Success Story: CORNET project 2SlaC
Two-step laser coating to protect 3D surfaces creating a thin layer and a smooth surface
Laser cladding with blown powder or wire is a well-known process to protect parts using wear and corrosion resistant layers. However, this process is limited when it comes to achieving thin layers as well as smooth surfaces.
The German-Wallonian CORNET project 2SlaC followed a two-step approach where a thin layer of slurry containing a metal powder is sprayed on the surface, dries in the air and is subsequently remelted and polished by a laser beam. Therefore, the main research objectives were firstly, the development of an aqueous based slurry in which the powder does not segregate and which can be easily sprayed on by an air spray gun, and secondly, the development of a laser remelting process which provides excellent bonding to the substrate and creates a smooth surface. The targeted thickness of the final layer was estimated to be in the range of 20 to 300 micrometres.
Spraying combined with laser processing leads to dense, bonded and smooth layers
The Wallonian project partner CRIBC (Centre de Recherches de L`Industrie Belge de la Ceramique) developed a slurry which consists of the previously mentioned metallic powder (70 %), water (27 %) and small additions of chemical additives including binders and antioxidants (3 %). After spraying on the layer, it dried in the air. The remelting was done via a laser scanner. Materials being investigated during the research process were stainless steel, a low hardness Ni-based alloy and a high hardness Co-based alloy. It turned out that these alloys did not require any shielding from atmosphere during the laser remelting since the melt pool is very small (less than 100 micrometres) and solidifies rapidly. All alloys showed few pores and were metallurgically bonded to the substrate. With adapted parameters, it was possible to minimise the dilution to less than 1 %. The surface roughness appeared to be less than 0.5 micrometres (the average difference between peak and valley on a surface). This result was achieved by combining the remelting process with a subsequent laser polishing process using a larger laser beam spot. The surface tension of the melt pool reduced the level of roughness. While remelting is a quick process, polishing requires more time and several runs to smoothen the surface.
To achieve a smooth surface, it takes a rather long time to coat large areas (accomplishing less than 3 cm2/min) because two laser processes are required. The project has shown that the coating speed can only be increased at the cost of an increase in surface roughness. Higher coating speeds can be achieved only with a higher surface roughness in the range of 3 to 4 micrometres.
High business potential in the tooling industry
The main market sector this project is targeted at is the tooling industry (molds, dies and special tools). Examples are extruder screws where a surface roughness of a few micrometres can be tolerated. Another field of application outside this sector are micro coolers for diode lasers where thin layers of a conductive material have to be applied.
Project Duration: 01.07.2015 – 30.06.2017
Dr. Andreas Weisheit, Fraunhofer Institute for Laser Technology (Fraunhofer ILT), email: email@example.com
Dipl. Ing. Dora Maischner, Fraunhofer Institute for Laser Technology (Fraunhofer ILT), email: firstname.lastname@example.org
Participants from Germany
DVS-FV – Forschungsvereinigung Schweisen und verwandte Verfahren e.V. des DVS (Participating Association)
Fraunhofer ILT – Fraunhofer Institute for Laser Technology (Research Performer)
Participants from Belgium/Wallonia
CRIBC – Centre de Recherches de L`Industrie Belge de la Ceramique (Coordinating Association and Research Performer)
Pictures: © Fraunhofer ILT