Extreme high-speed laser deposition welding coats components at over 250cm2 per minute

The extreme high-speed laser deposition welding​ (EHLA) method, which is significantly faster than conventional laser deposition welding, was developed and patented by the Fraunhofer Institute for Laser Technology (ILT) with the primary aim of executing coating processes very quickly with low layer thicknesses for rotationally symmetric components. Now, laser system manufacturer, TRUMPF is putting it into series production.

Antonio Candel-Ruiz, an expert for laser surface methods at Trumpf, explains: “For EHLA, we can draw on similar techniques that we’ve been using for laser deposition welding.”

A tried-and-true method in metal coating for many years, laser deposition welding delivers high-quality results. This method makes it possible to manufacture crack-free and virtually pore-free coatings with metallurgical bonding to the substrate out of a variety of materials.

“For large-area coating tasks, however, lasers have lacked the necessary speed,” says Candel-Ruiz. In addition, the minimum layer thickness was around 500 micrometres (μm); thinner layers simply were not possible.

How EHLA works

Marco Goebel, Trumpf’s industry manager for surface technologies, LMD, explains how laser deposition welding normally works: “We use a laser as a heat source to generate a melt pool, and we inject powder particles which are molten before the powder particles touch the melt pool on the substrate. By spinning out parts which are predominantly potentially asymmetric very rapidly we are able to deposit very thin, metrologically bonded coatings on the substrate with various kinds of different metals.”

Whereas normal laser deposition welding can coat 10 to 40cm2 per minute, the EHLA method can achieve rates of over 250cm2 per minute. In addition, thinner coatings with layer thicknesses of 10 to 300μm are possible. What’s more, EHLA permits a much finer laser focus as well as using higher heat to melt the powder, rendering the process considerably more energy efficient.

One application where TRUMPF is using the EHLA process is on brake discs to make them more durable and stop them shedding fine particles of brake dust that contributes to air pollution. If a hard, corrosion resistant layer is added there is less abrasion and therefore less fine dust. It also means that braking performance may last longer.

“The fine dust produced in our cities is 30% brake dust,” says Goebel. “It might come up in the future, and for sure it will come up in the European Union as a topic, that we have to reduce the fine dust. As always in politics you cannot really foresee when it will happen, but it will definitely happen.

“We strongly believe, and we have indication, that there might be a change in the next couple of years and that will mean that we are quite prepared. The European Union always takes a while, but when the law comes through usually you have to fulfil it immediately and that’s why we are preparing.”

Goebel goes on to say that there are many other applications in which EHLA can be used. For example, in the mining industry where hydraulic components are under high loads when making and supporting tunnels. Alternatively, in the oil and gas industry where hydraulics are used to keep drilling platforms level under extreme pressures.

These kinds of hydraulic components are ‘hard chromed’, which is achieved by electroplating. However, there are environmental concerns associated with disposal of the plating solution. EHLA is seen as a viable alternative.

“The advantage over hard chrome is you can deposit very thin layers from 50μm up to several millimetres, but usually we are fine with 200 or 300μm,” explains Goebel. “By doing that and by having a large selection of alloys which you can use and deposit on the material, which are ready to use and are nearly all qualified, you can create metrologically dense, diffusion dense layers to provide perfect corrosion and wear resistance that can be combined together and this is, for us, a very interesting new technology to look into where we can use our lasers and our expertise in the material process.”

Yet another advantage, Goebel says, is that the bonding strength of the coating will usually be at least on the same level as the substrate. For example, a steel substrate could be coated in steel powder, it will have the tensile strength of steel, so it will be extremely hard to remove. Metals that are considered too soft for certain applications – for example 316L stainless steel, which is austenitic – can also be hardened by around 50% due to EHLA’s rapid heating and cooling cycles. No extra chemicals are used to do this, and any powder overspray can be recycled and used again.

“Of course, there are certain limits,” says Goebel. “Steel and titanium will not go together. But we can use nearly every metal which is available as a powder for that process. We are still qualifying new materials.”

Available EHLA systems

Candel-Ruiz adds: “Our diode lasers and our disk lasers are suitable for EHLA, depending on the laser focus required.” With diode lasers, a focus of around 1mm is possible; with disk lasers, a focus as small as about 0.2mm. In addition to the laser beam source, another decisive factor is that the machine has a rotational axis that permits high speeds.

Depending on component size, Trumpf has various laser machines that are candidates for EHLA. The TruLaser Cell 3000 is suitable for small and medium-sized components, while the machines in the TruLaser Cell 7000 Series are suitable for large ones. Apart from these turnkey systems, manufacturers can integrate the EHLA method into their existing systems. The DepositionLine technology package from TRUMPF can also be equipped with powder feed nozzles developed by Fraunhofer ILT.

Looking forward Goebel says: “We are looking down several avenues. Of course, we are trying to be more productive. Going faster is definitely a goal for us, but what we’d really like is to have the high-speed laser cladding process in operation in as many applications as we see. And we of course try to enhance our system equipment so that the process will get even more flexible, for instance being able to do some high-speed cladding even on a 90° angle which is right now not feasible. So, in short: getting faster, more productive, more production, getting into more applications and enhancing our system technology.”

Author
Tom Austin-Morgan

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