High power diode lasers (HPDL) at the level of ? 6 kW are efficient cladding tools in heavy engineering applications where thick (up to 5 mm) wear and corrosion resistant coating layers are required. Large beam geometry makes possible the overlap of thick 20 mm wide cladding tracks side by side without coating defects. Compact size and closed cooling water circulation enable HPDL cladding process to take place also at a site of new or worn high-value machine parts, which have worn in operation or been damaged already during overseas transportation. Instead of moving parts of several tons’ weight, it would be perhaps more cost efficient to transport HPDL cladding unit.

Laser cladding is a surface treatment technology in which thick, dense and metallurgically adhered metallic layers are deposited on various structural steels with relatively low heat input, high accuracy and reproducibility. Laser cladding processes used in industrial cladding are largely based on the use of CO 2 or Nd:YAG lasers. High power diode lasers (HPDL) with rectangular beam spots are regarded as ideal laser sources for laser cladding processes, due to their compact size, high electrical to optical efficiency, easy operation, and low investment and running costs. In laser cladding of large surface areas, the affectivity of the laser cladding process becomes more important, i.e. high laser powers, wide laser beam spots, and high coating material feedrates are regarded as beneficial. In order to optimise the cladding process for such applications, special attention has to be put on devices used to deliver the coating power to the process. In the present work, various parameters in effective HPDL cladding are described and new approaches to optimised HPDL cladding process are described. The performance of a new HPDL cladding powder delivery nozzle will be presented and discussed. Abstract only; no full-text paper available.

Thermal spray processes are widely used to deposit high-chromium nickel-chromium coatings to improve high temperature oxidation and corrosion behaviour. However, in spite of the efforts made to improve the present spraying techniques, such as HVOF and plasma spraying, these coatings may still exhibit certain defects such as unmelted particles, oxide layers at splat boundaries, porosity and cracks, which are detrimental to corrosion performance in severe operation conditions. Due to low process temperature only mechanical bonding is obtained between the coating and substrate. Laser remelting of the sprayed coatings was studied in order to overcome the drawbacks of sprayed structures and to markedly improve the coating properties. The coating material was high-chromium nickel-chromium alloy, which contains small amounts of molybdenum and boron (53.3%Cr- 42.5%Ni - 2.5%Mo - 0.5%B). The coatings were prepared by high-velocity oxy-fuel spraying onto mild steel substrates. High power fiber coupled continuous wave Nd-YAG laser equipped with large beam optics was used to remelt the HVOF sprayed coating using different levels of scanning speed and beam width (10 mm and 20 mm). Coating remelted with the highest traverse speed tended to suffer cracking during rapid solidification inherent to laser processing. However, choosing appropriate laser parameters, non-porous, crack-free coatings with minimal dilution between coating and substrate were produced. Laser remelting resulted in the formation of dense oxide layer on top of the coatings and full homogenization of the sprayed structure. The coatings as-sprayed and after laser remelting were characterized by optical and electron microscopy (OPM, SEM). Dilution between coating and substrate was studied with EDS. The properties of the laser remelted coatings were directly compared with properties of as-sprayed HVOF coatings.

Thermal spray processes are widely used to protect materials and components against wear, corrosion and oxidation. Despite the use of the latest developments of thermal spraying, such as HVOF and plasma spraying, these coatings may in certain operation conditions show inadequate performance, e.g. due to insufficient bond strength and/or mechanical properties and corrosion resistance inferior to those of corresponding bulk materials. The main cause for a low bond strength in thermal sprayed coatings is the low process temperature, which results only in mechanical bonding. Mechanical and corrosion properties typically inferior to wrought materials are caused by the chemical and structural inhomogeneity of the thermal sprayed coating material. In order to overcome the drawbacks of sprayed structures and to markedly improve the coating properties, laser remelting of sprayed coating was studied in the present work. The coating material was nickel based superalloy Inconel 625, which contains chromium and molybdenum as the main alloying agents. The coating was prepared by high-velocity oxy-fuel spraying onto mild steel substrates. High power continuous wave Nd-YAG laser equipped with large beam optics was used to remelt the HVOF sprayed coating using different levels of power and scanning speed. The coatings as-sprayed and after laser remelting were characterized by optical and electron microscopy. Laser remelting resulted in full homogenization of the sprayed structure. This strongly influenced positively the performance of the laser remelted coatings in adhesion, wet corrosion and high temperature oxidations test. The properties of the laser remelted coatings were compared directly with the properties of as-sprayed HVOF coatings, and with PTA overlay coatings and wrought Inconel 625.