By Jari Tuominen, Jonne Näkki, Henri Pajukoski, Tuomo Peltola, Petri Vuoristo
Laser cladding is currently done with 3-6 kW gas and solid state lasers. Components to be clad or repaired are usually small or some discrete regions in larger components. Net deposition rates are typically 1-2 kg/h. In large area coating applications, conventional coating methods such as thermal spraying (HVOF, HVAF) and overlay welding (SAW) prevail due to higher cost efficiency based mainly on high productivity and low capital costs. For applications such as boiler tube panels in power generation and massive hydraulics in off-shore and mining industries, coating properties produced by conventional coating methods are often insufficient. For this reason, high power ~20 kW CO2 laser cladding has been utilized. However, CO2 lasers have such drawbacks as low process energy efficiency in cladding and high maintenance costs which make them poor in cost efficiency terms. In recent years, advances in laser technologies have resulted in development of increasingly powerful disc, fiber and diode lasers. These lasers delivering tens of kilowatts of laser power overcome limitations of gas lasers. Due to shorter wavelength of the laser beam, process energy efficiency is more than doubled in cladding. Together with high wall-plug efficiencies, negligible maintenance costs and flexible beam guidance with fibers, these lasers outperform CO2 lasers especially in cost efficiency. Until now, these new generation lasers, particularly disc and fiber lasers, have shown high potential in thick section welding. In cladding, they have not yet been studied to such extent but it is obvious that considerable enhancements in productivity could be reached opening up some new laser cladding applications such as large pressure vessels and chemical reactors in chemical & petrochemical industries, various drums and rolls and rollers in pulp & paper industries etc.
In order to study how these new high power lasers perform in cladding, a series of experiments were carried out by using 15 kW fiber laser equipped with linear scanner. Two different types of cladding experiments shown in Table 1 were conducted: 1) scanner cladding using powder feedstock and 2) linear traverse cladding using wires.
Representative example of single bead of Inconel 625 manufactured by off-axis powder nozzle is illustrated in Figure 1. With the laser power of 15 kW and triangular scanning, fusion-bonded beads with negligible dilution were obtained with net deposition rates of approximately 15 kg/h. Corrosion studies conducted on multi-track coatings proved that corrosion resistance is equivalent to corresponding wrought alloy and superior to PTA coating (Figure 2). Coatings manufactured by linear traverse cladding technique using single- and twin-wires were not as successful as those with powders. Controlling dilution was particularly difficult. By using strip consumables instead of wires enabled the production of low diluted coatings as displayed in Figure 3.
Besides increasing laser power, additional heat sources can be applied in laser cladding to improve productivity. Hot-wire and induction-assisted laser cladding techniques have shown significant improvements in deposition rates. Apart from off-axis wire feeding nozzles, there are already several coaxial wire heads available which can be used not only in brazing and welding but also in cladding. In this work, coaxial hot-wire cladding process was implemented by using coaxial wire head developed by Fraunhofer IWS and HighYAG, 4.4 kW YAG laser and low power capacity welding source. Example of multi-track coating manufactured by this method is displayed in Figure 4. Main benefits of the process over off-axis wire feeding are direction-independent cladding process, less parameters in wire alignment and extremely high process stability. This new cladding technique will have huge impact not only on laser cladding but also on laser additive manufacturing of complex 3D objects.