By: Eckhard Beyer
Fraunhofer IWS
Over the past decade laser buildup welding transitioned from mostly specialized laboratory efforts into an established industrial technology for high quality and precise surface coating deposition. Compared to traditional plasma powder buildup welding processes, laser cladding generates superior corrosion and wear protective coatings. The laser process can also generate localized surface functionalities. These combined traits of the process ultimately led to the industrial breakthrough of the technology. Today there are no acceptable alternatives to laser cladding for many applications including mining, oil and gas production and tool and die making.
However, so far the technology has proven insufficient in terms of deposition rates to coat large areas with protective and functional coatings such as, for example, those used for hydraulic cylinders. Industrial users desire the superior quality of the laser-produced coatings but simultaneously also require a high productivity and cost efficiency.
Currently there are two practical solutions to address this challenge with modern laser and laser-hybrid technologies. The commercial availability of high power diode lasers up to 10 kW has opened the way for significantly improved deposition rates. For example, with the laser as the only energy source in combination with powder jet forming nozzles, it is possible to achieve deposition rates of 9 kg/h of INCONEL625 corrosion protective coatings on large cylindrical components. A remarkable track width of 1 inch can be deposited using a specially designed flat-jet powder nozzle at 10 kW laser power. This nozzle can handle a powder throughput of up to 30 kg/h, and it is limited to the use of metallic alloys such as Inconel, Stellite, steel and bronze.
A hybrid processing head was developed to integrate a module for additional inductive heating during the laser process, see figure. This hybrid technology concept further improves the deposition rate and the energy efficiency. The localized inductive heating directly supports the laser beam and compensates heat losses, which leaves more laser energy to melt the powder. This resulting deposition rates increase 2-2.5 fold. The coaxial nozzle designed for this application can handle a powder throughput of up to 18 kg/h. Typical practical deposition rates for INCONEL 625 are about 14-16 kg/h with the simultaneous application of 8 kW laser and 12 kW induction power.
Another application potential of this hybrid process is to use the induction coupling to adapt coating strategies with tailored heat management. The additional heat leads to a significantly increased cooling time t8/5, as well as to decreased spatial temperature gradients. This configuration facilitates the crack free processing of especially hard and wear resistant metal alloys. An example is a protective coating made from Stellite 20 with a hardness exceeding 60 HRC.
The hybrid process also leads to a substantial cost reduction of the technology. The specific investment costs for the required energy sources are reduced by 50% while the overall energetic efficiency of the process is doubled.
