Monday, August 08, 2016

Additive manufacturing technique protects sensors

By Nick Flaherty

Researchers in Germany have used techniques more suited to 3D printing to protect sensors in harsh environments.

O.R. Lasertechnologie in Dieburg, Germany has used an additive manufacturing technique called DMD (“Direct Metal Deposition”) to protect the sensors with a hard alloy to significantly extend their lifetimes, for example in pipelines of the oil and gas industry.

Industrial sensors are deployed to precisely and reliably monitor temperatures, flow rates, and pressure over long periods of time and can be subject to extreme stresses while doing so. Each day, about a million barrels of crude oil, or 160,000 cubic meters, pass through a pipeline with a diameter of one meter. That is equivalent to 1850 liters per second. Onshore gas pipelines have an extremely high internal pressure of 100 bars, which can even reach 200 bars or more in offshore pipelines. Sensor elements used to monitor the flow suffer considerable wear as a result of corrosion and abrasion. This shortens their lifetimes and necessitates costly repairs.

The way to protect such sensors from wear is to coat it with a cobalt-chromium-based alloy called Stellite. This is usually applied in composite layers with a total thickness of several millimeters but the intense heat applied during the process results in considerable mingling of the sensor’s material with the cladding.

Instead, the DMD system uses relatively low laser output levels starting at 200 W, but with a high deposition rate of up to 5000 mm³/h. The laser only minimally melts the surface of the sensor, and only at scattered points. Metallic powder, with grain sizes between 45 and 90 µm, is fed coaxially to the laser beam and permanently fuses with the object’s surface. The advantages of this approach include precise deposition of the material, low heat penetration, and an undistorted, crack-free coating. Track widths between 200 µm and 2 mm are possible.

The team of the R&D department of OR LASER spent a year collaborating with the Fraunhofer Institute to develop a highly efficient, easy-to-install powder nozzle that works with high repeatability and is suitable for automated processes.

The coaxial arrangement also permits deposition of material independently of the direction of cladding, so that the workpiece can be freely rotated in all directions and, if required, even “grow” in three dimensions. Moreover, the laser parameters can be dynamically adjusted to changing conditions on the fly.

In order to prevent oxidation and the formation of tiny bubbles, the work is done in a shielding atmosphere of argon, a noble gas. The resulting surface quality is like new, free of pores and cracks, very close to the required final contours, and neat. The sensor itself is hardly affected by this “minimally invasive” technique, while its resistance to wear is greatly improved.

The system is completely manufactured in Germany, and the nozzle is the first of its kind to enable a combination of wire- and powder-based laser cladding.

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