Sharpest anti-corrosion, anti-wear repair technology

Sharpest anti-corrosion, anti-wear and repair technology

7:45 AM, 21st August 2017
Thomas Schopphoven, Graduate Engineer, Fraunhofer Institute for Laser Technology ILT, Aachen, Germany.
Thomas Schopphoven, Graduate Engineer, Fraunhofer Institute for Laser Technology ILT, Aachen, Germany.

In an interview, Thomas Schopphoven, Graduate Engineer, Fraunhofer Institute for Laser Technology ILT, Aachen, Germany with Chemical Today Magazine talks about the cutting-edge research on ultra-high-speed laser material deposition technology. The new process comes out on top not only in cost, quality and sustainability but also in retaining jobs in Europe.

Ultra-high-speed laser material deposition research.

Special coatings protect components against corrosion and wear. However, standard processes such as hard chrome plating, thermal spraying, laser material deposition or other deposition welding techniques have drawbacks. We have now developed the extreme high-speed laser material deposition process, known by its German acronym EHLA, to eliminate these drawbacks in an economical way. High-quality metal layers measured in tenths of a millimetre can be applied to large surfaces flexibly, efficiently and quickly.

Advantages and disadvantages of hard chrome plating, thermal spraying and laser material deposition methods.

The most common process for corrosion and wear protection is hard chrome plating: Chromium is taken from a chromic acid solution and deposited on a workpiece in an electrochemical bath at process temperatures of 50 to 65 degrees Celsius. However, this process is not optimal: Hard chrome layers are not metallurgically bonded to the base material and they delaminate easily. In addition, the layers show microcracks in the microstructure that reduce their resistance to corrosion and wear. Another disadvantage is that electrochemical processes consume a lot of energy and become less economical as electricity costs rise. Yet the biggest drawback concerns environmental protection. Due to its environmental impact, chromium (VI) has been included in the EU directive EC 1907/2006. From September 2017, an authorization or a special permit will be required to use it.

Thermal spraying likewise has disadvantages. With high-velocity oxygen fuel (HVOF) spraying, a liquid or gaseous fuel is fed into a combustion chamber, where it is ignited and burnt by adding oxygen. The powdery spray material is guided into the combustion chamber; there, the individual particles are heated, partly melted, accelerated to speeds of 600 to 1000 meters per second, and then sprayed onto the workpiece to be coated. Upon impact, the powder particles are plastically deformed and then bond to the substrate through mechanical clamping. The resulting bond is comparatively weak, which is why the substrate must be pretreated with blasting techniques.

In addition, the resulting layers have a porosity of one to two volume percent. This makes it necessary to apply several layers, each some 25 to 50 micrometres thick, atop one another in order to adequately protect the component. HVOF spraying is far from resource-efficient: Several hundred litres of gas are needed per minute and only about half of the material used ultimately coats the surface of the component.

Deposition welding processes are used to produce high-quality and firmly bonded coatings. With conventional processes such as tungsten inert gas (TIG) welding or plasma powder deposition, however, the layers of 2 to 3 millimetres are often far too thick and too much material is used as a result. As much of the coating material mixes with the base material, multiple layers often must be applied.

Laser material deposition already allows for far thinner layers – between 0.5 and 1 millimetre. Since laser material deposition requires considerably lower heat input compared to conventional processes, even a single layer can provide protection. One such example is wear protection for agricultural blades. Laser material deposition is, however, too slow for large components. Because the surface rate is only ten to fifty square centimetres per minute, it is used only for specific corrosion and wear protection applications.

In the past, increasingly powerful laser beam sources and optical systems, as well as wide-beam powder feed nozzles, were developed to achieve larger surface rates during laser material deposition. Although these efforts resulted in improved surface rates and especially deposition rates, their use for industrial coatings was insignificant. Key drawbacks are the relatively high-energy input, insufficient dimensional accuracy and the subsequent need for time-intensive reworking. In addition, lasers are expensive compared to competing for energy sources, such as arcs or plasma. That is why conventional material deposition processes are usually more cost-effective for thick layers.

In short, a flexible, resource-efficient and economical process for applying high-quality, metallurgically bonded coatings between 25 and 250 micrometres in thickness does not exist. The EHLA process fills this gap. It offers significant advantages over hard chrome plating, thermal spraying, deposition welding processes and conventional laser material deposition and is economical.

Steps followed in developing EHLA.

The following systematically derived steps were used to develop EHLA: A system for the powder-gas jet was developed and patented that accurately measures the number of particles, the position and diameter of the powder focus, as well as the speed of the powder particles layer by layer. The data obtained then serve as a basis for an in-house particle propagation model, which describes three-dimensional particle distribution: the position, direction and average velocity of the particles. We used the measured data to model the interaction of powder particles and laser radiation – which had never been done before. Based on the results, powder feeding nozzles were optimized so that they produce a small powder focus – making it possible to adjust particle velocity and trajectory and thus the interaction time.

EHLA process to be economical, sustainable and environmentally-friendly.

The chemical-free application makes the process very environmentally friendly. The resulting layers are non-porous, making pores and cracks a thing of the past. As a result, the layers protect the component much longer and more efficiently. In addition, the coating is firmly bonded to the base material and will not delaminate. Various materials – such as iron, nickel or cobalt base alloys as well as metal matrix composites (MMCs) with ceramics and carbides– can be used for these new coatings.

The EHLA process effectively utilizes approximately ninety percent of the material, making it far more resource-effective and economical. Since the layer is dense, one layer already offers adequate protection. It also solves the problem of layer thickness. Until recently, metallurgically bonded layers could not be thinner than 500 micro meters at elevated deposition rates. EHLA allows for layers that measure just 25 to 250 micrometres. In addition, the layers are smoother; roughness is now a mere tenth of that typical of laser material deposition.

A significant advantage lies in the heat input. Unlike laser material deposition where the powdery additive is melted in the melt pool, EHLA uses the laser beam to melt the powder particles while they are still above the melt pool. Since drops of liquid material fall into the melt pool instead of solid powder particles, less material must be melted – a few micrometres suffice instead of hundreds of micrometres. Using EHLA shrinks the heat-affected zone by a factor of one hundred: from between 500 and 1000 micrometres in conventional laser material deposition down to just 5 to 10 micrometres. Also, the EHLA powder nozzle is new, which can operate ten times longer than in conventional laser material deposition.

EHLA to have major potential in additive manufacturing.

Traditional manufacturing techniques are often characterized by a subtractive approach. Forged or cast blanks must be extensively reworked: As much as ninety percent of the original workpiece is machined and goes unused. This increases resource consumption as well as material and manufacturing costs. With EHLA volumes can be manufactured using a hybrid-additive approach – which is the additive manufacturing of volume elements on existing components – layer by layer. The result is a near net-shape component with almost 100% density and a property profile which meets the specifications of wrought or cast material or even exceeds them. The size of the components is only limited by the used handling system. The major potential lies in the combination of high deposition rates and high dimensional accuracy of the applied layers.

EHLA research to benefit various industries. 

EHLA’s application potential is enormous, not just in coating technology but also in additive manufacturing. In 2015, the worldwide market for hard chrome plating was estimated at $13.64 billion, while the market for thermal spraying amounted to $7.56 billion. If EHLA could capture a ten-percent share of the surface refining market – and that is a conservative estimate – this new process could account for an annual market volume of €2 billion.

Another advantage of EHLA is that it could be used for the large-scale coating of components nowadays used without coatings. This would make it possible to produce innovative components that do not wear out during a product’s lifecycle.

Moreover, EHLA could ensure that coating jobs remain in Europe – countering the trend of such jobs being outsourced to low-wage countries. The process also bears considerable potential for additive manufacturing methods, which have grown some thirty percent on average since 2011. EHLA can contribute significantly to this growth, particularly in the manufacturing of large components.

Future research plans. 

At the moment the advantages of EHLA can only be used for rotationally symmetric parts, where the high necessary feed rates of some hundred meters per minute are achieved by rotation of the parts. In a next step, we want to use all the benefits of the technology for the processing of 2D and freeform parts in the field of coating, repair as well as additive manufacturing. Therefore we are currently working on fast dynamics, high precision machines and components. Besides that, we are continuing to qualify EHLA for various new applications with different material combinations and specifications to fully utilize the enormous potential.

© Chemical Today Magazine


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