation-strengthened and two-phase alloys. Preliminary discussions with industrial partners suggest that the initial focus may be on types of stainless steel, red gold or other precious metals for watch manufacturers, and on nickel-based superalloys and titanium alloys for partners involved in heavy industry.

The researchers want to first carry out an extensive survey of materials’ state of the art, discuss technological goals of partners, extensively test chosen materials and ¬ finally use this insight to design powders or manufacturing strategies that improve the properties of the final product. The work will also involve five doctoral students and specialised courses.

The interest in the topic is not just academic. This Materials Challenge involves eight industrial partners, covering the whole spectrum of metallurgical applications, from heavy-duty applications to micromechanical parts. The team has included powder supplier Oerlikon Metco to simplify investigation of new compositions, as well as to be able to control characteristics of the powders more precisely.

Companies may have been hesitant to invest in the technology because of several issues. The project will investigate some of the issues related to materials science, Logé said, while others are likely to be solved elsewhere within the next five years.

“By the end of 2020, there should be a good knowledge platform for all partners, from which we could envision future investments,” he said. “Industry knows the additive manufacturing technology has now reached a stage where it can no longer be ignored.”

Text by Carey Sargent (March 2016).