Laser coating process better protects car brake discs

04 Sep 2019

Fraunhofer ILT-developed EHLA technique is reliable, efficient, and greener. To be presented at Frankfurt Motor Show.

More than many other car components, brake discs are subject to repeated mechanical loads. As a result of this continual abrasion, they produce fine particulate matter, which poses a substantial environmental threat. Now, however, a new coating process developed by the Fraunhofer Institute for Laser Technology (ILT) and RWTH Aachen University can significantly reduce this impact.

By using Extreme High-speed Laser Material Deposition, known by its German acronym EHLA, brake discs with an effective protection against wear and corrosion can be constructed in a procedure that is both fast and economical.

To date, say the partners, it has proved difficult to provide adequate protection for brake discs by means of conventional coating processes such as electroplating or thermal spraying. The problem with such processes is that they do not produce a metallurgical bond between the cast iron disc and the protective coating; moreover, they are expensive and use a lot of materials.

“The EHLA process is ideal for use in the automotive industry, especially for coating brake discs,” said Thomas Schopphoven, research fellow and team leader of Productivity and System Technology within the Laser Material Deposition group at the ILT.

“Conventionally, it’s very difficult to coat brake discs, because they have to withstand high loads, and there are cost and environmental considerations. But with EHLA, it’s now possible to apply coatings that from a metallurgical bond with the base material of the disc and therefore adhere very strongly. Unlike conventional coatings, these do not flake and chip.”

Advance on conventional processes

Coatings produced with conventional processes have pores and cracks. But with the EHLA process, the coating remains intact and therefore provides longer and more effective protection for the component. This process is suitable for a wide range of materials so it is possible to select an environmentally-friendly coating for each application.

The EHLA process is a new process variant on the established laser material deposition techniques, which have proved successful in areas such as the repair of turbine blades. EHLA does, however, have a number of decisive advantages.

With the EHLA process, the powder particles of the coating material are melted directly in the laser beam, rather than in a melt pool on the surface of the component. Since the melt pool now is fed by liquid drops of material rather than solid particles of powder, the coating process is much faster, rising from the 0.5–2 m/min with conventional laser material deposition to as much as 500 m/min.

This also substantially reduces the exposure to heat of the material being coated. Unlike conventional laser material deposition, where the heat affected zone can have a depth of one or more millimeters, thermal exposure with the EHLA process remains in the micrometer range. This enables the use of entirely new material combinations such as coatings for aluminum or – as with the brake discs – cast-iron alloys.

Firmly-bonded

The low heat input prevents the carbon to dissolve from the brake disc into the melt, otherwise resulting in brittle phases, pores, joining defects and cracks in the coating and bonding zone. In other words, it is now possible for the first time ever to provide brake discs made of gray cast iron with an effective coating that is firmly bonded with the base material.

With laser material deposition, the coating produced is usually at least half a millimeter in thickness. This consumes a lot of material and also makes finishing substantially more complicated. By contrast, the EHLA process produces very thin layers of between 25 and 250 micrometers. As a result, the coating is both purer and smoother, with roughness reduced to around one-tenth of its previous value.

Moreover, the EHLA process uses as much as 90 percent of the fed powder material. It is therefore extremely resource-efficient and more economic. The basic requirements for the use in an industrial, mass-production setting are at reach.

Industrial application could soon be a reality. Initial investigations have demonstrated that the EHLA process is capable of reliably producing coatings – with different material combinations – for conventional brake discs made of gray cast iron. A system that is ready for use in mass production, including a modified grinding process for finishing the components, is currently under construction at the Aachen-based company HPL Technologies.

Details on the EHLA process and other current trends from research and development will be presented, at the joint Fraunhofer booth (C12, in Hall 4.1) at this year’s IAA International Motor Show in Frankfurt, Germany.