From power plants to production plants, many metal components are under constant exposure to harsh environmental factors such as high humidity, drastic temperature variations, and aggressive chemicals. To help prevent wear and corrosion, these components are often clad in a coating of wear-resistant materials.
Industry has traditionally relied on electroplating techniques such as hard chrome plating to protect these components, but such approaches are facing mounting criticism due to the risks they pose to people’s health and the environment. This has led to heavier restrictions in recent years, for example the EU’s REACH (Registration, Evaluation, Authorisation and Restriction of Chemicals) regulation, which has significantly tightened the criteria for using hard chrome plating. This regulation offers protection against the health and environmental hazards posed by chemicals, and is enforced by the European Chemicals Agency (ECHA).
One alternative to hard chrome plating that has emerged recently is thermal spraying, which involves projecting molten metal particles onto a component’s surface. Thermal spraying is more flexible in terms of the diverse choice of coating materials, but the lack of a metallurgical bond between the coating and the substrate means that it tends to flake off sooner. There is also a risk of cracking and formation of pores.
A clean alternative that offers outstanding quality
Laser deposition welding sweeps away all these disadvantages, making it a popular choice for many modern coating applications. In the past, however, lasers lacked the necessary speed to tackle large surface-area cladding. What’s more, laser deposition welding was previously limited to a minimum layer thickness of around 500 micrometers, ruling out the option of thinner layers.
But now the Fraunhofer Institute for Laser Technology ILT has developed and patented a new process to overcome these limitations. The new method is known as EHLA, a German abbreviation for extreme high-speed laser deposition welding. It enables cladding processes to be executed very rapidly with low layer thicknesses for rotationally symmetric components.
How the two methods of laser deposition welding compare
Conventional laser deposition welding typically proceeds as follows: a laser generates a weld pool on the surface of a component into which metal powder is fed through nozzles arranged coaxially to the laser beam. The powder fuses with the surface to create a crack-free and pore-free coating with a metallurgical bond to the component.
The EHLA method works slightly differently. The powder nozzles are positioned higher up, which means the laser light strikes the powdery material above the weld pool – heating the material nearly to its melting point while it is still on its way to the component. Consequently, the particles melt faster in the weld pool, hugely accelerating the process of layer formation. That means the rotationally symmetric component can rotate correspondingly faster under the optics, which significantly increases the speed of the overall process.
While in normal laser metal deposition laser beam and powder meet on the surface of the workpiece … (Illustration: Fraunhofer ILT)
… in the EHLA-method the laser light strikes the powdery material above the weld pool. (Illustration: Fraunhofer ILT)
Conventional laser deposition welding can coat only 10 to 40 square centimeters per minute, whereas the EHLA method can achieve coating rates of over 250 square centimeters per minute. That, in turn, enables feed rates of between 10 and 500 meters per minute. What’s more, the higher process speed can produce much thinner coating layers – between 10 and 300 micrometers. In addition to all these improvements, the EHLA method still retains the two key advantages of laser deposition welding. Compatible with a remarkably broad range of cladding materials, it perfectly tailors the coating to the substrate material and feed configuration, forming a durable metallurgical bond between the coating layer and the substrate.
EHLA also permits a much finer laser focus because the beam strikes the powder at an earlier stage in the process. The standard focal diameter in conventional laser deposition welding is two to three millimeters, but EHLA can reduce this to slightly less than one millimeter. With disk lasers this drops even further to around 0.4 millimeters. The finer focus makes the process considerably more energy-efficient: EHLA consumes just two to four kilowatts compared to the four to 20 kilowatts required for conventional laser deposition welding.
Outlook for the future
Thanks to the introduction of EHLA, laser deposition welding now offers an exciting alternative approach for applying coating layers to components with large surface areas. EHLA provides the best of both worlds, offering the ability to apply high-quality coatings as thin as 10 micrometers at high processing speeds.
TRUMPF has many years of experience in providing solutions for laser deposition welding, and Fraunhofer ILT’s new processing optics can be integrated directly into existing TRUMPF systems, regardless of whether a diode or disk laser is used as a beam source. Depending on component size, TRUMPF offers various laser processing cells that are suitable for use with EHLA.
In terms of breaking boundaries, the next limitation engineers have in their sights is the feed rate. For the time being, the process speeds that can be attained with the EHLA method can only be achieved using stationary optics and with rotationally symmetric components. Processing large, flat components would require the processing head to travel above the component at high speeds. That’s where TRUMPF laser cutting machines could open up real potential. Specifically designed for this kind of fast linear motion across a surface, they are an ideal choice to meet the challenges of the EHLA process, though efforts to validate this are currently still in the pipeline.
Dr. Antonio Candel-Ruizis an expert on laser surface engineering at TRUMPF Laser- und Systemtechnik GmbH in Ditzingen.