Article published in METALL 4/2018
Intelligent combination of traditional and innovative processes creates real added value
Rosswag GmbH is a medium-sized family business with over 200 employees. Under the name Edelstahl Rosswag the company is known as the largest open-die forge in southern Germany, with over 100 years of experience in dealing with metal materials. In an integrated process chain, 400 different alloys are processed into heavy-duty forgings and ring-rolled products, including those for power engineering, the aerospace industry, power plant technology, pump construction and the optoelectronic industry. The portfolio has been supplemented for six years by the Rosswag Engineering division with a focus on the design and manufacture of complete components. Since many complex geometries can only be achieved using additive processes and often have to be built up on forged blanks, the technology of selective laser melting (SLM), also known as metal 3D printing, was soon introduced.
In-house metal powder production for optimized hybrid production
However, the Rosswag engineers soon discovered that powder metal with the exact same alloy as the forged part should ideally be used for the additive form build-up. Gregor Graf, Engineering Manager, likes to compare this with an example from the medical field: “When one’s own skin is transplanted, it is always better for a body than a stranger’s skin.”
It was obvious that an identical composition of the metal powder could only be guaranteed if the powder was produced in-house from the same batch. Also, the aspiration to uphold resource efficiency in the forging process, tipped the balance in favor of in-house powder production. After all, Rosswag is responsible for many tons of forging residue a year, which in the past produced relatively low revenue as metal scrap.
However, in order to be able to be used meaningfully for their own SLM production, a powder production plant had to meet very specific requirements. An essential condition was its suitability for small batches, so that powder can be manufactured specifically for individual components. Previously available plants were far too large for that, so small-scale production was not economically feasible.
At the end of 2015, contact was made with Indutherm Erwärmungsanlagen GmbH and its subsidiary BluePower Casting Systems GmbH in Walzbachtal, only 15 km away. The group had just developed a powder atomization plant for small amounts of metal powder in a joint research project with the University of Bremen. The existing AU 1000 plant was convincing with its powder quality and output rate, but with a melting temperature of around 1500°C was only suitable for a limited number of alloys. However, a high-temperature version suitable for Rosswag requirements was currently under development – as part of the ZIM (Central Innovation Program Mittelstand) research project “Calidus” with the University of Bremen Institute of Materials Engineering IWT. After promising test atomizations on the experimental setup at the University of Bremen, Rosswag and Indutherm/BluePower concluded a cooperation agreement – a happy decision for both sides, as it turned out: Rosswag’s know-how regarding steel alloys and requirements for the SLM suitability of the recovered powders could be incorporated into the development.
In the production of metal powder for SLM application, a high output rate of very fine powder is required. For steel alloys, this means high, precisely defined and maintained melting temperatures and the application of hot gas atomization. The metal emerging from the nozzle is thereby reduced by hot gas at high pressure into very fine particles. The challenge with atomization is adjusting the parameters to produce the desired particle size and particle shape. The important parameters here are the temperature and pressure of the atomizing gas, the temperature and mass flow of the melt during casting as well as the material properties (viscosity and surface tension) of the melt.
The joint development work was successful for all concerned: at the end of 2017, the new AU 3000 plant was completed, set up at Rosswag and commissioned. A practice-averaged output rate of over 60% SLM grade material is regarded as very high today.
New Value Creation Instead of Scrap Recycling
The success story continues: Rosswag Engineering is pleased not only with the expansion of its service portfolio and innovations in component production, but also with numerous awards (German Raw Materials Efficiency Award 2016, “Excellent Location in the Land of Ideas 2017”, “100 Companies for Resource Efficiency, Baden-Württemberg 2017”) as well as invitations to congresses and participation in national and international research projects.
The amount of scrap that goes into classic recycling has been significantly reduced; instead, stainless steel scrap now contributes to new added value. As far more forging residue can be processed into qualified powder metal than is needed in our own SLM production, Rosswag Engineering recently entered into partnerships with the recycling company Cronimet, Karlsruhe and with SLM Solutions. The latter exclusively distributes the various powder steel alloys produced by Rosswag.
Blue Power has full order books for its powder atomization facilities and is also involved in numerous new research projects. The Atomiser family of plants is already in the next expansion stage: explosion-proof plants, for example to produce powder from aluminum-based alloys. In addition, Blue Power is currently introducing the latest version of the Air Classifier AC 1000 G: the air classifier enables the separation of metal powders according to size classes, especially in ranges <50 μm, where classic sieving methods reach their limits. The system can now also be operated with virtually no air in the room under protective gas in order to minimize contamination with air humidity, oxygen or impurities.
left: filling in the forging residues – right: the forged/SLM hybrid component is built up in the SLM plant additively on the heavy duty, grain alignment optimized forged raw part.
Recycling is only the second-best solution because the production waste of high-quality single-grade alloys to sell for comparatively low revenue and then transport often thousands of kilometers by expensive and energy-intensive road, rail and ship freight to prepare for treatment and then re-import at a lower quality, seems questionable given the need for high quality alloys for new technologies. The Rosswag/BluePower Project is therefore an excellent example of raw material efficiency: Upcycling instead of downcycling – worthy of consideration for every metalworker.