Shavings to Savings

Ship propellers are among the largest individual industrial components – and their production generates vast amounts of metal shavings. Up to two tons of grinding waste are generated per propeller. What was once considered mere waste is now the focus of the circular economy: Researchers are developing processes to use these shavings as a resource for additive manufacturing. The goal: to use raw materials more efficiently, save energy, and make shipbuilding more sustainable.

From propeller to powder

Traditionally, metal shavings are melted down in order to be reused. However, melting is energy-intensive and causes high CO₂ emissions. Avoiding the costs associated with multi-stage processing turns what is actually a valuable material into worthless waste. That is unacceptable, especially in the context of a future circular economy. New approaches are therefore needed to effectively prevent this waste stream. For researchers at Fraunhofer IPK, this is where metal additive manufacturing comes into play.

Powder for additive manufacturing processes is today mainly produced via gas atomization. This is energy-intensive as well, because the raw material must first be melted before being atomized using a gas stream. Mechanical processes that operate without prior melting offer a potential alternative, saving energy and reducing emissions. The idea: The chips, for example from grinding processes, are mechanically processed to produce fine metal powder – the perfect raw material for additive manufacturing. This not only conserves resources but also closes material cycles directly within production.

Preparing the chips, however, is challenging. They vary in shape and size and are often contaminated with oils, greases, foreign metals, or even other materials such as ceramics. The researchers therefore rely on a multi-stage process chain: First, the chips are crushed and sieved, then cleaned using modern methods, and finally processed into a homogeneous powder. This powder can be used directly for additive manufacturing – for example, to produce replacement parts or new components, but also to repair existing components.

From waste to additive manufacturing

The advantage of this method is clear: Compared to traditional melting, mechanical processing saves a significant amount of energy. Studies show that energy consumption can be reduced by up to 60 percent. At the same time, CO₂ emissions drop significantly. Since the powder can be produced directly on-site, long transport routes and additional processing steps are also eliminated. Pilot projects at Fraunhofer IPK demonstrate how the processing works in practice. Experimental setups process chips directly into powder, which is then used for additive manufacturing via Directed Energy Deposition (DED).

Expansion to other processes such as Powder Bed Fusion (PBF) is also conceivable, precisely because the diverse processes place different demands on the powder used. For example, electron beam melting (PBF-EB) and laser beam melting (PBF-LB) require finer powder with a narrower particle size distribution, whereas the DED process can also be used with powder made up of larger particles. New possibilities that also raise questions for hybrid manufacturing processes: What could future, circular process chains look like in which additive manufacturing processes such as DED or PBF are combined with subtractive, conventional methods such as grinding or milling? Will new business models or combinations of powder production methods with additive manufacturing processes emerge that make optimal use of the shavings produced? For example, larger manufacturing companies could start recycling their own metal shavings. Smaller companies could derive greater value from their waste streams, the recycling business could grow, and powder manufacturers could source their raw materials from waste and thus reduce both their costs and overall emissions.

Optimized powder

The quality of the powder produced through this processing is quite comparable to that of conventional materials. The decisive factor, however, is which process is used to create the powder, as these processes have a significant influence on the powder’s subsequent properties: Particle size distribution, morphology (roundness and surface texture), flow properties, and chemical composition or purity/contamination vary depending on the technology that is used.

During the conventionally used gas atomization process, molten raw material is atomized using an inert gas, resulting in high-quality powder with spherical particles, good flow properties, and a narrow size distribution, making it ideal for use in PBF, DED, and other processes. The team at Fraunhofer IPK is researching three less common methods. In ball and impact whirl mills, the collected chips are mechanically processed without melting, while ultrasonic atomization works with molten material but produces high-quality powder that even surpasses the end product of gas atomization.

The scientists also see great potential in further improving the properties of the recycling powder through various post-processing steps, for example, to reduce contamination from the tools used during machining or from cooling lubricants. 

Full steam ahead

The various processes demonstrate how a circular economy can be specifically implemented in industrial applications: In the future, metal shavings will no longer be regarded as waste, but as a valuable resource. This can be applied in the shipbuilding industry, for example to repair worn components, but also far beyond.

A zero-waste cycle, in which waste from subtractive manufacturing is reused within the same facility in additive manufacturing (and the two thus complement each other perfectly), is still a vision for the future. However, in their work, the researchers are also examining other waste streams to make additive manufacturing fit for the circular economy. In a follow-up project, for example, they are investigating how to process overspray, a term used to describe powder that is not melted during the DED process. Recycling waste into powder as a raw material for additive manufacturing thus closes material loops and makes production more sustainable and economical. Scrap metal turns into a material with great potential – and shipbuilding, in particular, is poised for a more circular future.