Laser cladding, which involves applying protective coatings to components to protect them against friction, corrosion or heat, is becoming increasingly popular. Metal wire or metal powder emitted in a stream of protective gas is melted on the surfaces of components using a laser. The procedure not only protects against wear, but can also be used to repair wear and other damage.
High-performance solid-state lasers, some of them fiber-coupled, with a power output of 1 to 10 kW are used. Laser-based sensor systems monitor the process inline and collect digital data for a fully automated, quality assured coating process. Using 750 measurements per second, the lens position is controlled so as to ensure that the focal point of the processing laser remains at a constant distance from the surface being processed.
Researchers from the Fraunhofer Institute for Laser Technology (ILT) and the RWTH Aachen have further developed this procedure into “extreme high-speed Laser Material Deposition” (EHLA). It increases the previous feed speeds to up to 500 meters per minute – 100 to 250 times higher than before. At the same time, much thinner layers can be applied as a result. According to the ILT, the EHLA procedure allows metal layers just 25 to 250 micrometers thick to be applied homogeneously – much thinner than the typical layer thickness of around 0.5 millimeters. Thanks to this progress, laser cladding is becoming a serious competitor to established coating procedures. Unlike hard chrome plating, no chemicals are used and the laser procedure offers far greater material efficiency and economy than thermal spraying. However, the most important advantage lies in the quality: The coatings are firmly bonded, have no pores or cracks, and immunize surfaces against wear and corrosion for longer and more efficiently.
The researchers achieve a high processing speed and high surface quality by separating the melted powder from the component surface. The powder particles drip into the molten pool before being deposited on the component, thus making the layers are smoother. Their roughness is reduced by 90 percent compared to conventional laser cladding procedures. The process is also kinder to the components. “With EHLA, the material is influenced by heat only to a depth in the order of micrometers,” as the ILT reports. In the past, the heat penetrated several millimeters. As a result, even heat-sensitive components can be coated using lasers and difficult combinations with aluminum or cast iron alloys can be achieved. According to the researchers, the procedure could replace time-consuming, costly post-processing with a mortising machine in many cases. Instead, material is melted onto blanks which can be rotated if necessary. Blanks can be turned into wear and corrosion-resistant components ready for delivery in minutes instead of hours. TRUMPF has already started using the EHLA procedure in large-scale production.
AHC-Oberflächentechnik GmbH, part of the Dutch technology group Aalberts, is exploring different routes for protecting components against wear using lasers. Its patented LASOX-COAT® procedure hardens aluminum alloys with layers of aluminum oxide (corundum, Al2O3) approximately 10 micrometers thick and with Vickers hardnesses of up to 2,000 HV. This involves directing the laser beam at the component’s surface in an oxygen atmosphere. The laser causes alloy particles to melt and vaporize. Under the influence of the laser energy, these particles react with oxygen plasma to form corundum. All that is needed to provide a low-cost coating is to move the laser beam over the specific areas of the component where stress actually occurs. Here too, the protective coating does not need to be applied on the entire surface. Selectively generated corundum patterns are enough to provide protection against wear. Unlike dip coatings, the LASOX-COAT® process uses no chemicals and treats only the areas where protection is needed. As a result, it is highly efficient and environmentally friendly. On top of this, there is the degree of freedom typical of lasers – even rounded surfaces can be coated. This makes the process ideal for post-processing 3D printed components. Recent investigations have shown that post-processing accounts for 30 to 35 percent of the total costs in additive manufacturing. Innovative laser coating procedures have significant market potential in this area.