Matthias Möllenhoff, product manager for EHLA at Hornet Laser Cladding, discusses the benefits of the EHLA high-speed cladding technology, and how it is gaining traction in China
In recent years, many new manufacturing possibilities have emerged within laser technology, for example selective laser melting (SLM) – also known as laser powder bed fusion – in additive manufacturing. Less development has taken place in the area of laser cladding. However, the current development of extreme high-speed laser cladding – the German acronym being EHLA – by the Fraunhofer Institute for Laser Technology ILT in Aachen, represents a major step forward in this field.
Why is there a need for EHLA?
Common production techniques for applying protection coatings against wear and corrosion are thermal spraying, hard chrome plating or deposition welding. Hard chrome plating and thermal spraying, however, suffer from drawbacks such as high energy demand, or needing authorisation to use them because of their reliance on toxic chromium (VI). Also, there are often problems with the coating itself, such as high porosity, resulting in low corrosion resistance. While deposition welding does not face any of these disadvantages, due to its low deposition rates and high heat input on the work piece, it is only used for specific coating applications.
EHLA addresses these exact drawbacks. Thanks to the technique’s very high welding speed and low heat input, it offers deposition rates similar to thermal spraying, as well as low thermal effects on base metals, as is seen in hard chrome plating. Other advantages of EHLA are that it produces very strong bonding and dense metal coatings. Besides that, EHLA is also very resource efficient and free from toxic substances.
How does EHLA achieve these advantages?
To answer this question, one has to consider the conventional laser cladding process. Here, a metal powder is injected into a laser-induced melt pool on the substrate surface. The EHLA process differs as the metal powder is preheated by injecting it into the laser beam. The time taken for a powder particle to travel through the laser beam is long enough to melt it before reaching the substrate surface. Therefore, the process currently only works with powder material.
A large amount of energy of the laser beam dissipates by heating up the particles instead of heating the substrate. As a result the melt pool becomes thinner and the powder catchment efficiency – a value representing how much powder forms the coating – is more than 90 per cent. The process is capable of welding speeds of up to 500m/min, resulting in very thin layers of about 20-500µm. In addition, much less energy is put into the substrate, resulting in a very small heat-affected zone and low dilution.
EHLA uses a specific EHLA powder nozzle from Fraunhofer ILT, but apart from this the new process requires mostly the same parts as conventional laser cladding. Hornet Laser Cladding was therefore able to become the first system integrator for EHLA thanks to its history of building conventional laser cladding systems.
The beam quality of today’s diode lasers can achieve the requirements of some EHLA applications, so the systems are getting more interesting in terms of cost. On the system and process side there is a large amount of experience with regards to EHLA. But there is still much work to do.
On the application side there is demand for EHLA for cladding hydraulic components. It is therefore hardly surprising that the first EHLA system was set up for the offshore hydraulics industry two and a half years ago. IHC Vremac Cylinders was looking for a new coating solution with high corrosion resistance for offshore use, and thus became the first customer to use an EHLA system from Hornet. Since then we have seen rising demand for EHLA systems, in China, Australia and South Korea, driven by growing industry and environmental issues with current cladding technologies.
In cooperation with ACunity, a Fraunhofer ILT spin-off focusing on providing customised solutions for laser material processing for the Chinese market – including R&D consulting, system engineering, technical service, customer support and process development – Hornet delivered the first EHLA system in China at the Advanced Manufacture Technology Centre of the China Academy of Machinery Science and Technology (CAMTC) in Beijing. This was a starting point for a closer cooperation with ACunity that aimed to increase the penetration of EHLA technology in the Chinese market, and with this the two companies founded the EHLA consortium. The firms then began developing industrial EHLA machines integrated with advanced powder delivery technology from Fraunhofer ILT.
The EHLA process is capable of welding at speeds of up to 500m/min. Image: Fraunhofer ILT
The first big success of the EHLA consortium was the implementation of three more EHLA systems for Chinese customers for various types of application – for example one with an inside cladding head. However, there is more opportunity than simply replacing hard chrome plating processes with EHLA, as interest grows and information is spread thanks to innovation awards like the Fraunhofer prize in 2017, the Steel Innovation Award and, most recently, the Berthold Leibinger Innovationspreis award.
The range of applications that are already in industry or are being tested at Fraunhofer ILT varies from the replacement and enhancement of coating systems, to the repair and additive manufacturing of rotational symmetric structures. In the automotive and aviation industry, for example, there are possibilities of additively manufacturing lightweight rotational parts, like brake disk coatings and landing gear components.
One big step in the future will be the development of an EHLA system for freeform coatings, and with this, applications in 3D additive manufacturing. However, a plane surface EHLA system will be possible on a shorter timescale and will also offer a much wider range of applications.
To sum up, the EHLA process has been experiencing increasing demand worldwide since its introduction into industry more than two years ago. At the same time, future machining systems and further process development will open up new and interesting applications.
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