Metal foam and PFAS replacement improve environmental balance

Due to increasing regulatory requirements, production technology providers worldwide are facing the challenge of integrating high-performance and environmentally friendly materials. The solutions that already exist will be on show at EMO Hannover 2025, the world's leading trade fair for production technology, from September 22 to 26. The focus will be on metal foams and substitutes for perfluorinated and polyfluorinated alkyl substances (PFAS) in particular.

Metal foams help to make machines more efficient, lighter and at the same time more stable. The highly porous material has a cellular structure - similar to its natural counterparts bone or wood - that can absorb energy in the form of vibrations, impact or sound.

Like baking bread

Aluminum foam can be produced in a process that is similar in principle to baking bread. Take powder, propellant and heat, and the aluminum foam is ready. In detail, however, the production of the high-tech material is somewhat more complex. "To produce aluminum foams, an aluminum alloy powder and a blowing agent powder are mixed together, usually pre-compacted by axial pressing and then compacted into foamable strands by extrusion," explains Carsten Lies, head of the Functionally Integrated Lightweight Construction department at the Fraunhofer Institute for Machine Tools and Forming Technology (IWU) in Chemnitz. "To produce aluminum foam sandwiches, cut, foamable aluminum strands are placed between two cover sheets positioned at a distance from each other," the engineer continues, describing the production process.

In the subsequent heat treatment, the foamable aluminum expands many times over. The resulting foam bonds firmly with the two cover sheets to form a sandwich. After cooling, the sandwich is cut to the final dimensions. "Metal foams, especially aluminum foams, are mainly used as the core material in sandwiches," explains Lies. Their cover sheets are usually made of steel or aluminum. "The cover layers absorb the applied loads, while the core keeps the sheets at a constant distance," says the Fraunhofer researcher, explaining the special properties of the high-tech material. The bond between the cover layers and the core usually takes place in a metallic material bond.

Airy, light and rigid: sandwich with foam filling

"Depending on their design, sandwiches have a very high bending stiffness. This effect is used to make assemblies lighter while maintaining or even improving the rigidity of the assembly," says Lies. They replace solid elements of the conventional assembly. According to the researcher, depending on the optimization criteria, either significant weight savings can be achieved while maintaining the same rigidity (up to around 30 percent) or significant increases in rigidity can be achieved while maintaining the same weight. According to Lies, the specific advantages of using metal foam in the machine in terms of efficiency and sustainability are "significantly improved damping thanks to the foam core and significant weight savings through the use of sandwiches".

Another positive aspect for the environmental balance is that metal foams can be easily recycled. "As no adhesive is used for sandwich production, the material can be fed into existing cycles for processing scrap metal from steel and aluminum," says the researcher from Chemnitz.

A perfect fit from the 3D printer

Components made from metal foam - or, more precisely, components made from hybrid porous (HyPo) materials - can also be produced using 3D printing. The advantage of additively manufactured metal foam is that the air chambers can be arranged accurately. Components produced in this way can be optimized for specific applications, as the graded adjustment of the pore structure inside the component allows for more options than air bubbles in the metal, as they form when foaming with gas. This means that machine components can be produced in the 3D printer with a precise fit and precisely defined properties.

"A graded adjustment of pore structure and property profiles is difficult or even impossible in a monolithically produced material, as either the manufacturing process or the further processing up to the final component geometry do not match the final requirement conditions of the stress," explains Thomas Hassel from the Institute of Materials Science at Leibniz University Hannover (LUH). The doctor of engineering emphasizes that additive manufacturing makes it possible to manufacture components "close to the final contour" and at the same time to introduce the grading "in such a way that it is positioned precisely in the requirement profile".

The research focuses on specific applications in machine tool construction and how the innovative material can help to increase efficiency and sustainability in the factory. The focus here is on machine tool components (tool changers, tool holders, spindle slides) in terms of their rigidity, damping, thermoelastic behavior, unbalance, hardness and surface quality, explains Hassel. By implementing the HyPo components in a milling machine, for example, the advantages of the graded components are being researched. "The aim is to analyze the operating behavior during machining, as milling covers a wide range of different load cases," says Hassel. "This will make it possible to determine the influence of the HyPo component on the mechanical and thermal machine properties and significantly improve the performance of such machines."

Replacement for eternity chemicals

Greater sustainability through lightweight materials is one of many approaches to improving the environmental balance in industrial production. Meanwhile, environmentally friendly alternatives for so-called perpetual chemicals are increasingly coming into focus. Among other things, the focus is on environmentally harmful perfluorinated and polyfluorinated alkyl substances (PFAS), which are used in production, particularly where extreme conditions prevail: high temperatures, heavy abrasion or aggressive chemical conditions. PFAS are found in seals, pipes and fittings, among other things.

Whether a substitution of PFAS is possible depends on the individual application "and cannot be answered in general terms", says Frank Schönberger, Head of Synthesis and Formulation at the Fraunhofer Institute for Structural Durability and System Reliability LBF in Darmstadt. "A 1:1 replacement of fluoropolymers is generally not possible, but always depends on the individual requirements of the respective application."

There are cases in which a fluoropolymer can be replaced by another high-performance polymer (such as PEEK, PEI or PPS) depending on the requirements, for example when temperature and media requirements are moderate or in the field of tribological compounds. "But there are also areas of application in which the complex requirements - as things stand today - cannot be met by any other material," the researcher concludes. "Fluoropolymers have largely universal chemical resistance and high temperature resistance. In applications where this is required, such as in pumps or systems that have to withstand different media under different conditions, fluoropolymers are currently irreplaceable," Schönberger sums up, adding: "Opportunities may arise in applications where the full potential of fluoropolymers is not required and in situations where redesign is possible, for example."

PFAS substitution also relevant for the USA

According to Schönberger, the PFAS substitute is also relevant for markets outside Europe, particularly in the USA. In addition, there are regulations in the United States that depend in part on the respective state. This also shows that Greater sustainability in production technology is a global challenge to which factories in all industrialized nations must respond.

Daniel Schauber, specialized journalist