‘Green’ laser alternative to prohibited hard chrome plating

15 Aug 2017

High-speed laser material deposition technique is first certified for use by Dutch hydraulics manufacturer.

The strict conditions on the use of hexavalent chromium for corrosion and wear protection coatings, which come into effect in the EU on September 21st, 2017, will hit manufacturers of highly-stressed metal components particularly hard. One such company is IHC Vremac Cylinders, based in the Dutch city of Apeldoorn.

The hydraulic cylinders it manufactures, which often measure many meters in length, have to withstand harsh maritime conditions for years. With its choice of an award-winning alternative to hard chrome plating, this Dutch manufacturer has become the first company in the world to coat its components using the extreme high-speed Laser Material Deposition (“EHLA”) technique developed by the Fraunhofer Institute for Laser Technology in Aachen, Germany.

Because of the forthcoming restriction on coatings produced with hexavalent chromium, IHC Vremac Cylinders was looking for a rapid and economical alternative to hard chrome plating. The Dutch company ultimately opted for the EHLA technique, which was awarded the 2017 Joseph von Fraunhofer Prize in May.

Coating at 500 m/min

In the patented EHLA method, a laser beam melts the powder particles already above the melt pool. Because the particles no longer have to be heated and melted in the melt pool, process speeds can be accelerated from previous levels of between 0.5 and 2 meters per minute to as much as 500 meters per minute. EHLA can also reduce layer thickness: whereas the minimum thickness of layers used to be 500 micrometers, so layers as thin as 25-250µm can now be achieved cost-effectively. Moreover, the layers are smoother, with roughness reduced to a tenth of typical values for Laser Material Deposition.

The concept and the first industrial-standard system in Aachen impressed IHC Vremac Cylinders, so it contracted Hornet Laser Cladding, a manufacturer of Laser Material Deposition systems, based in Lexmond, Netherlands, to build an EHLA system.

“The advantage of using EHLA to coat rotationally symmetric parts is that the necessary components can be integrated into a lathe,” explains Thomas Schopphoven, head of the Productivity and System Technology team in the Laser Material Deposition group at Fraunhofer ILT, Aachen. It took them less than six months to procure and convert a lathe for the roughly 14-meter-long EHLA system, and thereafter install and commission it.

Faster than thermal spraying

“For a hydraulic cylinders, we currently apply a protective layer of Inconel 625, some 400 micrometers thick, which then requires some grinding rework. The final layer thickness is around 200 micrometers,” said Andres Veldman, Engineering Manager at IHC Vremac Cylinders. “EHLA is significantly faster than high-velocity oxygen fuel spraying and entails much less post-processing.”

The Dutch company is already using EHLA in production manufacturing. After completing several projects, Veldman is certain that EHLA currently costs roughly the same as thermal spraying. In his view, EHLA will become more economical once the rework processes have been optimized.

Veldman added, “The experts were very skeptical at the beginning, because we tested layers with a thickness of only 200 micrometers in the laboratory – before grinding them down further to between 150 and 100 micrometers. We did this to ensure that the coatings will offer good corrosion protection even after being subjected to wear.”

Hexavalent chromium

Chromium in its hexavalent form (Cr⁶⁺) is a hazardous chemical regulated under the Clean Air Act (US) and designated by EPA as 1 of 17 “high priority” toxic chemicals. It is a known human carcinogen and emits a toxic mist at elevated temperatures.

Chrome plating is used in a variety of heavy industrial applications to increase resistance to wear and corrosion on products such as cars and cutting tools, but it produces hazardous air emissions. New installations of Cr⁶⁺ platers are already banned in some US states (including California), and existing platers have strict monitoring and control requirements and must report to the EPA.