Conventional brake discs are made of cast iron containing graphite. They do have some advantages. These include good thermal conductivity and a very good capacity to store heat. They are also reasonably inexpensive. But their disadvantage is that they are prone to severe corrosion and excessive wear and tear of materials during operation. And this is precisely what leads to a substantial amount of fine particle emissions and enormous environmental pollution.
This could all soon change. In a joint project between the Fraunhofer Institute for Laser Technology ILT and the Department of Digital Additive Production DAP at RWTH Aachen University, scientists have developed a special coating process that reduces these factors considerably. Matthias Brucki, M.SC from the Fraunhofer Institute for Laser Technology ILT, explains the details:
“Protecting parts from both wear and corrosion is no simple matter. The usual processes, such as solid chrome plating and thermal spraying, have their disadvantages. Up until now, laser deposition welding has only been able to assert itself in this area in a few isolated instances. Which explains the main driving forces behind the demand for a more comprehensive solution. We have now developed an alternative, patent-protected process which is the Extreme High-speed Laser Deposition welding EHLA process, which eliminates the shortcomings of conventional processes related to coating technology and maintenance and repairs.”
The so-named Extreme High-speed Laser Deposition welding process (EHLA) is the first process to quickly and economically apply wear and corrosion protective coatings on brake discs. Thomas Schopphoven, research associate and team leader of “Productivity and Systems Technology” within the Laser Deposition Welding Group at the Fraunhofer ILT, explains further:
“The EHLA process is particularly suited to the automotive industry – e.g. for coating brake discs, which have previously been difficult to coat due to the high levels of pressure and the high demands placed on cost-effectiveness and environmental friendliness. EHLA makes it possible for the first time to apply coatings with good adhesion to brake discs that are firmly bonded to the base material and, unlike coatings produced by conventional processes, do not peel off.“
Whereas the layers of conventional processes have pores and cracks, the coatingss produced using the EHLA process are very dense. They therefore protect the part much more efficiently and over a longer period of time. This increases service life and prevents untimely failures due to damage to the surface of the friction areas. Brucki emphasizes:
“From an ecological and economic standpoint, the development of coatings on brake discs is aimed at preventing wear and corrosion. With EHLA, high-quality, pore-free and crack-free coatings with stronger metallurgical adhesion and less leakage can be fabricated from a wide range of materials. Functional properties of the brake disc shouldn’t be reduced by the coating.”
As a large range of materials can be used in this process, an application-specific coating using environmentally friendly materials is now possible.
New process design offers advantages
EHLA’s innovation is based on the well-established laser deposition welding process. This is used successfully, for example, as a repair method for turbine blades. But EHLA offers decisive advantages. In this process, the powder particles of the coating material are melted directly by the laser beam. In conventional processes, this only takes place in the molten state on the surface of the part. Since EHLA causes liquid drops of material instead of solid powder particles to enter the molten weld, the process speed is increased from 0.5 to 2 meters per minute by a staggering amount of up to 500 meters per minute.
The high process speed means that the heat effect on the material that is to be coated is reduced significantly. Whereas conventional laser deposition welding affects the material thermally down to the millimeter range, EHLA only affects the material in the micrometer range heat-wise. This enables completely new material combinations and applications. These include the coatings of aluminium or cast iron alloys – as is now the case with brake discs. But that’s not all, as Brucki notes:
“Due to compatible process conditions, EHLA can be used to process a wide range of materials. This offers the user a broad spectrum of coating materials that can be used to prevent wear and corrosion.”
In the conventional deposition welding process, the carbon from the brake disc dissolves in the weld. This results in brittle areas, pores, bonding defects and cracks in the coating or bonding zone. EHLA avoids all of this through the particular process. Brake discs made of grey cast iron can for the first time be reliably protected with firmly bonded coatings.
Resource-efficient and process-reliable with superior quality
Another advantage is offered as well. Only thick layers of half a millimeter or more are possible using the conventional welding process. This requires a lot of material. In addition, post-processing is very time-consuming. The EHLA process now makes it possible to apply very thin layers with thicknesses of 25 to 250 micrometers. Whereby the coating becomes cleaner and smoother. In this way, roughness can be reduced to about one tenth of the previous values. In addition, around 90 percent of the material is used in the new EHLA process. This makes the process extremely cost-effective and resource-friendly. Prerequisites for standard industrial use are thus met.
And this is all about to happen. Successful initial trials have shown that the EHLA process makes it possible to manufacture coated brake discs based on conventional cast iron discs using a variety of material combinations. A system technology suitable for series production with a modified grinding process for finish is currently being set up by HPL Technologies in Aachen.
Moreover, Brucki sees other areas of application as well:
“In addition to the major challenges facing the automotive industry in manufacturing coatings for brake discs subject both to great pressures and demands, other applications for the EHLA process are coatings for parts in the offshore industry which are exposed to the most severe corrosion and wear conditions, such as hydraulic cylinders. As well as corrosion and wear prevention, EHLA can be used to apply coatings with a defined surface topography, thereby giving the component additional functions.”
“The EHLA method also offers promising possibilities beyond just coating, e.g. with the hybrid-additive approach for the production of bulky elements on existing conventionally produced parts. Previous process chains have usually been characterized by a subtractive approach in which up to 90 percent of the original part is frequently mechanically removed. EHLA can help here: The machining of flanges and secure mountings for a pre-cast part on a shaft, for example, can take several hours. The same part produced by the hybrid EHLA process only takes a few minutes and the final finishing by lathe is also completed after just a few minutes.”
EHLA has meanwhile received three renowned innovation awards. Their technology was awarded the Joseph von Fraunhofer Prize in 2017, the Berthold Leibinger Innovation Prize in 2018 and the second prize in the ‘Steel in Research and Development’ category of the Steel Innovation Prize for its eco-friendly laser alternative to chromium(VI) coating.
Incidentally, details of the EHLA method and other current topics concerning research and development can be found at the joint Fraunhofer stand (C12 in Hall 4.1) at the International Motor Show 2019 in Frankfurt from September the 12th to the 22nd. More information on the new process can be found here.